Study On In-Situ Preparation And Mechanicial Properties Of Micro-Nano NbC Reinforcement Layer On Steel Substrate

As an alloy structural steel,bearing steel（GCr15）was widely used in bearing,cold punching die,cold roll and other components.However,the surface of bearing steel will appear some failure phenomenas of cracks,wear and peeling due to increasingly severe service conditions.In order to improve the surface comprehensive mechanical properties of bearing steel,the in-situ reaction technology was used to prepare NbC reinforcement layer with micro-nano structure on the surface of bearing steel,and the raw materials were bearing steel（GCr15）and high-purity Nb metal plate.Firstly,the possible chemical reactions and products of the system were predicted according to the function of Gibbs free energy（ΔG）.The growth rate（K）and activation energy（Q）of NbC reinforcement layer were calculated based on classical kinetic theory and the formula of Arrhenius.Thus,the functional relationship between the growth thickness of NbC reinforcement layer and time,temperature were obtained.The phase compositions,chemical valence of elements,microstructure,element distributions,crystal structure,phase distribution and phase orientation difference of NbC reinforcement layer were characterized by the methods of XRD、XPS、SEM、EDS、TEM、EBSD.The nano-hardness（H）,elastic modulus（E）,fracture toughness（KIC）and the failure of scratch of NbC reinforcement layer were tested and calculated by the methods of indentation and scratch.The main conclusions are as follows:（1）In the NbC reinforcement layer,the phase of NbC,Fe₃C,Cr₃C₂,Cr₇C₃ and Cr₂₃C₆ can be spontaneously generated by chemical reaction according to the calculation of Gibbs free energy function of each reaction product.The functional relation between the thickness of the NbC reinforcement layer,the holding time and the reaction temperature was obtained based on a series of experiments and kinetic analysis,which was convenient to predict the in-situ preparation of NbC reinforcement layer at different process conditions.（2）NbC reinforcement layer was obtained on the surface of bearing steel by in-situ reaction technology.It was divided into NbC dense layer and NbC gradient layer according to the volume fraction of NbC reinforcement phase.The volume fraction of NbC in the NbC dense layer was more than 90%,which was composed of closely arranged NbC ceramic particles.The volume fraction of NbC in the NbC gradient layer was 60～90%,which was composed of NbC phase and a small amount of α-Fe phase.And NbC gradient layer was composed of NbC,Fe₃C,α-Fe,and Cr₇C₃ phases.（3）The nano hardness and elastic modulus of NbC reinforcement layer show obvious gradient change.The hardness value of NbC reinforcement layer was in the range of 13.75～19.70 GPa,and the elastic modulus was in the range of 300.66～443.17 GPa.The results of nano indentation test show that the nano hardness H and elastic modulus E of NbC reinforcement layer are 1.5 times of that of bearing steel.（4）The results show that the fracture toughness of NbC reinforcement layer was 7.68～11.55 MPa·m^1/2 by using the Vickers hardness indentation method combined with the fracture type judgment and selecting the appropriate fracture toughness calculation formula.In the scratch testing,NbC dense layer was peeled off in a small area.However,there are only some micro-cracks and particle accumulation in the NbC gradient layer.There is no serious wear failure behavior,which has a good protective effect on the bearing steel.