| 14Cr14Co13Mo4 steel is a case-harden high temperature martensitic stainless steel,which is a new generation of bearing steel developed to improve the performance and service life of aero engine bearings.The design goals of 14Cr14Co13Mo4 steel is to make the case layer have a high-volume fraction and uniformly distributed carbides through carburizing heat treatment,and maintain good toughness in the core,so that 14Cr14Co13Mo4 steel has high red hardness,good wear resistance,strong resistance to fatigue spalling on the surface,good combination of strength and toughness between the case and the core.However,in practical applications,the carburized layer is prone to form a serious network carbide structure,and the hardness of the transition zone is lower than that of the core.In order to eliminate the network carbide structure in the carburized layer and improve the performance of the material,the law of carbon diffusion and carbide precipitation during the low-pressure pulse carburization of 14Cr14Co13Mo4 steel,the diffusion of carbon atoms during the quenching process and the law of carbide re-dissolution,and the influence of carbides on the hardness,friction and corrosion properties of carburized layers require in-depth research.Low-pressure pulse carburization is characterized by a series of boost stage and diffusion stage cycles.It is difficult to obtain the carbon diffusion law of 14Cr14Co13Mo4 steel in the low-pressure pulse carburizing process by the experimental method alone.In this paper,a low-pressure pulse carburizing model of 14Cr14Co13Mo4 steel based on multi-element complex phase dispersion system is established,two sets of carburizing processes are designed,and the accuracy of the model calculation results is verified by carburizing experiments.The results of microstructure analysis indicate that the type of carbides precipitated in the carburized layer is affected by the carbon concentration.In the high-carbon concentration region,M7C3 type carbides are mainly precipitated,and in the lowcarbon concentration region,M23C6 type carbides are mainly precipitated.The 14Cr14Co13Mo4 steel surface has a very strong ability to absorb carbon,and there is a problem that the rapid formation of a dense carbide layer hinders subsequent carbon diffusion.In order to avoid the formation of a dense carbide layer on the surface of steel,a new Ni buffer layer method was invented to adjust low-pressure pulse carburization,referred to as buffer layer method low-pressure pulse carburization,that is,14Cr14Co13Mo4 steel surface is pre-plated with different thickness of Ni film or Fe-Ni film,under the condition that the carburizing process is unchanged,the carbon diffusion behavior of 14Cr14Co13Mo4 steel under low pressure pulse carburizing process is adjusted.The research shows that pre-plating Ni film or Fe-Ni film with different thicknesses on the surface of 14Cr14Co13Mo4 steel,utilizing the low solid solubility of carbon in Ni film or Fe-Ni film and the time required for carbon atoms to pass through different thickness of film was different,affecting carbon atom flux into the sample surface during the low-pressure pulse carburization,which can reduce the carbon concentration on the surface of the carburized layer and make the distribution of carbides more uniform than the uncoated sample.After carburizing,the carburized layer has high carbide content,high hardness,and poor toughness.It needs to be quenched at high temperature to partially dissolve the carbides and improve the distribution of the carbides.In the process of hightemperature quenching,the re-dissolution and growth of carbides,the growth of core grains and the macroscopic diffusion of carbon atoms from surface to the core occur simultaneously,and the three processes are interconnected and interact with each other.The stability of carbides is affected by factors such as the type,size,chemical composition.The experimental results show that when the carbon concentration is in the range of 1.8 wt.% to 0.14 wt.%,the dissolution temperatures of M23C6,M6 C,and M7C3 are around 1080°C,1100°C,and 1270°C,respectively.During the quenching process,carbon atoms in the carburized layer diffuse to the center,which promotes the dissolution of carbides in the region where the carbon concentration decreases.Increasing the quenching temperature can promote the diffusion of carbon atoms and the re-dissolution of carbides in the carburized layer,which is conducive to the elimination of the network carbide structure,but will cause the growth of core grains.Quenching at 1060°C can reduce the residual austenite content in the transition zone and avoid softening in the transition zone.The quenched martensite has large stress and is easy to cause cracking,so it needs to be tempered in time.During the tempering and cold treatment,the quenched martensite and residual austenite are decomposed,and precipitation of tempered M23C6 carbide is accompanied.Nanometer-sized laves phase particles were observed in the carburized layer after tempering at 510°C.The carbides in the carburized layer of the uncoated sample were elongated and semi-continuously reticulated after heat treatment.After the pre-plated 5.2 ?m Ni film samples were carburized by A1 process,quenched at 1060°C,and tempered at 530°C,the carbides in the carburized layer were spherical and evenly distributed,eliminating the network carbide structure.The influence of the microstructure of the carburized layer on the properties of 14Cr14Co13Mo4 steel was studied.There is a linear relationship between the volume fraction of M7C3 in the carburized layer and the Vickers hardness.The hardness and elastic modulus of M7C3 are 21.2 GPa and 369.8 GPa,respectively.The volume fraction of M7C3 increased from 10.42% to 16.65%,the hardness increased from 713 HV to 803 HV,under dry sliding friction conditions,the wear rate decreased from 5.27×10-6 mm3/N·m to 1.30×10-6 mm3/ N·m,the wear mechanism is dominated by oxidative wear.The volume fraction of M7C3 increased from 10.42% to 16.65%,the corrosion potential in 3.5 wt.% Na Cl solution decreased from-0.484 V to-0.569 V,and the corrosion current density increased from 5.98×10-7 A/cm2 to 2.33×10-6 A/cm2,the polarization resistance decreased from 2.46×104 W/cm2 to 3.61×103 W/cm2.The corrosion mechanism of carburized layer in 3.5 wt.% Na Cl solution is mainly pitting corrosion.The M7C3 carbide on the surface of the carburized layer effectively increases the hardness of the surface of the carburized layer,thereby improving the wear resistance of the material,but reducing the corrosion resistance of the surface. |
PDF Full Text Request