Influence of Cold-Worked Structure on Electrochemical Properties of Austenitic Stainless Steels
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BECAUSE austenitic stainless steel (SS) is an fcc lattice structure, it is expected to deform by the movement of dislocations through slip during cold working. However, temperature, strain rate, and strain are the important parameters that can affect the deformation mechanism from slip to twinning and hence the deformation microstructure of austenitic SS.[1–3] Among the material properties, stacking fault energy (SFE) is one of the most important factors for producing changes in the deformation microstructure in austenite SS. Due to the difference in SFE, austenite SS produces a variety of deformation microstructures, such as tangled dislocations, dislocation pileups, stacking faults, and twins.[1,4,5] Further, it is reported that cryogenic deformation produces more stacking faults and twins compared to room-temperature deformation.[2,7] Even high speed[4] or heavy deformation at room temperature[6] found to produce twinned microstructure. Various metallurgical and processing variables dictate the corrosion resistance of stainless steels.[7] The austenitic grade is considered to be most resistant to industrial atmospheres including aggressive aqueous and nonaqueous acid media. However, as conditions become more severe, addition of several alloying elements is useful in promoting corrosion resistance and hence is desirable. Chromium >12 pct, for instance, improves the passivity of iron alloys, and molybdenum (>2 pct) promotes resistance to pitting corrosion.[8,9] Similarly, sensitization that occurs in stainless steels in B. RAVI KUMAR, Scientist, and B. MAHATO, Technical Assistant, Materials Science and Technology Division, and RAGHUVIR SINGH, Scientist, Applied Chemistry and Corrosion Division, are with the National Metallurgical Laboratory, Jamshedpur 831 007, India. Contact e-mail: [email protected] Manuscript submitted July 4, 2006. Article published online July 21, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
the temperature range 500 C to 870 C leading to intergranular corrosion can be improved by lowering carbon 70 pct thickness reductions. A low volume fraction of martensite phase in specimens without interpass cooling is due to adiabatic heating of the specimen on subsequent deformation during multipass rolling. This results in higher deformation temperatures with increased rolling and, therefore, inhibited formation of martensite phase. Thus, similar levels of cold working have produced different volume fractions of a¢-martensite. Kinetics of strain-induced martensite formation in 304L SS is well studied by Olson and Cohen,[25] Hecker et al.[24,32] and recently by others.[38,39] Hecker et al.[24] observed that the austenite fi a¢-martensite transfor-
mation can be described by Olson–Cohen analysis in uniaxial tension mode.[25] The volume fraction of a¢-martensite phase in this analysis is a sigmoidal function of strain under uniaxial tension loading, and the transformation is very sensitive to temperature in the vicinity of room temperature. The change in volume fraction of a¢-martensite obtained
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