Sensitization in austenitic stainless steels : quantitative prediction considering multicomponent thermodynamic and mass balance effects

Abstract

<div lang="en" class="abs"><p class="abs_header">Abstract</p><p>Austenitic stainless steels have wide range of different applications, but their corrosion resistance is compromised due to sensitization phenomenon. If these steels are subjected to heat treatments in the temperature regimes of 500–900 °C, chromium carbides are formed along the grain boundaries leading to chromium depletion in the vicinity of these carbides. This is called sensitization and makes the microstructure sensitive to intergranular corrosion. The sensitization behavior is greatly influenced by several metallurgical factors such as aging temperature, time, grain size and chemical composition. In order to study the effect of above-mentioned individual factors, a modeling prediction is needed, because of the cost and limitations involved with the lab experiments.</p><p>The CALPHAD modeling capability has evolved and is utilized in this thesis in estimating the sensitization by predicting the chromium concentration profiles across the grain boundary M₂₃C₆ carbide – austenite matrix interface by considering multicomponent effects. Thermo-Calc has been utilized in estimating the equilibrium calculations, TC-Prisma for the nucleation kinetics and DICTRA for solving the moving phase boundary problem. These are validated with diverse experimentally measured sensitization databases. Commercial stainless steel heats EN 1.4310, 1.4301, 1.4303, 1.4307 and 1.4318 have been subjected to heat treatments at various temperatures to induce varying levels of sensitization and the degree of sensitization was measured using the double loop electrochemical potentiokinetic reactivation test (DL-EPR) to develop the necessary quantitative database. Based on the chromium depletion profiles calculated and the experimentally determined DOS, a depletion parameter has been established that can quantitatively predict the sensitization and self-healing phenomenon in austenitic stainless steels. Models are proposed based on depletion parameter that accounts for the effects of thermal aging, grain size and chemical compositions.</p></div>

<div lang="fi" class="abs"><p class="abs_header">Tiivistelmä</p><p>Austeniittiset ruostumattomat teräkset soveltuvat monenlaisiin käyttökohteisiin, mutta niiden korroosionkestävyyttä on kompromisoitu herkistymisilmiön seurauksena. Mikäli näitä teräksiä kuumennetaan 500–900 °C lämpötiloissa, kromikarbideja alkaa muodostua raerajoille, mikä johtaa kromista köyhtymiseen karbidien läheisyydessä. Tätä kutsutaan herkistymiseksi, ja se altistaa mikrorakenteen raerajakorroosiolle. Herkistymiseen vaikuttavat olennaisesti mm. lämpötila, raekoko ja kemiallinen koostumus. Jotta näiden tekijöiden yksilöllisiä vaikutuksia voidaan tutkia, tarvitaan siihen tehokkaita mallinnusmenetelmiä.</p><p>Tämä väitöskirja hyödyntää CALPHAD mallinnusta herkistymisen arvioimisessa ennustamalla kromin konsentraatioprofiileja raerajalla olevan M₂₃C₆-karbidin ja austeniittimatriisin rajapinnan ylitse huomioiden multikomponenttiefektit. Thermo-Calc -ohjelmistoa käytetään tasapaino-olosuhteiden arvioimiseen, TC-Prismaa ydintymiskinetiikkaan ja DICTRA:a liikkuvan faasirajapinnan haasteen ratkaisemiseksi. Nämä tulokset validoidaan erilaisia kokeellisia tietokantoja hyödyntäen. Tutkimuksessa lämpökäsiteltiin kaupallisia austeniittisia ruostumattomia teräksiä EN 1.4310, 1.4301, 1.4303, 1.4307 ja 1.4318 eri herkistymisasteiden saavuttamiseksi, ja niiden herkistyminen mitattiin elektrokemiallisella potentiokineettisella uudelleenaktivointitestillä (DL-EPR) vaadittavan kvantitatiivisen tietokannan luomiseksi. Kromipitoisuusprofiilien perusteella laskettujen ja kokeellisesti määritettyjen herkistymisasteiden perusteella luotiin köyhtymisparametrit, jotka mahdollistavat herkistymisen ja toipumisen kvantitatiivisen ennustamisen austeniittisille ruostumattomille teräksille. Näihin köyhtymisparametreihin pohjautuvien mallien avulla voidaan arvioida lämpövanhenemisen, raekoon ja kemiallisen koostumuksen vaikutuksia.</p></div>

<div lang="en" class="abs"><p class="abs_header">Original papers</p><p>Original papers are not included in the electronic version of the dissertation.</p><ol type="I"><li><p>Kolli, S., Ohligschläger, T., & Porter, D. (2017). Continuous cooling sensitization and its evaluation in austenitic stainless steels EN 1.4310. European stainless steel conference & Duplex 2017, Bergamo, Italy</p><p><a href="https://urn.fi/urn:nbn:fi-fe201706197363">Self-archived version</a></p></li><li><p>Kolli, S., Ohligschläger, T., Kömi, J., & Porter, D. (2019). Sensitization and Self-healing in Austenitic Stainless Steel: Quantitative Prediction Considering Carbide Nucleation and Growth. ISIJ International, 59(11), 2090–2097. <a href="https://doi.org/10.2355/isijinternational.isijint-2019-264">https://doi.org/10.2355/isijinternational.isijint-2019-264</a></p><p><a href="https://urn.fi/urn:nbn:fi-fe2019111943207">Self-archived version</a></p></li><li><p>Kolli, S., Javaheri, V., Kömi, J., & Porter, D. (2019). On the Role of Grain Size and Carbon Content on the Sensitization and Desensitization Behavior of 301 Austenitic Stainless Steel. Metals, 9(11), 1193. <a href="https://doi.org/10.3390/met9111193">https://doi.org/10.3390/met9111193</a></p><p><a href="https://urn.fi/urn:nbn:fi-fe2019111538233">Self-archived version</a></p></li><li><p>Kolli, S., Javaheri, V., Ohligschläger, T., Kömi, J., & Porter, D. (2020). The importance of steel chemistry and thermal history on the sensitization behavior in austenitic stainless steels: Experimental and modeling assessment. Materials Today Communications, 24, 101088. <a href="https://doi.org/10.1016/j.mtcomm.2020.101088">https://doi.org/10.1016/j.mtcomm.2020.101088</a></p><p><a href="https://urn.fi/urn:nbn:fi-fe2020060841171">Self-archived version</a></p></li></ol></div>

<div lang="fi" class="abs"><p class="abs_header">Osajulkaisut</p><p>Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon.</p><ol type="I"><li><p>Kolli, S., Ohligschläger, T., & Porter, D. (2017). Continuous cooling sensitization and its evaluation in austenitic stainless steels EN 1.4310. European stainless steel conference & Duplex 2017, Bergamo, Italy</p><p><a href="https://urn.fi/urn:nbn:fi-fe201706197363">Rinnakkaistallennettu versio</a></p></li><li><p>Kolli, S., Ohligschläger, T., Kömi, J., & Porter, D. (2019). Sensitization and Self-healing in Austenitic Stainless Steel: Quantitative Prediction Considering Carbide Nucleation and Growth. ISIJ International, 59(11), 2090–2097. <a href="https://doi.org/10.2355/isijinternational.isijint-2019-264">https://doi.org/10.2355/isijinternational.isijint-2019-264</a></p><p><a href="https://urn.fi/urn:nbn:fi-fe2019111943207">Rinnakkaistallennettu versio</a></p></li><li><p>Kolli, S., Javaheri, V., Kömi, J., & Porter, D. (2019). On the Role of Grain Size and Carbon Content on the Sensitization and Desensitization Behavior of 301 Austenitic Stainless Steel. Metals, 9(11), 1193. <a href="https://doi.org/10.3390/met9111193">https://doi.org/10.3390/met9111193</a></p><p><a href="https://urn.fi/urn:nbn:fi-fe2019111538233">Rinnakkaistallennettu versio</a></p></li><li><p>Kolli, S., Javaheri, V., Ohligschläger, T., Kömi, J., & Porter, D. (2020). The importance of steel chemistry and thermal history on the sensitization behavior in austenitic stainless steels: Experimental and modeling assessment. Materials Today Communications, 24, 101088. <a href="https://doi.org/10.1016/j.mtcomm.2020.101088">https://doi.org/10.1016/j.mtcomm.2020.101088</a></p><p><a href="https://urn.fi/urn:nbn:fi-fe2020060841171">Rinnakkaistallennettu versio</a></p></li></ol></div>