Enhancement Mechanism And Aging Process Of Surface Hardened Layer By Nitrogen Carbon Complex Chemical Heat Treatment

Chemical heat treatment is one of the most important surface enhancement techniques,which has very extensive applications.However,there are many disadvantages of traditional chemical heat treatment such as the distortion of metal components due to the high quenching temperature of carburizing,the low carrying capability of the hardened layer by nitriding,the lack on enhancement post-processes of the chemical heart treatment,etc.Therefore,it is important to develop one complex chemical heat treatment process with the merits of wide range of applicable materials,high strength of the hardened surface layer,little distortion of metal components and residual compression stress in surface layer.In this thesis,many kinds of metallic materials such as low carbon steel,medium carbon alloy and multi-elements alloyed cast iron were selected as experimental materials.A series of complex chemical heat treatments was developed,i.e.carburizing is the main process at the first stage at high temperature,nitriding is the main one at the second stage at low temperature and followed by quenching and aging in heated salt bath furnace,etc.Moreover,this process aims to improve the strength of the surface hardened layer by adopting the mechanism of quenching-partitioning-tempering(Q-P-T).The microstructure of samples was observed by optical metallurgical microscope and scanning electron microscope(SEM).The hardness was tested by micro-hardness instrument.The crystal structure of the surface hardened layer was analyzed by X-ray diffraction(XRD)equipment.The precipitation of aging and the enhancement mechanism of the surface hardened lay by this process were studied and discussed based on references.Results show that:(1)the microstructure resulted from the nitrogen carbon complex chemical heat treatment was,successively from the surface to the core of metal component,compound layer,martensite+retained austenite(AR),transition layer and core material.The phase transformation in the martensite+AR layer with the highest hardness is complicated during the tempering.The hardness of this layer in 42 CrMo is about 900 HV;(2)diffusion layers without surface compound layers can be fabricated by adjusting the parameters of nitrogen carbon complex chemical heat treatment,which contains only martensite and AR at the surface with the average micro hardness about 650 HV whereas the micro hardness of the compound layer is about 500 HV;(3)the enhancement from the aging due to the precipitation of ?? from the ? is occurred in the compound layer,which is mainly composed of ? phase.The enhancement by the precipitation of micro hardness of this layer in Q235 steel is higher than 100 HV;(4)many phase transformations in the martensite and AR of multi-elements alloyed cast iron by the subsequent tempering after the quenching include carbon partition and precipitation of carbides,which are the two main composition and microstructure changes,and the transformation of AR.The hardness of the samples tempered at 420 ?,570 ? and 620 ? is 43.5 HRC,44.3 HRC and 38.3 HRC,respectively.The enhancement of the hardness at 570 ? is caused by the phase transformations mentioned above.