Low-alloy steel (SCr42O) bulk specimens were plasma-carburized in methane/hydrogen mixed gas. The carburizing gas near a specimen was collected by a sampling cylinder and a piston system, and the retained methane in carburizing gas was measured by gas chromatography. The influences of treating temperature, plasma energy and vacumm-pumping speed on the retained methane were studied.
The decomposing speed of methane being quite slow, the partial pressure of methane was extremely higher than the equilibrium partial pressure in plasma-carburizing. Since the methane was decomposed by heating, the retained methane decreased as treating temperature increased. It also decreased by plasma energy. As the retentive time of the carburizing gas in the furnace decreased with increasing pumping speed, the retained methane increased.
The influences of the retained methane, treating temperature, pressure, time and pumping speed on the surface carbon content of specimen were also studied. The surface carbon content of the specimen was determined by the retained methane in the temperature range of 1203 and 1373 K. It increased as the retained methane increased. Under the stable glow discharge, it increased as the chamber pressure, pumping speed and treating time increased.
The number of carbon atoms which were permeated into the specimen would agree with that of the dissociated carbon from the methane by plasma energy.
