Investigation of phase transformations of carbon steel scale during laser purification by nanosecond pulses in the evaporation mode

The phase transformations occurring in scale when exposed to nanosecond laser pulses are investigated. The initial phase composition of the scale and the phase composition of the surface layer exposed to the laser have been determined. The surface treatment of the samples was carried out in the evaporative mode of laser exposure and led to scale ablation. Two groups of samples from hot-rolled carbon steel sheets of the St3 grade (E235, Fe 360) were studied. The first group consisted of samples with an initial scale surface and samples with a mechanically ground surface. Based on these samples, the phase and elemental composition as well as the morphological parameters of the initial scale were studied. The second group includes samples with a scale surface treated with nanosecond laser pulses. A pulsed nanosecond ytterbium fiber laser with a maximum average power of 30 watts was used. A two-coordinate scanning system based on electroplating scanners was used to scan the surface of the samples with a laser beam. The phase composition of the scale was determined by RAMAN spectroscopy. The morphological parameters of the surface and the elemental composition of the samples were determined by scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersion analysis (EDX). Studies of the phase composition of the initial scale showed that it consists mainly of magnetite, while wustite was not detected in the scale. It has been established that during the processing of scale in the evaporation mode a crater is formed in the area affected by the laser pulse the surface of which is covered with a solidified melt of scale. A phase transformation occurs in the melt with the formation of wustite. Upon solidification, the melt cracks, which is associated with the phase transformation that has occurred. Thus, it is shown that in the process of laser purification, the evaporative mechanism of scale removal is accompanied by a phase transformation of a mixture of magnetite and metallic iron into wustite. The results obtained can be used as a basis for the creation of a new highly efficient technology for laser surface cleaning of steels from scale.