Study and comparison of thermal fatigue resistance of two roll materials

Rebozov, Nicolas Alejandro (author), Terčelj, Milan (mentor), Nagode, Aleš (comentor)

In 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.

• Language: English
• Keywords: Hi-Cr steel, CG HSS steel, Rolls, Hot deformation, Thermal fatigue
• Work type: Master's thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2020
• PID: 20.500.12556/RUL-124485
• Publication date in RUL: 26.01.2021

Secondary language

• Language: Slovenian
• Title: Študij in primerjava odpornosti na termično utrujanje dveh materialov za valje

Abstract

V tem magistrskem delu je bila izvedena raziskava nukleiranja in rasti razpok na površini delovnih valjev, ki nastanejo med procesom vročega valjanja. Pri omenjenem procesu so delovni valji izpostavljeni izmeničnim ciklom segrevanja in hlajenja. Na površini delovnega valja prihaja do hitrih temperaturnih sprememb zaradi stika valja z vročim obdelovancem in hladilnim sredstvom. Omenjeni neprekinjeni cikli na površini materiala delovnega valja povzročijo toplotno utrujenost materiala. To je pomemben dejavnik, ki vodi k razpadanju površine in s tem ogrozi življenjsko dobo valja. Dva valja iz različnih materialov (CG HSS in HRC) sta bila preizkušena na odpornost proti toplotni utrujenosti. Testiranje odpornosti na toplotno utrujenost je bilo izvedeno na termo-mehanskem simulatorju Gleeble 1500D. Valj iz materiala HCR je bil dodatno hladno deformiran (pri obremenitvah 0.23 in 0.41). Da bi pravilno razumeli pojave nukleacije in rasti razpok, smo vzorce ustrezno testirali pri 250, 750 in 2500 termičnih ciklih. Nato smo na prerezu izmerili dolžino razpok z optičnim mikroskopom. Nato smo izvedli SEM in EDS analize in preučili značilnosti nastanka razpok, njihove rasti in in materialni odred. Obstaja odvisnost med nukleacijo in rastjo razpok, številom obremenitvenih ciklov, velikostjo, obliko, orientacijo in porazdelitvijo evtektičnih karbidov. Pri obeh materialih smo ugotovili podobne mehanizme za nastanek nukleacije. Nukleacija razpok je v primerjavi z materialom CG HSS večja pri HCR materialu, saj opazimo večjo gostoto razpok. Rast razpok je večja pri materialu CH HSS, saj so se med preizkušanjem razvile večje razpoke. Primarni in evtektični karbidi na površini in tik pod njo so mesta kjer se najpogosteje pojavi nukleacija. Podolgovati karbidi pod površino pospešujejo rast razpok, medtem ko jo kroglasti karbidi zavirajo. Karbidi usmerjeni vporedno z ohajeno površino zmanjšujejo rast razpok. Deformacija preizkušanca iz materiala HCR pred ciklično toplotno obremenitvijo ponuja pozitivno obnašanje materiala tako pri materialu CG HSS, kot tudi HCR. Nukleacija in rast razpoke se zmanjša, če primerjamo matzeriale brez predhodne deformacije. To ugotovitev podpira dejstvo, da se je v primerjavi z predhodno deformiranim preizkurancem iz HCR (0.41), po 250 ciklih razpoka pri materialu CG HSS povečala za 3.98-krat. Pri 2500 ciklih pa je razpoka večja za 2.34-krat. Prav tako ima preizkušanec iz materiala CG HSS ima 1.3 krat večjo gostoto razpok pri 250 ciklih in 1.04-krat večjo gostoto razpok pri 2500 ciklih, če ga primerjamo z predhodno deformiranim preizkušancem iz materiala HCR (Pri obremenitvi 0.41). Magistersko delo prispeva k boljšemu razumevanju nukleacije, rasti razpok in pri pogovih, ki se pojavijo med procesom vročega valjanja.

• Keywords: Hi-Cr jeklo, CG HSS jeklo, Valji, Vroča deformacija, Termično utrujanje