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