The influence of martensite on the strength and impact behavior of steel

Abstract

The influence of high C martensite on the strength and impact behavior of C-Mn-Nb-Al steels has been determined for two distributions: films surrounding the ferrite grains and distinct colonies. In the former case, the impact behavior markedly deteriorated, this deterioration increasing with martensite level. The changes in impact behavior could be explained by regarding the films of martensite as being similar to the brittle grain boundary carbides that are present in ferrite-pearlite steels. These films readily crack, yet are always thick enough to produce a sufficiently wide crack to render crack propagation easy. The critical event in fracture then becomes the ability to propagate the cracks through the grain boundaries. The greater the grain boundary coverage by these films, the easier this becomes and the worse is the impact behavior. When the martensite is present as colonies, again the impact performance is seriously impaired, but to a lesser extent than when the martensite is present as films, this probably being related to the difficulty in cracking a thick colony except at its extremities. The influence of martensite on strength was found to be similar whether the martensite was in the form of colonies or films. Replacing pearlite with martensite led to a large increase in the yield strength. Increasing the amount of martensite to ∼7 pct caused the yield strength to fall to a minimum, after which it again increased. This behavior can be interpreted in terms of the manner in which the volume expansion accompanying the martensite transformation influences the generation of dislocations.