Vpliv dodatka niobija na mehanske lastnosti ter mikrostrukturo jekla X20CrMoV11-1

Poročnik, Matej (author), Nagode, Aleš (mentor)

V okviru magistrskega dela smo preučili vpliv dodatka legirnega elementa niobija na mehanske lastnosti ter mikrostrukturo nerjavnega konstrukcijskega jekla X20CrMoV11-1. Iz dosedanjih dognanj na področju mikrolegiranja vemo, da dodatek niobija izboljša mehanske lastnosti zaradi drobnih izločkov, ki utrdijo jekleno matico z mehanizmom izločevalnega utrjevanja, prav tako pa zavirajo rast kristalnih zrn med avstenitizacijo, kar je osnova za udrobnjeno mikrostrukturo martenzita v poboljšanem stanju. Niobijevi izločki prav tako povečajo odpornost jekla na lezenje. Glavni cilj naloge je bil preučiti vpliv niobija na trdnostne lastnosti jekla pri popuščanju na višjih temperaturah ter preučiti njegov vpliv na lezenje. V ta namen smo s pomočjo računalniškega programa ThermoCalc določili okvirne kemične sestave poskusnih šarž. V vakuumski indukcijski peči smo izdelali poskusne šarže z različnimi vsebnostmi niobija. Jeklo smo ulili v kokile, odlite ingote smo po strjevanju homogenizacijsko žarili, vroče plastično predelali (kovali) ter toplotno obdelali po različnih režimih. V okviru raziskav smo opravili kemično analizo, s katero smo potrdili uspešnost izdelave poskusnih šarž. Opravili smo dilatometrsko analizo ter naredili preizkus kalilne vrste, s katerima smo določili optimalno temperaturo avstenitizacije. Na poboljšanih vzorcih smo izmerili trdoto, opravili natezni trgalni preizkus, preizkus udarne žilavosti ter pospešen test lezenja. Mikrostrukturo smo pregledali na svetlobnem ter vrstičnem elektronskem mikroskopu, opravili smo tudi avtomatsko analizo izločkov ter vključkov. Ugotovili smo, da niobij v koncentracijah med 0,05 mas. % do 0,1 mas. % izboljša trdnost in odpornost jekla na lezenje, pri tem pa ne pride do bistvenega poslabšanja žilavosti. Boljšo odpornost na lezenje lahko pripišemo nastanku bolj stabilnih izločkov MX med lezenjem, ki dodatno ovirajo gibanje dislokacij, prav tako pa povzročajo izločevalno utrjevanje. Pri večji vsebnosti niobija (0,3 mas. %) iz taline pri strjevanju jekla nastanejo tudi veliki primarni izločki Nb(C,N). Ti imajo negativen vpliv na mehanske lastnosti jekla, še posebej na žilavost.

• Language: Slovenian
• Keywords: jeklo X20CrMoV11-1, mikrolegiranje z niobijem, mikrostruktura, mehanske lastnosti, lezenje
• Work type: Master's thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2020
• PID: 20.500.12556/RUL-121221
• COBISS.SI-ID: 31079683
• Publication date in RUL: 01.10.2020

Secondary language

• Language: English
• Title: Effect of niobium addition on mechanical properties and microstructure of X20CrMoV11-1 steel

Abstract

In the scope of this thesis, the influence of the addition of niobium on mechanical properties and microstructure of constructional stainless steel X20CrMoV11-1 was studied. Previous findings on microalloying have shown that niobium addition improves the mechanical properties of steel due to fine precipitates that harden the steel matrix with the mechanism of precipitation hardening. Moreover, these precipitates inhibit the growth of austenite crystal grains during austenitization, which leads to the finer martensitic microstructure of hardened steel. Niobium precipitates also improve the creep resistance of steel. The main aim of this work was to study the influence of niobium on the strength properties of steel that was tempered at higher temperatures and to study its influence on creep behaviour. For this purpose, a computer program for determination of approximate chemical compositions of laboratory melts was used. Laboratory melts were made in a vacuum induction furnace. Molten steel was cast into ingots, which were subsequently homogenisation annealed, forged and properly heat treated. As part of our research, a chemical composition analysis confirming the successfulness of melt fabrication was conducted. Furthermore, the optimal temperature of austenitization was determined using dilatometric analysis and sensitivity-to-grain growth hardenability test. Hardness, tensile strength and Charpy impact toughness of heat treated samples were determined. Moreover, an accelerated creep test was also undertaken. Lastly, the microstructure analysis was made using light and scanning electron microscope, and an automated SEM-EDS non-metallic inclusions and precipitates analysis was made. Results show that niobium in concentrations between 0,05 wt. % to 0,1 wt. % improves the strength and creep resistance of steel, without there being a significant decrease in impact toughness. Improvement of creep resistance can be attributed to the formation of more stable MX precipitates during creep, which represent an extra barrier for dislocation movement and improve strength due to precipitation hardening. At higher niobium concentrations (0,3 wt. %) primary Nb(C,N) precipitates have formed during melt solidification. These large precipitates have a negative impact on mechanical properties, especially impact toughness.

• Keywords: X20CrMoV11-1 steel, microalloying with niobium, microstructure, mechanical properties, creep