The Research Of Carbidic Austempered Ductile Iron

Grinding balls in wet ball mill are an important consumables in mine grinding equipment,which have poor wear resistance and large consumption.It is imperative to find excellent wear-resistant materials for the grinding balls.Carbidic Austempered Ductile Iron(referred to as CADI)was used as small and medium-sized wet ball mills.This grinding ball has the advantages of less wear,low crushing rate,power saving and low noise.However,the CADI grain boundaries are distributed with net-like eutectic carbides,which seriously damages the continuity of the matrix.In addition,the mechanism of corrosion wear and impact fatigue is lack of research due to complex phase composition and unclear mechanism of phase properties on improving performance.So CADI can't be applied to the grinding balls in large wet ball mill.Based on the above problems,with the support of the National Natural Science Foundation of China,the heterogeneous nucleation mechanism and adsorption mechanism of microalloying elements on M₃C carbides were analyzed by the first principles from the atomic scale in this paper firstly,which predicted the possibilities of microalloying to regulate the size,morphology and distribution of eutectic carbides in CADI.Then the effect of the microalloying on CADI was verified through experiments.The simulation results showed that after adding the modification elements Nb and Ti,Nb C and Ti C particles could be precipitated previously act as the effective heterogeneous nucleation of M₃C,destroyed the carbide network.The experimental results show that the net-like structure of carbides was destroyed by microalloying,which improved the impact toughness and the wear resistance of CADI.The thermodynamics and kinetics of the homogenization process and the isothermal transformation process are analyzed by a combination of modeling calculations and experiments.An equation about undissolved carbides'area S/S₀=1-0.14921?could be used to predict the area of carbides at 850°C for different holding time,and an equation about undissolved carbides'area ln(1-S/S₀)=14.2438-18267.3365/T could be used to predict the area of carbides at different austenitizing temperatures for 7200 s.The influence of austenitizing temperature and austempering temperature on the microstructure and properties of CADI was studied,which provided an experimental basis for regulating the CADI matrix by temperature.The results showed that as the increase of austenitizing temperature the volume of eutectic carbides gradually decreases and the quantity of retained austenite gradually increases.The carbon content of retained austenite was constant above 900°C.The size of bainite,the quantity of retained austenite,and the carbon content of retained austenite increases with the austempering temperature increases.When the austempering temperature was above 400°C,acicular ferrite preferentially nucleated around the graphite nodules and inside grains.However,acicular ferrite preferentially nucleated around the graphite nodules and the eutectic carbides at temperature below 400°C.This special nucleation phenomenon was explained from the perspective of thermodynamics and kinetics.As the increase of austenitizing temperature and austempering temperature the CADI impact toughness raised.A process of 900°C/120 min+300°C/120 min could be obtained an optimal comprehensive mechanical properties.A new type heat treatment process comprising super-high temperature pretreatment and austempering treatment(S?A treatment)was used to process CADI,which obtained the new CADI containing Fe₃C precipitated particles and a superfine ausferrite matrix(with a length of 1-3?m and a thickness of 50 nm).The CADI obtained by S?A treatment has an impact toughness 112%higher than the traditional CADI without sacrificing hardness,and has excellent wear resistance under high wear load(above 300N),which provides a new idea for further improving toughness of CADI.Finally,the failure mechanism of impact fatigue and corrosive wear was revealed by characterizing the sample surface and the cross-section after corrosive wear and impact fatigue.Corrosive wear failure mechanism:The matrix is first corroded,which led to breakage and exfoliation of carbides,causing CADI corrosive wear failure.Impact fatigue failure mechanism:a large number of impact fatigue cracks were produced due to the deformation of graphite nodules and the fracture of carbides,resulting in the intergranular fracture.In addition,the grains had no obvious preferred orientation after impact fatigue,but the grains had a tendency of the grain refinement and the increase of low-angle grain boundaries.The potential difference between graphite nodule and matrix was reduced by adding Cu element,which improved the corrosion resistance and corrosive wear resistance of CADI.