Vpliv toplotne obdelave na mehanske lastnosti in mikrostrukturo jekla PK993 : magistrsko delo

Češnjaj, Marko (author), Nagode, Aleš (mentor), Burja, Jaka (comentor)

V magistrski nalogi smo preučevali vpliv interkritičnega žarjenja na mehanske lastnosti in mikrostrukturo jekla PK993/1CH13N3. Trenutna toplotna obdelava jekla te kvalitete sestoji iz kaljenja na 1000 °C in popuščanja na 530 °C. Na ta način dosegamo vse zahtevane mehanske lastnosti z izjemo udarne žilavosti. Zaradi tega razloga se v sistem toplotne obdelave jekla PK993/1CH13N3 uvaja dodatno interkritično žarjenje za doseganje ustrezne kombinacije vseh zahtevanih mehanskih lastnosti. Vzorce, izdelane iz valjane palice s premerom 70 mm, smo razdelili v dve seriji. V prvi seriji smo vzorce kalili in nizkotemperaturno popuščali na različnih temperaturah. V drugi seriji pa smo med kaljenjem in nizkotemperaturnim popuščanjem izvedli še interkritično žarjenje vzorcev nad evtektoidno temperaturo AC1. S tem smo želeli ugotoviti vpliv interkritičnega žarjenja na mehanske lastnosti in mikrostrukturo jekla PK993/1CH13N3. Analiza mikrostrukture je potekala z uporabo svetlobnega in vrstičnega elektronskega mikroskopa. Za vsako od izvedb toplotne obdelave je sledila serija preizkusov za določanje napetosti tečenja, natezne trdnosti, trdote, udarne žilavosti in raztezka ter kontrakcije po pretrgu. Z dilatometrijskim preizkusom smo določili tudi temperature AC1, AC3, Ms in Mf. Dodatno se je izvedla tudi serija simulacij s programom Thermo-calc in XRD analiza. Na podlagi rezultatov smo zaključili, da se na temperaturi avstentizacije po mejah avstenitnih kristalnih zrn tvori tanka karbidna plast. Ta plast ostaja stabilna tudi po nizkotemperaturnem popuščanju vzorcev prve serije. Po interkritičnem žarjenju vzorcev druge serije ta karbidna plast ni več prisotna, dodatno se začnejo izločati karbidni izločki. S tem se zniža vsebnost ogljika in karbidotvornih elementov v trdni raztopini, kar zviša temperaturo Ms po ohlajanju s temperature interkritičnega žarjenja. Tvori se tudi nov, sveži martenzit. Spremembe v mikrostrukturi znižajo vrednosti vseh mehanskih lastnosti z izjemo udarne žilavosti. V splošnem tako dosegamo boljšo kombinacijo vseh mehanskih lastnosti. Poleg tega se nekoliko zmanjša tudi standardna deviacija posameznih merljivih vrednosti znotraj posamezne serije.

• Language: Slovenian
• Keywords: jeklo PK993/1CH13N3, martenzitna nerjavna jekla, mikrostrukturna analiza, interkritično žarjenje, mehanske lastnosti, udarna žilavost
• Work type: Master's thesis/paper
• Typology: 2.09 - Master's Thesis
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Place of publishing: Ljubljana
• Publisher: [M. Češnjaj]
• Year: 2021
• Number of pages: XIX, 78 f.
• PID: 20.500.12556/RUL-127969
• UDC: 669
• COBISS.SI-ID: 71546371
• Publication date in RUL: 30.06.2021

ČEŠNJAJ, Marko, 2021, Vpliv toplotne obdelave na mehanske lastnosti in mikrostrukturo jekla PK993 : magistrsko delo [online]. Master’s thesis. Ljubljana : M. Češnjaj. [Accessed 23 March 2025]. Retrieved from: https://repozitorij.uni-lj.si/IzpisGradiva.php?lang=eng&id=127969

Secondary language

• Language: English
• Title: Effect of heat treatment on mechanical properties and microstructure of PK993 steel : master's thesis

Abstract

In this study an effect of intercritical annealing was observed on an PK993/1CH13N3 martensitic stainless steel. Current heat treatment for PK993/1CH13N3 steel consists of quenching from 1000 °C and tempering at 530 °C. With this heat treatment all required mechanical properties are achieved, with an exception of impact toughness. Additional intercritical annealing was purposed to achieve better combination of all mechanical properties. To test that, two series of heat treatments were performed on samples taken from a rolled bar diameter 70 mm. In first series all samples were water quenched and tempered at different low temperatures. In second series an additional intercritical annealing above AC1 temperature was performed in between two previously mentioned steps of heat treatment. The study focuses on how this additional intercritical annealing effects microstructure and mechanical properties of PK993/1CH13N3 steel. Microstructure was observed using light and scanning electron microscope, while mechanical tests were performed to determine yield point, tensile strength, impact toughness, elongation, contraction and hardness. In addition to that, dilatometry tests were performed to determine AC1, AC3, Ms and Mf temperatures for PK993/1CH13N3 steel. For additional research Thermo-calc simulations and XRD analysis were performed. It has been concluded that a thin carbide film forms around austenitic grain bounders during austeitization. After only tempering at low temperatures this film stays stable. With intercritical annealing bigger spherical carbides form and the film is no longer present on the grain boundaries. Because more carbides form, Ms temperature raises after intercitical annealing. In addition to that, new and fresh martensite forms after cooling form an annealing temperature higher than AC1 point. Changes in microstructure lower all mechanical properties except impact toughness and provide a better combination of all mechanical properties. It also lowers standard deviations of all mechanical properties within a given heat treated series.

• Keywords: PK993/1CH13N3 steel, martensitic stainless steel, microstructural analysis, intercritical annealig, mechanical properties, impact toughness