State of hydrogen and susceptibility to hydrogen embrittlement of pre-strained tempered martensitic steel followed by hydrogen charging

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  • 予ひずみ後に水素添加した焼もどしマルテンサイト鋼での水素存在状態と水素脆化感受性
  • ヨヒズミ アト ニ スイソ テンカ シタ ヤキモドシ マルテンサイトコウ デ ノ スイソ ソンザイ ジョウタイ ト スイソゼイカ カンジュセイ

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Lath martensitic steels were pre-strained before hydrogen charging, and subjected to thermal desorption analysis (TDA) . The measured TDA curves was analyzed by peak separation with Gaussian distribution functions to know the state of hydrogen. Reasons for the effects of pre-straining were explored, together with the effects of pre-straining on the hydrogen embrittlement susceptibility. Pre-straining made hydrogen pick-up increased:It was trapped by dislocation and grain boundary, in combination with further formation of vacancy and vacancy cluster as hydrogen traps. They are originated by the cooperation of the charged hydrogen and internal stress introduced by pre-straining. These changes can be brought about through the link of the following four processes: (1) hydrogen trapping at dislocation, (2) vacancy production through dislocation interaction, followed by pairing with hydrogen, (3) agglomeration of vacancy-hydrogen pair to vacancy cluster-hydrogen complex, (4) transport of vacancy-hydrogen pair toward grain boundary to increase hydrogen along it. Processes (3) and(4) are in competition to each other. When the steel is aged at 200°C before hydrogen charging, the state of hydrogen was hardly affected by pre-straining. This is because the dislocation, which was newly formed by pre-straining, loses the ability of hydrogen trapping through strain aging with carbon atom, concerning to the process (1) in the above: The processes (2) to (4) are consequently lacked. When the strength of the steel is higher, pre-straining facilitates to increase the amount of hydrogen trapped by grain boundary, through localized dislocation density close to grain boundary. Thus the steels with higher strength become more susceptible to hydrogen embrittlement with grain boundary facet.

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