Secondary Hardening

Question 1

What is secondary hardening?

Answer 1

The hardening of certain alloy steels at moderate temperatures (250-650 C) by the precipitation of carbides.

The resultant hardness is greater than that obtained by tempering the steel at some lower temperature for the same time.

Question 2

What happens during the process of secondary hardening?

Answer 2

1. Cementite forms as carbon diffuses out of the martensite
2. The micro structure of the matrix coarsens

As this happens the strength and hardness falls. If tempering is continued then the cementite, which is forming, will then coarsen, causing a further loss of strength and hardness.

If the formation of cementite can be replaced by a more stable carbide, e.g. Mo or W carbide, then the softening in these tool steels at elevated temperatures can be greatly reduced.

If sufficient amounts of these alloying elements are present in the range ~500 - 600 C, re-hardening (secondary hardening) can take place.