Measurement And Simulation Of Residual Stress On Impacted CADI Gringing Balls

With the ball mills large-scale tendency in recent years,the demand for the grinding balls with large diameters is increasing.Consequently,the problem of partial spalling and fragmentation of the grinding balls becomes more prominent.The Carbidic Austempered Ductile Iron(CADI)grinding ball with 125 mm diameter was taken as the research object in this paper.The engineering background and research status in relevant fields at home and abroad were summarized.The residual stress in the surface of the grinding balls was measured by cutting method.The impact process of grinding balls was simulated by using finite element software ANSYS/LS-DYNA.A quantitative understanding was gained for the residual stress on the surface and inside the grinding balls.These might be instructive for revealing the failure mechanism of impacted grinding balls and improving the material design and manufacturing process level of grinding balls.The main research content is as follows:The basic elastic parameters and constitutive relation of grinding ball material were firstly determined by experiments.Cutting method were used to measure the residual stress on the surface of CADI grinding balls impact and un-impacted.The measurement results show that there is circumferential compressive residual stress about 500~900MPa,approximately equally in all directions on the surfaces of randomly impacted grinding balls.The value of residual stress increases with the number of impacting,but the increase tendency slows down after 20000 times.Within a certain extent,the higher the content of manganese is,the bigger the residual stress on the surface of grinding ball will be.Tensile or compressive residual stresses were also found on the surfaces of un-impacted grinding balls.But their values are much smaller than those from impacted grinding balls.In order to analyze the residual stress inside the impact grinding balls,the finite element model was established by simplifying the actual process of the falling ball fatigue test reasonably.Simulating computations were conducted for the impacted grinding balls from different heights.The results show that after a single impact a small range of tensile residual stresses are generated in the center of impacting contact zone.when the dropping height is 10 m,the maximum plastic strain occurs at the depth of 4.4mm from the surface.This is in good agreement with the fact that in actual engineering the thickness of spalling block of grinding balls is about 4~6mm.The higher the falling height is,the short the time of impacting contact,the greater the plastic zone,and the larger the contacting force will be but with a declining tendency.In order to analyze the effect of multiple impacts on the internal residual stress of grinding balls,simulations about multiple impacts at the same position,multiple impacts from different zenith angles and multiple impacts from different azimuth angles werecarried out.The results show that after multiple impacts at the same position,the range of plastic zone in grinding ball has not changed obviously,but the peak value of plastic strain increases in a descending manner with the increase of the number of impacts.After multiple impacts from different zenith angles,plastic zones generated by each impacting will be superimposed,and a plastic reinforced zone will be formed beneath the surface of the contact zone.After multiple impacts from different azimuth angles,the maximum equivalent plastic strain occurs at the depth of 3~5mm beneath the impacting surface.And the plastic zone reaches about 15 mm from the surface,showing no sign of deepening with the increase of impact times.