INFLUENCE OF GRAIN-BOUNDARY FILMS OF MARTENSITE ON STRENGTH AND IMPACT BEHAVIOR OF FERRITE

The influence of thin films of martensite at the ferrite grain boundar ies on the impact behaviour and strength of a high manganese-niobium c ontaining steel has been examined. Normalising followed by intercritic al annealing, at 730 degrees C for various times from 0 to 900 min, an d quenching introduced these martensite films. The percentage of marte nsite increased with increased holding time at 730 degrees C from 15 t o 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 be haviour markedly deteriorated when martensite was present, but when th e amount of martensite further increased above 1.5 vol.-% it only had a small detrimental influence. The changes in impact behaviour could b e explained by regarding the thin films of martensite as being similar to the brittle grain boundary carbides present in ferrite-pearlite st eels. 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 propag ate the cracks through the grain boundaries.