The effect of preheat on the structure and properties of the HAZ of a welded quenched and tempered steel plate

In the present research work the effect of preheat temperature (20°C, 80°C, 120°C, 2(X)°C) on cooling rate and consequently on the HAZ microstructures of bead - on - plate submerged arc welds have been investigated for quenched and tempered EM 812 plate steel. Due to the development of a microstructural gradient across the HAZ, it is difficult to carry out Charpy impact toughness testing on a particular structural region of the HAZ. Weld thermal cycle simulation has been conducted in the current work to reproduce in a bulk form similar microstructures to those in selected sub - regions of the actual weld HAZ. Hardness measurements were carried out for both actual and simulated welds and Charpy impact values were determined for the weld simulation samples. Microstructural comparison of the actual and simulated HAZs showed that simulation was successful. The grain coarsened heat affected zone( GCHAZ ) for both actual and simulated welds under pre - heat free conditions consisted of lath martensite or bainitic ferrite with martensite austenite ( MA ) islands, because of the relatively low ( 2.58 kJ / mm ) heat - input, and the high hardenabihty of the EM 812 steel. Despite the structural similarity, the prior austenite grain size in the simulated GCHAZ region was about double that in the actual welds. It was also found that increasing the preheat temperature from 20°C to 200°C caused a reduction in cooling rate, increased the austenitic transformation temperature and resulted in less lath shaped ferrite. Hardness traverses across the root of the HAZ in the actual welds and along the centre line of the simulated HAZs showed that the HAZ was generally harder than the base plate. Maximum hardness was observed in the grain refined heat affected zone (GRHAZ) instead of the GCHAZ due to transformation of both these regions to similar martensitic / bainitic microstructures and existence of much finer microstructure in GRHAZ. Minimum hardness was observed in the base metal due to over - tempering of the quenched and tempered base plate by coarsening of carbide and recovery of ferrite. The simulated GCHAZ showed the lowest toughness, with the loss of impact toughness being due to large prior austenite grain size which is resulted in a coarse microstructure and high hardness. Increase in the preheat temperature caused reduction in cooling rate with decrease in CVN value, mainly because of formation of a large volume fraction of coarse bainitic product in the GCHAZ. On the other hand, the maximum impact toughness was observed either in the grain refined region, or at the boundary of the partially transformed and grain refined region, which also exhibited the maximum hardness. Both of these effects are due to the structural refinement in this region. The investigation showed that for a heat input of 2.58 kJ / mm the best HAZ toughness is achieved for zero or low preheat, for conditions where hydrogen pick up is not significant.