Microstructures And Properties Of Nitrided Metastable 38CrMoAl Steel

38CrMoAl steel is an advanced nitriding steel with good nitriding application, and commonly used in the nitriding components with high wear resistance and high fatigue strength, such as shafts, gears, cylinder liners and bolts. Although the conventional preheat treatment before nitriding is quenching and tempering, the further research on the different preheat treatment before nitriding still have practical significance in order to expand the scope of application of nitriding process effectively, In this experiment,38CrMoAl steels were pretreated through the salt bath quenching, Quenching-Partioning (Q-P), oil quenching and conventional quenching and tempering and then hardened by the method of one-stage gas nitriding. Further, the microstructure, hardness and wear resistance of samples after nitriding were studied and the influence of metastable heat treatment on the microstructure and properties were discussed as well.The microstructure, thickness, phase structure, microhardness, surface hardness, nitriding depth, brittleness and wear performance of nitride layer were investigated by the optical microscopy, X-ray diffraction analysis, hardness tester, field emission scanning electron microscopy and wear test, respectively. Meanwhile, the influence of nitriding parameters and preliminary heat treatment on the nitrided layer was analyzed.It is shown that at 480 ℃,500 ℃, KN= 3, the nitrided layer was composed of compound layer and diffusion layer through the microstructure observation and phase analysis, while no compound layer was formed under the conditions of 450 ℃,480 ℃,500 ℃,KN=1 and 450 ℃,KN=3. The increasing nitrogen potential facilitated the formation of compound layer and its thickness increased with rising temperature. At 480 ℃, the nitriding compound layer after oil quenching and salt bath quenching have a smaller thickness. While the all the samples have the similar thickness at 500 ℃. The different temperature determined the phase content of compound layer. After quenching and tempering, the e phase of compound layer at 500 ℃ than at 480 ℃. Conversely, the sample after metastable pretreatment had more ε phase at 500 ℃ than at 480 ℃. Meanwhile, the sample after quenching and tempering possessed more e phase compared to sample after metastable pretreatment. When the compound layer was not formed on the surface of sample, the preheat heat treatment had little effect on the diffusion layer. Nitride levels were in line with the requirements of the important parts.Hardness analysis showed that the microhardness increased with the increase temperature, increased first and then decreased with increasing nitrogen potential. The sample after tempering presented low microhardness than that after metastable pretreatment. In general, the sample after Q-P had maximum microhardness. Nitriding depth increased with rising nitrogen potential and temperature. When no compound layer formed on sample surface, the nitriding depth was slightly affected by preheat heat treatment. At the presence of compound layer, the sample after tempering showed less nitriding depth than that after metastable pretreatment. For all samples, the surface hardness increased with increasing nitrogen potential. As for samples after the salt bath quenching, oil quenching,quenching and tempering, the surface hardness increased first and then decreased with rising temperature. The surface hardness of sample after Q-P decreased with rising temperature. The sample after tempering showed low surface hardness than that after metastable pretreatment. In general, the sample after Q-P possessed the maximum surface hardness.Oil lubrication wear test demonstrated that the wear resistance of sample whose surface had compound layer, the sample after quenching and tempering and Q-P showed the good wear resistance at 500 ℃, respectively. At the same temperature, the sample after quenching and tempering and Q-P owned a good wear resistance at low nitrogen potential.