Influence of grain boundary films of martensite on strength and impact behaviour of ferrite

The influence of thin films of martensite at the ferrite grain boundaries on the impact behaviour and strength of a high manganese–niobium containing steel has been examined. Normalising followed by intercritical annealing, at 730°C for various times from 0 to 900 min, and quenching introduced these martensite films. The percentage of martensite increased with increased holding time at 730°C from 15 to 33 vol.-%. The introduction of martensite removed the yield point but the yield strength was little affected, increasing only slightly with an increase in martensite volume fraction. In contrast, the impact behaviour markedly deteriorated when martensite was present, but when the amount of martensite further increased above 15 vol.-% it only had a small detrimental influence. The changes in impact behaviour could be explained by regarding the thin films of martensite as being similar to the brittle grain boundary carbides present in ferrite–pearlite steels. These martensite films readily crack yet are always thick enough to produce a sufficiently wide crack to render crack propagation easy and the critical event in fracture then becomes the ability to propagate the cracks through the grain boundaries.