Study On Friction And Wear Property Of Vanadium Carbide Layer Prepared On LD Steel By TD Salt-bath

LD steel, a new type of cold work die steel, has been widely used in mould manufacturing. However, the service life of the mould is not long, because the die steel could be abraded easily under high stress and impact force condition. Therefore, many surface strengthening technologies have been used to improve the hardness and wear resistance of die steel. TD salt-bath has become an effective way to improve wear resistance of mould among numerous surface strengthening methods with the advantages of simple equipment and good strengthening effect. In this paper, vanadium carbide layer was prepared on LD cold work die steel by TD salt-bath. The effects of different process parameters on microstructure, phase and properties of vanadizing layer were studied, and the formation mechanism of the vanadizing layer was analyzed. The wear resistance of vanadizing layers were evaluated by friction and wear testing under various wear conditions, in which the wear mechanism was studied. Meanwhile, effects of the rare earth content on the microstructure and properties of vanadizing layer were also studied.The results show that the best basic formula composed of salt bath is 78% Na2B4O7, 8% V2O5, 4% Al, 10% NaF(wt). The mainly phase of vanadizing layers, obtained by different temperature process and treatment time process, are VCx. The thickness and micro-hardness of vanadizing layer increase with the increasing of salt-bath treatment temperature. Meanwhile, the thickness and micro-hardness of vanadizing layer increase as the treating time increased. Finally, the optimal TD process temperature and time is 950℃ for 8h, and the thickness and microhardness of obtained vanadizing layer are 6.1 μm and 1975 HV, respectively. The microhardness of vanadizing layer is about 2.63 times as much as original specimen. The growth kinetics curve of vanadizing layer at 950 ℃ approximately follows similar parabolic equation: l2.632=13.657 t. The vanadium carbide grains is equiaxed grain.With the increasing of load and sliding velocity, both the friction coefficient of vanadizing and original specimen decrease gradually. The wear volume of vanadizing specimen is about 1/10~1/9 times as much as original specimen with load of 8.2 N. Moreover, the wear volume of vanadizing specimen is about 1/8~1/7 times as much as original specimen with different sliding velocity. So, the wear resistance of LD steel could be improved effectively by method of TD salt-bath. The wear mechanism of vanadizing layer is fatigue wear and adhesive wear.The optimum rare earth adding content in TD salt-bath at 950℃ and 8h is 4%, and thickness and microhardness of obtained layer is 7.5 μm and 2235 HV, respectively. Compared with common vanadizing, the thickness and hardness of vanadizing layer increase 29.3% and 22.8%, respectively. Also, the grain size of surface of vanadizing layer is about 0.32 μm, much less than the grain size of common vanadizing layer which the grain size is about 0.57 μm. It is concluded that grains could be fined obviously after adding rare earth.