THE INFLUENCE OF MARTENSITE ON THE STRENGTH AND IMPACT BEHAVIOR OF STEEL

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 c ase, the impact behavior markedly deteriorated, this deterioration inc reasing with martensite level. The changes in impact behavior could be explained by regarding the films of martensite as being similar to th e brittle grain boundary carbides that are present in ferrite-pearlite steels. These films readily crack, yet are always thick enough to pro duce a sufficiently wide crack to render crack propagation easy. The c ritical event in fracture then becomes the ability to propagate the cr acks through the grain boundaries. The greater the grain boundary cove rage by these films, the easier this becomes and the worse is the impa ct behavior. When the martensite is present as colonies, again the imp act performance is seriously impaired, but to a lesser extent than whe n the martensite is present as films, this probably being related to t he difficulty in cracking a thick colony except at its extremities. Th e influence of martensite on strength was found to be similar whether the martensite was in the form of colonies or films. Replacing pearlit e with martensite led to a large increase in the yield strength. Incre asing the amount of martensite to similar to 7 pet caused the yield st rength to fall to a minimum, after which it again increased. This beha vior can be interpreted in terms of the manner in which the volume exp ansion accompanying the martensite transformation influences the gener ation of dislocations.