| Grinding ball is the main wear medium in mine ball mill.Because of the huge consumption,its production cost and performance directly affect the cost and work efficiency of ore crushing process.However,most mines in China have not prepared suitable grinding ball products for different use environments or working conditions.Therefore,based on the difference of performance requirements under different working conditions,three new types of cast iron grinding balls are designed in this paper:low chromium cast iron grinding ball,bainitic spheroidal graphite cast iron grinding ball and high chromium cast iron grinding ball.By adjusting the microstructures and properties of three new cast iron grinding balls,analyzing their wear behavior and mechanism,and exploring their strengthening and toughening mechanism,a theoretical basis is provided for the selection and use of grinding balls under different working conditions.Low chromium cast iron grinding ball has the characteristics of low production cost and poor comprehensive performance.Therefore,a new low chromium cast iron grinding ball was designed with the composition of Fe-2.88C-1.02Si-1.19Mn1.53Cr(wt.%).The production cost was further reduced by reducing the Cr content,and its service performance was guaranteed by optimizing the heat treatment process.The effects of quenching and tempering temperatures on the properties of low chromium cast iron grinding ball were studied.It was found that quenching temperature mainly affected the macro hardness.With the increase of quenching temperature,the microstructure of the material is coarser and the comprehensive properties are lower.The tempering temperature mainly affected the impact toughness.With the increase of tempering temperature,the comprehensive properties of the material first decrease and then increase.The results show that the optimum heat treatment process is as follows:austenitizing at 940? for 1 h and air cooling at 200? for 1 h.The macro hardness is 50.3 HRC and the impact toughness at room temperature is 5.0 J/cm2,showing excellent mechanical properties.The wear behavior and mechanism of low chromium cast iron grinding ball under different impact loads were studied.It was found that fatigue and cutting wear were dominant under low load,while cutting and spalling wear were dominant under high load.After the optimal heat treatment,the wear resistance of the material is significantly improved compared with the as cast samples,which is due to the secondary phases formed during the heat treatment and their synergistic effects:the dispersion of external stress by secondary precipitates and the dispersion of internal stress by retained austenite.Therefore,the initiation and propagation of cracks between carbide and matrix could be slown down and the impact wear resistance of the material could be improved.Isothermal quenching heat treatment is often used to obtain bainitic spheroidal graphite cast iron grinding ball,but its production cost is high and the environmental pollution is serious.Therefore,a new type of bainitic spheroidal graphite cast iron grinding ball is designed with the composition of Fe-2.74C-3.75Si-1.85Mn0.02Mo-0.03Mg-0.018RE(wt.%).The ferrite+pearlite matrix is successfully transformed into bainite by simple water quenching and salt quenching processes,which effectively reduced the production cost and the environmental pollution.The effects of quenching temperature and cooling medium on the properties of bainitic ductile iron were studied.With the increase of quenching temperature,the hardness and impact toughness of the material first increase and then decrease,reaching the highest hardness of 53.6 HRC at 920?.And the highest impact toughness is 32.5 J/cm2 at 880 0C.At the same quenching temperature,the macro hardnesses of salt quenched samples are higher than that of water quenched samples,but the impact toughness is lower.The wear behavior and mechanism of bainitic ductile iron grinding balls under different impact loads were studied.With the increase of impact load,the cumulative weight loss of the material decreases.In the process of impact wear,retained austenite will undergo martensitic transformation,thus forming a hardened layer below the wear surface.Under the higher impact load,the retained austenite in the material is more prone to phase transformation,and a thicker hardening layer is formed below the wear surface,which has more obvious protective effect on the matrix,so that the wear loss is less.With the increase of wear cycle,the wear form of the material changes from the initial cutting wear to surface crack and even fatigue spalling.The properties of high chromium cast iron grinding ball can be effectively controlled by heat treatment.In addition,it is of great significance to clarify the deformation mechanism of materials at high temperature and the growth mechanism of eutectic carbides.The composition of high chromium cast iron grinding balls is designed to be Fe-3.18C-22.0Cr-1.05Si-0.81Mn(wt.%).The effect of quenching temperature on the microstructure and mechanical properties of high chromium cast iron was studied.It was found that with the increase of quenching temperature,the hardness of the material increased gradually,but the impact toughness decreased.The impact wear behavior and mechanism of high chromium cast iron were studied.It was found that the wear forms of the material mainly include furrow,chiseling,carbide crushing and spalling.With the increase of impact energy,the crushing and spalling of carbide become more and more serious,resulting in the obvious increase of cumulative weight loss.The guiding effect of eutectic carbides on austenite growth during solidification and their co orientation relationship are analyzed.Two possible growth models of eutectic carbides are proposed.The temperature effect and mechanism of deformation of high chromium cast iron were studied to provide theoretical basis for its wide application.The compression deformation of high chromium cast iron at room temperature is only 3%,which is attributed to the weak adhesion between carbide and austenite matrix.However,high chromium cast iron shows excellent high temperature mechanical properties,the compressive strength can reach more than 2000 MPa,and the compression ratio can reach more than 40%.Its plasticity comes from the austenite matrix softened at high temperature and its self-healing effect on cracks on primary carbides,which ensures the overall continuity of the material.The cracks of carbides initiate and propagate along the sub boundary of internal stacking faults at room temperature.However,at high temperature,the cracks initiate on the(001)surface activated by high temperature and fracture under shear stress.In addition,the hardness and strain hardening anisotropy of carbides are closely related to the stacking fault substructures with different internal sizes.Through the above researches on the microstructure and properties of three new cast iron grinding balls,the applicable service conditions are analyzed,which can provide a theoretical basis for the improvement of related grinding ball properties.Although the production cost of low chromium cast iron is low,its hardness and impact toughness are not excellent.It is suitable for small and medium-sized ball mill with low-hardness ore and low impact load.The hardness and plasticity of the new bainitic ductile iron are excellent,and the hardening caused by phase transformation will occur during impact wear,which effectively improves the wear resistance.It has the potential of application under high load,but there is no hard carbide in the microstructure,so it is suitable for large ball mill with high impact load and medium-hardness ore.High chromium cast iron contains high Cr content and hard carbides,which has very high hardness.However,its room temperature deformation ability is poor,and it is prone to serious spalling under high impact load.Therefore,high chromium cast iron grinding ball is suitable for medium and small ball mills with low impact load,hard ore and corrosive medium. |
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