Study On Microstructure And High Cycle Fatigue Of Welded Joint For45#Steel Ingot

Large forgings are the key components of kinds of large machinery and equipment, due to ingot quality, forging process and other factors, cracks will appear in the forging process. If the depth of the crack is large, but through the shovel and grinding can be removed, defects should be cleared after forging trimming, if cracks are deep and can not be repaired, generally forgings are scrapped. Therefore, the research in repair method of deep cracks has some theoretical and practical significance for practical production.In this paper, using0300mm45#steel ingot simulate surface ciseks repair of large forgings, research on microstructure, mechanical properties and high cycle fatigue performance of MAG welded joint of steel ingot under postwelding upsetting and postwelding upsetting+hardening and tempering process.Through microstructure analysis of the welded joint, the study shows that:the upsetting treatment of the welded joint is performed, weld organizations are equiaxed, grain of heat affected zone is coarse; the hardening and tempering treatment of the upsetting welded joint is performed, weld zone appears sorbite organizations, heat affected zone has been refined.The study about the mechanical properties of the original base metal and welded joint manifest that:For the original base metal, the tensile strength is580MPa, the yield strength is281MPa, reduction is39.5%, the hardness is HV199.6; For the upsetting sate welded joint, the tensile strength is527MPa, the yield strength is307MPa, reduction is45.8%, the weld hardness is HV158.9,the HAZ hardness is HV218.2,the base metal hardness is HV249.3; For the original base metal, the tensile strength is545MPa, the yield strength is318MPa, reduction is56.8%, the weld hardness is HV175.5,the HAZ hardness is HV264.8,the base metal hardness is HV284.5.The study about the fatigue properties of the original base metal and welded joint manifest that:In the2×106cycles, corresponding to the50%survival rate conditions, the fatigue strength of the upsetting weld specimen is197.2MPa, relative to the original base metal, the fatigue strength of the upsetting welded joint is reduced by17.6%; the fatigue strength of the upsetting+hardening and tempering welded joint is259.7MPa, relative to the original base metal, the fatigue strength of the upsetting+hardening and tempering welded joint is improved by8.6%, relative to the upsetting welded joint, the fatigue strength of the upsetting+hardening and tempering welded joint is improved by31.7%. So, hardening and tempering treatment can improve the fatigue strength of the upsetting welded joint.Fracture analysis manifests that:fatigue fracture of two kinds of welded joint are characteristics with cleavage, the fatigue crack initiates from the sample surface; fatigue crack propagation mainly composes of fatigue striation and tearing edges, local area can be observed second crack; the fracture morphology of transient fault zone has a characteristic of dimple.