Optimization Of Austenitizing Parameters And Isothermal Quenching Transformation Mechanism Of Ductile Iron

Austempered ductile iron(ADI)is regarded as a new generation of engineering structural materials,mechanical equipment lightweight materials and the most expected to achieve "iron substitute steel" materials because of its excellent mechanical properties.However,isothermal quenching heat treatment is the most effective way to prepare ADI products,and the austenitizing process in the first stage becomes an important link affecting the subsequent isothermal transformation.In particular,the amount of carbon content in high-temperature austenite will directly affect the transformation reaction in the subsequent isothermal process and the mechanical properties of ADI.However,there is a lack of data on the correlation between technological factors and the carbon content of high-temperature austenite after austenitization of ductile iron and the mechanical properties of ADI.At the same time,the description of the internal fine structure of ausferrite structure in the ADI matrix is not clear.This has greatly hindered the development and application of ADI in the domestic market.Therefore,it is urgent to optimize the austenitization process parameters of ductile iron,and to explore the influence of process factors on the structure and mechanical properties of ADI.By studying the influence of as-cast structure and austenitizing process parameters on carbon content in high-temperature austenite of ductile iron after austenitizing,deeply observing and analyzing the microstructure of the ausferritic obtained at different isothermal transformation temperatures and exploring the correlation between technological factors and mechanical properties of ADI materials,the following conclusions are drawn:(1)In the as-cast structure of nodular iron,the average carbon content of bovine eye ferrite was 0.54%.The carbon content of ferrite between two adjacent graphite spheres is distributed in the shape of "U".In addition,the austenitizing temperature(T?)is a significant factor that affects the carbon content in austenite during austenitizing process.At a lower austenitizing temperature(880?),the higher the number of pearlite in the matrix,the higher the carbon content in austenite.However,at higher austenitizing temperatures(920? and 960?),the higher the number of ferrites,the higher the carbon content in austenite.In the range of conventional austenitizing process parameters,the carbon content of high-temperature austenite ranges from 0.57%to 0.71%.The recommended austenitizing process parameter is 920?/2.0h.(2)The matrix structures of ADI obtained by austempering temperature at 280?,330?and 380? were ausferrite,ausferrite+strip austenite and ausferrite+striped austenite+bulk austenite,respectively.The prolongation of the austenitization holding time(1h?2h),the increase of the isothermal transition temperature,and the increase of the amount of ferrite in the as-cast structure all lead to different degrees of coarsening of the ausferrite structure in the ADI matrix.At the same time,the bundle of ausferrite structure observed under the standard light microscopes is actually composed of high-carbon austenite slices and ferrite slices which are roughly parallel in position or with the position angle of about 20°?25°.With the decrease of isothermal transition temperature,both high-carbon austenite slices and ferrite slices are refined to a certain extent.(3)There are clusters of thickness of about 1 ?m superfine ausferrite structures in the matrix of ADI under the isothermal transformation at lower temperature(280?,330?),which is composed of nano-sized high-carbon austenite sheets(thickness of about 36?57 nm)and nano-sized ferrite sheets(thickness of about 24?29 nm)interleaved that are roughly parallel in orientation.The number of superfine ausferrite structures increases with the decrease of isothermal transition temperature.Between the two adjacent clusters of superfine ausferrite structures,there is an inclusion of ausferrite structures with a direction angle of about 20°?25°.In addition,the austenite on both sides of the ferrite slice in the ADI matrix has a high carbon content.Along the direction of vertical ferrite needle growth,the carbon content in austenite decreases with the increase of the distance from the grain boundary of ferrite and austenite.However,the regularities of distribution of carbon content in the bulk austenite is "U" type.(4)After treatment of ductile iron with different as-cast structure at 920?/2h+280?/1.5h,ADI obtained by the ductile iron of Ms=0.25cm has the strongest resistance to elastic deformation.Its yield strength Rp0.2 can reach 1268.8MPa,the buckling ratio can reach 0.96.However,a comprehensive comparison showed that ADI obtained by the ductile iron of MS=0.50cm and Ms=0.75cm had better strength and toughness,and ADI obtained by the ductile iron of Ms=1.00cm and Ms=1.25cm had worse strength and toughness.Meanwhile,with the increase of ductile iron's Ms,the hardness of the ADI obtained after isothermal transformation decreases gradually,but the variation range is not large.(5)With the increase of isothermal transition temperature,the strength and hardness of ADI gradually decrease,while its plasticity and toughness continuously enhance.Meanwhile,when the austenitizing temperature is 920?,the tensile performance of ADI is better obtained by holding the temperature for 1h than that of holding the temperature for 2h.However,austenitizing holding time has little effect on the hardness of ADI matrix.In addition,ADI obtained from isothermal quenching treatment of ductile iron with wall thickness ?=7mm and 12mm has better mechanical properties,while ADI obtained from the ductile iron with ?=17mm has worse mechanical properties.