Gaseous nitriding process control: application of customised lehrer diagrams

The state of the art for controlling the nitriding process is to measure and control the nitriding potential in the atmosphere in order to define the nitrogen concentration and phase stability at the steel surface. The experimental Lehrer diagram for pure iron is widely used in industry to specify the nitriding potential for the nitriding process of steels. However, applying the pure iron Lehrer diagram for the steels can lead to erroneous results because of the different phase stabilities in the steels. A customised Lehrer diagram for AISI 4140 had been developed previously based on computational thermodynamics to predict the relationship between the nitriding potential and the phase stability as a function of temperature. The prediction is in excellent agreement with the experimental investigation and this proves the utility of using thermodynamic database to calculate the customised Lehrer diagrams for the steels. In the present work, a series of Lehrer diagrams are developed to analyse the effects of carbon and manganese on the nitriding process. These diagrams can be used to provide the nitriding process parameters for the specified steels but also pave a way for computational materials design of nitridable steels.