In quenched Fe-C (C: 0.0 similar to 2.0 wt.%) binary alloys, the body-centered cubic (BCC) {112}< 111 > type twin structure (density, size and morphology) in martensite was investigated by means of transmission electron microscopy (TEM). In the samples quenched to room temperature, the twin density increased as the carbon content increased. In the carbon free or pure iron sample, no twin structure was observed. In high carbon martensite, a high density of twins could be seen with twin thickness of 1 nm-2 nm, which is of the scale of the smallest alpha-Fe grain. The twin density variance is discussed based on a detwinning process, which occurs upon cooling. The twin, as an initial product of martensitic transformation, would experience a higher temperature auto-tempering process in low carbon alloys than in high carbon samples. A noticeable detwinning process takes place in low carbon alloys and results in a low density of twins observed at room temperature. Martensite starting (M-s) temperature plays a crucial role in the detwinning or auto tempering effect on the twins. (C)2022 The Author(s). Published by Elsevier B.V.

Nanosized (∼2 nm) ω-Fe3C particles with hexagonal structures have been observed only at body-centered cubic (BCC) {112}〈111〉-type twinning boundaries in twinned Fe-C martensite of the Fe-C alloy system. However, these ultrafine ω-Fe3C particles never grow large enough to be observed easily. The present structural modeling and electron diffraction calculations reveal that the formation of the new carbide (ω′-Fe3C) during coarsening of the ultrafine ω-Fe3C particles is inevitable. Coarsening or aggregation of fine ω-Fe3C particles may result in a phase transition due to the arrangement of interstitial carbon atoms. A ω-Fe3C → ω′-Fe3C transition was analyzed at the atomic scale. The ω′-Fe3C phase can exhibit an orthorhombic structure with lattice parameters aω′ = 4.033 Å, bω′ = 2.470 Å, and cω′ = 6.986 Å based on aω′ = aω, bω′ = cω, and cω′=3aω for abcc or aα-Fe = 2.852 Å (aω=2abcc, cω=3/2abcc). The simulated ω′-Fe3C electron diffraction patterns were experimentally confirmed. The ω-Fe3C → ω′-Fe3C transition can explain why the ω-Fe3C phase never becomes larger than several nanometers in carbon steel.

The hydrogen-induced cracking (HIC) and blistering behaviour of A537 steel was investigated by electrochemical hydrogen permeation measurements, electrochemical hydrogen-charging tests, and surface characterisation techniques. The results indicate that at ambient temperatures the diffusivity of hydrogen in A537 steel varies along three perpendicular directions. The diffusivity of hydrogen along the through-surface (S) direction was observed to be lower than that along each of the other two directions. The hydrogen concentration of A537 steel increased with increasing charging current density, which promoted the HIC susceptibility of the A537 steel. HIC initiated primarily at the interface of ferritic/pearlitic bands and nucleated at non-metallic inclusions in the A537 steel.