Effect of cooling rate on microstructural evolution and hardness of self-shielded arc weld deposits containing 1 wt% aluminium
The self-shielded arc welding is based on the presence of flux elements that protect the weld from oxygen and nitrogen. The hardfacing alloy investigated contained about 1 wt% Al to prevent weld porosity by combining with oxygen and nitrogen. Thermodynamic calculations of the phase equilibria for the alloy predicted a weld metal microstructure consisting of δ-ferrite and austenite at high temperatures. Since the alloy had a high hardenability, the austenite transformed to martensite on cooling to provide a hardfacing deposit microstructure that is resistant to metal-to-metal wear. Nevertheless, the high Al content ensured that δ-ferrite was present as the minor microstructural component in all of weld deposits examined. The cooling rate after welding is a key variable that influences the volume fractions of δ-ferrite and martensite in the weld deposit. Dilatometric studies reported in this paper show that increasing the cooling rate of samples subjected to a weld thermal cycle designed to simulate the effect of pre-heat resulted in increases in volume percentage martensite, the magnitude of the transformation volume change and the MS temperature. Using a pre-heat temperature higher than MS resulted in the isothermal formation of bainite, as well as martensite on subsequent cooling. Despite the multiphase microstructure of bainite, martensite and δ-ferrite, the weldment hardness was considerably more uniform than for deposits produced using lower temperature, conventional, pre-heats. In general, deposition at the higher temperature served the purpose of normalising the hardness by reducing differences in dislocation density and carbide precipitation throughout the various regions of the weld deposit.