THE FORGING OF STEEL
So much depends upon the forging of steel that this operation must
be carefully supervised. This is especially true because of the
tendency to place unskilled and ignorant men as furnace-tenders
and hammer men. The main points to be supervised are the slow and
careful heating to the proper temperature; forging must be continued
at a proper rate to the correct temperature. The bar of stock from
which a forging was made may have had a fairly good structure, but
if the details of the working are not carefully watched, a seamy,
split article of no value may easily result.
HEATING.--Although it is possible to work steels cold, to an extent
depending upon their ductility, and although such operations are
commonly performed, "forging" usually means working _heated_ steel.
_Heating_ is therefore a vital part of the process.
Heating should be done slowly in a soaking heat. A soft "lazy"
flame with excess carbon is necessary to avoid burning the corners
of the bar or billet, and heavily scaling the surface. If the
temperature is not raised slowly, the outer part of the metal may
be at welding heat while the inner part is several hundred degrees
colder and comparatively hard and brittle.
The above refers to muffle furnaces. If the heating is done in
a small blacksmith's forge, the fire should be kept clean, and
remade at intervals of about two hours. Ashes and cinders should
be cleaned from the center down to the tuyere and oily waste and
wood used to start a new fire. As this kindles a layer of coke
from the old fire is put on top, and another layer of green coal
(screened and dampened blacksmiths' coal) as a cover. When the
green coal on top has been coked the fire is ready for use. As
the fuel burns out in the center, the coke forming around the edge
is pushed inward, and its place taken by more green coal. Thus the
fire is made up of three parts; the center where coke is burning
and the iron heating; a zone where coke is forming, and the outside
bank of green coal.
STEEL WORKED IN AUSTENITIC STATE.--As a general rule steel should
be worked when it is in the austenitic state. (See page 108.) It
is then soft and ductile.
As the steel is heated above the critical temperature the size of
the austenite crystals tends to grow rapidly. When forging starts,
however, these grains are broken up. The growth is continually
destroyed by the hammering, which should consequently be continued
down to the upper critical temperature when the austenite crystals
break up into ferrite and cementite. The size of the final grains
will be much smaller and hence a more uniform structure will result
if the "mother" austenite was also fine grained. A final steel
will be composed of pearlite; ferrite and pearlite; or cementite
and pearlite, according to the carbon content.
The ultimate object is to secure a fine, uniform grain throughout
the piece and this can be secured by uniform heating and by thoroughly
rolling it or working it at a temperature just down to its critical
point. If this is correctly done the fracture will be fine and
silky. Steel which has been overheated slightly and the forging
stopped at too high a temperature will show a "granular" fracture.
A badly overheated or "burned" steel will have iridescent colors
on a fresh fracture, it will be brittle both hot and cold, and
absolutely ruined.
STEEL CAN BE WORKED COLD.--As noted above, steel can be worked cold,
as in the case of cold-rolled steel. Heat treatment of cold-worked
steel is a very delicate operation. Cold working hardens and strengthens
steel. It also introduces internal stresses. Heat-treatments are
designed to eliminate the stresses without losing the hardness
and strength. This is done by tempering at a low heat. Avoid the
"blue" range (350 to 750°C.). Tempering for a considerable time just
under the critical is liable to cause great brittleness. Annealing
(reheating through the critical) destroys the effect of cold work.
FORGING
HIGH-SPEED STEEL.--Heat very slowly and carefully to from 1,800
to 2,000°F. and forge thoroughly and uniformly. If the forging
operation is prolonged do not continue forging the tool when the
steel begins to stiffen under the hammer. Do not forge below 1,700°F.
(a dark lemon or orange color). Reheat frequently rather than prolong
the hammering at the low heats.
After finishing the forging allow the tool to cool as slowly as
possible in lime or dry ashes; avoid placing the tool on the damp
ground or in a draught of air. Use a good clean fire for heating.
Do not allow the tool to soak at the forging heat. Do not heat any
more of the tool than is necessary in order to forge it to the
desired shape.
CARBON TOOL STEEL.--Heat to a bright red, about 1,500 to 1,550°F.
Do not hammer steel when it cools down to a dark cherry red, or
just below its hardening point, as this creates surface cracks.
OIL-HARDENING STEEL.--Heat slowly and uniformly to 1,450°F. and
forge thoroughly. Do not under any circumstances attempt to harden
at the forging heat. After cooling from forging reheat to about
1,450°F. and cool slowly so as to remove forging strains.
CHROME-NICKEL STEEL.--Forging heat of chrome-nickel steel depends
very largely on the percentage of each element contained in the
steel. Steel containing from 1/2 to 1 per cent chromium and from
1-1/2 to 3-1/2 per cent nickel, with a carbon content equal to
the chromium, should be heated very slowly and uniformly to
approximately 1,600° F., or salmon color. After forging, reheat
the steel to about 1,450° and cool slowly so as to remove forging
strains. Do not attempt to harden the steel before such annealing.
A great deal of steel is constantly being spoiled by carelessness
in the forging operation. The billets may be perfectly sound, but
even if the steel is heated to a good forging heat, and is hammered
too lightly, a poor forging results. A proper blow will cause the
edges and ends to bulge slightly outwards--the inner-most parts
of the steel seem to flow faster than the surface. Light blows
will work the surface out faster; the edges and ends will curve
inwards. This condition in extreme cases leaves a seam in the axis
of the forging.
Steel which is heated quickly and forging begun before uniform
heat has penetrated to its center will open up seams because the
cooler central portion is not able to flow with the hot metal
surrounding it. Uniform heating is absolutely necessary for the
best results.
Figure 16 shows a sound forging. The bars in Fig. 17 were burst
by improper forging, while the die, Fig. 18, burst from a piped
center.
Figure 19 shows a piece forged with a hammer too light for the size
of the work. This gives an appearance similar to case-hardening,
the refining effect of the blows reaching but a short distance
from the surface.
While it is impossible to accurately rate the capacity of steam
hammers with respect to the size of work they should handle, on
account of the greatly varying conditions, a few notes from the
experience of the Bement works of the Niles-Bement-Pond Company
will be of service.
[Illustration: FIG. 16.--A sound forging.]
[Illustration: FIG. 17.--Burst from improper forging.]
For making an occasional forging of a given size, a smaller hammer
may be used than if we are manufacturing this same piece in large
quantities. If we have a 6-in. piece to forge, such as a pinion or
a short shaft, a hammer of about 1,100-lb. capacity would answer
very nicely. But should the general work be as large as this, it
would be very much better to use a 1,500-lb. hammer. If, on the
other hand, we wish to forge 6-in. axles economically, it would
be necessary to use a 7,000- or 8,000-lb. hammer. The following
table will be found convenient for reference for the proper size
of hammer to be used on different classes of general blacksmith
work, although it will be understood that it is necessary to modify
these to suit conditions, as has already been indicated.
[Illustration: FIG. 18.--Burst from a piped center.]
[Illustration: FIG. 19.--Result of using too light a hammer.]
Diameter of stock Size of hammer
3-1/2 in. 250 to 350 lb.
4 in. 350 to 600 lb.
4-1/2 in. 600 to 800 lb.
5 in. 800 to 1,000 lb.
6 in. 1,100 to 1,500 lb.
Steam hammers are always rated by the weight of the ram, and the
attached parts, which include the piston and rod, nothing being
added on account of the steam pressure behind the piston. This makes
it a little difficult to compare them with plain drop or tilting
hammers, which are also rated in the same way.
[Illustration: FIG. 20.--Good and bad ingots.]
Steam hammers are usually operated at pressures varying from 75
to 100 lb. of steam per square inch, and may also be operated by
compressed air at about the same pressures. It is cheaper, however,
in the case of compressed air to use pressures from 60 to 80 lb.
instead of going higher.
Forgings must, however, be made from sound billets if satisfactory
results are to be secured. Figure 20 shows three cross-sections
of which _A_ is sound, _B_ is badly piped and _C_ is worthless.
PLANT FOR FORGING RIFLE BARRELS
The forging of rifle barrels in large quantities and heat-treating
them to meet the specifications demanded by some of the foreign
governments led Wheelock, Lovejoy & Company to establish a complete
plant for this purpose in connection with their warehouse in Cambridge,
Mass. This plant, designed and constructed by their chief engineer,
K. A. Juthe, had many interesting features. Many features of this
plant can be modified for other classes of work.
[Illustration: FIG 21.--Cutting up barrels.]
[Illustration: FIG. 22.--Upsetting the ends.]
The stock, which came in bars of mill length, was cut off so as to
make a barrel with the proper allowances for trimming (Fig. 21).
They then pass to the forging or upsetting press in the adjoining
room. This press, which is shown in more detail in Fig. 22, handled
the barrels from all the heating furnaces shown. The men changed
work at frequent intervals, to avoid excessive fatigue.
[Illustration: FIG. 23.--Continuous heating furnace.]
Then the barrels were reheated in the continuous furnace, shown
in Fig. 23, and straightened before being tested.
The barrels were next tested for straightness. After the heat-treating,
the ends are ground, a spot ground on the enlarged end and each
barrel tested on a Brinell machine. The pressure used is 3,000 kg.,
or 6,614 lb., on a 10-millimeter ball, which is standard. Hardness
of 240 was desired.
The heat-treating of the rifle blanks covered four separate operations:
(1) Heating and soaking the steel above the critical temperature
and quenching in oil to harden the steel through to the center;
(2) reheating for drawing of temper for the purpose of meeting the
physical specifications; (3) reheating to meet the machine ability
test for production purposes; and (4) reheating to straighten the
blanks while hot.
A short explanation of the necessity for the many heats may be
interesting. For the first heat, the blanks were slowly brought
to the required heat, which is about 150°F. above the critical
temperature. They are then soaked at a high heat for about 1 hr.
before quenching. The purpose of this treatment is to eliminate
any rolling or heat stresses that might be in the bars from mill
operations; also to insure a thorough even heat through a cross-section
of the steel. This heat also causes blanks with seams or slight
flaws to open up in quenching, making detection of defective blanks
very easy.
The quenching oil was kept at a constant temperature of 100°F.,
to avoid subjecting the steel to shocks, thereby causing surface
cracks. The drawing of temper was the most critical operation and
was kept within a 10° fluctuation. The degree of heat necessary
depends entirely on the analysis of the steel, there being a certain
variation in the different heats of steel as received from the mill.
MACHINEABILITY
Reheating for machine ability was done at 100° less than the drawing
temperature, but the time of soaking is more than double. After
both drawing and reheating, the blanks were buried in lime where
they remain, out of contact with the air, until their temperature
had dropped to that of the workroom.
For straightening, the barrels were heated to from 900 to 1,000°F.
in an automatic furnace 25 ft. long, this operation taking about 2
hr. The purpose of hot straightening was to prevent any stresses
being put into the blanks, so that after rough-turning, drilling
or rifling operations they would not have a tendency to spring
back to shape as left by the quenching bath.
A method that produces an even better machining rifle blank, which
practically stays straight through the different machining operations,
was to rough-turn the blanks, then subject them to a heat of practically
1,0000 for 4 hr. Production throughout the different operations is
materially increased, with practically no straightening required
after drilling, reaming, finish-turning or rifling operations.
[Illustration: FIG. 24. FIG. 25.
FIGS. 24 and 25.--Roof system of cooling quenching oil.]
This method was tested out by one of the largest manufacturers and
proved to be the best way to eliminate a very expensive finished
gun-barrel straightening process.
[Illustration: FIG. 26.--Details of the cooler.]
The heat-treating required a large amount of cooling oil, and the
problem of keeping this at the proper temperature required considerable
study. The result was the cooling plant on the roof, as shown in
Figs. 24, 25 and 26. The first two illustrations show the plant as
it appeared complete. Figure 26 shows how the oil was handled in
what is sometimes called the ebulator system. The oil was pumped
up from the cooling tanks through the pipe _A_ to the tank _B_.
From here it ran down onto the breakers or separators _C_, which
break the oil up into fine particles that are caught by the fans
_D_. The spray is blown up into the cooling tower _E_, which contains
banks of cooling pipes, as can be seen, as well as baffies _F_. The
spray collects on the cool pipes and forms drops, which fall on
the curved plates _G_ and run back to the oil-storage tank below
ground.
The water for this cooling was pumped from 10 artesian wells at the
rate of 60 gal. per minute and cooled 90 gal. of oil per minute,
lowering the temperature from 130 or 140 to 100°F. The water as
it came from the wells averaged around 52°F. The motor was of a
7-1/2-hp. variable-speed type with a range of from 700 to 1,200
r.p.m., which could be varied to suit the amount of oil to be cooled.
The plant handled 300 gal. of oil per minute.