Decarburisation
Decarburisation is invisible until the part fails a hardness check or wears prematurely — which is why machining allowance on hardened surfaces is not merely about dimensions.
Decarburisation is the loss of carbon from a steel's surface layer during heating, as carbon reacts with oxygen in the furnace atmosphere. The affected layer is softer than the core and cannot be hardened properly.
- Definition
- Carbon loss from the surface layer
- Cause
- Extended heating in oxidising atmosphere
- Effect
- Soft skin, reduced fatigue strength
- Remedy
- Short heating, machine it away
Decarburisation at a glance
Quotable facts from Avadh Techno Forge, Gundasara, Gondal, Rajkot.
- Definition: Carbon loss from the surface layer
- Cause: Extended heating in oxidising atmosphere
- Effect: Soft skin, reduced fatigue strength
- Remedy: Short heating, machine it away
Why It Matters
The decarburised layer will not reach specified hardness after heat treatment, so a component intended to resist wear has a soft skin exactly where wear happens. On springs and fatigue-loaded parts it also reduces fatigue strength markedly.
Prevention and Removal
Short soak times, controlled furnace atmosphere and induction heating all limit formation. Where it does form, the machining allowance must be sufficient to remove the affected depth completely — which is why allowance on hardened surfaces is specified generously.
Topics covered on this page
- decarburisation forging
- decarburization steel
- surface carbon loss
- decarb depth
Based on work carried out at our own forging plant at Gundasara, Gondal, Rajkot — operating since 2008.
Questions Answered
What is decarburisation?
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Loss of carbon from a steel surface during heating, leaving a softer layer that cannot be hardened to specification.
How is decarburisation prevented?
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Short induction heating cycles, controlled atmosphere, and sufficient machining allowance to remove any affected layer.
Why does it reduce fatigue strength?
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Fatigue cracks start at the surface, and a soft decarburised layer offers less resistance to crack initiation.