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Title: Effect of ageing on phase evolution, mechanical and corrosion properties of a high tungsten super-duplex stainless steel
Author: Obi, Udoka
ISNI:       0000 0004 5366 6535
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2015
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Super duplex stainless steels (SDSSs) with lower nickel content are cost effective substitutes for higher alloyed austenitics and nickel alloys in demanding environments such as oil and gas production tubulars and pipelines due to their excellent corrosion resistance and high strength. The overall properties of SDSS are derived from its dual microstructure of equal ferrite and austenite, higher alloying additions of chromium, molybdenum, nitrogen and tungsten and its thermomechanical history. Higher alloying renders SDSS prone to secondary phase precipitation such as sigma phase during improper welding operations or fabrication, affecting the materials properties. Reports suggest that tungsten additions in SDSS delays sigma phase precipitation, hence the development of tungsten based SDSS such as UNS S39274. However, secondary phases cannot be entirely avoided in SDSS. Secondary phase evolution in DSS and the mechanical properties/corrosion behaviour of SDSS has been studied extensively yet their interaction is still not clear. In-service failures of SDSS components have identified gaps in the understanding of the link between secondary phase evolution and material properties, thus limiting the safe and efficient use of SDSS. The work presented in this thesis explored and quantified experimentally the role of aging on secondary phase evolution and ensuing effects on the mechanical properties, corrosion behaviour and toughness of UNS S39274 SDSS. The results revealed that chi phase precipitation occurred preferentially before the sigma phase, although chi phase was metastable in the studied alloy. Numerical modelling established that the measured concentration of the precipitated sigma phase follows the prediction by the Johnson-Mehl-Avrami kinetic model. The time-temperature -transformation was computed using experimental data, the results were compared with theoretical predictions. Results established that increase in both sigma and chi phase led to significant drop in the uniform strain and enhancement of the modulus, hardness and yield and tensile strengths. We note that the sigma phase was attacked by corrosion in comparison to other grades of 25Cr SDSS where the sigma phase remains inert to corrosion attack. Pitting corrosion resistance was influenced more by sigma phase than the chi phase composition. Chi phase was more damaging on the toughness than sigma phase. Another key finding is that the corrosion behaviour and fracture behaviour is more sensitive to lower secondary phase volume fraction than the tensile properties.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Duplex stainless steel