Study Of Hydrogen Embrittlement Of Precipitation Strengthened Austenitic Stainless Steel Weldment

Precipitation strengthened austenitic stainless steels are widely used in hydrogen related applications due to its acceptable resistance to hydrogen embrittlement, high strength and good formability. In the engineering application, the welding process generally causes differences between the weld and matrix microstructures. And, these differences are always difficult to be eliminated by heat treatment or other ways. Because of the short time to stay, the possible problems in the service process of the precipitation strengthening austenitic stainless steel weldment is not clear enough. Therefore it is essential to study the hydrogen embrittlement sensitivity and hydrogen induced cracking mechanism of precipitation strengthened austenitic stainless steel weldment, thus it can provide some data and theoretical support to these areas.In this study, the plate tensile test, small sample charging without any applied stress, tensile tests under constant loading condition with charging et al. were employed to investigate the hydrogen damage and the service safety of the precipitation strengthening austenitic stainless steel weldment. And the results are as follows:For the precipitation strengthened austenitic stainless steel, the matrix shows typical austenite with average grain size of40-50μm and some annealing twins. The width of the weld is ahout2mm.The weld shows typical cast microsture with chear fusion lines, columnar grains and small equiaxed grains. There is no heat-affected zone with large grains near the matrix/weld interface.The weld is more sensitive to fracture and hydrogen-induced failure than the matrix. Hydrogen damage (microcracks) appears on the surface of the matrix and the weld during charging with very large current and long time even without any applied stress. Hydrogen-induced delayed failure occurred during charging under constant load and the normalized threshold stress σth/σb decreased exponentially with the increase of the defined time tc (hr), i.e. σth/σb=0.21exp(-tc/2712.68)+0.71. An estimation of the threshold stress occurring without hydrogen-induced failure during forty years was σth=713MPa, which is greater than the service stress. And the safety factor for hydrogen-induced fracture for the hydrogen storage tank is high enough to keep the tank safe during designed service time. The apparent diffusion coefficient of hydrogen in the weld is estimated as DH=1.1×10-11cm2/s.Microstructure of a precipitation strengthened austenitic stainless steel electron beam weldment was studied using SEM and TEM. The mechanism of hydrogen-induced cracking of the weldment was systematically studied by tensile tests under constant loading condition with hydrogen and the fracture surface was observed by SEM. In-situ TEM tensile experiments with hydrogen charged specimens were carried out to verify the microscopic mechanism. The results show that the density of curved dislocations in the weld was very high and these dislocations could act as hydrogen traps. There were a lot of large size precipitates (including η, G and carbides) in the weld. They located at the grain boundaries and inside the grains, surrounded by high density dislocations. They were major hydrogen traps and preferential microcrack nucleation sites. The columnar grain zone had a superior mechanical strength than the equiaxed grain zone along loading direction due to the texture. After post-weld aging treatment, precipitates in the base metal and the weld metal were both y’(Ni3(Al, Ti)) phase. The γ’ precipitates in the weld were3times larger in average size and were more dispersed than that in the base metal. These results explained the strengthening effect and the lower microhardness for the weld metal. The equiaxed grain zone was the most ductile part of the weldment. In the base metal, dislocations slipped and sheared through the γ’precipitates. While, in the weld metal, dislocations were bended and dislocation loops formed around γ’precipitates, resulting in dislocation entangles. Lattice images of the precipitate in the weld show the disordered γ’/γ interface owning a loss of coherency. It indicates that the sensitivity of studied material to fracture was determined by the size, type and distribution of precipitates. Hydrogen-induced cracking mechanism in the weld was not always the same. It depends on the value of the applied stress. When the applied stress was high, brittle transgranular fracture was the dominant. With the decreased of the applied stress, brittle intergranular fracture gradually became the dominant.