Contribution Oral Presentation
TAILORING THE MECHANICAL PROPERTIES THROUGH THE CONTROL OF HEAT TREATMENTS IN A PRECIPITATION HARDENING MESTABLE STAINLESS STEEL
- David SAN-MARTIN
- Dr. Carola CELADA-CASERO (Delft University of Technology)
Ultrafine grained austenitic microstructures (~280-440 nm) have been obtained after applying isochronal heat treatments (0.1, 1, 10 and 100 ºC/s) to a cold-rolled (CR) metastable stainless steel. A detailed investigation has been carried out to characterize the heavily deformed initial microstructure and to understand how the martensite-to-austenite transformation takes place. Several complementary experimental techniques have been used in the microstructural characterization; microhardness Vickers tests (Hv), electron probe microanalysis (EPMA), magnetization measurements, scanning and transmission electron microscopy (SEM, TEM) and electron backscattered diffraction (EBSD). Besides, the mechanical behavior of partially and fully transformed austenitic microstructures has been characterized by tensile testing on sub-size samples. The transformation occurs diffusionally and in two-steps for all heating rates, which is attributed to the chemical banding present in the initial microstructure. The shear reversion mechanism has not been observed even for the highest heating rates. The severe deformation of the martensite present in the initial microstructure along with the formation of Chi-phase and Ni3(Ti,Al) nano-precipitates are responsible for the grain refinement, especially upon slowly heating at 0.1 °C/s. The mechanical response of mixed martensite/austenite microstructures is influenced by the volume fractions of these phases, the mechanical stability of the austenite (TRIP effect), and the presence of Ni3(Ti,Al) nano-precipitates in the martensite phase. A wide range of strength (2.1-1.1 GPa) and elongation (3-25%) values can be obtained through the control of the microstructure. In fully austenitic microstructures the strength can drop to 0.35 GPa and the elongation increases to 40% depending on the grain size. This investigation provides a better understanding of the processing-microstructure-properties relationship in metastable stainless steels.