Nitriding is achieved by heating steel parts in vacuum furnace, filling a small amount of gas, infiltrating reactive nitrogen atoms into steel under low pressure and diffusing them into steel, while ion nitriding is achieved by bombarding the surface of steel parts with reactive N ions produced by flashing discharge and heating only the surface of steel parts, and chemical reaction occurs to form nucleides. Hardened, through the box furnace, atmosphere furnace infiltration of nitrogen, to achieve the density of hardening, so that just a more hard texture. When infiltrating atmosphere, after exhausting the vacuum furnace to a higher vacuum of 0.133 PA (1 x 10-3 Torr), the workpiece is raised to 530-560 C. At the same time, a variety of composite gases containing active substances, mainly ammonia, are fed into the vacuum furnace, and the feeding amount of various gases is accurately controlled. The pressure of the vacuum furnace is controlled at 0.667 Pa (5 Torr), and the heat preservation time is 3-5 hours. Inert gas in furnace is used for rapid cooling.
The hardened layer with a penetration depth of 20-80 um and a hardness of 600-1500HV can be obtained from different materials. Vacuum nitriding, known as vacuum exhaust nitrocarburizing, is characterized by the activation and purification of metal surface through vacuum technology. During the whole heat treatment process of heating, heat preservation and cooling, impure trace gases are discharged and pure composite gases containing active substances are fed in, which makes it possible to adjust and control the surface layer structure, improve the quality and improve the efficiency. X-ray diffraction analysis shows that the compound layer in vacuum nitriding layer is epsilon single-phase structure, and there are no other brittle phases (such as Fe3C, Fe3O4), so it has high hardness, good toughness and good distribution. The hardness and material composition of the "white layer" single-phase EPI compound layer are related. The higher the Cr content in the material is, the higher the hardness is. When Cr13% is added, the hardness can reach 1200HV; when Cr18% is added, the hardness can reach 1500HV; and when Cr25% is added, the hardness can reach 1700HV. The wear resistance of single-phase epsilon compound layer without brittle phase is higher than that of gas nitrocarburizing structure. The wear resistance of single-phase epsilon compound layer without brittle phase is better than that of gas nitrocarburizing structure. However, the existence of the "white layer" is also disadvantageous to some dies and parts. It is easy to cause cracks in the early forging stage of the forging die, and easy to generate pinholes in welding repair.
Another advantage of vacuum nitriding is that by controlling the type and quantity of compound gases containing active substances fed into the furnace, the structure of compound layer (white layer) can be obtained, but only diffusion layer. This may be due to the formation of a vacuum furnace exhaust to 0.l33Pa (1 x 10-3Torr). Another reason is the structure formed by the diffusion of composite gases with active substances into steel in a short time. The advantages of this structure are excellent heat shock resistance and cracking resistance. Therefore, for hot working dies tempered at high temperature, such as high-speed steel or 4Cr4MoSiV (H13) steel dies, the comprehensive properties of high surface hardness, good wear resistance, good heat shock resistance, crack resistance and toughness can be obtained. However, the anti-occlusion, anti-cladding and melting properties of the dies are not good enough when only the diffusion layer structure is used. Due to the different service conditions and performance requirements of the die or mechanical parts, it is necessary to adjust the structure and performance of the surface layer during surface heat treatment.
Vacuum nitriding is not only applied to tools and dies, but also has obvious effect on improving the performance of precision gears, mechanical parts requiring wear and corrosion resistance, springs and so on. The acceptable materials are also widely used.
Plasma Nitriding Furnace
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