Nitriding is to use vacuum furnace to heat iron and steel parts as a whole, fill a small amount of gas, produce living nitrogen atoms in low-pressure state to infiltrate and diffuse into the steel to achieve hardening; while ion nitriding is to rely on the active N ions produced by the glow discharge to bombard and only heat the surface of iron and steel parts, and generate nuclear compounds through chemical reaction to achieve hardening, through box furnace, atmosphere furnace to infiltrate nitrogen Gas, to achieve the density of hardening, so that the texture just harder.
In the process of air infiltration, exhaust the vacuum furnace to a higher vacuum degree of 0.133Pa (1 × 10-3torr), raise the workpiece to 530-560 ℃, and simultaneously send a variety of composite gases containing active substances, mainly ammonia, and accurately control the amount of various gases. The pressure of the vacuum furnace is controlled at 0.667pa (5torr), and after holding for 3-5h, use inert gas in the furnace for rapid cooling. The hardened layer with the depth of 20-80 μ m and the hardness of 600-1500 HV can be obtained by using different materials.
Vacuum nitriding is called vacuum exhaust nitrocarburizing, which is characterized by the activation and purification of metal surface by vacuum technology. In the whole heat treatment process of heating, heat preservation and cooling, impure trace gas is discharged and pure composite gas containing active substance is sent, which makes it possible to adjust and control the structure of surface layer, improve the quality and efficiency. X-ray diffraction analysis shows that the compound layer in the vacuum nitriding layer is ε single phase structure, and there are no other brittle phases (such as Fe3C and Fe3O4), so it has high hardness, good toughness and good distribution. The hardness of "white layer" single phase ε compound layer is related to the material composition. The higher the Cr content, the higher the hardness. When Cr13%, the hardness can reach 1200hv; when Cr18% (mass fraction, the same for the rest), the hardness can reach 1500hv; when Cr25%, the hardness can reach 1700hv. The wear resistance of the single phase ε compound layer without brittle phase is higher than that of the gas nitrocarburizing structure. It has excellent anti friction burn, anti thermal bonding, anti deposition and anti melt loss properties. However, the existence of the "white layer" is also detrimental to some molds and parts, which is easy to cause cracking in the early stage of forging, and to generate pinholes during welding and repair.
Another advantage of vacuum nitriding is that there is almost no compound layer (white layer) but only the structure of diffusion layer through the control of the type and quantity of compound gas containing active substance sent into the furnace. The reason may be the formation after the vacuum furnace exhaust to 0.133Pa (1 × 10-3torr). The other reason is the structure formed by the diffusion of the composite gas with active substances in the steel in a short time. This kind of structure has the advantages of heat shock resistance and crack resistance.
Therefore, for the hot working die with high temperature tempering, such as the high-speed steel or 4cr4mosiv (H13) steel die, the comprehensive properties of high surface hardness, good wear resistance, good heat shock resistance, crack resistance and toughness can be obtained; however, when there is only diffusion layer structure, the die's bite resistance, deposit resistance and melting loss resistance are not good enough. Due to the different service conditions and performance requirements of the mold or mechanical parts, the structure and performance of the surface layer must be adjusted during the surface heat treatment.
In addition to the application of vacuum nitriding in tooling, it has obvious effect on improving the performance of precision gear, mechanical parts requiring wear-resistant and corrosion-resistant, spring and so on, and the materials that can be treated are more widely.
On 2018-08-08 11:45:46
