ステンレス鋼ＦＣＡＷ溶接金属における再熱割れに関する研究 ３ 定速度引張試験による溶接金属の高温延性の評価

Abstract

According to previous reports, the main factor causing reheat cracking in stainless steel FCAW weld metal is bismuth added in the flux to improve slag detachability. The cracks, whose surfaces have a dendritic appearance, propagate along the columnar grain boundary. In this report, the constant speed tensile test at elevated temperature in an inert gas atmosphere was performed and the ductility of the FCAW weld metal was evaluated to clarify the temperature range of weld metal embrittlement, and characterization of the fractured surfaces was performed to categorize the fracture type. Also, the effect of initial ferrite content and precipitates such as the σ phase on the ductility was evaluated.As for the bismuth-free weld metal, a significant ductility drop was not observed at any test temperature and the fractured surface showed a dimple pattern which suggests a ductile and transgranular fracture. On the other hand, in the case of the bismuth-containing weld metal, ductility is remarkably depressed at temperatures above 873 K and the fractured surfaces showed a dendritic appearance. Moreover, at temperatures above 1173 K, the fractured surface showed sighs of Equation at higher magnification. The ferrite content in as-welded condition and aging time at 973 K had no effect on the ductility at elevated temperature. Therefore bismuth is the main factor that causes the grain boundary embrittlement at elevated temperature. However, in the case of the high ferrite content under as-welded conditions, the aged weld metal showed slightly depressed ductility compared with the low ferrite content material, therefore, the precipitates in ferrite are considered to be a secondary factor for reheat cracking.