Mod.9Cr1Mo강에서 발생되는 일시적 취성현상 (Temporal Brittleness of the Mod.9Cr-1Mo Steel)

서지정보

· 발행기관 : 한국재료학회
· 수록지 정보 : 한국재료학회지 / 21권 / 11호 / 592 ~ 595페이지
· 저자명 : 허성강, 구지호, 신기삼, Yinsheng He, 신종호

초록

It is well known that modified 9Cr-1Mo steel has a low thermal expansion and high thermal conductivity with excellent high temperature properties compared to austenitic stainless steel. For these advantages, the steel is very popular for the boiler tube of thermal power plants. Normalizing is commonly utilized to obtain martensite in this steel, which shows an unusual toughness for martensite. However, some accidents related to this steel have been reported recently, opening the necessity for further study. As a particular behavior of the steel, an abrupt drop of the impact value has been identified upon tempering at 750℃ for about 1 hour. It is well known that Fe_3C forms during autotempering and turns to Cr_2C at an early stage and then transforms to Cr_23C_6. In this study, the cause of the abrupt drop of the impact value was investigated with an impact test, microstructural observation, nanodiffraction and phase analyses using instruments such as optical and transmission electron microscopes (TEM) with an extraction carbon replica of the carbides. The analyses revealed that the M2C that formed when retained for about 1 hour at 750℃ causes a drastic decrease in the mechanical properties. The sharp drop in mechanical properties, however, disappeared as the M_2C transformed into M_23C_6 with longer retention.

영어초록

It is well known that modified 9Cr-1Mo steel has a low thermal expansion and high thermal conductivity with excellent high temperature properties compared to austenitic stainless steel. For these advantages, the steel is very popular for the boiler tube of thermal power plants. Normalizing is commonly utilized to obtain martensite in this steel, which shows an unusual toughness for martensite. However, some accidents related to this steel have been reported recently, opening the necessity for further study. As a particular behavior of the steel, an abrupt drop of the impact value has been identified upon tempering at 750℃ for about 1 hour. It is well known that Fe_3C forms during autotempering and turns to Cr_2C at an early stage and then transforms to Cr_23C_6. In this study, the cause of the abrupt drop of the impact value was investigated with an impact test, microstructural observation, nanodiffraction and phase analyses using instruments such as optical and transmission electron microscopes (TEM) with an extraction carbon replica of the carbides. The analyses revealed that the M2C that formed when retained for about 1 hour at 750℃ causes a drastic decrease in the mechanical properties. The sharp drop in mechanical properties, however, disappeared as the M_2C transformed into M_23C_6 with longer retention.

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