Vacuum Nitriding Process and Application

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Vacuum nitriding is to use a vacuum furnace to heat steel parts as a whole, fill a small amount of gas, and generate active N atoms to infiltrate and diffuse into the steel under low pressure to achieve hardening; while ion nitriding is generated by glow discharge. The active N ions bombard and only heat the surface of the steel part, and a chemical reaction occurs to form nitrides to achieve hardening.

Process flow:

During vacuum nitriding, the vacuum furnace is exhausted to a higher degree of vacuum (1×10-1Pa), and then the workpiece in the vacuum furnace is raised to (530-560) ℃, and at the same time, ammonia-based, active A variety of composite gases of substances are used, and the feeding amount of various gases is accurately controlled. The furnace pressure is controlled at 665Pa, and the rapid cooling of the inert gas in the furnace is carried out after holding for (3-5) h. Depending on the material, a hardened layer with a hardness of (20 to 80) μm and a hardness of (600 to 1500) HV can be obtained after this treatment.

Application:

After vacuum nitriding, the wear resistance of aluminum extrusion die is improved by about 2 to 6 times compared with liquid nitrocarburizing and gas nitriding. Whether it is a slot die or a hollow profile die, the surface of the extruded aluminum profile is bright and smooth, the unit weight increase rate is low, and the qualified rate of the finished product increases. This is because the tiny gaps of the mold can be hardened to achieve the required hardness.

Die-casting molds are often made of 4Cr5M oSiVl steel. When the section size is large (>120mm), or when the temperature of the die-casting alloy is high, such as higher than 600 ℃ or even 700 ℃, precipitation hardening steel is used. Japan has YHD steel, and my country has similar grades. 2Cr3M o2NiVSi. When the die-casting mold is working, due to the concentrated rapid cooling and rapid heating, the service conditions are too cold, and the surface is prone to defects such as thermal shock cracks, thermal adhesion, melting loss, and erosion. For the die-casting cavity mold, it is mainly to improve the thermal shock resistance to prevent the occurrence of cracks; for the casting core, gate, ejector, etc., it is mainly to improve the resistance to adhesion, melting loss and erosion. There are reports that the life of foreign die-casting model cores is extended by 5 to 8 times after vacuum nitriding. In an automobile manufacturing enterprise, the number of molds formally accepted by vacuum nitriding reaches 45t/month. The largest single weight is 500kg, and the largest size is 1200mm × 750mm × 750mm.

The commonly used materials for vacuum nitriding of plastic molds are all kinds of stainless steel, Cr-containing quenched and tempered steel (such as 3Cr2M o steel) and maraging steel (P21 steel or its improved type). After the vacuum nitriding treatment, because the dimensional change is small, about (2 to 5) μm, it can be carried out after the mold is manufactured. Other hardening methods, such as high frequency quenching and dimensional changes after vacuum quenching, make processors worry and trouble, so vacuum nitriding is an ideal heat treatment method for plastic molds. For example, after the stainless steel 1Cr18Ni9 mold is processed, the hardness can reach (1200-1600) HV without changing the size. An auto parts manufacturing company, die-casting a thermosetting resin forming part, the thimble near the gate is very hot and wears, the original use of H13 steel nitrocarburizing treatment, after 4000 pieces of die-casting, can not be used due to dimensional changes. Now the stainless steel 1Cr18Ni9 steel is used for vacuum nitriding, and there is no dimensional change after 30,000 times of die casting. Vacuum nitriding also makes it easy to obtain the mirror surface effect of the mold, because the surface roughness after treatment is within 0.5μm, and the mirror surface state of the mold surface can be easily obtained again with diamond abrasive paste.