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.

The cooling history of carburized heat-treated gears plays a significant role in developing microstructure, hardness, and residual stress in the tooth that influences the fatigue performance of the gear. Evaluating gear carburizing heat treatment should include a microstructure and hardened depth evaluation. This can be done on an actual part or with a test piece. The best practice for a test piece is to use a section size that closely approximates the cooling rate at the gear flank of the actual gear. This study furthers work already presented showing the correct test piece size that should be used for different gear modules (tooth thicknesses). Metallurgical comparisons between test pieces, actual gears, and FEA simulations are shown.

Modeling of as-tempered hardness in steel is essential to understanding final properties of heat-treated components. Most of the tempering mathematical models derive a tempering parameter using Hollomon-Jaffe formulation. Some recent models incorporate chemical composition into the general Hollomon-Jaffe relationship. This paper compares model predictions with a substantial set of actual tempered Jominy End Quench bars and the hardness data from them. Improvements to the models and direction for future work are discussed.

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.

Test pieces are often used for hardened depth and microstructure checks on carburize and harden heat treatment processes so that actual parts are not destroyed for the sake of quality assurance. For gear heat treatment, this is especially important because of costly prior processing. This paper reports on a study to determine the proper size and material of a cylindrical test piece that could be used as an appropriate indicator of the hardened depth and microstructure of the actual gear. Heat treat simulation is used to examine cooling rates for various diameters of test pieces to see how they compare to different modules of tooth sizes on gears. The surface cooling rates up to depths of 1.5 mm are examined to size the test piece correctly.

Developing heat treat systems and control plans that produce consistent direct harden (quench and temper) results with a high percent martensite and the corresponding proper mechanical properties is challenging for large components or large batch sizes. In this study, large section bars in alloys suitable for water quenching were austenitized and quenched under controlled flow conditions. The bars were primarily examined by several as-quenched hardness versus depth traverses in order to be sure localized non-martensitic regions (soft areas) would be detected. The tests allowed for some key insights into defining the adequacy of direct harden water quench systems, including the idea of agitation thresholds required for each alloy grade or hardenability level to prevent soft spots (spotty hardening) on large section steel components.

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.