Through Hardening
Heating a medium or high carbon steel above its critical temperature and quenching so that hardness is achieved through the full section rather than only at the surface.
Section size is the limit
An EN8 bar hardens usefully to maybe 20 mm from the surface; past that the quench cannot pull heat out fast enough and the centre transforms to softer pearlite. This is the reason a 120 mm EN8 shaft tested at the centre disappoints. EN19 extends the depth, EN24 further again.
Tempering
As-quenched martensite is hard and brittle and must be tempered. Low tempering (200 °C) keeps hardness for wear parts; high tempering (550-650 °C) trades hardness for the toughness a shaft needs. The quenched and tempered hardness band on the order is what actually gets checked.
Related terms
Where this applies
Alloy Steel Bright Bars, Carbon Steel Bright Bars
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