SUS 316L 溶接金属の透過電子顕微鏡観察及びミクロ分析

オーステナイトステンレス鋼溶接金属の低温じん性(第2報)

抄録

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 incoherent.
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 Charpy 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 σ phase were confirmed.