The purpose of the thesis was to investigate the initiation and growth of cracks on the surface layer of steel for rolls which are subjected to thermal fatigue. Testing was carried out on thermo-mechanical simulator of metallurgical states Gleeble 1500D. Chemical composition of used steel for rolls was: 1.65 wt. % C, 1.940 wt. % Ni, 11.279 wt. % Cr, 1.169 wt. % Mo. As a result of thermal fatigue initiatiation of cracks at carbides spots was observed while their growth takes place using carbides pathways. Alternately heating and cooling causes alternately performing tensile and compressive stresses which results in fatigue of the material. The samples were heated to three different maximal temperatures, i.e. 500 °C, 600 °C and 700 °C, respectively, whereas the specimen were internally cooled with water. For each test temperature the testing was interrupted at 200, 500, 1000 and 2500 cycles. Under the microscope we counted all the cracks, measured their lengths and depth. We calculated the average length of all cracks and crack density. Thus also seven longest cracks were determined.
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