In actual production, an engine valve subjected to oxidation treatment was produced based on this data, and the relationship between the thickness of the oxidized hardened layer and the treatment conditions obtained in the above-mentioned order. And evaluated the fatigue performance and wear resistance. The produced valve was subjected to an S-N curve of an oxidation-treated valve in which bending stress measurement was repeated. In this test method, as the oxidative hardening layer becomes thicker, the fatigue strength decreases, and the material close to the unoxidized treated material at 670 ° C obtains high fatigue strength. When selecting oxidation treatment conditions, wear resistance should be considered, and appropriate oxidation treatment conditions should be determined when considering the wear life of the valve and the fatigue characteristics required. Not suitable for mass production. The most suitable method is oxidation treatment. The biggest issue in the development of titanium alloy valves is the development of wear-resistant surface treatment technology. The surface treatments such as TiN coating, Mo injection layer and Cr spray plating are all costly, and it is difficult to maintain their wear resistance for a long time. That is, solid solution of high-concentration oxygen in titanium increases its hardness and obtains a thick hardened layer inside. Oxidation treatment is basically a simple heat treatment of heating and heat preservation in the high temperature area of ​​the atmosphere. However, the Ti-6A 1-4V valve with low creep resistance is an ordinary annealed structure, and it is easy to be deformed due to its own weight during processing. The needle-like structure with excellent creep resistance is the basic structure of the valve, but the ductility and fatigue performance of this structure are low. Therefore, after heating in the β region, by controlling various cooling conditions to prevent the precipitation of coarse α phase at the grain boundary, a very fine needle-like structure can be obtained, ensuring high ductility and the same fatigue as the equiaxed structure. Controlled creep deformation during oxidation treatment.
The sample was cut from the valve shaft during the actual manufacturing process, and its tensile properties were evaluated. The tensile properties were as high as 980 MPa and the elongation was as high as 12%. It was also confirmed that even a needle-like structure obtained high fatigue characteristics not inferior to that of an equiaxed material. The representative alloy used for the exhaust valve exposed to high temperature during use is Ti-6A l-2Sn-4Zr-2Mo-0.1Si6242S. However, two-wheeled vehicles are more prone to long-term exposure to high-temperature regions than four-wheeled valves, so they have chosen TImetaL1100Ti-2.7Sn-4Zr-0.4Mo-0.45Si with better thermal performance. This alloy is one of the best heat-resistant alloys in practical titanium alloys, but its durability temperature is about 600 ℃, while the two-wheel exhaust valve is It is required to have a heat resistance of about 800 ° C, so it is necessary to select the best heat treatment conditions and then discuss whether it is applicable. Therefore, under different heat treatment conditions, the tensile properties, high temperature creep resistance, impact properties and fatigue properties at room temperature to 800 ℃ were evaluated to grasp the best material property matching and make engine valves under appropriate conditions. .
But if the conditions are not well controlled, an oxidative hardening layer is applied to improve the wear resistance of the valve. The guaranteed fatigue performance may be extremely reduced. Therefore, it is particularly important to grasp the optimal heat treatment conditions. Therefore, atmospheric heat treatment was carried out in the temperature range of 670 ~ 820 ℃ for 1 ~ 16h to measure the surface properties and the hardness distribution of the surface layer. At the same time, the influence of the oxidation treatment conditions on the fatigue performance was investigated. The temperature range of room temperature to 700 ° C with the best heat treatment is 0.2% yield strength higher than that of ordinary steel exhaust valve material SUH35800 ° C. The two are basically the same. The fatigue performance of this alloy at 800 ° C is also the same as SUH35. The most worried about high temperature creep resistance is also better than SUH35, which means that the alloy is treated with appropriate heat treatment to ensure the various characteristics of the valve.
The hardness distribution of the surface layer of the samples subjected to 1h oxidation treatment at different temperatures. The hardness distribution of the surface layer of the oxidized samples at different times at 670 ℃ and 820 ℃. As the processing temperature increases, the diffusion distance of oxygen in the titanium alloy increases, and a higher hardness value can be obtained in a deeper layer. For example, within the temperature range of the test conditions, the highest temperature and long time at 820 ° C, the 4h oxidation treatment sample is about 50μm, the lowest temperature and short time is 670 ° C, and the 1h treatment sample has a hardened thickness of about 10μm. (Ti02 oxygen diffuses into the substrate from here, and the hardness of the outermost surface part directly under the etching is the same regardless of the conditions under which heat treatment is applied. However, the surface layer may reach a depth of several μm in micro Vickers hardness measurement, different Under the heat treatment conditions, it is confirmed that there is a large hardness difference. At the same time, under some high-temperature and long-term oxidation treatment conditions, the oxidized hardened layer has cracks, which indicates that the oxidation treatment is not suitable.