Preparation And Properties Of SHS Hard Coatings On Steel Matrix Synthesized By V-EPC

This paper puts forward a new process for the production of hard coatings on the surface of steel matrix by combing SHS coating technology with V-EPC technology, in order to eliminate such defects as blow hole, slag inclusion, lower adhesion strength at interface and narrow thickness range existing in the coating during the production process of casting infiltration hard coatings on the surface of steel matrix. The results show that the steel-matrix SHS hard coatings have the characteristics of wide thickness range, high adhesion strength, good wear resistance, good impact resistance and high surface quality, and it is suitable for large-scale industrial production.This paper designs two SHS reaction systems, i.e. Ti-C and Ti-W-C. Using XRD, OM, SEM, EDS, TEM and mechanical property test, the effect of composition proportion, coating method, V-EPC process parameters, additives and heat treatment process of different reaction systems on the structure and property of hard coatings are studied, and the formation mechanism of hard coatings and particle phase are analyzed. The main conclusions obtained are as follows.Basic proportion of two SHS reaction systems, i.e. Ti-C and Ti-W-C, are designed. The Ti/C molar ratio ranges from 0.75 to 1.0, and the molar ratio of W/Ti ranges from 0.5 to 1.0. The structure of steel-matrix hard coatings of Ti-C system is mainly composed of TiC particles through in-situ synthetic reaction. The structure of steel-matrix hard coatings of Ti-W-C system consists of inner and outer layers. The particle phases of inner layer are TiC particle and fine (Ti, W) C particle, while the particle phases of outer layer are coarse WC particle and fine TiC particle.The internal quality of hard coatings after compression and degumming is high. With the increase of press power, the TiC size through reaction synthesis becomes finer, and the particles are uniform distribution. The optimum V-EPC process parameters obtained through orthogonal experiments are:high manganese steel casting temperature is 1520℃, and the vacuum degree ranges from 0.03MPa to 0.05MPa. The grain size of titanium metal power is 200-300 mesh, and 2% PVA water solution is used as adhesion agent.With TiC and WC contents increasing, the structural of hard coatings becomes more compact. The TiC particle formed through reaction synthesis is finer. TiC particle distribution is more even. The volume fraction of particle phase is increased considerably and the microhardness increase. With addition of TiC particles, the hard coating structure is composed of TiC particle formed through reaction synthesis, added TiC particle and substrate phase. The hard coatings structure with addition of WC particle is composed of TiC particle formed through reaction synthesis, (Ti, W) C annular phase featuring nucleus/shell structure and matrix phase. After heat treatment, the (Ti, W) C annular phase is dissolved and precipitated, and the thickness of annular phase precipitated becomes smaller, and part of W element has dissolved in substrate phase, leading to solution strengthening. The maximum of microhardness of hard coatings added with TiC is up to 3100HV. The optimum addition amount of TiC is 10wt.%, and the addition amount of WC is controlled with a range of 10wt.%～15wt.%.The addition of Rare Earth CeO2 can refine the coating structure, and improve density. Coarse dendritic microstructure in hard coatings can also be eliminated. TiC particle agglomeration is reduced and uniform and fine granular grain structure is formed. The addition amount of Rare Earth CeO2 is lower than 1.5wt.%.With the increase of TiC and WC addition, the adhesion strength of hard coatings interface is increased, the impact toughness of hard coatings is reduced, and the amplitude of reduction in impact toughness of coating after addition of WC is relatively higher. The impact fracture of coating is brittle cleavage fracture. The abrasion resistant capability is increased, and the amplitude of abrasion resistance of hard coatings after addition of TiC is relatively higher, and its abrasion resistant capability is 8～10 times of the high manganese steel, representing 1.5～ 3 times of hard particle without addition. With the increase of TiC particle addition, the ploughing in the wear morphology of hard coatings becomes shallow till completely disappearing, and the particle detachment becomes less, representing superior abrasion resistant capability.Mathematic model of formation process is established by combining the molten mass casting infiltration rate with Fourier heat transfer model. The TiC particle in the molten mass through reaction synthesis includes two periods, i.e. TiC nucleus formation and growth; higher of Ti and C content in the molten mass is, longer the TiC nucleus formation period is, and more TiC nucleus quantity formed. The TiC particle growth features faceted smooth growth.Helical blade with SHS hard coatings has been prepared. Fine surface quality and good metallurgical bonding can be obtained. The coating has compact structure. The production of helical blade can meet industrialization production requirements. The problem of rupture during working and local wear resistance of helical blade has been solved.