Effects of process-generated hydrogen on RPV walls

2.4. Conclusion

avoided in order not have a sensitive microstructure. The formation of these phases is mainly determined by the cooling rate from the austenitization temperature. [24] Similarly, the higher the toughness of the material, the less susceptible it is to hydrogen flaking. Higher inclusion volume fractions generally result in a decreased toughness and therefore one would expect it to result in an increased susceptibility to hydrogen flaking. However, as these inclusions, mainly MnS precipitates, also act as hydrogen traps, they also influence the local hydrogen concentration. There are numerous reports referring to these hydrogen traps as the initiation point for hydrogen flakes. [23, 24, 26, 28, 29] However, simply reducing the number of these strong traps is not a solution as in that case a larger amount of dissolved hydrogen will be accumulated over a lesser volume of hydrogen traps, resulting in higher local hydrogen recombination. Therefore, there is a more complex relation between the inclusion volume fraction and the susceptibility of the material. [23] Also the grain size can affect the probability of hydrogen flaking. As larger grains result in a lower material toughness, this will again increase the susceptibility. On top of that, larger grains mean that the total grain boundary surface area will decrease and therefore result in an increase of the hydrogen concentration in the grain boundary, since these grain boundaries can also act as hydrogen traps as explained later. [23] Of course the hydrogen content in the material is one of the most influencing parameters for hydrogen flaking. In literature, there is consensus on the existence of a threshold hydrogen concentration for hydrogen flaking. This threshold is determined by the materials microstructural susceptibility and the presence of hydrogen traps in the material. In the 1970’s and 1980’s, this threshold was thought to be 1.5 to 2 ppm, however in this period hydrogen flakes were found in ultra clean steels, containing less then 20 ppm sulfur and 20 ppm oxygen and lower hydrogen concentrations. [23, 24] Furthermore, it is difficult to determine such a threshold as it is the local hydrogen concentration which determines the formation of hydrogen flakes. Therefore, a different degree of segregation can result in a different threshold for the global hydrogen concentration in the material. Fruehan [23] tried to visualize the threshold hydrogen concentration in steel as a function of the sulphur content (Figure 2.11). Furthermore, there is the existence of local stresses which is an important pa- rameter. In the absence of stresses, there will be no cracking. These stresses can have a different origin, e.g. residual stresses, transformation stresses, the pressure by buildup of hydrogen gas. These transformation stresses can become very high due to the γ to α transformation in the material. When the total force is high enough for the initiation or propagation, a hydrogen flake will form. [23, 24] 2.4 Conclusion Multiple arguments have lead to pointing to hydrogen flakes as the origin for the indications found. One of the main reasons is the clustering of the indications in the macrosegregated areas. One can see that the A-segregations in Figure 2.7 correspond to the position of the found indications (Figure 2.6). This is completely as expected 19