Abstract
Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.
Highlights
Austenitic stainless steels are characterized by superior corrosion resistance and are widely applied in different fields, such as the food, biomedical, and chemical processing industries [1]
Low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor
It can be proposed that the difference in the total consumed power in the cases of the steel AS and the CFC-AS material might be related to peculiarities of the AS construction influencing the heat exchange between the AS and the reactor wall
Summary
Austenitic stainless steels are characterized by superior corrosion resistance and are widely applied in different fields, such as the food, biomedical, and chemical processing industries [1]. Plasma-assisted thermochemical diffusion treatment has drawn significant attention for its capacity to enhance the surface properties of stainless steels and its simple process implementation, low cost, and low environmental impact. Different plasmaassisted treatments, such as plasma nitriding (PN) [4,5], plasma carburizing (PC) [6,7], and plasma nitrocarburizing (PNC) [8,9], have been realized by introducing nitrogen- and/or carbon-containing precursor gases to the process atmosphere. The expanded austenite layer generated by the simultaneous incorporation of nitrogen and carbon atoms consists of a near-surface nitrogen-expanded austenite layer (γN) and a subjacent carbon-expanded austenite (γC) layer [10,23]
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