Primerjava kinetike mehčanja dveh obrabo-odpornih jekel HB400

Žerjav Jereb, Blaž (author), Fajfar, Peter (mentor)

Namen diplomske naloge je bil primerjati odzive dveh različnih obrabo-odpornih jekel HB400 na proces statične rekristalizacije med dvostopenjskim tlačnim preizkusom. V teoretičnem delu smo spoznali osnovne lastnosti visokotrdnih malo-legiranih jekel (HSLA) in ultra visoko trdnostnih jekel (UHSS), mikrolegirne sisteme teh jekel ter vplive določenih legirnih elementov. Nadalje smo opisali procese utrjevanja jekel in predvsem procese mehčanja, do katerih pride med in po plastični deformaciji. Za razumevanje vpliva pogojev deformacije in mikrolegirnih elementov smo izvedli preizkuse na simulatorju termomehanskih stanj Gleeble 1500D. Podatke meritev smo uredili in izrisali krivulje prava napetost – prava deformacija dvostopenjskih preizkusov. Nato smo določili maksimalno napetost prve deformacije ter napetosti tečenja prve in druge deformacije. Od tod smo določali stopnjo mehčanja in izrisali poteke deleža mehčanja v odvisnosti od desetiškega logaritma časa. Variirali smo čas izotermnega držanja med prvo in drugo deformacijo ter temperaturo, pri kateri smo izvedli preizkus. Poleg dvostopenjskih preizkusov smo izvedli metalografsko analizo vzorcev na svetlobnem mikroskopu. Ta nam ponudi dodatne informacije o vzorcih, s pomočjo katerih lažje interpretiramo ostale rezultate. Naredili smo tudi analizo povprečne velikosti zrn pri različnih časih izotermnega držanja pri temperaturi 950 °C za oba vzorca. Nazadnje smo opravili še meritve trdot po Vickersu. Izkaže se, da je kinetika mehčanja obeh jekel primerljiva, trdota pa je višja v primeru jekla z dodanim titanom, molibdenom in višjo vsebnostjo bora. Zaradi ohlajevanja na zraku je bila vidnost prvotnih avstenitnih zrn slaba, kar je onemogočilo analizo velikosti prvotnih zrn. Zato smo naredili analizo velikosti feritnih zrn. V končni mikrostrukturi imamo ferit in perlit, v nekaterih primerih pa večinsko bainit.

• Language: Slovenian
• Keywords: Mehčanje, HSLA, UHSS, statična rekristalizacija, obrabo-odporna jekla
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2020
• PID: 20.500.12556/RUL-120959
• COBISS.SI-ID: 34571779
• Publication date in RUL: 28.09.2020

Secondary language

• Language: English
• Title: Comparison of softening kinetics of two HB400 wear-resistant steel grades

Abstract

The purpose of this diploma work was to compare reactions of two HB400 wear-resistant steel grades to static recrystallization between a double-pass compression test. In the theoretical part we met the basic qualities of high-strength low-alloy steels (HSLA) and ultra high strength steels (UHSS), microalloying systems of these steels and the effects of certain alloying elements. We further described the processes of steel strengthening and predominantly processes of softening, which occur between and after plastic deformation. To understand the effects of deformation conditions and microalloying elements we carried out the experiments on a thermomechanical simulator Gleeble 1500D. We arranged the information we got from our measurements and constructed true stress – true strain curves (flow curves) for the double-pass compression tests. We then determined the maximal stress of the first deformation and the yield stress of the first and second deformation. From here we determined the softening fracture and drew the evolution of the degree of softening in dependence with the common logarithm of time. We varied the time of the isothermal holding between the first and second deformation, and the temperature, at which the test took place. In addition to the two-stage tests we carried out a metallographic analysis of the samples on a light microscope. This offers additional information on the samples, which help us interpret the others results in an easier manner. We also made an analysis of the average grain size for different times of isothermal holding at a temperature of 950 °C for both samples. Lastly, we carried out Vickers hardness tests. It turns out that the softening kinetics are comparable for both steels, however the hardness is higher in the steel with added titanium, molybdenum and higher boron content. Because the samples were air cooled, the visibility of primary austenite grains was bad. This made primary grain size analysis impossible and therefore, we rather did a ferrite grain size analysis. In the final microstructure we have ferrite and perlite, but in some cases we have mostly bainite.

• Keywords: softening, HSLA, UHSS, static recrystallization, wear-resistant steels