At what temperature is steel stress relieved?

Usually about 550–650 °C, kept below the temperature at which the steel's structure changes. Hardened parts are stress relieved below their tempering temperature so they do not soften.

Why does a welded frame need stress relieving before machining?

Welding locks stresses into the frame. When machining removes metal, those stresses rebalance and the frame moves, so flat faces and bore positions go out. Stress relieving first keeps them where they were machined.

Does stress relieving reduce hardness?

Very little on normal unhardened steel. On hardened steel it must stay below the tempering temperature, or the hardness will drop.

Stress relieving heat treatment: temperature, when to use

Stress relieving heats a part to below its transformation temperature, about 550–650 °C for steel, holds it and cools it slowly. It removes the stresses left by welding, heavy machining, bending or casting, so the part does not warp later during finish machining or use. It does not change the hardness much.

Quick facts

Steel temperature: About 550–650 °C (below the hardening range)
Holding time: Roughly 1 hour per 25 mm (0.984 inch) of section, at least 1 hour
Cooling: Slowly, in the furnace or in still air
Effect on hardness: Little change on unhardened steel
Common uses: Welded frames, rough-machined parts, castings
Usually priced: Per kg or per furnace load

When a part needs stress relieving

Heavy welded fabrications that will be machined afterwards
Parts rough-machined from a large block before finish machining
Long shafts and thin plates that bend after machining
Castings with uneven wall thickness
Pressure parts where the code asks for post-weld heat treatment

Stress relieving, annealing or normalising?

	Stress relieving	Annealing	Normalising
Temperature (steel)	About 550–650 °C	Above about 750–900 °C	Above about 850–950 °C
Changes the structure	No	Yes, softens	Yes, refines grain
Main aim	Remove internal stress	Make soft and machinable	Uniform structure

What to tell the heat treater

Material grade and whether it is already hardened
Weight, largest size and the thickest section
Temperature and holding time if the drawing or code gives them
Whether a temperature chart (recorder graph) is needed
How the part should be supported so it does not sag in the furnace
Machined faces that must not scale (inert gas or vacuum may be needed)

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Questions

At what temperature is steel stress relieved?: Usually about 550–650 °C, kept below the temperature at which the steel's structure changes. Hardened parts are stress relieved below their tempering temperature so they do not soften.
Why does a welded frame need stress relieving before machining?: Welding locks stresses into the frame. When machining removes metal, those stresses rebalance and the frame moves, so flat faces and bore positions go out. Stress relieving first keeps them where they were machined.
Does stress relieving reduce hardness?: Very little on normal unhardened steel. On hardened steel it must stay below the tempering temperature, or the hardness will drop.

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 18 October 2026.