What is the difference between hardening and tempering?

Hardening (heating and quenching) makes steel hard but brittle; tempering (reheating to a lower temperature) reduces the brittleness and sets the final hardness and toughness.

How should I specify hardness on a drawing?

As a range on a named scale, for example 28–32 HRC or 250–300 HB, with the test location if it matters. A single value without a range is hard to meet.

Can every steel be hardened?

No. Mild steel and free-cutting steel have too little carbon to harden much (they can be case hardened instead). Medium- and high-carbon, alloy and tool steels harden well.

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Hardening and tempering

Also called: heat treatment job work, hardening job work, quenching and tempering, through hardening, heat treatment of steel

Hardening and tempering is a heat treatment that makes steel strong and hard: the part is heated, quenched in oil, water or gas, then reheated (tempered) to the hardness and toughness needed. It is used for shafts, gears, tools, dies and wear parts.

Quick facts

Suits: Medium- and high-carbon steels, alloy steels, tool steels
Steps: Heat (austenitise), quench, temper
Hardness measured: HRC (Rockwell C) for hard parts, HB (Brinell) for tougher ones
Common furnaces: Sealed quench, pit, vacuum and salt bath
Usually priced: Per kg, with a minimum charge per batch
After treatment: Grinding brings critical sizes back to tolerance

How it works

Heating: the steel is heated above its hardening temperature (often about 800–1050 °C, depending on the grade)
Quenching: cooled fast in oil, water, polymer or gas so it becomes hard
Tempering: reheated to a lower temperature to reduce brittleness and set the final hardness
Higher tempering temperatures give lower hardness but more toughness

Through hardening or surface hardening?

Through hardening and tempering: the whole section is strengthened (EN19, EN24, tool steels)
Case carburising: a hard case on a tough low-carbon core (EN353, 20MnCr5) for gears
Induction hardening: only the surface where needed (EN8, EN9, EN19)
Nitriding: a thin hard surface with very little distortion

What to tell the heat-treatment shop

Exact material grade (and the test certificate if you have it)
Required hardness as a range, e.g. 28–32 HRC or 250–300 HB
Where hardness should be tested, if it matters
Areas that must stay soft, or thread areas to protect
Quantity and part weight
Whether you need a hardness report

Distortion and cracking

Quenching can bend long parts and crack sharp corners or thin sections. Generous radii, even wall thickness, stress relieving before final machining, and leaving a grinding allowance on critical sizes all reduce problems.

Need hardening and tempering done?

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Related job work

Machines

CNC lathe

Materials

Questions

What is the difference between hardening and tempering?: Hardening (heating and quenching) makes steel hard but brittle; tempering (reheating to a lower temperature) reduces the brittleness and sets the final hardness and toughness.
How should I specify hardness on a drawing?: As a range on a named scale, for example 28–32 HRC or 250–300 HB, with the test location if it matters. A single value without a range is hard to meet.
Can every steel be hardened?: No. Mild steel and free-cutting steel have too little carbon to harden much (they can be case hardened instead). Medium- and high-carbon, alloy and tool steels harden well.

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 8 October 2026.