In order to shorten the nitriding cycle and make the nitriding process not restricted by the steel type, two new nitriding processes have been developed on the basis of the original nitriding process in the past one to twenty years.
Soft nitriding is essentially a low-temperature carbonitriding based on nitriding. At the same time as the nitrogen atomization of steel, there is also a small amount of carbon atom infiltration. Compared with the aforementioned general gas nitrogen, the hardness of the nitrided layer is higher. It is low and brittle, so it is called soft nitriding.
1. Soft nitriding method, soft nitriding method is divided into gas soft nitriding and liquid soft nitriding. At present, the most widely used in domestic production is gas nitrocarburizing. <,br>Gas nitrocarburizing is a low-temperature carbon and nitrocarburizing in an atmosphere containing activated carbon and nitrogen atoms. Commonly used co-permeating media are urea, formamide and triethanolamine, which generate heat at the nitrocarburizing temperature. The decomposition reaction produces activated carbon and nitrogen atoms.
The activated carbon and nitrogen atoms are absorbed by the surface of the workpiece and penetrate into the surface of the workpiece through diffusion, thereby obtaining a nitrogen-based carbonitriding layer.
The gas nitrocarburizing temperature is usually 560-570℃, because the hardness of the nitrided layer is the highest at this temperature. The nitriding time is usually 2-3 hours, because more than 2.5 hours, the depth of the nitriding layer increases slowly with time.
2. The structure and characteristics of nitrocarburizing layer: After the steel is nitrocarburized, the outermost layer of the surface can obtain a white layer of several microns to tens of microns, which is composed of ε phase, γ`phase and nitrogen-containing Carbon body is composed of Fe3 (C, N), and the sublayer is a 0.3-0. 4 mm diffusion layer, which is mainly composed of γ`phase and ε phase.
Soft nitriding has the following characteristics:
(1) The processing temperature is low, the time is short, and the deformation of the workpiece is small.
(2) Not limited by steel type, carbon steel, low alloy steel, tool and die steel, stainless steel, cast iron and iron-based powder metallurgical materials can all be soft-nitrided. The surface hardness of the workpiece after nitrocarburizing is related to the nitriding process and material.
3. It can significantly improve the fatigue limit, wear resistance and corrosion resistance of the workpiece. It also has anti-abrasion and anti-seize properties under dry friction conditions.
4. Since there is no brittle ξ phase in the soft nitrided layer, the nitrided layer is hard and has certain toughness and is not easy to peel off.
Therefore, nitrocarburizing has been widely used in the treatment of wear-resistant workpieces such as molds, measuring tools, high-speed steel tools, crankshafts, gears, and cylinder liners.
It should be noted that the current problem of gas nitrocarburizing is that the thickness of the iron-nitrogen compound layer in the surface layer is relatively thin (0.01-0.02mm), and the hardness gradient of the nitride layer is steep, so it is not suitable to work under heavy load conditions. In addition, during the nitriding process, toxic gases such as HCN will be generated in the furnace. Therefore, attention must be paid to the sealing of the equipment during production to prevent the furnace gas from leaking out and polluting the environment.