Annealing and normalising are heat treatments that reset steel's structure. Annealing softens steel so it machines and forms easily; normalising refines the grain for even strength; stress relieving removes internal stress from welding or heavy machining without changing hardness much.
Quick facts
- Annealing
- Heat above the critical temperature, cool very slowly in the furnace
- Normalising
- Heat above the critical temperature, cool in still air
- Stress relieving
- Heat to about 550–650 °C, hold, cool slowly
- Effect
- Softer, more machinable, more uniform, less stress
- Used on
- Forgings, castings, bars, welded fabrications
- Usually priced
- Per kg, per furnace load
Which one, when
| Treatment | Main purpose | Typical use |
|---|---|---|
| Full annealing | Softest, most machinable state | Tool steels and alloy steels before machining |
| Spheroidise annealing | Soft, easy to machine high-carbon steel | EN31, D2 and other tool or bearing steels |
| Normalising | Fine, even grain and strength | Forgings, castings, before hardening |
| Stress relieving | Remove stress without softening much | Welded frames, parts before final machining |
Good practice
- Clean parts and, where scale matters, use a controlled atmosphere or allow for scale removal
- Stress relieve heavy fabrications before final machining so they do not move later
- Ask for a hardness check (HB) on sample pieces after annealing
What to tell the heat treater
- Steel grade and the treatment wanted
- Required hardness after treatment, if any (e.g. max 229 HB)
- Part size and weight, and quantity
- Whether surface scale is acceptable
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Questions
- What is the difference between annealing and normalising?
- Both heat steel above its critical temperature. Annealing cools it very slowly in the furnace for maximum softness; normalising cools it in air, giving a finer grain and slightly higher strength and hardness.
- Why stress relieve a welded frame?
- Welding locks stress into the steel. If the frame is machined without stress relieving, it can slowly bend or move later. Stress relieving at about 550–650 °C lets it settle first.
- Does annealing remove hardness completely?
- It brings steel close to its softest state for that grade. A hardened part can be annealed to machine it again, then re-hardened.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 15 October 2026.