Papers by Author: Yoshiki Mikami

Abstract: This study investigated a method for estimating hardness distribution in welds, considering the effect of phase transformation and weld thermal cycles. Hardness distribution in welds was estimated from fractions and hardness of each microstructure by using rule of mixture. Finite element heat conduction analysis was performed to calculate weld thermal cycles. Microstructures formed corresponding to the thermal cycle were also calculated based on the continuous cooling transformation (CCT) diagram. The method mentioned above was applied to welds of Ultra-Narrow Gap welding process, which was developed for welding of ultrafine-grained steels. The calculated thermal cycles in the welds corresponded with measured results. Moreover, the estimated hardness distribution in the welds, which were estimated by using calculated thermal cycles and the phase fraction of each microstructure, was also in good agreement with measured values.

Abstract: Joint performances such as tensile strength and hardness in multi-pass welds are induced from both metallurgical and mechanical heterogeneity due to the difference of welding conditions. Hardness distribution in multi-pass weld metal is evaluated with a numerical simulation considering multiple heat cycles and phase transformation. Hardness of multipass weld metal is calculated with the rule of mixture by using fraction and hardness of each microstructure. In order to calculate fraction of each microstructure, CCT diagram was used. Conventional CCT diagrams of weld metal is not available even for single pass weld metal, thus new diagrams for multi-pass weld metals are created in this study. Modified diagrams for multi-pass weld metals with reheating effect were more dependent on the maximum temperature in reheating than the welding conditions. Hardness distribution is precisely predicted when the created CCT diagram for the multipass weld metal was used and the detailed calculation of weld thermal cycle is done.