Design Of Homogeneous Welding Process For Cast Iron

Do to the eutectic solidification of cast iron has duality,the homogeneous weld zone of gray cast iron is prone to white iron and hardened structure.In production,thermal welding with preheating temperature of 600? to 700? and semi-thermal welding of preheating at 350? to 400? are often used to repairing castings.Most of people usually use experience to determine the preheating temperature,which often results in insufficient preheating temperature or waste of energy.Based on the calculation of the critical cooling rate of phase transition of cast iron,this paper uses the finite difference method to establish the cast iron welding temperature field model,visually simulates the cast iron welding thermal process,the mothed of combining numerical simulation of temper field and welding experiments is used to analyze the influence of welding process and main parameters on the cooling rate of welding zone;determine and optimize welding thermal process parameters,in order to achieve the purpose of saving casting remanufacturing costs.The calculation results show that the critical cooling rate Vc for HT250 without white iron is 66.1?/s,the critical cooling time t8/5 for no hardened structure is 30s;the critical cooling rate Vc for QT450-10 without white iron is 41.2?/s,the critical cooling time t8/5 for no hardened structure is 47s.The results of simulation and numerical analysis show as follow:the position of largest solidification cooling rate in the welding zone is located at the fusion zone,and the position of largest cooling rate of continuous solid phase transformation is located at the overheating zone.Welding plate thickness,preheating temperature,welding current and post-heating temperature all have a greater impact on the welding temperature field of cast iron.The larger the weldment plate thickness,the faster the welding zone will cool.By increasing the welding current,the preheating temperature and increasing the prefabricated defects size of the weldment can effectively reduce the cooling rate of the weld zone.For thin-walled gray cast irons with a plate thickness of less than 15mm,if the welding time is longer than 35s,the cooling rate of the fusion zone can be less than the critical cooling rate without preheating before welding;for gray cast irons with a thickness of more than 30mm,the welding time is greater than 35s,and it needs to be preheated to 250? to avoid the production of white iron.Ductile iron castings cannot be repaired by homogenous welding under cold welding conditions.The parts of ductile iron with a thickness of less than 15mm need to be preheated above 150? and the welding duration is greater than 35s;ductile iron castings with a thickness greater than or equal to 30mm need to be heated above 480?,white iron and hardened structure can be avoided.The test results show that for the gray iron HT250 weldment with a thickness of 20mm and a prefabricated conical defect size of ?20mm×8mm,continuous welding with a 200?preheating temperature and 215 A current welding process can avoid the white iron and hardened structure in the welding zone;Increase the preheating temperature to 300?,or increase the welding current to 240A,or increase the size of the weldment prefabrication defect size to?24mm×8mm,which can increase the ferrite content of the welding zone and decrease the hardness;air cooling cannot avoid the appearance of martensite in the fusion zone,with the slow cooling of the furnace at 200? after welding,the ferrite content in the matrix can be increased and reduced the hardness of the fusion zone to 246HBW.The white iron tendency of ductile iron is greater than that of gray cast iron.The ductile iron of QT450-10 with a thickness of 20mm and a weldment prefabricated defect size of ?20mm×8mm will still produce ledeburite under preheating temperature at 400? and welding current at 250A.The zone hardness is as high as 300HBW.Continue to increase the preheating temperature to 550?,the white iron in the welding zone basically disappears,and the hardness of the fusion zone reduce to 253HBW;with the increase of the slow cooling temperature after welding,the ferrite content in the heat-affected zone increases.