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Influence of multi-stage deformation processing and stacking fault energy on near-surface slip and martensite formation in austenitic stainless steels
Davis, Skyler L.
Davis, Skyler L.
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2023
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Abstract
Metastable austenitic stainless steel is used to produce billions of cannula per year. Austenitic stainless steel is well suited for cannula due to its biocompatibility, excellent corrosion resistance, good formability, and low cost. The composition of the AISI Type 304 stainless steel used to produce 80% of the world's cannula differs somewhat from the 304 stainless steels used in most other applications, and that has been studied most extensively in the international technical literature. It has higher nickel content and lower carbon content.
Reported first in this dissertation are the results of cold rolling experiments to show the evolution of deformation-induced microstructures in the distinct 304 stainless steel alloy used in cannula. Electron backscatter diffraction, x-ray line profile analysis, x-ray texture goniometry, and magnetic permeability measurements were used to characterize the microstructures. Additionally, an innovative image processing technique was developed and applied to evaluate how the distribution of martensite nucleation sites depends on grain boundary misorientation.
Two other stainless steels, AISI Types 301 and 305, having either slightly lower or higher stacking fault energy ( 10 mJ/m2), were subject to the same cold rolling and characterized to document how variation in stacking fault energy influences microstructure evolution. The combined results from rolling 301, 304, and 305 stainless steels were then applied to analyze and interpret the microstructures produced in the 304SS seam-welded tubing subject to the industrial drawing and sinking processes to make cannula. The interpretation of stress-dependent and strain-dependent microstructural effects were enabled by corresponding large-strain Arbitrary Lagrangian-Eulerian finite element simulations to compare the gradients and relative magnitudes of normal and shear stress and strain components during rolling, tube drawing, and tube sinking.
The leading result of this research is the demonstration that stacking fault energy is a useful parameter for understanding and controlling the microstructures in metastable stainless steels. It provides an integrative parameter to understand the interplay between the effects of alloy composition and deformation processing conditions on microstructures. A modest increase in stacking fault energy of approximately 10 mJ/m2 reduced the rate of transformation and the volume fraction of ’-martensite, increased the density of deformation-induced twins, increased the maximum dislocation density, and decreased the average spacing between the slip bands in the austenite. Decreasing the stacking fault energy had comparable but opposite effects.
X-ray line profile analyses showed how the multiaxiality of the stress states and changes in strain path influence the storage of dislocations and the formation of martensite. The plane strain deformation mode during rolling results in a progressive increase in the proportion of edge-type dislocations. In contrast, the change in strain path and stress components after the shift between drawing and sinking produces larger populations of screw-type dislocations. Furthermore, drawing and sinking produced reversals in the sign of the shear stress component, which activates more slip systems and creates more intragrain slip band intersections to nucleate martensite and alters the dislocation density evolution. Additionally, changes in austenite dislocation density are believed to be altered by martensite formation, as the regions of the highest dislocation density are removed from the austenite as they transform into martensite.
Crystallographic texture evolution was not strongly affected by the stacking fault energy. However, a moderately high fraction of Goss texture, a common texture component that forms during annealing of low stacking fault energy alloys, was measured after deformation of recrystallized sheet and tubing.
Finally, empirical approaches, such as the Md30 criterion, to predict martensite phase transformation based solely on alloy content or mechanistic models based on analyses of monotonic uniaxial deformation do not address the effects of stacking fault energy on slip band spacing, twinning, or dislocation density. The stacking fault energy better represents the influence of grain size, grain boundary misorientation, temperature, multiaxial stress states, or strain path changes. This work quantifies some of these effects so that they can be subsequently incorporated into more comprehensive predictive models.
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