Microstructure And Properties Of Butt Welding Joints Of High Strength Chain

The ring chain has special transmission mode and great carrying capacity,so it is widely used in mining machinery,marine ships,military vessels and other fields.The size and performance of the chain need to be further improved due to the terrible working environment and the background of development.The study on the correlation among the welding technology,microstructure and properties of joints will provide theoretical basis and practical guidance for improving the quality of chain.In this paper,20Mn2,SAE8620H and 23MnNiMoCr54 steels commonly used in the ring chain were welded by flash butt welding and resistance butt welding.Quenching and tempering were carried out after welding.Then the microstructure and element distribution of the joints were analyzed by OM,SEM,EDS and XRD.The mechanical properties of the joints were tested by microhardness tester and tensile tester.The evolution of the microstructure of the joints in the process of welding and heat treatment was investigated,and the correlation among technology,microstructure and properties was analyzed.At present,resistance butt welding is mainly applicable to chain under Φ18mm and the larger size chain(≥Φ18mm)will use flash butt welding.The strip structure produced by rolling can be observed in all investigative steels.There are bright lines in the center of joints of high strength ring chain welded by flash butt welding.The microstructure of SAE8620H is proeutectoid ferrite and granular bainite in this area.In the heat affected zone,the proeutectoid ferrite is reticulated and gradually refined as it approaches the base metal.The bright area of 23MnNiMoCr54 is mainly composed of granular bainite,which has coarser bainite ferrite than that of HAZ.The lath martensite structure affected by the distribution of carbon element was formed in the original banded structure.The contact resistance decreases rapidly after the resistance butt welding begins,and the heat generated by self resistance is relatively high.The difference is that the heat production of contact resistance is relatively high during flash butt welding.The numerical analysis of the temperature field of the resistance butt welding joint showed that the clamping position of the electrode can affect the distribution of the electric field in the ring chain,causing a funnel-shaped heat affected zone.It has no significant effect on weld by the process.The microstructure of the weld of 20Mn2 steel is ultra-low carbon bainite.The influence of process on heat affected zone is obvious.More carbides precipitated from bainite ferrite during the resistance butt welding leading to the banded structure which was more continuous.Resistance butt welding has a smaller hardness gradient in the transition range between the weld and the heat affected zone.The welding microstructure being similar to that after heat treatment of 20Mn2 steel,so it is hard to be strengthened by heat treatment due to the element burned loss during butt welding.The intergranular quenching crack would initiate and expand after quenching.On the contrary,the joint of 23MnNiMoCr54 steel enhanced greatly by heat treatment.The microstructure was transformed into tempered martensite and tempered troostite with the increase of microhardness from 280HV0.5 to 380HVo.s.XRD diffraction results showed that the lattice constants were larger after heat treatment because more carbon atoms and alloying elements were dissolved in solid solution,which effectively improved its strength.