| At present,the middle trough of a scraper conveyor is often welded by wear resistant steel and ZG30SiMn through semi-automatic C02 welding or MAG welding.However,due to the poor weldability of the medium-thick plate wear resistant steel and ZG30SiMn resulting from dissimilar steel welding,there are disadvantages such as low production efficiency and unstable quality in actual production.Robot welding has the advantages of high production efficiency and stable welding quality.Its application in the middle slot welding can better improve the above problems,becoming the development trend of coal mine machines enterprises at home and abroad.In this dissertation,common wear resistant steels of Hardox450,NM450,JFE-EH400 and NM400 were used as base metal of middle trough,and the welding wires of GHS-70 and SM700 were selected through comparative analysis.The difference between welding process controlled by technician and general welder was analyzed by artificial welding,robot welding,microstructure analysis,ultrasonic nondestructive testing and mechanical property testing.Then,the robot MAG welding process was optimized.Finally,based on the established welding process,the microstructure and properties of different welded joints of wear resistant steel/ZG30SiMn under the same conditions were studied.At the same time,the feasibility of the robot instead of the artificial welding of the middle trough was evaluated.The results showed that under the condition of 200℃ preheating temperature,interlayer temperature above 100℃,and slow cooling after welding,the welded joint of Hardox450/ZG30SiMn adopted GHS-70 wire obtained by welding technician had a tensile strength greater than 700MPa and a impact energy at-20℃ more than 24J,in line with the requirements of the welded joint performance evaluation indicators.Welding technician had better welding quality than general welder.The welding seam obtained by general welder has a relatively high occurrence probability of pores,cracks,welds,and incomplete welding defects.In comparison,the welded joints obtained by welding technician had more layers than that obtained by general welder.The number of welding layers by robot was between welding technician and general welder.Compared with the welding technician,the welding current of central trough by robot MAG welding was larger when the bottom welding is performed.When performing filling and cap welding,the welding current of central trough by robot MAG welding welding was lower.With GHS-70 welding wire,the welded joint of Hardox450/ZG30SiMn obtained by robot welding had the same or higher quality and performance than welding technician’s welded joint.The welding HAZ of wear resistant steel/ZG30SiMn consisted of quenching zone,incomplete quenching zone and tempering zone.The phases compositions of quenching zone were mainly:martensite or bainite structure.The incomplete quenching zone was mainly composed of martensite or bainite and ferrite mixed structure.The tempering zone mainly had a mixed structure of ferrite and pearl ite.The HAZ microhardness of the wear resistant steel showed obvious softening with a lower hardness level.With two kinds of welding wire,four kinds of steel plates of JFE-EH400/ZG30SiMn,NM400/ZG30SiMn,Hardox450/ZG30SiMn and NM450/ZG30SiMn formed eight different welded joints.The impact energy of theirs-20℃ was greater than 24J,in line with the performance evaluation criteria of middle groove joint.The impact fracture at wear resistant steel HAZ and weld seam was ductile fractures with good crack resistance.The impact fracture at ZG30SiMn HAZ was brittle fracture with poor crack resistance.The impact energy at-20℃ of weld seam obtained by GHS-70 wire was higher than that obtained by SM700 wire.In summary,it was feasible for the robot welding to replace the artificial welding of middle trough. |
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