White Layer Control in Gas Nitriding Process Using an Oxygen Sensor

Abstract

There are many advantages in nitriding process, but the formation of white layer sometimes results in trouble. The formation of white layer can be reduced by controlling the nitriding atmosphere appropriately. In this experiment, the nitriding atmosphere is prepared by mixing NH3 and H2. An oxygen sensor is used to detect the condition of the atmosphere, and the value of output voltage (EMF) is used as a signal for controlling the flow rate of H2. The experimental results show that the thickness of white layer can be reduced effectively by controlling the flow rate of H2 through the voltage reading of the atmosphere. Meanwhile, the hardness and the depth of nitriding layer could still be maintained. For nitriding at 550 °C, no white layer is formed when EMF is controlled above 1160 mV and a satisfying hardness distribution of the nitriding layer can be obtained when EMF is controlled at 1140 mV.