Effect of cooling rate on microstructural evolution and hardness of self-shielded arc weld deposits containing 1 wt% aluminium

Authors

Milo Dumovic, University of WollongongFollow
Druce Patrick Dunne, University of WollongongFollow
John Norrish, University of WollongongFollow
Hui Jun Li, University of WollongongFollow
Brian J. Monaghan, University of WollongongFollow

RIS ID

128431

Publication Details

Dumovic, M., Dunne, D., Norrish, J., Li, H. & Monaghan, B. J. (2018). Effect of cooling rate on microstructural evolution and hardness of self-shielded arc weld deposits containing 1 wt% aluminium. Welding in the World, 62 685-697.

Abstract

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 M_S temperature. Using a pre-heat temperature higher than M_S 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.