A comparative study of monotonic fracture, environmentally assisted fracture and fatigue behaviour of cast and forged high strength steels

Abstract

The microstructure and fracture characteristics of a high strength cast steel and a forged steel with the same chemical composition and strength and both subjected to the same heat treatment were studied. It was found that the cast steel has better fracture mechanics characteristics ( K Ic , K ISCC , dd a d t and d a d N although the conventional mechanical properties of the cast steel (tensile ductility, notch toughness, fatigue strength etc.) are poorer than those of the forged steel. The roles that the microstructure of the cast steel plays in the two kinds of tests are discussed. The microporosity causes the conventional properties to deteriorate and in particular leads to much more scatter in these properties but it does not appear to have any effect on the fracture mechanics parameters. The dendrite segregation exerts a very harmful influence on both the conventional and the fracture mechanics characteristics. The inclusion distribution state (the secondary dendrite arm spacing) and the weaker inclusion-matrix interfacial bonding are the main reasons why the cast steel has a higher fracture toughness; in particular, the weak interface between the inclusions and the matrix is to a large extent responsible for the lower hydrogen embrittlement, the susceptibility to stress corrosion cracking and the slightly lower fatigue crack growth rate.