Experimental study on the martensitic transformation in AISI 304 steel sheets subjected to tension under wide ranges of strain rate at room temperature

Abstract

a b s t r a c tIn this work, the martensitic transformation occurring in AISI 304 steel sheets subjected to tension at room temperature has been experimentally studied. Tensile tests performed on AISI 304 specimens are split into two different types; in situ tensile tests and macroscopic tensile tests. The former are conducted mounting the sample in a tensile micromachine originally developed in ENSAM/Metz within the range of strain rates 10 −5 s −1 ≤ε ≤ 10 −3 s −1 . The latter are performed under static and dynamic conditions of deformation within the range of strain rates 10 −3 s −1 ≤ε ≤ 10 2 s −1 . Using X-rays diffraction technique, in situ tensile tests have allowed measuring the stress of the phases during loading by placing the tensile micromachine under the range of a PROTO goniometer. Additionally, the martensitic transformation has been recorded via mounting the tensile micromachine into a scanning electron microscope. The volume fraction of martensite has been measured in the post mortem specimens, V˛ ≈ 70%. Moreover, the static macroscopic tensile tests, 10 −3 s −1 ≤ε ≤ 10 −1 s −1 , have been recorded using a high speed infrared camera. It has been proven that martensitic transformation takes place in AISI 304 steel for a temperature increase over T > 140 K. The macroscopic dynamic tensile tests are performed within the range of strain rates, 1 s −1 ≤ ε ≤ 100 s −1 . A considerable amount of martensite has been detected in the post mortem samples, V˛ ≈ 35%. This proves that plastic deformation is the dominant mechanism responsible for the transformation phenomenon in this steel. The free energy supplied by straining the sample relegates to a secondary role the rise in temperature taking place during the course of plastic deformation.

Dynamic necking in materials with strain induced martensitic transformation

“…In the case of the SIMT material $ 95% of the austenite has been transformed to martensite whereas in the case of the SIMTT material the final volume fraction of martensite is $ 26%. Similar values have been experimentally measured in austenitic steels tested in tension under dynamic (adiabatic) conditions, see Rodríguez-Martínez et al (2011).…”

“…once the transformation is complete the SIMT material shows slightly lower flow stress than martensite. Similar volume fractions of martensite have been experimentally measured in austenitic steels tested in tension under isothermal conditions of deformation, see Rodríguez-Martínez et al (2011).…”

“…2. Adiabatic loading process: both the stability analysis and the finite element modeling take into account the thermomechanical coupling in the phase transformation kinetics according to the experimental observations reported elsewhere (Olson and Cohen, 1975;Tomita and Iwamoto, 1995;Rodríguez-Martínez et al, 2011;Zaera et al, 2012Zaera et al, , 2013. Thus, this section examines the effect of temperature on the transformation rate which, in turn, affects the transformation hardening and consequently the processes of necking inception and energy absorption.…”

“…The results obtained with the second method (necking strain, necking wavelength, energy dissipated by plastic flow) at different strain rates and for different kinetics of transformation are rationalized with the first one, permitting to determine the conditions for the development of plastic instabilities. The work considers a metastable austenitic stainless steel as reference material, since this type of grade shows a large amount of transformed martensite even under adiabatic conditions (Rodríguez-Martínez et al, 2011). The study allows to analyze the range of strain rates at which the beneficial effect of transformation hardening on material ductility and energy absorption capacity becomes masked by the inertial effects.…”

On the Taylor–Quinney coefficient in dynamically phase transforming materials. Application to 304 stainless steel

“…Additional phenomena may be induced by high rate straining of crystalline solid like dynamic recrystallization (Khan et al, 2007;Rittel et al, 2008;Osovski et al, 2012;Cerreta et al, 2012;Brown and Bammann, 2012). Also during phase transformations such as the conversion of austenite to martensite, well known to occur in a reversible way in pure iron (Rittel et al, 2006) or to develop in many ferrous alloys such as metastable austenitic steels of the 3XX series (Rodríguez-Martínez et al, 2011) or with high manganese content. In such cases, the measured temperature rise comprises the effects of exothermal phase transformations during which latent heat is released.…”