Study On Crack Sensitivity Of GMAW No-preheat Welded Joint Of Excavator Bucket

Excavators play an important role in mining,construction sites and other aspects.Their tasks are complex and arduous.Bucket is a critical part of excavator which is in direct contact with excavating medium.When the excavator works,the bucket not only bears certain impact load,but also has serious wear.Xiaosong Shantui Construction Machinery Co.,Ltd.used SHT1080 high strength steel as a strengthening wear-resistant plate on the excavator to improve the performance of the bucket.The bottom plate of the bucket is made of Q460 high strength steel.Pre-welding and post-welding heat treatment are used to reduce the cold cracks of bucket,which is not only complicated in technology,but also high in production cost.This paper studies the effect of welding parameters on the cold crack sensitivity,structure and performances of overlap joints of SHT1080 and Q460 dissimilar high strength steels welded by gas metal arc welding(GMAW)under low strength matching,low temperature and no-preheating conditions.The study reveals the relationship between welding heat input,microstructure and the initiation and propagation of cold cracks in the welding joint zones,The study analyzes the fracture mechanism of overlap joints so that it avoides the cold cracks in the shovel bucket bottom plate and wear-resistant plate,and provides experimental data and theoretical support for the welding technology and application of high strength wear-resistant steel shovel bucket.The cold crack sensitivity of SHT1080/Q460 dissimilar steel welded joints under different GMAW parameters and no-preheating conditions was studied by field welding crack test of bucket.With the increase of heat input,the cold crack rate of the joint decreases first and then increases.The optimum welding heat input range for bucket welding is determined to be 13-16kJ/cm.With the increase of lap gap,the residual stress and stress concentration coefficient of the joint increase,and the cold crack sensitivity of the joint increases accordingly.Therefore,to ensure that the cold crack is not easy to appear,the lap gap should be controlled within 1 mm.Microstructure and fine structure of SHT1080/Q460 high strength steel no-preheated joints are observed by metallographic microscope and scanning electron microscopy.The composition of structure in the area around the cold crack was determined by X-ray diffractometer to study the influence of structure distribution and element composition on cold crack sensitivity.With the increase of heat input,the content of proeutectoid ferrite and Side lath ferrite in the weld structure first decreases and then increases,while the content of acicular ferrite first increases and then decreases.The proeutectoid ferrite and Side lath ferrite are easy to weaken the crack resistance of joints due to their small angle grain boundaries,while acicular ferrite can improve the toughness of joints.The increase of heat input also leads to the coarsening of proeutectoid ferrite,side-lath ferrite and martensite on SHT1080 side and Q460 side of joints,and then the strip-like proeutectoid ferrite is transformed into blocks.With the increase of heat input,the island M-A structure which seriously destroy the toughness of the joints are possibly formed.The ferrite band also becomes wider when the temperature increases.The wider the hardened zone in the fusion zone of SHT1080 side,the easier to induce cold cracks.Therefore,it is necessary to ensure that the welding heat input of SHT1080/Q460 dissimilar high strength steel is not too large.It was found that the distribution of sulfide inclusions of microstructure elements around the cold crack was too dense,which was the main reason for the cold crack initiation.The hardness of the welded joints is the highest in the HAZ,and the fusion zone is easy to initiate cold cracks because it is in the transition zone of abrupt hardness change.The mechanism of high strength steel welding cold crack initiation and propagation was studied.It was found that the cold crack was relatively flat at the initiation site.The shape of the cold crack on the Q460 side of the joint judges the extended form,and there is a crack turning in the direction of the extension during the forward extension process.There will be small crack branches in the cold crack.The welding cold crack of high strength steel is formed by many single intermittent microcracks joining together.Thus,it can be judged that the process of cold crack propagation is intermittent.The cold cracks on the Q460 side of the joint initially propagate along the fusion boundary line,and then deflect into the coarse grain region and begin to propagate.Cold cracks are easy to propagate along coarse martensite lathes in coarse grain region and parallel ferrite lathes in upper bainite region.The expansion of cold cracks on the SHT1080 side of the welding joint is also ladder-like,and the cold cracks begin to propagate along the fusion boundary line,then deflect and propagate to the weld.SHT1080 side cold crack propagates into the weld and then passes through or along the pre-eutectoid ferrite.When the propagating encounters acicular ferrite,it deflects due to the increase of resistance.The test results indicate that cold cracks can be prevented under no-preheating GMAW condition by controlling welding heat input below 20kJ/cm.The crack resistance of SHT1080/Q460 high strength steel is guaranteed to meet the welding requirements of excavator bucket in low temperature environment.