Study On The Mechanism Of The Variation Of Microstructure And Properties In The Repair Welding Zone Of ZG06Cr13Ni4Mo Stainless Steel

ZG06Cr13Ni4Mo is a kind of low carbon martensitic stainless steel.This steel is widely used in hydroelectric power generation,nuclear pump unit impeller and offshore drilling platform.Because large castings are easy to form cracks,inclusions,shrinkage holes,porosities,and other defects during the casting process,it is necessary to repair the casting defects with repair welding technology,so as to ensure the performance and quality of castings and meet the requirements of safe use.In this paper,once repair welding and twice repair welding of ZG06Cr13Ni4 Mo stainless steel were carried out by GTAW.The microstructure and mechanical properties of the original casting and once repair welding and twice repair welded joints were tested.The repair welding effects in the actual production and manufacturing process were simulated.In order to explore the evolution of welded joint structure during repair welding,the microstructure,grain size,crystal defects and grain growth direction of welded joints with different repair welding times were observed by optical microscope,X-ray diffractometer,field emission scanning electron microscope and transmission electron microscope.The hardness,tensile properties,impact toughness,residual stress and fatigue properties of original casting,once repair welding and twice repair welded joints were tested so as to obtain the influence of repair welding times on mechanical properties of welded joint.Besides that,the effects of deep cryogenic treatment on microstructure and mechanical properties of original casting and once repair welding joint were studied to provide technical support for the technique of ZG06Cr13Ni4 Mo repair welding.The results show that:(1)After once repair welding,the microstructure of the welding joint is no longer like that of the original casting which is dominated by lath martensite.The dislocation density and texture concentration density of once repair welding joint are reduced.More grain boundaries and sub-grain boundaries can be observed in once repair welding joint,and the grain growth is no longer obvious directionality.After twice repair welding,the texture of the heat affected zone is the same as that of once repair welding.Dislocations and twins exist in the crystal,and the texture concentration density continues to decrease.With the increase of repair welding times,the grain structure is refined.(2)After repair welding,the hardness and fatigue limit strength of the welded joint are all increased.But the increase of tensile strength and yield strength is not obvious.The residual compressive stress of twice repair welded joint is lower than that of once repair welded joint and the plasticity and toughness are improved.(3)With the increase of cryogenic time,the grain size of the original casting and once repair welding samples are gradually refined,and the residual compressive stress value is increased.The lath martensite in the original casting changes to needle-like martensite,the content of residual austenite is gradually reduced.The tensile strength and yield strength are significantly increased when the original casting is treated for 18 h in deep cryogenic environment.The strength of once repair welding sample is significantly reduced after cryogenic for 18 h,and the yield strength is close to the critical value when it is treated for 36 h.The plastic index of the original casting and once repair welding joint have been reduced to the lowest value required by the standard when they are treated for 36 h.They cannot meet the normal use requirements.