However, accompanying these heat treatment benefits is the undesirable side effect of distortion. Control of residual stress is a growing issue as the operating stress in critical parts continues to increase, making improvement in properties such as fatigue strength important. The achievement of desirable surface compressive stresses in the hardened and dimensionally correct part is a goal of all heat treaters.
Past practices used by heat treaters and designers in determining the proper process routing have included experience-based rules, statistical models, and costly shop trials. By combining mechanics, thermal analysis, and phase transformation behavior with the powerful numerical methods of finite element analysis, new software-based tools have emerged for heat treat simulation.
This heat treatment simulation software predicts distortion, residual stress, hardness, and metallurgical phase percentages during hardening of both carburized parts and through-hardened steel parts. It is also used to assess the effect of the quenching method, allowing comparisons between different quench processes, such as immersion quenching in oil, salt, polymer or brine, high pressure gas quenching, intensive quenching and press quenching. Tempering effects, including the size and residual stress changes due to tempering, are also predicted.
The resident database contains properties for many commonly used steels, allowing the software to be used to quantitatively assess the effect of alloy substitution on part hardness.
Developed under a Cooperative Research and Development Agreement CRADA between a group of automotive-related companies and the Department of Energy, the project addressed the common problem of distortion due to phase transformations that occur during heat treatment of precision, critically stressed components. An equally important problem was control of residual stress so that part performance could be extended by heat treatment, i.
As these subroutines were being developed to address the diffusive and martensitic phase transformations that occur during heating and cooling of steel components, supporting material and process data were developed from many experiments, both for use in simulations and to validate model accuracy.
The end result was an accurate simulation tool that offered heat treaters and designers an alternative and cost effective method for modifying existing heat treat processes or implementing new product designs and heat treat processes instead of the traditional method of shop floor try-outs. Examples of the latter, thermochemical processes, would be diffusion and coating processes , such as carburization, case hardening, nitrating, boriding. The mechanisms used for heat treatment vary considerably between different groups of materials, e.
Therefore, the material group has to be taken into account. These demands cannot be met without a targeted change of material properties by heat treatment. A simulation of heat treatment is primarily about the prediction of the material properties PreMaP that can be achieved by heat treatment. Predictions of distortions caused by the heat treatment can also be of interest.
A high-alloy steel that has been incorrectly heat-treated can have technological properties inferior to a low-alloy steel that has been correctly heat-treated. High-quality, correctly treated steel will, however, always have properties better suited for its intended use. To manufacture high quality products , the efficient control of these energy-intensive processes is important. This includes the appropriate use of controls and regulators, burner technology, and an attuned combination of various refractory products such as refractory bricks or high temperature wool.
Heat treatment is an energy-intensive process. All pressure parts regardless of product form are applicable unless exempted. UCS e f g give exemptions. Thanks metengr and David for your views and feed back on the subject.
I am not clear on this. Quote: 2. From the foregoing, I understand that ASME with certain valid exemptions, considers that the mechanical properties of the materials can affect as a result of Heat treatment after various fabrication methods including welding, and accordingly it specified SIM HT of such materials to ensure that the the strength of materials used did not decrease below the limits of the basis of allowable stress criteria used in the design.
Am I right in such assumption? This plate will be used for forming a dished head. This dished head will have to undergo heat treatment after forming. Test coupon is only straight plate. Is it right to carry out heat treatment on straight i. The simulated heat treatment need only be performed on the actual heat of material.
Forming only introduces cold work that is removed upon heat treatment so the concern is the heat treatment only. The adjustment has to be done only once for each steel and the results stored in a database for future use. The major steels used in heat treatment are already available in the database.
The thermal, metallurgical, and mechanical material properties of a heat-treated steel depend on temperature, phases, and carbon content.
A comprehensive material database includes the major steels that are used for case hardening, surface hardening, and through hardening. The values in the database are average values extracted from experiments and literature; missing values have been completed by best simulation engineering practice.
In addition, steel properties depend on the manufacturer, the year, and the country, etc. Therefore, the material properties represent an average material that will give good tendencies; the data, in any case, do not fit precisely to an individual steel.
A graphical user interface heat treatment advisor allows an intuitive and process-driven methodology to set up simulations Figure 8. Once a dedicated project is defined and stored then parts, process, and material parameters can be exchanged within the project and a computation of a variant can be started in less than one minute.
With the help of the advisor, case hardening and through hardening processes can be fully defined. In case of surface hardening, a few additional simple operations with the standard capabilities of the software are needed to adjust the energy input through the surface. Heat treatment problems are solved automatically, covering all related complex mathematics and material physics. Depending on temperature, phase proportions, and proportion of chemical elements, thermal and mechanical properties are computed, including large strains.
Isotropic and kinematic hardening including phase transformations , transformation plasticity, nonlinear mixture rules for the yield stress of phases, phase-dependent strain hardening, restoring of strain hardening during diffusion-controlled phase transformations, melting and solidification of material, material properties depending on temperature, phases and proportion of chemical elements, and all features dedicated to the methodology of finite elements are taken into account. The user need not be familiar with the mathematics involved to perform heat treatment computations, but only needs to load the project and to start the solver Figure 9.
Examples of some of these capabilities are shown in Figs. Figure The system also provides the capability to review movies of the step by step evolution of results on the surface or through the structure. The simultaneous display of the evolution of results gives a deep understanding of process and computed results.
The user defines the chemical composition of the steel, and the computation of the Jominy test is done automatically. The most important results—e. The Jominy test is key to a precise heat treatment simulation. When the computed hardness coincides well with the measured hardness it ensures that the CCT-diagram of the steel being examined is numerically well implemented for a full range of possible cooling rates.
In case of discrepancies the CCT diagram can be modified to precisely meet the measured hardness profile. Because the formulas used for the hardness computation are empirically approved, existing CCT diagrams can be tuned following hardness measurements. Based on the optimized CCT diagram the core hardness of complex parts can be precisely predicted, which is of utmost importance for the lifetime of parts and components under dynamic loads.
About the Authors Dr. Frederic Boitout, Dr. For more information contact Olivier Morisot at or olivier. Visit online at [ www.
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