Heat Treating Stainless Steels

Wrought stainless steels are solution annealed after processing and hot worked in order to dissolve carbides and sigma. Carbides may form during heating in the 425 to 900C (800 to 1650F) range or during slow cooling through this range. Sigma tends to form at temperatures below 925C (1700F). Specifications normally require solution annealing to be done at 1035C (1900F) with a rapid quench. The molybdenum-containing grades are frequently solution annealed at somewhat higher temperatures in the 1095 to 1120C (2000 to 2050F) in order to better homogenize the molybdenum.

Stainless steels may be stress relieved. There are several stress relief treatments. Guidelines follow.

Stress redistribution at 290 to 425C (550 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 290 - 425C (550 - 800F), there is a significant redistribution of peak stresses and an increase in both tensile and yield strength. Stress redistribution heat treatments at 290 - 425C (550 - 800F) will reduce movement in later machining operations and are occasionally used to increase strength. Since stress redistribution treatments are made at temperatures below 425C (800F), carbide precipitation and sensitization to intergranular attack (IGA) are not a problem for the higher carbon grades.

Stress relief at 425 to 595C (800 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 425C (800F) as the higher carbon grades are sensitized to IGA when heated above about 425C (800F).

Stress relief at 815 to 870C (1500 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 900C (1650F) minimum for 1 to 10 hours are occasionally employed for assemblies that are to be used in the 400 to 900C (750 to 1650F) temperature range. Thermal stabilization is intended to agglomerate the carbides, thereby preventing further precipitation and intergranular attack (IGA). As with 815 to 870C (1500 to 1600F) stress relief, it is best to test for susceptibility to IGA per ASTM A262.

"Heat Treating, Cleaning and Finishing," Metals Handbook, 10th edition, Vol. 4 in the section entitled "Heat Treatment of Stainless Steels and Heat-Resisting Alloys".

 

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