Takahiro Osuki

Effects of chemical compositions and microstructure on hydrogen embrittlement of austenitic stainless steel weld metal in high-pressure gaseous hydrogen environment

The effect of chemical compositions and microstructures on hydrogen embrittlement of austenitic stainless steel weld metals in high-pressure hydrogen gas was surveyed by using the slow strain rate test (SSRT). As a result, hydrogen embrittlement of the weld metal was hardly influenced by δ ferrite in the weld metal, but by stability of austenite phase, which was estimated by Md30 value or Ni equivalent. In the weld metal with poor stability of austenite, α′-martensite was formed near a crack induced by SSRT. In addition, although the crystal structure of α′-martensite is as same as δ ferrite, susceptibility of hydrogen embrittlement became higher with the increase in α′-martensite. The mechanism to explain the difference between δ ferrite and α′-martensite was considered as following. The hardness, which increases the hydrogen embrittlement susceptibility in bcc structure, is higher in α′-martensite than in δ ferrite. In addition, α′-martensite might be formed continuously with propagation of a crack. Therefore, the effect of α′-martensite on hydrogen embrittlement could be larger compared with δ ferrite.

Analysis of the solidification process in Fe–36%Ni weld metal with NbC crystallization

Abstract When austenitic high-alloy steel weld metal sustains single-phase solidification generally described as A-mode solidification, this is well-known to result in heightened solidification cracking susceptibility.1–3 To reduce the solidification cracking susceptibility of austenitic stainless steel, it is known to be effective to undertake component modification such as to obtain a solidification mode called the AF mode or FA mode involving the ä phase being crystallized or retained.1, 2 To obtain a complete γ solidification mode in the case of high Nibase alloys, such as Fe–36%Ni alloy, however, it is necessary to arrange for high Cr addition in order to achieve component modification facilitating AF or FA mode solidification such as affects austenitic stainless steel. The result of such addition, however, is that impairment of base metal properties also self-evidently cause heavy loss of hot workability in a way that makes this approach difficult to describe as effective.