High pressure effect on the substructure and hardness of IF steel during martensitic transformation

Effect of High-pressure Quenching on Pure-iron Martensite Transformation and Its Strengthening Mechanism

“…Hence, pressure can shift the Time-Temperature-Transformation to the right side and lower the critical cooling rate, so that diffusion transformations such as ferrite and pearlite are suppressed and martensitic transformation is able to occur in industrial pure iron with low cooling rate. Several excellent experiments have confirmed the credibility of this theory in Fe-15 wt.%Cr and IF steel, 28,29) thus pressure is critical for martensitic transformation at a low cooling rate in pure iron. The martensite transformation temperature (Ms) plays vital roles in the morphology and performance of iron and steels, thus considerable research efforts have been devoted to the calculation of Ms by the difference in free energy of the parent and martensite phase.…”

{112} 〈111〉 Twins or Twinned Variants Induced by Martensitic Transformation?

“…The sample was held for 30 min in a hexahedral multi-anvil pressure device with a pressure of 4 GPa and a temperature of 1100 ℃, and then cooled to room temperature at a speed of 10 ℃/s. See [8] for details of the high-pressure thermal cycling experiment. The microstructure and crystallographic information of the samples were characterized based on bright field imaging, dark field imaging, SAED and high-resolution TEM imaging (HRTEM) in TEM (Talos F200X, FEI).…”

“…There are different explanations on these extra diffraction spots, for instance, from the remained austenite [11], carbide [12] or ω-phase [13]. However, extra diffraction spots with high intensity were also observed from the twinned substructure induced in pure iron [7], IF steel [8] and Fe-Cr [9], where retained austenite, carbide and other extra phases are not presented. This means that such special diffraction behavior should have the intrinsic cause related to the martensitic transformation.…”

“…For instance, in a water quenched Fe-0.2wt%C alloy, twinned areas are observed in dislocation lath martensite that was induced having a typical hierarchical structure of prior austenite grain-packet-block-lath [5]. As the martensitic transformation was proceeded under high pressure of 3-5 GPa, lath martensite was formed in pure iron [7], interstitial free (IF) steel [8], Fe-Cr alloy [9], and the substructure was even predominated by twinned crystals. The twinned substructure within lath martensite seems unable to be explained by the traditional origin of {112} 〈111〉 twins.…”

“…In order to rule out the influence of the extra phases like austenite, carbide, etc., IF steel was chosen as the experimental material. To precede martensitic transformation and to induce twinned substructure in IF steel, the thermal cycle was performed under high pressure, which has been demonstrated by our previous investigation [8]. Theoretic simulating by Material Studio was supplemented to the structural characterization to visualize the spatial arrangement for bcc {112} 〈111〉 twins and twinned variants.…”