Study of phase transformations during tempering of AISI L6 low alloy tool steel by means of calorimetry
Douglas I. Quiñones-Salinas1*, Rafael D. Mercado-Solís1, Luis A. Leduc-Lezama1,
Florentino Fernandez-Guzman2, Rene Cerda-Rojas2.
1Universidad Autonoma de Nuevo León, México
2Frisa Forjados S.A. de C.V., México
The purpose of this work was to establish the influence of heating rate on the reaction kinetics during double tempering of AISI L6 tool steel by means of differential scanning calorimetry. Steel samples in the as-quenched condition were double-tempered inside the calorimeter to 700 °C at heating rates 5, 15, 20, 35 and 50 °C/min. In each experiment the energy involved in the reactions was measured and the temperatures at which they occurred were established. The activation energies were calculated using the Kissinger-like method. The presence of the reaction phases during the tempering were identified by EDS, also the particle morphology and size were measured. The hardness of the as-quenched and the several heating rates for tempering conditions were measured by Vickers method. In the first tempering two reactions were identified. The first one occurred at 300 - 350 °C and was associated to carbon clustering, while the second one occurred at 500 – 550 °C due to the transformation of the retained austenite into cementite. In the second tempering no reactions were identified, possibly due to the full transformation of retained austenite into cementite from the first tempering. Based on the calculations, the activation energies for the reactions were not influenced by the heating rates. Differential scanning calorimetry is a powerful method for analyze the phase transformations during the tempering. The calorimetry reactions showed that first tempering mainly lead the retained austenitic transformation and the beginning of the phase of cementite. The phenomena of growing and distribution of carbides at the second tempering do not appear to be detected by the calorimetry technique. Other complementary techniques such as dilatometry could be used to measure these reactions.
Keywords: tempering heat treatment, low alloy tool steel, precipitation, phase transformations, differential scanning calorimetry, Kissinger, microstructure, hardness.