Research On Welding Repair Technology Of Casting Die Steel

Materials A and B are casting die steels, applied to automobile panel dies.Content of alloy elements in the two materials, such as Cr, Mo and V, is high. Theycan be quenched in air. One hand, in processing of automobile panel dies, there maybe some defects such as subcutaneous blowholes; another hand, there are failureproblems such as wear-out in applications of automobile panel dies. In order to reducecost, we can use weld to solve these problems.Materials A and B were welded by arc welding process. In the lab, we weldedtest steel plates with V groove using different currents. Then, those weld joints wereheat-treated by appropriate processes. Microstructure and mechanical properties anddry tribological property of weld joints were studied. According to microstructure andproperties of weld joints, we got the best current and the applicability of base metalheat treatment process.The tests found that, through welding currents of70-90A,90-110A,110-140A,current of70-90A has poor weld formability for materials A and B. There is lots ofslag in weld metal. Another two currents have good weld formability for materials Aand B.According to the experimental data, we got some of the conclusions.(1) Afterquenching and tempering treatment, test steel plates of material A were welded. Thepost-weld heat treatment of tempering at500℃for2hours,can improve themicrostructure of weld joint and enhance the strength of weld joint, also can increasewear resistance of the weld metal. When welding current is90-110A, strength of theweld joint is highest, and wear resistance of the weld metal is best. After temperingtreatment, the strength of weld joint is higher than700MPa, over60%of material A.Its fracture mechanism is quasi cleavage, same as base metal. If friction distance iscertain, the wear resistance of weld metal is much better than base metal when weincrease load or speed. The wear mechanism of weld metal is mainly abrasive wear,accompanying adhesive wear, same as base metal. Current of90-110A is the best.(2) When material A is on annealed state, we welded the test steel plates. After post-weldheat treatment of air quenching at1040℃for30min and tempering at500℃for2hours, microstructures of base metal and former HAZ are the same. Also, thehardnesses of base metal and weld metal are the same, about HRC60. When weldingcurrent is110-140A, strength of the weld joint is higher than800MPa, about70%-80%of material A. Fracture mechanism of the weld joint is quasi cleavage, sameas base metal. The influence of welding current on the wear resistance of weld metalis not obvious. After post-weld heat treatment, if glide distance is certain, wearresistance of the weld metal is just slightly higher than base metal when F=100N,V=1.0m/s, but if we increase load or speed, the wear resistance of weld metal is muchbetter than base metal. The wear mechanism of weld metal is abrasive wear andadhesive wear, same as base metal. Current of110-140A is the best.(3) Whenmaterial A is on as-cast state, we welded the test steel plates. After post-weld heattreatment of annealing at860℃for2hours then at760℃for4hours, we found lotsof reheat cracking in former quenching zone. There are two factors causing reheatcracking. One factor is that the content of alloy elements is high, another one is thatannealing process is in the sensitive temperature range. Annealing process of basemetal can’t be applied to the heat treatment of weld joint.For material B, test steel plates were welded after quenching and tempering. Aswelded, the hardness of weld metal is the same as base metal, about HRC58-62.Hardness of tempering zone is the lowest in weld joint. After tempering at520℃for2hours, the hardness of weld metal is reduced, about HRC55, which can’t meet theperformance requirements. But strength of weld joint is improved after tempering.When welding current is90-110A, the strength of weld joint is higher than600MPa,about55%of material B. Fracture mechanism of the weld joint is quasi cleavage,same as base metal. The influences of tempering treatment and welding current on thewear resistance of weld metal are not obvious. If glide distance is certain, wearresistance of the weld metal is much better than base metal when we increase load orspeed. Wear mechanism of weld metal is mainly abrasive wear, accompanyingadhesive wear, same as base metal. Current of90-110A is the best. Tempering process of base metal can’t be used to the heat treatment of weld joint.