Effect of 2.5 heat treatment on corrosion resistance of coating Electroless Ni-P alloy plating is a method to effectively improve the corrosion resistance of materials. Numerous literatures have shown that corrosion resistance is best when the coating contains a high amount of phosphorus and an amorphous structure. At the same time, the coating indicates that the influence of the morphology on the corrosion resistance can not be ignored. The cell surface without defects, uniform size and smooth surface is better than the uneven size, indicating that the cell structure of the rough, convex and concave is good. Further research shows that for high-phosphorus coatings, increasing the heat treatment temperature and prolonging the heat treatment holding time have a considerable influence on the corrosion resistance of the coating. With the increase of heat treatment temperature and the prolongation of holding time, the corrosion resistance of high-phosphorus coating decreases first. The corrosion resistance is the worst at 500 °C, and then the corrosion resistance is gradually enhanced. The corrosion resistance is best at 750 °C. . There are three main reasons for this. After the coating is heat treated at 500 °C, the microstructure of the coating changes from amorphous to crystalline, and crystal defects such as grain boundary and phase boundary appear. Due to precipitation of Ni3P, the volume shrinkage of the coating leads to an increase in grain boundaries, and the Ni3P phase is uniformly dispersed at around 500 °C. The distribution causes the phosphorus content in the coating matrix to decrease, which makes the potential of the coating negatively shift and easily forms a galvanic cell, which aggravates corrosion. At the same time, the precipitation of Ni3P changes the lattice size of the alloy coating, resulting in stress concentration. The above factors cause the corrosion resistance of the coating to be slightly lowered after heat treatment at 500 °C. When the heat treatment is performed at 750 °C, the coating of the crystalline structure is heated and insulated at a high temperature, and the coating undergoes aggregation and growth, and the total area of ​​the grain boundary is reduced, thereby reducing the possibility of corrosion; secondly, the coating is a high temperature annealing in air. It shows that a dense and thick oxide film is formed to prevent the corrosion from proceeding. Third, the high temperature and long time heat treatment forms a wide continuous and strong Ni-Fe diffusion layer between the coating and the steel substrate, strengthening The combination of the plating layer and the substrate increases the corrosion resistance of the plating layer. The level of mold production technology is not only an important indicator to measure the level of product manufacturing in a country, but also largely determines the country's product quality, efficiency and new product development capabilities. The low level of mold in China has long been recognized as a fact. China's current mold development and manufacturing level is at least 10 years worse than the international advanced level. As mentioned above, the electroless Ni-P alloy has good wear resistance, corrosion resistance and lubricating properties. After heat treatment, it has high hardness (400 ° C insulation for 1 h up to 1100 HV), and the coating has good adhesion to the substrate (process Reasonable to 1200Mpa), and the coating is uniform, uniform coating can be obtained for blind holes, grooves, threads, etc. These advantages are very suitable for mold-forming parts with complicated shapes, so the application of electroless plating on molds has become more and more extensive over the years. 3.1 Application of electroless Ni-P alloy on plastic mold The application of electroless Ni-P alloy on plastic molds is the most extensive. At present, it has been widely used in molds such as injection molds, extrusion molds, and tensile molds, and has achieved good application results. Gao Hongxia and others performed Ni-P-SiC-PTFE chemical composite plating on the surface of 45 steel plastic molds, and obtained a composite coating with excellent self-lubricating and wear resistance. Fu Jian et al. conducted an electroless plating test on a 45 steel quenched and tempered substrate and achieved good results. The Ni-P alloy electroless plating method for the working face of the mold was successfully applied to a plastic factory branch in Chengdu. According to feedback from users, Ni-P electroless plating is very helpful for improving the service life of the mold and the brightness of the work surface, and also saves costs. Na Mingjun and others used the electroless plating process to replace the traditional molding process, which not only reduced the material cost by 10% to 50%, shortened the molding cycle and reduced the processing cost, but also showed that the technology was produced by the Harbin Craft Label Factory. There is a promotion and application value. Previous page next page
3 Application of electroless composite heat treatment in mold surface treatment
Gao Jin et al. conducted an electroless Ni-P test on the surface of a plastic molding die, and compared it with other surface treatments. The results show that the electroless Ni-P alloy coating has good surface hardness and wear resistance. In addition, the coating also has good toughness and good adhesion to the substrate.