高強度低合金鋼溶接熱影響部の600℃付近での粒界割れ感受性におよぼす微量元素の影響

抄録

Stress-annealing tests (constant load, or constant displacement) on smooth bars which were given a prior treatment to simulate a heat-affected zone in a welded joint were used to characterize the susceptibility to intergranular fracture at about 600°C of various heats of a pressure vessel steel, SA 508-2.
Micro-alloying to cerium or zirconium in the steel proved to have the effect of suppressing the intergranular fracture, whereas phosphorus, arsenic, antimony, or copper did not have any substantial effect on the susceptibility. Remarkable enrichment of sulfur was found on the intergranular facets with an Auger spectrometer, which is quite dissimilar to the grain boundary segregation in temper-embrittle low alloy steels. The extent of the sulfur enrichment proved to reduce with lowering of the quenching temperature in a prior treatment (two step austenitizing), corresponding to the decrease in the susceptibility to the intergranular fracture.
On the basis of these results, a new model is proposed to explain the mechanism that the intergranular fracture is promoted by the enriched sulfur in cavity surfaces.