Heat treatments of steels and alloys: which to use and what for
Each heat treatment combines temperature, time and cooling method to reach a property: more hardness, more toughness, more machinability or more corrosion resistance.
Comparison of the treatments
| Treatment | Typical temperature* | Cooling | Purpose | Alloys |
|---|---|---|---|---|
| Annealing | Above the critical range (carbon steel ~800–900 °C) | Slow, in the furnace | Soften and ease machining | Carbon and alloy steels |
| Normalizing | ~850–950 °C | In air | Refine and homogenize the grain | A105, A350 LF2, alloy steels |
| Quenching | Austenitizing (4140 ~840–870 °C) | Water, oil or polymer | Harden (martensite) | 4140, 4340, F22, stainless 410/420/431 |
| Tempering | ~150–700 °C, depending on hardness | In air | Trade hardness for toughness | Every quenched steel |
| Solution annealing | Austenitic stainless ~1040–1120 °C; duplex ~1020–1100 °C | Fast, in water | Dissolve carbides and sigma phase, restore corrosion resistance | 304L, 316L, 904L, duplex, super duplex, 625, C-276 |
| Aging | 17-4PH 480–620 °C (H900 to H1150) | In air | Precipitation hardening | 17-4PH, 15-5PH, 718, K-500 |
| Stress relief | Carbon steel ≥ 595 °C (post-weld) | Slow, controlled | Reduce welding and machining stresses | Fabrications, welded parts |
* Typical ranges. Temperature, time and medium come from the material standard or the customer specification.
Precautions by family
- Duplex (including super duplex): never leave the part too long between ~600 and 1000 °C; it forms sigma phase and loses toughness and corrosion resistance. Cool quickly after solution annealing.
- Austenitic stainless: between ~450 and 850 °C sensitization occurs; hence solution annealing and the use of "L" grades.
- Cr-Mo steels (F11, F22, F91): F91 requires normalizing and tempering with strict temperature control.
- Confirm the result: hardness, tensile and Charpy in the laboratory.
Frequently asked questions
What is the difference between tempering and annealing?
Tempering comes after quenching and reduces part of the hardness to gain toughness. Annealing is done on non-quenched material to soften it and ease machining or forming.
Why solution anneal duplex and stainless after forging?
Forging and slow cooling form carbides and intermetallic phases that reduce corrosion resistance and toughness. Solution annealing dissolves these phases and the fast cooling prevents them from returning.
Heat treatment with reportOwn furnaces, right after forging and machining.
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