Study On Microstructure And Properties Of Knuckle Made Of Grade E Cast Steel Prepared By Squeeze Casting With Multi Cavity And Multidirectional Pressurization

In the current study,a knuckle made of grade E cast steel prepared by squeeze casting with multi cavity and multidirectional pressurization was developed to solve the problems of difficult forming,casting defects,coarse grain,poor surface quality and high cost of al oy steel parts with complex shape,thick differences on wall and difficult feeding during processing in the field of rail transit.Firstly,the effects of pressure on microstructure and mechanical properties of grade E cast steel was studied.On this basis,the squeeze casting die with multi cavity and multidirectional pressurization and its process parameters were designed.The trial production of coupler knuckle prepared by squeeze casting was carried out.The effects of design dimensions and process parameters on the thermal deformation and fit clearance of shot sleeve and punch were studied.The effect of pressure on the microstructure and mechanical properties of grade E cast steel in the range of 0 to 156 MPa was investigated by experiments.The results showed that the as-cast microstructure of grade E cast steel is ferrite and pearlite,and the heat treatment microstructure is tempered troostite.With the increase of pressure,the grains of grade E cast steel were refined significantly and the content of ferrite increase.When the pressure is 156 Mpa,the secondary dendrite arm spacing(SDAS)of grade E cast steel reduced by 45.3%,and the content of ferrite increased by 39.1% compared with gravity casting.The impact toughness at-40 °C exhibited their optimal values when the pressure is 38 MPa,which is 65.4% higher than that of gravity casting.The lowtemperature impact energy of grade E cast steel under the pressure of 38 MPa is 31.79 J.The impact fracture morphology were mainly dimples,which indicated that the fracture mechanism was dominated by ductile fracture.The wear resistance was similar to that of gravity casting.And the wear mechanism was dominated by impact wear,abrasive wear,and adhesive wear.The impact energy decreases linearly and the fracture mechanism changes from toughness to brittleness when the pressure rise above 38 MPa,while the brinell hardness and wear resistance increase.The mechanism of toughening and embrittlement of grade E cast steel by pressure is as follows: The phenomenon of pressure toughening is caused by fine grain toughening and the increase of ferrite content,while the widmanstatten and solution strengthening aggravate the trend of embrittlement with the increase of pressure.The grade E cast steel coupler knuckle was taken as the object,then the process scheme,mould and process parameters of squeeze casting were designed.The process scheme of squeeze casting with multi cavity and multidirectional pressurization combines indirect pressure feeding and local direct pressure feeding was determined according to the characteristics of coupler knuckle.Then,the injection system,feeding system,forming parts,unloading mechanism,side core pulling mechanism and cooling system were designed.The assembly drawing of the mould is drawn and the technical requirements and technological process were determined.The process parameters were calculated theoretically and verified by Pro CAST,which can provide a reference for the trial produce of knuckle prepared by squeeze casting.The trial produce was carried out according to the technological process of knuckle.The problems found in the test,such as uneven fil ing of multi cavities,supplement not in place and feeding crack,were analyzed theoretically,and the solutions were put forward.The reason for uneven fil ing is that the pre solidified layer is formed in shot sleeve before the liquid metal fil ing,which is pushed up and piled up by punch to block the Inner gate during fil ing.When the solidified layer on one side was broken through randomly under the fil ing pressure,the liquid metal will release the pressure and fill this cavity and there is not enough pressure to break through the cavity on the other side at the same time.The reason why the feeding head does not reach the feeding stroke is that the solidified layer of metal melt under the head is thick with poor plastic deformation capability.The formation mechanism of the crack at the corner of the feeding pressure position is that the metal melt around the feeding head has poor fluidity,and the friction between solidified layer and cavity is too large,which leads to local cracks.The effects of four factors including the ratio of height to diameter of shot sleeve,the design gap,the pouring temperature of liquid metal and the mold temperature on the deformation,the change of gap and the contact stress of shot sleeve and punch were studied by simulated orthogonal test.The results showed that the pouring temperature has the most significant effect on the maximum stress of shot sleeve and punch.The deformation of shot sleeve and punch are significantly affected by the ratio of height to diameter of shot sleeve by changing the contact area between liquid metal and them.The most significant effect on the change rate of fit clearance of the contact surface between shot sleeve and punch is the design gap.The mold temperature has a significant effect on the contact stress of shot sleeve and punch.The analysis showed that the fit clearance of the contact surface between shot sleeve and punch is not uniform,which have both local space and the press and friction caused by local deformation.The design of deformation compensation for punch can reduce the maximum contact stress of the contact surface by 36.4% and effectively reduce the friction loss of squeeze pressure according to its deformation direction and amount by theoretical analysis and simulation.