Abstract

    Distortion induced by heat treatment is a major industrial problem because it critically affects the dimensional accuracy of precision components . Prediction of distortion is difficult because it requires detailed knowledge of the material properties which are normally lacking and difficult to evaluate,especially at high temperatures. The present work describes the development of a computer model for prediction of the material properties required for distortion prediction in steels. The success of the model is based on accurate description of all the major phase transformations taking place, as well as an accurate calculation of the properties of different phases formed during heat treatment. The model calculates a wide range of physical, thermophysical and mechanical properties, all as a function of time/temperature/cooling rate. Jominy harden-ability prediction was also performed and shows excellent agreement with experimental data. All the present calculations can be carried out via a user friendly graphical interface.

  Introduction

    Heat treatments are widely used in various manufacturing processes to enhance the quality of a product.However, heat treatment can generate unwanted distortion. This is a major industrial problem because it critically affects the dimensional accuracy of precision components, which may considerably increase the cost and time required for product development and decrease the quality of core parts. If distortion can be predicted and controlled, then corrections can be made during the earlier machining stage so that the components reach their final desired shape and dimension after heat treatment.Prediction of distortion induced by heat treatment has generally been based on prior experience or by a trial an error approach. In recent years with the significant improvement of computing power, finiteelement (FE) tools have been developed to tackle this problem. While being successful in some cases, almost all of the FE-modelling packages suffer from one common problem: the lack of accurate material property data. This is because distortion prediction requires detailed knowledge of the material properties as a function of alloy com-position and heat treatment procedures, whereas such properties are normally unavailable especially at elevated temperatures. This problem is fatal for FE-modelling because simulation based on inaccurate material information is simply not trustworthy.In summary, the following infor-mation on materials properties has to be known for distortion prediction:

• Phase transformation kinetics, i.e. TTT and CCT diagrams.

• Temperature and microstructure dependent thermophysical properties, such as density, thermal expansion coefficient, and thermal conductivity.

• Temperature and microstructure dependent mechanical properties, including tensile strength,yield strength, hardness, and stress-strain curves.

    In the present paper, the development of a computer program is reported, which can calculate the above material properties for general steels. The success of the model is based on an accurate description of all the major phase transformations taking place during heat treatment, as well as an accurate calculation of the properties of different phases formed in steels. Jominy hardenability calculations have also been carried out to meet industrial interests. The program has been incorporated into JMatPro, a computer software for materials property simulation, which allows the required calculations to be readily carried out via a user friendly interface.

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