Materials Transactions Online

Materials Transactions, Vol.52 No.04 (2011) pp.691-698
© 2011 The Japan Society for Heat Treatment

Effect of Martensitizing Temperature on Creep Strength of Modified 9Cr Steel

Manabu Tamura1, Takuya Kumagai1, Nobuhiro Miura2, Yoshihiro Kondo2, Kei Shinozuka1 and Hisao Esaka1

1Department of Materials Science and Engineering, National Defense Academy, Yokosuka 239-8686, Japan
2Department of Mechanical Systems Engineering, National Defense Academy, Yokosuka 239-8686, Japan

A modified 9Cr-1Mo steel was cooled to a temperature between the Ms point and room temperature from the normalizing temperature and then directly heated to the tempering temperature. It was found that the time to rupture at 650 and 700°C of the steel heat-treated by the new heat treatment increased 2–3 times longer than that of the steel conventionally normalized and tempered. The microstructure of the improved steel was tempered martensite and the size of martensite blocks was larger than for the conventional heat treatment. The hardness of the improved steel was fully recovered after tempering. This strengthening is not caused by fresh martensite, but caused by the refining of M23C6 and V(C,N) particles in addition to the increase in the martesite block size, where M denotes metallic elements.

(Received 2010/10/7; Accepted 2011/1/24; Published 2011/3/9)

Keywords: modified 9Cr steel, creep, strengthening mechanism, heat treatment, block size, precipitates

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