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　　According to the requirements of depth and hardness of nitriding layer, one, two or three stages of nitriding treatment can be carried out. At the same time, reasonable selection should be made according to the material and technical requirements of seamless steel pipe

　　The three nitriding processes have their own characteristics. The surface hardness of isothermal nitriding (or one-stage nitriding) is about hv1000 ~ 1200, the deformation is small, the brittleness is low, the process is simple and convenient, but the process cycle is long, the cost is high, and the nitriding layer is shallow. It is mostly used for seamless steel pipes with shallow nitriding layer, large size, precision and high hardness; Compared with isothermal nitriding, the surface hardness of two-stage nitriding is slightly medium, and the deformation is slightly increased, but the nitriding rate is fast. It is mostly used for seamless steel pipes with deep nitriding layer and large batch; The three-stage nitriding is faster than isothermal nitriding in hardness, brittleness and deformation. Therefore, when nitriding treatment is needed for seamless steel pipe parts, the best nitriding process can only be determined after comprehensive evaluation according to the technical requirements, working characteristics, production efficiency, manufacturing cost and other aspects of seamless steel pipe.

　　It should be noted that for carbon steel and cast iron seamless steel pipes, nitriding in order to improve the corrosion resistance of workpiece: I is called corrosion resistance nitriding. The depth of nitriding layer is 0.02 ~ 0.04mm, and a thin and dense white nitriding layer (e phase is 0.015 ~ o.ooomm) is formed on the bead surface. J has the characteristics of high chemical stability The products of gas combustion sensing and weak alkali solution have different corrosion resistance. The seamless steel pipe with corrosion resistant nitriding can be normalized or quenched and tempered. Corrosion resistant nitriding can replace nickel plating, zinc plating, bluing, anodizing and other surface treatment methods, sometimes even alloy steel and stainless steel. The process temperature is about 550-650 ° C, the time is 1-3 h, and the ammonia decomposition rate is 45% - 70%.

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