無粒界腐食型25Cr-35Ni系超高純度ステンレス鋼の高温割れ感受性

Abstract

The hot cracking susceptibility of type 25Cr-35Ni extra high-purity stainless steels was evaluated by the transverse-Varestraint test with varying the contents of Cr as well as P and S. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was reduced as the amount of Cr in steels increased, and 25Cr-35Ni stainless steel was negligibly susceptible to solidification cracking. On the other hand, the ductility-dip cracking susceptibility adversely increased with an increase in Cr content as well as P and S contents in steels. There was a good linear relationship between the compositional parameter of (P+1.22S) and the DTR, as well as between (P+1.19S) and the BTR in 25Cr-35Ni stainless steel welds. Accordingly, the quantitative influence of S to an increase in the ductility-dip as well as solidification cracking susceptibility was approx. 1.2 times as large as that of P. The amount of P+1.22S in steels should be limited to approx. 90ppm in order to obtain the sufficiently low ductility-dip cracking susceptibility in 25Cr-35Ni stainless steel welds. Numerical simulation of segregation behaviours of P and S revealed that they were segregated at the grain boundary in the ductility-dip temperature range during welding. Molecular orbital analysis suggested that ductility-dip cracking was attributed to the grain boundary embrittlement due to the grain boundary segregation of P and S. In order to further inhibit the ductility-dip cracking in 25Cr-35Ni extra high-purity stainless steel welds, the effect of La addition on hot cracking susceptibility was investigated. The solidification and ductility-dip cracking susceptibilities could be improved by adding 20-70ppmLa to the extra high-purity steel containing P and S of 6-8ppm. The ductility-dip cracking susceptibility was decreased as a result of the desegregation of P and S to grain boundaries due to the scavenging effect of La.