Influence of Cooling Rates on Microstructure and Mechanical Performance of High-Strength Steel SMAW Joints for High-Pressure Applications

Abstract

This manuscript intends to carry out a comprehensive study on the microstructural evolution, mechanical properties and performance of DP780 dual-phase steel joints under the influence of the post-weld cooling rates during Shielded Metal Arc Welding (SMAW). DP780 has already been used for thin-gauge automotive components; in this study we evaluate the suitability of the 6 mm thick plates in high-pressure industrial applications requiring weight reduction. The cooling mode was set to three different modes including water quench (rapid), still air (moderate), and thermal insulation blankets (gradual). Specific microstructural characterization of the Fusion Zone (FZ) and Heat Affected Zone (HAZ) indicated that rapid cooling leads to a large volume fraction of lath martensite with a maximum microhardness of 350 HV and ultimate tensile strength (UTS) of 728 MPa. In comparison, moderate cooling enables a balanced phase distribution of bainite and polygonal ferrite, resulting in considerable tensile strength versus ductility trade-off, attaining a yield strength of 624 MPa. The observed gradual cooling leads to the development of a coarser ferritic-pearlitic structure, which reduces the hardness and load-bearing capacity considerably. Tensile testing also revealed high dependence upon the phase transformations due to cooling at the FZ/HAZ interface. This reinforces the importance of accurate thermal control for high-strength steel joint safety due to peak loading conditions.