3.7.2 Heat Treating Stainless Steel

Stainless steels may be divided into four groups to distinguish their response to heat treatments. The ferritic, austenitic and martensitic grades are described in Section 4.2. The precipitation hardening grades, designated with a PH suffix (e.g.15-5 PH) constitute a group which, unlike the other three, achieves both high strength and corrosion resistance. Following is a brief overview of the heat treatment capabilities of the four groups.

Ferritic Stainless Steels — Ferritic stainless steels are not hardened by quenching. The only heat treatment applied to ferritics is annealing because they develop minimum hardness and maximum ductility, impact toughness and corrosion resistance in the annealed and quenched condition. Annealing also relieves stresses developed during welding or cold working and provides a more homogeneous microstructure.

Austenitic Stainless Steels — Conventional austenitic stainless steels will work harden with cold working, but cannot be hardened by heat treatment. Annealing is employed to optimize corrosion resistance, softness and ductility; consequently post-annealing may be specified after welding or thermal processing. This group is usually purchased in the annealed or cold worked condition.

Martensitic Stainless Steels — The maximum hardness achievable in martensitic stainless steels depend primarily on their carbon content. Therefore heat treating this group is essentially the same as for plain carbon or low alloy steels, but process parameters are different because the higher alloy content of the group makes them respond more sluggishly. They exhibit excellent hardenability so that maximum hardness is achieved in the center of sections up to 30 mm (12 in.) thick by air cooling. There are two principal types of heat treatment.

  1. Annealing reduces hardness and increases ductility. Full annealing is expensive and time consuming, and should be specified only when required for severe forming.
  2. Austenitizing, quenching and tempering are employed to increase strength and hardness. These processes also affect corrosion resistance, sometimes requiring balancing of the heat treatment parameters to optimize product strength and corrosion resistance requirements.

Precipitation Hardening (PH) Stainless Steels — The most commonly forged grades of PH stainless steels are 15-5 PH and 17-7 PH. These grades combine the high corrosion resistance of austenitic grades with the strengths achievable in martensitic grades. Procedures are available for homogenization, austenite conditioning, transformation cooling and precipitation hardening (age-tempering) these grades.

Duplex Stainless Steels — The typical forging grades contain a mixture of austenite and ferrite in their microstructures. They are generally not heat treated other than annealing.

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Stainless steels may be divided into four groups to distinguish their response to heat treatments. The ferritic, austenitic and martensitic grades are described in Section 4.2. The precipitation hardening grades, designated with a PH suffix (e.g.15-5 PH) constitute a group which, unlike the other three, achieves both high strength and corrosion resistance. Following is a brief overview of the heat treatment capabilities of the four groups.

Ferritic Stainless Steels — Ferritic stainless steels are not hardened by quenching. The only heat treatment applied to ferritics is annealing because they develop minimum hardness and maximum ductility, impact toughness and corrosion resistance in the annealed and quenched condition. Annealing also relieves stresses developed during welding or cold working and provides a more homogeneous microstructure.

Austenitic Stainless Steels — Conventional austenitic stainless steels will work harden with cold working, but cannot be hardened by heat treatment. Annealing is employed to optimize corrosion resistance, softness and ductility; consequently post-annealing may be specified after welding or thermal processing. This group is usually purchased in the annealed or cold worked condition.

Martensitic Stainless Steels — The maximum hardness achievable in martensitic stainless steels depend primarily on their carbon content. Therefore heat treating this group is essentially the same as for plain carbon or low alloy steels, but process parameters are different because the higher alloy content of the group makes them respond more sluggishly. They exhibit excellent hardenability so that maximum hardness is achieved in the center of sections up to 30 mm (12 in.) thick by air cooling. There are two principal types of heat treatment.

  1. Annealing reduces hardness and increases ductility. Full annealing is expensive and time consuming, and should be specified only when required for severe forming.
  2. Austenitizing, quenching and tempering are employed to increase strength and hardness. These processes also affect corrosion resistance, sometimes requiring balancing of the heat treatment parameters to optimize product strength and corrosion resistance requirements.

Precipitation Hardening (PH) Stainless Steels — The most commonly forged grades of PH stainless steels are 15-5 PH and 17-7 PH. These grades combine the high corrosion resistance of austenitic grades with the strengths achievable in martensitic grades. Procedures are available for homogenization, austenite conditioning, transformation cooling and precipitation hardening (age-tempering) these grades.

Duplex Stainless Steels — The typical forging grades contain a mixture of austenite and ferrite in their microstructures. They are generally not heat treated other than annealing.

Return to Table of Contents

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Stainless steels may be divided into four groups to distinguish their response to heat treatments. The ferritic, austenitic and martensitic grades are described in Section 4.2. The precipitation hardening grades, designated with a PH suffix (e.g.15-5 PH) constitute a group which, unlike the other three, achieves both high strength and corrosion resistance. Following is a brief overview of the heat treatment capabilities of the four groups.

Ferritic Stainless Steels — Ferritic stainless steels are not hardened by quenching. The only heat treatment applied to ferritics is annealing because they develop minimum hardness and maximum ductility, impact toughness and corrosion resistance in the annealed and quenched condition. Annealing also relieves stresses developed during welding or cold working and provides a more homogeneous microstructure.

Austenitic Stainless Steels — Conventional austenitic stainless steels will work harden with cold working, but cannot be hardened by heat treatment. Annealing is employed to optimize corrosion resistance, softness and ductility; consequently post-annealing may be specified after welding or thermal processing. This group is usually purchased in the annealed or cold worked condition.

Martensitic Stainless Steels — The maximum hardness achievable in martensitic stainless steels depend primarily on their carbon content. Therefore heat treating this group is essentially the same as for plain carbon or low alloy steels, but process parameters are different because the higher alloy content of the group makes them respond more sluggishly. They exhibit excellent hardenability so that maximum hardness is achieved in the center of sections up to 30 mm (12 in.) thick by air cooling. There are two principal types of heat treatment.

  1. Annealing reduces hardness and increases ductility. Full annealing is expensive and time consuming, and should be specified only when required for severe forming.
  2. Austenitizing, quenching and tempering are employed to increase strength and hardness. These processes also affect corrosion resistance, sometimes requiring balancing of the heat treatment parameters to optimize product strength and corrosion resistance requirements.

Precipitation Hardening (PH) Stainless Steels — The most commonly forged grades of PH stainless steels are 15-5 PH and 17-7 PH. These grades combine the high corrosion resistance of austenitic grades with the strengths achievable in martensitic grades. Procedures are available for homogenization, austenite conditioning, transformation cooling and precipitation hardening (age-tempering) these grades.

Duplex Stainless Steels — The typical forging grades contain a mixture of austenite and ferrite in their microstructures. They are generally not heat treated other than annealing.

Return to Table of Contents

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