Prof. Tatsuo Inoue passed away on September 23, 2023, at the age of 83. He held a professorship at Kyoto University from 1983 to 2003 and made significant contributions to the theory of heat treatment simulation, which is now widely used. His theory was reported at an international conference in Linkoping, Sweden in 1984. Fundamental equations in his theory cover metallurgical coupling effects caused by changes due to phase transformation, temperature, and inelastic stress/strain as well as carbon diffusion during the carburizing process. Prof. Inoue designated these effects as “metallothermo- mechanical coupling”. Software applying his theory was presented at ASM International’s 1st International Conference on Quenching and the Control of Distortion in 1992, where its advanced nature was recognized. In 1994, Prof. Inoue published a paper on the application of heat treatment simulation to the quenching of Japanese swords, revealing changes in temperature, curving, microstructure, and stress/strain in their model during the traditional quenching process. In 2017, he published “The Science of Japanese Swords” with Sumihira Manabe, a swordsmith, to communicate his specific achievements to the general public.

From concept and design to production readiness, introducing a new product or service can be expensive. Because of the lower cost of smart technologies, it is now possible to validate designs with less rounds of prototyping and testing. This leads to savings in the design phase, improved product quality, and less materials and energy being used to bring the product or service to market. The sum total of these savings’ lower startup costs.

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.

Heat treatment simulation has progressed to the point where commercial software is widely available, and validations of simulation functions using experimental results have played a big role in getting here. For this reason, the author presents a number of validation cases and explains what relatively simple experiments can reveal about the complex phenomena of heat treating. In the case of validating basic functions, such as heat transfer and phase transformation, the author uses experimental results of the inverse hardening of quenched steel cylinders. When validating software at the stage where stress and strain functions are added, the author uses measurement data corresponding to length and diameter changes and residual stress distributions in normally quenched steel cylinders. Other cases presented include the validation of curving in long specimens cooled unevenly and the validation of distortions and residual stresses in carburized and quenched, induction hardened and nitrided steels.

A physics-based software model is being developed to predict the nitriding and ferritic nitrocarburizing (FNC) performance of quenched and tempered steels with tempered martensitic microstructure. The microstructure of the nitrided and FNC steels is comprised of a white compound layer of nitrides (ε and γ’) and carbides below the surface with a hardened diffusion zone (i.e., case) that is rich in nitrogen and carbon. The composition of the compound layer is predicted using computational thermodynamics to develop alloy specific nitriding potential KN and carburizing potential KC phase diagrams. The thickness of the compound layer is predicted using parabolic kinetics. The diffusion in the tempered martensite case is modeled using diffusion with a reaction. Diffusion paths are also developed on these potential diagrams. These model predictions are compared with experimental results.

Commercially, carbon steels are induction heated at heating rates on the order of 100 to 1,000 °C·s -1 for surface hardening. The high precision DIL 805L dilatometer employs induction heating and is often used to study transformation characteristics and prepare test specimens for metallurgical analysis. However, heating the commonly used 4 mm diameter by 10 mm long specimens at rates above 50 °C·s -1 results in non-linear heating rates during transformation to austenite and large transient temperature variations along the specimen length. These limitations in heating rate and variances from ideal uniform heating can lead to inaccurate characterization of the transformation behavior compared to commercial induction hardening practices. In this study it is shown that changing the specimen design to a thin wall tube allows faster heating rates up to 600 °C·s -1 and modifies the pattern of temperature variations within the test sample. The response of selected specimen geometries to induction heating in the dilatometer is characterized by modelling and tests using multiple thermocouples are used to verify the models. It is demonstrated that the use of properly designed tubular test specimens can aid in more accurately establishing transformation characteristics during commercial induction hardening.

Heat treaters are adopting more and more Industry 4.0 techniques and solution packages to improve production processes and product quality. Proper specification, measurement, and control of heat-treating atmospheres are always critical to achieving the desired metallurgical and microstructural results. The combination of atmosphere measurements and other furnace operating parameters (e.g., furnace temperature and pressure) can provide a better view of the whole production. Thermodynamic calculations and field experiences can be integrated into the smart solution to provide process engineers more capabilities to manage and optimize production. In this article, our recent research and development work on smart solutions for the heat-treating industry will be presented and discussed.

AISI 52100 is a high carbon alloy steel typically used in bearings. One hardening heat treatment method for AISI 52100 is austempering, in which the steel is heated to above austenitizing temperature, cooled to just above martensite starting (Ms) temperature in quench media (typically molten salt), held at that temperature until the transformation to bainite is completed and then cooled further to room temperature. Different austempering temperatures and holding times will develop different bainite percentages in the steel and result in different mechanical properties. In the present work, the bainitic transformation kinetics of AISI 52100 were investigated through experiments and simulation. Molten salt austempering trials of AISI 52100 were conducted at selected austempering temperatures and holding times. The austempered samples were characterized and the bainitic transformation kinetics were analyzed by Avrami equations using measured hardness data. The CHTE quench probe was used to measure the cooling curves in the molten salt from austenitizing temperature to the selected austempering temperatures. The heat transfer coefficient (HTC) was calculated with the measured cooling rates and used to calculate the bainitic transformation kinetics via DANTE software. The experimental results were compared with the calculated results and they had good agreement.

Computer simulations are increasingly being used in the automotive industry to evaluate the state of stress in cylinder blocks during casting and heat treat processes. With recent advancements, it is now possible to model casting and quenching processes as well as residual stress and high cycle fatigue. However, calculating the final stress in cylinder blocks requires the integration of several software tools with different meshing topologies, numerical methods, data structures, and post-processing capabilities. The intent of this research is to develop an integrated virtual engineering environment that combines casting simulation, computational fluid dynamics, and finite element methods in order to simulate the manufacturing process from the beginning of casting, through water quenching heat treatment, to engine dynamometer testing. The computational environment is built on three CAE tools, Magmasoft, AVL Fire, and Abaqus, and required considerable amounts of research and development to validate each numerical method and the tools that facilitate data exchange between them.

The knowledge of the thermal boundary conditions helps to understand the heat transfer phenomena that takes place during heat treatment processes. Heat Transfer Coefficients (HTC) describe the heat exchange between the surface of an object and the surrounding medium. The Fireworks Algorithm (FWA) method was used on near-surface temperature-time cooling curve data obtained with the so-called Tensi multithermocouple 12.5 mm diameter x 45 mm Inconel 600 probe. The fitness function to be minimized by a Fireworks Algorithm (FWA) approach is defined by the deviation of the measured and calculated cooling curves. The FWA algorithm was parallelized and implemented on a Graphics Processing Unit architecture. This paper describes the FWA methodology used to compare and differentiate the potential quenching properties of a series of vegetable oils, including cottonseed, peanut, canola, coconut, palm, sunflower, corn, and soybean oil, versus a typical accelerated petroleum oil quenchant.

Excessive distortion was observed in many small components made from 1080 steel that was neutral hardened following stamping. A study was then undertaken to determine how to reduce the distortion of the heat-treated parts while maintaining proper hardness and microstructure. A numerical simulation based on Simheat software was conducted to determine the effect of elevated temperature on the quenching oil used and its impact on distortion and microstructure. A second oil designed to operate at higher temperatures was also examined. Using Simheat software, the two oils were compared based on predicted distortion, hardness, and microstructure and the results were subsequently validated using empirical methods. It was concluded that a significant improvement in distortion could be achieved by using a different oil and higher quench temperatures.

This paper revisits a case study presented at Heat Treat 2009, investigating the failure of induction coils used for heat treating automotive wheel hubs. At the time, computer simulation was beginning to allow for virtual prototyping of heat treat applications as an alternative to experimental testing. As explained in the original paper on p. 86 of the 2009 HTS conference proceedings, although simulation helped in the development of a more robust coil, it was not used to pinpoint the cause of failure. In this current work, the authors tackle the same problem aided by more than a decade of improvements in compute power and finite element analysis techniques. To highlight the leaps made in virtual prototyping, the induction hardening coil previously analyzed using an axisymmetric 2D model is now examined using more precise 3D electromagnetic and thermal models while accounting for the rotation of the part.

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.

The work presented in this paper addresses a data gap that continues to be a hinderance to users of precipitation modeling tools, particularly those based on Langer-Schwartz theory. Thermodynamic and kinetic data required for precipitation models can be obtained from CALPHAD databases, but interfacial energies between the bulk and precipitate phases are not available for many alloy systems. In this work, a number of matrix-precipitate interfacial energies have been determined for influential precipitates in alloys of industrial importance, for example, carbides in Grade 22 low-alloy steels, delta phase in Ni 625 and 718, S-phase in Al 2024, and Q’ and β’’ in Al 6111.

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.

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.

In this work, the authors employ computer modeling to investigate a quenching process recently demonstrated at Karlsruhe Institute of Technology. A matrix of models was run to assess the effects of heat transfer and phase transformation kinetics on residual stress and microstructure in a relatively thick walled tube. The experiments at Karlsruhe were conducted using a high pressure water quench to produce martensite and residual compressive stress in the bore of a 4140 steel tube. Results show that the timing and rate of martensite formation and bainite kinetics have a significant effect on both the in-process and residual stress state.

For over two decades, heat treat modeling has progressed from an academic concept to a mature production tool. This presentation will discuss many barriers that have been mitigated with a wide range of developments. Early limitations included solver speed and robustness, material data, specialized heating and the requirement to include microstructure development models over a series of dissimilar operations and processes. Solver improvements ranging from parallel processing to specialized iteration methods allow models with millions of elements to run on a personal computer. Additional degrees of freedom have greatly improved solution accuracy. Meshing techniques allow users to identify critical regions for a finer mesh, such as the surface of gear teeth that will be carburized. Rotational (and other) symmetry is frequently used to further refine many models. Driven by the demand for modeling data, sources for quality material properties have increased over the years. Additionally, tools to approximate required data based on chemistry are available and maturing. Radiant, convective, electrical resistance and induction heating effects are incorporated into heat treat simulation systems. Integrated simulation systems also include large deformation behavior to capture the effects of forging, coining or other mechanical processes on the microstructure. A vision of the future will include the use of Design of Experiments (DOE) and optimization in heat treat simulation. How companies will model the entire process chain to build a more accurate fatigue model for the part in service will be discussed. In terms of TRL (technology readiness level), heat treat simulation was in the 2 – 3 range in the 1990’s. Today it is in the 7 – 8 range and moving quickly.

Heat treaters are encountering an ever-increasing need for practical process design and troubleshooting methods to effectively address quality, cost and production time requirements for thermal treatment of steel parts. Over the last two decades, substantial advances have been made in heat treatment process modeling, now permitting user-friendly and robust means for process engineers, designers, and other heat treatment technical professionals to readily apply advanced modeling technology to address complex, “real-life” heat treatment challenges. DANTE modeling software has now been implemented for ready application to carburizing and hardening processes with the consideration of phase transformation, following the process parameters input from heat treaters. This paper highlights a user-friendly and advanced modeling tool now available for solving practical heat treatment challenges. Several case studies using DANTE will cover induction hardening, press quenching and plug quenching, and low pressure carburizing. Also shown are the important benefits received from this technology, including minimization of the costly “trial and error” approach to troubleshooting, and evaluating the effect of process parameters on part quality.

Dilatometry test systems are commonly used for characterizing the transformation behavior in steels and induction heating is commonly the heating source. In these systems, the steel test article is assumed to have a uniform temperature throughout the sample. This is a good assumption for slow heating rates with small samples, however, for induction hardening cycles this may or may not be accurate. Using computer models, it is possible to predict the temperature dynamics of the sample, both radially and axially, during the thermal processing cycle (heating and cooling). O1 tool steel was utilized to characterize and model heating and cooling temperature gradients. Specimens instrumented with multiple thermocouples were induction heated and gas quenched. The test data and geometry were evaluated with 1- D and 2-D models to characterize transient temperature gradients. The goal of the modeling is to better characterize temperature corrections required when rapid heating and cooling processes are used to determine transformation behavior in induction hardenable steels.