Nitriding is a chemical heat treatment method in which nitrogen atoms penetrate into the surface of metal and alloy workpiece to form a layer of nitride. The three basic processes of nitriding are the generation of active nitrogen atoms, surface absorption and diffusion of nitrogen atoms. Surface modification of metals by nitriding process can significantly improve the hardness, corrosion resistance, oxidation resistance, fatigue resistance and thermal stability of metal materials (such as stainless steel, aluminum and alloys). Through the improvement of these properties, the application field of these metal materials can be greatly expanded.
The precision casting parts discussed by ZHY casting are gear shifting mechanism components in a gearbox. During gear shifting, the synchronizer is pushed to shift through the part finger to achieve the purpose of gear shifting. The surface hardness and wear resistance of the parts are required to be high. With the rapid development of automobile industry in recent years, the product quality and performance stability of automobile industry have the requirements of standardization and standardization to a great extent. Because the surface hardness of parts after plasma nitriding can not reach customer 600 hv0 5 (hv0.5 means that the load is 0.5 kg during the test), and the customer refuses to use unqualified parts, resulting in economic losses to the enterprise. In order to improve the competitiveness of enterprises and customer satisfaction, the method of experimental design (DOE) is used to find the causes of problems.
Doe method is widely used in the automotive field, including process design, production, packaging, transportation, assembly, user use, recycling, etc. it can also provide services for new product development, product or process improvement, and installation. Doe finds out the factors of high quality, high yield and low consumption through fewer tests, so as to achieve the purpose of improvement.
By comparing the microstructure of machined parts and precision casting parts, ZHY casting finds out the reasons for the low surface hardness of precision casting parts, finds out the defective process through the research of process flow, and improves its process.
The customer requires that the measured surface hardness is Vickers hardness, and the surface roughness Rz of precision casting parts is 23 μ M or so. The measurement engineer conducted repeated measurement on the same part and found that the surface hardness of the cast part became worse, which was close to 100 hv0 5. Fluctuation range; The surface hardness of machined parts is only 30 hv0 5. Fluctuation range. Therefore, the researchers decided to solve the problem of measurement first, and then find the important factors affecting the poor surface hardness through DOE method.
Due to the lack of isolation of oxygen in the operating environment, the oxidation phenomenon of precision casting parts occurs in the casting process, resulting in the loose white bright layer and oxidation in the diffusion layer after plasma nitriding. In order to effectively avoid oxidation of parts during casting, it is necessary to seal and protect the cast parts, put charcoal and engine oil into the box in advance for full combustion, so as to remove the oxygen component in the box, so as to ensure that the surface hardness of parts after nitriding can meet the requirements of customers.