Abstract
In this work the effects of long-term ageing of 9Cr-1Mo (T91) heat-resistant steel on its dynamic hydrogen embrittlement resistance in impact loading conditions were investigated. Altogether three heat-treated material states were studied, namely the initial normalized and tempered (N&T) state, i.e., 1050 °C /0.5 h + 750 °C /1 h and two long-term aged states, i.e., 600 °C /5000 h and 650 °C /5000 h. Charpy V-notch (CVN) impact bending tests were performed at room temperature for all the heat-treated material states in conditions either without or with electrochemical hydrogen charging. A brittle fracture resistance of the investigated material in individual material conditions was evaluated by the determination of Embrittlement Index (EI) values in terms of relative changes in CVN impact toughness with respect to variously selected initial and final material conditions. This approach enabled to differentiate the effects of long-term ageing, hydrogen charging, and their mutual superposition on the resulting brittle fracture resistance. The obtained results indicated excellent brittle fracture resistance of studied T91 steel in both the aged and hydrogen-charged material states at impact loading conditions.

Key words
9Cr-1Mo steel, ageing, cathodic hydrogen charging, impact toughness, fracture micro-mechanism

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