Effect of Martensitic Transformation and Grain Misorientation on Surface Roughening Behavior of Stainless Steel Thin Foils

Abstract

The surface roughening (Ra), martensitic phase transformation (MPT), and grain misorientation (GMO) behavior of stainless steel 304 and 316 in various grain sizes (Dg) were studied experimentally, including five cycles of sequential uniaxial tensile stress testing and Scanning Electron Microscope-Electron Back Scattered Diffraction (SEM-EBSD) investigation. The MPT and GMO characteristics were sequentially investigated using tensile testing and SEM-EBSD analysis. The correlation between MPT, GMO, martensitic volume fraction (Mf), and Ra behavior were investigated. The experimental results showed that increasing the total strain from 5.0% to 25.0% increased the MPT, GMO, and Mf, which were transformed from the metastable austenitic phase in stainless steel (SUS) 304. The increasing total strain increased Ra for all kinds of Dg. Furthermore, SUS 304 and SUS 316 were used to compare the roughening mechanism. The MPT was very high and spread uniformly in fine grain of SUS 304 thin foil, but the MPT was low and not uniform in coarse grain of SUS 304 thin foil. There was no MPT in SUS 316 thin foil, both in coarse and fine grain. The GMO in fine grains, both in SUS 304 and SUS 316 thin foils, spread uniformly. The GMO in coarse grains, both in SUS 304 and SUS 316 thin foils, did not spread uniformly. Surface roughness increased higher in coarse grain than fine grain for both of SUS 304 and SUS 316 thin foil. SUS 304 increased higher than SUS 316 thin foil. The effect of inhomogeneous deformation due to the MPT is a more important factor than GMO in coarse grain.

Constitutive Model of the Surface Roughening Behavior of Austenitic Stainless Steel

“…In this research, the author uses SUS 304 and SUS 316 thin metal foils of SUS 304 and SUS 316. The material condition for room [20] and elevated temperature [21] was similar to previous results, such as sample dimension, microstructure, grain size, and chemical composition. Room temperature was used to investigate the effect of MPT and GMO on the surface roughening behavior.…”

“…The surface roughening increases with the strain level. Furthermore, the surface roughening in SUS 304 increases higher than in SUS 316 TMF coarse grain at room temperature [20]. The main reason for this is that the MPT spreads inhomogeneously in the case of coarse grains.…”

“…The methodology of using room temperature [20] and elevated temperatures [21] has been explained in previous research. The methodology, in general, involves using uniaxial tensile stress state for five steps, and surface roughness is measured every step.…”

“…It has been generally reported that surface roughening linearly increases with plastic strain [14][15][16][17] and is affected not only by plastic deformation conditions such as the strain ratio or strain path but also by material factors such as the grain size crystal structure, crystal orientation and texture [15,[17][18][19]. When the austenitic stainless steel is subjected to plastic deformation, martensitic phase transformation (MPT) occurs [15,[20][21][22]. MPT occurs as a result of the slip band intersection.…”

“…Thus, it is not enough if the surface roughening depends on the grain size and strain level. MPT is the one factor in SUS 304 thin metal foil (TMF) that enhances the surface roughening compared to SUS 316 TMF, in which the surface roughening in SUS 304 increases higher than SUS 316 TMF with the same grain size (Dg) and the same strain level at room temperature [20]. Furthermore, the influential factor of grain size does not explain the surface roughening behavior in thin metal foils since the influential factors such as MPT and GMO are also related to the grain size of the material.…”