Dilatation measurements of plain carbon steels and their thermodynamic analysis

Dilatation of the low-carbon steels with small additions of mass contents of Mn (up to 1.50 %), Si (up to 0.347 %), Nb (up to 0.053 %), and V (up to 0.082 %), was measured at a heating rate of 3 degrees C/min, and the experimental results were compared with calculations based on thermodynamic models. The differences between experiments and calculations were analysed. It was found that the thermal expansion of pearlite and of austenite in the steels exhibits almost linear temperature dependencies, and these dependencies are described very well by the present calculations. During the transformation of pearlite to austenite, contraction of the steels may occur due to the dissolution of cementite within a narrow temperature range. The dilatation of the steels during the transformation of ferrite to austenite depends on the competition between the thermal expansion and the transformation process, and it finally leads to an increase in the length change to a maximum followed by a decrease down to the temperature at which the transformation is completed. For some steels, however, a certain amount of ferrite may remain in the samples during heating even at temperatures well above that of the minimum dilatation. This will affect the determination of the A(3) temperature, and makes the expansion of the steels deviate from the true expansion behaviour of austenite.