Razvoj mikrostrukture in mehanskih lastnosti jekla UTOPNiCu

Lečnik, Miha (author), Markoli, Boštjan (mentor)

Tema magistrskega dela je preiskava izločevalno utrjevalnega jekla UTOPNiCu v različnih toplotno obdelanih stanjih. Vzorce smo izrezali iz različnih mest odkovka prečnih dimenzij 405 x 185 mm, jih avstenitizirali pri 900 (1 uro) in umetno starali pri 550 °C (1, 2, 4, 8, 16, 24 in 48 ur). Predmet raziskave je predstavljal predvsem vpliv toplotne obdelave na mikrostrukturne spremembe in spremembe mehanskih lastnosti po različnih časih izločevalnega utrjevanja. V okviru eksperimentalnega dela smo zato opravili preizkušanje jekla z nateznim preizkusom, merili udarno žilavost, trdoto, opazovali mikrostrukture s svetlobnim mikroskopom, ugotavljali delež zaostalega avstenita z rentgensko fazno analizo ter izvedli dilatometrske preiskave z oblikovanjem CCT (angl. Continuous-Cooling-Transformation) diagrama. Ugotovili smo, da z izbranim režimom toplotne obdelave dosežemo ustrezno velikost prvotnega avstenitnega zrna in kombinacijo mehanskih lastnosti med 4 in 8 urami izločevalnega utrjevanja pri 550 °C. Zaostalega avstenita v jeklu nismo našli. Z zabeleženimi dimenzijskimi spremembami med dilatometrskimi preiskavami smo nakazali potek ključnih procesov med prvo uro zadrževanja na temperaturi umetnega staranja ter medsebojen vpliv izločanja in popuščanja martenzitne osnove. Z izdelavo CCT diagrama smo omogočili širše razumevanje lastnosti, faznih transformacij in toplotne obdelave jekla UTOPNiCu.

• Language: Slovenian
• Keywords: jeklo UTOPNiCu, toplotna obdelava, mikrostruktura, mehanske lastnosti, CCT diagram
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2018
• PID: 20.500.12556/RUL-99509
• COBISS.SI-ID: 1713503
• Publication date in RUL: 30.01.2018

LEČNIK, Miha, 2018, Razvoj mikrostrukture in mehanskih lastnosti jekla UTOPNiCu [online]. Master’s thesis. [Accessed 27 August 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=99509

Secondary language

• Language: English
• Title: Evolution of microstructure and mechanical properties of UTOPNiCu steel

Abstract

The thesis investigates the precipitation hardened steel UTOPNiCu properties and behaviour in various heat-treated conditions. First, samples were cut out from various parts of the forging with transverse dimensions of 405 x 185 mm, which were then austenitized at 900 °C (for 1 hour) and aged at 550 °C (for 1, 2, 4, 8, 16, 24 and 48 hours). The focal point of the research was the influence of heat treatment on microstructure and mechanical properties at various durations of precipitation hardening. Therefore, in the experimental part, the following procedures were carried out: tensile material testing, impact toughness and hardness measuring, microstructure observing using an optical microscope, determining the retained austenite volume fraction using the X-ray phase analysis as well as dilatometry with a CCT (Continuous-Cooling-Transformation) diagram construction. It was discovered that both the appropriate austenite grain size and combination of mechanical properties are achieved after 1 hour of austenitizing at 900 °C and between 4 and 8 hours of precipitation hardening at 550 °C. There was no trace of retained austenite found in steel. The observed dimensional changes during dilatometry indicated the key processes' outcome during the first hour of aging at 550 °C as well as the interaction between precipitation and tempering of martensitic base. The construction of a CCT diagram enabled a broader understanding of steel UTOPNiCu properties, phase transformations and heat treatment.

• Keywords: steel UTOPNiCu, heat treatment, microstructure, mechanical properties, CCT diagram