Analysis of the impact of isothermal hardening on the mechanical properties and abrasion resistance of welded joints on Hardox Extreme steel

The object of the research reported in this study was a welded joint of Hardox Extreme steel, made using submerged arc welding (SAW) and subjected to thermal treatments involving isothermal hardening in various temperature–time variants. This treatment serves as an alternative to conventional hardening, enabling the achievement of high mechanical indices in selected cases due to the formation of fine-lath martensite or lower bainite microstructures. Heat-treated joints were analyzed macro- and microstructurally using stereoscopic, light (LM), and scanning electron microscopy (SEM). The study also determined selected mechanical properties, such as hardness, tensile strength, relative elongation, and reduction of area at break, as well as impact toughness at ambient and reduced temperatures. A separate section was dedicated to characterizing abrasion resistance in the presence of loose abrasive, along with determining the relationship between this parameter and the identified mechanical characteristics. Based on the analysis of the microphotographic images obtained, the main wear mechanisms were also characterized. The analysis of the results allowed the conclusion that in the case of isothermal hardening, the factor determining the obtained microstructural and mechanical properties is the temperature of the performed thermal operations. Furthermore, after the conducted thermal treatments, the parameters characterizing the ductility of the welded joint improved by several percent compared to the state immediately after welding. Therefore, the main goal of the technological operations conducted on welded joints of high-strength steels can be defined as improving ductility, which is justified in applications considering alternatives even to structural steels.