Abstract
Two types of duplex stainless steels were deformed by torsion at a temperature range of 900 to 1200 °C and strain rate of 1.0 s-1 and their final microstructures were observed. The austenite volume fraction of steel A (26.5Cr - 4.9Ni - 1.6Mo) is approximately 25% at room temperature, after conventional annealing, while that of steel B (24Cr - 7.5Ni - 2.3Mo) is around 55%. Experimental data show that steel A is ductile at high temperatures and displays low ductility at low temperatures, while steel B has low ductility in the entire range of temperatures studied. At high temperatures, steel A is essentially ferritic and shows dynamic recrystallized grains after deformation. When steel A is strained at low temperatures and displays low austenite volume fraction, microstructural observations indicate that failure is triggered by grain boundary sliding due to the formation of an austenite net structure at the ferrite grain boundaries. At intermediate volume fraction, when austenite forms a dispersed second-phase in steels A and B, failure begins at the ferrite/ferrite boundaries since some of the new ferrite grains may become immobilized by the austenite particles. When steel B is strained at volume fraction of around 50% of austenite and both phases percolate the microstructure, failure occurs after low straining as a consequence of the different plastic behaviors of each of the phases. The failure characteristics of both steels are correlated not only with the volume fraction of austenite but also with its distribution within the ferrite matrix, which limits attainable strain without failure.
Highlights
Duplex stainless steels have become more attractive than single-phase austenitic and ferritic grades in many industrial applications owing to their high strength and corrosion resistance in chloride-containing media[1,2]
Recent research has shown that the ductility of this kind of steel depends on the Materials Research deformation conditions, the behavior of the constituent phases, and the volume fraction of austenite in the ferrite matrix[9,11,12]
Attention has been paid to the softening mechanisms of the ferrite and austenite phases and on deformation conditions, very few systematic studies have focussed on the identification of the relationship between the microstructure and failure characteristics of these steels during high temperature deformation
Summary
Duplex stainless steels have become more attractive than single-phase austenitic and ferritic grades in many industrial applications owing to their high strength and corrosion resistance in chloride-containing media[1,2]. In addition to the work hardening and softening mechanisms required to deform each phase, the boundaries play an important role in duplex microstructures; the accommodation of macroscopic deformation depends on the plastic characteristics of both phases and interfaces[4,5]. It is a well known fact that during high temperature deformation, after a certain amount of work hardening, In spite of the complex deformation behavior of duplex stainless steels, it has been found that the ferrite phase continues to show intense dynamic recovery while the austenite phase undergoes dynamic recrystallization[8,9,10]. Attention has been paid to the softening mechanisms of the ferrite and austenite phases and on deformation conditions, very few systematic studies have focussed on the identification of the relationship between the microstructure and failure characteristics of these steels during high temperature deformation
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