Termodinamični model visoko temperaturne oksidacije orodnih jekel za delo v vročem : doktorska disertacija

Balaško, Tilen (author), Medved, Jože (mentor), Vončina, Maja (comentor)

V doktorski disertaciji smo preiskovali visoko temperaturno oksidacijo treh orodnih jekel za delo v vročem (Dievar, UTOPMO1 in W600). Omenjena jekla se uporabljajo predvsem v poboljšanem stanju, kar je razlog, da smo visoko temperaturno oksidacijo jekel preučevali v dveh toplotnih stanjih, in sicer v mehko žarjenem in poboljšanem. Najprej so bili s termodinamičnim modeliranjem po metodi CALPHAD narejeni izračuni ravnotežnih sestav nastalih oksidnih plasti. Nato smo del vzorcev oksidirali v komorni peči in ostale v napravi za simultano termično analizo. Prvi so bili uporabljeni za metalografsko analizo sestave, debeline in drugih lastnosti oksidnih plasti. Druge smo uporabili za analizo kinetike visoko temperaturne oksidacije s termogravimetrično analizo. Hkrati je bila izvedena tudi XRD-analiza, z namenom določanja tipa nastalih oksidov v oksidni plasti. Na podlagi vseh zbranih rezultatov smo izdelali termodinamične modele za vsako analizirano jeklo pri posamezni temperaturi. Omenjeni modeli opisujejo kinetiko visoko temperaturne oksidacije, ki je razložena in opisana tudi z zbranimi rezultati drugih analiz (SEM, XRD, Thermo-Calc itd.). Glede na to, da smo simulirali realne pogoje, je vsak model sestavljen iz dveh delov, in sicer prvi podaja kinetiko oksidacije pri pogojih ogrevanja in drugi pri izotermnih pogojih. Kinetiko preiskovanih jekel lahko opišemo s tremi matematičnimi funkcijami: eksponentno, parabolno in kubično funkcijo. Katera funkcija najboljše opiše kinetiko, je odvisno od temperature oksidacije in kemijske sestave jekla. Ugotovljeno je bilo, da imajo dodani legirni elementi velik vpliv na kinetiko oksidacije. Istočasno ima vpliv na kinetiko tudi izhodna mikrostruktura oziroma toplotna obdelava. Dokazano je bilo, da med ogrevanjem nastala oksidna plast vpliva na kinetiko oksidacije pri izotermnih pogojih, zato je relevantno tudi, ali ogrevanje poteka v zaščitni atmosferi ali ne.

• Language: Slovenian
• Keywords: visoko temperaturna oksidacija, orodna jekla za delo v vročem, termodinamično modeliranje, termogravimetrična analiza, kinetika oksidacije
• Work type: Doctoral dissertation
• Typology: 2.08 - Doctoral Dissertation
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [T. Balaško]
• Year: 2021
• Number of pages: XXVIII, 246 str.
• PID: 20.500.12556/RUL-128244
• UDC: 669
• COBISS.SI-ID: 71658499
• Publication date in RUL: 07.07.2021

Secondary language

• Language: English
• Title: Thermodynamic model for high temperature oxidation of hot work tool steels : Ph. d. thesis

Abstract

In this dissertation, the high temperature oxidation of three hot work tool steels (Dievar, UTOPMO1 and W600) was investigated. These steels are mainly used in the heat-treated conditions; therefore, the high temperature oxidation of these steels was investigated in two thermal conditions, namely, the soft-annealed and the heat-treated. First, calculations of the equilibrium compositions of the oxide layers formed were carried out by thermodynamic modelling using the CALPHAD method. Then, half of the samples were oxidised in a chamber furnace and the rest in a STA device. The samples from the chamber furnace were used for metallographic analysis of the composition, thickness, and other properties of the oxide layers formed. Others were used for analysis of the kinetics of high-temperature oxidation by thermogravimetric analysis. At the same time, we also performed XRD analysis to determine the oxides formed in the oxide layers. Based on all collected results, we created thermodynamic models for each analysed steel at an individual temperature. These models describe the kinetics of high temperature oxidation, which is additionally explained and described using the collected results of other analyses (SEM, XRD, Thermo-Calc, etc.). Since we simulated real conditions, the model consists of two parts, the first reflecting the kinetics of oxidation under heating conditions and the second under isothermal conditions. The kinetics of the studied steels can be described by three mathematical functions, namely: exponential, parabolic, and cubic. However, which function best describes the kinetics depends on the oxidation temperature and the chemical composition of the steel. In fact, it was found that the added alloying elements have a great influence on the oxidation kinetics. At the same time, the heat treatment conditions also affect the kinetics. It also matters whether the heating takes place in a protective atmosphere or not. This is because, as has been shown, the oxide layer formed during heating influences the oxidation kinetics under isothermal conditions.

• Keywords: high temperature oxidation, hot work tool steels, thermodynamic modelling, thermogravimetric analysis, oxidation kinetics