A study of the weld heat affected zone of carbon manganese microalloyed steel

Abstract

The toughness and the microstructure of the weld Heat Affected Zone (HAZ) of a Quenched and Tempered High Strength Low Alloy Steel containing microadditions of niobium and vanadium, have been investigated by Charpy Impact tests and Metallographic examination. The real HAZs were obtained from bead-on-plate deposits made by Submerged Arc Welding at 3.6 and 6.3KJ/mm. These experiments provided toughness, microstructural and welding thermal cycle data. Welding Simulation, combined with Dilation techniques, were employed for the majority of the investigation. This involved toughness, microstructural and phase transformation data corresponding to heat inputs equivalent to cooling times from 800 to soo0c of 25, 90 and 220s respectively. Simulation thermal cycles were mainly obtained from finite element heat flow calculation. The nominal peak temperatures tested ranged from 700 to 1400°c, thus providing detailed information for all the regions of the HAZ. A variation of the HAZ toughness relative to the base metal was confirmed. An improvement of the toughness with the increase in the peak temperature up to 1000°c was found, and it was attributed to a refinement of the prior austenite grain size. Deterioration in toughness was observed at peak temperatures ).11so0c. This embrittlement increased with the increase in heat input and was associated with the presence of proeutectoid ferrite and upper bainite. No precipitation of microalloys was evident but the practice of Post Weld Heat Treatment provoked further embrittlement in the tested samples. The nature of the transformed microstructures was considered to be the major embrittling mechanism of the HAZ, and an experimental CCT diagram was produced for predicting the weld HAZ microstructures in the coarse grained region.