Abstract
The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process. Laser absorption spectroscopy was used to determine the resulting process gas composition at different levels of oxygen admixture using O2 and CO2, respectively, as well as the long-term trends of the concentration of major reaction products over the duration of a material treatment of ARMCO® iron. The short-term studies of the resulting process gas composition, as a function of oxygen addition to the process feed gases N2 and H2, showed that a stepwise increase in oxygen addition led to the formation of oxygen-containing species, such as CO, CO2, and H2O, and to a significant decrease in the concentrations of hydrocarbons and HCN. Despite increased oxygen concentration within the process gas, no oxygen enrichment was observed in the compound layer of ARMCO® iron; however, the diffusion depth of nitrogen and carbon increased significantly. Increasing the local nitrogen concentration changed the stoichiometry of the ε-Fe3(N,C)1+x phase in the compound layer and opens up additional degrees of freedom for improved process control.
Highlights
Plasma nitrocarburizing (PNC) is an industrially established process applied in order to improve corrosion resistance and fatigue behavior, as well as the wear resistance of engineering materials like plain carbon or tool steels [1]
The effect of a controlled oxygen admixture to a plasma nitrocarburizing process using active screen technology and an active screen made of carbon was investigated to control the carburizing potential within the plasma-assisted process
As the admixture of oxygen-containing species impacted the concentration of reactive species in the process, we further studied its effect on the material treatment results
Summary
Plasma nitrocarburizing (PNC) is an industrially established process applied in order to improve corrosion resistance and fatigue behavior, as well as the wear resistance of engineering materials like plain carbon or tool steels [1]. In order to stabilize the ε-Fe3(N,C)1+x phase and to avoid the formation of the γ’-Fe4N phase within the compound layer, carbon-containing gas, such as CH4, is admixed to nitrogen-rich plasmas. In addition to the positive effect of small CH4 additions on the nitriding reaction, in the case of excess carbon, the undesired cementite Fe3C phase is formed in the compound layer. Such a cementite formation within nitride layers is detrimental under tribological and fatigue loads and limits the process windows of PNC treatments using carbon-containing gases [3]. By using a solid-phase carbon source, Dalke et al showed that on AISI 4140 steel, compound layers can form, which consist of nearly single-phase ε-Fe3(N,C)1+x without
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