Carbon Effect on Thermal Ageing Simulations in Ferrite Steels
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Carbon Effect on Thermal Ageing Simulations in Ferrite Steels Fabio Nouchy, Antoine Claisse and Pär Olsson Reactor Physics, KTH, AlbaNova University Centre, 106 91 Stockholm, Sweden
ABSTRACT Two major causes of hardening and subsequent embrittlement in ferrite steels are the spinodal decomposition of the binary Fe-Cr solid solution and the carbide formation due to the presence of carbon as foreign interstitial atoms. In the present work, simulations of the microstructure evolution due to thermal ageing are performed by means of a kinetic Monte Carlo code and using a state-of-the-art interatomic potential based on density functional theory (DFT) predictions and experimental data. The main issues concern the possibility to perform thermal ageing simulations in an acceptable computational time frame and to reproduce a realistic behavior of carbon kinetics and carbide formation. The simulations on the binary system show the microstructural evolution during thermal ageing and allowed to find an exponential trend related to the acceleration as a function of temperature. With the insertion of carbon in the model, the chromium precipitation tends to accelerate. The carbon clustering, analyzed separately, is faster with higher C concentrations and in lattices with segregated chromium. INTRODUCTION Ferrite structures are found in a considerable percentage in welding steels in order to avoid microfissures which occur in fully austenitic deposits [1]. Because of the body-centered cubic (bcc) structure, with a low atomic density, and the high chromium content, ferrite steels suffer thermal ageing processes on a larger extent than other steels, ending up with a brittle behavior after long periods at high temperatures [2]. In FeCr bcc alloys a Cr-rich Į' phase tends to segregate from an Fe-rich Į phase, according to Cr concentration. The driving force depends on the position inside the miscibility gap, which recently has been under intense scrutiny and reassessment [3,4,5]. For concentrations of interest, the occurring phenomenon is a spinodal decomposition [6]. At the same time, other elements may precipitate: particular attention must be paid to the carbide formation [7]. Spinodal decomposition and carbide formation were identified among the most relevant causes of hardening of the steel [8] and consequent embrittlement which raises the likelihood of weld failure. Spinodal decomposition and Cr precipitation have been studied recently by several authors using kinetic Monte-Carlo (KMC) [9, 10, 11, 12]. In the two latter references, the cohesive model providing the driving force was based on density functional theory (DFT) and experiments [12]. In addition, the vacancy migration properties were determined ab initio [13]. For the FeCrC system carbide formation and cohesive energies for different phases have been investigated using DFT [14]. Carbon self-interactions in Fe, as well as interactions with other species and migration energies, have also been investigated [15,16]. In the present study, a further analysis to perform accele
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