Study On Process Of Vanadizing In Salt-Bath On Cold-Work Die Steels

Mould is one of important equipments in mass production of manufacture, which quality is a key factor influencing the product quality and the economic benefits for enterprises. One of main reasons is the poor surface properties of moulds resulting in wear failure. The mechanical properties and the service life of moulds can be improved largely by appropriate surface strengthening technique. Thus the surface strengthening technique plays a very important role in the mould industry, and attracts widely attentions. It is necessary to find an advanced surface modification technology to improve the mould life and retrench the cost or increase the productivity. Salt bath vanadising by TD method belongs to surface alloying. The coating formed by this process is very stable at room temperature and shows excellent wear resistance. The process has been given more and more attention. Therefore, it is widely applied in cold work die in home and abroad. However, some key technologies, such as short service life of salt bath, severe erosion of device, large deformation of work piece and difficult cleaning of remained salt agent, constraint the extensive application of process at home. The vanadizing technology, microstructure, properties and mechanism have not been adequately studied at home.In this dissertation, the microstructure was systematically studied by OP, SEM/EDS and XRD on the base of successful development of TD process. The micro-hardness of coating was determined by Victor hardness tester. The wear resistance of vanadium carbide coating was tested by MM2000 tester and the wear mechanism was discussed according to the observation of worn surface of samples. The foramtion mechanism of coating was analyzed from the points of thermodynamics and kinetics. The main highlights are as follows:1. The increase of immersion temperature can evidently increase vanadizing rate. The experimental results showed that under the same processing parameters, the thickness of coating was nearly linear in relation to immersion temperature and the growth rate of coating was gradually decreased with the extension of processing time.2. The concentrations of vanadium and carbon slightly changed from interface to coating surface and immersion temperature influenced the concentrations of C and V.3. The surface hardness of coating increased with the enhance of substrate hardness and the thickness of coating. The cross-sectional micro-hardness gradually increased from the coating surface to interface and decreased sharply at interface.4. Vanadium carbide coating can greatly increase wear resistance and corrosion resistance. Its wear mechanism was the abrasive due to fatigue peeling.5. The coating formed by TD method had many advantages, such as uniform thickness, high microhardness and good bonding strength with matrix.