Study On The Fast Nitriding Technology Of Active Screen Plasma Nitriding

Nitriding may enhance the hardness and wear-resistance of steel surface remarkably. The distortion of the nitrided workpieces is also quite small because the nitriding is carried out below the phase-transform temperature. However, the nitriding time is very long because of the low diffuse rate of nitrogen atom at the nitriding temperature. In this dissertation, the fast nitriding technology of active screen plasma nitriding(ASPN) was studied systemtically. Nitrided layers were observed and analyzed by field emission scanning electron microscope (SEM), X-ray diffraction (XRD), optical microscope (OM), microhardness tester. It was discussed about the effect of high nitriding temperature, nitriding time at high temperature, low nitriding temperatrue, and nitriding time at low temperature on the morphologies, phase structure, microstructure, microhardness, chemical component of the nitrided layer. Experimental results indicate that the three factors that nitriding high temperature, nitriding time at high temperature and nitriding time at low temperature are important to thickness of nitrided layer.The fast nitriding process parameters has been optimized through orthogonal experiment. Choosing the fitting process parameters can increase the nitriding speed. Experimental results indicate that the nitrided layer thickness of fast optimized process is 30~50% thicker than the nitrided layer thickness of traditional process in the same nitriding time. The new nitriding technology can be explained with"absorption-diffusion"model.Based on the facts that the nitrogen solubility inγphase is higher and the diffusion speed of nitrogen inαphase is higher, the fast nitriding technology is advanced. It can form the nitrogen-rich layer above the eutectoid temperature and consume the nitrogen-rich layer below the eutectoid temperature . The thickness of nitrogen-rich layer is the key of the fast ASPN technology.Nitrogen concertration and nitrogen concertration gradient of the optimized process by fast ASPN have been calculated according to Fick Second Law. Results show that nitrogen concentration gradient of surface and substrate by optimized process is more than 20 times than traditional nitriding process's. The fast nitriding technology's main characteristic is that increasing the solid diffusion speed of nitrogen because of high concentration gradient. High concentration gradient is come from nitrogen-rich layer.