| CGHAZ(Coarsened Grain HAZ) which is the sub-zone closest to the weld metal is the region where many weld difficulties appear such as cracks, low toughness due to grain growth, etc. So, prediction of grain growth and microstructure under severe welding thermal cycle is very useful research to remove the defects forming in the HAZ. Therefore, in this study, welding thermal cycle was predicted by FDM mathematical analysis for three dimensional bead on plate model, and to predict the grain growth, the metallurgical modeling for the grain growth behavior was developed on the basis of phase transformation and grain growth theory. Phase transformation phenomena which appears between A₁ and A₃ temperature was analyzed by Ham`s theory and classical transformation theory. The grain growth phenomena was analyzed by the general grain growth equation. The general grain growth equation established by many researchers contains the relationship between the grain size and grain boundary mobility and grain growth driving force. In this study, such relationship was examined more precisely, and the grain size variation was predicted by the computer simulation based on this equation. For more precise analysis, the variation of the phase transformation temperature range with the heating rate was considered by the dilation experimental data. And then, through these mathematical approach and experimental data, phase transformation model and grain growth model were constructed. The prediction results were very good coincident with the HAZ microstructure experiment results. From the results of simulation, the HAZ grain size diagram was drawn up, in which we can acquire very many informations about welding condition, material selection for welding and defect forming part, etc. It is concluded that the grain growth modeling of this study is proper for the prediction of prior austenite grain size variation in the HAZ and general enough to apply for other heat treatment processes. |
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