Effect of Delta Ferrite on Low Temperature Toughness of Type 308 L Weld Metal —Low Temperature Toughness of Austenitic Stainless Steel Weld Metal (Report 4)—

Effect of delta ferrite on low temperature toughness of molbdenum-bearing weld metal (Type 316L) was reported in authors’ previous papers, which revealed that notch toughness of as-welded metal was considerably dependent on a solidification mode and in addition, the amount of molybdenum within the delta ferrite which was strongly affected by the solidification mode was closely related to the weld toughness. In this study, a similar relation on molybdenum-free weld metal (Type 308L) was investigated in detail by using a transmission electron microscopy (TEM) and an analytical electron microscopy (STEM/EDX). From notch toughness test at 77K of as-welded metal, the toughness decreased monotonously and slightly with increasing amount of delta ferrite. This result differed from the previous data of Type 316L weld metal and revealed that toughness of Type 308L weld metal was basically unaffected by solidification mode. In eutectic delta ferrite, a loss of coherency between crystallographic orientation of delta ferrite and that of adjacent austenite matrix reduced as compared with previous data of Type 316L weld metal. Moreover, STEM/EDX analysis suggested that chromium content within eutectic delta ferrite was about 4 percent more than that within primary delta ferrite. This was considered to be due to difference of solidification mode. From above results, lack of molybdenum in the weld metal would increase coherency at eutectic delta ferrite/austenite interface. As a result, marked degradation of toughness in eutectic delta ferrite weld metal was considered to not occur.