Analysis of DTA and dilatometric data used to study the behaviour of a mould flux

Abstract

In the present work a casting powder containing ≈ 36 wt% SiO2, ≈ 31 wt% CaO, 13 wt% Na2O, and 10 wt% CaF2, was used as starting material. A mass of 10 g of the mould powder was placed in a graphite crucible and melted at 1300°C. After 15 minutes at this temperature, the melt was poured onto a stainless steel inclined plate. By this method, solid glass layers were produced, which were sectioned into bars 10-15 mm in length to be used in dilatometric tests. Some of these bars were ground to powder to perform DTA runs. Both tests (DTA and dilatometry) were carried out at different heating rates: 5, 10 and 15 ºC/min, in air atmosphere, up to 900°C. Data of glass transition temperature (Tg), and crystallization peak temperature (Tc) were obtained from DTA and dilatometric runs. From these data, several non-isothermal methods based on Kissinger, Augis-Bennet, Ozawa, and Cheng models, were used to study the crystallization kinetics. Based on these models, the activation energy of crystallization (E), the frequency factor () and the crystallization rate constant (k) were calculated. A subsequent study was followed to determine the Avrami exponent and the crystallized fraction. The crystallographic phases were determined by XRD and the microstructure of samples, treated between 400-900 ºC, was observed by optical and electronic microscopy.