The Fracture Analysis for Hydrogen Embrittlement of Super Duplex Stainless Steel Weld Metal
Katsuyuki NAKADE and Toshio KURODA
Abstract:The influence of sigma phase on the hydrogen embrittlement in the 329J4L super duplex stainless weld metal was investigated by 3-dimensional fracture analysis. In the case of ferrite-austenite as-welded specimen, a little reduction of notch tensile strength was recognized since a little hydrogen cause the hydrogen embrittlement of ferrite. On the basis of the microstructure examination, hydrogen cracking preferentially occurred in the ferrite, while widmanstatten austenite showed ductile fracture. On the one hand, when hydrogen charging was carried out for weld metal precipitated 40% sigma phase (sigma phase-austenite microstructure), the notch tensile strength drastically dropped compared to hydrogenated as-welded specimen. This strength loss had a dependency of the amount of sigma phase. The susceptibility to hydrogen embrittlement of sigma phase was very high and it was clearly found that sigma phase strongly enhanced hydrogen embrittlement. The fracture surface was corresponded to typical widmanstatten microstructure. The area of sigma phase was always lower than other ductile area on the 3-dimensional fracture surface since sigma phase strongly promoted hydrogen embrittlement. From the viewpoint of microstructure, the ferrite almost decomposed and 40% sigma phase was present. Consequently, it could be concluded that sigma phase itself and sigma/widmanstatten austenite phase boundaries were preferential hydrogen cracking sites in the 329J4L weld metal. Key Words:Super duplex stainless steel weld metal, Sigma (ƒÐ) phase, Widmanstatten austenite (ƒÁ), Ferrite (ƒ¿), Hydrogen embrittlement, Fracture surface, 3-dimensional image