Transmission Electron Microscopic Observation and Microanalysis of Type 316L Weld Metal —Low Temperature Toughness of Austenitic Stainless Steel Weld Metal (Report 2)—

An authors’ previous paper which reported on the influence of delta-ferrite on low temperature toughness of Type 316L austenitic stainless steel weld metal, revealed that the toughness of as-welded specimen was dependent not only on the delta-ferrite content but also on the solidification mode. In this paper, various factors being affected on the toughness such as concentration difference, coherency, etc., were investigated in detail by using a transmission electron microscopy (TEM), an analytical electron microscopy (STEM/EDX) and a scanning electron microscopy equipped with an energy dispersive X-ray spectrometer (SEM/EDX). From an electron diffraction analysis, a crystallographic orientation of primary delta-ferrite was coherent with that of its adjacent austenite matrix but in the eutectic delta-ferrite, the relation was in-coherent. The amount of molybdenum contained in the eutectic delta-ferrite was 1.8 times as that in the primary delta-ferrite and moreover, a cell or a cellular dendrite boundary adjacent to the eutectic delta-ferrite, was enriched in molybdenum slightly. Probably, in these molybdenum-rich sites, a lattice strain was considered to be higher than that in the primary delta-ferrite. From the SEM/EDX analysis on the fractured surface of the Chirpy tested eutectic delta-ferrite specimen, the chromium and molybdenum rich zones were measured, where these compositions were equivalent to that of delta-ferrite or cellular dendrite. However, in the primary delta-ferrite specimen, such a zone could not be detected. From results above, when the eutectic delta-ferrite specimen was Charpy-tested, a crack initiation occurred at the delta-ferrite/austenite boundary or within the delta-ferrite and subsequently, the crack propagated preferentially along the cell or the cellular dendrite boundary or the delta-ferrite. In the specimen heat-treated at 923 K for 100 hrs, the concentration of the principal elements such as chromium, molybdenum and nickel within the prior delta-ferrite fluctuated widely, independent of its morphology, in which furthermore, the presence of carbide M₂₃C₆ and a phase were confirmed.