Search

The ultrafine grain with the effective grain size of 1.10 μm was fabricated by intercritical deformation on low-carbon microalloyed steel, which was finer than the deformation induced ferrite transformation (DIFT) at 1.55 μm. During intercritical deformation, the DIFT process was enhanced, showing a colony distribution of the refined grains. Meanwhile, the continuous dynamic recrystallization of pro-eutectoid ferrite was developed by both the subgrains progressively trapping dislocations and the serrated boundary pinching off the elongated grains. These processes were directly observed through the grain average misorientation maps of electron back-scattered diffraction and confirmed by transmission electron microscopy micrographs. [ABSTRACT FROM AUTHOR]

The effect of intercritical annealing temperature (IAT) on the multiphase microstructure evolution of 0.02C-7Mn steel was investigated using scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), X-ray diffraction (XRD), and transmission electron microscope (TEM), as well as Thermo-Calc and Dictra software simulations. The results indicated that an hexagonal closed-packed phase (ε-martensite) mainly appeared on reversed austenite by air cooling after the steel was annealed between 650 °C to 675 °C for 1 h. The multiphase microstructure contained retained austenite (RA), athermal ε-martensite, annealed martensite, and athermal α-martensite, but both their content and morphology strongly depended on the IAT. In contrast to athermal ε-martensite, RA mainly existed after annealing at 600 °C to 650 °C. The maximum volume fraction of RA is 24.32% after annealing at 650 °C, while it was 28.54% for athermal ε-martensite after annealing at 675 °C. RA displayed a granular and thin lath-like morphology after annealing at 620 °C and a larger granular and coarse lath-like morphology after annealing at 650 °C. The athermal ε-martensite morphology was granular and lath-like at 650 °C, and block-like at 675 °C. Decreasing the C and Mn contents, and increasing the lath size of reversed austenite decreased the thermal stability and the stacking fault energy (SFE), which induced martensitic transformation with increasing IAT. As a result, the reversed austenite remained at low IAT, transformed into athermal ε-martensite at medium IAT, and induced athermal α-martensite at high IAT. • A thermal ε-martensite appeared during the air cooling of reversed austenite in 0.02C-7Mn steel. • Content and morphology of multiphase microstructure strongly depended on the annealing temperature. • The decreasing and heterogeneousC and Mn decreased thermal stability, which induced martensitic transformation. • Athermal martensitic transformation was due to the negativeSFE (n=2). [ABSTRACT FROM AUTHOR]

In the transition towards green energy, long distance transportation of gaseous hydrogen by pipeline is considered to be an economically attractive solution. For safe transportation of hydrogen, it is important to conduct relevant mechanical tests in representative hydrogen environments and working conditions of pipelines to demonstrate acceptable hydrogen embrittlement resistance of the steel. One important issue is the testing condition for the ratio of gaseous hydrogen volume over specimen surface area required in mechanical testing since this has not been specified in any testing standards and codes or investigated. This study investigated this issue by performing slow strain rate tensile (SSRT) testing of an X52 pipeline steel in gaseous hydrogen with different ratios of hydrogen volume/specimen surface area, R v/s , ml/mm2. It was found that, with increasing R v/s ratio, the area reduction of the steel decreased and hydrogen embrittlement sensitivity increased in the range of the R v/s ratios investigated. The saturating R v/s ratio appeared to be ∼2.0 beyond which the effect of R v/s ratio on hydrogen embrittlement sensitivity was relatively small. The macroscopic and microscopic observations on both the fracture and side surfaces of the SSRT tested specimens also found that the tendency to cleavage fracture increased with increasing R v/s ratio. The results of the ab initio molecular dynamics (AIMD) simulations performed also predicted a dependence of hydrogen embrittlement on hydrogen volume when R v/s ratio is relatively small. • The effect of the ratio of gaseous hydrogen volume over specimen surface area, R v/s , on HE was investigated the first time in the world. • The threshold R v/s ratio, beyond which the influence of the R v/s ratio on hydrogen embrittlement sensitivity is small, was determined. • Tendency to cleavage fracture increased with increasing R v/s ratio. • The increased tendency to hydrogen embrittlement under higher R v/s ratios was verified by the AIMD simulations. [ABSTRACT FROM AUTHOR]