Load sharing between austenite and ferrite in a duplex stainless steel during cyclic loading

Abstract

The load sharing between phases and the evolution of micro-and macrostresses during cyclic loading has been investigated in a 1.5-mm cold-rolled sheet of the duplex stainless steel SAF 2304. X-ray diffraction (XRD) stress analysis and transmission electron microscopy (TEM) show that even if the hardness and yield strength are higher in the austenitic phase, more plastic deformation will occur in this phase due to the residual microstresses present in the material. The origin of the microstresses is the difference in coefficients of thermal expansion between the two phases, which leads to tensile microstresses in the austenite and compressive microstresses in the ferrite. The microstresses were also found to increase from 50 to 140 MPa in the austenite during the first 100 cycles when cycled in tension fatigue with a maximum load of 500 MPa. The cyclic loading response of the material was, thus, mainly controlled by the plastic properties of the austenitic phase. It was also found that initial compressive macrostresses on the surface increased from Ϫ40 to 50 MPa during the first 10 3 cycles. After the initial increase of microstresses and macrostresses, no fading of residual stresses was found to occur for the following cycles. A good correlation was found between the internal stress state and the microstructure evolution. The change in texture during cyclic fatigue showed a sharpening of the deformation texture in the ferritic phase, while no significant changes were found in the austenitic phase.

An in situ high-energy X-ray diffraction study of micromechanical behavior of multiple phases in advanced high-strength steels

“…This type of dislocation structure observed in ferrite has previously been reported in Ref. [21]. Meanwhile, other changes in the microstructure are also observed in this TRIP steel, i.e., a denser dislocation accumulation close to the phase boundary between bainite and ferrite and the grain boundaries within ferrite, suggesting that stress concentration near the phase or grain boundaries can be compensated by some dislocations.…”

Load Partitioning and Strain-Induced Martensite Formation during Tensile Loading of a Metastable Austenitic Stainless Steel

“…[35] Load transfer and stress interaction between phases has also been demonstrated for duplex steels. [36,37] However, in individual austenite grains, the stress levels may deviate significantly from the average stress; this will influence the phase transformation. [38] The two slower strain rates display similar behavior, in which the austenite stress follows the macroscopic stress quite closely until approximately 0.5 true strain, when the austenite deviates from the macroscopic stress curve.…”

SKPFM measured Volta potential correlated with strain localisation in microstructure to understand corrosion susceptibility of cold-rolled grade 2205 duplex stainless steel

“…In duplex stainless steels, strain localisation is complex due to the mismatch of mechanical properties between body-centred-cubic ferrite (δ -bcc) and face-centred-cubic austenite ( -fcc), which manifests itself during plastic deformation. Load sharing between both crystallographic phases is observed leading to heterogeneous deformation within the microstructure, with micro-yielding near grain boundaries in the ferrite, augmented by plastic deformation and dislocation pile-ups in the austenite [2,3]. Pitting corrosion [4,5] and crack nucleation [6] is often triggered at such sites, but cracks have also been observed to nucleate at slip bands due to the presence of local differences in elastic anisotropy [7].…”