When the nitriding temperature is 500°C, the surface layer has high strength, and the strength decreases beyond this temperature. The reason is that the agglomeration of nitrogen compounds below 500°C is not obvious and the dispersion is large. In addition, the relationship between the nitriding temperature and the strength, the depth of the nitriding layer, and the deformation of the seamless steel pipe is fully considered, and the nitriding temperature is generally controlled at 480-560 °C.
After nitriding for a certain period of time, the strength of the surface layer gradually increases, and the strength decreases slightly after increasing the time. For example, the nitriding temperature increases, and the longer the time is, the lower the strength value is.
3)Ammonia decomposition rate
The decomposition rate of ammonia is the percentage of hydrogen and nitrogen in the furnace gas volume caused by ammonia dissolution. The decomposition rate is high and the hydrogen concentration in the furnace is high, so that the nitrogen atoms are in a stagnant state, that is, the penetration of nitrogen atoms is blocked; vice versa. The rate is related to the working pressure in the furnace, the total flow of ammonia, the state of the surface of the seamless steel pipe, and whether it has metal catalysts and other factors, so the decomposition rate should be controlled.
According to the requirements of the depth and strength of the nitriding layer, the nitriding treatment of seamless steel pipes is divided into three types
One-stage nitriding process, the surface hardness is high about HV1000~1200, the deformation is small, the brittleness is low, the process is simple, and the handling is convenient, but the process cycle is long, the cost is high, and the infiltration layer is shallow. seam steel pipe;
Compared with the first-stage nitriding process, the surface hardness is slightly different (HV850~1000), and the deformation is slightly increased, but the infiltration rate is fast, and it is mostly used for deep nitriding and large batches of seamless Steel Pipe.
The three-stage nitriding process has a fast infiltration rate, but the hardness, brittleness, deformation and other aspects are worse than isothermal nitriding.
Nitriding treatment refers to the chemical heat treatment process in which nitrogen atoms infiltrate the surface of the steel pipe in a certain medium at a certain temperature. The common ones include liquid nitriding, gas nitriding, and ion nitriding. The purpose is to improve the surface hardness and wear resistance of the steel pipe. Improve fatigue strength, corrosion resistance, etc.