Effects of Volume Fraction and Carbon Concentration of Austenite on Formation of Ultrafine Grained Ferrite/Austenite Duplex Microstructure by Warm Compression

Dynamic Recrystallization and Dynamic Precipitation Behaviors of a 17Ni–0.2C Martensite Steel Studied by In Situ Neutron Diffraction

“…This precipitation speed is consistent with that estimated by the microstructure observation where the austenite amount increases from 9.5 % before tempering to about 18 % after 60 min tempering at 500°C. 17) These results suggest that the neutron diffraction technique is convincing and suitable to characterize the austenite precipitation during isothermal holding.…”

“…Since the warm deformation at a temperature just below the austenite transformation starting temperature accelerated the austenite transformation significantly, a little low temperature (500°C) was selected in this study in order to investigate the dynamic recrystallization of ferrite. Here, 60 min pre-tempering treatment at 500°C was carried out to obtain about 18 vol% austenite 17) in the initial microstructure. Cylinder specimens with 8 mm diameter and 20 mm length were prepared by spark cutting and surface grinding.…”

“…The neutron diffraction spectra obtained from the 60 min pre-tempered specimen with martensite-austenite duplex microstructure were collected at room temperature and 100°C for 10 min separately, in order to obtain information about the initial microstructure and compare it with that obtained by electron backscattering diffraction (EBSD). 17) Then the specimen was heated up to 500°C and each neutron diffraction spectrum during the isothermal holding was collected for 1 min to investigate the effect of the tempering process on the microstructure evolution. After 10 min isothermal holding at 500°C, the specimen was compressed at a strain rate of 8.3ϫ10…”

“…Region Ib and Region II are speculated to be related to the dynamic precipitation of new austenite particles with preferred orientation and the selective growth of dynamically precipitated austenite, respectively. The austenite orientation color maps of an eϭ0.6 warm compressed specimen after 60 min pre-tempering 17) (Fig. 9) show that preferably orientated austenite grains exist locally, as marked by white lines.…”

“…It has been reported that such ultrafine grained microstructures can be obtained in various steels by static recrystallization through annealing after cold working or by dynamic recrystallization through warm deformation at a lower temperature in the aϩg region. 6,[13][14][15][16][17] Microstructural observation of warm compressed specimens with different amounts of pre-existing austenite has shown that a fully recrystallized microstructure can be obtained in a 17Ni-0.2C martensite steel at a small strain (eՅ0.6) and that the existence of austenite plays an important role in accelerating the dynamic recrystallization. 17) However, no in situ crystallographic data have been published to characterize the dynamic recrystallization behavior quantitatively and further to investigate the effect of the second phase particles on dynamic recrystallization.…”