TREATMENT OF TOOL STEEL
=85. Selecting and Working Steel.=—In making a tool, the differences
in quality of steel should be considered, because steel suitable for a
razor would not do for a cold chisel or any battering tool. (See sec.
181.)
If the steel at hand is not exactly suitable, but the selection must
be made from it, then that should be chosen which will most nearly
meet the requirements, and tempering must be relied upon to make up
the deficiency. In most large factories all grades of steel are kept
on hand and are assorted in the stock room so that there need be no
difficulty in making the proper selection.
The percentage of carbon in steel represents the amount of carbon it
contains. A steel that is called a 75-point carbon steel is one that
contains (.75) seventy-five one hundredths of one per cent, each point
representing (.01) one one hundredth of one per cent.
Some steel makers use the word “temper” to indicate the amount of
carbon, expecting the user of the steel to be familiar with the
amounts of carbon each different temper represents. For instance, a
razor-temper steel represents one that contains 1.50 per cent carbon
and a tool-temper steel represents one containing about 1.25 per cent.
The word “temper” as used in this connection should not be confused
with the word as it is used in the art of tempering, where it
indicates the operation of reducing the hardness of the metal in order
to make it less brittle and more suitable for some particular use.
=86. Uses of Different Grades of Steel.=—As the percentage of carbon,
and consequently the quality of steel, will vary somewhat with
different makes, it is rather difficult to give a rule that will
apply generally, but the following list of different grades of carbon
will give a general idea of how steel should be selected, forged, and
hardened.
Steel of 0.7 to 0.8 per cent carbon should be used for snaps, rivet
sets, cupping tools, etc. This grade of steel should be forged at a
light red heat. It can be welded easily and will harden at a light red
heat.
Steel from 0.8 to 0.9 per cent carbon should be used for drop-forging
dies, hammers, cold sets, track chisels, blacksmith’s tools, well
drills, etc. It should be forged at a light red heat; it welds easily
and hardens at a light red heat.
Steel from 0.9 to 1 per cent carbon should be used for large hand
chisels, large punches, shear blades, dies, etc. Forging should be done
at a light red heat. It welds readily and hardens at a bright red heat.
Steel from 1 to 1.1 per cent carbon should be used for hand chisels,
punches, punch dies, small shear blades, etc. Forging should be done at
a light red heat. It welds readily and hardens at a bright red heat.
Steel from 1.1 to 1.2 per cent carbon should be used for screw-cutting
dies, large cutting and trimming dies, small punches, small hand
chisels, large milling cutters, cups, cones, etc. Forging should be
done at a light red heat. It welds readily when care is taken in
heating, and hardens at a bright red heat.
Steel from 1.2 to 1.3 per cent carbon should be used for drills, taps,
reamers, milling cutters, circular cutters, cutting and trimming dies,
mill picks, engraving tools, twist drills, etc. Forging should be done
at a bright red heat. Welding can be done when precaution is taken
against overheating and burning. It hardens at a dull red heat.
Steel from 1.3 to 1.4 per cent carbon should be used for small drills,
taps, cutters, boring tools, etc. Forging should be done at a bright
red heat; welding can be done with care against overheating. It hardens
at a dull red heat. This steel should be handled carefully.
Steel from 1.4 to 1.5 per cent carbon should be used for tools for
working chilled castings or locomotive wheel tires, lathe and planer
tools, razors, or any tools required to cut hard materials. Forging
should be done at a dull red heat. Welding can scarcely be accomplished
with this grade of stock. Hardening should be done at a dark red heat.
=87. Injuries.=—One of the most common injuries to steel comes from
carelessness in the heating for forging. It is one of the important
operations, for unless the metal is uniformly heated, violent strains
are liable to occur, and, when hardened, the steel will show these
strains by cracking. These defects are known as fire cracks.
The smith should always have plenty of fuel surrounding the metal while
it is in the fire so that the cold-air blast will not come in direct
contact with the metal. The air should be heated by passing through a
bed of hot coals before it strikes the steel. It is always necessary to
heat steel thoroughly to make it plastic, being careful not to overheat
or burn any part of the metal. If it is overheated or burned, it
cannot be completely restored to its former state; the grain becomes
coarse and the structure weak.
Never let steel lie in the fire to soak up heat after it is hot enough
to work. If for any cause it cannot be worked when it is ready, it
should be taken from the fire and left to cool, then reheated when it
can be worked. By this precaution injury to the steel will be prevented.
If steel is heated so that the outer parts are hotter than the center,
the metal will forge unevenly. The outer portion will be forged by the
hammer blows, while the center remains almost in the original form.
This will also cause an uneven grain, sure to produce cracks when the
tool is hardened. Forging at too low a heat will injure the steel in
the same manner as uneven heating.
After the steel has been properly heated, and forging has begun, the
first blows should be struck rather heavily and followed by lighter
ones as the heat vanishes. The forging should cease when the steel gets
too cold, but it may be reheated as often as necessary to complete the
work.
=88. Annealing.=—After the steel has been forged to the desired shape,
it usually is necessary to do some finishing upon it before it can be
hardened and tempered; in order to do this, it must be annealed or
softened so that it can be machined or filed into shape. _Annealing
is the process of softening steel._ It is done by heating the steel
slowly to an even low red heat and placing it in an iron box containing
unslaked lime or fine charcoal and leaving it there until perfectly
cold. The object of this process is to retain the heat and prolong the
cooling. The box is usually of cast iron, but sheet steel is equally
good. It should be placed in a perfectly dry place and rest on bricks,
if necessary, to avoid any dampness.
If an annealing box is not at hand, small steel forgings can be
softened very satisfactorily by placing them between two boards, then
completely covering all with dry ashes and leaving them there until
entirely cold. Precaution should be taken here, also, to leave them in
a dry place.
Another method, which is sometimes used, is called _water annealing_.
Some mechanics claim to have had good results with it, while others
condemn it entirely. By this method the article is heated to a dull red
and allowed to cool partly, out of any direct current of air. When all
redness has disappeared as it is held in a dark place, it is plunged
into water and left there until perfectly cold.
The first method mentioned above is always the best; the second is
nearly as good; and only when there is not sufficient time to allow the
metal to cool slowly, should water annealing be attempted.
Such tools as cold chisels and lathe tools may be heated and laid in
or on warm ashes until nearly cold, when they may be ground, hardened,
and tempered. Quite frequently, if not generally, these tools are not
treated in this manner, but it is no doubt the course to pursue to get
the best results.
=89. Hardening and Tempering.=—When steel has been properly heated,
forged, finished, or ground, the next two steps are hardening and
tempering. These two processes are often understood as one, but they
are entirely different in their results. The confusion arises because
the two operations are sometimes performed with one heating of the
steel as in hardening and tempering a cold chisel, or other similar
tools.
As the steel has been subjected to severe strains during the heating
and forging operations, its structure may have been somewhat altered.
It can be restored to the proper crystalline structure by the
hardening, scientifically known as refining. The hardening or refining
heat is always lower than the forging heat, and should be only as high
as is necessary to harden the steel to the required density by sudden
cooling. Then this first operation of cooling will harden and refine
the steel at the same time.
Extreme hardness is always accompanied by extreme brittleness, a
quality undesirable in any cutting tool, and especially so in a tool
required to withstand sudden shocks. As the hardness is reduced by
subsequent heating, the toughness increases. This modification, called
tempering, is accomplished by reheating the hardened portion of the
tool until a sufficient toughness has been obtained, when the process
is stopped by again plunging the tool into cold water. The heat for
tempering may be supplied from the uncooled portion of the tool as in
tempering a cold chisel, from the forge fire, from another hot piece of
metal, or from a carefully heated furnace.
It has been found that the colored oxides formed on the surface of a
piece of polished steel or iron represent a definite temperature in
that metal. These colors have been used, therefore, to determine the
desired temperature in tempering a tool. When we say “temper a tool to
a light straw,” we mean that the hardened tool is to be heated again to
a degree which will produce that color; namely, about 430 degrees Fahr.
The colors as they appear are light straw, dark straw, bronze, bronze
with purple spots, purple, dark blue. The light color appears first. Do
not allow the colors to pass too quickly, as will happen if the heat
applied is too intense.
[Illustration: FIG. 70.—HARDENING A CHISEL.]
There are two distinct methods of hardening and tempering. The one
generally followed in tempering cold chisels, lathe and various other
tools, requires only one heating. The tool is heated to a proper
hardening temperature at the end, where hardness is desired, and
also over an excess area to supply the heat for tempering. About 2
inches of the cutting end is heated; about 1 inch of this is plunged
perpendicularly into water, as shown in Fig. 70; it is then kept in
motion perpendicularly between the places indicated at _a_ and _b_,
while the end is cooling. This will prevent a fixed cooling point
and prevent a fracture that might possibly occur if it were held in
one position while cooling. The portion between _b_ and _c_ should
retain sufficient heat to produce the necessary temper. When the end
is perfectly cold it should be removed and immediately polished with
sandstone or emery cloth to remove the scale of oxide so that the
different colors may be more readily seen as they move from _b_ toward
the point. The heat in the portion between _b_ and _c_ flows toward
the point, causing the colors to appear as the heat extends. When the
desired color covers the point, it should again be plunged into the
water and left there until entirely cold. In this method the first
cooling is the hardening, and the second the tempering. A comparative
color chart is appended to this chapter for guidance in obtaining the
tempers for various tools.
[Illustration: FIG. 71.—HARDENING A REAMER.]
By the second method the steel is heated as in the first method,
then it is cooled off entirely by immersing the tool exactly
perpendicularly, as shown in hardening a reamer in Fig. 71; after this
it is polished. The temper is then drawn by holding the tool in contact
with a piece of heated metal, cast iron preferably. In Fig. 72 the
reamer is shown inside of a heated bushing, which is a more practical
way than laying it on top of a heated flat plate. The bushing will
impart sufficient heat to the tool to produce the desired color, when
it should be again cooled. This method is used mostly for tempering
plane bits, wood chisels, milling cutters, taps, reamers, and various
other tools of a like nature.
Sometimes tools having sharp protruding edges, as milling cutters,
taps, reamers, etc., are very liable to crack by the sudden cooling
in water; this difficulty is avoided by using oil for hardening and
tempering. Any so treated are called oil-tempered tools.
[Illustration: FIG. 72.—TEMPERING A REAMER.]
The above methods of tempering are such as are ordinarily used when
only a common shop equipment is at hand, and the operator must depend
entirely upon his judgment of the colors which represent the proper
forging, annealing, hardening, and tempering heats. The degree of
accuracy that has been attained in this practice is most surprising.
In large manufacturing establishments where many duplicate pieces
are to be tempered, a more modern as well as scientific apparatus is
employed to relieve the operator of dependence upon his discernment of
colors. Here the steel is heated in a furnace, to which is attached
a pyrometer that registers the exact degree of temperature. In this
manner all pieces can be heated uniformly for any of the four required
heats.
[Illustration: FIG. 73.—SECTIONAL VIEWS OF TOOL STEEL, SHOWING THE
EFFECTS OF PROPER AND IMPROPER TREATMENT.
_A._ Natural Bar.
_B._ Refined.
_C._ Too hot.
_D._ Burned.]
The views in Fig. 73 were photographed from the same grade or bar of
steel to show the various granular structures produced by different
heat treatments. _A_ shows the condition of the natural bar, which
was broken to be photographed just as it was received from the steel
makers. The lower left side shows where it was nicked with the cutter
to be broken. _B_ shows the structure when proper conditions of
heating and hardening have been maintained. Notice how much finer the
structure here appears to be; this effect was caused by, and previously
referred to as, the refining heat of steel. A similar condition should
be produced with any tool steel under correct treatment. _C_ shows
a much coarser structure; it was heated too hot and hardened in the
same manner. If a tool were made thus, its weakness would be hardly
noticeable at the time, but the structure shows that it is considerably
weaker. _D_ shows the condition of the stock after being burned. It has
produced from a quality of steel that was valuable, a metal worthless
for any kind of tool.
=90. Casehardening.=—Another method of hardening, called casehardening,
is used for wrought iron and low carbon or soft steel parts which are
to be subjected to considerable friction. Neither of these metals
could be hardened by the other methods mentioned. This process adds
carbon to the exterior surfaces only, and for that reason is called
casehardening, as the outside is made extremely hard, while the inner
portion or core remains in a condition like that produced by sudden
cooling, thus providing a hard wearing surface and great strength at
the same time. It is similar to the old cementation process of steel
making, but is not prolonged sufficiently to allow the hardening to
continue through the entire structure.
The articles to be hardened are packed in a box somewhat similar to
an annealing box. This should be partly filled with charred leather,
ground bone, or wood or bone charcoal, all of which are highly
carbonaceous materials; then the articles are placed in and entirely
surrounded with a thin coating of cyanide of potassium, especially if
iron is being hardened. The remaining space in the box is filled with
the leather, bone, or pieces of charcoal. The box should be provided
with a lid that will drop loosely between the outer projecting rims.
The outer edges of this lid should be luted with clay to keep it as
air-tight as possible. If a few small holes are provided in the center
of the lid, test wires can be inserted; by removing a wire and cooling
it, the progress of the operation may be known. These wires should
be inserted before the box is placed in the furnace. The box and its
contents are then placed in a suitable furnace and kept thoroughly
heated from 6 to 15 hours, depending upon the depth of hardness
required. Then it is withdrawn, the lid removed, and the articles
quickly plunged into a large tank of water. This will complete the
hardening.
When a number of very small articles are to be hardened, it is
advisable to connect them with strong bailing wire before they are
placed in the box so that they can all be removed at once. Beside
holding the articles together, the wire will provide a means of testing
the depth and quality of the process.
If only a thin coating of hardness is needed, or the labor and expense
are excessive, the following method may be used: The article is heated
thoroughly and evenly to about a bright red and thoroughly sprinkled
with, or rolled in, cyanide of potassium. Then it is reheated so that
the cyanide may penetrate as deeply as possible, after which it is
quickly chilled in cold water. This is a good method of hardening small
tack hammers made of soft steel, set screws, nuts, and very small tools.
TEMPERATURE AND COLOR CHART TO BE USED IN TEMPERING
Tools Temperature (Fahr.) Color
_Degrees_
Scrapers for brass 430 Very pale yellow
Light turning tools 430 Very pale yellow
Lathe and planer tools for steel 430 Very pale yellow
Steel engraving tools 430 Very pale yellow
Milling and boring cutters 460 Straw yellow
Screw-cutting dies 460 Straw yellow
Taps and reamers 460 Straw yellow
Punches and dies 480 Dark straw
Penknives 480 Dark straw
Twist drills 500 Bronze
Plane irons 500 Bronze
Surgical instruments 530 Dark purple
Cold chisels for steel 540 Dark purple
Cold chisels for cast iron 550 Dark blue
Cold chisels for wrought iron 550 Dark blue
Springs 570 Very dark blue
SUITABLE TEMPERATURE (FAHR.) FOR:
_Degrees_
Annealing tool steel 900
Forging tool steel 1200 to 1500
Hardening tool steel 1200 to 1400
Casehardening iron or soft steel 1300 to 1500
COLORS AND CORRESPONDING TEMPERATURES (FAHR.) FOR IRON
Bright red in dark 750 to 760
Red hot in twilight 880 to 890
Dark red hardly visible in daylight 970
Red visible by daylight 1070
Brighter red by daylight 1300
Cherry red by daylight 1450
Bright cherry red by daylight 1650
Light cherry red by daylight 1800
Orange 2000
Yellow 2150
White heat 2350
White welding heat 2600
White welding and dazzling 2800
QUESTIONS FOR REVIEW
What is meant by the carbon contents of steel? Why is steel graded
according to its carbon content? Explain the cause of fire cracks.
How can they be prevented? Why should steel be thoroughly healed? If
steel is overheated or burned, what is the effect? Why should steel
never be left in the fire to soak up heat? How does steel forge if
it is unevenly heated? How should the blows be delivered in forging
steel? What is annealing? Describe three methods of annealing. Is it
best to anneal cold chisels and lathe tools? Explain the process of
hardening steel. What effects does hardening have? Are the forging
and the hardening heats the same? Why is steel polished after it
is hardened? Explain the process of tempering. What is the effect
of tempering? How may the heat be supplied for tempering? Name the
colors in order as they appear in heating steel. Explain the methods
of hardening and tempering. Why should a cold chisel be kept in
motion when it is being hardened? What is meant by oil-tempering?
What is meant by casehardening? Explain different methods of
casehardening.