オーステナイトステンレス鋼熱影響部における結晶粒界挙動に関する研究(第1報)

Abstract

A series of experiments on stainaless steels have been conducted in order to understand the phenomenon and mechanism of weld hot cracking, especially due to grain-boundary liquation, in heat-affected zone of some steel weldments.In previous report (the lst report), some new knowledge of liquation cracking was obtained for Type 310 and Type 304 austenitic stainless steels.In this report, some experiments on Type 347 and Type 321 stabilized austenitic stainless steels were conducted with a high temperature microscope and an electron probe X-ray microanalyser to obtain a more detail understanding of grain-boundary liquation cracking in the weld heat-affected zone.The main results obtained in this study are as follows:1) In Type 347 stainless steel, the appearance of liquid phase occurred at temperature lower by about 70°C than the solidus in both cases of rapid and slow heating.This phenomenon is probably attributed to eutectic reaction between niobium carbide and r-iron.2) Grain-boundary liquation was observed in the heat-affected zone of Type 347 stainless steel. The mechanism causing grain-boundary liquation is presumed to be as follows:Grain-boundary migration accompanying rapid heating occurs and these migrating grain boundaries are pinned by niobium precipitates which remain indissolved in spite of heating at high temperature above 1300°C. Continuously, with an increasing temperature NbC-r-iron eutectic reaction occurs and then eutectic liquid produced in close vicinity of niobium precipitates penetrates into the grain boundaries which have been pinned and fixed here.3) Narrow ferrites extended in the rolling direction of the base metal were liable to promote liquation hot cracking in HAZ.4) It was presumed that Type 321 stabilized steel has less tendency to grain-boundary liquation than Type 347 stabilized steel and then has less susceptibility to hot cracking in HAZ.