5.1 This practice is used to provide steel phase transformation data required for use in numerical models for the prediction of microstructures, properties, and distortion during steel manufacturing, forging, casting, heat treatment, and welding. Alternatively, the practice provides end users of steel and fabricated steel products the phase transformation data required for selecting steel grades for a given application by determining the microstructure resulting from a prescribed thermal cycle.

5.1.1 There are available several computer models designed to predict the microstructures, mechanical properties, and distortion of steels as a function of thermal processing cycle. Their use is predicated on the availability of accurate and consistent thermal and transformation strain data. Strain, both thermal and transformation, developed during thermal cycling is the parameter used in predicting both microstructure and properties, and for estimating distortion. It should be noted that these models are undergoing continued development. This process is aimed, among other things, at establishing a direct link between discrete values of strain and specific microstructure constituents in steels. This practice describes a standardized method for measuring strain during a defined thermal cycle.

5.1.2 This practice is suitable for providing data for computer models used in the control of steel manufacturing, forging, casting, heat-treating, and welding processes. It is also useful in providing data for the prediction of microstructures and properties to assist in steel alloy selection for end-use applications.

5.1.3 This practice is suitable for providing the data needed for the construction of transformation diagrams that depict the microstructures developed during the thermal processing of steels as functions of time and temperature. Such diagrams provide a qualitative assessment of the effects of changes in thermal cycle on steel microstructure. Appendix X2 describes construction of these diagrams.

5.2 It should be recognized that thermal and transformation strains, which develop in steels during thermal cycling, are sensitive to chemical composition. Thus, anisotropy in chemical composition can result in variability in strain, and can affect the results of strain determinations, especially determination of volumetric strain. Strains determined during cooling are sensitive to the grain size of austenite, which is determined by the heating cycle. The most consistent results are obtained when austenite grain size is maintained between ASTM grain sizes of 5 to 8. Finally, the eutectoid carbon content is defined as 0.8 % for carbon steels. Additions of alloying elements can change this value, along with Ac₁ and Ac₃ temperatures. Heating cycles need to be employed, as described below, to ensure complete formation of austenite preceding strain measurements during cooling.