Effect of Strain Rate on the Formation of Strain-Induced Martensite in AISI 304L Stainless Steel
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TENITIC stainless steels consists of metastable austenite phase c, which on plastic deformation, transforms to strain-induced martensite referred to as a¢-martensite. The mechanism and kinetics of transformation for c to strain-induced a¢-martensite has been In investigated and well documented.[1–4] metastable austenitic stainless steels the potential nucleation sites for a¢-martensite were found to be the intersection of microscopic shear bands, which comprise of stacking faults, twins, and hcp e-martensite.[5–7] The extent of martensitic transformation in these steels is governed by several factors, such as chemical composition, temperature, strain rate and deformation mode. Of these the strain rate plays an important role that has not been completely explained. It has been reported that an increase in strain rate results in a lower content of strain-induced a¢-martensite in metastable austenitic stainless steels.[8–13] This is explained based on the increased temperature, due to adiabatic heating
SAURAV SUNIL and RAJEEV KAPOOR are with the Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Mumbai 400085, India and also with the Division of Engineering Sciences, Homi Bhabha National Institute, Anushakti Nagar, Mumbai 400094, India. Contact e-mail: [email protected] Manuscript submitted May 23, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
occurring at high strain rates, that reduces the driving force for the formation of strain-induced a¢-martensite, and hence leads to suppression of the c to a¢ transformation. However, there have been contrasting reports on the effect strain rate has on the formation of a¢-martensite. Hecker et al.[8] reported that for strains lower than 0.25 the a¢-martensite content at strain rate of 103 s1 were higher than at low strain rates, but at higher strains the a¢-martensite content at high strain rate was lower. Murr et al.[12] investigated microstructural evolution of AISI 304 (also referred to as SS 304) and found that up to low strains there were more shear bands when tested at high strain rates than when tested at low strain rates. However at larger strains, the adiabatic heating in the specimen at high strain rates becomes dominant and is thought to be responsible for the lower a¢ content as compared to that at lower strain rates. Lee and Lin[14] reported an increase in the number of shear bands and in the a¢ content with increase in strain rate for tests in the range of 102 to 59103 s1. Isakovet et al.[10] reported a retardation in the a¢ transformation as function of strain rate, although this could not be explained solely on the basis of adiabatic heating. In the recent work by Vazquez-Fernandez et al.,[15] an effort was made to decouple the effect of strain rate and adiabatic heating on the a¢ transformation in 301LN stainless steel. They inferred that apart from the effect of adiabatic heating, strain rate may have a direct role to play in the formation of a¢-martensite. Most of the above-mentioned experimental studies
suggest that strain-induced c to a¢-m
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