Kinetics And Thermo-Stability Of V(N,C) And Nb(N,C) Coatings On Pre-nitrocarburized Aisi H13Steel Produced By V-Bb-Re Deposition Process

AISI H13steel is widely used for aluminum extrusion dies. Because of performing at severe working conditions such as high pressure, high temperature and strong friction environments, the extrusion dies often lose effectiveness from its surface in form of wear and fatigue. Therefore, the dies’surface strengthening treatment is the key to extend its life. Composite surface processing technique is one of the most front and effective ways to research material surface strengthening. These studies combine conventional surface chemical treatment technique with modern thermo-reaction deposition process. The previous studies have been obtained good microstructure and excellent performance characteristics layer. In the present studies, scanning electron microscopy, energy dispersive, X-ray diffraction technique and microhardness measurement are employed in order to further study the coating’s kinetics growth law and structure stability in high temperature.The formation mechanism analyses of coatings show that the nitrogen and carbon elements infiltrate into the substrate in form of interstitial diffusion and spreading along crystal defects, because of the chemical potential difference between penetrating agents and H13steel substrate. The formation of vanadium niobium carbonitride relies mainly on nitrogen and carbon atoms diffusion from substrate to coating. The coating growth rate depends on nitrogen and carbon atoms diffusion speed in the matrix and vanadium niobium carbonitride layer. The relationship among the diffusion layer thickness, treatment time and temperature was calculated by Fick’s laws and Arrhenius equation, revealing the kinetics formation laws of the vanadium niobium carbonitride deposition layer. These laws can be used to predict and control the thickness of the coating on H13steel and the process of composite treatment, and also guide experiment and production. The obtained diffusion activation energy Q and growth rate constant K showed that the pre-nitrocarburizing can accelerate the formation of vanadium niobium carbonitride coating. The Q value of vanadium and niobium co-deposition was the minimum value of single deposition vanadium or niobium. Rare earths with unique structure and properties can accelerate the vanadium and niobium deposition rate and reduce the Q value. So it benefits the formation of vanadium niobium carbonitride coatings.According to the change of coating hardness, phase composition and grain size with holding time, thermo-stability of V(N,C) and Nb(N,C) coatings was studied. It was found the following rules. The value of microhardness of the coatings was up to1230HV0.1after heating at560℃for16h. Due to the reduction of dislocations and other defects in the coating grain, the diffusion and shedding of nitrogen and carbon atoms, the formation of interstitial phases such as V4C2.67and Nb4N3.92, and the oxidized of vanadium and niobium atoms, the hardness decreases rapidly in the early stage. But the hardness decreases gentle in the later. At this time, grain growth is also the one of the reasons for the decline of thermo-stability performance. The grain growth follows a certain kinetics equation at the experimental temperature range. Each of the permeation elements has different effects on thermo-stability performance of the coating in N-C-V-Nb-RE TRD process. Pre-nitrocarburized can significantly increase the thermo-stability performance of the coating. The performance of the coating produced by co-deposition vanadium and niobium is higher than single deposition vanadium or niobium. Rare earth elements which segregated in the grain boundary can slow down the rate of grain growth and hinder the diffusion of nitrogen and carbon atoms. Therefore, the thermo-stability performance of the coating with addition the rare earth element is greater than that of without added.