Plasma nitriding of Fe30Ni alloy, synthesized from elementary powder and processed by slip casting

Full text: Plasma nitriding is a thermochemical treatment for surface hardening, which allows modifying the mechanical behaviour of steels. Plasma-assisted nitriding treatments on austenitic alloys at low temperature produce the so-called 'expanded austenite' [1]. Even if the debate is still open, it is now well accepted by the scientific community that below 420 °C, a plasma-assisted nitriding (PAN) treatment of austenitic stainless steel forms a peculiar phase usually called expanded austenite and noted γ_N in this communication. The γ_N phase is a metastable nitrogen supersaturated solid solution with a disordered fcc structure and a distorted crystal lattice. The objective of this paper is evaluating the conditions of plasma nitriding of the Fe30Ni alloy processed by colloidal rote. In this case, was produced a water suspension with 40 wt.% of solids and its stability was controlled adding 2.0 wt.% (Fe) and 0.6 wt.% (Ni) of dispersant with pH adjusted in 10. The compressed samples was sintered in 900 deg C and 1100 deg C during 1 hour under the one flow of Ar/5%H₂ and was evaluated the different conditions of plasma nitriding on the Fe30Ni alloy using elementary powder. The parameters of nitriding were 1 hour of heating on atmosphere of Ar and H₂ and 2 hours of nitriding in 500 deg C, 3 torr of pressure, with atmosphere 60% N₂, 20% H₂ and 20% Ar. The instruments of characterization were optical microscopy, scanning electronic microscopy and x-ray diffraction. Preliminary results have showed dissolution of Ni to into the reticulated of Fe, stabilizing the phase Fe-γ with different nitrogen rich phases. (author)