Microstructures and fatigue crack growth of EH36 TMCP steel weldments

The microstructures and fatigue crack growth rates (FCGRs) of EH36 thermo-mechanical control process (TMCP) steel weldments were investigated. The small increase in hardness of the heat-affected zone (HAZ) in the as-welded condition could be attributed to the low carbon equivalent (CE) of the steel. Hence, the improved HAZ toughness of EH36 welds could be anticipated. Microstructural observations indicated that the coarse-grained HAZ was composed of mainly lower bainite with some upper bainite. Fine-grained HAZ consisted of refined ferrite and bainite, together with interlath microphases. Although the impact toughness of specimens varied significantly with orientations with respect to the rolling direction, however, only minor change in FCGRs of the TMCP steel plate had been found. The lower FCGRs of the HAZ than those of the steel plate could be attributed to the formation of low-carbon bainite with high toughness, regardless of the postweld heat treatment (PWHT). The evidence showed that the TMCP steel weld after tempering at 600°C for 2 h possessed a slightly better resistance to crack growth than the steel plate.