1. Heat to 1,475-1,525°F. (Hold at this temperature one-half hour,
to insure thorough heating.)
2. Cool slowly.
3. Machine.
4. Reheat to 1,375-1,425°F.
5. Quench.
6. Reheat to 250-550°F. and cool slowly.
_Heat Treatment R_
After forging:
1. Heat to 1,500-1,550°F.
2. Quench in oil.
3. Reheat to 1,200-1,300°F. (Hold at this temperature three hours.)
4. Cool slowly.
5. Machine.
6. Reheat to 1,350-1,450°F.
7. Quench in oil.
8. Reheat to 250-500°F. and cool slowly.
_Heat Treatment S_
After forging or machining:
1. Carbonize at a temperature between 1,600 and 1,750°F.
(1,650-1,700°F. Desired.)
2. Cool slowly in the carbonizing mixture.
3. Reheat to 1,650-1,750°F.
4. Quench.
5. Reheat to 1,475-1,550°F.
6. Quench.
7. Reheat to 250-550°F. and cool slowly.
_Heat Treatment T_
After forging or machining:
1. Heat to 1,650-1,750°F.
2. Quench.
3. Reheat to 500-1,300°F. and cool slowly.
_Heat Treatment U_
After forging:
1. Heat to 1,525-1,600°F. (Hold for about one-half hour.)
2. Cool slowly.
3. Machine.
4. Reheat to 1,650-1,700°F.
5. Quench.
6. Reheat to 350-550°F. and cool slowly.
_Heat Treatment V_
After forging or machining:
1. Heat to 1,650-1,750°F.
2. Quench.
3. Reheat to 400-1,200°F. and cool slowly.
RESTORING OVERHEATED STEEL
The effect of heat treatment on overheated steel is shown graphically
in Fig. 65 to the series of illustrations on pages 137 to 144. This
was prepared by Thos. Firth & Sons, Ltd., Sheffield, England.
[Illustration: FIG. 65.--Chart of changes due to heating and cooling.]
The center piece Fig. 65 represents a block of steel weighing about
25 lb. The central hole accommodated a thermo-couple which was attached
to an autographic recorder. The curve is a copy of the temperature
record during heating and cooling. Into the holes in the side of
the block small pegs of overheated mild steel were inserted. One
peg was withdrawn and quenched at each of the temperatures indicated
by the numbered arrows, and after suitable preparation these pegs
were photographed in order to show the changes in structure taking
place during heating and cooling operations. The illustrations here
reproduced are selected from those photographs with the object
of presenting pictorially the changes involved in the refining of
overheated steel or steel castings. Figures 66 to 79 with their
captions show much that is of value to steel users.
[Illustration: FIG. 66.--The structure of overheated mild steel
from which all the pegs were made (magnified 25 diameters). The
pegs withdrawn at 720°C., or earlier, had this structure and were
quite soft.]
[Illustration: FIG. 67.--Peg withdrawn at 750°C. (magnified 25
diameters). The structure is apparently unaltered, but the peg was
hard and, unlike the earlier ones, would not bend double.]
[Illustration: FIG. 68.--A portion of 66 magnified 200 diameters
to show that the dark (pearlite) areas are laminated.]
[Illustration: FIG. 69.--A portion of 67 magnified 200 diameters,
showing that pearlite areas are no longer laminated and providing
reason for observed hardness.]
[Illustration FIG. 70.--Peg withdrawn at 780°C. (magnified 25
diameters), showing inter-diffusion of transformed pearlite and
ferrite areas.]
[Illustration: FIG. 71.--Peg withdrawn at 800°C. (magnified 25
diameters), showing inter-diffusion so far advanced that the original
outline of the crystals is now only faintly suggested.]
[Illustration: FIG. 72.--Peg withdrawn at 850°C. (magnified 100
diameters) after inter-diffusion was completed. Note the regular
outlines and the small size of the crystals as compared with 67.]
[Illustration: FIG. 73.--To facilitate comparison 67 was enlarged
to the same magnification as 62, and the one superimposed on the
other. The single large crystal occupied as much space as 8,000
of the smaller ones.]
[Illustration: FIG. 74.--The peg withdrawn on cooling at 800°C.
(magnified 100 diameters) shows the first reappearance of free
ferrite. All pegs withdrawn at higher temperatures were like Fig.
72.]
[Illustration: FIG. 75.--Peg withdrawn after cooling to 760°C. The
increased amount of free ferrite arranges itself about the crystals
as envelopes.]
[Illustration: FIG. 76.-Peg withdrawn after cooling to 740°C.]
[Illustration: FIG. 77.--Peg withdrawn after cooling to 670°C.
(magnified 800 diameters). Just at this moment the lamination of
pearlite, which now occupied its original area, was taking place.
In some parts the lamination was perfect, in other parts the iron
and iron-carbide were still dissolved in each other.]
[Illustration: FIG. 78.--Any peg withdrawn after 670°C. on cooling
(magnified 100 diameters).]
[Illustration: FIG. 79.--Structure of overheated steel before (left)
and after refining (right).]
CHAPTER IX
HARDENING CARBON STEEL FOR TOOLS
For years the toolmaker had full sway in regard to make of steel
wanted for shop tools, he generally made his own designs, hardened,
tempered, ground and usually set up the machine where it was to
be used and tested it.
Most of us remember the toolmaker during the sewing machine period
when interchangeable tools were beginning to find their way; rather
cautiously at first. The bicycle era was the real beginning of
tool making from a manufacturing standpoint, when interchangeable
tools for rapid production were called for and toolmakers were in
great demand. Even then, jigs, and fixtures were of the toolmaker's
own design, who practically built every part of it from start to
finish.
The old way, however, had to be changed. Instead of the toolmaker
starting his work from cutting off the stock in the old hack saw,
a place for cutting off stock was provided. If, for instance, a
forming tool was wanted, the toolmaker was given the master tool
to make while an apprentice roughed out the cutter. The toolmaker,
however, reserved the hardening process for himself. That was one
of the particular operations that the old toolmaker refused to
give up. It seemed preposterous to think for a minute that any
one else could possibly do that particular job without spoiling
the tools, or at least warp it out of shape (most of us did not
grind holes in cutters 15 to 20 years ago); or a hundred or more
things might happen unless the toolmaker did his own hardening
and tempering.
That so many remarkably good tools were made at that time is still
a wonder to many, when we consider that the large shop had from 30
to 40 different men, all using their own secret compounds, heating
to suit eyesight, no matter if the day was bright or dark, and then
tempering to color. But the day of the old toolmaker has changed.
Now a tool is designed by a tool designer, O.K.'d, and then a print
goes to the foreman of the tool department, who specifies the size
and gets the steel from the cutting-off department. After finishing
the machine work it goes to the hardening room, and this is the
problem we shall now take up in detail.
THE MODERN HARDENING ROOM.--A hardening room of today means a very
different place from the dirty, dark smithshop in the corner with
the open coal forge. There, when we wanted to be somewhat particular,
we sometimes shoveled the coal cinders to one side and piled a great
pile of charcoal on the forge. We now have a complete equipment;
a gas- or oil-heating furnace, good running water, several sizes
of lead pots, and an oil tank large enough to hold a barrel of
oil. By running water, we mean a large tank with overflow pipes
giving a constant supply. The ordinary hardening room equipment
should consist of:
Gas or oil muffle furnace for hardening.
Gas or oil forge furnace.
A good size gas or oil furnace for annealing and case-hardening.
A gas or oil furnace to hold lead pots.
Oil tempering tank, gas- or oil-heated.
Pressure blower.
Large oil tank to hold at least a barrel of oil.
Big water tank with screen trays connected with large pipe from bottom
with overflow.
Straightening press.
The furnace should be connected with pyrometers and tempering tank with
a thermometer.
Beside all this you need a good man. It does not make much difference
how completely the hardening department is fitted up, if you expect
good work, a small percentage of loss and to be able to tackle anything
that comes along, you must have a good man, one who understands
the difference between low- and high-carbon steel, who knows when
particular care must be exercised on particular work. In other
words, a man who knows how his work should be done, and has the
intelligence to follow directions on treatments of steel on which
he has had no experience.
Jewelers' tools, especially for silversmith's work, probably have
to stand the greatest punishment of any all-steel tools and to
make a spoon die so hard that it will not sink under a blow from
an 1,800-lb. hammer with a 4-ft. drop, and still not crack, demands
careful treatment.
To harden such dies, first cover the impression on the die with
paste made from bone dust or lampblack and oil. Place face down
in an iron box partly filled with crushed charcoal, leaving back
of die uncovered so that the heat can be seen at all times. Heat
slowly in furnace to a good cherry red. The heat depends on the
quality and the analysis of steel and the recommended actions of
the steel maker should be carefully followed. When withdrawn from
the fire the die should be quenched as shown in Fig. 80 with the
face of die down and the back a short distance out of the water.
When the back is black, immerse all over.
[Illustration: FIG. 80.--Quenching a die, face down.]
If such a tank is not at hand, it would pay to rig one up at once,
although a barrel of brine may be used, or the back of the die
may be first immersed to a depth of about 1/2 in. When the piece
is immersed, hold die on an angle as in Fig. 81.
[Illustration: FIG. 81.--Hold die at angle to quench.]
This is for the purpose of expelling all steam bubbles as they
form in contact with hot steel. We are aware of the fact that a
great many toolmakers in jewelry shops still cling to the overhead
bath, as in Fig. 82, but more broken pieces and more dies with
soft spots are due to this method than to all the others combined,
as the water strikes one spot in force, contracting the surface
so much faster than the rest of the die that the results are the
same as if an uneven heating had been given the steel.
TAKE TIME FOR HARDENING.--Uneven heating and poor quenching has
caused loss of many very valuable dies, and it certainly seems
that when a firm spends from $75 to $450 in cutting a die that
a few hours could be spared for proper hardening. But the usual
feeling is that a tool must be hurried as soon as the hardener
gets it, and if a burst die is the result from either uneven or
overheated steel and quenching same without judgment, the steel
gets the blame.
[Illustration: FIG. 82.--An obsolete method.]
Give the steel a chance to heat properly, mix a little common sense
with "your 30 years experience on the other fellows steel." Remember
that high-carbon steel hardens at a lower heat than low-carbon
steel, and quench when at the right heat in the two above ways,
and 99 per cent of the trouble will vanish.
When a die flies to pieces in quenching, don't rush to the
superintendent with a "poor-steel" story, but find out first why it
broke so that the salesman who sold it will not be able to harden
piece after piece from the same bar satisfactorily. If you find
a "cold short," commonly called "a pipe," you can lay the blame
on the steelmaker. If it is a case of overheating and quenching
when too hot, you will find a coarse grain with many bright spots
like crystals to the hardening depth. If uneven heating is the
cause, you will find a wider margin of hardening depth on one side
than on the other, or find the coarse grain from over-heating on
one side while on the other you will find a close grain, which
may be just right. If you find any other faults than a "pipe,"
or are not able to harden deep enough, then take the blame like
a man and send for information. The different steel salesmen are
good fellows and most of them know a thing or two about their own
business.
For much work a cooling bath at from 50 to 75°F. is very good both
for small hobs, dies, cutter plates or plungers. Some work will
harden best in a barrel of brine, but in running cold water, splendid
results will be obtained. Cutter plates should always be dipped
corner first and if any have stripper holes, they should first
be plugged with asbestos or fire clay cement.
In general it may be said that the best hardening temperature for
carbon steel is the lowest temperature at which it will harden
properly.
CARBON IN TOOL STEEL
Carbon tool steel, or "tool steel" as it is commonly called, usually
contains from 80 to 125 points (or from 0.80 to 1.25 per cent)
of carbon, and none of the alloys which go to make up the high
speed steels. This was formerly known also as crucible or "cast"
steel, or crucible cast steel, from the way in which it was made.
This was before the days of steel castings. The advent of these
caused so much confusion that the term was soon dropped. When we
say "tool steel," we nearly always refer to carbon-tool steel,
high-speed steel being usually designated by that name.
For many purposes carbon-steel cutters are still found best, although
where a large amount of material is to be removed at a rapid rate,
it has given way to high-speed steels.
CARBON STEELS FOR DIFFERENT TOOLS
All users of tool steels should carefully study the different qualities
of the steels they handle. Different uses requires different kinds of
steel for best results, and for the purpose of designating different
steels some makers have adopted the two terms "temper," and "quality,"
to distinguish between them.
In this case temper refers to the amount of carbon which is combined
with the iron to make the metal into a steel. The quality means
the absence of phosphorous, sulphur and other impurities, these
depending on the ores and the methods of treatment.
Steel makers have various ways of designating carbon steels for
different purposes. Some of these systems involve the use of numbers,
that of the Latrobe Steel Company being given herewith. It will
be noted that the numbers are based on 20 points of carbon per
unit. The names given the different tempers are also of interest.
Other makers use different numbers.
The temper list follows:
LATROBE TEMPER LIST OF CARBON TOOL STEELS
No. 3 temper 0.60 to 0.69 per cent carbon
No. 3-1/2 temper 0.70 to 0.79 per cent carbon
No. 4 temper 0.80 to 0.89 per cent carbon
No. 4-1/2 temper 0.90 to 0.99 pet cent carbon
No. 5 temper 1.00 to 1.09 per cent carbon
No. 5-1/2 temper 1.10 to 1.19 per cent carbon
No. 6 temper 1.20 to 1.29 per cent carbon
No. 6-1/2 temper 1.30 to 1.39 per cent carbon
No. 7 temper 1.40 to 1.49 per cent carbon
USES OF THE VARIOUS TEMPERS OF CARBON TOOL STEEL
DIE TEMPER.--No. 3: All kinds of dies for deep stamping, pressing
and drop forgings. Mining drills to harden only. Easily weldable.
SMITHS' TOOL TEMPER.--No. 3-1/2: Large punches, minting and rivet
dies, nailmakers' tools, hammers, hot and cold sets, snaps and
boilermakers' tools, various smiths' tools, large shear blades,
double-handed chisels, caulking tools, heading dies, masons' tools
and tools for general welding purposes.
SHEAR BLADE TEMPER.--No. 4: Punches, large taps, screwing dies,
shear blades, table cutlery, circular and long saws, heading dies.
Weldable.
GENERAL PURPOSE TEMPER.--No. 4-1/2: Taps, small punches, screwing
dies, sawwebs, needles, etc., and for all general purposes. Weldable.
AXE TEMPER.--No. 5: Axes, chisels, small taps, miners' drills and
jumpers to harden and temper, plane irons. Weldable with care.
CUTLERY TEMPER.--No. 5-1/2: Large milling cutters, reamers, pocket
cutlery, wood tools, short saws, granite drills, paper and tobacco
knives. Weldable with very great care.
TOOL TEMPER.--No. 6: Turning, planing, slotting, and shaping tools,
twist drills, mill picks, scythes, circular cutters, engravers'
tools, surgical cutlery, circular saws for cutting metals, bevel
and other sections for turret lathes. Not weldable.
HARD TOOL TEMPER.--No. 6-1/2: Small twist drills, razors, small
and intricate engravers' tools, surgical instruments, knives. Not
weldable.
RAZOR TEMPER.--No. 7: Razors, barrel boring bits, special lathe
tools for turning chilled rolls. Not weldable.
STEEL FOR CHISELS AND PUNCHES
The highest grades of carbon or tempering steels are to be recommended
for tools which have to withstand shocks, such as for cold chisels
or punches. These steels are, however, particularly useful where
it is necessary to cut tempered or heat-treated steel which is
more than ordinarily hard, for cutting chilled iron, etc. They are
useful for boring, for rifle-barrel drilling, for fine finishing
cuts, for drawing dies for brass and copper, for blanking dies for
hard materials, for formed cutters on automatic screw machines
and for roll-turning tools.
Steel of this kind, being very dense in structure, should be given
more time in heating for forging and for hardening, than carbon
steels of a lower grade. For forging it should be heated slowly
and uniformly to a bright red and only light blows used as the
heat dies out. Do not hammer at all at a black heat. Reheat slowly
to a dark red for hardening and quench in warm water. Grind on a
wet grindstone.
Where tools have to withstand shocks and vibration, as in pneumatic
hammer work, in severe punching duty, hot or cold upsetting or
similar work, tool steels containing vanadium or chrome-vanadium
give excellent results. These are made particularly for work of
this kind.
CHISELS-SHAPES AND HEAT TREATMENT[1]
[Footnote 1: Abstract of paper by HENRY FOWLER, chief mechanical
engineer of the Midland Ry., England, before the Institution of
Mechanical Engineers.]
In the chief mechanical engineer's department of the Midland Ry.,
after considerable experimenting, it was decided to order chisel
steel to the following specifications: carbon, 0.75 to 0.85 per
cent, the other constituents being normal. This gives a complete
analysis as follows: carbon, 0.75 to 0.85; manganese, 0.30; silicon,
0.10; sulphur, 0.025; phosphorus, 0.025.
The analysis of a chisel which had given excellent service was as
follows: carbon, 0.75; manganese, 0.38; silicon, 0.16; sulphur,
0.028; phosphorus, 0.026. The heat treatment is unknown.
[Illustration: FIG. 83.--Forms of chisels standardized for the
locomotive shops of the Midland Ry., England.]
At the same time that chisel steel was standardized, the form of
the chisels themselves was revised, and a standard chart of these
as used in the locomotive shops was drawn up. Figure 83 shows the
most important forms, which are made to stock orders in the smithy
and forwarded to the heat-treatment room where the hardening and
tempering is carried out on batches of fifty. A standard system
of treatment is employed, which to a very large extent does away
with the personal element. Since the chemical composition is more
or less constant, the chief variant is the section which causes
the temperatures to be varied slightly. The chisels are carefully
heated in a gas-fired furnace to a temperature of from 730 to 740°C.
(1,340 to 1,364°F.) according to section. In practice, the first
chisel, is heated to 730°C.; and the second to 735°C. (1,355°F.);
and a 1 in. half round chisel to 740°C., because of their varying
increasing thickness of section at the points. Upon attaining this
steady temperature, the chisels are quenched to a depth of 3/8
to 1/2 in. from the point in water, and then the whole chisel is
immersed and cooled off in a tank containing linseed oil.
The oil-tank is cooled by being immersed in a cold-water tank through
which water is constantly circulated. After this treatment, the
chisels have a dead hard point and a tough or sorbitic shaft. They
are then tempered or the point "let down." This is done by immersing
them in another oil-bath which has been raised to about 215°C.
(419°F). The first result is, of course, to drop the temperature
of the oil, which is gradually raised to its initial point. On
approaching this temperature the chisels are taken out about every
2°C. rise and tested with a file, and at a point between 215 and
220°C. (428°F.), when it is found that the desired temper has been
reached, the chisels are removed, cleaned in sawdust, and allowed
to cool in an iron tray.
No comparative tests of these chisels with those bought and treated
by the old rule-of-thumb methods have been made, as no exact method of
carrying out such tests mechanically, other than trying the hardness
by the Brinell or scleroscope method, are known; any ordinary test
depends so largely upon the dexterity of the operator. The universal
opinion of foremen and those using the chisels as to the advantages
of the ones receiving the standard treatment described is that
a substantial improvement has been made. The chisels were not
"normalized." Tests of chisels normalized at about 900°C. (1,652°F.)
showed that they possessed no advantage.
Tools or pieces which have holes or deep depressions should be
filled before heating unless it is necessary to have the holes
hard on the inside. In that case the filling would keep the water
away from the surface and no hardening would take place. Where
filling is to be done, various materials are used by different
hardeners. Fireclay and common putty seem to be favored by many.
Every mechanic who has had anything to do with the hardening of
tools knows how necessary it is to take a cut from the surface of
the bar that is to be hardened. The reason is that in the process
of making the steel its outer surface has become decarbonized.
This change makes it low-carbon steel, which will of course not
harden. It is necessary to remove from 1/16 to 1/4 in. of diameter
on bars ranging from 1/2 to 4 in.
This same decarbonization occurs if the steel is placed in the
forge in such a way that unburned oxygen from the blast can get at
it. The carbon is oxidized, or burned out, converting the outside
of the steel into low-carbon steel. The way to avoid this is to use
a deep fire. Lack of this precaution is the cause of much spoiled
work, not only because of decarbonization of the outer surface
of the metal, but because the cold blast striking the hot steel
acts like boiling hot water poured into an ice-cold glass tumbler.
The contraction sets up stresses that result in cracks when the
piece is quenched.
PREVENTING DECARBONIZATION OF TOOL STEEL
It is especially important to prevent decarbonization in such tools
as taps and form cutters, which must keep their shape after hardening
and which cannot be ground away on the profile. For this reason
it is well to put taps, reamers and the like into pieces of pipe
in heating them. The pipe need be closed on one end only, as the
air will not circulate readily unless there is an opening at both
ends.
Even if used in connection with a blacksmith's forge the lead bath
has an advantage for heating tools of complicated shapes, since
it is easier to heat them uniformly and they are submerged and
away from the air. The lead must be stirred frequently or the heat
is not uniform in all parts of the lead bath. Covering the lead
with powdered charcoal will largely prevent oxidization and waste
of lead.
Such a bath is good for temperatures between 620 and 1,150°F. At
higher temperatures there is much waste of lead.
ANNEALING TO RELIEVE INTERNAL STRESSES
Work quenched from a high temperature and not afterward tempered
will, if complex in shape, contain many internal stresses which may
later cause it to break. They may be eased off by slight heating
without materially lessening the hardness of the piece. One way
to do this is to hold the piece over a fire and test it with a
moistened finger. Another way is to dip the piece in boiling water
after it has first been quenched in a cold bath. Such steps are
not necessary with articles which will afterward be tempered and
in which the strains are thus reduced.
In annealing steels the operation is similar to hardening, as far
as heating is concerned. The critical temperatures are the proper
ones for annealing as well as hardening. From this point on there
is a difference, for annealing consists in cooling as slowly as
possible. The slower the cooling the softer will be the steel.
Annealing may be done in the open air, in furnaces, in hot ashes
or lime, in powdered charcoal, in burnt bone, in charred leather
and in water. Open-air annealing will do as a crude measure in
cases where it is desired to take the internal stresses out of
a piece. Care must be taken in using this method that the piece
is not exposed to drafts or placed on some cold substance that
will chill it. Furnace annealing is much better and consists in
heating the piece in a furnace to the critical temperature and
then allowing the work and the furnace to cool together.
When lime or ashes are used as materials to keep air away from
the steel and retain the heat, they should be first heated to make
sure that they are dry. Powdered charcoal is used for high-grade
annealing, the piece being packed in this substance in an iron box
and both the work and the box raised to the critical temperature
and then allowed to cool slowly. Machinery steel may be annealed in
spent ground-bone that has been used in casehardening; _but tool
steel must never be annealed in this way_, as it will be injured
by the phosphorus contained in the bone. Charred leather is the
best annealing material for high-carbon steel, because it prevents
decarbonizing taking place.
DOUBLE ANNEALING
Water annealing consists in heating the piece, allowing it to cool
in air until it loses its red heat and becomes black and then
immediately quenching it in water. This plan works well for very
low-carbon steel; but for high-carbon steel what is known as the
"double annealing treatment" must be given, provided results are
wanted quickly. The process consists in heating the steel quickly to
200° or more above the upper critical, cooling in air down through
the recalescence point, then reheating it to just above the critical
point and again cooling slowly through the recalescence, then quenching
in oil. This process retains in the steel a fine-grained structure
combined with softness.
QUENCHING TOOL STEEL
To secure proper hardness, the cooling of quenching of steel is
as important as its heating. Quenching baths vary in nature, there
being a large number of ways to cool a piece of steel in contrast
to the comparatively few ways of heating it.
Plain water, brine and oil are the three most common quenching
materials. Of these three the brine will give the most hardness,
and plain water and oil come next. The colder that any of these
baths is when the piece is put into it the harder will be the steel;
but this does not mean that it is a good plan to dip the heated
steel into a tank of ice water, for the shock would be so great
that the bar would probably fly to pieces. In fact, the quenching
bath must be sometimes heated a bit to take off the edge of the
shock.
Brine solutions will work uniformly, or give the same degree of
hardness, until they reach a temperature of 150°F. above which
their grip relaxes and the metals quenched in them become softer.
Plain water holds its grip up to a temperature of approximately
100°F.; but oil baths, which are used to secure a slower rate of
cooling, may be used up to 500° or more. A compromise is sometimes
effected by using a bath consisting of an inch or two of oil floating
on the surface of water. As the hot steel passes through the oil,
the shock is not as severe as if it were to be thrust directly
into the water; and in addition, oil adheres to the tool and keeps
the water from direct contact with the metal.
The old idea that mercury will harden steel more than any other
quenching material has been exploded. A bath consisting of melted
cyanide of potassium is useful for heating fine engraved dies and
other articles that are required to come out free from scale. One
must always be careful to provide a hood or exhaust system to get
rid of the deadly fumes coming from the cyanide pot.
The one main thing to remember in hardening tool steel is to quench
on a rising heat. This does not mean a rapid heating as a slow
increase in temperature is much better in every way.
THE THEORY OF TEMPERING.--Steel that has been hardened is generally
harder and more brittle than is necessary, and in order to bring
it to the condition that meets our requirements a treatment called
tempering is used. This increases the toughness of the steel, _i.e._,
decrease the brittleness at the expense of a slight decrease in
hardness.
There are several theories to explain this reaction, but generally
it is only necessary to remember that in hardening we quench steel
from the austenite phase, and, due to this rapid cooling, the normal
change from austenite to the eutectoid composition does not have
time to take place, and as a consequence the steel exists in a
partially transformed, unstable and very hard condition at atmospheric
temperatures. But owing to the internal rigidity which exists in
cold metal the steel is unable to change into its more stable phase
until atoms can rearrange themselves by the application of heat.
The higher the heat, the greater the transformation into the softer
phases. As the transformation takes place, a certain amount of heat
of reaction, which under slow cooling would have been released in
the critical range, is now released and helps to cause a further
slight reaction.
If a piece of steel is heated to a certain temperature and held
there, the tempering color, instead of remaining unchanged at this
temperature, will advance in the tempering-color scale as it would
with increasing temperature. This means that the tempering colors
do not absolutely correspond to the temperatures of steels, but the
variations are so slight that we can use them in actual practice.
(See Table 23, page 158.)
TEMPERATURES TO USE.--As soon as the temperature of the steel reaches
100°C. (212°F.) the transformation begins, increasing in intensity
as the temperature is raised, until finally when the lower critical
range is reached, the steel has been all changed into the ordinary
constituents of unhardened steels.
If a piece of polished steel is heated in an ordinary furnace, a
thin film of oxides will form on its surface. The colors of this
film change with temperature, and so, in tempering, they are generally
used as an indication of the temperature of the steel. The steel
should have at least one polished face so that this film of oxides
may be seen.
An alternative method to the determination of temper by color is
to temper by heating in an oil or salt bath. Oil baths can be used
up to temperatures of 500°F.; above this, fused-salt baths are
required. The article to be tempered is put into the bath, brought
up to and held at the required temperature for a certain length
of time, and then cooled, either rapidly or slowly. This takes
longer than the color method, but with low temperatures the results
are more satisfactory, because the temperature of the bath can
be controlled with a pyrometer. The tempering temperatures given
in the following table are taken from a handbook issued by the
Midvale Steel Company.
TABLE 23.--TEMPERING TEMPERATURES FOR STEELS
----------------------------------------------------------------------------
Temperature | | Temperature |
for 1 hr. | | for 8 min. |
---------------| Color |---------------| Uses
Deg. F.|Deg. C.| |Deg. F.|Deg. C.|
-------|-------|------------|-------|-------|-------------------------------
370 | 188 |Faint yellow| 460 | 238 |Scrapers, brass-turning tools,
| | | | |reamers, taps, milling cutters,
| | | | |saw teeth.
390 | 199 |Light straw | 510 | 265 |Twist drills, lathe tools,
| | | | |planer tools, finishing tools
410 | 210 |Dark straw | 560 | 293 |Stone tools, hammer faces,
| | | | |chisels for hard work, boring
| | | | |cutters.
430 | 221 |Brown | 610 | 321 |Trephining tools, stamps.
450 | 232 |Purple | 640 | 337 |Cold chisels for ordinary work,
| | | | |carpenters' tools, picks, cold
| | | | |punches, shear blades, slicing
| | | | |tools, slotter tools.
490 | 254 |Dark blue | 660 | 343 |Hot chisels, tools for hot
| | | | |work, springs.
510 | 265 |Light blue | 710 | 376 |Springs, screw drivers.
----------------------------------------------------------------------------
It will be noted that two sets of temperatures are shown, one being
specified for a time interval of 8 min. and the other for 1 hr. For
the finest work the longer time is preferable, while for ordinary
rough work 8 min. is sufficient, after the steel has reached the
specified temperature.
The rate of cooling after tempering seems to be immaterial, and
the piece can be cooled at any rate, providing that in large pieces
it is sufficiently slow to prevent strains.
KNOWING WHAT TAKES PLACE.--How are we to know if we have given a
piece of steel the very best possible treatment?
The best method is by microscopic examination of polished and etched
sections, but this requires a certain expense for laboratory equipment
and upkeep, which may prevent an ordinary commercial plant from
attempting such a refinement. It is highly recommended that any
firm that has any large amount of heat treatment to do, install
such an equipment, which can be purchased for from $250 to $500.
Its intelligent use will save its cost in a very short time.
The other method is by examination of fractures of small test bars.
Steel heated to its correct temperatures will show the finest possible
grain, whereas underheated steel has not had its grain structure
refined sufficiently, and so will not be at its best. On the other
hand, overheated steel will have a coarser structure, depending
on the extent of overheating.
To determine the proper quenching temperature of any particular
grade of steel it is only necessary to heat pieces to various
temperatures not more than 20°C. (36°F.) apart, quench in water,
break them, and examine the fractures. The temperature producing
the finest grain should be used for annealing and hardening.
Similarly, to determine tempering temperatures, several pieces
should be hardened, then tempered to various degrees, and cooled
in air. Samples, say six, reheated to temperatures varying by 100°
from 300 to 800°C. will show a considerable range of properties,
and the drawing temperature of the piece giving the desired results
can be used.
For drawing tempers up to 500°F. oil baths of fresh cotton seed
oil can be safely and satisfactorily used. For higher temperature
a bath of some kind of fused salt is recommended.
HINTS FOR TOOL STEEL USERS
Do not hesitate to ask for information from the maker as to the
best steel to use for a given purpose, mentioning in as much detail
as possible the use for which it is intended.
Do not heat the steel to a higher degree than that fixed in the
description of each class. Never heat the steel to more than a
cherry red without forging it or giving it a definite heat treatment.
Heating steel at even moderate temperature is liable to coarsen the
grain which can only be restored by forging or by heat treating.
Let the forging begin as soon as the steel is hot enough and never
let tool steel soak in the fire. Continue the hammering vigorously
and constantly, using lighter blows as it cools off, and stopping
when the heat becomes a very dull red or a faint brown.
Should welding be necessary care should be taken not to overheat
in order to make an easy weld. Keep it below the sparkling point
as this indicates that the steel is burnt.
Begin to forge as soon as the welds are put together, taking care
to use gentle strokes at first increasing them as the higher heat
falls, but not overdoing the hammering when the steel cools. The
hammering should be extended beyond the welding point and should
continue until the dull red or brown heat is reached.
PREVENTING CRACKS IN HARDENING
The blacksmith in the small shop, where equipment is usually very
limited, often consisting of a forge, a small open hard-coal furnace,
a barrel of water and a can of oil must have skill and experience.
With this equipment the smith is expected to, and usually can,
produce good results if proper care is taken.
In hardening carbon tool steel in water, too much cannot be said in
favor of slow, careful heating, nor against overheating if cracks
are to be avoided.
It is not wise to take the work from the hardening bath and leave
it exposed to the air if there is any heat left in it, because
it is more liable to crack than if left in the bath until cold.
In heating, plenty of time is taken for the work to heat evenly
clear through, thus avoiding strains caused by quick and improper
heating, In quenching in water, contraction is much more rapid
than was the expansion while heating, and strains begin the moment
the work touches the water. If the piece has any considerable size
and is taken from the bath before it is cold and allowed to come to
the air, expansion starts again from the inside so rapidly that the
chilled hardened surface cracks before the strains can be relieved.
Many are most successful with the hardening bath about blood warm.
When the work that is being hardened is nearly cold, it is taken
from the water and instantly put into a can of oil, where it is
allowed to finish cooling. The heat in the body of the tool will
come to the surface more slowly, thus relieving the strain and
overcoming much of the danger of cracking.
Some contend that the temper should be drawn as soon as possible
after hardening: but that if this cannot be done for some hours, the
work should be left in the oil until the tempering can be done. It
is claimed that forming dies and punch-press dies that are difficult
to harden will seldom crack if treated in this way.
Small tools or pieces that are very troublesome because of peculiar
shape should be made of steel which has been thoroughly annealed.
It is often well to mill or turn off the outer skin of the bar,
to remove metal which has been cold-worked. Then heat slowly just
through the critical range and cool in the furnace, in order to
produce a very fine grain. Tools machined from such stock, and
hardened with the utmost care, will have the best chance to survive
without warping, growth or cracking.
SHRINKING AND ENLARGING WORK
Steel can be shrunk or enlarged by proper heating and cooling.
Pins for forced fits can be enlarged several thousandths of an
inch by rapid heating to a dull red and quenching in water. The
theory is that the metal is expanded in heating and that the sudden
cooling sets the outer portion before the core can contract. In
dipping the piece is not held under water till cold but is dipped,
held a moment and removed. Then dipped again and again until cold.
Rings and drawing dies are also shrunk in a similar way. The rings
are slowly heated to a cherry red, slipped on a rod and rolled
in a shallow pan of water which cools only the outer edge. This
holds the outside while the inner heated portion is forced inward,
reducing the hole. This operation can be repeated a number of times
with considerable success.
TEMPERING ROUND DIES
A number of circular dies of carbon tool steel for use in tool
holders of turret lathes were required. No proper tempering oven
was available, so the following method was adopted and proved quite
successful.
After the dies had been hardened dead hard in water, they were
cleaned up bright. A pair of ordinary smiths' tongs was made with
jaws of heavy material and to fit nicely all around the outside of
the die, leaving a 3/32-in. space when the jaws were closed around
the die. The dies being all ready, the tongs were heated red hot, and
the dies were picked up and held by the tongs. This tempered them
from the outside in, left the teeth the temper required and the
outside slightly softer. The dies held up the work successfully
and were better than when tempered in the same bath.
THE EFFECT OF TEMPERING ON WATER-QUENCHED GAGES
The following information has been supplied by Automatic and Electric
Furnaces, Ltd., 6, Queenstreet, London, S. W.:
Two gages of 3/4 in. diameter, 12 threads per inch, were heated
in a Wild-Barfield furnace, using the pyroscopic detector, and
were quenched in cold water. They were subsequently tempered in a
salt bath at various increasing temperatures, the effective diameter
of each thread and the scleroscope hardness being measured at each
stage. The figures are in 10,000ths of an inch, and indicate the
change + or - with reference to the original effective diameter
of the gages. The results for the two gages have been averaged.
TABLE 24.--CHANGES DUE TO QUENCHING
----------------------------------------------------------------
| After |Tempering temperature, degrees Centigrade
Thread |quenching|-----------------------------------------
| | 220 | 260 | 300 | 340 | 380 | 420
------------|---------|------|------|------|------|------|------
1 | +25 | +19 | +17 | +15 | +13 | +11 | +11
2 | +18 | +12 | +11 | + 9 | + 6 | + 5 | + 5
3 | +12 | + 6 | + 5 | + 3 | 0 | 0 | 0
4 | +10 | + 4 | + 4 | + 2 | ... | 0 | - 1
5 | + 9 | + 4 | + 4 | + 2 | 0 | 0 | 0
6 | + 9 | + 4 | + 3 | + 2 | 0 | 0 | 0
7 | +10 | + 5 | + 5 | + 3 | + 2 | + 1 | + 2
8 | + 8 | + 4 | + 3 | + 2 | 0 | 0 | + 1
9 | + 9 | + 4 | + 3 | + 2 | + 1 | + 1 | + 1
10 | + 9 | + 5 | + 5 | + 3 | + 2 | + 2 | + 2
11 | + 7 | + 4 | + 4 | + 2 | + 1 | + 1 | + 1
12 | + 9 | + 5 | + 5 | + 5 | + 4 | + 4 | + 3
| | | | | | |
Scleroscope | 80 | 70 | 70 | 62 | 56 | 53 | 52
----------------------------------------------------------------
Had these gages been formed with a plain cylindrical end projecting
in front of the screw, the first two threads would have been prevented
from increasing more than the rest. The gages would then have been
fairly easily corrected by lapping after tempering at 220°C. Practically
no lapping would be required if they were tempered at 340°C. There
seems to be no advantage in going to a higher temperature than
this. The same degree of hardness could have been obtained with
considerably less distortion by quenching directly in fused salt. It
is interesting to note that when the swelling after water quenching
does not exceed 0.0012 in., practically the whole of it may be
recovered by tempering at a sufficiently high temperature, but when
the swelling exceeds this amount the steel assumes a permanently
strained condition, and at the most only 0.0014 in. can be recovered
by tempering.
TEMPERING COLORS ON CARBON STEELS
Opinions differ as to the temperature which is indicated by the
various colors, or oxides, which appear on steel in tempering.
The figures shown are from five different sources and while the
variations are not great, it is safer to take the average temperature
shown in the last column.
TABLE 25.--COLORS, TEMPERATURES, DEGREES FAHRENHEIT
----------------------------------------------------------
| _A_ | _B_ | _C_ | _D_ | _E_ | Average
------------------|-----|-----|-----|-----|-----|---------
Faint yellow | 430 | 430 | 430 | 430 | 430 | 430
Light straw | 475 | 460 | 450 | ... | 450 | 458
Dark straw | 500 | 500 | 470 | 450 | 470 | 478
Purple (reddish) | 525 | 530 | 520 | 530 | 510 | 523
Purple (bluish) | ... | 555 | 550 | 550 | 550 | 551
Blue | 575 | 585 | 560 | 580 | 560 | 572
Gray blue | ... | 600 | ... | 600 | 610 | 603
Greenish blue | ... | 625 | ... | ... | 630 | 627
----------------------------------------------------------
TABLE 26.--ANOTHER COLOR TABLE
----------------------------------------------------------
Degrees |
Fahrenheit | High temperatures judged by color
------------|---------------------------------------------
430 | Very pale yellow \
460 | Straw-yellow |
480 | Dark yellow |
500 | Brown-yellow > Visible in full daylight
520 | Brown-purple |
540 | Full purple |
560 | Full blue |
600 | Very dark blue /
752 | Red heat, visible in the dark
885 | Red heat, visible in the twilight
975 | Red heat, visible in the daylight
1,292 | Dark red
1,652 | Cherry-red
1,832 | Bright cherry-red
2,012 | Orange-red
2,192 | Orange-yellow
2,372 | Yellow-white
2,552 | White welding heat
2,732 | Brilliant white
2,912 | Dazzling white (bluish-white)
----------------------------------------------------------
These differences might easily be due to the difference in the light
at the time the colors were observed. It must also be remembered
that even a thin coating of oil will make quite a difference and
cause confusion. It is these possible sources of error, coupled
with the ever present chance of human error, that makes it advisable
to draw the temper of tools in an oil bath heated to the proper
temperature as shown by an accurate high-temperature thermometer.
Another table, by Gilbert and Barker, runs to much higher temperatures.
Beyond 2,200°, however, the eye is very uncertain.
TABLE 26.--COLORS FOR TEMPERING TOOLS
-----------------------------------------------------------------------
Approximate |
color and | Kind of tool
temperature |
--------------|--------------------------------------------------------
Yellow | Thread chasers, hollow mills (solid type) twist drills
430 to 450°F.| centering tools, forming tools, cut-off tools, profile
| cutters, milling cutters, reamers, dies, etc.
--------------|--------------------------------------------------------
Straw-yellow | Thread rolling dies, counterbores, countersinks. Shear
460°F. | blades, boring tools, engraving tools, etc.
--------------|--------------------------------------------------------
Brown-yellow | Taps, Thread dies, cutters, reamers, etc.
500°F. |
--------------|--------------------------------------------------------
Light purple | Taps, dies, rock drills, knives, punches, gages, etc.
530°F. |
--------------|--------------------------------------------------------
Dark purple | Circular saws for metal, augers, dental and surgical
550°F. | instruments, cold chisels, axes.
--------------|--------------------------------------------------------
Pale blue | Bone saws, chisels, needles, cutters, etc.
580°F. |
--------------|--------------------------------------------------------
Blue | Hack saws, wood saws, springs, etc.
600°F. |
-----------------------------------------------------------------------
CHAPTER X
HIGH-SPEED STEEL
For centuries the secret art of making tool steel was handed down
from father to son. The manufacture of tool steel is still an art
which, by the aid of science, has lost much of its secrecy; yet
tool steel is today made by practical men skilled as melters,
hammer-men, and rollers, each knowing his art. These practical
men willingly accept guidance from the chemist and metallurgists.
A knowledge of conditions existing today in the manufacture of
high-speed steel is essential to steel treaters. It is well for
the manufacturer to have steel treaters understand some of his
troubles and difficulties, so that they will better comprehend the
necessity of certain trade customs and practices, and, realizing
the manufacturer's desire to cooperate with them, will reciprocate.
The manufacturer of high-speed steel knows and appreciates the
troubles and difficulties that may sometimes arise in the heat-treating
of his product. His aim is to make a uniform steel that will best
meet the requirements of the average machine shop on general work,
and at the same time allow the widest variation in heat treatment
to give desired results.
High speed steel is one of the most complex alloys known. A
representative steel contains approximately 24 per cent of alloying
metals, namely, tungsten, chromium, vanadium, silicon, manganese,
and in addition there is often found cobalt, molybdenum, uranium,
nickel, tin, copper and arsenic.
STANDARD ANALYSIS
The selection of a standard analysis by the manufacturer is the
result of a series of compromises between various properties imparted
to the steel by the addition of different elements and there is a
wide range of chemical analyses of various brands. The steel, to
be within the range of generally accepted analysis, should contain
over 16 per cent and under 20 per cent tungsten; if of lower tungsten
content it should carry proportionately more chromium and vanadium.
The combined action of tungsten and chromium in steel gives to it the
remarkable property of maintaining its cutting edge at relatively high
temperature. This property is commonly spoken of as "red-hardness."
The percentages of tungsten and chromium present should bear a
definite relationship to each other. Chromium imparts to steel
a hardening property similar to that given by carbon, although
to a less degree. The hardness imparted to steel by chromium is
accompanied by brittleness. The chromium content should be between
3.5 and 5 per cent.
Vanadium was first introduced in high-speed steel as a "scavenger,"
thereby producing a more homogeneous product, of greater density
and physical strength. It soon became evident that vanadium used
in larger quantities than necessary as a scavenger imparted to
the steel a much greater cutting efficiency. Recently, no less an
authority than Prof. J. O. Arnold, of the University of Sheffield,
England, stated that "high-speed steels containing vanadium have
a mean efficiency of 108.9, as against a mean efficiency of 61.9
obtained from those without vanadium content." A wide range of
vanadium content in steel, from 0.5 to 1.5 per cent, is permissible.
An ideal analysis for high-speed steel containing 18 per cent tungsten
is a chromium content of approximately 3.85 per cent; vanadium, 0.85
to 1.10 per cent, and carbon, between 0.62 and 0.77 per cent.
DETRIMENTAL ELEMENTS.--Sulphur and phosphorus are two elements known
to be detrimental to all steels. Sulphur causes "red-shortness"
and phosphorus causes "cold-shortness." The detrimental effects
of these two elements counteract each other to some extent but
the content should be not over 0.02 sulphur and 0.025 phosphorus.
The serious detrimental effect of small quantities of sulphur and
phosphorus is due to their not being uniformly distributed, owing
to their tendency to segregate.
The manganese and silicon contents are relatively unimportant in
the percentages usually found in high-speed steel.
The detrimental effects of tin, copper and arsenic are not generally
realized by the trade. Small quantities of these impurities are
exceedingly harmful. These elements are very seldom determined
in customers' chemical laboratories and it is somewhat difficult
for public chemists to analyze for them.
In justice to the manufacturer, attention should be called to the
variations in chemical analyses among the best of laboratories.
Generally speaking, a steel works' laboratory will obtain results
more nearly true and accurate than is possible with a customer's
laboratory, or by a public chemist. This can reasonably be expected,
for the steel works' chemist is a specialist, analyzing the same
material for the same elements day in and day out.
The importance of the chemical laboratory to a tool-steel plant
cannot be over-estimated. Every heat of steel is analyzed for each
element, and check analyses obtained; also, every substance used
in the mix is analyzed for all impurities. The importance of using
pure base materials is known to all manufacturers despite chemical
evidence that certain detrimental elements are removed in the process
of manufacture.
The manufacture of high-speed steel represents the highest art
in the making of steel by tool-steel practice. Some may say, on
account of our increased knowledge of chemistry and metallurgy,
that the making of such steel has ceased to be an art, but has
become a science. It is, in fact an art; aided by science. The
human element in its manufacture is a decided factor, as will be
brought in the following remarks:
The heat treatment of steel in its broad aspect may be said to
commence with the melting furnace and end with the hardening and
tempering of the finished product. High-speed steel is melted by
two general types of furnace, known as crucible and electric. Steel
treaters, however, are more vitally interested in the changes that
take place in the steel during the various processes of manufacture
rather than a detailed description of those processes, which are
more or less familiar to all.
In order that good high-speed steel may be furnished in finished
bars, it must be of correct chemical analysis, properly melted and
cast into solid ingots, free from blow-holes and surface defects.
Sudden changes of temperature are to be guarded against at every
stage of its manufacture and subsequent treatment. The ingots are
relatively weak, and the tendency to crack due to cooling strains
is great. For this reason the hot ingots are not allowed to cool
quickly, but are placed in furnaces which are of about the same
temperature and are allowed to cool gradually before being placed
in stock. Good steel can be made only from good ingots.
Steel treaters should be more vitally interested in the important
changes which take place in high-speed steel during the hammering
operations than that of any other working the steel receives in
the course of its manufacture.
QUALITY AND STRUCTURE
The quality of high-speed steel is dependent to a very great extent
upon its structure. The making of the structure begins under the
hammer, and the beneficial effects produced in this stage persist
through the subsequent operations, provided they are properly carried
out. The massive carbides and tungstides present in the ingot are
broken down and uniformly distributed throughout the billet.
To accomplish this the reduction in area must be sufficient and the
hammer blows should be heavy, so as to carry the compression into
the center of the billet; otherwise, undesirable characteristics
such as coarse structure and carbide envelopes will exist and cause
the steel treater much trouble. Surface defects invisible in the
ingot may be opened up under the hammering operation, in which
event they are chipped from the hot billet.
Ingots are first hammered into billets. These billets are carefully
inspected and all surface defects ground or chipped. The hammered
billets are again slowly heated and receive a second hammering,
known as "cogging." The billet resulting therefrom is known as
a "cogged" billet and is of the proper size for the rolling mill
or for the finishing hammer.
Although it is not considered good mill practice, some manufacturers
who have a large rolling mill perform the very important cogging
operation in the rolling mill instead of under the hammer. Cogging
in a rolling mill does not break up and distribute the carbides and
tungstides as efficiently as cogging under the hammer; another objection
to cogging in the rolling mill is that there is no opportunity to
chip surface defects developed as they can be under the trained eye
of a hammer-man, thereby eliminating such defects in the finished
billet.
The rolling of high-speed steel is an art known to very few. The
various factors governing the proper rolling are so numerous that
it is necessary for each individual rolling mill to work out a
practice that gives the best results upon the particular analysis
of steel it makes. Important elements entering into the rolling
are the heating and finishing temperatures, draft, and speed of
the mill. In all of these the element of time must be considered.
High-speed steel should be delivered from the rolling mill to the
annealing department free from scale, for scale promotes the formation
of a decarbonized surface. In preparation of bars for annealing,
they are packed in tubes with a mixture of charcoal, lime, and
other material. The tubes are sealed and placed in the annealing
furnace and the temperature is gradually raised to about 1,650°F.,
and held there for a sufficient length of time, depending upon the
size of the bars. After very slow cooling the bars are removed
from the tubes. They should then show a Brinnell number of between
235 and 275.
The inspection department ranks with the chemical and metallurgical
departments in safeguarding the quality of the product. It inspects
all finished material from the standpoint of surface defects, hardness,
size and fracture. It rejects such steel as is judged not to meet
the manufacturer's standard. The inspection and metallurgical
departments work hand in hand, and if any department is not functioning
properly it will soon become evident to the inspectors, enabling
the management to remedy the trouble.
The successful manufacture of high-speed steel can only be obtained
by those companies who have become specialists. The art and skill
necessary in the successful working of such steel can be attained
only by a man of natural ability in his chosen trade, and trained
under the supervision of experts. To become an expert operator
in any department of its manufacture, it is necessary that the
operator work almost exclusively in the production of such steel.
As to the heat treatment, it is customary for the manufacturer
to recommend to the user a procedure that will give to his steel
a high degree of cutting efficiency. The recommendations of the
manufacturer should be conservative, embracing fairly wide limits,
as the tendency of the user is to adhere very closely to the
manufacturer's recommendations. Unless one of the manufacturer's
expert service men has made a detailed study of the customer's
problem, the manufacturer is not justified in laying down set rules,
for if the customer does a little experimenting he can probably
modify the practice so as to produce results that are particularly
well adapted to his line of work.
The purpose of heat-treating is to produce a tool that will cut so
as to give maximum productive efficiency. This cutting efficiency
depends upon the thermal stability of the complex hardenites existing
in the hardened and tempered steel. The writer finds it extremely
difficult to convey the meaning of the word "hardenite" to those that
do not have a clear conception of the term. The complex hardenites
in high-speed steel may be described as that form of solid solution
which gives to it its cutting efficiency. The complex hardenites are
produced by heating the steel to a very high temperature, near the
melting point, which throws into solution carbides and tungstides,
provided they have been properly broken up in the hammering process
and uniformly distributed throughout the steel. By quenching the
steel at correct temperature this solid solution is retained at
atmospheric temperature.
It is not the intention to make any definite recommendations as to
heat-treating of high-speed steel by the users. It is recognized
that such steel can be heat-treated to give satisfactory results
by different methods. It is, however, believed that the American
practice of hardening and tempering is becoming more uniform. This
is due largely to the exchange of opinions in meetings and elsewhere.
The trend of American practice for hardening is toward the following:
_First_, slowly and carefully preheat the tool to a temperature
of approximately 1,500°F., taking care to prevent the formation
of excessive scale.
_Second_, transfer to a furnace, the temperature of which is
approximately 2,250 to 2,400°F., and allow to remain in the furnace
until the tool is heated uniformly to the above temperature.
_Third_, cool rapidly _in oil_, dry air blast, or lead bath.
_Fourth_, draw back to a temperature to meet the physical requirements
of the tool, and allow to cool in air.
It was not very long ago that the desirability of drawing hardened
high-speed steel to a temperature of 1,100° was pointed out, and it
is indeed encouraging to learn that comparatively few treaters have
failed to make use of this fact. Many treaters at first contended
that the steel would be soft after drawing to this temperature and
it is only recently, since numerous actual tests have demonstrated
its value, that the old prejudice has been eliminated.
High-speed steel should be delivered only in the annealed condition
because annealing relieves the internal strains inevitable in the
manufacture and puts it in vastly improved physical condition. The
manufacturer's inspection after annealing also discloses defects
not visible in the unannealed state.
The only true test for a brand of high-speed steel is the service that
it gives by continued performance month in and month out under actual
shop conditions. The average buyer is not justified in conducting a
test, but can well continue to purchase his requirements from a
reputable manufacturer of a brand that is nationally known. The
manufacturer is always willing to cooperate with the trade in the
conducting of a test and is much interested in the information
received from a well conducted test. A test, to be valuable, should be
conducted in a manner as nearly approaching actual working conditions
in the plant in which the test is made as is practical. In conducting
a test a few reputable brands should be allowed to enter. All tools
entered should be of exactly the same size and shape. There is much
difference of opinion as to the best practical method of conducting
a test, and the decision as to how the test should be conducted
should be left to the customer, who should cooperate with the
manufacturers in devising a test which would give the best basis
for conclusions as to how the particular brands would perform under
actual shop conditions.
The value of the file test depends upon the quality of the file and
the intelligence and experience of the person using it. The file
test is not reliable, but in the hands of an experienced operator,
gives some valuable information. Almost every steel treater knows
of numerous instances where a lathe tool which could be touched
with a file has shown wonderful results as to cutting efficiency.
Modern tool-steel practice has changed from that of the past, not
by the use of labor-saving machinery, but by the use of scientific
devices which aid and guide the skilled craftsman in producing a
steel of higher quality and greater uniformity. It is upon the
intelligence, experience, and skill of the individual that quality
of tool steel depends.
HARDENING HIGH-SPEED STEELS
We will now take up the matter of hardening high-speed steels. The
most ordinary tools used are for lathes and planers. The forging
should be done at carbon-steel heat. Rough-grind while still hot
and preheat to about carbon-steel hardening heat, then heat quickly
in high-speed furnace to white heat, and quench in oil. If a very
hard substance is to be cut, the point of tool may be quenched in
kerosene or water and when nearly black, finish cooling in oil.
Tempering must be done to suit the material to be cut. For cutting
cast iron, brass castings, or hard steel, tempering should be done
merely to take strains out of steel.
On ordinary machinery steel or nickel steel the temper can be drawn
to a dark blue or up to 900°F. If the tool is of a special form
or character, the risk of melting or scaling the point cannot be
taken. In these cases the tool should be packed, but if there is
no packing equipment, a tool can be heated to as high heat as is
safe without risk to cutting edges, and cyanide or prussiate of
potash can be sprinkled over the face and then quenched in oil.
Some very adverse criticism may be heard on this point, but experience
has proved that such tools will stand up very nicely and be perfectly
free from scales or pipes. Where packing cannot be done, milling
cutters, and tools to be hardened all over, can be placed in muffled
furnace, brought to 2,220° and quenched in oil. All such tools,
however, must be preheated slowly to 1,400 to 1,500° then placed in
a high-speed furnace and brought up quickly. Do not soak high-speed
steel at high heats. Quench in oil.
We must bear in mind that the heating furnace is likely to expand
tools, therefore provision must be made to leave extra stock to
take care of such expansion. Tools with shanks such as counter
bores, taps, reamers, drills, etc., should be heated no further
than they are wanted hard, and quench in oil. If a forge is not
at hand and heating must be done, use a muffle furnace and cover
small shanks with a paste from fire clay or ground asbestos. Hollow
mills, spring threading dies, and large cutting tools with small
shanks should have the holes thoroughly packed or covered with
asbestos cement as far as they are wanted soft.
CUTTING-OFF STEEL FROM BAR
To cut a piece from an annealed bar, cut off with a hack saw, milling
cutter or circular saw. Cut clear through the bar; do not nick or
break. To cut a piece from an unannealed bar, cut right off with
an abrasive saw; do not nick or break. If of large cross-section,
cut off hot with a chisel by first slowly and uniformly heating
the bar, at the point to be cut, to a good lemon heat, 1,800 to
1,850°F. and cut right off while hot; do not nick or break. Allow
the tool length and bar to cool before reheating for forging.
LATHE AND PLANER TOOLS
FORGING.--Gently warm the steel to remove any chill, is particularly
desirable in the winter, then heat slowly and carefully to a scaling
heat, that is a lemon heat (1,800 to 2,000°F.), and forge uniformly.
Reheat the tool for further forging directly the steel begins to
stiffen under the hammer. Under no circumstances forge the steel
when the temperature falls below a dark lemon to an orange color
about 1,700°F. Reheat as often as is necessary to finish forging
the tool to shape. Allow the tool to cool after forging by burying
the tool in dry ashes or lime. Do not place on the damp ground
or in a draught of air.
The heating for forging should be done preferably in a pipe or
muffle furnace but if this is not convenient use a good clean fire
with plenty of fuel between the blast pipe and the tool. Never
allow the tool to soak after the desired forging heat has been
reached. Do not heat the tool further back than is necessary to
shape the tool, but give the tool sufficient heat. See that the
back of the tool is flatly dressed to provide proper support under
the nose of the tool.
HARDENING HIGH-SPEED STEEL.--Slowly reheat the cutting edge of
the tool to a cherry red, 1,400°F., then force the blast so as
to raise the temperature quickly to a full white heat, 2,200 to
2,250°F., that is, until the tool starts to sweat at the cutting
face. Cool the point of the tool in a dry air blast or preferably
in oil, further cool in oil keeping the tool moving until the tool
has become black hot.
To remove hardening strains reheat the tool to from 500 to 1,100°F.
Cool in oil or atmosphere. This second heat treatment adds to the
toughness of the tool and therefore to its life.
GRINDING TOOLS.--Grind tools to remove all scale. Use a quick-cutting,
dry, abrasive wheel. If using a wet wheel, be sure to use plenty
of water. Do not under any circumstances force the tool against
the wheel so as to draw the color, as this is likely to set up
checks on the surface of the tool to its detriment.
FOR MILLING CUTTERS AND FORMED TOOLS
FORGING.--Forge as before.--ANNEALING.--Place the steel in a pipe,
box or muffle. Arrange the steel so as to allow at least 1 in.
of packing, consisting of dry powder ashes, powdered charcoal,
mica, etc., between the pieces and the walls of the box or pipe.
If using a pipe close the ends. Heat slowly and uniformly to a
cherry red, 1,375 to 1,450°F. according to size. Hold the steel at
this temperature until the heat has thoroughly saturated through
the metal, then allow the muffle box and tools to cool very slowly
in a dying furnace or remove the muffle with its charge and bury
in hot ashes or lime. The slower the cooling the softer the steel.
The heating requires from 2 to 10 hr. depending upon the size of
the piece.
HARDENING AND TEMPERING.--It is preferable to use two furnaces
when hardening milling cutters and special shape tools. One furnace
should be maintained at a uniform temperature from 1,375 to 1,450°F.
while the other should be maintained at about 2,250°F. Keep the
tool to be hardened in the low temperature furnace until the tool
has attained the full heat of this furnace. A short time should be
allowed so as to be assured that the center of the tool is as hot
as the outside. Then quickly remove the tool from this preheating
furnace to the full heat furnace. Keep the tool in this furnace only
as long as is necessary for the tool to attain the full temperature
of this furnace. Then quickly remove and quench in oil or in a
dry air blast. Remove before the tool is entirely cold and draw
the temper in an oil bath by raising the temperature of the oil
to from 500 to 750°F. and allow this tool to remain, at this
temperature, in the bath for at least 30 min., insuring uniformity
of temper; then cool in the bath, atmosphere or oil.
If higher drawing temperatures are desired than those possible
with oil, a salt bath can be used. A very excellent bath is made
by mixing two parts by weight of crude potassium nitrate and three
parts crude sodium nitrate. These will melt at about 450°F. and
can be used up to 1,000°F. Before heating the steel in the salt
bath, slowly preheat, preferably in oil. Reheating the hardened
high-speed steel to 1,000°F. will materially increase the life
of lathe tools, but milling and form cutters, taps, dies, etc.,
should not be reheated higher than 500 to 650°F., unless extreme
hardness is required, when 1,100 to 1,000°F., will give the hardest
edge.
INSTRUCTIONS FOR WORKING HIGH-SPEED STEEL
Owing to the wide variations in the composition of high-speed steels
by various makers, it is always advisable to follow the directions
of each when using his brand of steel. In the absence of specific
directions the following general suggestions from several makers
will be found helpful.
The Ludlum Steel Company recommend the following:
CUTTING-OFF.--To cut a piece from an annealed bar, cut off with
a hack saw, milling cutter or circular saw. Cut clear through the
bar; do not nick or break. To cut a piece from an unannealed bar,
cut right off with an abrasive saw; do not nick or break. If of
large cross-section, cut off hot with a chisel by first slowly
and uniformly heating the bar, at the point to be cut, to a good
lemon heat, 1,800°-1,850°F. and cut right off while hot; do not nick
or break. Allow the tool length and bar to cool before reheating
for forging.
LATHE AND PLANER TOOLS
TO FORGE.--Gently warm the steel to remove any chill is particularly
desirable in the winter. Then heat slowly and carefully to a scaling
heat, that is a lemon heat (1,800°-2,000°F.), and forge uniformly.
Reheat the tool for further forging directly the steel begins to
stiffen under the hammer. Under no circumstances forge the steel
when the temperature falls below a dark lemon to an orange color:
about 1,700°F. Reheat as often as is necessary to finish forging
the tool to shape. Allow the tool to cool after forging by burying
the tool in dry ashes or lime. Do not place on the damp ground
or in a draught of air.
The heating for forging should be done preferably in a pipe or
muffle furnace, but if this is not convenient use a good clean
fire with plenty of fuel between the blast pipe and the tool. Never
allow the tool to soak after the desired forging heat has been
reached. Do not heat the tool further back than is necessary to
shape the tool, but give the tool sufficient heat. See that the
back of the tool is flatly dressed to provide proper support under
the nose of the tool.
HARDENING.--Slowly reheat the cutting edge of the tool to a cherry
red, 1,400°F., then force the blast so as to raise the temperature
quickly to a full white heat, 2,200°-2,250°F., that is, until the
tool starts to sweat at the cutting face. Cool the point of the
tool in a dry air blast or preferably in oil; further cool in oil,
keeping the tool moving until the tool has become black hot.
To remove hardening strains reheat the tool to from 500° to 1,100°F.
Cool in oil or atmosphere. This second heat treatment adds to the
toughness of the tool and therefore to its life.
GRINDING.--Grind tools to remove all scale. Use a quick cutting,
dry, abrasive wheel. If using a wet wheel, be sure to use plenty
of water. Do not under any circumstances force the tool against
the wheel so as to draw the color, as this is likely to set up
checks on the surface of the tool to its detriment.
The Firth-Sterling Steel Company say:
INSTEAD OF PRINTING ANY RULES ON THE HARDENING AND TEMPERING OF
FIRTH-STERLING STEELS WE WISH TO SAY TO OUR CUSTOMERS: TRUST THE
STEEL TO THE SKILL AND THE JUDGEMENT OF YOUR TOOLSMITH AND TOOL
TEMPERER.
The steel workers of today know by personal experience and by
inheritance all the standard rules and theories on forging, hardening
and tempering of all fine tool steels. They know the importance of
slow, uniform heating, and the danger of overheating some steels,
and underheating others.
The tempering of tools and dies is a science taught by heat, muscle
and brains.
The tool temperer is the man to hold responsible for results. The
tempering of tools has been his life work. He may find suggestions
on the following pages interesting, but we are always ready to
trust the treatment of our steels to the experienced man at the
fire.
HEAT TREATMENT OF LATHE, PLANER AND SIMILAR TOOLS
FIRE.--For these tools a good fire is one made of hard foundry
coke, broken in small pieces, in an ordinary blacksmith forge with
a few bricks laid over the top to form a hollow fire. The bricks
should be thoroughly heated before tools are heated. Hard coal
may be used very successfully in place of hard coke and will give
a higher heat. It is very easy to give Blue Chip the proper heat
if care is used in making up the fire.
FORGING.--Heat slowly and uniformly to a good forging heat. Do
not hammer the steel after it cools below a bright red. Avoid as
much as possible heating the body of the tool, so as to retain
the natural toughness in the neck of the tool.
HARDENING.--Heat the point of the tool to an extreme white heat
(about 2,200°F.) until the flux runs. This heat should be the highest
possible short of melting the point. Care should be taken to confine
the heat as near to the point as possible so as to leave the annealing
and consequent toughness in the neck of the tool and where the tool
is held in the tool post.
COOL in an air blast, the open air or in oil, depending upon the
tools or the work they are to do.
For roughing tools temper need not be drawn except for work where
the edge tends to crumble on account of being too hard.
For finishing tools draw the temper to suit the purpose for which
they are to be used.
GRIND thoroughly on dry wheel (or wet wheel if care is used to prevent
checking).
HEAT TREATMENT OF MILLING CUTTERS, DRILLS, REAMERS, ETC.
THE FIRE.--Gas and electric furnaces designed for high heats are
now made for treating high-speed steels. We recommend them for
treating all kinds of Blue Chip tools and particularly the above
class. After tools reach a yellow heat in the forge fire they must
not be allowed to touch the fuel or come in contact with the blast
or surrounding air.
HEATING.--Tools of this kind should be heated to a mellow white
heat, or as hot as possible without injuring the cutting edges
(2,000 to 2,200°F.). For most work the higher the heat the better
the tool. Where furnaces are used, we recommend preheating the
tools to a red heat in one furnace before putting them in a white
hot furnace.
COOLING.--We recommend quenching all of the above tools in oil
when taken from the fire. We have found fish oil, cottonseed oil,
Houghton's No. 2 soluble oil and linseed oil satisfactory. The
high heat is the important thing in hardening Blue Chip tools.
If a white hot tool is allowed to cool in the open air it will be
hard, but the air scales the tool.
DRAWING THE TEMPER.--Tools of this class should be drawn considerably
more than water-hardening steel for the same purpose.
HEAT TREATMENT OF PUNCHES AND DIES, SHEARS, TAPS, ETC.
HEATING.--The degree to which tools of the above classes should
be heated depends upon the shape, size and use for which they are
intended. Generally, they should not be heated to quite as high a
heat as lathe tools or milling cutters. They should have a high
heat, but not enough to make the flux run on the steel (by pyrometer
1,900 to 2,100°F.).
COOLING.--Depending on the tools, some should be dipped in oil
all over, some only part way, and others allowed to cool down in
the air naturally, or under air blast. In cooling, the toughness
is retained by allowing some parts to cool slowly and quenching
parts that should be hard.
DRAWING THE TEMPER.--As in cooling, some parts of these tools will
require more drawing than others, but, on the whole, they must
be drawn more than water hardening tools for the same purpose or
to about 500°F. all over, so that a good file will just "touch"
the cutting or working parts.
BARIUM CHLORIDE PROCESS.--This is a process developed for treating
certain classes of tools, such as taps, forming tools, etc. It is
being successfully used in many large plants. Briefly the treatment
is as follows:
In this treatment the tools are first preheated to a red heat,
but small tools may be immersed without preheating. The barium
chloride bath is kept at a temperature of from 2,000 to 2,100°F.,
and tools are held in it long enough to reach the same temperature.
They are then dipped in oil. The barium chloride which adheres
to the tools is brushed off, leaving the tools as dean as before
heating.
A CHROMIUM-COBALT STEEL
The Latrobe Steel Company make a high-speed steel without tungsten,
its red-hardness properties depending on chromium and cobalt instead
of tungsten. It is known as P. R. K-33 steel. It does not require
the high temperature of the tungsten steels, hardening at 1,830 to
1,850°F. instead of 2,200° or even higher, as with the tungsten.
This steel is forged at 1,900 to 2,000°F. and must not be worked
at a lower temperature than 1,600°F. It requires soaking in the
fire more than the tungsten steels. It can be normalized by heating
slowly and thoroughly to 1,475°F., holding this for from 10 to 20
min. according to the size of the piece and cooling in the open
air, protected from drafts.
A peculiarity of this steel is that it becomes non-magnetic at or
above 1,960°F. and the magnetic quality is not restored by cooling.
Normalizing as above, however, restores the magnetic qualities. This
enables the user to detect any tools which have been overheated,
with a horseshoe magnet.
It is sometimes advantageous to dip tools, before heating for hardening,
in ordinary fuel or quenching oil. The oil leaves a thin film of
carbon which tends to prevent decarbonization, giving a very hard
surface.
For other makes of high-speed steel used in lathe and planer tools
the makers recommend that the tools be cut from the bar with a
hack saw or else heated and cut with a chisel. The heating should
be very slow until the steel reaches a red after which it can be
heated more rapidly and should only be forged at a high heat. It
can be forged at very high heats but care should be taken not to
forge at a low heat. The heating should be uniform and penetrate
clear to the center of the bar before forging is begun. Reheat
as often as necessary to forge at the proper heat.
After forging cool in lime before attempting to harden. Do not
attempt to harden with the forging heat as was sometimes done with
the carbon tools.
For hardening forged tools, heat slowly up to a bright red and
then rapidly until the point of the tool is almost at a melting
heat. Cool in a blast of cold, dry air. For large sizes of steel,
cool in linseed oil or in fish oil as is most convenient. If the
tools are to be used for finishing cuts heat to a bright yellow
and quench in oil. Grind for use on a sand wheel or grindstone
in preference to an emery or an artificial abrasive wheel.
For hardening milling and similar cutters, preheat to a bright
red, place the cutter on a round bar of suitable size, and revolve
it quickly over a very hot fire. Heat as high as possible without
melting the points of the teeth and cool in a cold blast of dry
air or in fish oil.
Light fragile cutters, twist drills, taps and formed cutters may
be heated almost white and then dipped in fish oil for hardening.
Where possible it is better to give an even higher heat and cool
in the blast of cold, dry air as previously recommended.
SUGGESTIONS FOR HANDLING HIGH-SPEED STEELS
The following suggestions for handling high-speed steels are given
by a maker whose steel is probably typical of a number of different
makes, so that they will be found useful in other cases as well.
These include hints as to forging as well as hardening, together
with a list of "dont's" which are often very useful. This applies
to forging, hardening of lathe, slotting, planing and all similar
tools.
[Illustration: FIG. 84.--All-steel, 5/8 in. square, 1/2 X 1 in.,
and larger is usually mild finished, and can be cut in a hack saw.
If cut off hot, be sure to heat the butt end slowly and thoroughly
in a clean fire. Rapid and insufficient heating invariably cracks
the steel. If you want to stamp the end with the name of the steel,
it is necessary that this is done at a good high orange color heat,
as it is otherwise apt to split the steel. (Take your time, do
not hurry.)]
HARDENING HIGH-SPEED STEEL
In forging use coke for fuel in the forge. Heat steel slowly and
thoroughly to a lemon heat. Do not forge at a lower heat. Do not
let the steel cool below a bright cherry red while forging. After
the tool is dressed, reheat to forging heat to remove the forging
strain, and lay on the floor until cold. Then have the tool rough
ground on a dry emery wheel.
[Illustration: FIG. 85.--Be sure to have a full yellow heat at the
dotted line. Remember this is a boring mill tool and will stand
out in the tool-post, and if you do not have a high thorough lemon
heat, your tool will snap off at the dotted line. (Ninety-five
per cent of all tools which break, have been forged at too low a
heat or at a heat not thorough to the center.)]
[Illustration: FIG. 86.--Keep your high lemon forging heat up.
If you forge under a steam hammer, take light blows. Do not jam
your tool into shape. Put frequently back into the fire. Never
let the high lemon color go down and beyond the dotted line.]
For built-up and bent tools special care should be taken that the
forging heat does not go below a bright cherry. For tools 3/4 by
1-1/2 or larger where there is a big strain in forging, such as
bending at angles of about 45 deg. and building the tools up, they
should be heated to at least 1,700°F. Slowly and without much blast.
For a 3/4 by 1-1/2 tool it should take about 10 min. with the correct
blast in a coke fire. Larger tools in proportion. They can then be
bent readily, but no attempt should be made to forge the steel
further without reheating to maintain the bright cherry red. This
is essential, as otherwise the tools crack in hardening or while
in use.
[Illustration: FIG. 87.--Be sure that the tool is absolutely straight
at the bottom, so as to lie flat in the tool-post.]
[Illustration: FIG. 88.--This is the finished forged tool, and
let this grow cold by itself, the slower the better. It is well to
cool the tool slowly in hot ashes, to remove all forging strain.
You can now grind the tool dry on a sharp emery wheel. The more
you now finish the tool in grinding, the less there is to come
off after hardening.]
In hardening place the tool in a coke fire (hollow fire if possible)
with a slow blast and heat gradually up to a white welding heat
on the nose of the tool. Then dip the white hot part only into
thin oil or hold in a strong cold air blast. When hardening in
oil do not hold the tool in one place but keep it moving so that
it cools as quickly as possible. It is not necessary to draw the
temper after hardening these tools.
[Illustration: FIG. 89.--This tool is ground, ready for hardening.
Never harden from the forging heat.]
[Illustration: FIG. 90.--Heat the nose of the tool only up to dotted
line, very slowly and thoroughly to an absolutely white welding
heat, so that it shows a trifle fused around the edges, and be
very sure that this fusing has gone thoroughly through the nose,
otherwise the fusing effect will be taken off after the second
grinding. Note the difference of the nose between this and Fig. 86.]
[Illustration: FIG. 91.--Shows unnecessary roasting and drossing.
Such hardening requires a great amount of grinding and is not good.
After hardening grind carefully on a wet emery wheel, and be sure
that the wheel is sharp with a plentiful supply of water. Do not
force the grinding, otherwise the cold water striking the steel
heated up by friction, will crack the nose. Be sure that the grinding
wheel is sharp.]
In grinding all tools should be ground as lightly as possible on
a soft wet sandstone or on a wet emery wheel, and care should be
taken not to create any surface cracks, which are invariably the
result of grinding too forcibly. The foregoing illustrations, Figs.
84 to 91, with their captions, will be found helpful.
Special points of caution to be observed when hardening high-speed
steel.
DON'T use a green coal fire; use coke, or build a hollow fire.
DON'T have the bed of the fire free from coal.
DON'T hurry the heating for forging. The heating has to be done
very slowly and the forging heat has to be kept very high (a full
lemon color) heat and the tool has to be continually brought back
into the fire to keep the high heat up. When customers complain
about seams and cracks, in 9 cases out of 10, this has been caused
by too low a forging heat, and when the blacksmith complains about
tools cracking, it is necessary to read this paragraph to him.
DON'T try to jam the tool into shape under a steam hammer with one
or two blows; take easy blows and keep the heat high.
DON'T have the tool curved at the bottom; it must lie perfectly
flat in the tool post.
DON'T harden from your forging heat; let the tool grow cold or
fairly cold. After forging you can rough grind the tool dry, but
not too forcibly.
DON'T, for hardening, get more than the nose white hot.
DON'T get the white heat on the surface only.
DON'T hurry your heating for hardening; let the heat soak thoroughly
through the nose of the tool.
DON'T melt the nose of the tool.
DON'T, as a rule, dip the nose into water; this should be done
only for extremely hard material. It is dangerous to put the nose
into water for fear of cracking and when you do put the nose into
water put just 1/2 in. only of the extreme white hot part into the
water and don't keep it too long in the water; just a few seconds,
and then harden in oil. We do not recommend water hardening.
DON'T grind too forcibly.
DON'T grind dry after hardening.
DON'T discolor the steel in grinding.
DON'T give too much clearance on tools for cutting cast iron.
DON'T start on cast iron with a razor edge on the tool. Take an
oil stone and wipe three or four times over the razor edge.
DON'T use tool holder steel from bars without hardening the nose
of each individual tool bit.
AIR-HARDENING STEELS.--These steels are recommended for boring,
turning and planing where the cost of high-speed seems excessive.
They are also recommended for hard wood knives, for roughing and
finishing bronze and brass, and for hot bolt forging dies. This
steel cannot be cut or punched cold but can be shaped and ground
on abrasive wheels of various kinds.
It should be heated slowly and evenly for forging and kept as evenly
heated at a bright red as possible. It should not be forged after
it cools to a dark red.
After the tool is made, heat it again to a bright red and lay it
down to cool in a dry place or it can be cooled in a cold, dry
air blast. Water must be kept away from it while it is hot.
CHAPTER XI
FURNACES
There are so many standard furnaces now on the market that it is
not necessary to go into details of their design and construction
and only a few will be illustrated. Oil, gas and coal or coke are
most common but there is a steady growth of the use of electric
furnaces.
[Illustration: FIG. 92.--Standard lead pot furnace.]
TYPICAL OIL-FIRED FURNACES.--Several types of standard oil-fired
furnaces are shown herewith. Figure 92 is a lead pot furnace, Fig.
93 is a vertical furnace with a center column. This column reduces
the cubical contents to be heated and also supports the cover.
[Illustration: FIG. 93.--Furnace with center column.]
A small tool furnace is shown in Fig. 94, which gives the construction
and heat circulation. A larger furnace for high-speed steel is
given in Fig. 95. The steel is supported above the heat, the lower
flame passing beneath the support.
For hardening broaches and long reamers and taps, the furnace shown
in Fig. 96 is used. Twelve jets are used, these coming in radially
to produce a whirling motion.
[Illustration: FIG. 94.--Furnace for cutting tools.]
[Illustration: FIG. 95.--High-speed steel furnace.]
Oil and gas furnaces may be divided into three types: the open
heating chamber in which combustion takes place in the chamber
and directly over the stock; the semimuffle heating chamber in
which combustion takes place beneath the floor of the chamber from
which the hot gases pass into the chamber through suitable openings;
and the muffle heating chamber in which the heat entirely surrounds
the chamber but does not enter it. The open furnace is used for
forging, tool dressing and welding. The muffle furnace is used for
hardening dies, taps, cutters and similar tools of either carbon
or high-speed steel. The muffle furnace is for spring hardening,
enameling, assaying and work where the gases of combustion may
have an injurious effect on the material.
[Illustration: FIG. 96.--Furnace for hardening broaches.]
[Illustration: FIG. 97.--Forging and welding furnace.]
[Illustration: FIG. 98.--Semi-muffle furnace.]
[Illustration: FIG. 99.--Muffle furnace.]
Furnaces of these types of oil-burning furnaces are shown in Figs.
97, 98, and 99; these being made by the Gilbert & Barker Manufacturing
Company. The first has an air curtain formed by jets from the large
pipe just below the opening, to protect the operator from heat.
[Illustration: FIG. 100.--Gas fired furnace.]
[Illustration: FIG. 101.--Car door type of annealing furnace.]
Oil furnaces are also made for both high- and low-pressure air,
each having its advocates. The same people also make gas-fired
furnaces.
Several types of furnaces for various purposes are illustrated
in Fig. 100 and 101. The first is a gas-fired hardening furnace
of the surface-combustion type.
A large gas-fired annealing furnace of the Maxon system is shown
in Fig. 101. This is large enough for a flat car to be run into
as can be seen. It shows the arrangement of the burners, the track
for the car and the way in which it fits into the furnace. These
are from the designs of the Industrial Furnace Corporation.
Before deciding upon the use of gas or oil, all sides of the problem
should be considered. Gas is perhaps the nearest ideal but is as a
rule more expensive. The tables compiled by the Gilbert & Barker
Manufacturing Company and shown herewith, may help in deciding
the question.
TABLE 27.--SHOWING COMPARISON OF OIL FUEL WITH VARIOUS GASEOUS FUELS
Heat units
per thousand
cubic feet
Natural gas 1,000,000
Air gas (gas machine) 20 cp 815,500
Public illuminating gas, average 650,000
Water gas (from bituminous coal) 377,000
Water and producer gas, mixed 175,000
Producer gas 150,000
Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the
following comparisons may evidently be made. At 5 cts. a gallon,
the equivalent heat units in oil would equal:
Per thousand
cubic feet
Natural gas at $0.375
Air gas, 20 cp at 0.307
Public illuminating gas, average at 0.244
Water gas (from bituminous coal) at 0.142
Water and producer gas, mixed at 0.065
Producer gas at 0.057
Comparing oil and coal is not always simple as it depends on the
work to be done and the construction of the furnaces. The variation
rises from 75 to 200 gal. of oil to a ton of coal. For forging
and similar work it is probably safe to consider 100 gal. of oil
as equivalent to a ton of coal.
Then there is the saving of labor in handling both coal and ashes,
the waiting for fires to come up, the banking of fires and the dirt
and nuisance generally. The continuous operation possible with
oil adds to the output.
When comparing oil and gas it is generally considered that 4-1/2
gal. of fuel oil will give heat equivalent to 1,000 cu. ft. of
coal gas.
The pressure of oil and air used varies with the system installed.
The low-pressure system maintains a pressure of about 8 oz. on the
oil and draws in free air for combustion. Others use a pressure
of several pounds, while gas burners use an average of perhaps
1-1/2 lb. of air to give best results.
The weights and volumes of solid fuels are: Anthracite coal, 55 to
65 lb. per cubic foot or 34 to 41 cubic feet per ton; bituminous
coal, 50 to 55 lb. per cubic foot or 41 to 45 cubic feet per ton;
coke, 28 lb. per cubic foot or 80 cubic feet per ton--the ton being
calculated as 2,240 lb. in each case.
A novel carburizing furnace that is being used by a number of people,
is built after the plan of a fireless cooker. The walls of the
furnace are extra heavy, and the ports and flues are so arranged
that when the load in the furnace and the furnace is thoroughly
heated, the burners are shut off and all openings are tightly sealed.
The carburization then goes on for several hours before the furnace
is cooled below the effective carburizing range, securing an ideal
diffusion of carbon between the case and the core of the steel
being carburized. This is particularly adaptable where simple steel
is used.
PROTECTIVE SCREENS FOR FURNACES
Workmen needlessly exposed to the flames, heat and glare from furnaces
where high temperatures are maintained suffer in health as well as
in bodily discomfort. This shows several types of shields designed
for the maximum protection of the furnace worker.
Bad conditions are not necessary; in almost every case means of relief
can be found by one earnestly seeking them. The larger forge shops
have adopted flame shields for the majority of their furnaces. Years
ago the industrial furnaces (particularly of the oil-burning variety)
were without shields, but the later models are all shield-equipped.
These shields are adapted to all of the more modern, heat-treating
furnaces, as well as to those furnaces in use for working forges;
and attention should be paid to their use on the former type since
the heat-treating furnaces are constantly becoming more numerous
as manufacturers find need of them in the many phases of munitions
making or similar work.
The heat that the worker about these furnaces must face may be
divided in general into two classes: there is first that heat due
to the flame and hot gases that the blast in the furnaces forces
out onto a man's body and face. In the majority of furnaces this
is by far the most discomforting, and care must be taken to fend
it and turn it behind a suitable shield. The second class is the
radiant heat, discharged as light from the glowing interior of
the furnace. This is the lesser of the two evils so far as general
forging furnaces are concerned, but it becomes the predominating
feature in furnaces of large door area such as in the usual
case-hardening furnaces. Here the amount of heat discharged is
often almost unbearable even for a moment. This heat can be taken
care of by interposing suitable, opaque shields that will temporarily
absorb it without being destroyed by it, or becoming incandescent.
Should such shields be so constructed as to close off all of the
heat, it might be impossible to work around the furnace for the
removal of its contents, but they can be made movable, and in such
a manner as to shield the major portion of the worker's body.
First taking up the question of flame shields, the illustration,
Fig. 102, is a typical installation that shows the main features
for application to a forging machine or drop-hammer, oil-burning
furnace, or for an arched-over, coal furnace where the flame blows
out the front. This shield consists of a frame covered with sheet
metal and held by brackets about 6 in. in front of the furnace.
It will be noted that slotted holes make this frame adjustable
for height, and it should be lowered as far as possible when in
use, so that the work may just pass under it and into the furnace
openings.
Immediately below the furnace openings, and close to the furnace
frame will be noted a blast pipe carrying air from the forge-shop
fan. This has a row of small holes drilled in its upper side for
the entire length, and these direct a curtain of cold air vertically
across the furnace openings, forcing all of the flame, or a greater
portion of it, to rise behind the shield. Since the shield extends
above the furnace top there is no escape for this flame until it has
passed high enough to be of no further discomfort to the workman.
In this case fan-blast air is used for cooling, and this is cheaper
and more satisfactory because a great volume may be used. However,
where high-pressure air is used for atomizing the oil at the burner,
and nothing else is available, this may be employed--though naturally
a comparatively small pipe will be needed, in which minute holes
are drilled, else the volume of air used will be too great for
the compressor economically to supply. Steam may also be employed
for like service.
[Illustration: FIGS. 102 to 108.--Protective devices for furnace
fronts.]
The latest shields of this type are all made double, as illustrated,
with an inner sheet of metal an inch or two inside of the front.
In the illustration, _A_, Fig. 102, this inner sheet is smaller,
but some are now built the same size as the front and bolted to
it with pipe spacers between. The advantage of the double sheet
is that the inner one bears the brunt of the flame, and, if needs
be, burns up before the outer; while, if due to a heavy fire it
should be heated red at any point, the outer sheet will still be
much cooler and act as an additional shield to the furnace man.
HEAVY FORGING PRACTICE.--In heavy forging practice where the metal
is being worked at a welding heat, the amount of flame that will
issue from an open-front furnace is so great that a plain, sheet-steel
front will neither afford sufficient protection nor stand up in
service. For such a place a water-cooled front is often used. The
general type of this front is illustrated in Fig. 103, and appears to
have found considerable favor, for numbers of its kind are scattered
throughout the country.
In this case the shield is placed at a slight angle from the vertical,
and along the top edge is a water pipe with a row of small holes
through which sprays of water are thrown against it. This water runs
down in a thin sheet over the shield, cooling it, and is collected
in a trough connected with a run-off pipe at the bottom. The lower
blast-pipe arrangement is similar to the one first described.
There are several serious objections to this form of shield that
should lead to its replacement by a better type; the first is that
with a very hot fire, portions in the center may become so rapidly
heated that the steam generated will part the sheet of water and
cause it to flow from that point in an inverted V, and that section
will then quickly become red hot. Another feature is that after
the water and fire are shut down for the night the heat of the
furnace can be great enough to cause serious warping of the surface
of the shield so that the water will no longer cover it in a thin,
uniform sheet.
After rigging up a big furnace with a shield of this type several
years ago, its most serious object was found in the increase of
the water bill of the plant. This was already of large proportions,
but it had suddenly jumped to the extent of several hundred dollars.
Investigation soon disclosed the fact that this water shield was one
of the main causes of the added cost of water. A little estimating
of the amount of water that can flow through a 1/2-in. pipe under
30-lb. pressure, in the course of a day, will show that this amount
at 10 cts. per 1,000 gal., can count up rather rapidly.
Figure 103 is a section through a portion of the furnace front and
shield showing all of the principal parts. This shield consists
essentially of a very thin tank, about 2-1/2 in. between walls,
and filled with water. Like other shields it is fitted with an
adjustment, that it may be raised and lowered as the work demands.
The tank having an open top, the water as it absorbs heat from
the flame will simply boil away in steam; and only a small amount
will have to be added to make up for that which has evaporated. The
water-feed pipe shown at _F_ ends a short distance above the top
of the tank so that just how much water is running in may readily
be seen.
An overflow pipe is provided at _O_ which aids in maintaining the
water at the proper height, as a sufficient quantity can always be
permitted to run in, to avoid any possibility of the shield ever
boiling dry; at the same time the small excess can run off without
danger of an overflow. The shield illustrated in Fig. 104 has been
in constant use for over two years, giving greater satisfaction
than any other of which the writer has known. It might also be
noted that this shield was made with riveted joints, the shop not
having a gas-welding outfit. To flange over the edges and then
weld them with an acetylene torch would be a far more economical
procedure, and would also insure a tight and permanent joint.
The water-cooled front shown in Fig. 105 is an absurd effort to
accomplish the design of a furnace that will provide cool working
conditions. This front was on a bolt-heating furnace using hard
coal for fuel; and it may be seen that it takes the place of all
of the brickwork that should be on that side. Had this been nothing
more than a very narrow water-cooled frame, with brickwork below
and supporting bricks above, put in like the tuyeres in a foundry
cupola, the case would have been somewhat different, for then it
would have absorbed a smaller proportion of the heat.
A blacksmith who knows how a piece of cold iron laid in a small
welding furnace momentarily lowers the temperature, will appreciate
the enormous amount of extra heat that must be maintained in the
central portion of this furnace to make up for the constant chilling
effect of the cold wall. Moreover, since there would have been
serious trouble had steam generated in this front, a steady stream
of water had to be run through it constantly to insure against
an approach to the boiling point. This is illustrated because of
its absurdity, and as a warning of something to avoid.
Water-cooled, tuyere openings, as mentioned above, which support
brick side-walls of the furnace, have proved successful for coal
furnaces used for forging machine and drop-hammer heating, since
they permit a great amount of work to be handled through their
openings without wearing away as would a brick arch. Great care
should be exercised properly to design them so that a minimum amount
of the cold tuyere will be in contact with the interior of the
furnace, and all interior portions possible should be covered by
the bricks. However, a discussion of these points will hardly come
in the flame-shield class, although they can be made to do a great
deal toward relieving the excessive heat to be borne by the furnace
worker.