Effect Of Ultrasonic Impact Treatment On Corrosion And Fatigue Properties Of 30CrMnSi Steel Welded Joint

As the main energy source in China,coal should serve the healthy development of economy and society,especially in the implementation of the"double carbon"target,ensuring the safe and efficient mining of coal mines is the basic requirement of the development of coal industry.In recent years,low alloy high strength steel has been widely used in coal mine hydraulic support due to its excellent comprehensive mechanical properties.Hydraulic support in service process is affected by cyclic load and complex environment in the mine,resulting in its failure of safety accidents often occur.In this paper,aiming at the welded joint,the weak position in the welding structure of hydraulic support,ultrasonic impact treatment method is used to improve the stress state and microstructure of the welded joint position.The influence of ultrasonic impact on the corrosion and fatigue properties of welded joints of 30Cr Mn Si steel in complex environment was studied.The main conclusions are as follows:After ultrasonic impact treatment of 30Cr Mn Si steel welded joints,the geometry of the weld toe position was changed,the stress concentration was reduced,and the welding residual tensile stress on the weld toe surface was converted into residual compressive stress.Plastic deformation layer was formed on the surface by ultrasonic impact strengthening,resulting in the occurrence of grain refinement,dislocation density and microhardness were improved.Because of the changes in the surface structure and properties after ultrasonic impact treatment,the fatigue life of welded joints of 30Cr Mn Si steel was increased about 7 times under the rated working stress of hydraulic cylinder 245MPa,and the weak fatigue position was also shifted.After ultrasonic impact treatment of 30Cr Mn Si steel welded joints and base metal,the corrosion rate in mine water was reduced,the number of surface corrosion defects was less,the capacitive arc radius and polarization resistance of electrochemical corrosion were larger,and the polarization current was reduced.Ultrasonic impact refining the surface grain will produce more dense passivation film on the surface of the material,hinder the contact between the corrosive medium such as chloride ion and the metal matrix,and improve the corrosion resistance.The analysis of the surface topography of welded joints at different corrosion times showed that the first-order exponential function can predict the dynamic law of welded joints corrosion pit depth and diameter distribution parameters.The residual compressive stress and grain refinement layer introduced by ultrasonic impact can improve the corrosion fatigue property of welded joints,and the corrosion fatigue strength corresponding to 2×10~6 cycles was increased by about51.16%.Especially at the low stress level,the specimen fractured at the base metal.This is because the compressive stress is formed at the weld toe by ultrasonic impact treatment,the tensile stress is balanced at the base metal,and the tensile stress gathers hydrogen and leads to fracture at the base metal.