Study Of Hot Working Die Remanufacturing Surfacing Electrode

With the improvement of industrial automation, the status of hot working die become more and more important in the automotive, machinery and other industries, and the performance develops to the direction of large-scale, high precision, high efficiency, long life and so on. In the working process, hot working die is not only worked at heavy load, but also contacted with the workpiece metal as high as 1200℃, making the surface temperature of the mold cavity keepping 450～600℃ all the time. And hot working die withstands periodic alternating stress and friction effect, often failures for wear, deformation, or fatigue. Repairing failure die by surfacing welding is a method of quality reliable, low cost, flexible application.This paper designs a new hot working die surfacing welding electrode by adding Ti, V, Mo,W, Cr, Ni and other alloying elements to Crl3 martensite stainless steel surfacing welding electrode coat. Then added CrN to realize nitrogen alloying, and combining thermodynamic calculation to analyze the carbonitride’s precipitation and reinforcement. The surfacing metal is also studied under the environment of high temperature oxidation between the sliding wear characteristics, and the wear mechanism is analyzed. The results show that the microstructure of the new hot working die surfacing metal are martensite, a small amount of residual austenite, and diffused carbides. The carbides are composited by Ti, V, Mo, Cr, and other alloying elements. The combination of the carbides and the martensite matrix with strong toughness will have excellent wear resistance. In the nitrogen alloying process, Ti, V, Mo, Cr, Mn and other elements can increase nitrogen’s solid solubility by improving the nitrogen solubility in the liquid pool. And alloy elements can combine with C and N, to form a complex carbonitride particles, further improving the nitrogen content of surfacing welding metal, and making it possible to nitrogen alloying. The microstructure of the surfacing welding metal by nitrogen alloying is as still as martensite, a small amount of residual austenite, and tiny carbonitrides distributed on the grain boundary and the martensite matrix. And along with the increase of nitrogencontent, carbon nitrides in surfacing metal also gradually increased and distributed uniformity. The weldability of martensite stainless steel surfacing metal is poor. In order to avoid the cracks in the process of welding, welding preheating before welding is required. Combining with the experience formula, Ms temperature can be declined to about 191℃ by nitrogen alloying. So lower preheating temperature and interpass temperature can be choose, and it also can save resources effectively. In addition, because the austenitizing elements such as N and Ni, there are a small amount of residual austenites distributed in the surfacing metal, and it is benefited to the toughness of surfacing metal.Through thermodynamic calculation, TiN can satisfy the thermodynamic conditions and precipitate in liquid phase, TiC precipitates in the coexistence of solid and liquid phase, and the rest of the carbons and nitrogens precipitate under the solidus temperature. And the precipitation sequence is:TiN> T,(1740.6K)> TiC> TS(1627.4K)> VN> Cr7C3> VC0.875>Mo2N> Cr23C6> VC> Mo2C> WC> V2C> MoC> W2C. The strengthening mechanism of nitrogen alloying mainly include interstitial solid solution strengthening, fine-grain strengthening and precipitation strengthening. Interstitial solid solution strengthening is that a large number of nitrogen, carbon atoms dissolve in matrix lattice space with the rapid cooling of weld. Fine-grain strengthening is that TiN and TiC can precipitate respectively in liquid phase and solidification, and they can refine the grain size as the spontaneous nucleation core. Precipitation strengthening is that through tempering treatment, a large number of carbonitrides precipitate along the grain boundary, dislocation or martensite boundaries, producing secondary hardening effect. According to the high temperature friction and wear test, at the temperature of 300℃,wear failure mechanisms of the surfacing metal without nitrogen alloying are mainly abrasive wear and a small amount of oxidation wear. And the main wear mechanisms of the nitrogen alloying surfacing metal are oxidation wear and a small amount of abrasive wear. At 600℃ temperature, main wear mechanisms of the surfacing metal with or without nitrogen alloying are all oxidation wear, peeling off caused by plastic deformation, and abrasive wear caused by the oxidation film falling off. But nitrogen alloying surfacing metal has higher strength and thermal stability, and the abrasion resistance is better than the surfacing welding without nitrogen alloying.