On the spallation of oxide scales in high-strength low-alloy (HSLA) hot-rolled steels

Abstract

In this work, results on the causes that could promote the abnormal spallation of the oxides formed on the surface of high-strength low-alloy (HSLA) steels are presented. By means of Rietveld refining of X-ray diffraction spectra, scanning electron microscopy analyses and calculations, it was found that the value of the thermal stress experienced by the oxide scale reached a maximum when the oxide scale was comprised by 65% wt magnetite Fe3O4 and 24% wt wustite FeO this, due to the incomplete transformation of the latter phase to Fe3O4 and α-Fe from cooling from 670 °C to ambient temperature. Contrarily, it was found that when a balance in the amount of Fe3O4 and FeO was 46.4 and 46.5%wt respectively, the calculated thermal stress was reduced, and oxide spallation was not that severe. The reasons for oxide scale detachment from the surface of the steels are explained in terms of the adhesion energy of the bulk oxide scale, the amount of magnetite Fe3O4 present in the oxides and the chemical composition of the steel particularly the elements chromium and titanium.