Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

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Ruscak et al. (1991) also studied the trapping of hydrogen and hydrogen embrittlement at the heat-affected zones of the weldments in CrMoV and CrNiMoV steels. The heat-affected zone is significantly more able to absorb hydrogen and consequently its residual hydrogen content is always higher than that of the base material. Owing to nickel, Ni-steel also has the ability to dissolve more hydrogen into the microstructure of both the base material and HAZ. Tensile tests showed the effect of hydrogen content on the mechanical properties of the base material and both the untreated and the post-weld annealed (665°C, CrMoV steel as well as 625 and 650°C, CrNiMoV steel) HAZ. Figure 26 shows that the ductilities of the different zones have almost identical dependence on the hydrogen content for CrMoV steel. On the other hand, thermally untreated HAZ, in particular, is susceptible to hydrogen embrittlement of CrNiMoV steel. The hydrogen contents of HAZ identified earlier, i.e., 2 and 3.5 ppm for CrNiMoV and CrNiMoV steel, already cause the reduction of ductility and if the hydrogen content exceeds 5 ppm at the HAZ of CrNiMoV steel and 10 ppm at the HAZ of CrNiMoV steel, a total loss of ductility may occur. In summary, Czechoslovakia has provided a lot of research into the hydrogen embrittlement of VVER-type pressure vessel steel in static and dynamic tests for both irradiated and unirradiated states taking also into account the different weld zones (e.g., Koutsky 1991). Studies show that hydrogen plays a major part in the test results, if the hydrogen content exceeds 2 ppm, which according to the Czech studies may occur, for instance, at different stages of operation in the cladding/base material boundary. These studies have also clearly shown that the elevated hydrogen content promotes intergranular cracking so that hydrogen may play an important role in understanding the anomalous cracking behaviour of survaillance specimens, which is associated with intergranular cracking.