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HEATREAT.TXT
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1991-01-26
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Stainless steels are solution annealed after processing and hot worked in order
to dissolve carbides and sigma. Carbides are formed during heating in the
800F to 1650F range or during slow cooling through this range. Sigma tends
to form in the low carbon and the columbium and molybdenum containing grades
at temperatures below 1700F. Specifications normally require solution
annealing to be done at 1900F with a rapid quench. The molybdenum containing
grades are frequently solution annealed at somewhat higher temperatures in
the 2000F to 2050F in order to better homogenize molybdenum.
Stainless steels may be stress relieved. There are several stress relief
treatments. Guidelines follow.
Stress redistribution at 550F to 800F, which is below the sensitization range.
When stainless steel sheet and bar are cold reduced greater than about 30% and
subsequently heated to 550 - 800F, there is a significant redistribution of
peak stresses and an increase in both tensile and yield strength. Stress
redistribution heat treatments at 550F - 800F will reduce movement in later
machining operations and are occasionally used to increase strength. Since
stress redistribution treatments are made at temperatures below 800F, carbide
precipitation and sensitization to intergranular attack (IGA) are not a
problem for the older higher carbon grades.
Stress relief at 800F to 1100F is normally adequate to minimize distortion that
would otherwise exceed dimensional tolerances after machining. Only the low
carbon "L" grades or the stabilized 321 and 347 grades should be used in
weldments to be stress relieved above 800F as the older higher carbon grades
are sensitized to IGA when heated above about 800F.
Stress relief at 1500F to 1600F is occasionally needed when a fully stress
relieved assembly is required. Only the low carbon "L" grades, 321 and 347
should be used in assemblies to be heat treated in this range. Even though
the low carbon and stabilized grades are used, it is best to test for
susceptibility to IGA per ASTM A262 to be certain there was no sensitization
during stress relief treating in this temperature range.
Thermal stabilization treatments at 1650F minimum for 1 to 10 hours are
occasionally employed for assemblies that are to be used in the 750F to 1650F
temperature range. Thermal stabilization is intended to agglomerate the
carbides thereby preventing further precipitation and IGA. As with 1500F to
1600F stress relief, it is best to test for susceptibility to IGA per
ASTM A262.
Further information on heat treatment of stainless steels may be found in the
ASTM specifications for the product form of interest in:
"Heat Treating, Cleaning and Finishing", Metals Handbook, 8th edition,
Vol. 2 in the section entitled "Heat Treatment of Stainless Steels and
Heat-Resisting Alloys".