Abstract
An extra low carbon martensitic stainless steel with 16% ultrafine grained metastable reverted austenite was subjected to uniaxial tensile testing and investigated with in-situ energy-dispersive synchrotron X-ray diffraction (XRD) and in-situ electron backscatter diffraction (EBSD) to reveal the complex interplay between stress, strain and martensitic transformation. In-situ XRD demonstrated that, upon surpassing the yield strength, the fraction of reverted austenite declined linearly with increasing true stress, which was associated with transformation-induced plasticity (TRIP). EBSD and XRD consistently showed that the texture of martensite evolved from an initially weak texture towards a strong 110α′ fiber parallel to the tensile axis. For the first time, stress partitioning between (remaining) reverted austenite and the martensite matrix was determined quantitatively during in-situ XRD by averaging over the stress values obtained from lattice strains for multiple reflections. Martensite accommodates the majority of the applied load while reverted austenite is severely plastically deformed. XRD shows strong plastic anisotropy in austenite. In-situ forward-scatter electron imaging and advanced variant analysis of the EBSD data indicate that plastic deformation and strain-induced austenite-to-martensite transformation is concentrated along boundaries between martensite blocks and packets which are inclined up to ∼55° with respect to the tensile direction. These regions were preferred sites for strain-induced martensite formation.
Highlights
Extra low carbon stainless steels, combining the families of softand super-martensitic stainless steels [1], are among the most popular alloys for pipeline applications in the oil and gas industry
The obtained lattice strains determined by Xray diffraction (XRD) are converted into phase specific mechanical stress to quantify the partitioning of the applied stress over martensite and austenite
The present study investigated the interplay between stress, strain and martensitic transformation in an intercritically annealed extra-low carbon martensitic stainless steel during uniaxial tensile loading by combining in-situ results from synchrotron XRD, forward-scatter electron (FSE) imaging and electron backscatter diffraction (EBSD)
Summary
Extra low carbon stainless steels, combining the families of softand super-martensitic stainless steels [1], are among the most popular alloys for pipeline applications in the oil and gas industry. The alloy properties, and in particular the mechanical properties, are obtained by an inter-critical annealing In this treatment, the tempering of lath martensite leads to segregation of interstitial elements to lattice defects [5] and a reduction in dislocation density by an order of magnitude [4]. The tempering of lath martensite leads to segregation of interstitial elements to lattice defects [5] and a reduction in dislocation density by an order of magnitude [4] Most importantly, it triggers the formation of ultrafine grained reverted austenite on the various boundaries in the hierarchical microstructure that characterizes martensite [6]. This is commonly referred to as “austenite memory” [7,9,10]
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