Toughness recovery of modified 9Cr–1Mo steel weld metals after long-term thermal ageing and its evaluation by electrochemical measurement

Abstract

Recently, thermal power plants have tended to operate under higher temperature and pressure steam conditions for CO2 reduction. Modified 9Cr–1Mo steels are used in the ultra-super critical power plant because of their excellent creep properties. However, the toughness of base metal decreases after long-term thermal ageing at operation temperature. Moreover, toughness of weld metal is lowest in the MIG weldment. In this study, metallurgical factors of toughness of the modified 9Cr–1Mo steel weld metals by thermal ageing at operation temperature were investigated. And the evaluation method of toughness by the electrochemical measurement used with 5% sulphuric acid aqueous solution was investigated. The weld metal that received post-welding heat treatment (PWHT: 1023 K and 5.4 ks) decreased toughness by thermal ageing in 873 K and 31.5 Ms. However, toughness of the welds after thermal ageing was recovered to the same level as PWHT welds when the thermal aged welds received the same thermal history as PWHT. A lot of the large Laves phase was observed in the weld metal after the thermal ageing. However, almost all of the Laves phase dissolved in parent phase by the same thermal history as PWHT. Therefore, it seemed that the decrease and the recovery of toughness were mainly determined by the behaviour of precipitation of the Laves phase. The peak of the current density (Ip) in the electrochemical measurement appeared in the weld metals that precipitated a lot of large Laves phase. It seemed that appearance of Ip was caused by the dissolution of the Laves phase. A good correlation between toughness and Ip was observed. Therefore, it can be said that toughness of the weld metals is supposed to be predicted using an Ip of the electrochemical measurement.