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We may conclude based on these observations, that the light areas (A to E) in the Figs. l(a) and (b) are methane bubbles formed during the hydrogen exposure. The sizes of the bubbles were 200 to 1000 A and they are small enough to be inside the foils. In all the observations bubbles are found to be nucleated on the surfaces of grain boundary precipitates. In Cr-Mo steels carbides of the types Fe3C, M7C3, M6C, M23C 6, M a C b (Fe2MoC) and M02C can be expected depending on the composition and heat treatment. 9 In the as received material for the present work at least Fe3C, M02C and M23C 6 w e r e found to exist by using electron diffraction techniques and MnS sulfides were identified by using an EDS analyzer connected with a SEM. The structure of MnS is usually cubic (NaCl-type) although two other structures are also reported? ~ Carbides M23C6 and Fe3C usually gave quite strong and easily observable diffraction spots and they were easy to identify. After 20 day hydrogen exposure at 600 ~ cementite had almost totally disappeared but other carbides observed in the as received material as well as MnS sulfide were common. The diffraction patterns of carbides shown in Fig. 1 were weak and they did not fit to any other expected carbide (or MnS) except to Fe2MoC (MaCb). The sequence of precipitation of carbides during thermal aging in c h r o m i u m - m o l y b d e n u m steels has been reported to follow for instance Fe3C --~ M02C MaC b ---) chromium carbides with increasing chemical stability where one or both of the intermediate phases can be missing. 9 The almost total absence of Fe3C in hydrogen exposed specimens can be the result of thermal aging a n d / o r decarburization. The observed bubbles could be nucleated on Fe3C type of carbides because of their low chemical stability prior to its conversion to Fe2MoC. They could also be nucleated directly on Fe2MoC type of carbides because of their stability compared to higher chromium carbides. In no instances were bubbles observed to be associated with the latter type of carbides. MnS has low chemical stability and has the potential to react with dissolved hydrogen to form hydrogen sulfide. Figure 2 shows some bubbles nucleated around

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