Effect of Titanium Carbides on the Ductility and Toughness of High Carbon 30 mass% Chromium Steels

Effects of Ti carbide precipitates on the ductility and toughness of 30 mass%Cr steels containing above 0.1%C and 1%Ti were investigated by tensile and impact tests at low temperatures. Two kinds of heat treatments of aging after solution treatment (process SA) and annealing without solution treatment (process A) were performed for the test pieces. The ductility and toughness were evaluated by ductile-brittle transition temperature (DBTT) in reduction of area and DBTT in absorption energy, respectively. Both the processes SA and A produce coarse globular Ti carbide particles within the grains in the 2 (mu)m size range and cluster-like coarse globular Ti carbides at grain boundaries. The process SA does not bring about numerous fine Ti carbides at grain boundaries for the specimens containing above 0.1%C, which is contrary to those for the specimens containing below 0.07%C. The DBTTs determined by the tensile test (tensile DBTTs) for the specimens with above 0.1%C after the process SA are lower than the tensile DBTTs for the specimens in the range from 0.04% to 0.07%C, while the impact DBTTs for the former specimens are higher than the impact DBTTs for the latter specimens. The effect of the morphology of Ti carbides on the tensile DBTT and that of the morphology of Ti carbides on the impact DBTT are different. The numerous fine Ti carbides at grain boundaries exert detrimental effects on the tensile DBTT. On the other hand, the numerous fine Ti carbides at grain boundaries and the coarse globular Ti carbides exert detrimental effects on the impact DBTT.