Study On Vacuum Carburizing Heat Treatment Process Of 12Cr2Ni4A Steel

As a clean,efficient and energy-saving advanced heat treatment technology,low pressure vacuum carburizing has become a key technology popularized in the current heat treatment industry due to its advantages such as no internal oxidation,blind hole carburization and good uniformity of the infiltrating layer.After more than thirty years of technological innovation and technological development,low pressure vacuum carburizing has been widely used in automotive,aerospace and other fields.But the domestic research related technology and equipment is relatively backward,research the related mechanism of low pressure vacuum carburizing complete heat treatment process is not deep enough,around all kinds of low pressure vacuum carburizing steel less basic research,lack of research on the related material surface carbon flux and the diffusion coefficient and other parameters of the process and primitive accumulation.This paper focuses on the pulse low pressure vacuum carburizing technology of 12Cr2Ni4A steel as the research object,on the surface of carbon flux,diffusion coefficient,austenite saturated carbon concentration,and the carburizing experiments under different temperatures,and the infiltration of quenching and tempering layer properties were studied.The main research work are as follows:(1)Based on the linear relationship between the martensitic transformation temperature and the carbon concentration in the test steel,the transformation position of the martensite was observed in real time during the quenching and cooling of the infiltrated layer by using the high-temperature metallographic observation technique.A determination method of carbon concentration gradient were obtained,and were applied to the carbon concentration detection of steel layer.(2)Under the conditions of carburizing pressure 300Pa and carburizing temperature 930?,the change of carbon flux during the carburizing time between 5s and 64s with time was measured.The results show that the surface carbon flux is a constant.The change of the quality and appearance of the sample is analyzed,it was proposed that when the acetylene was rapidly decomposed,the attachment layer was formed on the surface of the sample during the carburizing and charging instant.During the holding process,the carbon atoms entered the metal through the layer,and the carbon layer diffused into the steel after the gas was extracted,until it disappeared completely.The effects of carburizing temperature and carburizing pressure on the surface carbon flux were investigated at different carburizing pressures of 100Pa,200Pa and 300Pa and 930?,950? and 980?carburizing temperatures.The results show that the effect of carburizing temperature on the surface carbon flux is less,the effect of carburizing pressure on the surface carbon flux is greater.At the same time,the surface carbon flux under various conditions was determined.(3)Study on the process parameters of carburizing.The growth behavior of austenite grain was observed at different temperatures of 850?,930,? 950?,1000?,1050?,1150? and diffrent time of 1h,2h,4h.The carburizing temperature of the experimental steel could not over 1000?.The chemical composition of experimental steel was substituted into the empirical formula of diffusion coefficient related to temperature,carbon concentration and alloy composition,and the diffusion coefficient of experimental steel was obtained when carburized at 930?,950?,980?.Using thermodynamic calculation software,the austenitic saturated carbon concentration of experimental steel at 930?,950?,980? degrees is 1.1%,1.15%and 1.26%,and the UH value of carburizing at different temperatures is determined.(4)Combined with the principle of acetylene pulse vacuum carburization,the pulverization process with depth of 0.8mm and 1.2mm respectively at 930 0C,950? and 980? was obtained by using the C-N carburization model of Fick's law.Six carburizing experiments were carried out.The reliability of carburizing process was verified by comparing the hardness gradient of infiltrated layer,the gradient of carbon concentration in infiltrated layer with the expectation of the process,and the feasibility and high efficiency of carburizing at high temperature were pointed out.(5)Using high-temperature confocal microscopy,quenching at different temperatures of 830?,880?,930? and different cooling rates of 1?/s,3.5?/s,10?/s,15?/s and 20?/s,combined with the thermodynamic calculation of the infiltration CCT curve,the hardness of the infiltration layer hardness gradient and the impact of the organization were studied.The results show that different cooling rates have a great influence on the types and proportion of each phase in the microstructure.When the cooling rate is more than 10?/s,almost all the martensite in the microstructure can be quenched.Quenching at different temperatures of 830?,880?,930? can obtide the same organization,and can get a qualified quenching result.(6)The changes of microstructure and hardness gradient curves of 1.2mm layer which was obtained by carburizing at 930? and oil quenched were observed after tempering 2h and 4h.respectively.The results show that the hardness of the layer after low temperature tempering is the result of the coaction of martensitic decomposition and the transformation of retained austenite.