The Formation Rule Of Microstructure And Control Of Hardness Of Cast Iron Welded Zone

Cast iron has good mechanical properties and low cost.So it has been widely used in chemical metallurgy,mechanical manufacture and other fields.Welding repair for defective castings is an important measure for increasing capital and energy-saving of enterprises.The weldability of cast iron is poor,the main problem is that the welding area is easy to produce white cast iron,hardening structure and crack.The microstructures of micro-alloying cast iron electrode is the preferred material for high quality repair of cast iron.Because of the existence of duality of phase transformation in cast iron welding area.The formulation of welding process,especially the control of cooling speed is particularly important.In this paper,according to the transformation characteristics of cast iron solidification and continuous cooling,it is determined the critical cooling rate in the solidification process of the molten pool does not produce white iron and the heat-affected zone does not produce hardening structure.Then,the 3D finite difference method is used to establish the temperature field model of are welding for gray and nodular iron.The cooling rate of the cast iron welding area under different technological conditions is calculated by means of the control variable method.And in this way,to make the welding process.Experimental study on the formation rule of microstructure and the control measures of hardness in welding area.It has been explored to the inherent relationship between the preheating temperature,welding current and the thickness,welding time of obtain the homogeneous welded seam of cast iron.The results show that:Using the microstructures of micro-alloying cast iron core to repair the HT250 and QT450-10 defects,the critical cooling rate of the molten pool no white iron is 37.7?/s and 21.5?/s.The critical cooling time t8/5 of the superheated area no hardening structure is 30s and 47s.Considering the changes of material thermal and physical properties and the latent heat of crystallization,the simulation results of gray and ductile iron arc welding temperature field agree well with the measured values.The effect of process parameters and conditions on the cooling rate of the welding area is that The large thickness of the welding piece is,the faster the cooling rate is.And the cooling rate will be reduced with the increase of welding current,preheating temperature and welding time.Through the welding tests of gray iron and ductile iron specimens with different thickness and welding process.The different welding zones have been obtained.Under the condition of 300? preheating temperature and 200A welding current to continuous welding for 30s,the gray iron weldment with thickness of 20mm can obtain the microstructure of pearlite matrix of welded zone.The hardness of weld seam and fusion zone is 238HBW and 260HBW in respective.On this basis,enlarge the preheating temperature to 400?,or increased the welding current to 240A,or extend the welding time to 45s can make the content of ferrite in welding area rised.The hardness of weld decreased to 198HBW?210HBW.Use the process of slow cooling at 300? after welding,it takes that the proportion of ferrite in the matrix is up to 45%,and the hardness of the fusion zone is reduced to 214HBW,which is basically consistent with base metal.Ductile iron welding is more inclined to be white iron.Under the condition of 400? preheating tempurature and 240A welding current to continuous welding for 30s,the ductile iron weldment with thickness of 20mm is still exist the chill of white iron.And the hardness of the fusion zone is up to 300HBW.Continue to enlarge the preheating temperature to 550?,or extend the welding time to 45s,the white iron of welding area disappeared,and the hardness of the fusion zone is reduced to 250HBW?260HBW.To hold temperature and slow cooling over 550? after welding can gain the matrix structure with more than 90%content of ferrite.The increase of welding current can reduce the preheating temperature of the melting zone without the appearance of white iron.Using the larger diameter electrode work in with the process of continuous welding large current has a good returns effect on the controlling of the white iron and hardened phase in welded zone.For homogeneous welding of gray cast iron,if the amount of deposited metal reaches a certain level,arc action time exceeds to 45s,and the thickness of weldment is less than 15mm,using cold welding method can realized the welded zone without white iron.If the thickness is greater than 20mm,it need to preheating the temperature to 200?250? before the welding.Nodular cast iron cannot to achieve the homogeneous repair in cold welding.So the weldment is used to preheat properly.When the thickness is greater than 30mm,it need to preheating the temperature to 500? before the welding.The thickness of weldment reduced per 1mm,the preheating temperature can be reduced by 20?.