Study of the microstructure and mechanical property relationships of gas metal arc welded dissimilar Hardox 450 and S355J2C+N steel joints

Abstract

This research reports the investigation of the dissimilar welding of Hardox 450 and S355J2C+N steels, which have two different superior properties, by the gas metal arc welding (GMAW) method in terms of microstructure and mechanical properties. Experiments were performed using mechanical tests such as Vickers microhardness (HV 0.1), bending, Charpy V-Notch impact, wear, and tensile test. Optical microscopy (OM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) analyzes have been implemented. A variation in the welded joint microstructure was observed, which affect the mechanical properties. Grain size distribution in area fraction was determined from the EBSD maps of S355J2C+N steel, welded zone, heat affected zone (HAZ) and Hardox 450 steel. The hardness value was measured 311 ± 8 HV for Hardox 450 base material. While the hardness of the S355J2C+N base metal was ∼187 ± 8 HV, the hardness of the weld center was measured as 249 ± 8 HV. Mechanical strength analysis revealed that the yield strengths of base metals were 513 ± 8 MPa and 1464 ± 8 MPa for S355J2C+N and Hardox 450, respectively. The highest tensile strength of the sample extracted from the longitudinal direction of the welded joint is 899 ± 8 MPa. The highest tensile strength of the transverse sample, which was subjected to the tensile tests, was determined as 699 ± 7 MPa and it fractured from the S355J2C+N steel which has less strength. The joint efficiency was determined as ∼133%. The weld zone has sufficient toughness and the change in toughness values in the HAZ is small. The grain size increased from martensitic Hardox 450 steel region to S355J2C+N region, in addition to this, the hardness decreased with the effect of transition from martensitic structure region to ferritic structure region. The welding zone is compatible with two different materials and has a high plastic deformation capability. With the welding procedure followed in the study, Hardox 450 and S355J2C+N dissimilar materials were successfully combined, and it was determined that this welded joint was scientifically and industrially applicable.