| LI Nan,HONG Yue,WU Cui-lan,XU Qiang.Gaseous Nitrocarburizing and Post-quenching of Low Carbon Steel[J],47(11):9-16 |
| Gaseous Nitrocarburizing and Post-quenching of Low Carbon Steel |
| Received:August 27, 2018 Revised:November 20, 2018 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2018.11.002 |
| KeyWord:nitrocarburizing quench compound layer multiphase strengthened layer martensite austenite |
| Author | Institution |
| LI Nan |
School of Material Science and Engineering, Hunan University, Changsha , China |
| HONG Yue |
School of Material Science and Engineering, Hunan University, Changsha , China |
| WU Cui-lan |
School of Material Science and Engineering, Hunan University, Changsha , China |
| XU Qiang |
School of Material Science and Engineering, Hunan University, Changsha , China |
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| Abstract: |
| The work aims to decrease the shortcomings of numerous pores and poor performance in the martensitic layers fabricated by traditional nitrocarburizing and post-quenching and find out the optimized process parameters to obtain a streng?thened layer with few pore, high strength and high tenacity on low carbon steel. X-ray diffraction (XRD), scanning elec??tron mi?cr?oscopy (SEM), transmission electron microscopy (TEM) and microhardness tester were used to characterize the microstruc?tures and the properties of the strengthened layers formed at different process parameters. Nitrocarburized compound layer was transformed into a porous martensitic layer after re-heating and oil-quenching at 850 ℃. The quantity of pores on the surface quenched specimen could be decreased by lowering quenching temperature, reducing nitrocarburizing time, decreasing gas fluxes of NH3 and increasing the gas fluxes of CO. After post-quenching at 680 ℃, the nitrocarburized compound layer could be transformed into a multiphase strengthened layer composed of martensite and retained austenite. The multiphase strengthened layer had the hardness up to 1000HV and good toughness. Even after tempering at 120 ℃, the multiphase strengthened layer could still keep the high hardness and good toughness. Compared with increasing of the gas fluxes of CO during the nitrocar???burizing process, decreasing of gas fluxes of NH3 not only decreased the pores and kept high hardness and good toughness of the strengthened layers, but also reduced the nitrocarburizing time and costing. The optimized nitrocarburizing and post-quenching process can be used to fabricate a multiphase strengthened layer composed of martensite and retained austenite and reduce the surface pores. The widely distributed austenite-blocks in the strengthened layers can greatly improve the toughness. Therefore, the multiphase strengthened layer has advantages of high hardness, high toughness and few pores. |
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