Thermal Spraying Strengthen Of Power Station Valve Steel

Relative to cobalt-based self-fluxing coating, ni-based composite coating is widelyused in industrial production owing to its outstanding performance on corrosionresistance and wear resistance and resistance to high temperature oxidation within600℃, and also its low price.Two methods of surface strengthen were carried out（plasma spray and oxy-acetylene welding）on the12CrMoV and25#steel, and themicrostructure, wear resistance, corrosion resistance of the coatings were test.The layer powder was self-fluxing ni-based tungsten carbide.The analysisexperiment such as OM, SEM, XRD was adopted to investigate the microstructure andphase composition of the layers. Results revealed that PS coating possessed lamellarstructure with a certain number of hole and oxide inclusion,and the poriness was3.5%.The bond structure between coating and matrix was Mechanical Bonding; Themicrostructure of OAW coating was densification and uniformity with a poriness of2%and the lamellar structure disappeared. The bond structure between coating andmatrix was Metallurgical Bonding; The coatings of two technology had a similar phasecomposition, mainly includeγ-Ni、WC、W₂C and Cr₇C₃.Micro-hardness tester was used to test the hardness of coating surface and the crosssection between coating and matrix, as a result, the hardness of PS layer was HV870while the OAW layer’s hardness was HV930and the hardness distribute uniformallywithin the coatings; The wear resistance of room temperature and high temperature wastested, the results show that,the COF（coefficient of friction） on room temperature ofthe PS and OAW coatings were approximate,as a number as0.3; nevertheless the hightemperature COF differed largely with the PS coating as0.65and the OAW as0.5; WC（tungsten carbide） fell off from the layer, part of which work as abrasive particle inthe wearing test, which aggravate the danage of wear; The OAW coating performentedwell in the wear test, which wear resistance was stable. As a result, the wear mechanismof two coatings was Oxide Adhesion Wear.The CHI604C Electrochemical Workstation was adopted to investigate thecorrosion resistance of the two coatings in three kinds of mediums which were acidic（NaCl+HCl）, alkali（NaCl+NaOH）and saline（NaCl）. It turned out that thepolarization curve of anode（Tafel curve） of two coatings both existed passivationregion. The Tafel curve of three kinds of mediums were highly coincidence, and theECORR of PS coating was lower than the OAW coating while the corrosion currentdensity of PS coating was higher. The PS coating corroded locally, which mainly happend at the defect such as hole, flaw and oxide inclusion. The corrision of OAWcoating were uniform attack, and the corrision mechanism was pitting corrosion.