Improving Hydro-formability of a Ferritic Stainless Steel Tube Through Severe Plastic Deformation
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I.
INTRODUCTION
FERRITIC stainless steels (FSSs) typically ensure a higher resistance to stress corrosion cracking as well as lower prices in comparison to austenitic stainless steels. Thus, they are promising candidates for corrosive environments, especially those containing the chlorides ion.[1,2] However, the formability of these steels is lower than that of their austenitic counterparts.[3–5] For instance, FSSs usually suffer from surface roughening during tensile forming processes like deep-drawing and hydro-forming. This type of defect, often called ridging, has mainly been attributed to the appearance of a specific texture during the casting and hot-rolling of FSSs. More specifically, it has been reported that the h100i orientations of the large columnar grains of an FSS slab are parallel to the slab’s normal direction, owing to the directional solidification that occurs during slab casting. This solidification texture fiber is complemented by two other relatively strong texture fibers during the subsequent hot-rolling process in which the {111} planes of the grains are almost identical to the rolling plane and/or the h110i orientations of the grains
M.H. FARSHIDI is with the Department of Materials Science and Metallurgical Engineering, Ferdowsi University of Mashhad, Azadi Square, Mashhad, Iran. Contact e-mail: [email protected] Y. KITANO, M. YUASA, and H. MIYAMOTO are with the Department of Mechanical Engineering, Doshisha University, Kyotanabe city, Kyoto, Japan. Manuscript submitted March 12, 2018. Article published online September 25, 2018 6052—VOLUME 49A, DECEMBER 2018
are almost parallel to the rolling direction. These special hot-rolling texture fibers are usually called c-fiber and a-fiber, respectively. Since grains oriented on the texture fibers formed during the casting and rolling of FSSs show different Schmid factors, their slip behaviors tend to differ, and therefore, they induce ridging during subsequent tensile forming processes.[4–7] The other reason for the poor formability of FSSs is their small R-value. The average R-value of FSS sheets is typically reported to be between 1.0 and 1.5 depending on the processing conditions. However, the exact R-value is dependent on the direction of tensile forming. For example, the R-value can be as low as 0.5 for one specific direction of a rolled FSS sheet.[7–10] The differences in R-value are mainly related to the appearance of the specific texture fibers mentioned above. For instance, the R-values of the different directions of a sheet oriented exactly on the {111}h110i texture component are found to be between 1.5 and 2.2 by using a crystal plasticity finite element (CPFE) simulation. The corresponding values for a sheet oriented exactly on the {100}h110i texture component are found to be between 0.1 and 0.9.[10,11] Thus, it can be inferred that modification of the texture of an FSS is key to obtaining a favorable formability. Modification of the conventional texture that appeared during casting and rolling of an FSS is difficult since it does not undergo the austenit
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