A study of the effect of propane addition on plasma nitrocarburizing for AISI 1045 steel

• Propane was selected as carbon bearing gas in plasma nitrocarburizing due to its lower ionization energy compared with methane.
• The thickness of compound layer and surface hardness reached the maximum with optimum addition (1.5%) of propane.
• Cementite hindered the nitrogen and carbon up-take, which contributed to a decrease in case depth and surface hardness.

Propane was used as carbon-bearing gas in plasma nitrocarburizing due to its lower ionization energy and the effect of its concentration on the nitrocarburizing rate and phase compositions for AISI 1045 steel was investigated. The surfaces of nitrocarburized samples were analysed by optical microscopy, X-ray diffraction and by using a microhardness tester. The results showed that the case depth increased initially with the increasing concentration of propane, and then decreased when it exceeded a limit of 1.5%. The maximum case depth of 40 μm was achieved at this optimum concentration of 1.5% when nitrocarburizing at 510 °C for 4 h, and a maximum surface hardness of 7.63 GPa and desirable phases of dominant ε nitride with a little cementite were obtained. The amount of cementite increased with the increasing concentration of propane above 1.5%, which hindered the nitrogen and carbon up-take, thus resulting in the decrease of case depth and surface hardness.