Heat treatment of steels is a process of modifying the mechanical properties by solid-state phase transformations or microstructural changes through heating and cooling. The material volume changes with phase transformations, which is one of the main sources of distortion. The thermal stress also contributes to the distortion, and its effect increases with solidstate phase transformations, as the material stays in the plastic deformation field due to the TRIP effect. With the basic understanding described above, the sources of distortion from a quench hardening process can be categorized as: 1) nonuniform austenitizing transformation during heating, 2) nonuniform austenite decomposing transformations to ferrite, pearlite, bainite or martensite during quenching, 3) adding of carbon or nitrogen to the material, and forming carbides or nitrides during carburizing or nitriding, 4) coarsening of carbide in tempered martensite during tempering, 5) stress relaxation from the initial state, 6) thermal stress caused by temperature gradient, and 7) nonhomogeneous material conditions, etc. With the help of computer modeling, the causes of distortion by these sources are analyzed and quantified independently. In this article, a series of modeling case studies are used to simulate the specific heat treatment process steps. Solutions for controlling and reducing distortion are proposed and validated from the modeling aspect. A thinwalled part with various wall section thickness is used to demonstrate the effectiveness of stepped heating on distortion caused by austenitizing. A patented gas quenching process is used to demonstrate the controlling of distortion with martensitic transformation for high temperature tempering steels. The effect of adding carbon to austenite on size change during carburizing is quantified by modeling, and the distortion can be compensated by adjusting the heat treat part size.

A gas quenching method was developed by DANTE Solutions, in conjunction with the U.S. Army Combat Capabilities Development Command Aviation & Missile Center (DEVCOM AvMC), to control distortion in difficult to quench geometries. This new method addresses the nonuniform cooling inherent in most gas quenching processes. A prototype unit was constructed and tested with the aim of controlling the martensite formation rate uniformity in the component being quenched. With the ability of the DANTE Controlled Gas Quenching (DCGQ) unit to control the temperature of the quench gas entering the quench chamber, thermal and phase transformation gradients are significantly reduced. This reduction in gradients yields a more uniform phase transformation, resulting in reduced and predictable distortion. Being able to minimize and predict distortion during gas quenching, post heat treatment finishing operations can be reduced or eliminated, and as such, fatigue performance can be improved. This paper will discuss the prototype unit performance. Mechanical testing and metallographic analysis were also performed on Ferrium C64 alloy steel coupons and will be discussed. The results obtained showed that the slower cooling rate provided by the prototype did not alter the microstructure, hardness, strength, ductility, toughness, or residual stress of the alloy.

This paper presents a computational approach for assessing the potential for distortion when using high pressure gas to quench steel parts. It explains how to account for component geometry, heat transfer coefficient, gas temperature and velocity, heating and cooling rates, and phase transformations. The authors employ finite element modeling methods to determine local phase fraction and displacement in a Ferrium C64 disk for different quench pressures. Simulations at timed intervals show how distortion and phase fraction progress in different areas of the disk and along the edges of an off-center bore. The causes of distortion are examined and explained using the model, with insights into why the cooling rate has a nonlinear relation with distortion.

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.

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.

In single-shot hardening applications, heat treated components often feature a variety of geometric complexities including variable wall thicknesses, sharp diameter transitions, lightening holes, slots, etc. Due to the inherent 3D electromagnetic nature of single-shot coils and the complex geometry components which they must accommodate, the design and optimization of single-shot hardening coils is typically a demanding and intricate task. This paper presents combined electromagnetic-thermal and thermal-mechanical FEA simulation results for the single-shot induction hardening of a power transmission shaft. The simulation results, including electromagnetic, thermal, metallurgical, and mechanical data demonstrate the value of computer simulation in the design and development of single-shot induction hardening systems.

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.

Carburizing grades of high strength steels, such as Ferrium C- 64 alloy, contain strong carbide forming elements such as chromium and molybdenum. Alloys with high amounts of strong carbide formers can form stable carbides during carburization that effectively block carbon diffusion and retard the carburization process. This is especially true for low pressure carburization. To achieve the desired case depth, the low pressure carburization process consists of a series of rapid boost and longer time diffusion cycles. One problem is how to determine an acceptable carburization schedule. This paper will discuss a methodology used to develop the data for Ferrium C-64 so that a proper low pressure carburizing schedule could be determined. Integral parts of this methodology are experiments to determine carbon diffusion rates, carbide formation kinetics, and carbide dissolution kinetics, and use of these data in computer software to simulate the process and to determine the proper schedule.

Previous work was reported on the induction hardening process for a 1541 steel axle shaft. This presentation compares the previous results with the stress formation dynamics in the same shaft made from steels with lower hardenability. Hardened using a scan heating method and a trailing PAG spray quench, several steels having lower hardenability were modeled using the same heating schedule so that the depth of austenite formation is similar in all cases. During spray quenching, the hardened case is shallower as steel hardenability is reduced. This leads to differences in the magnitude of compressive and tensile stresses and their distributions. In turn, the potential for internal cracking is reduced as the stress transition zone is altered by the thickness of the diffusive phase layer between the martensitic case and the ferrite-pearlite core of the shaft. The next step is to investigate these effects on the torque carrying ability of the shaft.

Press quenching is a specialized quenching technique used in heat treating operations to minimize the distortion of complex components such as spiral bevel gears and high quality bearing races. The quenching machine is designed to control the geometrical characteristics of components such as out-of-round, flatness, and (if the tooling is designed to accommodate it) taper. The achievement of final dimensional tolerances is accomplished through a trial and error process where the incoming machined sizes of the components are adjusted based upon measurement data taken from the initial sets of quenched and tempered components that have already been processed through the press quenching operation. Oil flow rates can be altered during the different stages of the quenching cycle, and through the use of specialized tooling the oil flow pathways can be selectively adjusted to meter the oil flow towards specific areas of the part surface while baffling it away from others in order to provide a more uniform overall quench. Complex metallurgical changes take place during austenitizing and quenching, resulting in corresponding mechanical property changes. Accompanying these changes are the generation of thermal and transformation induced stresses, which produce in-process and final residual stresses. During press quenching, dimensional restrictions add additional complexity to the combined effects of thermal and mechanical process sensitivities on these stresses. And if the stresses are severe enough, quench cracking can result. In this investigation the quench cracking of an asymmetrical AISI 52100 bearing ring is evaluated through physical experiments and through corresponding heat treatment process modeling using DANTE. The effects of quench rate, die load pulsing, and several other process variables are examined experimentally and/or analytically to illustrate how they can impact the resulting stresses generated during the press quenching operation.

During the liquid quenching process, there are three main phases between the solid and the liquid interface: film boiling where vapor blanket covers the entire solid structure, transition or nucleate boiling, and single phase convection. The type of the quenching media, the agitation, and the flow pattern of a quench tank have significant effect on the cooling behavior during these three phases, which will affect the cooling rate, phase transformation, stress evolution and shape change of the quenched components. In this paper, transient CFD analysis using AVL FIRE is coupled with heat treatment analysis using DANTE to simulate an oil quench hardening process of a test gear made of Pyrowear 53. The gear is carburized prior to quench hardening. During the coupling analyses, the heat flux between the gear and the oil calculated in the CFD model is applied to the solid heat treatment model, and the gear surface temperature predicted by the heat treatment model is passed back to the transient CFD model. The aforementioned CFD tool is capable of considering the entire quenching domains without considering phase transformations in the quenched components. In the present case the gear is treated with a finite element tool in combination with DANTE to account for the latent heat release, which slows down the cooling. The relations between carbon content, temperature field, phase transformation, internal stress, and shape change during quenching are explained from the heat treatment modeling results. The coupling of CFD and heat treatment analyses provides a more robust application of computer modeling in the heat treatment industry.