Effects of Plasma-Chemical Composition on AISI 316L Surface Modification by Active Screen Nitrocarburizing Using Gaseous and Solid Carbon Precursors

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.

Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part I.: Applicability of the Beer–Lambert law and interpretation of spectroscopic data

“…The line HCN 1356.854 21 cm −1 is marked by a star symbol. A spectral line at this position was consistently assigned to the C 2 H 2 1356.855 432 cm −1 transition in previous publications [10,[16][17][18]20], due to incomplete HITRAN line lists at that time. Moreover, at higher carbon content a weaker line around 1356.88 cm −1 (based on experimental accuracy) appeared, which was assigned previously to C 2 H 2 1356.888 13 cm −1 with plausible intensity ratio relative to the line C 2 H 2 1356.855 432 cm −1 .…”

“…Only a difference of 20 K causes already qualitative changes in the treatment results, see for example [22]. For investigation of heat treatment processes the major part of the LAS diagnostics path is aligned through the active reactor zone [10,[15][16][17][18]20]. The new empirical observations, made in the current work, indicate inconsistencies with previous interpretation of the LAS data.…”

“…However, for the current ASPNC reactor a uniform temperature was expected, as the AS is the main heat source and heats the major part of the reactor volume, see figure 1. For this reason, it was assumed in previous works [10,[16][17][18]20] that for the evaluation of the densities using equation ( 8) the gas temperature is equal to the process temperature. The observed difference in T eff for each species as reported in this work calls for another interpretation of the LAS measurements.…”

“…Monitoring the composition of the discharge is important for the process control and optimization as well as for understanding of the chemical kinetics. Three approaches are broadly reported: optical emission spectroscopy (OES) [2,10,11], mass spectrometry (MS) [12][13][14] and laser absorption spectroscopy (LAS) [1,3,10,[15][16][17][18][19][20]. OES monitors exclusively electronically excited species and quantitative interpretation of OES data requires complex collisional-radiative models.…”

Laser absorption spectroscopy for plasma-assisted thermochemical treatment. Part II.: Impact of the carbon and water contaminants on a low-pressure N2–H2 discharge

“…Rembges and Oppel [13] suggest that the presence of oxygen can be compensated for with a flow of carbon-containing gas, which is recommended to be controlled with 0.1% accuracy. Also, the most abundant carbon-containing reaction product in ASPNC processes, HCN, shows a strong hysteresis in its concentration, about a factor of 2 despite a stabilization time of 20 min between measurements [14]. The effect of contaminated inner reactor surfaces on HCN densities was pointed out in [15] and continuous in-line monitoring of the process gas composition is suggested to overcome this issue.…”