Study On The Fracture Analysis And Welding Repair Process Of Electric Shovel Bucket Rod In Low Temperature Condition

The main excavation and loding equipment in domestic large-scale open-pit is the electric shovel.As the key part of the working device of the electric shovel,the force of the bucket rod is complicated.Moreover,cracks and even fracture occur frequently in mining construction in high and cold areas.It is of great significance to study the welding process of bucket rod under low temperature.It can greatly improve the service life after welding and improve the economic benefit of mine.In this paper,the force and fracture of the electric bucket rod were analyzed.The weldability between the 16 Mn steel same materials and between the 16 Mn steel and ZG28 NiCrMo steel dissimilar material were studied.The influence of welding position,filling material,welding environment temperature and pre-welding heat treatment on joint structure and performance was analyzed.The microstructure and fracture morphology of the joints were observed by optical microscope(OM),scanning electron microscope(SEM)and electronic dispersive spectrometer(EDS),and the mechanical properties of the joints were tested by microhardness tester,universal tensile testing machine and impact tester.Combined with the particularity environment of the electric shovel,the low temperature impact performance of the joints was emphatically analyzed.The results show that:1)The stick is cracked under long-term fatigue loading.Improperly repaired materials and processes after welding repair lead to decreased impact properties of welded joints,especially low-temperature impact performance.The defects in the welds and the low-temperature environment further lead to the occurrence of brittle fracture at low temperatures at the welded joints.The impact of the cyclical impact load during the use of the stick is the main reason leading to the crack propagation and even fracture of the stick.2)The microstructure analysis of 16 Mn steel welded joints,16 Mn steel and ZG28 NiCrMo dissimilar steel joints shows that.The welded seam microstructures of J506 and J607 are ferrite and pearlite.The welded seam microstructures of A102 and A307 are primary austenite and austenite interdendritic worm-like ferrite.The nickel-base alloy welding rod Ni327-3 welded seam microstructure is a thin white columnar solid solution and dispersion of dianilized carbide precipitates between dendrites.The HAZ of 16 Mn steel CGHAZ organization is bainite and widmanstatten structure,FGHAZ and ICHAZ organization is pearlite and ferrite.A layer of ferrite columnar crystal zone appears in the near-fusing zone of ZG28 NiCrMo cast steel,and the HAZ microstructure is ferrite and pearlite.And there is a certain width of the single-phase austenite region in the fusion zone of the joint.3)By comparing the mechanical properties of different welding materials,the welding between the same material of the bucket rod and the welding material between the bucket rod and the bucket tooth can be unified as A307 welding rod.Welding of 16 Mn steel at low temperature for A307 welding electrode has the same room temperature impact toughness as that of J506 and J607 electrodes,but the impact toughness of low temperature at-40? is 9 times that of J506 weld and 3.6 times that of J607 weld.The weld zone has the lowest impact toughness at room temperature and low temperature,but it can reach 114.7J and 83 J respectively.The room temperature and low temperature impact energy of the HAZ and the base metal are higher than the weld zone.A307 electrode welding 16 Mn steel and ZG28 NiCrMo steel,the joints at low temperatures without preheating obtained at room temperature and low temperature impact toughness are higher than A102 and nickel-based electrodes.The room temperature and low temperature impact toughness of the 16 Mn side HAZ is 60% and 36% higher than that of the original A102 and the center of the weld is 1.9 and 2.2 times than that of the A102.The impact-toughness at room temperature and low temperature in the HAZ of the cast steel increased by 52% and 49% respectively,compared with the HAZ of cast steel in A102 welded joints.The tensile strength of the joint is about 530 MPa,and the fracture position is in the position of 16 Mn steel base metal.The hardness distribution trend is roughly the same.The hardness of the cast steel base metal is the highest at about 270 HV,followed by the weld seam.The 16 Mn steel base metal hardness is the lowest between 160~180 HV,and no hardened microstructure is generated.