Abstract
Duplex stainless steels are susceptible to the formation of sigma phase at high temperature which could potentially be responsible for catastrophic service failure of components. Thermal treatments were applied to duplex stainless steels in order to promote the precipitation of different fractions of sigma phase into a ferrite-austenite microstructure. Quantitative image analysis was employed to characterize the microstructure and Charpy impact tests were used in order to evaluate the mechanical degradation caused by sigma phase presence. The fracture morphology of the Charpy test specimens were thoroughly observed in SEM, looking for a correlation between the microstructure and the fracture types in UNS S32205 duplex stainless steel. The main conclusion is the strong embrittlement effect of sigma phase since it is possible to observe a transition from transgranular fracture to intergranular fracture as increases the percentage of sigma phase. Thus, the mixed modes of fracture are predominant in the present study with high dependence on sigma phase percentages obtained by different thermal treatments.
Highlights
Duplex stainless steels appeared as an alternative to austenitic steels for numerous components due to their excellent combination of properties such as higher strength, excellent resistance to stress corrosion cracking and localized corrosion, such as pitting and crevice
Samples were aged in accordance with the thermal treatments at 850 °C for [5, 15, 45] and 135 minutes followed by water quenching, in order to produce different sigma phase percentages at ferriteaustenite microstructure
Final microstructures of specimens aged at 850 °C during 15 and 45 minutes are presented in Figures 1a, b) respectively
Summary
Duplex stainless steels appeared as an alternative to austenitic steels for numerous components due to their excellent combination of properties such as higher strength, excellent resistance to stress corrosion cracking and localized corrosion, such as pitting and crevice. These advantages are due to a phase-balanced ferrite-austenite defect-free microstructure, but these ideal features are practically impossible due to the formation of deleterious secondary phases during high temperature processing such as hot forming, heat treatments and welding. Sigma phase is a hard, brittle phase, which is generally formed between 600 and 950 °C, with rapid kinetic formation; its nucleation is preferentially at the ferrite-austenite interfaces and presents different morphologies depending on thermal treatments[12,13].
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