Compression tests were carried out on a 0.06wt%C-0.3wt%Mn-0.01wt%Si steel at temperatures high in the austenite phase field. Eight deformation temperatures were selected in the range from 1000 to 1350 ⁰C at 50 ⁰C intervals. The quenched samples were examined using optical microscopy and EBSD techniques. It was observed that dynamic transformation took place and that the volume fraction of transformed ferrite first decreased with temperature (up to 1050 ⁰C) and then increased as the delta ferrite temperature domain was approached. The EBSD results revealed the presence of Widmanstätten ferrite plates under all testing conditions, right up to 1350 ⁰C.

In this work, the effect of strain rate on the dynamic transformation (DT) of austenite to ferrite during high temperature forging of an as-cast medium-carbon low-alloy steel was investigated.Hot deformation experiments were carried out in the 1150 °C to 1200 °C temperature range (400-450 °C above Ae3) and 0.25 s -1 to 2 s -1 strain rate range using a Gleeble 3800 ® thermomechanical simulator. The critical strains for DT and dynamic recrystallization (DRX) were determined to be in the 0.08 to 0.18 strain range. A microstructural analysis was conducted using optical and electron microscopy. The kernel average misorientation (KAM) technique was applied to electron back-scattered diffraction (EBSD) images to quantify the internal misorientation of grains for the characterization of DT ferrite. Furthermore, it was found that an increase in strain rate decreased the amount of dynamically formed ferrite under the same applied strain and testing temperature.The obtained results were correlated with the influence of deformation parameters on carbon diffusivity and its impact on the growth of dynamically formed ferrite. It was found that an increase in the strain rate decreased the diffusion distance of carbon, which was the responsible mechanism for decreased ferrite formation at higher strain rates.

In this work, the microstructure and mechanical properties of an additively manufactured X3NiCoMoTi18-9-5 maraging steel were determined. Optical and electron microscopies revealed the formation of melt pool boundaries and epitaxial grain growth with cellular dendritic structures after the laser powder bed fusion (LPBF) process. The cooling rate is estimated to be around 106 °C/s during solidification, which eliminates the nucleation of any precipitates. However, it allows the formation of austenite with a volume fraction of about 5% and dendritic structures with primary arm spacing of 0.41 ± 0.23 µm. The electron backscatter diffraction analysis showed the formation of elongated grains with significant low-angle grain boundaries (LAGBs). Then, a solutionizing treatment was applied to the as-printed samples to dissolve all the secondary phases, followed by aging treatment. The reverted austenite was evident after heat treatment, which transformed into martensite after tensile testing. The critical plastic stresses for this transformation were determined using the double differentiation method. The tensile strength of the alloy increased from 1214 MPa to 2106 MPa after the aging process due to the formation of eta phase. The experimental data were complemented with thermodynamic and mechanical properties simulations, which showed a discrepancy of less than 3%.

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In this work, the microstructural features and mechanical properties of an additively manufactured 316L stainless steel have been determined. Three types of samples were characterized: (i) as printed (AP), (ii) annealing heat treated (AHT), and (iii) hot isostatic pressed and annealing heat treated (HIP + AHT). Microstructural analysis reveals that the AP sample formed melt pool boundaries with nano-scale cellular structures. These structures disappeared after annealing heat treatment and hot isostatic pressing. The AP and AHT samples have similar grain morphologies; however, the latter has a lower dislocation density and contains precipitates. Conversely, the HIP + AHT sample displays polygon-shaped grains with twin structures; a completely different morphology compared to the first two samples. Optical micrography reveals that the application of hot isostatic pressing reduces the porosity generated after laser processing. The tensile strengths of all the samples are comparable (about 600 MPa); however, the elongation of the HIP + AHT sample (48%) was superior to that of other two samples. The enhanced ductility of the HIP + AHT sample, however, resulted in lower yield strength. Based on these findings, annealing heat treatment after hot isostatic pressing was found to improve the ductility of as-printed 316L stainless steel by as much as 130%, without sacrificing tensile strength, but the sample may have a reduced (40%) yield strength. The tensile strength determined here has been shown to be higher than that of the hot isostatic pressed, additively manufactured 316L stainless steel available from the literature.