Ｃｒ‐Ｍｏ鋼の再熱割れに関する研究 ＶＩ 鋼の再熱割れにおよぼすＰ，Ｃｒ及びＭｏの相互作用

Abstract

Combined effect of phosphorus, chromium and molybdenum on the reheat cracking sensitivity was investigated using synthetic Cr-Mo steels melted in our laboratory. The cracking sensitivity was evaluated in the terms of critical restraint stress obtained by the modified implant method applied for the reheat cracking. The results are summarized as follows. (1) In the case of a constant phosphorus content of 0.01%, the combined effect of chromium and molybdenum (0 to 3%Cr, 0.5 to 1.4%Mo) was shown by menas of the contour lines of the critical restraint stress in the Cr-Mo content diagram (Fig. 3). (2) In the case of five sets of Cr-Mo contents, the effect of phosphorus in a content range of 0.004 to 0.1% was examined. There exists a critical phosphorus content Pcrit, from which the cracking sensitivity increases with the increase of phosphorus content. Pcrit values are: above 0.02%, 0.01%, 0.008%, 0.014% and below 0.01% for 0%Cr-0.5%Mo, 0.5%Cr-0.5%Mo, 1%Cr-0.5%Mo, 1.3%Cr-0.5%Mo and 2%Cr-l% Mo steels, respectively. (3) Phosphorus segregation at the prior-austenite grain boundary was estimated by a grain boundary etching method. Phosphorus has segregated there already in as-welded condition, and the degree of segregation corresponds to the cracking sensitivity. (4) Grain boundary concentration of phosphorus becomes the maximum by tempering at about 500°C, which corresponds to the temperature of crack initiation.