Research On The Content Of Austenite In Austempered Ductile Iron Based On Magnetic Method

The content of austenite in austempered ductile iron has an important influence on its hardness,wear resistance and other properties.On the one hand,austenite will transform into martensite under certain stress,which will increase the hardness and wear resistance of austempered ductile iron.On the other hand,excessive austenite will reduce its hardness and wear resistance.At the same time,the volume expansion caused by the martensitic transformation will destroy the dimensional accuracy of the workpiece and even cause spalling and cracking.In order to ensure the quality of the materials,it is necessary to accurately measure the content of austenite in the austempered ductile iron.The AMI-21 austenitic measuring instrument based on the magnetic method has the advantages of low cost,fast measurement speed,good stability,zero material damage,etc.It has been successfully applied to bearing steels,but has not been applied to austempered ductile iron.Therefore,the content of austenite in austempered ductile iron was measured based on the magnetic method.In this paper,first of all,the microstructure,mechanical properties and magnetic properties of ADI,CADI and ultra-low carbon ADI at different austempering temperatures and time were studied.And then,the AMI-21 austenitic measuring instrument was used to measure the unbalanced current of austempered ductile iron.At last,the linear regression analysis was performed between the measured data and the XRD test results to obtain a relationship between the content of austenite and the unbalanced current in the magnetic measurement.The results show that:(1)When carbides are not precipitated,the hardness of ADI,CADI,and ultra-low carbon ADI decreases with the increase of austenite content,and the impact toughness increases with the increase of austenite content.When carbides are precipitated,carbides have a greater impact on properties.Carbides can increase the hardness and decrease the impact toughness of the materials.(2)ADI,CADI,and ultra-low carbon ADI are ferromagnetic materials,and the three materials can reach saturation when the magnetic field intensity is 1 T.With the increase of the content of austenite,the saturation magnetization of ADI,CADI,and ultra-low carbon ADI decreases.(3)The current and austenite content measured by the magnetic method are affected by other phases(such as graphite and carbide).Even if the content of austenite in the two materials is the same,the measured current and the content of austenite will also exhibit large differences due to the different microstructure.(4)The relationship between the magnetic austenite content and the unbalanced current is substituted into the instrument program,and the predicted austenite content is compared with the actual content of austenite.The results are similar and consistent with metallographic structure,mechanical properties and magnetic properties.The austenitic measuring instrument has a small measurement error after being properly modified and can be used for actual measurement.