A Proposal to Mechanism of Embrittlement of 9Cr-2Mo Heat Resisting Steel during Long-term Aging

A trial has been made on the clarification of mechanism of embrittlement of 9Cr-2Mo dual phase steels during aging in the temperature range from 500 to 600°C. The focus is placed mainly on the relationship between toughness and precipitates of Laves phase and/or carbides.The precipitation of Laves phase causes a considerable decrease in toughness in rather short-term aging (600°C×100h for example). In the case of long-term aging (600°C×1000 h), however, the toughness depends mainly on the total amounts of the precipitates including both Laves phase and carbides. The preciptation of the Laves phase is markedly retarded by the decrease of Si content, resulting in excelent toughness. A harmful effect of P-addition on toughness is also observed. It is found, however, that no intergranular fracture is observed at below transition temperature, which probably suggests that the embrittlement is caused by some different mechanism from the so-called temper embrittlement.