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Duplex stainless steel (DSS) is vulnerable to changes in the α/γ phase fraction due to the dissolution and precipitation of various compounds during multi–pass welding. Thus, prediction of this dissolution and precipitation behavior is key for the development of effective manufacturing strategies. In this study, the kinetics of the dissolution and precipitation phenomena of the γ phase in Fe-Cr-Ni alloy were investigated using the kinetic constants derived by the phase–field method, and the α/γ phase fraction of the multi-pass welds was theoretically investigated. The kinetics of the dissolution and precipitation phenomena were evaluated theoretically and experimentally through optical and scanning electron microscopy. DSSs with different compositions were evaluated to achieve satisfactory correlation. The temperature dependences of the experimental and calculated values were found to be in good agreement, indicating that it is possible to theoretically predict the dissolution and precipitation phenomenon of the γ phase by using the phase-field method presented herein. With regard to the α/γ phase fraction, there was a remarkably high correlation between the results predicted using the kinetic constant of the experimental value and those predicted using the kinetic constant derived from the phase–field method. Therefore, the kinetics-based theoretical results of this study could serve as a resource for predicting the γ phase amount of multi–pass welds in DSSs.

In order to change the cooling rate of weld pool, in-situ laser post-heating was irradiated behind the weld pool during gas tungsten arc welding in a U-type hot cracking test. The solidification cracking occurrence based on the cooling rate change was observed. The high temperature ductility curve and thermal strain curve were calculated to investigate the relationship between solidification cracking behaviour and cooling rate. The high temperature ductility curve was obtained based on the combination of the critical strain and solidification brittleness temperature range. An in-situ observation technique was used to measure the critical strain. The solidification initiation and completion temperatures were estimated by the metallurgical analysis model. The thermal strain curve was calculated using a finite element simulation method. The high temperature ductility curve did not significantly changed according to the welding conditions. However, the decrease of cooling rate deteriorated yield strength distribution and increased both the stress applied to the weld bead and the slope of thermal strain curve, increasing vulnerability to solidification cracking.

A computer simulation of the α/γ phase transformation in a multi-pass weld of duplex stainless steel (DSS) was made to predict the distribution of the γ phase fraction in the heat-affected zone (HAZ). The kinetic equations including rate constants of the dissolution behavior as well as the precipitation behavior of the γ phase were determined by an isothermal heat treatment test. Based on the kinetic equations determined, the distribution of the γ phase fraction in a multi-pass weld of lean DSS was calculated by applying the incremental method combined with a heat conduction analysis of the welding process. The γ phase fraction was reduced in the higher temperature HAZ; however, that in the reheated HAZ was increased and recovered to the base metal level. Microstructural analysis revealed that the calculated results of the γ phase fraction in the multi-pass weld were consistent with experimental ones. Based on the computer prediction, the microstructural improvement welding (“reheat bead welding”) process, an analogous concept to the temper bead welding technique, is newly proposed for recovering the γ phase fraction in the weld just as in the as-welded situation.