Microstructural analysis of HAZ in the welded joints of 9Cr heat resistant ferritic steels for fast reactor

Iwata, Mitsunao*; Sugiyama, Yuichi*; Murata, Yoshinori*; Takaya, Shigeru  

Microstructural states of metallic materials can be expressed by system free energy, and the estimation of the system free energy is useful for damage analysis of the materials. In this study, change with creep time in the system free energy of the FG-HAZ in 9Cr heat resistant ferritic steels is estimated on the basis of a series of experiments such as chemical analysis by using extracted residues, X-ray diffraction, and scanning transmission observations. The change in the system free energy is expressed quantitatively by rate constants depending on applied stress. It is observed that the steel ruptures when the applied stress exceeds the allowable stress. The relationship between allowable stress and system free energy makes it possible to predict the rupture time in the long term.