Izdelava CCT diagrama za jeklo PT181

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

Namen diplomske naloge je bil konstruirati kontinuirni premenski diagram (CCT) za jeklo PT181. V ta namen smo uporabili dilatometrijo, svetlobno in vrstično elektronsko mikroskopijo ter izvedli merjenje trdot. Eksperimentalno določene temperature premen smo primerjali s temperaturami, ki smo jih izračunali z empiričnimi enačbami iz literature ter računalniškimi programi. Z dilatometrijo smo določili premenske temperature Ac1 = 773 °C, Ac3 = 880 °C, Ms = 310 °C ter Mf = 250 °C. Prav tako smo določili temperature premen avstenita pri ohlajanju s hitrostmi med 2 K/min in 6000 K/min. Ferit se v mikrostrukturi pojavi pri nizkih ohlajevalnih hitrostih med 2 in 100 K/min. Perlit nastane le pri najpočasnejših ohlajevalnih hitrostih med 2 K/min in 8 K/min v zelo majhnih deležih. Bainit se pojavi pri vseh hitrostih ohlajanja, pri hitrostih nad 30 K/min ga je v mikrostrukturi vselej nad 95 %. Pri 20 K/min sta deleža ferita ter bainita enaka, najmanj bainita se pojavi pri najpočasnejšem ohlajanju z 2 K/min. Martenzit nastane v zelo majhnih deležih (do 5 %) le pri zelo hitrem ohlajanju nad 1000 K/min. Vzajemno z mikrostrukturno sestavo se spreminjajo tudi trdote, ki naraščajo s povečevanjem ohlajevalne hitrosti – pri 2 K/min znaša trdota 172 HV0,5; pri 6000 K/min pa 399 HV0,5. Določili smo tudi temperaturni razteznostni koeficient, ki pri 100 °C znaša 12,526·10-6 K-1, pri 700 °C pa 14,659·10-6 K-1. Ugotovili smo, da so empirične enačbe in računalniški programi uporabno orodje za določanje parametrov toplotne obdelave, vendar je njihovo zanesljivost vselej potrebno potrditi z eksperimentalnimi metodami.

• Language: Slovenian
• Keywords: kontinuirni premenski diagram (CCT), jeklo PT181, dilatometrija, mikrostrukturna analiza, trdota
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2017
• PID: 20.500.12556/RUL-96218
• COBISS.SI-ID: 1700703
• Publication date in RUL: 27.09.2017

Secondary language

• Language: English
• Title: Construction of CCT diagram for PT181 steel

Abstract

The purpose of the thesis is to construct continuous cooling transformation (CCT) diagram for PT181 steel. For this purpose, dilatometric analysis, light microscopy, scanning electronic microscopy and hardness testing were used. Experimentally defined transformation temperatures were compared to transformation temperatures determined with empirical equations from literature and computer programs. Using dilatomeric method, transformation temperatures Ac1 = 773 °C, Ac3 =880 °C, Ms = 310 °C and Mf = 250 °C were defined. Using the same method, transformation temperatures of austenite at different cooling rates between 2 K/min and 6000 K/min were specified. Ferrite formed at lower cooling rates between 2 K/min and 100 K/min. Perlite formed only in small quantities at the lowest rates of cooling between 2 K/min and 8 K/min. Bainite formed at all cooling rates, at rates higher than 30 K/min, its share was always over 95%. At cooling rate of 20 K/min, shares of bainite and ferrite were the same, smallest amount of bainite formed at slowest cooling rate of 2 K/min. Martensite formed only by very rapid cooling, at rates greater than 1000 K/min. Reciprocally with microstructural composition increased hardness of analysed steel, being lowest at cooling rate of 2 K/min (172 HV0,5) and highest at rate of 6000 K/min (399 HV0,5). Coefficient of thermal expansion was defined at 100 °C, being 12,526·10-6 K-1, and at 700 °C being 14,659·10-6 K-1. Empirical equations and computer programs can be a useful tool for defining parameters of heat treatment, however, the results showed that their accuracy must always be proven with experimental methods.

• Keywords: continuous cooling transformation diagram, PT181 steel, dilatometry, microstructure analysis, hardness