On the enhancement of transformation induced plasticity of a AISI 304

I. ÁLVAREZ-ARMAS; V. FUSTER; F. FARÍAS

Darmstadt

Congreso; Materials Science and Engineering Congress (MSE 2018); 2018

Darmstadt University of Technology

Austenitic stainless steel AISI 304 is in a metastable state up to ambient temperature and can transform into martensite due to plastic deformation. Cyclic loading-induced martensitic transformation and cyclic hardening-softening behavior during low cycle fatigue has been studied in as-received and thermal treated samples of Type 304 stainless steel bars. The steel was receivedin the form of hot-rolled cylindrical bars. Samples obtained from the bar were tested in the as-received (AR) condition and as solution treated (ST) at 1050°C for 1h to remove any inhomogeneities followed by water quenching to room temperature. Cyclic deformation tests were carried out under plastic strain control using a fully reversed triangular form signal. The specimens were tested in air at room temperature with different plastic strain amplitudes,0.3, 0.4, 0.5 and 0.6 % with a constant total strain rate of 2 x 10-3s-1. The hysteresis loops were digitally recorded and by means of a linear regression analysis and from the unloading part of each hysteresis loop the elastic modulus was calculated for each cycle using a computer program applied to the data recorded directly from the test. The cyclic hardening-softening behavior observed for AR and ST samples low cycle fatigued with different plastic strain rates indicates a huge difference in cyclic hardening rate between samples cycled in the as-received condition and the thermal treated samples. These samples present the typical hardening produced by the austenite-martensite transformation induced by cycling. From the performed analysis it is clear that as-received samples will induce a larger amount of the martensite fraction under cycling. X-ray diffraction was used to measure the volume fraction of martensite of selected specimens. From this analysis it was corroborated that fatigued as-received samples can reach up to a 50% in wt. percent of martensite, while for thermal treated samples the martensite fraction reached only up to 10%.It is also interesting to observe the elastic modulus behavior during cycling. The elastic moduli,calculated from the hysteresis loops, change during cycling. But, it decreases for thermal treated samples and increases for as-received samples. In this work, it will be discussed the influence of previous dislocation plasticity and deformation twinning present on the as-received bars have on the observed results.