Effect of prior martensite on bainite transformation in nanobainite steel

Monitoring of Bainite Transformation Using in Situ Neutron Scattering

“…Diffraction profiles provide the insights on volume fractions of constituents, which show good agreement with the results obtained by dilatometry [17], carbon contents in austenite [23], texture [19], and dislocation density [20]. Line broadening analysis for the ND profile provides "coherently diffracting mosaic size" probably related to dislocation cell size in engineering steels.…”

“…As seen, the dilatometry result in Figure 7a is quite similar to that in Figure 2. Changes in austenite 111 and ferrite 110 diffraction profiles are presented in Figure 7b, in which the appearance of two populations of austenite with different amounts of carbon concentration found in the previous studies [19,20] is well confirmed because of the higher resolution of the back-scatter detector at the iMATERIA. The ferrite volume fraction (V α ) was calculated using Equation (1) [32,33] from the hkl diffraction intensities determined with the Z-Rietveld software (J-PARC, Tokai, Japan) [34].…”

“…Though the change in specimen length can be converted to the ferrite volume fraction, i.e., kinetics of bainite transformation, the insights on chemical, crystallographic and microstructural features cannot be found. Therefore, as was described above, in situ neutron scattering/diffraction has been performed; in situ ND gave us the information on not only the change in the ferrite volume fraction but also the formation of two populations of austenite (the higher carbon concentration region and the lower one) [19,23], texture evolution [19], and dislocation density [20], as was mentioned above. Here, the size of the transformed product is expected to be monitored by SANS measurement.…”

“…However, the acceleration of the transformation must enlarge the application of nano-bainite steels. We have found that a small amount of low temperature ausforming (e.g., at 300˝C) [18,19] or partial quenching below Ms temperature [20] is effective to accelerate nano-bainite transformation. The dislocation structure introduced in austenite at low temperatures is found to assist bainite transformation with strong variant selection where partial dislocations introduced by ausforming play an important role for bainite transformation [19].…”

“…For studying microstructure evolution, in situ observations during processing using synchrotron X-ray [21,22] or neutron diffraction (ND) [19,20] have successfully been employed so far. Diffraction profiles provide the insights on volume fractions of constituents, which show good agreement with the results obtained by dilatometry [17], carbon contents in austenite [23], texture [19], and dislocation density [20].…”