| Gas nitriding(GN) is a thermochemical treatment used to improve the surface hardness, corrosion property and wear resistance of mechanical components. The treatment is usually performed at 500-580°C with a flowing NH3 gas media. During nitriding, the adsorbed nitrogen atoms can be dissolved in the solid of the surface and subsequently diffuse to larger depths in the substrate. Conventional gas nitriding(CGN) is generally performed at a total nitriding pressure of about 1 atm for a very long duration(typically 20~80 h), which causes excessive energy consumption and low production efficiency in manufacturing industries. Moreover, a compound layer composed of ε-Fe2-3N and γ′-Fe4 N is formed at the surface. The internal stresses make it friable and brittle, which can often lead to spalling during service. They all greatly limits applications of this technique in industries. Accelerating the GN kinetics, reducing energy consumption, and improving the quality of the nitrided layer have become the main direction of development of GN.In this study, the pressure parameter was introduced into the GN process. The pressurized gas nitriding(PGN) behaviors of pure iron and two alloy structural steels were systematically investigated. The effects of nitriding pressure on the microstructure, phase structure, mechanical properties, wear resistance and corrosion resistance of the nitrided layer were analyzed by means of OM, XRD, SEM, TEM and wear test etc. The mechanisms of formation and rapid thickening of nitrided layer were discussed. Moreover, the combined treatment of PGN and cold rolling(PGNC), and GN with circular pressures were proposed to improve the performance of the nitrided layer. The obtained main results are as follows.The GN of pure iron with flowing NH3 gas was conducted at 500°C for 5 h under pressures ranging from 1 to 5 atm. The effect of nitriding pressures on microstructures,hardness and thickness of nitrided iron samples was identified. The nitriding potentials under different pressures were calculated, and the variations of nitriding potentials with pressures were clarified. It was found that the microstructure, surface hardness and nitriding potential were directly controlled by nitriding pressure, and the GN kinetic process could be significantly accelerated by PGN. The nitriding potential, surface hardness and ratio of ε-Fe2-3N phase to γ′-Fe4 N phase, were all increased with increasing pressure from 1 to 3 atm, and turned to decrease when the pressure was higher than 3 atm. Increasing the nitriding pressure from 1 atm to 5 atm significantly increased the nitrided layer thickness from about 210 μm to 1100 μm.The mechanisms of formation and rapid thickening of nitrided layer on pure iron were analyzed basing on the thermodynamic and kinetic theory. The kinetic energies of ammonia molecules increase with increasing pressure, which enhances the reactions at the surface. The fluctuations of the nitriding potential and viscous airflow, which usually exist in CGN, can be avoided. Furthermore, the ammonia decomposition is strongly affected by gas pressure, so that the active control of nitriding potential can be achieved.The GN characteristics for steel 38 CrMoAlA in flowing NH3 gas at 510°C for 5 h under nitriding pressures ranging from 0 to 0.50 MPa were investigated. The surface microstructure, mechanical properties, wear resistance and corrosion resistance of the nitrided samples were systematically characterized. The results show that, the nitrided layer thickness(410 μm) of the sample nitrided at 510°C under 0.50 MPa for 5 h was almost equal to that(440 μm) of the CNG for 50 h. The wear/corrosion resistance of the nitrided layer decreased/increased with increasing pressure from 0 to 0.25 MPa, and then increased/decreased when the pressure was higher than 0.25 MPa.A new approach to rapid preparation of highly strengthened and toughened nitrided layer for alloy structural steel was proposed. The microhardness, toughness, wear resistance and thermal fatigue performance of the modified surface layer on the CGN sample(520°C, 40 h, NH3 flux of 0.50 L/min), PGN sample(520°C, 5 h, NH3 flux of 0.20~0.30 L/min), and PGNC sample(PGN+30% Cold rolling) have been comparatively evaluated. It was found that the CGN sample exhibited relatively higher surface hardness(1160 HV) and poorest toughness, whereas the PGN sample showed relatively lower surface hardness(1060 HV) and excellent toughness. The PGNC sample had the most excellent surface hardness(1065 HV) and toughness. The combined treated sample showed the best bearing capacity and wear resistance under heavy loads. After thermal fatigue test(20°C~600°C) for 300 cycles, the surface layer in the PGNC sample can significantly delay the crack nucleation and prevent crack propagation, relative to those of the CGN sample and PGN sample.The GN behaviors of steel 42 CrMo in flowing NH3 gas at 530°C for 6 h with cyclic pressures(0/0.25 MPa) were investigated. The microstructure characteristics, microhardness and thickness of the nitrided layer under different pressure cycles were identified. The mechanisms of rapid thickening of nitrided layer and brittleness control of surface layer were analyzed. Under the same nitriding temperature and duration, both the total thickness of the nitrided layer and surface toughness was increased with increasing the pressure cycle numbers. The formation and growth of the compound layer can effectively suppressed by GN with circular pressures, which would results in elimination of N diffusion barrier and acceleration in GN kinetic process. |
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