| The steel used for this research,Cronidur? 30 is a high-interstitial Mn-and Ni-free stainless steel,and it is conventionally used in the quenched and tempered martensitic state.The first part of this work focuses on t he possibility of obtaining fully austenitic microstructures by solution annealing at high temperatures.For this purpose,in order to study the phase transformation behavior of the alloy,dilatometry tests were performed,especially changes in the Ms temperature,after solution annealing at various temperatures.As the annealing temperature increases from 950 ? to 1150 ?,the apparent Ms temperature decreases from around 275 ? to room temperature.Accordingly,when the annealing temperature rised,the fraction of austenite increased as well.In order to study the tensile properties of Cronidur? 30,t he as-received steel was annealed at 1250 ? for 5 m in(1st series),followed by tensile tests in the temperature range of 25–230 ?.The tensile curves showed that the elongation increased as the tensile temperature increased.Light optical microscope observations were performed to study the microstructures of the tensile specimens.The micrographs and the hardness profile measurement near the surface indicated the presence of a martensitic layer near the outer surface of tensile specimens.Because a thick martensite layer comes to appearance on the surface of the steel,tensile properties were not as good as expected.To obtain tensile specimens with a smaller extent of decarburization and denitrding during heat treatments and therefore containing less martensite near the surface,another series of specimens were annealed at 1240 ? for 2 m in(2nd series).The annealing temperature of 1240 ? was reached at a faster heating rate and this reduced decarburization/denitriding.After tensile tests from 25 ? to 230 ?,the tensile elongation was found to progressively enhance at higher tensile test temperatures.This behavior was also observed in the 1st series specimens except that the elongations were higher and the strength levels were lower compared to the 1st series.As the tensile test temperature increased,the formation of deformation-induced martensite was hindered due to a reduced driving force of the transformation(higher SFE).The delated formation of deformation-induced martensite was found to lead to the enhancement of tensile ductility.XRD and EBSD measurements were performed to investigate the mechanism behind ductility enhancement at higher temperatures.There was clearly no ?-martensite formation during tensile tests.Deformation twinning occurred in the austenite after deformation at 230 ?.Twinning was more pronounced in the 2nd s eries specimens because they experienced a l arger tensile elongation.The occurrence of deformation twins was indicative of a high glide planarity even at the raised tensile test temperature of 230 ?. |
PDF Full Text Request