As reheating temperature of coarse grain HAZ is around 1623 K, TiN and austenite compositions are reported to reach equilibrium. This means that size and a volume fraction of the TiN precipitate could be predicted by thermodynamic data. As austenite grain size has close relation with average size and a volume fraction of TiN, prediction of the austenite grain size is also possible. In order to establish a prediction way of sizes of TiN and austenite grain of coarse grain HAZ, the relationship between size of the TiN and composition of the TiN that was calculated by thermodynamic data was investigated. The relation between the TiN precipitate morphology and size of the austenite was also examined. The effect of microstructure and the TiN size on coarse grain HAZ toughness of Ti containing steels was also investigated. The size of the TiN showed close relationship with chemistry of the TiN and the austenite that equilibrate each other. The austenite grain size showed to follow the Zener's relation with the TiN size and a volume fraction. These results show that the prediction of the austenite grain size of coarse grain HAZ is possible by calculating equilibrium chemistry of TiN and austenite. HAZ toughness of Ti containing steels reveals close relation with the grain size and the TiN size.
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