to orange and then cooled slowly, their fractures will be so different
as to enable an expert to place them properly in their order of carbon,
and to classify them as mild, hard, and harder; beyond that he could
not go; if he attempted to give them their temper numbers, he would be
likely to miss by four or five numbers either way, and a correct mark
would be only a lucky guess.
Hammering and rolling heated steel affect the grain or structure
profoundly; a high steel may be worked so that the grain will look
mild, and a mild steel may be so worked that the grain will look hard.
It is common to see a bar of steel with a fine grain at one end and a
coarse grain at the other, and this state of things often frightens a
consumer, who imagines that he has received a very irregular, uneven
article, and he is as often astonished when it is shown to him that at
the same proper heat the two ends will refine and harden equally well,
and be exactly alike. In such a bar one end has been finished a little
hotter than the other, and the grain is due to the heat in each case.
This uneven heating may have been incidental or careless; with skilful
workers it is rare.
One end might have been finished so cold as to crush the grain, and the
other end so hot as to cause incipient disintegration, but a competent
inspector would discover either condition at once and reject the bar.
There is, then, a specific structure due to temperature; it is modified
by carbon and by treatment under the hammer or in the rolls. If a bar
of steel be heated up to the highest plastic limit, just so that it
will not fall to pieces, and then cooled slowly without disturbance,
and a fracture be taken, it will be found to be coarse and with an
exceedingly brilliant lustre. Now let it be heated again to a bright
lemon color, but still plastic, and cooled as before; it will be found
to be coarse, with bright lustre, but neither so coarse nor so bright
as the first piece. Then let it be treated in this way to lemon color,
light orange, medium orange, dark orange, and orange red; as the heats
go down the grain will be finer and the lustre will be less, until at
about medium orange the lustre will be absent.
If any number of bars of even composition be heated in this way, the
fractures will all be alike for each temperature.
If a series of bars of the different full tempers, about seven in all,
be treated in this way, the structures due to a given temperature will
all be similar, but there will be no two exactly alike, because high
steel is much more profoundly affected by heat than low steel.
Seven tempers are mentioned here, because that is the number of full
tempers in common use. Steel is graded out into fifteen tempers
ordinarily by the interpolation of half numbers; this is easy and sure
in the ingot inspection. In the above experiment the differences due
to carbon are not quite so delicate, and the work is hampered in the
heating by the personal equation, so that the use of seven full tempers
is refinement enough. There is a difference due to every separable
quantity of carbon, which could be shown if all of the operations of
the experiment were exact.
If when a bar is broken cold the fracture is uneven, with coarse grain
in one part and fine grain in another, it shows that there has been
uneven heating. If one side has large grain and the other side is fine,
the bar has been a great deal hotter on the side having coarse grain
than on the other: the heater has let the bar lie in the furnace with
one side exposed to a hot flame and the other protected from the flame
in some way; he has neglected to turn the bar over and heat it evenly.
If the outside of the bar is fine and the centre is coarse, the bar
has been very hot all through and has been finished by light blows
of the hammer or by light passes in the rolls; it has been worked
superficially and not thoroughly.
If the outside of the bar is coarse and the centre is fine, the steel
has been heated on the surface too hot and too quickly; it has not
had time to get hot through, and it has had too little work in the
finishing.
If the grain is dark, with the appearance of a rather heavy india-ink
tint, the steel has been finished too cold, and it will be found to be
brittle.
If the grain is very dark, especially about the middle, looking almost
black, then it has been finished altogether too cold: the grain is
disintegrated, and the bar is fit only for the scrap heap.
A bar of this kind containing enough carbon to harden will harden
thoroughly, and often appear to be sound and fine, but it is not sound
and will not do good work; if it be brought up to a proper heat and
forged to a point, it will almost certainly burst, showing that the
integrity of the steel has been destroyed.
If a bar, or plate, or beam shows cracks on the surface or at the
corners, with rough, torn surfaces, the steel has either been
superficially burned or it is red-short. In either case it should be
rejected, for the cracks, although small, will provide starting-points
for ultimate fractures, whether it be tool-steel that is to be
hardened, or structural steel that is to be strained without hardening.
If the steel is to be machined, so that all of the cracks can be cut
out, then in machinery-steel the removal of these surface defects
might leave the finished piece sufficiently sound and good. If,
however the steel is to be hardened, and the defects should be due to
red-shortness, the piece would almost certainly break in the hardening;
and if it were not red-short, then unless the cracks were cut away
entirely, if the least trace of the crack is there, although it may
not be visible, that trace will be sufficient to start a crack when
the piece is hardened.
EFFECTS OF COOLING.
Increase of heat causes increase of softness up to the liquid condition.
Decrease of heat--cooling--increases hardness up to the hardness of
glass.
As an invariable rule the rate of cooling fixes the degree of hardness
to be had in the cold piece within the limits of obtainable hardness or
softness.
Slow cooling retains softness, so that when annealing is to be done the
slower the cooling the better. Cooling is always a hardening process,
but when it is carried on slowly more softness, will be retained than
when the cooling is quick.
Rapid cooling produces hardness, and the more nearly instantaneous it
is the greater the hardness will be. This property of hardening is of
such extreme importance that it will be treated fully in a separate
chapter.
There is an apparent exception to this rule shown in the operation
called water-annealing. It is common, when work is hurried, to heat a
piece of steel carefully and uniformly up to the first color, that is,
until it just begins to show color, and then to quench it in water.
This is called water-annealing; and many believe that because a
piece so treated is left softer than it was before treatment, the
water-cooling had something to do with it. The fact is that hammering
and rolling are hardening processes. When the increment of heat due
to the work is less than the decrement of heat due to radiation, the
compacting of the grain increases hardness.
This process leaves the piece harder than does the quenching in
water-annealing; the decrease in hardness due to water-annealing is the
difference between the effects of the two operations. Let two pieces of
the same bar be heated exactly the same for water-annealing; let one
be quenched in water, and the other be allowed to cool in the air in a
dry place. Then the superior softness of the air-cooled piece will show
that the so-called water-annealing furnishes no exception to the rule.
There is one extremely important matter connected with cooling that
should be noted carefully.
It is a common practice among steel-workers when they get a part of
a piece of steel too hot to partially quench that part, and then go
on with their heating; or if they are in a hurry to get out a big
day’s work, or if the weather is hot, and a pile of red-hot bars is
uncomfortable, to dash water over the pile and hurry the cooling.
This practice means checks in the steel, hundreds of them.
[Illustration]
A bar breaks and has this appearance. The dark spot is the check; it
did not show in the bar, no inspector could see it, but it broke the
bar. Any one can prove this to his own satisfaction in a few minutes.
Take a bar of convenient size, about one inch by one eighth; heat it
carefully to an even medium orange color and quench it completely; then
snip it with a hand-hammer over the edge of an anvil, snipping away
until satisfied that it is sound steel. There are no checks.
Now beat a similar length of the same bar in the same way, and pass it
through the stream from the bosh-pipe, or submerge it for a moment in
the bosh, not long enough to produce more than the slightest trace of a
change in the color; then put it back in the fire and bring it gently
to the uniform color used before, and quench it completely. Now when it
is snipped over the anvil it will show numerous checks, dozens of them.
In this experiment the complete submersion for a moment may not
produce checks at every trial, because the complete submersion permits
practically uniform cooling, which if continued to complete cooling
would be simply the ordinary hardening process. Still it will produce
checks in the majority of cases, indicating that starting the changes,
strains, or whatever they are of the quenching process and then
stopping them suddenly while the steel is in the plastic condition does
cause disintegration, so that the operation is dangerous and should
not be tolerated. Passing the hot steel through a stream of water
or dashing water over it must cause different rates of cooling, and
necessarily produce local strains resulting in checks. These latter
ways of injuring, therefore, rarely fail to produce the ruinous checks.
If this positive destruction is produced in this way, in steel
containing enough carbon to harden it is clear that similar, although
not so pronounced, results will be produced in the mildest steels when
they are treated in the same manner.
The rule, then, should be: Never allow water to come in contact with
hot steel, and never allow hot steel to be laid down upon a damp floor.
Even the spray from water which is run upon roll-necks may cause these
checks in steel that is passing through the rolls, so that it is better
to put up a guard to deflect such water away from the body of the roll.
A hammerman may sweep a bar with a damp broom to cause the vapor to
explode with violence when the hammer comes down, and so tear away all
rough scale and produce a beautiful finish. A careful, skilful man may
be permitted to do this, but as surely as he gets his broom too wet,
so that drops of water will fall on the steel and whirl around in the
spheroidal condition, just so surely will he check the steel.
The best way is to have the broom not wet enough to drip, and then
to strike it up against the top die when it is ready to descend;
sufficient moisture will be caught upon the die to cause a loud
explosion when it strikes the hot steel; it is a violent explosion and
will drive off every particle of detachable scale, leaving as beautiful
a surface as that which is peculiar to Russia sheet iron.
It is common in rolling tires to run jets of water over the tire to
break up the scale and produce a clean surface. Tire-makers assert that
experience shows that the water does no harm. There are two reasons
for this if it be true: first, the steel is of medium carbon and more
inert than high steel, and it has been hammered and compacted before
rolling; second, the tires are usually turned, and this would cut away
any little checks that might occur on the surface.
The magnetic properties of steel are well known. Soft steel, like soft
wrought iron, cannot be magnetized permanently; higher carbon steel
will retain magnetism a long time, and hardened steel will retain it
still longer. Hardened-steel magnets are the most permanent.
The permanency and the efficiency of a magnet increase with the
quantity of carbon up to about 85 carbon; steel of higher carbon than
this will not make magnets of so good permanency. The efficiency of a
magnet of 85 carbon is increased largely by the addition of a little
tungsten; a little less than .05% is sufficient.
It has been shown that tungsten has the property of retaining the
hardness of steel up to a relatively high temperature; this additional
power of retaining magnetism may indicate a close relation between the
conditions set up by magnetism and by hardening.
It has been stated that maximum physical properties, except as to
compression, are found at from 90 to 100 carbon; now we find maximum
magnetic properties in the same region. Prof. Arnold has found by
microscopic tests the same point of saturation; he fixes it at 89
carbon and deduces from it an unstable carbide of Fe₂₄C.
The magnetic maximum was found by magnet-makers by actual use in large
numbers of magnets. Prof. J. W. Langley found the same maximum in a
series of careful and delicate experiments undertaken to determine the
best composition and the best treatment for the production of permanent
magnets. Magnetism is affected by temperature, and it is found that
steel becomes non-magnetic at or about the point of recalescence. This
is important to electricians, as it marks the limit of temperature that
is available to them. It is of interest to the scientists, as it is
another indication of the importance of the changes that take place at
this temperature. Later, recalescence will be found to be an equally
important point to the steel-worker, especially to the temperer.
It has been stated that if a bar of steel be heated to any visible
temperature and then be cooled without disturbance there will be a
resulting grain or structure that is due to the highest temperature to
which the bar was subjected. As a rule the highest temperature leaves a
grain that appears to the eye to be the largest, or coarsest, whether
the microscope shows it to be composed of larger crystals or not.
Let the following squares represent the apparent sizes of the grains:
[Illustration]
1. The natural bar, untreated.
2. Grain due to dark orange or orange red.
3. “ “ “ medium orange.
4. “ “ “ bright orange.
5. “ “ “ dark lemon.
6. “ “ “ medium lemon.
7. “ “ “ bright lemon.
8. “ “ “ very bright lemon, or creamy.
These designations are used because steel in cooling down, or in
heating up, runs through a series of yellow tints, not reds. It is
common to see the expression “glowing white” applied to steel that is
not even melted, when as a matter of fact melted wrought iron is not
quite white. An occasional heat of steel may be seen that could fairly
be called white, and then the melter knows that it is altogether too
hot, and that he must cool the steel or make bad ingots. “Glowing
white,” like “cherry red,” will do for ordinary talk, but not for
accurate description, although “cherry red” comes nearer to describing
the dying color than “glowing white” comes to describing the highest
heat.
An arc light may be “glowing white,” and sunlight is “glowing white,”
and when either light falls upon melted steel it shows how far the
steel is from being “glowing white.”
Referring to the squares: If a bar that has been heated to No. 8 be
re-heated to No. 2 and be kept at that color a few minutes to allow
the steel to arrange itself, in other words, to provide for lag, and
then be cooled, it will be found to have grain No. 2. Sometimes in
performing this experiment the fracture will be interspersed with
brilliant spots as if it were set with gems; this shows that not quite
enough time was allowed for lag. Another trial with a little more time
will bring it to a complete No. 2 fracture. If now it be heated to No.
4, or 5, or 6 in the same way, it will be found to have when cold the
grain due to No. 4, or 5, or 6 temperature.
This may be repeated any number of times, and the changes may be rung
on all of the numbers, until the disintegrating effect of numerous
heatings begins to destroy the steel. This property of registering
temperature, this steel thermometer, is of great value, and it will be
referred to frequently.
EFFECTS OF MECHANICAL WORK.
When an ingot is heated and then hammered, rolled, or pressed hot, its
density will be increased, as well as its strength when cold under all
strains.
If it be hammered carefully, with heavy blows at first, and with
lighter and quicker blows at the last, the grain will become very close
and fine; it is called “hammer-refined.”
When down to the so-called cherry red, orange red, great care is
needed, and when black begins to show through the red much caution must
be used; any heavy blows will crush the grain and produce the dark or
black color mentioned before.
Fine-tool makers attach great importance to this hammer-refining; some
of the most expert will not have a rolled bar if a well-hammered one
can be had. At first thought this would seem to be a mere notion, but
the testimony in favor of hammering is so universal among those who
know their business that it would seem as if it must be based upon some
reason. If it have any scientific basis of fact, it is that the shocks
or vibrations of the hammer keep the carbon in more intimate union with
the iron, whether it be combination or solution, than either rolling
or pressing will do. After considering the phenomena of hardening,
tempering, annealing, etc., it may be concluded that there is something
in this. It is easy to laugh at and to deride shop prejudices, and
there are enough of them that deserve ridicule; again, there are
some that will not down, and they compel the scientist to hunt for
explanations. But after all, ridicule is dangerous; it is possible
that a careful comparison of some of the laws laid down by the highest
scientists would tend to excite the risibles. If the hand-worker
sometimes flounders in the mud, the scientist is sometimes enveloped
and groping in mist.
Hot-rolling produces results similar to those of hot-hammering; it
makes the grain finer, increases density, and adds to the strength.
The same precautions are needed in rolling as in hammering. Heavy
passes with rapid reduction may be used to advantage while the steel
is hot and thoroughly plastic; as the heat falls the passes should be
lighter to avoid crushing the grain.
Overrolling, like too much hammering, may be more injurious than too
little work; a coarse, irregular structure due to too little work may
be rectified and made fine and even by annealing, while if the grain
be crushed by overwork the damage cannot be cured by annealing; the
annealed grain may appear to be all right, but on testing, the strength
will be found impaired.
By care and light passes steel may be rolled safely down to a black
heat and be made elastic and springy. It is common to roll spring-steel
in this way so that it may be formed into a spring and have all of the
properties of a tempered spring without going through the operations of
hardening and tempering. This is often desirable for spring-makers, as
it saves them considerable expense; but it is hazardous work, because
it is so difficult to heat every piece exactly to the same temperature,
and secure every time the same number of passes and the same pressure
in each. The best roller will get some pieces too hard and brittle, and
some too soft and ductile. A careful steel-maker will shun such work.
Cold-hammering, cold-rolling, and cold-drawing reduce specific-gravity
and increase tensile, transverse, compressive, and torsional strength.
They increase hardness and brittleness, reducing ductility. The
hardness due to cold-working is different from that due to hot-work or
quenching; the latter operations produce great elasticity as well as
hardness.
The hardness due to cold-working might be described as harshness; the
steel is not truly springy; of course it will bend farther without
permanent set than an annealed piece, but it never has the true
spring elasticity. If it be worked far enough to be really springy,
it will bear the same relation to a hot-worked spring that a piece
of cross-grained, brashy oak bears to a piece of well-seasoned,
straight-grained hickory.
The hammering of round sections between flat dies tends to burst the
bars in the centre; great care must be used to avoid this, and the most
skilful and careful hammermen will often turn out bursted bars. The
bursts do not show on the surface; the bars are true to size, round,
smooth, and sound on the outside. The safest plan is to hammer in a
V-die, or in rounded swedges.
Radial rolling will produce the same results, and it is on this
principle that the celebrated Mansmann tubes are made. The explanation
seems to be simple, as the following exaggerated sketches will show:
[Illustration]
No. 1 has been struck; it is then turned up to position No. 2 and
knocked into shape No. 3. The rapid hammering of a bar, turning it a
little at a time, must burst it if the blows are heavy enough to deform
the whole section. Heavy radial rolling produces the same results.
The concluding pages of this chapter will be devoted to a few examples
showing by tests the effects of heat and work upon specific-gravity,
tensile strength, elasticity, and ductility; they are not to be taken
as fixing exact limits in any case; they are given merely to illustrate
the truth of the general properties stated, and to show the wide ranges
of strength that are attainable by varying carbon and work.
TABLE I.
---------------+----------------------------------------------
Character | Ingot Numbers.
of |-------+-------+-------+-------+-------+------
Steel. | 1 | 2 | 3 | 4 | 5 | 6
---------------+-------+-------+-------+-------+-------+------
Carbon | .302 | .490 | .529 | .649 | .801 | .841
Silicon | .019 | .034 | .043 | .039 | .029 | .039
Phosphorus | .047 | .005 | .047 | .030 | .035 | .024
Sulphur | .018 | .016 | .018 | .012 | .016 | .010
Sp. gr. ingots.| 7.855 | 7.836 | 7.841 | 7.829 | 7.838 | 7.824
Sp. gr. bars, | | | | | |
burned, 1 | | | 7.818 | 7.791 | | 7.789
2 | | | 7.814 | 7.811 | | 7.784
3 | | | 7.823 | 7.830 | | 7.780
4 | | | 7.826 | 7.849 | | 7.808
5 | | | 7.881 | 7.806 | | 7.812
cold, 6 | | | 7.844 | 7.824 | | 7.829
Diff. 6-1 | | | .025 | .034 | | .040
Mean diff. | | | | | |
of carbon | .071 | | | | |
---------------+-------+-------+-------+-------+-------+------
| 7 | 8 | 9 | 10 | 11 | 12
---------------+-------+-------+-------+-------+-------+------
Carbon | .867 | .871 | .955 | 1.005 | 1.058 | 1.079
Silicon | .057 | .053 | .059 | .088 | .120 | .039
Phosphorus | .014 | .024 | .070 | .034 | .064 | .044
Sulphur | .018 | .012 | .016 | .012 | .006 | .004
Sp. gr. ingots.| 7.819 | 7.818 | 7.813 | 7.807 | 7.808 | 7.805
Sp. gr. bars, | | | | | |
burned, 1 | | 7.752 | | 7.744 | | 7.690
2 | | 7.755 | | 7.749 | | 7.741
3 | | 7.758 | | 7.755 | | 7.769
4 | | 7.773 | | 7.789 | | 7.798
5 | | 7.790 | | 7.812 | | 7.811
cold, 6 | | 7.825 | | 7.826 | | 7.825
Diff. 6-1 | | .073 | | .082 | | .135
Mean diff. | | | | | |
of carbon | | | | | |
---------------+-------+-------+-------+-------+-------+------
The twelve ingots treated here were first selected by ocular inspection
for carbons; the carbons were then determined by combustion analyses.
It will be seen that the inspection was correct, and that the mean
difference in carbon between consecutive numbers is .007. Between
Nos. 7 and 8 there is a difference of only .004; when the analyst
discovered this, he asked for a reinspection, not giving any reason for
his request. The inspectors made new fractures, examined the ingots
carefully in good light, and reported that they erred the first time,
that both ingots belonged in the same temper number, but that if there
were any difference No. 8 was the harder. It is not claimed that a
difference of .004 is really observable.
The contents of silicon, phosphorus, and sulphur show clearly that the
controlling element is carbon. This experiment has been repeated a
number of times, and always with the same result, showing that there is
no uncertainty in this method of separating tempers.
Parts of these ingots were reduced to ¾-inch round bars. The specific
gravities of the ingots were taken, showing generally a reduction
of sp. gr. for an increase of carbon. No. 3 and 5 are anomalous; an
explanation of this could doubtless have been found if a careful
investigation had been made, but there was no re-examination.
The sp. gr. No. 6 are of the ¾-inch bars as they came from the rolls;
they are all heavier than the ingots except No. 4, and they are of
nearly uniform sp. gr.; this is due doubtless to the fact that the
higher carbon steels are so much harder than the low-carbon steels
that it required much more work to reduce them to the bars, and as
hot-working increases density, the densities of the higher carbons were
increased more than those of the lower.
The bars were nicked six times at intervals of about ¾-inch and then
heated so that the ends were scintillating, ready to pass into the
granular condition, and the heat was so regulated as to have each piece
less hot than the piece next nearer to the end, the last piece, No. 6,
being black and as nearly cold as possible.
It is manifest that this operation is subject to the error of
accidentally getting No. 2, for instance, hotter than No. 1, and so on,
so that perfect regularity is not to be expected; to obtain a true rule
of expansion it would be necessary to make hundreds of such experiments
and use the mean of all.
It will be noticed that No. 4 is abnormal in the ingot series, and that
the No. 6 piece of No. 4 is abnormal in being lighter than the ingot;
probably this No. 6 of No. 4 was hot when it was intended to be cold.
Also No. 2 of ingot No. 3 is lighter than its No. 1, showing another
irregularity in heating.
Taking the whole list of No. 1 pieces, they are all lighter than
their respective No. 6 pieces; the differences of sp. gr. 6-1 are
progressive, being only .025 for the No. 3 ingot and .135 for the No.
12 ingot. This shows clearly that expansion due to a given difference
in temperature is much greater in high steel than in low steel.
This clears away the mystery of the so-called treachery of high steel,
its tendency to crack when hardened. There is no treachery about it; it
is very sensitive to temperature, and it must be treated accordingly.
A few examples will now be given to show the changes of tensile
strength, ductility, etc., that may be had by differences of carbon,
and by differences of treatment, annealing, hardening, and tempering.
TABLE II.
-------------------+-----------+----------+---------+--------+------
| | | | |
Character of | | | | |
Steel. | O. H. | Crucible | O. H. | O. H. | O. H.
| | Sheet | | |
| | | | |
-------------------+-----------+----------+---------+--------+------
Carbon | .09 to .12| .435 | .50 | .60 | .70
Silicon | .008 | .014 | .025 | |
Phosphorus | .007 | .050 | .016 | |
Sulphur | .026 | .028 | .028 | |
Manganese | .055 | .204 | .325 | |
Tensile strength, | | | | |
lbs. per sq. in. | 46800 | 73142 | 84220 |1 08800 |117400
Elastic limit | 30900 | | 63560 | 71500 | 69980
Elongation {| in 2 | in 1 | 25% | 14.5% | 11.5%
{| in. 41% | in. 42% | | |
Reduction of area | 75.85% | 62.3% | 29.91% | 13.55% | 8.59%
{| | | | |
{| | | | |
{| silky | | | |
Fracture {| ½ | | | |
{| cup | | | |
{| | | | |
-------------------+-----------+----------+---------+--------+------
-------------------+-----------+----------+------------+---------
| Crucible | Crucible | Crucible | Crucible
Character of | Eye-bar, | Eye-bar, | Eye-bar, | ½-in.
Steel. | 2″ × 1″. | 2″ × 1″. | 2″ × 1″. | Drawn
| | | | Wire.
-------------------+-----------+----------+------------+---------
Carbon | .96 | 1.35 | 1.40 | 1.15
Silicon | .156 | | | < .02
Phosphorus | .008 | | | < .02
Sulphur | .015 | | | trace
Manganese | .24 | | | < .30
Tensile strength, | | | |
lbs. per sq. in. | 124800 | 100733 | 117710 | 141500
Elastic limit | 65000 | 85078 | 69850 | 92420
Elongation {| 4.75% | .5% | 7.28 at | 2%
{| | | 2.85 in 2½ |
Reduction of area | | | 13.03% | 2.42%
{| broke | | |
{| in neck | broke | | broke
{| slight | in head | | in
Fracture {| flaw, | close | | grip
{| fine | grain | |
{| grain | | |
-------------------+-----------+----------+------------+---------
O. H. is the abbreviation for open hearth.
Second column is mean of 24 analyses and 24 tests of boiler-sheets.
TABLE III.
---------------------------+---------+--------+------------+--------
|Tensile |Elastic | Elongation.| Reduc-
Cold-drawn Wire, |Strength,|Limit, |----+-------+ tion of
½-inch Diam. |lbs. per |lbs. per|In 3| Per | Area,
|sq. in. |sq. in. | in.| cent. | per ct.
---------------------------+---------+--------+----+-------+--------
Cold-drawn, broke in grip | 141,500 | 92,400 | .06| 2.00 | 2.42
Same bar drawn black | 138.400 |114,700 | .18| 6.00 | 12.45
“ “ annealed | 98,410 | 68,110 | .30| 10.00 | 11.69
“ “ hardened and then | | | | |
drawn black | 248,700 |152,800 | .25| 8.33 | 19.7
---------------------------+---------+--------+----+-------+--------
Analysis of this bar is given in Table II in the last column.
A test of ½-inch wire to show effect of cold-drawing, tempering,
annealing, and hardening and tempering. Four pieces were cut from the
same bar. It is probable that the first piece would have given a little
higher tensile if it had not broken in the grip; it was clamped too
tight. The second piece was heated until it passed through all of the
temper colors and turned black, technically called “drawing black,” or
drawing out all of the temper. It is not quite annealing; the idea was
to find the effect of temper-drawing upon a cold-hardened drawn wire.
The effect of this operation was to lower the ultimate and raise the
elastic strength, increasing also the ductility.
The third piece was heated carefully to the recalescence-point, and
cooled slowly, thus annealing it completely, and giving the normal
strength of a bar of this composition.
The fourth piece was heated to recalescence and quenched, hardening and
refining it thoroughly; it was then tempered through all of the colors
until it turned black; the result shows the enormous potencies there
are in the hardening and tempering operations.
The cases given in Table II were selected indiscriminately, so as to
show better the effect of carbon, as we here have tests of ordinary
test-bars, boiler-sheet, small eye-bars, and drawn wire.
The 96-carbon eye-bar and the 115-carbon ½-inch wire are the nearest
to the 100-carbon saturation limit mentioned before, and they show
the highest strength. The 96-carbon eye-bar had a slight flaw in the
fracture, which doubtless caused it to break below its real strength.
The 135-carbon eye-bar broke in the head in a way to indicate that
there was some local strain there, due to forging.
These examples are not given as establishing any general law; they are
illustrations of what all experience shows to be the fact, that the
strength of steel is affected profoundly by the quantity of carbon
present, and also by heat and by mechanical work. From 46,800 lbs. to
248,700 lbs. tensile strength per square inch is an enormous range, and
these figures probably represent pretty closely the ultimate limits at
present attainable.
An inspection of the analyses makes it clear that the other elements
present in addition to carbon were not there in sufficient quantity or
variety to have had much effect upon the results.
VI.
HEATING FOR FORGING; FOR HARDENING; FOR WELDING.
BURNING, OVERHEATING, RESTORING.
From what has been said already about the effects of heat it follows
without further argument that heating is one of the most important, or
perhaps more properly the most important of all, of the operations to
which steel has to be subjected.
The first and vital thing to be borne in mind is that all heating
should be uniform throughout the mass. It has been shown that
heat affects the grain, the structure, as surely as it moves the
mercury-column, and such being the case it is plain that as perfect
uniformity as it is possible to attain is the first essential for all
heating, no matter what the ultimate object may be.
In heating for forging the limit lies between the point of
recalescence, the beginning of true plasticity, and the granular
condition, the end of plasticity; these temperatures lie between dark
or medium orange for all steels and medium or light lemon on the upper
limit, depending on the carbon content, or lower if it be an alloy
steel.
If there is much work to be done upon a piece of steel, it is well to
heat at first to as high a temperature as is safe, and then to forge
or work heavily at the higher heat, reducing the blows or passes as
the piece is reduced and the temperature falls. Although this high
heating will raise the grain of the steel, the heavy working will bring
it back to a fine, compact structure.
If little work is to be done, then it is better to heat as low as may
be safe, and allow the work to be done without letting the heat down
below orange red, so that the steel may not be crushed in the grain.
Below orange red, the so called “dark cherry,” steel should not be
forged, except that in forging for fine tools it is well to give
many light and rapid blows until black begins to show in order to
hammer-refine it; this must be done with extreme care so as not to
crush the steel and cause cracking in the subsequent hardening, or
crumbling in the hardened tool.
HEATING FOR HARDENING.
When a piece of steel is to be hardened by quenching in water or any
quick-cooling medium, it should be heated with great care to the exact
temperature to produce the required hardness.
After forging, no piece of steel should be quenched without first being
heated uniformly to the proper temperature. Ede in his book recommends
quenching immediately after forging in some cases. The so-called Harvey
patent recommends cooling from a high heat down to the required heat
and then quenching.
Both practices are bad. In the Ede case this is believed to be the only
bad piece of advice in his very valuable book--in every other respect
the most practical and useful book upon the manipulation of steel known
to the author.
The reason for objecting to the quenching after forging without
re-heating is that forging always sets up uneven strains in the mass;
the flow is easier from the sides than from the middle of the piece,
and therefore the amount of work done upon one part is greater than
upon another; also it is impossible to hammer or press a piece of steel
with exact uniformity throughout, so that it follows that after forging
there is never exact uniformity of texture or temperature, and such
uniformity is the one essential thing to insure good and even hardening.
The practice of allowing a highly heated piece to cool down to a given
color and then quenching is objectionable, because it produces a coarse
and brittle grain due to the higher heat.
Referring to the illustration on page 67 of the squares representing
grains due to different temperatures: Assume that square No. 3
represents the heat at which quenching is to take place, and No. 6 is
the heat to which the piece has been subjected; then the piece when it
has cooled to No. 3 will not have the grain due to No. 3 heat: it will
have a larger, coarser grain that formed as the piece cooled from No.
6. If now it be quenched, it will have only the hardness due to No. 3,
with a much coarser and more brittle grain than No. 3 heat should give.
The way to manage such a case is to let the piece cool completely and
assume the No. 6 grain; then re-heat carefully to exactly No. 3 and
no hotter; keep the piece at that heat for a few minutes, or moments,
according to its size, to allow for lag; then it will have the finer
grain due to No. 3 heat, and when quenched it will be as hard as under
the other method, and it will be much finer and stronger.
The same rule applies to any two temperatures.
As an expression of exactness as to evenness of heat, it may be said
that the piece should be as uniform in color as if it had been dipped
into a pot of paint. When such uniformity is attained, a break from
quenching is rare, unless the piece has been shamefully overheated so
that the strains of quenching are greater than the tenacity of the
steel.
HEATING FOR WELDING.
When an ingot is to be forged or rolled, it is well to take the highest
heat possible--that immediately below the heat of granulation. Such a
heat may be taken safely by keeping the steel covered with a surface
flux to protect it from the flame. Ordinary red clay, dried and
powdered, is an excellent flux for the purpose, and the cheapest known.
Melted and powdered borax is the best of known fluxes, but it is so
expensive that, as a rule, it is used only on the finest tool-steel,
or on some of the alloy steels where the highest heat possible is not
above a bright orange color, or hardly so high.
A good flux, intermediate in cost between common red clay and powdered
borax, is an earth or mineral barite, or heavy spar. This material
fuses more readily than red clay and not quite so easily as borax. It
forms a good protective covering on the steel, and it is nearly or
quite as efficient as borax.
The object in heating so high is to make the steel as soft and plastic
as it may be, so that the subsequent working will close up all porosity
as far as possible. Nearly all ingots have in them a greater or less
number of cavities, commonly called blow-holes, that are caused by the
separation of occluded gases during cooling. If such porosities are
not oxidized on the surface they will disappear under heavy working at
a high heat. It is probable that under the compression of the work
the gases are redisseminated in the mass and the walls of the cavities
are reunited. If there be the slightest oxidation of the surface of a
cavity the walls will not reunite: there will be left in the mass a
little flat film of oxide which will prevent the union.
In mild steels used for machinery or structural purposes these little
films may do no harm, the factor of safety being sufficient to more
than cover any weakening effect. In tool-steel that is to be hardened
such little films are almost certain to cause fracture. Dies as large
as twelve inches square and six to eight inches thick, having been
heated and quenched with the greatest care, have split fairly in two,
and have revealed in the fracture a little film no larger than half an
inch in diameter and of inappreciable thickness. At the same time the
perfectly uniform grain and hardness showed that the highest skill had
been used. This is only one illustration of the fact that every break
in the continuity of the grain in steel forms a starting-point for
fracture under heavy stress.
From what has been said it is plain that to weld two pieces of steel
together is a difficult matter; still it can be done if great care
be used. In general it is better to avoid such welding except in
cases of necessity. The welding of steel tubing, and the electric
welding of rails, frogs, switches, etc., is done on a large scale and
satisfactorily, so that it will not do to say that steel cannot be
welded. It can be welded or pasted together, and it is a good operation
to avoid in all high steel. In case steel is to be hardened a weld will
reveal itself almost certainly.
BURNING IN HEATING.
When a piece of steel breaks and shows a coarse, fiery fracture, it
is common to say that it is burned. This is not necessarily the case.
There are several degrees in the effects of heat. The first is the
raising of the grain; the second, in high steel, is the decarbonizing
or burning out of carbon from the surface in, the depth of the
decarbonizing depending upon time and temperature; the third is
oxidizing, or actual burning in the common acceptance of the term.
All of these operations go on to a slight extent every time a piece of
steel is heated, but when the heating is done carefully there is only
a small film of steel that is decarbonized and oxidized, and this film
flies off when the piece is quenched for hardening. When the steel is
forged or rolled this skin will be united firmly to the steel, and it
will be thinner or thicker, according to the number of heatings and
the time of exposure to the fire. In tool-making this skin must always
be removed. Many an expensive tool is made perfectly worthless by not
having this skin all removed, owing usually to mistaken economy. The
steel is expensive, and the tool-maker does not wish to cut it up into
worthless chips.
When a tool costing, say, twenty-five dollars is made useless by
failure to cut away twenty-five cents’ worth of useless skin, the
economy of such an operation requires no discussion. It is impossible
to forge a piece of steel without producing such a skin, and it is well
known that decarbonized iron will not harden.
Ordinarily a cut of ¹/₁₆ of an inch should remove such a skin on
straight rolled or hammered bars. In the case of a shaped forging where
many re-heatings have been required the forgeman will have done good
work if the cutting away of ⅛ of an inch will present a good surface:
tool-makers should consider this and allow for it. On the other hand,
if a tool-maker finds that the removal of ⅛ of an inch from a bar, or ¼
of an inch from a forging will not yield him a good, hard surface, he
should hold the steel-maker responsible for bad work.
Actual burning reveals itself in rough tears, and cracks at the surface
and corners of the piece. Such a piece should go to the scrap heap.
Overheated steel of coarse, fiery grain has been injured, and not
necessarily destroyed. Such a piece may be restored to any fineness of
grain by heating to the right temperature--medium orange for the best
grain--keeping it at that heat for, say, one minute for a little piece,
and five to ten or fifteen minutes for a large piece. The heat should
penetrate the whole mass, and it should not be allowed to run above
the given color in any part, not even for a moment. It should then be
allowed to cool in a dry place, without disturbance. The grain will now
be fine and uniform, and the steel may be worked in the ordinary way.
This simple operation is all that is necessary to restore to a fine
grain any piece of steel that has been overheated, provided that the
piece has not been actually burned nor ruptured.
VII.
ANNEALING.
It has been shown that the grain or structure of steel is profoundly
affected by heat, so that any difference of heat-color that is visible
to the naked eye will cause a difference of grain that is also visible
to the naked eye.
Specific-gravity tests and delicate magnetic tests have proved that for
every variation in grain there is a difference of specific-gravity,
which means, of course, a difference in volume; from this it is clear
that if in any one piece of steel there exists a variety of grain due
to uneven heating, there must necessarily be in the mass internal
destructive strains. These strains become manifest when a piece of
unevenly heated steel cracks in hardening; in this case the strains are
greater than the tenacity of the steel.
It is well known, also, that all working of steel, such as forging or
rolling, has a hardening effect, so that ordinary bars or forgings
cannot be machined readily in the condition in which they are left by
these operations.
If there were no remedy for these conditions of internal stress and
initial hardness, the general use of steel would be very difficult, and
its application would be limited seriously.
Fortunately, there are three properties of steel which furnish an easy
and efficient remedy.
First, the fact that steel will assume by mere heating a grain or
structure due to any temperature, no matter what its previous structure
may have been, makes it a simple matter to remove practically all
irregularities of grain and stress, by heating the mass to a perfectly
uniform color and allowing it to cool uniformly.
Second, as heating is a softening process always, the mere heating of
any piece of steel will soften it, and the amount of this softness that
can be retained when the piece is cold is a direct function of the
length of time of cooling, so that by sufficiently slow cooling any
steel can be left reasonably soft.
This does not apply to Hadfield’s manganese steel, which cannot be made
soft when cold by any of the known processes of annealing.
Third, by reference to the specific-gravity table No. I, Chap. V,
it will be seen that the change in volume due to differences of
temperature is much less in mild steel than in high steel. This fact
does not rest upon the evidence of this table alone; it is a fact of
common knowledge to all steel-makers that mild steel is much more inert
than high steel; therefore differences of heat and working that produce
serious results in high steel are hardly appreciable in mild steel. As
a rule all structural steels are comparatively mild, therefore they
are generally in a fit condition for use when they leave the rolls
or forge. In cases of special forging, where one part is heated and
another is left cold, as in the forging of the heads of eye-bars, it
would seem to be wiser to anneal such pieces to remove the area of
strain that must exist between the unheated parts and those that were
heated and forged.
The operation of removing strains and hardness by careful, uniform
heating and slow cooling is known as _annealing_.
Annealing should not be confused with tempering. Tempering is the
partial softening of hardened steel, to remove some of the exceeding
brittleness of hardened steel, and so to make it strong and highly
elastic while it is still very hard.
Annealing is the complete softening of a piece of steel; that is to
say, as a rule, the obtaining of the utmost softness that is possible;
or in any case to have the steel softer than any tempering would leave
it.
Annealing, and tempering are frequently used synonymously. Such misuse
of terms in speaking of technical matters leads to confusion of ideas
and misunderstandings.
As a rule, the best heat to use for annealing is that which gives a
medium orange color; it is a good heat to quench from; it is a little
above the heat of recalescence, about 655° Cent. This heat is that
which gives the finest grain to steel when it is hardened, and is known
as the refining heat.
As steel is thoroughly plastic and soft at this heat, and as it yields
the best and strongest grain when cooled from this heat, it is clear
that there is nothing to be gained by heating any higher for annealing.
In annealing, the steel should be brought up to the right color, medium
orange, and left at that heat until it is hot through, care being taken
that the heat does not run any higher in any part of the piece. If the
corners or edges or any part be allowed to run up to bright orange, or
to medium or bright lemon, as is often done, then there is bad work;
the result will be uneven grain and internal strains.
When steel is to be hardened afterwards, there may be no harm in
heating up to an even lemon color; but where is the use in applying
this excess of heat merely to make a coarse grain, when the lower,
medium orange color will give just as good softness and a much better
grain?
The time necessary for good annealing depends upon the size of the
piece; a wire may be brought up to the right heat in five minutes or
less, and heated through in another minute; then it should be removed
from the fire, as every additional moment of heating will only injure
the steel.
A block six or eight inches cube may require three to five hours to
bring it up to the color and have it heated through, and sufficient
time should be given; but as soon as it is hot through it should be
removed from the fire.
A six-inch block may be brought up to a medium orange color in twenty
minutes or less in a hot furnace, and then if it be kept in such a
furnace until it is hot all through, the surface and edges will almost
certainly be brought to a bright lemon color, with bad results. To
do good annealing a piece should never be hotter in one part than in
another, and no part should be hotter than necessary, usually the
medium orange color. Annealing, then, is a slow process comparatively,
and sufficient time should be allowed.
There are many ways of annealing steel, and generally the plan used
is well adapted to the result desired; it is necessary, however, to
consider the end aimed at and to adopt means to accomplish it, because
a plan that is excellent in one case may be entirely inefficient in
another.
Probably the greatest amount of annealing is done in the manufacture of
wire, where many tons must be annealed daily.
For annealing wire sunken cylindrical pits built of fire-bricks are
used usually; the coils of wire are piled up in the cylinders, which
are then covered tightly, and heat is applied through flues surrounding
the cylinders, so that no flame comes in contact with the steel. For
all ordinary uses this method of annealing wire is quick, economical,
and satisfactory. The wire comes out with a heavy scale of oxide on the
surface; this is pickled off in hot acid, and the steel should then be
washed in limewater, then in clean water, and finally dried.
If it be desired to make drill-wire for drills, punches, graving-tools,
etc., this plan will not answer, because under the removable scale
there is left a thin film of decarbonized iron which cannot be pickled
off without ruining the steel, and which will not harden. It is
plain that this soft surface must be ruinous to steel intended for
cutting-tools, for it prevents the extreme edge from hardening--the
very place that must be hard if cutting is to be done.
Tools for drills, lathe-tools, reamers, punches, etc., are usually
annealed in iron boxes, filled in the spaces between the tools with
charcoal; the box is then looted and heated in a furnace adapted to the
work. This is a satisfactory method generally, because the tools are
either ground or turned after annealing, removing any decarbonized film
that may be found; the charcoal usually takes up all of the oxygen and
prevents the formation of heavy scale and decarbonized surfaces, but it
does not do so entirely, and so for annealing drill-wire this plan is
not satisfactory. It is a common practice in annealing in this way to
continue the heating for many hours, sometimes as many as thirty-six
hours, in the mistaken notion that long-continued heating produces
greater softness, and some people adhere to this plan in spite of
remonstrances, because they find that pieces so annealed will turn as
easily as soft cast iron. This last statement is true; the pieces may
be turned in a lathe or cut in any way as easily as soft cast iron, for
the reason that that is exactly what they are practically. When steel
is made properly, the carbon is nearly all in a condition of complete
solution; it is in the very best condition to harden well and to be
enduring.
When steel is heated above the recalescence-point into the plastic
condition, the carbon at once begins to separate out of solution and
into what is known as the graphitic condition. If it be kept hot long
enough, the carbon will practically all take the graphitic form, and
then the steel will not harden properly, and it will not hold its
temper. To illustrate: Let a piece of 90-carbon steel be hardened and
drawn to a light brown temper; it will be found to be almost file hard,
very strong, and capable of holding a fine, keen edge for a long time.
Next let a part of the same bar be buried in charcoal in a box and
be closed up air-tight, then let it be heated to a medium orange, no
hotter, and be kept at that heat for twelve hours, a common practice,
and then cooled slowly. This piece will be easily cut, and it will
harden very hard, but when drawn to the same light brown as the other
tool a file will cut it easily; it will not hold its edge, and it will
not do good work.
Clearly in this case time and money have been spent merely in spoiling
good material. There is nothing to be gained, and there is everything
to be lost, in long-continued heating of any piece of steel for any
purpose. When it is hot enough, and hot through, get it away from the
fire as quickly as possible.
This method of box-annealing is not satisfactory when applied to
drill-wire, or to long thin strands intended for clock-springs,
watch-springs, etc.
The coils or strands do not come out even; they will be harder in one
part than in another; they will not take an even temper. When hardened
and tempered, some parts will be found to be just right, and others
will have a soft surface, or will not hold a good temper. The reason of
this seems to be a want of uniformity in the conditions: the charcoal
does not take up all of the oxygen before the steel is hot enough to be
attacked, and so a decarbonized surface is formed in some parts; or it
may be that some of the carbon dioxide which is formed comes in contact
with the surface of the steel and takes another equivalent of carbon
from it. Whatever the reaction may be, the fact is that much soft
surface is formed. This soft surface may not be more than .001 of an
inch thick, but that is enough to ruin a watch-spring or a fine drill.
Again, it seems to be impossible to heat such boxes evenly; it is
manifest that it must take a considerable length of time to heat a
mass of charcoal up to the required temperature, and if the whole be
not so heated some of the steel will not be heated sufficiently; this
will show itself in the subsequent drawing of the wire or rolling of
the strands. On the other hand, if the whole mass be brought up to
the required heat, some of the steel will have come up to the heat
quickly, and will then have been subjected to that heat during the
balance of the operation, and in this way the carbon will be thrown out
of solution partly. This is proven by the fact that strands made in
this way and hardened and tempered by the continuous process will be
hard and soft at regular intervals, showing that one side of the coil
has been subjected to too much heat. This trouble is overcome by open
annealing, which will be described presently.
When steel is heated in an open furnace, there is always a scale of
oxide formed on the surface; this scale, being hard, and of the nature
of sand or of sandstone, grinds away the edges of cutting-tools,
so that, although the steel underneath may be soft and in good
cutting condition, this gritty surface is very objectionable. This
trouble is overcome by annealing in closed vessels; when charcoal is
used, the difficulties just mentioned in connection with wire- and
strand-annealing operate to some extent, although not so seriously,
because the steel is to be machined, removing the surface.
The Jones method of annealing in an atmosphere of gas is a complete
cure for these troubles.
Jones uses ordinary gas-pipes or welded tubes of sizes to suit the
class of work. One end of the tube is welded up solid; the other end is
reinforced by a band upon which a screw-thread is cut; a cap is made to
screw on this end when the tube is charged. A gas-pipe of about ½-inch
diameter is screwed into the solid end, and a hole of ¹/₁₆- to ⅛-inch
diameter is drilled in the cap.
When the tube is charged and the cap is screwed on, a hose connected
with a gas-main is attached to the piece of gas-pipe in the solid end
of the tube; the gas-pipe is long enough to project out of the end of
the furnace a foot or so through a slot made in the end of the furnace
for that purpose.
The gas is now turned on and a flame is held near the hole in the cap
until the escaping gas ignites; this shows that the air is driven out
and replaced by gas.
The pipe is now rolled into the furnace and the door is closed, the
gas continuing to flow through the pipe. By keeping the pipe down to
a proper annealing-heat it is manifest that the steel will not be any
hotter than the pipe. By heating the pipe evenly by rolling it over
occasionally the steel will be heated evenly. A little experience will
teach the operator how long it takes to heat through a given size of
pipe and its contents, so that he need not expose his steel to heat any
longer than necessary.
There is not a great quantity of gas consumed in the operation, because
the expanding gas in the tube makes a back pressure, the vent in the
cap being small. This seems to be the perfection of annealing. A tube
containing a bushel or more of bright, polished tacks will deliver
them all perfectly bright and as ductile as lead, showing that there
is no oxidation whatever. Experiments with drill-rods, with the use of
natural gas, have shown that they can be annealed in this way, leaving
the surface perfectly bright, and thoroughly hard when quenched. This
Jones process is patented.
Although the Jones process is so perfect, and necessary for bright
surfaces, its detail is not necessary when a tarnished surface is not
objectionable.
The charcoal difficulty can be overcome also. Let a pipe be made like
a Jones pipe without a hole in the cap or a gas-pipe in the end. To
charge it first throw a handful of resin into the bottom of the pipe,
then put in the steel, then another handful of resin near the open end,
and screw on the cap. The cap is a loose fit. Now roll the whole into
the furnace; the resin will be volatilized at once, fill the pipe with
carbon or hydrocarbon gases, and unite with the air long before the
steel is hot enough to be attacked.
The gas will cause an outward pressure, and may be seen burning as it
leaks through the joint at the cap. This prevents air from coming in
contact with the steel. This method is as efficient as the Jones plan
as far as perfect heating and easy management are concerned. It reduces
the scale on the surfaces of the pieces, leaving them a dark gray color
and covered with fine carbon or soot. For annealing blocks or bars it
is handier and cheaper than the Jones plan, but it will not do for
polished surfaces. This method is not patented.
OPEN ANNEALING.
Open annealing, or annealing without boxes or pipes, is practised
wherever there are comparatively few pieces to anneal and where a
regular annealing-plant would not pay, or in a specially arranged
annealing-furnace where drill-wire, clock-spring steel, etc., are to be
annealed.
For ordinary work a blacksmith has near his fire a box of dry lime or
of powdered charcoal. He brings his piece up to the right heat and
buries it in the box, where it may cool slowly. In annealing in this
way it is well not to use blast, because it is liable to force all
edges up to too high a heat and to make a very heavy scale all over the
surface. With a little common-sense and by the use of a little care
this way of annealing is admirable.
It is a common practice where there is a furnace in use in daytime and
allowed to go cold at night to charge the furnace in the evening, after
the fire is drawn, with steel to be annealed, close the doors and
damper, and leave the whole until morning. The furnace does not look
too hot when it is closed up, but no one knows how hot it will make
the steel by radiation: the steel is almost always made too hot, it is
kept hot too long, and so converted into cast iron, and there is an
excessively heavy scale on it.
Many thousands of dollars worth of good steel are ruined annually in
this way, and it is in every way about the worst method of annealing
that was ever devised.
To anneal wire or thin strands in an open furnace the furnace should
be built with vertical walls about two feet high and then arched to a
half circle. The inports for flame should be vertical and open into the
furnace at the top of the vertical wall; the outports for the gases of
combustion should be vertical and at the same level as the inports and
on the opposite side of the furnace from the inports. These outflues
may be carried under the floor of the furnace to keep it hot.
The bottom of the door should be at the level of the ports to keep
indraught air away from the steel. The annealing-pot is then the whole
size of the furnace--two feet deep--and closed all around.
The draught should be regulated so that the flame will pass around
the roof, or so nearly so as to never touch the steel, not even in
momentary eddies.
In such a furnace clock-spring wire not more than .01 inch in diameter,
or clock-spring strands not more than .006 to .008 inch thick and
several hundred feet long, may be annealed perfectly. The steel is
scaled of course, but the operation is so quick and so complete that
there is no decarbonized surface under the scale.
This plan is better than the Jones method or any closed method,
because the big boxes necessary to hold the strands or coils cannot
be heated up without in some parts overheating the steel; all of which
is avoided in the open furnace, because by means of peep-holes the
operator can see what he is about, and after a little practice he can
anneal large quantities of steel uniformly and efficiently.
VIII.
HARDENING AND TEMPERING.
For nearly all structural and machinery purposes steel is used in the
condition in which it comes from the rolls or the forge; in exceptional
cases it is annealed, and in some cases such as for wire in cables or
for bearings in machinery, it is hardened and tempered.
For all uses for tools steel must be hardened, or hardened and
tempered. The operations of hardening and tempering, including the
necessary heating, are the most important, the most delicate, and the
most difficult of all of the manipulations to which steel is subjected;
these operations form an art in themselves where skill, care, good
judgment, and experience are required to produce reliable and
satisfactory results. It is a common idea that all that is necessary
is to heat a piece of steel, quench it in water, brine, or some pet
nostrum, and then warm it to a certain color; these are indeed the only
operations that are necessary, but the way in which they are done are
all-important.
An experienced steel-maker is often amazed at the confidence with which
an ignorant person will put a valuable tool in the fire, rush the heat
up to some bright color, or half a dozen colors at once, and souse it
into the cooling-bath without regard to consequences. That such work
does not always result in disastrous fractures shows that steel does
possess marvellous strength to resist even the worst disregard of rules
and facts.
On the other hand, the beautiful work upon the most delicate and
difficult shapes that is done by one skilled in the art cannot but
excite the surprise and admiration of the onlooker who is familiar with
the physics of steel, and who can appreciate the delicacy of handling
required in the operation.
There are a few simple laws to observe and rules to follow which will
lead to success; they will be stated in this chapter as clearly as may
be, in the hope of giving the reader a good starting-point and a plain
path to follow; but he who would become an expert can do so only by
travelling the road carefully step by step. The hair-spring of a watch,
or a little pinion or pivot, so small that it can only be seen through
a magnifying-glass, the exquisitely engraved die costing hundreds or
thousands of dollars, and the huge armor-plate weighing many tons,
must all be hardened and tempered under precisely the same laws and in
exactly the same way; the only difference is in the means of getting at
it in each case.
Referring now to properties mentioned in the previous chapters, we have
first to heat the piece to the right temperature and then to cool it
in the quickest possible way in order to secure the greatest hardness
and the best grain. In doing this we subject the steel to the greatest
shocks or strains, and great care must be used.
The importance of uniformity in heating for forging and for annealing
has been stated, and it has been shown how an error in this may be
rectified by another and a more careful heating; when it comes to
hardening, this uniformity must be insisted upon and emphasized, for as
a rule an error here has no remedy.
There may be cases of bad work that do not cause actual fracture
that can be remedied by re-heating and hardening, but these are
rare, because even if incurable fracture does not occur the error is
not discovered until the piece has been put to work and its failure
develops the errors of the temperer.
If the error is one of merely too low heat, not producing thorough
hardening, it will generally be discovered by the operator, who will
then try again and possibly succeed; but if the error be of uneven
heat, or too much heat, the probabilities are that it will not be
discovered until the piece fails in work, when it will be too late to
apply any remedy.
Referring to Table I, Chap. V, treating of specific gravities, it is
clear that all steel possesses different specific gravities, due to
differences of temperature, and that these differences of specific
gravity increase as the carbon content increases; it follows that if a
piece of steel be heated unevenly, internal strains must be set up in
the mass, and it is certain that if steel be quenched in this condition
violent strains will be set up, even to the causing of fractures.
The theory of this action, as of all hardening, is involved in
discussion which will be considered later; in this chapter the facts
will be dealt with. When a piece of steel is heated, no matter how
unevenly or to what temperature below actual granulation, and is
allowed to cool slowly and without disturbance, it will not break or
crack under the operation. If a piece be heated as unevenly as, say,
medium orange in one part and medium lemon in another, and is then
quenched, it will be almost certain to crack if it contains enough
carbon to harden at all in the common acceptance of the term, that is
to say, file hard or having carbon 40 or higher.
This fact is too well known to be open to discussion; therefore the
quenching of hot steel, the operation of hardening, does set up violent
strains in steel, no matter what the true theory of hardening may be.
Referring to Chap. V, to the series of squares representing the
apparent sizes of grain due to different temperatures, similar
results follow from hardening, with the exceptions that the different
structures are far more plainly marked, and the squares should be
arranged a little differently; they are shown as continuously larger in
Chap. V, from the grain of the cold bar up to the highest temperature;
this is true if a bar has been rolled or hammered properly into a fine
condition of grain. Of course if a bar be finished at, say, medium
orange it will have a grain due to that heat--No. 3 in the series of
squares. Then if it be heated to dark orange and cooled from that heat
it will take on a grain corresponding to square No. 2, and No. 1 square
will be eliminated.
The series of squares to represent hardened grain will be as follows:
[Illustration]
The heat colors being the same as before, viz.:
1. The natural bar--untreated.
2. Quenched at dark orange or orange red.
3. “ “ medium orange--_refined_.
4. “ “ bright orange.
5. “ “ dark lemon.
6. “ “ medium lemon.
7. “ “ bright lemon.
8. “ “ very bright lemon or creamy.
Heats 6, 7, 8 will almost invariably produce cracks although the pieces
be evenly heated.
[Illustration]
[Illustration]
These squares do not represent absolute structures with marked
divisions; they are only the steps on an incline, like the temper
numbers in the carbon series; thus, the carbon-line is continuous,
but the temper divisions represent steps up the incline. So with the
series of squares, the changes of grain or structure are continuous,
as represented by the doubly inclined line; the squares being only the
steps to indicate easily observed divisions. The minuteness of the
changes is illustrated by the fact that in a piece heated continuously
from creamy to dark orange and quenched, differences of grain have been
observed unmistakably on opposite sides of pieces broken off not more
than ⅛ inch thick.
In practice the differences due to the colors given in the list above
are as plain and surely marked as are the differences in the structure
of ingots due to the different temper carbons already described.
In this hardened series each carbon temper gives its own peculiar
grain; in low steel, say 40 carbon compared to 1.00 carbon or higher,
No. 3 will be larger and No. 8 will be smaller in the low temper than
in the high--another illustration of the fact that low steel is more
inert to the action of heat than high steel. All grades and all tempers
go through the same changes, but they are more marked in the high than
in the low steel.
The grain of hardened steel is affected by the presence of silicon,
phosphorus, and manganese, and doubtless by any other ingredients,
these three being the most common.
It is in the grain of hardened steel that the conditions described
in Chap. V as “sappy,” “dry,” and “fiery” are the most easily and
frequently observed, although the same conditions obtain in unhardened
steel in a manner that is useful to an observing steel-user. But it is
in this hardened condition that the excellences or defects of steel are
brought out and emphasized.
When a piece of steel is heated continuously from “creamy,” or
scintillating, down to black, or unheated, and is then quenched, the
grain will be found to be coarsest, hardest, and most brittle at the
hottest end, and with the brightest lustre, even to brilliancy, and
to become finer down to a certain point, noted as No. 3 in the series
of squares, or at a heat which shows about a medium orange color;
here the grain becomes exceedingly fine, and here the steel is found
to be the strongest and to be without lustre. Below this heat the
grain appears coarser and the steel is less hard, until the grain and
condition of the unheated part are reached. This fine condition, known
as the _refined_ condition, is very remarkable. It is the condition to
be aimed at in all hardening operations, with one or two exceptions
which will be noted, because in this state steel is at its best; it
is strongest then, and it would seem to be clear without argument
that the finest grain and the strongest will hold the best at a fine
cutting-edge, and will do the most work with the least wear, although
a coarser grain may be a little harder, the coarser and more brittle
condition of the latter more than counterbalancing its superior
hardness.
The advantages of this refined condition are so great that it is found
to be well to harden and refine mild-steel dies, and battering- and
cutting-tools that are to be used for hot work, although the heat
will draw out all of the temper in the first few minutes, because the
superior strength of the fine grain will enable the tool to do twice to
twenty times more work than an unhardened tool.
The refining-heat, like most other properties, varies with the carbon;
the medium orange given is the proper heat for normal tool-steel of
from about 90 to 110 carbon. Steel of 150 carbon will refine at about a
dark orange, and steel of 50 to 60 carbon will require about a bright
orange to refine it.
This range is small, but it must be observed and worked to if the best
results are desired.
A color-blind person can never learn to harden steel properly.
In studying this phenomenon of refining, the conclusion was reached
that it occurred at or immediately above the temperature that broke
up the crystalline condition of cold steel and brought it fairly into
the second, the plastic condition. Farther observation led to the
conclusion that the coarser grain and greater hardness caused by higher
heats were due to the gradual change from plastic toward granular
condition that takes place as the heat increases. Later investigations
have given no reason for changing these conclusions.
When the phenomenon of recalescence was observed and investigated by
Osmond and others, different theories were advanced in explanation.
Langley concluded that if recalescence occurred at the change from a
plastic to a crystalline condition, then the heat absorbed and again
set free during such changes would account for the visible phenomenon
of recalescence.
Again, if it should prove that recalescence occurred at the refining
point, the conjunction of these phenomena would indicate strongly,
first, that refining does occur at the point where this change of
structure is complete in the reverse order, from crystalline to
plastic; and second, the first being true, recalescence would be
explained as stated, as indicating the inevitable absorption and
emission of heat due to such a change.
Langley fitted up an electric apparatus for heating steel, in a box so
placed that the light was practically uniform, that is, so that bright
sunlight, or a cloudy sky, or passing clouds would not affect seriously
the observation of heat-colors.
Pieces of steel were heated far above recalescence, up to bright lemon,
and then allowed to cool slowly; in this way recalescence was shown
clearly.
It was found to occur at the refining heat in every case, shifting for
different carbons just as the refining heat shifts.
Immediately under the pieces being observed was a vessel of water
into which the pieces could be dropped and quenched. After observing
the heating and cooling until the eye was well trained, pieces were
quenched at different heats and the results were noted. It was found
that in the ascending heats no great hardness was produced until
the recalescence heat was reached or passed slightly; and in the
descending heat excessive hardening occurred at a little below the
recalescent heat, although no such hardening occurred at that color
during ascending heats. This apparent anomaly is due simply to lag. If,
in ascending, the piece be held for a few moments at the recalescent
point, no increase being allowed, and then it be quenched, it will
harden thoroughly and be refined. If, in descending, the cooling be
arrested at a little below the recalescence for a few moments, neither
increase nor decrease being allowed, and then the piece be quenched,
it will not harden any better than if it be quenched immediately upon
reaching the same heat in ascending.
Time must be allowed for the changes to take place, and lag must be
provided for.
These experiments show that refining and recalescence take place at the
same temperature.
AS TO HARDNESS.
Prof. J. W. Langley showed by sp. gr. determinations that steel
quenched from 212° F. in water at 60° F. showed the hardening effect of
such quenching, the difference of temperature being only 152° F.
Prof. S. P. Langley, of the Smithsonian, proved the same to be true by
delicate electrical tests, and these again were confirmed by Prof. J.
W. Langley in the laboratory of the Case School of Sciences.
A piece of refined steel will rarely be hard enough to scratch glass.
A piece of steel quenched from creamy heat will almost always scratch
glass. The maximum hardness is produced by the highest heat, or when
temperature minus cold is a maximum; the least hardness is found
by quenching at the lowest heat above the cooling medium, or when
temperature minus cold is a minimum--the time required to quench being
a minimum in both cases.
What occurs between these limits? Is the curve of hardness a straight
line, or an irregular line?
Let a piece of steel be heated as uniformly as possible from a creamy
heat at one end to black at the other, and then be quenched.
Now take a newly broken hard file and draw its sharp corner gently and
firmly over the piece, beginning at the black-heated end. The file
will take hold, and as it is drawn along it will be felt that the
piece becomes slightly harder as the file advances, until suddenly it
will slip, and no amount of pressure will make it take hold above that
point. The piece has become suddenly file hard.
Next try the same thing with a diamond; the diamond will cut easily
until the point is reached where the file slipped, then there will be
found a great increase of hardness.
From this point to the end of the piece it is observed readily by the
action of the diamond that there is a gradual increase of hardness from
the hump to the end of the piece to the creamy-heated end. Attempts
were made to measure this curve of hardness by putting a load on the
diamond and dragging it over the piece; but no diamond obtainable would
bear a load heavy enough to produce a groove that could be measured
accurately by micrometer. An examination of such a groove, through a
strong magnifying-glass revealed the conditions plainly; the groove of
hardness may be illustrated on an exaggerated scale; thus:
[Illustration]
The next question was, Where does this hump occur, and what is the
cause of it?
Careful observation showed that it occurred at the point of
recalescence, at the refining-point. This word point must not be taken
as space without dimension in this connection; it is used in the
common sense of at or adjacent to a given place. There is of course a
small allowable range of temperature above any given exact point of
recalescence, such as 655° C. or 1211° F.
By superimposing Langley’s curves of cooling and of hardening (see
Trans. Am. Soc. Civ. Eng., Vol. XXVII, p. 403), the relation between
recalescence and the hardening-hump is obvious.
[Illustration]
It is safe to say that experience proves that the _refined_ condition
is the best for all cutting-tools of every shape and form.
It seems to be obvious; the steel is then in its strongest condition,
and when the grain is finest, the crystals the smallest, a fine edge
should be the most enduring, because there is a more intimate contact
between the particles. That a steel will refine well, and be strong in
that condition is the steel-maker’s final test of quality.
No steel-maker who has a proper regard for the character of his product
will accept raw material upon mere analysis; analysis is of the utmost
importance, for material for steel-making must be of a quality that
will produce a certain quality of steel, or the result will be an
inferior product. This applies to acid Bessemer and open-hearth, and to
crucible-steel especially; the basic processes admit of a reduction of
phosphorus not obtainable in the others.
In making fine-tool steel a bad charge in the pot inevitably means a
bad piece of steel. It may happen also that an iron of apparently good
analysis will not produce a really fine steel; then there must be a
search for unusual elements, such as copper, arsenic, antimony, etc.,
or for dirt, left in the iron by careless working. The refining-test
then is as necessary as analysis, for if steel will not refine
thoroughly it will not make good tools. Battering-tools, such as
sledges, hammers, flatters, etc., should be refined carefully, for
although their work is mainly compressive they are liable to receive,
and do get, blows on the corners and edges that would ruin them if they
were not in the strongest condition possible.
The reasons for refining hot-working tools have been stated already.
Engraved dies for use in drop-presses where they are subjected to heavy
blows are undoubtedly in the most durable condition when they are
refined, but they are subjected not only to impact, but to enormous
compression, and therefore they must be hardened deeply. When a
die-block is heated so as to refine, and then is quenched, it hardens
perfectly on the surface and not very deeply, and it is quite common in
such a case to see a die crushed by a few blows: the hardened part is
driven bodily into the soft steel below it, and the die is ruined; thus:
[Illustration]
To avoid this, such a die should be heated to No. 5, or a dark lemon,
and quenched suddenly in a large volume of rushing water.
It will then have the enormous resistance to compression that is so
well known in very hard steel, and it will be hardened so deeply that
the blow of the hammer will not crush through the hard part. This is
the best condition, too, of an armor-plate that is to resist the impact
of a projectile.
It will be brittle, a light blow of a hammer will snip the corners,
but it cannot be crushed by ordinary work. Dies made in this way have
turned out thousands of gross of stamped pieces, showing no appreciable
wear.
To harden a die in this way is a critical operation, because the
strains are so enormous that a very trifling unevenness in the heat
will break the piece, but the skill of expert temperers is so great
that they will harden hundreds of dies in this way and not lose one if
the steel be sound.
HEATING FOR HARDENING.
A smith can heat an occasional piece for hardening, in his ordinary
fire by using care and taking a little time. Where there are many
pieces to be hardened, special furnaces should be used.
For thousands of little pieces, such as saw-teeth or little springs, a
large furnace with a brick floor, and so arranged that the flame will
not impinge on the pieces, is good.
The operator can watch the pieces, and as soon as any come to the right
color he can draw them out, letting them drop into the quenching-tank,
which should be right under the door or close at hand.
For twist-drills, reamers, etc., a lead bath, or a bath of melted salt
and soda, is used. The lead bath is the best if care be taken to draw
off the fumes so as not to poison the heaters. Because a bath of this
kind is of exactly the right color at the top it is not to be assumed
that pieces can be heated in it and hardened without further attention.
Thousands of tools are ruined, and thousands of dollars are thrown
away annually, by unobserving men who assume that because a lead bath
appears to be exactly the right color at the surface it is therefore
just right.
A dark orange color surface may have underneath it an increasingly
higher temperature, up to a bright lemon at the bottom, and tools
heated in such a bath will have all of the varying temperatures of
the bath; then cracked tools, twisted tools, brittle tools, tools too
hard at one end and not hard enough at the other, will come out with
exasperating regularity.
All of this can be avoided by a simple thorough stirring of the bath,
to be done as often as may be necessary to keep it uniform.
In heating toothed tools, taps, reamers, milling-cutters, and the like,
care should be taken that the points of the teeth never get above the
refining-heat, the dark or medium orange required. It is no easy matter
to do this except in a uniform bath, but it must be done. If the teeth
are bright lemon, or even bright orange, when the body of the tool is
at medium orange refining-heat, the probabilities are that they will
shell off from the hardened tool as easily as the grains from a cob of
corn.
Even if they are not so bad, if they do not crack off, they will be
coarse-grained and brittle; they will not hold a good edge, and they
will not do good work. If a long tool, such as a drill, etc., be heated
medium orange on one side and bright orange on the other,--a difference
of 100° to 200° F.,--and be quenched, it will come out of the bath
curved; it must be curved. In quenching a long tool which it is desired
to have straight it should be dipped vertically, so as to cool all
around the axis simultaneously. If such a tool be dipped sideways, it
will come out bent. In heating edge-tools of all kinds it is best to
heat first the thicker part, away from the edge, and then when the body
has come up to the refining-heat to draw the edge into the fire and let
it come up last; as soon as a uniform color is reached quench promptly.
If the edge be exposed to the fire in the beginning of the operation,
it will almost certainly become too hot before the thicker parts are
hot enough.
When a smooth, cylindrical piece is to be hardened, it should be
rolled around from time to time while heating, unless it is in a lead
bath; if it be left to lie quietly in a furnace until it is hot, it
will have a soft streak along the part that was uppermost.
The cause of this is not clear; the fact is as certain as hundreds
of tests can make any fact. The experiment can be made by re-heating
the piece with the soft streak down; then the original soft streak
will come out hard, and another soft streak will be found on top. The
changes can be rung upon this indefinitely.
A maker of roller-tube expanders had great trouble with his
expander-pins; they cut, and wore out on one side. He tried many makes
and many tempers of steel with the same result. He was told to turn
his pins over and over as he heated them and his troubles would end.
He replied: “Why, of course; I can see the reason and sense in that.”
If he did see the reason, he is the only person known, so far, who has
done so. His pins worked all right from that time.
_In hardening_ ROUND SECTIONS it is necessary to use great care to
have the heat perfectly uniform and not too high, because the circular
form is the most rigid, offering the greatest resistance to change.
For this reason a round piece will be almost certain to split if it be
heated above a medium orange, or if it be heated unevenly. Many a round
piece is cracked by a heat, or by a little unevenness of heat, that
another section would endure safely. A roll with journals is perhaps
the most difficult of all tools to harden successfully; the most expert
temperers will not be surprised at losing as many as one roll in five.
_Engraved dies_ require to be hardened without oxidizing the engraved
face, so that the finest lines will be preserved clear and clean.
This is done by burying the engraved face in carbonaceous material
in such a way as to prevent the flame or any hot air from coming in
contact with it.
There are many ways of doing this, and many different carbonaceous
mixtures are used; one simple, and known to be satisfactory, plan
will be explained as sufficient to give any intending operator a good
starting-point.
The carbonaceous material preferred is burnt leather powdered--and
the older it is the better--until it is reduced to ash, so that the
material should be saved after each operation to be used again mixed
with enough new material to make up the necessary quantity.
[Illustration]
_D_ is the die to be heated; _B_ is an open box about two inches deep
and one inch larger each way than the die; _L_ is the burnt leather
packed in thoroughly, and as full as the box will hold. The engraved
face is down, embedded in the burnt leather, and secure from contact
with flame or air.
Sometimes powdered charcoal is used, with or without a mixture of tar,
according to the fancy of the operator.
Some operators prefer to have the box so high as to leave only the top
surface of the embedded die exposed, but the most successful workers
prefer the plan sketched, because they can see more of the die, and so
regulate better the even heating.
The die and box are put in the furnace, and the heating is watched,
the die being turned and moved about in the furnace so as to obtain a
perfectly even heat.
When the right temperature is reached, the whole is withdrawn from the
furnace; the die is lifted out of the box and plunged into the water
immediately. There must be no delay at this point whatever; a few
moments’ exposure of the hot die to the air will result in oxidation
and scaling of the engraving.
In heating such a die a furnace should be used. It can be done in a
smith’s fire, but it is a hazardous plan, and gives many chances for a
failure.
A furnace with an even bed of incandescent coke is good, and such a
furnace is very useful for many other purposes.
Where many dies are to be hardened, the handiest appliance is a little
furnace with brick floor and lining, and heated by petroleum or gas, so
arranged that the flames will not impinge upon the piece to be heated.
Such furnaces are now made to work so perfectly that illuminating-gas
is found to be an economical fuel.
For quenching there should be plenty of water. For small dies that can
be handled easily by one man a large tub or tank of water will answer
if the operator will keep the die in rapid motion in the water.
Running water is the best. A handy plan is to have the inlet-pipe
project vertically a short distance through the bottom of the tank,
producing a strong upward current which will strike directly against
the face of the submerged die.
Some prefer a downward stream; others a side stream; others, again,
prefer a shower-bath; and, again, some use side jets.
A very efficient tank has a partition running from a few inches from
the bottom to within a few inches of the surface of the water, and so
placed as to separate, say, nine tenths of the tank from one tenth. In
the smaller compartment there is an Archimedean screw driven at a speed
of 200 to 300 revolutions; this drives the water under the partition
and out over the top in a violent current. The steel is quenched in
the larger space. Where water is an item of expense, this plan is
economical, and it is certainly efficient.
An excellent way of quenching large faces, such as anvils, is to have a
tank raised twelve to fifteen feet from the floor. In the bottom of the
tank is a pipe with a valve, to be operated by a lever. The whole is
enclosed in a sort of closet with a door in one side. When the piece is
hot, it is placed immediately under the pipe, the door is closed, the
valve is opened, and a great body of water is dashed down upon the face
that is to be hardened.
A slight modification of this plan is used in hardening armor-plates,
where many jets are used to insure even quenching of the large surface.
This plan is supposed to be patented, or, more properly, it is
patented; but as it is very old and well known the patent should not be
allowed to disturb anybody.
Water only has been mentioned so far as a quenching medium, because it
is the simplest and the cheapest generally. Oil is used frequently
where extreme hardness is not necessary and toughness is desirable. Oil
gives a good hardness with toughness, and it is used almost universally
for springs, and it is sometimes used to toughen railroad axles and
similar work. The oil acts more slowly than water and leaves the piece
in more nearly a tempered condition; it is neither so hard nor so
brittle as it would be if quenched in water. Straits fish-oil is good
and cheap; lard-oil gives greater hardness than fish-oil; mineral oil
is too fiery to use safely; but there are mixed oils in the market made
expressly for hardening which are cheap and efficient.
If it is desired to get the greatest hardness, brine will harden harder
than fresh water; and mercury will give the greatest hardness of all.
It is a rather expensive cooling medium.
Acid added to water increases its hardening power; but those who know
the effects of acids will be very chary of using them.
As to heating, too much emphasis cannot be given to the importance of
even temperature throughout the mass. The illustration of the painted
piece mentioned in connection with heating for forging applies more
forcibly here. Every piece that is to be quenched should look as if
it were covered with a perfectly even coat of paint of the exact tint
necessary to give the best result.