The major goal of this work was to study the effect of rapid heating and fast cooling on the transformation behavior of 22MnB5 steel. The effect of the initial microstructure (ferrite + pearlite or fully spheroidized) on the transformation behavior of austenite (during intercritical and supercritical annealing) in terms of heating rates (2.5, 30 & 200 °C/s) and fast cooling, i.e., 300 °C/s rate, were studied. As expected, the kinetics of austenite nucleation and growth were strongly related to the heating rates. Similarly, the carbon content of the austenite was higher at lower intercritical annealing temperatures, particularly when slower heating rates were used. The supercritical temperatures used in this study were similar to those used during commercial hot stamping operations, i.e., 845 and 895 °C, respectively, followed by a fast cooling rate. The prior austenite grain size (PAGS) was not strongly influenced by the nature of the initial microstructure, heating rate, reheating temperatures (845 or 895 °C), at 30 s holding time. The decomposition of austenite using fast cooling rates was examined. The results showed that 100% martensite was not obtained. The observed low temperature transformation products consisted of mixtures of martensite-bainite plus undissolved Fe₃C carbides and small amounts of martensite-austenite (M-A). At higher supercritical temperatures, i.e., 1000 °C and 1050 °C, the final microstructure showed an increase in the volume fraction of martensite and a decrease in the volume fraction of bainite. The Fe₃C and the M-A microconstituent were not observed. The best combination of tensile properties was obtained on samples reheated in the lower temperature range (845 to 895 °C). Interestingly, when the samples where reheated at the higher temperature range (1000 to 1050 °C) and fast cooled, the results of the mechanical properties did not exhibit significantly higher strength levels independent of heating rate or initial microstructural condition. This can be attributed to the change in the microstructural balance %martensite+%bainite as the reheating temperature increases. The results of this study are presented and discussed.

The relationship between precipitate morphology and dissolution on grain coarsening behavior was studied in two Ti-Nb microalloyed Linepipe (LP) Steels. The developed understanding highlights the importance of the complex relationship between precipitate constitutive make-up, dissolution mechanism and grain boundary (GB) pinning force. Equilibrium-based empirical solubility products were used to calculate precipitate volume fractions and compared to experimental measurements. Scanning Electron Microscopy (SEM), Electron Backscatter Diffraction (EBSD) and Electron Probe Micro-Analysis (EPMA) were conducted on bulk samples. Transmission Electron Microscopy (TEM)-based techniques were used on C-replica extractions and thin-foils. A retardation in the grain-coarsening temperature compared to the predicted coarsening temperature based on equilibrium calculations was noticed. In addition, a consistent NbC epitaxial formation over pre-existing TiN was observed. The resulting reduction in total precipitate/matrix interface area and the low energy of the TiN/NbC interface are pointed to as responsible mechanisms for the retardation in the kinetics of precipitates’ dissolution. This dissolution retardation mechanism suggests that a lower Nb content might be effective in controlling the grain coarsening behavior of austenite.

The dissolution behavior of δ-ferrite in two commercial stainless steels, 15-5PH and M-154, was studied. In this work, a new approach combining hot deformation and additional post-treatment was investigated and compared with conventional annealing heat treatments for the dissolution of δ-ferrite. The results showed the acceleration in the dissolution of δ-ferrite using the new methodology. Samples from each steel were subjected to conventional annealing heat treatments at 1000 °C and 1150 °C, with soaking times of 1, 2, and 3 h. A second set of samples was subjected to hot compression experiments at 900 °C, under different strain rates, followed by post-processing heat treatments at 1000 °C and 1150 °C, while keeping the holding time constant for 10 min. Advanced microstructural characterization techniques such as Scanning Electron Microscopy (SEM), and Electron Probe Micro-Analysis (EPMA) were employed to investigate δ-ferrite dissolution in terms of changes in area fraction and chemical composition. The results indicated a strong correlation between the dissolution behavior of δ-ferrite and the processing parameters. In addition, thermodynamic calculations using Thermo-Calc software (version 2021.2.87071-368) were used to assess the diffusion of elements during the dissolution of δ-ferrite as a function of temperature and time.

This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.