Effect of Partial Solution Treatment on Toughness of 12%Cr-0.3%C Steels

In 12%Cr-0.3%C martensitic steels, the effect of partial solution (PS) treatment in the (γ + carbide) two phase region was investigated in terms of the relation between microstructure and impact toughness. PS treatment is very effective for refining γ grains, because insoluble carbide particles works as obstacles to grain growth. The γ grain refining results in a significant lowering of Ductile-Brittle Transition Temperature (DBTT). This is mainly due to the reduction of grain boundary stress concentration which is caused by pile-up of dislocations to grain boundary. Besides, PS treatment contributes to homogeneous dispersion of carbide particles: In specimens with full solution (FS) treatment, carbide particles precipitate along lath boundary of martensite during tempering, and this causes early crack initiation in impact testing. On the other hand, in specimens with PS treatment, carbides preferentially precipitate on insoluble carbide particles which have homogeneously dispersed within the martensite matrix. This results in an increase of upper shelf energy in impact testing. It was also found that PS treatment suppresses precipitation of grain boundary carbide which occurs during slow cooling from γ phase region, because the insoluble carbide particles provide preferential precipitation site within γ grains. As a result, DBTT is not dependent on cooling rate in PS treated specimens, although it is greatly raised in FS treated specimens with slow cooling because of the grain boundary precipitation of carbide.