On the influence of carbon contamination of reactor parts in active screen plasma nitrocarburizing processes

Abstract

Compared to conventional plasma nitrocarburizing the active screen plasma nitrocarburizing technology reduces significantly the risk of soot production and cementite precipitation in the compound layer of nitrocarburized materials. However, in long-time treatments, the uncontrollable contamination even up to full saturation of the metallic active screen and of the walls of the used plasma reactor with carbon compounds is still a remaining factor of uncertainty. It may result in an increased carbon concentration or even in an appearance of the cementite phase in the compound layer of the treated steel surface, as in the case of 42CrMo4 (AISI 4140). The absolute concentration of hydrogen cyanide (HCN), in situ monitored by tunable diode laser absorption spectroscopy, and the emission intensity of the CN (0–0) band of the violet system at a wavelength of λ = 388 nm, recorded by optical emission spectroscopy, were found to be reliable parameters to control the carburizing potential of the nitrocarburizing plasma. A close relationship was found between (i) the concentration of HCN and the CN band emission intensity with (ii) the level of the methane admixture in the feed gas and the time-dependent contamination effects of the active screen and other inner reactor surfaces with carbon compounds. Therefore, this relation could serve as a basis for an in-line control of the carburizing activity in plasma nitrocarburizing processes with methane admixture using an active screen.