The Basic Study On Microstructure And Property Of CADI Containing Chromium

As an increasingly developed metal material,austempered ductile iron(ADI)has many excellent mechanical properties,such as light weight,excellent impact toughness,low production cost,high hardness and wear resistance.And most ADI is used in the mechanical structure parts of automobile,engine,engineering machinery and other fields.As a wear-resistant material,however,the wear resistance of ADI still needs to be improved.In recent years,material scientist have,on the basis of ADI,developed carbide austempered ductile iron(CADI),which is a new type of wear-resistant material.CADI has many advantages of ADI,such as light weight,good impact toughness and low production cost,at the same time,the existence of carbides greatly improves the hardness and wear resistance of the material.It is a very promising wear-resistant material for CADI.At the same time,it is also very suitable for making grinding balls for ball mill.However,the strength and toughness of CADI are better than white cast iron,but it still can be improved greatly compared with ADI.By means of optical microscope,X-ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),Rockwell hardness tester,pendulum impact testing machine and ring-on-block wear tester,the CADI was studied in this study.Through the regulation of Cr content and heat treatment process parameters,the number and morphology of carbide in the matrix,acicular ferrite size,the number of high carbon austenite and the carbon content of high carbon austenite can be controlled better,thereby improving the wear resistance of CADI on the basis of good toughness.And so it can reveal the regulating mechanism of the CADI microstructure and properties.The results show that the content of Cr has a great influence on the microstructure and properties of CADI in as-cast and heat treated states.Cr is a strong carbide forming element,and the content of carbides in CADI is very sensitive to the change of Cr content,so it is necessary to strictly control the Cr content in the microstructure.With the increase of Cr content in the cast state,the content of carbides increases obviously,and gradually develops into huge reticular from strip or massive.The volume ratio and spheroidization rate of graphite gradually decrease,and the graphite diameter increases first and then decreases.After austempering process,a typical ausferrite is formed in the microstructure,and the carbides arepartially dissolved.When the specimens are austenitized at 900? for 100 min,the proeutectoid ferrite begins to appear when Cr content is over 1.42 wt.%.And it has a great influence on the hardness and wear resistance of CADI.With the increase of Cr content,the hardness and wear resistance first increase and then decrease,but the impact toughness decreases gradually.When the Cr content is 0.96 wt.%,the comprehensive performance is the best.With the increase of austempering temperature,the acicular ferrite becomes thicker and bigger,and the content of high carbon austenite increases gradually,the hardness decreases,and the impact toughness increases.The austenitizing process has a great effect on the microstructure and mechanical properties of CADI.It shows that with the increase of austenitizing temperature the acicular ferrite and high carbon austenite gradually become thicker and bigger,and the content of high carbon austenite increases.When the austenitizing temperature is 850?,there is a lot of proeutectoid ferrite in the microstructure,which significantly reduces the hardness and wear resistance of the material.With the increase of austenitizing temperature,the proeutectoid ferrite disappears,and the carbon content in high carbon austenite increases gradually,which makes the high carbon austenite more stable,and the transformation rate into martensite during the wear process gradually decreases.Although martensite has higher hardness,it is beneficial to the generation and propagation of cracks,and so reduces the abrasion resistance.When the temperature rises to 950? and 1000?,the carbides dissolve partially and the hardness decreases,but the distribution becomes more uniform.In addition,the impact toughness increases greatly due to the mass production of high carbon austenite.The wear resistance is related to both hardness and impact toughness of the material.With the increase of austenitizing temperature,the wear resistance increases first and then decreases.The effect of austenitizing time on microstructure and properties of CADI is negligible.