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ONCE FORGING IS COMPLETE, THE QUESTION OF
handling the part must be considered. Generally, except for
the austenitic stainless steels, before the part is allowed to
cool and transform, several factors have to be taken into account.
These include the type of steel used, whether or not
vacuum degassing was employed in making it, the size of the
forging, and the type of post forge heat treatment, if any that
the part will be given.
Austenitic stainless steel forgings are generally permitted
to cool in air after completion of forging, unless they are
small enough for a direct solution-annealing quench from
the forging temperature. However, this implies a high finishing
temperature, since the required solution treatment temperature
will be upwards of 1900~ (1040~ Some product
specifications will permit this practice, provided that the finishing
temperature for forging was higher than the minimum
solution treatment temperature for the grade of steel
involved. This saves the expense of a separate solution annealing
treatment downstream in the manufacturing sequence.
The specification requirements in this regard must
be watched, however, since some do not allow the practice
at all and some prohibit it for grades intended for use at
elevated temperatures such as the "H" grades. This is especially
true for forgings that will be used under the ASME
Boiler and Pressure Vessel Code.
The ferritic forgings must be permitted to transform on
cooling after forging, but there are dangers in doing this.
Bearing in mind that flake can only occur after an incubation
period following the austenite transformation, it follows that
the post forge cycle must allow this transformation to occur
safely. Prior to the introduction of vacuum degassing, this
often involved controlled cooling of the forging below the
lower critical temperature, perhaps in a furnace, under an
insulated hood or under a refractory insulating medium.
This was then followed by an extended subcritical heat treatment
cycle designed to facilitate the diffusion of hydrogen
from within the part. This cycle was frequently very long,
running for many hours per inch of cross-sectional area, so
that for a large forging in a nickel-chromium-molybdenum
alloy steel such as SAE 4340, about two weeks could elapse
before the forging would be ready for further processing.
Sometimes the cycle would involve reaustenitizing the forging
to refine the microstructure before embarking on the
subcritical part of the cycle.
Vacuum degassing of the steel significantly simplifies the
post forge handling by reducing the risk of flake, and for
many grades it is possible to forego the flake heat treatment
cycle and simply cool in still air and proceed with the rest
of the processing. Since flake can affect carbon as well as
alloy steels, this was a very significant improvement. However,
vigilance must be maintained on the vacuum degassing
operation during steel making, ensuring that only minimal
ladle additions are made after vacuum degassing, for example, and that the degassing equipment is properly mainrained.
It should be noted, however, that unless the forge
facility has its own melt shop, the condition of the suppliers
vacuum degassing equipment and their practices may not be
known.
For some vacuum degassed alloy steel forgings, cooling
in still air on completion of forging has been found to be
successfial. The 2.25 % Chromium--1% Molybdenum alloy
known as F22 in the ASTM Specifications A 182/A 182M
Forged or Rolled Alloy Steel Pipe Flanges, Forged Fitting and
Valves and Parts for High Temperature Service, and A 336/
A336M Alloy Steel Forgings for Pressure and High Temperature
Parts, is a good example. If cooled slowly, or annealed,
this material tends to have poor machinability, being described
as "gummy," but if cooled after forging in still air the
machining response is very much improved. A common alloy
steel, SAE 4130 is also amenable to air-cooling after forging.
For steels of higher hardenability, such as those of the
nickel-chromium-molybdenum type, simple air cooling after
forging is generally unsatisfactory, for several reasons:
1. Depending on the shape of the forging, especially when
section sizes vary widely, the transformation products
may vary considerably from one part of the forging to
another. At the least, this may only adversely affect machining.
2. Depending on the forging section size, composition, and
the delay before further processing, it is possible that
cracking may occur during heating for subsequent quality
heat treatment.
3. Depending on the section size, vacuum degassing efficiency,
and particnlarly composition, the risk of flake
damage.