| Abstract |
| Plasma nitrocarburizing (PNC) treatment of quenching and tempering (QT)-treated SNCM220 steel samples was carried out at 570 ℃ for 5 hours in plasma-nitriding equipment. The PNC in a gas mixture of N2:H2:CH4 (85:13.75:1.25 by vol%) produced a compound layer composed mainly of ε-Fe2-3(N,C)-phase on the surface of SNCM220 steel. The thickness of the compound layer decreased with increasing CH4 content when N2 composition was fixed to 85%. The amount of ε-Fe2-3 (N,C)-phase in the compound layer increased with increasing CH4 gas content up to 1.25 vol%. When there was more than 1.5% CH4 in the gas mixture, the cementite phase started to form, which reduced the thickness of the compound layer. The gas carbo nitriding (GCN) treatment was performed at 860 ℃ for 2 hours followed by oil quenching at 130 ℃ and tempering at 180 ℃ for 2 hours. The GCN treatment produced a hardened layer with a martensite structure containing high carbon content (0.6 wt%). The effective case-depth (550 HV0.3) was about 300 μm. Compared with the GCN, the PNC and QT-treated SNCM220 steel had a higher surface- hardness (by 150-200 HV0.1), but lower near-surface hardness and bulk hardness (by 300-400 HV0.1). However, PNC treatment of QTtreated SNCM220 steel showed better wear resistance. This was due to a smaller worn-wear-track section and a lower friction-coefficient, compared with the result from GCN treatment. Therefore, in the very near future, it is expected that gas carbo nitriding treatment could be replaced by the combined QT+Plasma nitrocarburizing process for manufacturing vehicle parts. |
|
|
| Key Words |
| plasma nitrocarburizing, metals, surface modification, wear, hardness test |
|
|
 |
|
