Investigation on the Evolution of Stagewise Microstructure During the QLT Process of 7 Pct Ni Steel
摘要
The study investigates the stagewise microstructural evolution in the quenching, lamellarization, and tempering (QLT) heat treatment of the 7 pct Ni steel. The intercritical lamellarization temperature was varied, and microstructures were evaluated in each stage to understand in detail. DICTRA was used to understand elemental partitioning and distribution of phases in the microstructure. Dilatometric data provide information about the phase changes during the heat treatment conditions. Optical microscopy, XRD, SEM, EBSD, and TEM were used to characterize the microstructures. Epsilon (ε) martensite was observed in the intercritical quenched and tempered sample due to enrichment of Ni and Mn in the austenite according to DICTRA calculation. Stacking fault energy in range of below 16 mJ/m2 at temperatures of 0 °C to 100 °C can provide epsilon martensite phase near the retained austenite region due to medium enrichment of austenite stabilizer. Lamellarization at 700 °C (L-stage) reduces the volume of retained austenite compared to lamellarization at 650 °C. Subsequent tempering at 590 °C (T-stage) increases the retained austenite volume, making it finer and more uniformly distributed in the QLT-700 + 590 sample compared to QLT-650 + 590 sample. The phenomenon was understood from dilatometric analysis, microstructural characterization and DICTRA prediction of alloying distribution.