EXAMINING THE STRUCTURE OF MILL ROLLS METAL WITH LASER SURFACING FOR THEIR QUALITY CONTROL

The authors have investigated microstructure and crystal struc­ture of the steel samples of 9Kh2MF and 8Kh3SMFA steel with la­ser surfacing. The samples were taken from working shafts of reverse rolling mill in conditions of OJSC «Uralmashzavod». Brewing surface cracks in rolls with the use of laser is considered as an effective method of parts restoring in small-scale production. The research was carried out to control the quality of steel products with laser surfacing. Quality control of working rolls of rolling mills with laser surfacing is aimed at identifying the defects of metallurgical origin (nonmetallic inclu­sions, discontinuities, regions with heterogeneity of chemical compo­sition) in zones of surfacing and thermal influence and is performed by ultrasonic method. Metallographic study of the microstructure and crystal structure of steel samples with laser surfacing was necessary to develop an ultrasonic testing technique. The main way to detect de­fects of metallurgical origin in steels is scanning electron microscopy with functions of micro-X-ray spectral analysis (EDS-analysis) and diffraction of backscattered electrons (EBSD-analysis). The metallo­graphic study was carried out using a scanning electron microscope Carl Zeiss AURIGA CrossBeam equipped with analytical systems for studying the elemental surface composition by X-ray spectral analysis (EDS) and the crystal structure of the surface by diffraction of back­scattered electrons (EBSD). As a result of metallographic examination, steel-laser welded samples taken from the working rollers of the re­verse rolling mill were found to have defects of metallurgical origin along the surfacing boundary. The size of microinhomogeneities for 9Kh2MF steel is 10 – 50 μm; the elemental composition includes Mn, Si and O. The size of microinhomogeneities for 8Kh3SMFA steel is 1 – 3 μm, and the elemental composition includes Mn, Cr and Mo. It was established that metal on melting is less textured and has more homogeneous acoustic characteristics than base metal, it must be taken into account at ultrasonic quality control of steel products with laser surfacing. At ultrasonic inspection of laser-surfaced working rolls, we recommend setting the signal fixation level with reflectivity equivalent to the flat-bottom hole diameter of 1.5 mm.