Simulations of Transformation Kinetics in a Multi-Pass Weld

Abstract

This paper discusses the thermal welding cycle's influence on microstructural changes in the weld metal. We examined the kinetics of austenite formation by the application of simulated microstructures. Special attention was dedicated to the thermal welding cycle's influence on the formation of microstructures with very low toughness, which could be potential triggers of brittle fracture. The simulated microstructures were prepared by the application of several simulated thermal cycles with different peak temperatures, on samples of real single-pass weld metal. The results of this research show that martensite-austenite constituents form in the overcritical temperature regions (over 950°C), too. The dissolution of carbides (particularly cementite) occurs at temperatures much higher than their expected solubility temperature, particularly over a short heating time. The nucleation rate of austenite in ferrite is not very rapid, in spite of the great amount of interfacial area on which nucleation can occur. The surface energies associated with austenite, ferrite, and carbide are not favorable for nucleation. Nucleation occurred at high angle boundaries where the surface energies are more favorable. The rate of austenite formation is controlled by the rate of growth of the austenite grains. The kinetics of austenite transformation in the weld metal are slower compared to the kinetics of austenite transformation in the heat affected zone. The weld metal is very stable and less sensitive to the thermal influence of the subsequent weld passes. The simulation of kinetics of austenite transformation in the weld metal enables the determination optimal of welding parameters, particularly the cooling rate from 800°C to 500°C.