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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.

Microstructure and mechanical properties of friction stir welded lean duplex stainless steel

“…These materials typically comprise approximately equal amounts of bodycentered cubic (bcc) ferrite, a, and face-centered cubic (fcc) austenite, g, phases in their microstructures [1,2]. There are three basic categories of DSSs: low-alloy or lean, intermediate alloy, and highly alloyed, or superduplex stainless steel (SDSS) grades, which are grouped according to their pitting resistance equivalent number (PREN) [3].…”

Hot working behavior of 2205 austenite–ferrite duplex stainless steel characterized by constitutive equations and processing maps

“…At higher strains, the load is transferred from ferrite to austenite leading to an increase in dislocation density and work hardening till DRX is triggered. It is understood that dissimilar plastic behavior and restoration mechanism of austenite and ferrite during the hot deformation of duplex SS can lead to a drastic decrease in the hot ductility of these alloys [25,28]. In order to define the safe regions of deformation and also to avoid the occurrence of flow localization and therefore premature fracture, a practical approach is plotting a processing map.…”

Hot Workability of 2304 and 2205 Duplex Stainless Steels

“…The present 2205 steel exhibited a similar temperature dependence but lower strength by -30 MPa than one of similar composition but a varying from 31 to 50% over 1100 to 1300 °C [59]. Moreover, with a somewhat higher Cr (eq)/Ni (eq) ratio (yet not dissimilar composition) than a published steel [64,65], it has a similar a content but a higher strength by +20 MPa and a similar ductility. The differences arise due to a much more extensive preheating treatment than the present industrially based one.…”

“…In 2205, the creep activation energy is 299 kJ/mol [67]. The present 2304, having a similar ratio to a published steel with higher Ni and higher Mo but notably lower N, has a lower a phase by -18% and a higher strength by +20 MPa, but a much lower ductility by a factor of 10 [64,65].…”