Low-pressure hollow cathode plasma source carburizing of AISI 304L austenitic stainless steel at low temperature

Abstract

A carburizing treatment of AISI 304L austenitic stainless steel was performed at a low temperature of 450 °C using the low-pressure hollow cathode plasma source technique. The hollow cathode plasma was produced in a cathodic cage made of stainless steel screen inlaid with a stainless steel tubes array. A pulsed high voltage power, with a peak voltage of 800 V and a pulse frequency of 30 kHz, was applied at a low gas pressure of 50 Pa. The austenitic stainless steel samples were carburized with a direct current bias of −200 V under the CH4/H2 ratio changing in a range from 0.11 to 9, for a carburizing time of 4 h. Under the lower CH4/H2 ratio of 0.11, a diamond-like carbon film of approximately 260 nm thick formed on the stainless steel surface. As the CH4/H2 ratio increased to 0.67 and 1, both carburizing cases possessed a similar thickness of about 11 μm, with the surface carbon concentration being 6 to 6.5 at.%. A layer of carbon supersaturated face-centered-cubic (γC) phase formed, together with carbon-rich (Fe,Cr)5C2 carbide precipitation. Under the highest CH4/H2 ratio of 9, a single γC phase layer was obtained in a carburizing case of similar thickness, with a high surface carbon concentration of 7.5 at.%. The maximum surface hardness value of HV0.25 N 7.1 GPa occurred on the single γC phase layer. No pitting corrosion occurred on the single γC phase in the polarization curve. The higher passive film resistance and the lower donor and acceptor concentrations and flat band potential indicated that a more compact passive film is grown on the γC phase layer. The (Fe,Cr)5C2 carbide precipitates in the γC phase matrix degraded the passivity of the γC phase layer due to the reduced compactness of the passive film. A carbon transport competition mechanism between carbon adsorption onto the surface and carbon inward diffusion that was mainly dependent on the heavier CHn+ ion bombardment, was proposed to explain the transition with increasing CH4/H2 ratio from carbon film deposition, to a carburizing case composed of ae single γC phase, with both increased hardness and higher corrosion resistance.