Application of Probe Method to Carbon Content Control in Plasma Carburizing Low Carbon Steel

In the plasma carburizing process, the influence of electron temperature and electron density measured by a langmuir probe on carbon content was studied on low carbon steel foils of 0.25 mm in thickness. The increases in plasma current and plasma voltage cause the decreases in electron temperature. The carbon content decreases curvilinearly as the electron temperature increases. Generally the carbon content approximately 0.8 mass% is targeted to obtain an effective mechanical property. But the decreasing rate of carbon content to electron temperature at around 0.8 mass%C is large, so a highly accurate control with electron temperature is difficult. The increase in plasma current and plasma voltage cause the increases in electron density. The carbon content increases linearly as electron density increases. Negative charges balance with positive charges in plasma, so the density of positively charged carbon ions increases as the electron density increases, hence the carbon content increases. Even if the electron temperature or electron density is constant, the carbon content increases as the premixed methane ratio increases. Because the increase in premixed methane ratio causes the increased retained methane ratio and the increased carbon potential. Even if the plasma current or plasma voltage changes in plasma carburizing, the carbon content can be predicted and controlled by the electron density.