Fracture characteristics and mechanisms in cast steel

The current practices of the UK steel foundry industry have been described and a review has been made of the fracture characteristics and mechanisms which commonly occur in steel castings. Emphasis has been placed on intergranular embrittlement phenomena caused by segregation and precipitation effects, and in particular the role of aluminium nitride has been considered. The problem of A1N-induced intergranular fracture (I. F.) has been examined using experimental 0.30C-1.50bh castings with varying Al and N contents, and commercially-produced material. The mechanism of fracture has been studied using various optical and electron optical techniques and the existence of two forms of A1N (a plate-like form and a dendritic form) has been confirmed. Directional solidification work has shown that the dendritic A1N forms in the residual liquid at a late stage of solidification. The plates are precipitated at a later stage when the casting is fully solidified. The plates nucleate at the grain boundaries and grow into the adjacent austenite grains with an orientation relationship of the form <1010> (0001)// // <110> {111} Y A1N Y. The effects of some processing variables on the susceptibility to IF have been examined. IF is favoured by a high Al and N content and a slow cooling rate. The state of heat treatment was also found to be important, with quenched and lightly-tempered samples being more susceptible to IF. A model has been proposed which is based on a comparison of the relative yield strength, cleavage strength and intergranular strength of a casting. This can be used to explain the variation in fracture behaviour with testing temperature. A programme of mechanical testing has shown that different testing techniques and conditions can influence the severity of IF observed quite significantly. Although there may be a considerable decrease in Charpy impact energy as a result of intergranular embrittlement, there is little effect on the initiation value of the crack opening displacement (Si). The C. O. D. at maximum load, (ö), however, max is decreased and the implications with regard to engineering design practices are considered.