Hg70 Low Alloy High Strength Steel Welding Research

In the present investigation, welding wires HS-70 were chosen according to "low strength match" and the welding procedure of HG70 high strength low alloy steel was instituted. Calculations of carbon equivalents, cold crack sensitivity indexes, implant tests and Y-groove cracking tests were performed to evaluate the cold crack sensitivity of HG70 high strength low alloy steel. Weld tests were conducted on condition of no preheat, preheat at 100℃, and preheat at 150℃using CO₂ gas shielded arc welding. The microstructures and mechanical properties of welded splices obtained in the three types of welding procedures above-mentioned were analyzed. The phase compositions, cell parameters and micro-strain of different zones of the welded splices were analyzed by X-ray diffraction analysis.The results showed that welding wires HS-70 and mother metals HG70 steel matched well. HG70 steel plates with a thickness of 10mm had some hardenability and cold crack sensitivity, thus low temperature preheating should to conduct before weld. Preheating could heighten the critical fracture stress and lower the sensitivity of the cold crack. The preheating temperature should be controlled between 80 to 120℃. HAZ of CO₂ gas shielded arc welding welded splice was the weak part, but its intensity and toughness indexes could also meet the corporation manufacturing requirement. The microstructure of weld seam, HAZ coarse grain zone, HAZ refined zone were respectively composed of pro-eutectoid ferrite and acicular ferrite, lath martensite and granular bainite, tempered sorbite, which guaranteed the high strength and good ductility of HG70 steel. With the increase of preheating temperature, the amount of lath martensite in HAZ coarse grain zone reduced and the granular bainite increased, as a result the hardness, the trend of harden quenching of the welded splices decreased and the plasticity, ductility increased, but the tensile strength remained the same. The results of XRD showed that with preheating 100℃the magnitude of lattice distortion in different zones of the welded splice was small and well-proportioned. Preheating made the micro-strain in different zones of the welded splices tended to homogeneous, which accordingly might reduce the residual internal stress.