Microstructure and Mechanical Properties of MIG Welds between 6252 Armor Steel and Q550D HSLA Steel

The microstructure and mechanical properties of the welded joints of 6252 armor steel and Q550D high-strength low-alloy (HSLA) steel welded by MIG welding were studied. ER70S-G and ER140S-G were used as fillers to obtain welded joints with good formation and no faults. The joint microstructure (OM) analysis showed that a large Widmanstatten structure was observed at the fusion line on the Q550D side, and the apparent grain sizes changed on the 6252 side. Cylindrical ferrite growth along the bainite grain boundary was observed in the ER70S-G filler weld zone, while the ER140S-G filler weld zone was occupied by lower bainite structures. The XRD phase analysis showed that more Fe-Ni-Cr compounds and less ferrite were formed in the ER140S-G filler weld. The hardness test showed that the hardness of the HAZ on the 6252 side was significantly higher than that of the BM and the WZ, and the welded joint obtained by the ER140S-G filler had a higher hardness. The tensile strength test showed that WZ (>772 MPa) had a higher strength than Q550D BM, and the tensile fracture (SEM) was primarily a ductile fracture. The impact test results showed that the welded joint had better impact resistance at room temperature, but the impact absorption energy of the weld and the heat-affected zone was strongly affected by changes in temperature, and brittle fracture occurred easily at low temperatures.

Automated summary

The microstructure and mechanical properties of welded joints between 6252 armor steel and Q550D high-strength low-alloy steel were investigated. The use of ER70S-G and ER140S-G fillers resulted in well-formed and defect-free welded joints. Microstructural analysis revealed the presence of a large Widmanstatten structure at the fusion line on the Q550D side, with apparent grain size changes on the 6252 side. Cylindrical ferrite growth along the bainite grain boundary was observed in the ER70S-G filler weld zone, while the ER140S-G filler weld zone exhibited lower bainite structures. XRD phase analysis showed the formation of more Fe-Ni-Cr compounds and less ferrite in the ER140S-G filler weld. Hardness testing indicated significantly higher hardness in the heat-affected zone on the 6252 side, with the ER140S-G filler resulting in higher hardness in the welded joint. Tensile strength testing demonstrated higher strength in the weld zone compared to the Q550D base material, with ductile fracture being the primary mode of failure in the tensile fracture (SEM) analysis. Impact testing showed improved impact resistance in the welded joint at room temperature, but the impact absorption energy in the weld and heat-affected zone was highly influenced by temperature changes, with brittle fracture occurring easily at low temperatures.