This study systematically examined the abnormal grain growth (AGG) behavior of ferrite in pure iron subjected to cold torsion followed by annealing. Continuous shear strain in the radial direction of the cylindrical test specimen was introduced by cold torsion. Two torsion angles 1300° and 3700° were applied, and the resulting microstructural change was characterized through hardness distribution, optical microscopy, electron backscatter diffraction (EBSD), and inclusion particle analysis. Annealing at 1123 K revealed that AGG consistently occurred at regions with a shear strain of approximately 1.5, independent of the torsion angle or radial position. The abnormal grains formed in a circumferential chain, reaching diameters of 6 mm and 2 mm for 1300° and 3700° torsion, respectively. The recovery and recrystallization behavior were influenced by the shear strain. The emergence of AGG was successfully interpreted using the defect-model proposed by Hillert, which defines AGG conditions based on the balance between the driving force of grain growth and the pinning force exerted by dispersed particles.

Effects of cementite on brittle fracture of fine-grained martensitic steel were examined through fractographic and cross-sectional observation. Despite the grain size of the steel with coarse cementite being smaller than that of the steel without coarse cementite, the absorbed energy of the steel with coarse cementite decreased. In the fine-grained martensitic steels, it is considered that fracture progresses through the generation of micro-cracks, including cleavage and quasi-cleavage cracks, which are halted at grain boundaries in front of the crack tip and subsequently connected. In this fracture mode, several micro-cracks are generated before crack propagation, resulting in the reduction in the connected area of ductile fracture. Thus, the proportion of ductile fracture area was lower in the steel containing coarse cementite than in the steel without coarse cementite. In the fine-grained martensitic steels, particularly in the specimen tested at low temperature, ductile fracture is exhibited at tear ridges, which are the connected regions between micro-cracks. In this shear-type fracture, void formation due to debonding reduces the critical fracture strain, resulting in a decrease in absorbed energy per unit area of ductile fracture. Therefore, coarse cementite lowers toughness of fine-grained martensitic steel by decreasing the ductile fracture area ratio and the absorbed energy per unit area of ductile fracture.

Overcoming delayed fracture under atmospheric corrosion environment is a critical challenge for the practical application of ultra-high-strength bolts. This study investigated the delayed fracture behavior of 1700 MPa-class quenched and tempered bolts fabricated from a 0.4%C-2%Si-1%Cr-1%Mo steel (mass%) by means of outdoor exposure tests, accelerated tests and thermal desorption spectrometry. In a 10-year outdoor exposure test, the delayed fracture behavior of the ultra-high-strength bolts was categorized into two types: rupture at the thread runout and the first thread root in nut, and fracture at the thread root in nut and the under-head fillet. Outdoor exposure tests using notched bolts could reproduce the delayed fracture at the thread runout. By measuring the stain change for the shank of notched bolt during the outdoor exposure test, we successfully observed the delayed fracture process: the initiation of intergranular fracture along the boundaries of prior-austenite grains, followed by unstable fracture, and culminating in the formation of a shear lip. The delayed fracture of the bolts was attributed to hydrogen embrittlement. Slow-strain-rate testing for hydrogen-pre-charged notched specimens showed a linear relationship between the fraction of intergranular fracture (IGF) area and the peak value of the maximum principal stress σ_max* at the notch root. The delayed fracture mechanism of the ultra-high-strength bolts was discussed based on the σ_max* and the IGF area fraction at the fracture locations.