Nitriding surface hardening is commonly used on steel components for high wear, fatigue and corrosion applications. Case hardening results from white layer formation and coherent alloy nitride precipitates in the diffusion zone. This paper evaluates the microstructure development in the nitrided case and its effects on the hardness in both the white layer and the substrate for two industry nitriding materials, Nitralloy 135M and AISI 4140. Computational thermodynamic calculations were used to identify the type and amount of stable alloy nitrides precipitation and helped explain the differences in the white layer hardness, degree of porosity at the surface, and the hardening effect within the substrate. Some initial insights toward designing nitriding alloys are shown.

Nitriding is a surface hardening treatment used on steel components to improve their resistance to corrosion, fatigue, and wear. Iron nitrides at the nitrided steel surface form a compound layer known for its high hardness but also for its brittle nature. It is not uncommon for this layer to chip or break away during metallurgical sample preparation, making it difficult to accurately characterize the microstructure of the nitrided load. This paper presents the results of several studies that assess the effect of cutting and polishing operations along with polishing pressure, the use of foils, and Ni plating. A best practice procedure has been developed to prevent damage to nitrided samples and minimize uncertainty when evaluating part quality.

Microstructural examination of a nitrided part is the most commonly used method for evaluating nitriding material and process performance. Microstructural evaluation also helps to validate that the process ran as intended and produced the desired nitrided case characteristics. However, sample preparation is often complicated by the partial or complete breakaway of the compound layer and may affect the accuracy of the conclusions made. A set of experiments was performed to evaluate the effect of two saw cutting methods, the use of metal foil for sample mounting, and the use of Ni plating before cutting. Microstructures of 12 experimental conditions were analyzed. Recommendations were made for the nitrided sample preparation best practice to analyze compound layer uniformity and thickness.

Accurate assessment of heat treat (HT) growth on carburized ring gears is of critical importance when developing new gears or implementing various design/process changes on current production gears. The traditional approach has been to conduct expensive and time consuming HT trials with green and after- HT measurements on a case-by-case basis. An advancement of this process was to create an extensive database in order to develop a predictive model. Various statistical analyses were performed using Minitab. Ring gear HT growth on measurements between pins expressed in % growth gave better predicting power than delta (mm) growth. The best subset model with green hardness data utilizes 7 factors (material, key geometrical features) and yields 98.3% R 2 . The model developed from a larger dataset without green hardness yields 89.8% R 2 . On-going work includes continuously updating the database and refining the model. This work will help minimize the number of trials needed for new product launches and shortening of the development cycle.

Ring gear distortion from heat treatment processes has been a challenge to gear manufacturers. To provide good post heat treatment dimensions, efforts have been made to optimize post heat treat controls such as quench apparatus, quench severity and post heat treatment grinding. However, the impact of prior-to-heat treatment gear microstructure is not fully understood. In this study, ring gears with different prior-to-heat treatment (initial) microstructures were compared against post-heat treatment dimensional measurements. Statistical analyses were performed on data collected over 2 years at a production facility. Heat treat growth was found to be strongly affected by the initial microstructures and green hardness. Recommendations were made to help reduce heat treat growth variation.