Increasing power density and rotational speed pose significant challenges for transmission design, especially in the aerospace and electro mobility sectors. Due to increased energy input and reduced heat dissipation, higher operating temperatures occur in high performance gears. At higher temperatures, the hardness and microstructure of conventional bearing and gear materials are affected by annealing effects, which can reduce the load capacity of these components. Therefore, increased operating temperatures can only be considered if the components are made of special heat-resistant, high-performance material systems. Heat treatment is essential to achieve the required performance. Today, high performance gears are typically case hardened to achieve the best performance in service. Due to the meta-stable properties of martensite and retained austenite, especially for low alloy case hardening steels, the microstructure can degrade in service if the temperature equals or exceeds the previous tempering. As a result, the hardness and performance of the components will decrease. Alternative steel grades with increased alloy content can mitigate but are in most cases more expensive. Therefore, an increase in temperature resistance through heat treatment of the low-alloy steels would be of increased interest. To achieve a more stable microstructure state, new heat treatments and alternative microstructures must be considered. This presentation will address the tempering behavior of martensitic and bainitic microstructures under long-term thermal stress above typical tempering conditions at 210 °C for up to 200 hours. The microstructure degradation and hardness change are shown.

As Computational Fluid Dynamics (CFD) methods evolve and mature, more engineering problems are being solved through computer simulation to reduce reliance on the costly and time-consuming experimental methods. This trend is also occurring in the gear manufacturing industry, where an increasing number of quality issues related to the oil quenching process are being investigated by CFD methods to find solutions. However, while the CFD theory and numerical methods have made significant advancements, gaps still exist between the academic research and industrial applications. In the case of the oil quenching processes, the prospect of using CFD methods to visualize and study the oil flow pattern in the gear quenching tank is promising yet challenging. The obstacle to simulating the oil quenching process using CFD methods lies not in the numerical method itself for solving the Navier-Stokes equation, but in building a computer simulation model that encompasses all the geometrical details of the quenching tank, fixtures, centrifugal pumps, and gears, including all the gear teeth. This task is particularly challenging for Finite Volume Method (FVM) CFD solvers, as the computation mesh could take days or weeks to build. In this research, a new solution method based on Smoothed Particle Hydrodynamics (SPH) is introduced to simulate the oil flow in the gear quenching tank. Since SPH is a mesh-free Lagrangian method, it not only greatly simplifies the mesh generation task for building the computational models but also handles the complex physics of the free surface flow and fluid-structure interaction with great ease. In addition, the oil flow in the gear quenching tank usually is driven by centrifugal pumps whose dynamics can be simulated directly in SPH methods, as opposed to FVM methods which require complicated moving mesh computation.

Heat-treatment simulation is a powerful tool for gear design and process troubleshooting, but many times the predicted gear distortion is difficult to compare to physical gear measurements and to required specification charts or measurements. To help ease this burden, two software programs are utilized to provide powerful gear analyses to heat-treatment simulation results. This paper briefly describes the software used, DANTE and Integrated Gear Design (IGD), and presents a simple case study. The stress and deformation from the heat treatment of a small gear made of SAE 10B22 are predicted using DANTE. The distorted gear geometry is then imported into IGD and the predicted distortion is compared to the actual measurements of the gear.

As consumers embrace Electric Vehicle (EV) technology, the automotive industry is moving quickly into replacing internal combustion engines (ICE) and traditional transmissions. The change to electrically driven vehicles offers new challenges to the gear manufacturing world, and most importantly new specifications to heat treat these gears - specifically quieter gear sets and higher torque ratings. Today’s EVs have a much lower tolerance for noise from the gear set to power the vehicle; therefore, this continues the need for even quieter and stronger gears. This technical presentation will illustrate the heat treat and distortion specifications for these new gears, along with answering the “why” of selecting low pressure vacuum carburizing (LPC) for new programs around the world.

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.

Vacuum carburizing 9310 gear steel followed by austenitizing, oil quench, cryogenic treatment, and tempering is known to impact the residual stress state of the material. Residual stress magnitude and depth distribution can have adverse effects on part distortion during intermediary and finish machining steps. This study provides residual stress measurement, microstructural, and mechanical property data for test samples undergoing a specific heat treat sequence. Test rings of 9310 steel are subjected to a representative gear manufacturing sequence that includes normalizing, rough machining, vacuum carburizing to 0.03”, austenitizing, quench, cryo-treatment, temper, and finish machining. The rings along with metallurgical samples are characterized after each step in order to track residual stress and microstructural changes. The results presented here are particularly interesting because the highest compressive residual stresses appear after removal of copper masking, not after quenching as expected. Data can be used for future ICME models of the heat treat and subsequent machining steps. Analytical methods employed include X-ray diffraction, optical and electron microscopy, and hardness testing.

This paper compares and contrasts heat treat processes and equipment typically used to harden gears. It discusses the basic design and operation of vacuum, controlled atmosphere, and hybrid furnaces and process techniques such as carburizing, carbonitriding, nitriding, nitrocarburizing, and neutral hardening. It also includes information on operating and maintenance costs, using batch integral quench furnaces as the base case for comparison. A discussion on when to consider continuous furnace types is included as well.

The objective of this work is to develop the material and numerical models needed to simulate the carburizing process of an automotive gear. The paper discusses the factors that influence calculation time and accuracy and presents important equations and material property data. It describes how the simulation predicts local carbon content based on diffusion and how quenching computation provides information on stress states and residual stresses. It also explains how to account for the effects of grain growth, volume variation due to phase changes, and transformation plasticity.

In this contribution, we attempt to optimize the heat process of a gear. The goal is to evaluate the best use of two frequencies to complete the heating phase in the least amount of time achieving a uniform distribution of temperature in the treated area of the gear. Assuming the application of the appropriate cooling, this temperature distribution will lead to the appropriate hardness along and between the teeth of the gear. A 3D model of part of the gear wheel is modelled in a multiphysics magneto-thermal analysis. The two frequencies used for the heating are first evaluated. In the optimization process, the duration of the application for each frequency is a parameter. Temperatures are evaluated through the heating process at selected locations as constraints of the optimization process.

Low pressure carbonitriding (LPCN) has the potential to improve the impact and fatigue strength of steel components through the enrichment of nitrogen and the effect of carburizing at higher temperatures. The work described in this paper investigates the influence of boron on the LPCN response of 20MnCr5 steel and the effect of niobium on that of 8620. LPCN treatments were developed to achieve a surface hardness of ~700 HV and case depth of 0.65-0.75 mm in four alloys: 20MnCr5, 20MnCr5 + B, 8620, and 8620 + Nb. The hardness and case microstructure of treated and quenched test samples are correlated with bending fatigue measured in Brugger fatigue specimens, which simulate the root of a gear tooth.

A number of modifications were made to a batch quenching process for pinion gears to reduce the amount of size change in the ID. This paper assesses the impact of adding vertical plates to the load elevator to better condition oil flow to the stacked part baskets. Data collected from pinion gears before and after the modification show a reduction in the average and range of ID bore change, indicating an improvement in quench uniformity. CFD analyses suggest that improvement is due to a significant reduction in turbulence, resulting from the addition of the vertical plates. As the authors explain, high levels of turbulence promote collapse of the vapor film that occurs at the start of the quench process, and disparity in the timing causes unwanted variation in part size change throughout the load.

Press quenching is often used to harden parts that are sensitive to distortion, but it is a difficult process to control due to the effects of tooling and the relatively large number of process parameters. In this paper, the authors show how they use finite element analysis to optimize the process and tooling design for a spiral bevel gear made of carburized 9310 steel. Several designs adaptations are assessed, one of which is shown to minimize radial shrinkage and taper distortion in the inner diameter of the bore.

Application of 3D finite element method (FEM) simulation for induction hardening of gears is still a time-consuming and expensive task. The significant cost of a simulation remains in the manual preparation of the 3D description of geometry. In the current work, we propose to complement the numeric simulations with automatic geometry generation based on a parametric representation of a gear and an induction coil. The parameters used to describe a gear are module, pitch diameter, and pressure angle. The circular coil is described by the height, external and internal diameters. FEM computations are implemented to solve magneto-quasi-static Maxwell’s equations. A demonstration of the possibilities of the proposed approach via a parametric study is presented by varying the module of a gear while keeping a constant number of teeth. A heuristic tuning of heating power frequency- time is presented here and compared to the classical semi-analytical equations and 2D simulations.

Quench hardening is a transient thermal stress process with phase transformations. It is inevitable that a component will go through plastic deformation due to phase transformations, which will lead to distortion in the hardened part. Understanding the sources of distortion is necessary in designing the heat treat process and component configuration to obtain a product with greater dimensional accuracy. It is worth mentioning that consistent distortion can be compensated by adjusting the part dimensions prior to hardening. The possible sources of distortion include residual stresses prior to hardening, heating rate, austenitizing temperature, soaking time, quenching rate and uniformity, and possible tooling constraints, etc. The significance of these effects varies according to the part geometry and heat treatment process. Characterization of material properties and the development of computer modeling made it possible to understand the material and component responses during quench hardening, which is the key to process improvement and part configuration optimization. In this paper, the hardening process of a simplified bevel gear with thin-wall feature made of AISI 9310 is analyzed using DANTE, and the effect of tooling used in a press quench on distortion is investigated. The causes of distortion are analyzed through the material response aspect using the modeling results.

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.

Case hardening by carburizing is the most common heat treatment in mass production, which relies on atmosphere, or vacuum carburizing followed by oil or gas quenching and finally by tempering. Parts being heat-treated undergo the process in a configuration of a batch consists of hundreds or even thousands pieces. Under these circumstances, individual parts can’t help but be exposed to different process parameters in terms of temperature, atmosphere and quenching depends on their position within the batch. Parts near the outer portion of the load see a more rapid rise in temperature, are first exposed to the carburizing atmosphere and are more effectively quenched than parts located in the center of the batch. This can lead to significant variation from part to part and load to load; the resultant effective case depth deviation can be as high as 50%. Similarly, during quenching from hardening temperature distortion becomes highly unpredictable and unrepeatable. Modern industry demands greater precision and repeatability of results beyond those achievable by so-called traditional batch or continuous technologies and their associated equipment. Elimination of batches and focus on individual parts is the only true way to advance the industry. The article will introduce the first operational system for truly single-piece flow method for case hardening by low-pressure carburizing and hardening by high-pressure gas quench. The system treats each part individually and as such provides virtually identical process parameters, which results in extremely accurate and repeatable results. Quenching one part at a time in a specially design chamber, achieves more precise control and significantly reduces distortion so as to all make it possible to avoid post heat treatment hard machining operations. This single-piece flow heat treatment method is easily adapted into manufacturing and can be directly integrated into in-line manufacturing operations, working directly with machining centers. Materials handling and logistical issues are eliminated thus saving time and reducing unit cost. The results achieved on series of automotive gears will be reported and demonstrate incredible accuracy and repeatability, while significantly reducing distortion. Productivity and process costs prove the system to be highly competitive with other technologies. These proven advantages and savings.

Low pressure carburizing (LPC) in a vacuum furnace is increasingly the preferred method of case hardening for aerospace gears, and acetylene is often one of the gases used in the process. Selective case hardening is common with gears, where certain sections of a part are “stopped off” or “masked” to prevent carburization at those locations. For aerospace parts, the masking used is typically copper electroplating. The low pressures and high temperatures used in LPC lead to copper evaporation, which contaminates the vacuum furnace hot zone and components. In a worst-case scenario, deposited copper can lead to short-circuiting of power feedthroughs. This study looks at the effect of vacuum and partial pressure gases on copper evaporation and its application in production processes.

The development of residual stress in an induction hardened small spur gear is numerically simulated. A full scale 3D simulation is utilized to obtain the results, providing the possibility to evaluate the complete distribution of residual stress in the hardened component. Electromagnetic and thermal solutions under induction heating conditions are obtained with Cedrat Flux 3D, whereas EDF Code Aster software is used for thermal simulation during the quenching stage, phase transformation, and stress-strain simulations. The simulated induction heating isotherms and distribution of residual stress are compared with experimental investigations done by Larregain et al. and Savaria et al.

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.

Eddy current is a non-destructive testing technique proven for use in heat treat and material structure verification. Modern multi-frequency eddy current instruments can test for conditions such as misplaced case, shallow case, short heat, short quench, and delayed quench. Eddy current testing offers many benefits over traditional heat treat validation methods. Unlike sample testing processes using cut, polish, etch, and visual inspection techniques, eddy current testing provides a clean, fast, and repeatable process that can perform in-line inspections of all parts produced. Eddy current inspections have traditionally focused on symmetrical parts such as wheel bearings and gears. However, advances in robotics have paved the way for cost-effective inspection of non-symmetrical, complex components that would have previously required multiple test stations. Robotics also provides a low-cost way to retest, null, and periodically proof the testing process using multiple conditions of masters. This has been difficult and expensive with other types of automation and operator involvement.