3.7.5 Heat Treating Heat Resistant Alloys

Three types of heat resistant alloy groups are commonly forged and heat treated:

• iron based
• nickel based
• cobalt based.

Heat Treating Iron Based Heat Resistant Alloys Iron base heat resistant alloys can be heat treated by one of three principal types, depending on chemical composition, fabrication requirements and anticipated service.

1. Annealing is performed at temperatures between 705 and 980^°C (1300 and 1800^°F) to soften and stress relieve a work hardened forging.
2. Solution Treating is normally applied to age-hardenable alloys before the aging treatment to put age-hardening constituents and carbides into solid solution.
3. Age Hardening, or precipitation hardening, is performed at temperatures of 425 to 700^°C (800 to 1300^°F) to develop maximum design strength, sometimes at two different ageing temperatures.

Heat Treating Nickel Based Heat Resistant Alloys Nickel alloys can be heat treated by one of six principal process types, depending on chemical composition, fabrication requirements and anticipated service. Selection of the optimum heat treatment depends on the desired objective and the capability of the alloy to respond.

1. Annealing is used to produce a recrystallized grain structure and soften work-hardened alloys. It usually requires temperatures between 705 and 1205^°C (1300 and 2200^°F), depending on alloy composition and degree of work hardening.
2. Solution annealing is a high-temperature anneal, performed at temperatures between 1150 and 1315^°C (2100 and 2400^°F) of certain nickel alloys to put carbides into solid solution and produce a coarse grain size for enhanced stress-rupture properties.
3. Stress relieving is used to remove or reduce stresses in work-hardened, non-age-hardenable alloys without producing a recrystallized grain structure. It is performed at temperatures between 425 and 870^°C (800 and 1600^°F) depending on alloy composition and the degree of work hardening.
4. Stress equalizing is a low temperature heat treatment used to balance stresses in cold-worked forgings without an appreciable decrease in the mechanical properties produced by cold working.
5. Solution treating is a high-temperature heat treatment used to put age-hardening constituents into solid solution. It is normally applied to age-hardenable materials before the aging treatment.
6. Age hardening, or precipitation hardening, is a heat treatment performed at intermediate temperatures of 425 to 870^°C (800 to 1600^°F) on certain alloys to develop maximum strength by precipitation of a dispersed phase throughout the matrix.

Heat Treating Cobalt Based Heat Resistant Alloys Cobalt based alloys are generally supplied in the solution (annealed) condition only. They are generally not hardenable by aging. Solutioning after forging minimizes residual stresses. Strength and hardness can be increased only by working at typical forging temperatures.