Plastic Deformation and Corrosion in Austenitic Stainless Steels

This thesis contains study of localized corrosion behavior (sensitization and passivation) of coldworked austenitic stainless steel (SS). This has been covered in three parts: the first part deals with in-grain misorientation and sensitization, second part deals with effects of deformed microstructure on passivation, and the third part deals with study of Cr₂O₃ characterization at machined sub-surfaces. Three different grades of austenitic SS grades (Sanicro 28^{TM 1} hereinafter called as alloy A, AISI (American Irons and Steel Institute) 316L and 304L) were selected in this study. In the first part, AISI type 304L austenitic SS was cold rolled (25ºC) and warm rolled(300ºC) followed by isothermal sensitization. Quantification of near boundary gradient zone was done, partially automated, by appropriate computer algorithms. One-to-one microstructural correlation was achieved by electron backscattered diffraction (EBSD) and white light interferometry (WLI). Grains with visible fragmentation, or clear reductions in size, showed a poor resistance to sensitization. However, non-fragmented deformed grains with clear presence of near boundary orientation gradients provided an improved resistance.
 
   For the second part, alloy A, 316L and 304L were subjected to anodic potentiodynamic polarization test in 0.5M H₂SO₄ at room temperature after plane strain compression test. Deformation microstructures developed in these grades, after plain-strain compression tests, include strain-induced martensite. Alloy A showed the poor corrosion performance among three alloys. Combination of microtexture measurements and Fourier transform infrared spectroscopy (FTIR)-imaging revealed that the presence of strain-induced martensite promoted postpassivation stability or retention of a protective Cr₂O₃ film.
 
   In the third part, alloy A, 316L and 304L of austenitic SS were subjected to vertical milling. These alloys exhibited difference in stacking fault energy and thermal conductivity. Anodic potentiodynamic polarization tests did not reveal differences (between machined specimens) in sub-surface machined layers. However, such differences were revealed in surface roughness, sub-surface residual stresses, misorientations, and detection of subsurface Cr₂O₃ passive films. It was shown, quantitatively, that higher machining speed reduced surface roughness & the effective depths of the affected subsurface layers.

¹ Sanicro 28 is an alloy marketed by Sandvik®. It is sold under the trademark