Decarburisation
Loss of carbon from the surface layer of steel during hot rolling or heat treatment, leaving a soft skin that will not harden with the rest of the bar.
Why it ruins a hardened part
A decarburised skin has less carbon than the core, so on quenching it does not transform to full-hardness martensite. The result is a soft surface exactly where hardness was wanted, and a part that fails in wear or fatigue despite a correct core hardness reading.
How bright bar handles it
Turning, peeling and grinding physically cut the decarburised layer away, which is why hardened components are made from turned or ground bar rather than cold drawn. EN 10277 sets permissible decarburisation depths by class, measured to ISO 3887.
Related terms
Where this applies
Case Hardening & Bearing Steel Bars, Alloy Steel Bright Bars
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