Hardening
Hardening heats steel above its critical temperature and quenches it rapidly in oil, water, polymer or gas, forming hard martensite for strength and wear resistance. It is always followed by tempering to reduce brittleness to the required hardness.
What it is
Through hardening affects the whole cross-section and suits medium- and high-carbon and alloy steels such as EN8, EN19, EN24, EN31 and tool steels. The achievable hardness depends on carbon content; the depth depends on alloy content and section size (hardenability).
Tempering at a chosen temperature brings hardness down to the target range while improving toughness.
Typical applications
- Shafts, pins and axles
- Gears and sprockets
- Tools, punches and dies
- Bearing races and wear plates
- Fasteners (high-tensile)
Suitable materials
- Carbon steels (EN8, EN9, C45)
- Alloy steels (EN19, EN24, EN31, 42CrMo4)
- Tool steels (D2, H13, O1)
Points to consider
- Low-carbon steels (like EN1A, mild steel) cannot be through hardened; use case hardening.
- Sharp corners and sudden section changes risk cracking during quenching.
- Large sections may not harden through; choose an alloy steel with enough hardenability.
- Specify a hardness range, not a single value.
Frequently asked questions
Which steels can be hardened?
Medium- and high-carbon steels and alloy steels such as EN8, EN19, EN24, EN31 and tool steels.
Why is tempering needed after hardening?
As-quenched steel is very hard but brittle. Tempering reduces brittleness and sets the final hardness.
Can mild steel be hardened?
Not by through hardening. It can be case hardened (carburised) to give a hard surface.