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Annealing, Normalizing, Tempering and Stress Relieving Steel

Guide · Updated October 28, 2026

Hardening gets the attention, but most of the heat treatment a machinist meets is softening or stabilizing: annealing a bar so it machines, normalizing a forging so it is uniform, stress relieving a weldment before finish machining, and tempering a hardened part so it is tough rather than brittle. This guide covers carbon and low alloy steels. Temperatures are typical values from the ASM Heat Treater's Guide and ASM Handbook Volume 4; always follow the supplier's or drawing's heat treat specification. Tool steels have their own procedures, covered in the tool steel heat treating guide.

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Step by step

Know the goal: Soft and machinable: anneal. Uniform, refined grain: normalize. Stable after welding or roughing: stress relieve. Tough after hardening: temper.
Find the grade data: Look up the critical temperatures and recommended cycle for the exact grade.
Heat evenly: Bring the part up slowly and soak about one hour per inch of thickness at temperature, as a common rule.
Cool the right way: Anneal: slow furnace cool. Normalize: still air. Stress relieve: slow furnace or still air. Temper: air after soaking.
Check the result: Test hardness, check for scale and decarburization, and allow stock for cleanup.

Process comparison

Full anneal: heat about 30–50 °C (50–90 °F) above the upper critical temperature (Ac3) for hypoeutectoid steels, soak, then furnace cool slowly. Result: softest, coarse pearlite, best machinability for medium and high carbon steels.
Spheroidize anneal: hold just below or cycle around the lower critical (Ac1, about 700–730 °C / 1290–1350 °F), long hold. Result: spheroidal carbides, preferred for high carbon steel such as 52100 before machining.
Normalize: heat about 55 °C (100 °F) above Ac3, then cool in still air. Result: finer, more uniform grain, slightly harder and stronger than annealed.
Stress relieve: heat below Ac1, typically 550–650 °C (1000–1200 °F), soak, cool slowly. Result: reduced residual stress with little change in hardness.
Temper: reheat a hardened part to 150–650 °C (300–1200 °F) depending on grade and target hardness, soak, then cool. Result: lower hardness, much higher toughness.

Critical temperatures shift with carbon and alloy content, so always use the figure for the grade, not a generic number.

Typical temperatures by grade (ASM Heat Treater's Guide)

1018: normalize about 890–940 °C (1635–1725 °F), still-air cool; anneal about 870–910 °C (1600–1670 °F), furnace cool (AZoM AISI 1018 data)
1045: normalize about 870–920 °C (1600–1690 °F), still-air cool; anneal about 800–850 °C (1470–1560 °F), furnace cool (AZoM AISI 1045 data)
4140: normalize about 870 °C (1600 °F); anneal about 845 °C (1550 °F), furnace cool to about 755 °C (1390 °F) then slower; harden 845 °C oil quench
4340: normalize about 870 °C (1600 °F); anneal about 845 °C (1550 °F), furnace cool (AZoM AISI 4340 data); harden roughly 830–845 °C, oil quench — confirm against your supplier sheet
Stress relief of carbon and low alloy steel: 550–650 °C (1000–1200 °F), at least 25 °C below any previous tempering temperature

These are typical published figures; the ranges differ between sources and with section size. For example, AZoM's AISI 4140 data sheet lists normalizing at about 913 °C (1675 °F) and annealing at about 872 °C (1600 °F), somewhat higher than the ASM figures above.

Tempering and hardness

Higher tempering temperature means lower hardness and higher toughness. As a rough guide for 4140 oil quenched to about 55 HRC: tempering at 205 °C (400 °F) leaves roughly 50–55 HRC, at 425 °C (800 °F) roughly 45 HRC, at 540 °C (1000 °F) roughly 28–35 HRC, and at 650 °C (1200 °F) roughly 20–28 HRC; results vary with section size and source. For reference, AZoM's AISI 4140 data sheet gives about 225 ksi tensile when tempered at 316 °C (600 °F) and about 130 ksi at 538 °C (1000 °F). Avoid holding many alloy steels in the 260–370 °C (500–700 °F) range, where tempered martensite embrittlement lowers impact toughness. Temper promptly after quenching to reduce cracking risk.

Effect on machining

Annealed medium carbon steel is the easiest to rough but can be gummy in low carbon grades; normalized 1018 or cold drawn often machines better than fully annealed 1018. Prehardened 4140 at 28–32 HRC (quenched and tempered) machines well with carbide and gives a better finish than annealed. Stress relieving between roughing and finishing is the standard fix for parts that move after unclamping or that were welded.

Worked example: weldment that moves

A 1018 welded base plate warps 0.006 in after facing. Stress relieve at 595–650 °C (1100–1200 °F), soak about one hour per inch of thickest section, then cool slowly in the furnace to below about 315 °C (600 °F) before air cooling. Leave 0.030–0.060 in per side for cleanup, then finish machine. Hardness changes very little, but the plate stays flat after machining.

Worked example: soak time

A 4140 bar 3 in in diameter is being normalized. Using the common rule of about one hour per inch of thickness at temperature, soak about three hours at 870 °C (1600 °F) after the center reaches temperature, then cool in still air away from drafts. A thermocouple on the part, not just the furnace setpoint, tells you when the soak starts.

Common problems

Using annealing and normalizing as if they were the same, quenching from a stress relief, tempering below the service temperature, skipping a temper after hardening, and machining to final size before heat treat without allowing for scale and distortion.

FAQ

What is the difference between annealing and normalizing?

Both heat above the critical range; annealing cools slowly in the furnace for maximum softness, normalizing cools in air for a finer, more uniform grain.

What does tempering do?

It reduces hardness and brittleness of a hardened part and increases toughness.

What temperature is stress relieving steel?

Typically 550–650 °C (1000–1200 °F) for carbon and low alloy steels.

Does stress relieving change hardness?

Very little, when it is done below the previous tempering temperature.

How long should steel soak?

A common rule is about one hour per inch of thickness at temperature.

Can I anneal prehard 4140?

Yes; it returns to the soft annealed condition, but you lose the prehardened strength.

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General shop information only — follow your drawing, tool maker's data and your quality system.