Study and comparison of thermal fatigue resistance of two roll materialsRebozov, Nicolas Alejandro (Avtor) Terčelj, Milan (Mentor) Nagode, Aleš (Komentor) Hi-Cr steelCG HSS steelRollsHot deformationThermal fatigueIn this master work, an investigation was carried out regarding the nucleation and growth of cracks on the surface of work rolls materials that occur during hot rolling. During hot rolling, work rolls are exposed to successive heating and cooling cycles. Their surface consequently suffers rapid temperature changes due to being in contact with hot rolled material and cold water from cooling. These successive and continuous heating and cooling cycles induce thermal fatigue in the material, which is a very important factor leading to surface deterioration and thus jeopardizing the roll life. Two hot roll materials (CG HSS and HCR) were tested regarding their thermal fatigue resistance. Testing of thermal fatigue resistance was carried out on Gleeble 1500D thermo-mechanical simulator. HCR was also additionally pre-hot deformed (strain of 0.23 and 0.41). In order to understand properly the degradation phenomena of crack nucleation and growth, samples were properly tested at 250, 750 and 2500 thermal cycles, respectively, and then on cross-section by optical microscope length of cracks was measured. SEM analysis and EDS analysis were also performed to study characteristics of crack initiation, their growth and spalling of materials. There is a dependence between crack nucleation and growth, and the number of cycles performed, the size, shape, orientation and distribution of the primary and eutectic carbides. Similar mechanisms for crack nucleation were found in both materials. Nucleation of cracks is higher in as-cast HCR material in comparison to as-cast CG HSS steel, as its crack density is higher. However, crack growth is higher in as-cast CG HSS as it developed larger cracks. Primary and eutectic carbides at the surface or close to it are preferred locations for crack nucleation. Elongated carbides under the surface accelerate crack growth while globular inhibit it. Carbides oriented parallel to the cooled surface reduce the growth of cracks. The deformation of the HCR prior to the thermal cycling offers a positive combination of thermal behaviours between as-cast CG HSS and as-cast HCR. Crack nucleation and growth are diminished when compared to as-cast HCR and as-cast CG HSS respectively. Firstly, this supported by the fact that at 250 cycles CG HSS developed 3.98 times larger crack than HCR deformed (0.41), whereas at 2500 cycles 2.34 times. Secondly, CGHSS has 1.3 times greater crack density than deformed HCR (0.41 strain) at 250 cycles, and 1.04 at 2500. The master work contributes to better understanding of crack nucleation, growth and spalling during the hot rolling conditions.20202021-01-26 09:00:03Magistrsko delo/naloga124485VisID: 78176sl