ANNEALING
There is no mystery or secret about the proper annealing of different
steels, but in order to secure the best results it is absolutely
necessary for the operator to know the kind of steel which is to
be annealed. The annealing of steel is primarily done for one of
three specific purposes: To soften for machining purposes; to change
the physical properties, largely to increase ductility; or to release
strains caused by rolling or forging.
Proper annealing means the heating of the steel slowly and uniformly
to the right temperature, the holding of the temperature for a given
period and the gradual cooling to normal temperature. The proper
temperature depends on the kind of steel, and the suggestions of the
maker of the special steel being used should be carefully followed.
For carbon steel the temperatures recommended for annealing vary
from 1,450 to 1,600°F. This temperature need not be long continued.
The steel should be cooled in hot sand, lime or ashes. If heated in
the open forge the steel should be buried in the cooling material
as quickly as possible, not allowing it to remain in the open air
any longer than absolutely necessary. Best results, however, are
secured when the fire does not come in direct contact with the
steel.
Good results are obtained by packing the steel in iron boxes or
tubes, much as for case-hardening or carbonizing, using the same
materials. Pieces do not require to be entirely surrounded by carbon
for annealing, however. Do not remove from boxes until cold.
Steel to be annealed may be classified into four different groups,
each of which must be treated according to the elements contained in
its particular analysis. Different methods are therefore necessary
to bring about the desired result. The classifications are as follows:
High-speed steel, alloy steel, tool or crucible steel, and high-carbon
machinery steel.
ANNEALING OF HIGH-SPEED STEEL
For annealing high-speed steel, some makers recommend using ground
mica, charcoal, lime, fine dry ashes or lake sand as a packing
in the annealing boxes. Mixtures of one part charcoal, one part
lime and three parts of sand are also suggested, or two parts of
ashes may be substituted for the one part of lime.
To bring about the softest structure or machine ability of high-speed
steel, it should be packed in charcoal in boxes or pipes, carefully
sealed at all points, so that no gases will escape or air be admitted.
It should be heated slowly to not less than 1,450°F. and the steel
must not be removed from its packing until it is cool. Slow heating
means that the high heat must have penetrated to the very core of
the steel.
When the steel is heated clear through it has been in the furnace
long enough. If the steel can remain in the furnace and cool down
with it, there will be no danger of air blasts or sudden or uneven
cooling. If not, remove the box and cover quickly with dry ashes,
sand or lime until it becomes cold.
Too high a heat or maintaining the heat for too long a period,
produces a harsh, coarse grain and greatly increases the liability
to crack in hardening. It also reduces the strength and toughness
of the steel.
Steel which is to be used for making tools with teeth, such as
taps, reamers and milling cutters, should not be annealed too much.
When the steel is too soft it is more apt to tear in cutting and
makes it more difficult to cut a smooth thread or other surface.
Moderate annealing is found best for tools of this kind.
TOOL OR CRUCIBLE STEEL
Crucible steel can be annealed either in muffled furnace or by
being packed. Packing is by far the most satisfactory method as it
prevents scaling, local hard spots, uneven annealing, or violent
changes in shape. It should be brought up slowly to just above
its calescent or hardening temperature. The operator must know
before setting his heats the temperature at which the different
carbon content steels are hardened. The higher the carbon contents
the lower is the hardening heat, but this should in no case be
less than 1,450°F.
ANNEALING ALLOY STEEL
The term alloy steel, from the steel maker's point of view, refers
largely to nickel and chromium steel or a combination of both. These
steels are manufactured very largely by the open-hearth process,
although chromium steels are also a crucible product. It is next
to impossible to give proper directions for the proper annealing
of alloy steel unless the composition is known to the operator.
Nickel steels may be annealed at lower temperatures than carbon
steels, depending upon their alloy content. For instance, if a
pearlitic carbon steel may be annealed at 1,450°C., the same analysis
containing 2-1/2 per cent nickel may be annealed at 1,360°C. and
a 5 per cent nickel steel at 1,270°.
In order that high chromium steels may be readily machined, they
must be heated at or slightly above the critical for a very long
time, and cooled through the critical at an extremely slow rate.
For a steel containing 0.9 to 1.1 per cent carbon, under 0.50 per
cent manganese, and about 1.0 per cent chromium, Bullens recommends
the following anneal:
1. Heat to 1,700 or 1,750°F.
2. Air cool to about 800°F.
3. Soak at 1,425 to 1,450°F.
4. Cool slowly in furnace.
HIGH-CARBON MACHINERY STEEL
The carbon content of this steel is above 30 points and is hardly
ever above 60 points or 0.60 per cent. Annealing such steel is
generally in quantity production and does not require the care that
the other steels need because it is very largely a much cheaper
product and a great deal of material is generally removed from
the outside surface.
The purpose for which this steel is annealed is a deciding factor
as to what heat to give it. If it is for machineability only, the
steel requires to be brought up slowly to just below the critical and
then slowly cooled in the furnace or ash pit. It must be thoroughly
covered so that there will be no access of cool air. If the annealing
is to increase ductility to the maximum extent it should be slowly
heated to slightly over the upper critical temperature and kept at
this heat for a length of time necessary for a thorough penetration
to the core, after which it can be cooled to about 1,200°F., then
reheated to about 1,360°F., when it can be removed and put in an
ash pit or covered with lime. If the annealing is just to relieve
strains, slow heating is not necessary, but the steel must be brought
up to a temperature not much less than a forging or rolling heat
and gradually cooled. Covering in this case is only necessary in
steel of a carbon content of more than 40 points.
ANNEALING IN BONE
Steel and cast iron may both be annealed in granulated bone. Pack the
work the same as for case-hardening except that it is not necessary
to keep the pieces away from each other. Pack with bone that has
been used until it is nearly white. Heat as hot as necessary for
the steel and let the furnace cool down. If the boxes are removed
from furnace while still warm, cover boxes and all in warm ashes
or sand, air slaked lime or old, burned bone to retain heat as
long as possible. Do not remove work from boxes until cold.
ANNEALING OF RIFLE COMPONENTS AT SPRINGFIELD ARMORY
In general, all forgings of the components of the arms manufactured
at the Armory and all forgings for other ordnance establishments
are packed in charcoal, lime or suitable material and annealed
before being transferred from the forge shop.
Except in special cases, all annealing will be done in annealing
pots of appropriate size. One fire end of a thermo-couple is inserted
in the center of the annealing pot nearest the middle of the furnace
and another in the furnace outside of but near the annealing pots.
The temperatures used in annealing carbon steel components of the
various classes used at the Armory vary from 800°C. To 880°C. or
1,475 to 1,615°F.
The fuel is shut off from the annealing furnace gradually as the
temperature of the pot approaches the prescribed annealing temperature
so as to prevent heating beyond that temperature.
The forgings of the rifle barrel and the pistol barrel are exceptions
to the above general rule. These forgings will be packed in lime
and allowed to cool slowly from the residual heat after forging.