Microstructural defects and failure mechanisms in dissimilar welds of austenitic stainless steel and carbon steel

The structural integrity of weld joints between dissimilar metals, such as austenitic stainless steel and carbon steel, is critical for ensuring the durability and performance of engineering components. This study systematically examines the microstructural evolution and mechanical behavior of these welds, with particular focus on identifying potential sources of failure across different weld regions. Continuous cooling transformation (CCT) diagrams were generated using ThermoCalc software to predict phase transformations and microstructural changes in response to welding heat inputs. Detailed microstructural analyses revealed distinct characteristics within the weld metal, heat-affected zones (HAZ) and fusion boundary. The weld metal exhibited a uniform ferrite distribution within an austenitic matrix, while the HAZ on the stainless steel side consisted of retained austenite and martensite, and the HAZ on the carbon steel side was predominantly martensitic. Optical and scanning electron microscopy (SEM) highlighted critical microcrack sources and subtle defects that may escape detection by conventional ultrasonic testing, particularly at the fusion boundary and within the carbon steel HAZ. Energy-dispersive spectroscopy (EDS) line scans confirmed the presence of deep cracks along element boundaries and identified impurities in the weld metal. Tensile testing demonstrated that fracture primarily occurred in the carbon steel HAZ, with failure patterns directly associated with microstructural features such as carbide formation and phase transformations. These findings provide comprehensive insights into the interactions between welding parameters and material behavior, offering a deeper understanding of failure mechanisms in dissimilar metal welds. The study suggests targeted strategies to enhance weld quality and reliability, contributing to the advancement of welding techniques for critical structural applications requiring high durability and performance.