APPLICATION OF PROBE METHOD TO CARBON CON TENT CONTROL IN PLASMA CARBURIZING LOW-CARBON STEEL

In the plasma carburizing process, the influence of electron temperatu re 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 in creases in plasma current and plasma voltage cause the decreases in el ectron temperature. The carbon content decreases curvilinearly as the electron temperature increases. Generally the carbon content approxima tely 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 v oltage cause the increases in electron density. The carbon content inc reases linearly as electron density increases. Negative charges balanc e with positive charges in plasma, so the density of positively charge d carbon ions increases as the electron density increases, hence the c arbon content increases. Even if the electron temperature or electron density is constant, the carbon content increases as the premixed meth ane ratio increases. Because the increase in premixed methane ratio ca uses the increased retained methane ratio and the increased carbon pot ential. Even if the plasma current or plasma voltage changes in plasma carburizing, the carbon content can be predicted and controlled by th e electron density.