中，高炭素低合金鋼の溶接熱影響部における低温割れの研究 ＶＩ 焼割れ的低温割れ感受性に及ぼす燐の粒界偏析およびセリウムによるその改善効果

Abstract

In order to investigate the harmful effect of phosphorus on quenching crack type cold cracking in weld HAZ of medium/high carbon low alloy steels, fracture behavior and grain boundary segregation have been studied by the simulated cold cracking test and Auger electron spectroscopic analysis with vacuum induction mélted JIS SNCM447 containing low (about 0.001wt.%) and high level (about 0.030wt.%) of phosphorus and sulphur. The effect of cerium on the harmful effect of phosphorus has been also studied by the same procedures.Phosphorus enhances the intergranular embrittlement and greatly reduces fracture stress. On the other hand, sulphur does not the detrimental effect.The AES results clearly indicate that phosphorus was segregated at grain boundaries. Carbon was also found to be segregated at the grain boundaries. The phosphorus segregation to grain boundary is fairly increased by grain boundary liquation and decreased with increasing in cooling time. These tendencies are closely correlated with the fracture stress and the fraction of intergranular fracture. The decrease of phosphorus segregation in relation to increasing cooling time occurred at the temeprature region below Ms during cooling, however, this is inconsistent with general tendency of equilibrium segregation described in Fe-P binary alloy. Therefore, it was thought that some complex mechanisms seem to play a role in the temperature region.Cerium has a beneficial effect to reduce the harmful effect of phosphorus due to reducing the degree of phosphorus segregation at grain boundaries.