Alloying Factors and Parameter of Alloying Elements for Carbon and Nitrogen Uptake during Carbonitriding as Basis for Simulation

Abstract

Carbonitriding serves to increase the strength and wear properties of steel components. The carbon and nitrogen concentration and the depth distribution decisively determine the resulting properties. Optimal profiles create an ideal microstructure of martensite, residual austenite, finely distributed nitrides, carbonitrides and inherent compressive stress in the surface zone. The reliability of carbonitriding heat treatment process is strongly dependent on the possibilities of process control. Previous investigations aimed at measuring the nitrogen potential of the atmosphere by means of an ammonia sensor in the exhaust gas as well as by a wire sensor. Carbon potential is conventionally controlled using an oxygen probe. In order to further increase the process reliability of carbonitriding, simulation of the carbon and nitrogen profiles and of any precipitation of carbides and nitrides are necessary. The first step is the determination of interdependent alloying influences on the carbon and nitrogen contents. The carbon and nitrogen activities in the atmosphere and the alloy surface are near equilibrium after long-time carbonitriding. Depending on the composition of the material, significantly different effects can be described. These influences must be considered. In addition to the reached equilibrium content, the diffusion, the position of phase boundaries in the phase diagrams and the formation of precipitation with influence of carbon and nitrogen as well as interactions with other alloying elements are to be worked out in order to further develop controlled carbonitriding for the reliable adjustment of the heat treatment results.

Development of a Prediction Method for Carbonitrided Surface Carbon and Nitrogen Contents by Computational Thermodynamics and Validation by Carbonitriding for Pure Iron

“…Thus, although the NH 3 gas content remained constant, an increase in H 2 gas content resulted in a decrease in the nitrogen content. Therefore, as demonstrated previously, 12) both the NH 3 and H 2 gas contents affect the nitrogen content after carbonitriding.…”

“…The relationship between the nitrogen content and the thermodynamic properties (e.g., the chemical potential and activity) can be calculated using these methods. Furthermore, we note that the nitriding potential, K N , during the carbonitriding process can be controlled using hardware such as sensors; 4,5,12) hence, the nitrogen content can be accurately predicted. Table 1 presents the chemical composition of the foil employed for this study.…”

“…4,5) In addition, numerous prediction methods have been reported for the carbon and nitrogen contents of alloyed steels through use of the carbon and nitrogen potentials of the atmosphere. 11,12) In these methods, the effects of alloying elements on the equilibrium contents were also taken into account, and these methods were proven to exhibit sufficient accuracy to predict the carbon and nitrogen contents in the carbonitriding of alloyed steels under specific conditions. 11,12) However, the influence of the alloying elements can vary with the temperature and the constituent phases; therefore, a more effective method is required.…”