The Role of Splitting Phenomenon under Fracture of Low-Carbon Microalloyed X80 Pipeline Steels during Multiple Charpy Impact Tests

Abstract

The ambiguity of the splitting effect on X80 low-carbon microalloyed pipeline steels’ tendency towards brittle fracture prompted an experimental study of impact toughness scattering based on multiple Charpy impact tests in a temperature range from 20 °C to −100 °C. A fractographic analysis of a large number of fractured samples was carried out. The relationships between impact toughness, deformability and splitting characteristics were studied. A number of common features of three X80 low-carbon microalloyed pipeline steel fractures were revealed. It was experimentally established that the reason for the scattering of the impact toughness values during completely ductile fracture of specimens, as well as during fracture accompanied by the splitting formation, is the local inhomogeneity of plastic properties. The higher the susceptibility to the formation of splits for a particular steel, the lower the impact toughness. Using the electron backscatter diffraction (EBSD) technique, an uneven distribution of local plasticity in the plastic zone of impact-fractured specimens was established. A comparative analysis of specimens with equal impact toughness values at different test temperatures makes it possible to identify the mechanism of negative splitting influence compensation by the increased plasticity of certain specimen.

The Cold-Brittleness Regularities of Low-Activation Ferritic-Martensitic Steel EK-181

“…In contrast to brittle fracture, ductile fracture is characterized by a continuous nucleation of multiple sources in the field of plastic deformation, which moves along with the growing macrocrack [46,47]. In our opinion, particles of the second phases play an important role in the implementation of the mechanism of ductile dimple fracture of EK-181 steel.…”

“…In [46], using EBSD methods, it was established that under the conditions of fracture of a ferritic steel in the cold-brittleness range, ductile microcracks appear along with the cleavage microcracks. This work [46] shows that the microstructure of the plastic zones under the brittle and ductile components of the fracture formed by the main crack is significantly different.…”

“…If the stresses here significantly exceed the yield strength, then the crack development will be associated with the plastic deformation, and the work of this deformation may turn out to be so large that, in accordance with the Griffith criterion, the crack can no longer propagate as a brittle one [34]. It is believed that brittle fracture initiates from a single source and propagates continuously from facet to facet [46]. However, according to [46], for the applied stress to achieve the value of peel resistance even in the absence of general plastic deformation, a small amount of local plastic deformation is required.…”

“…It is believed that brittle fracture initiates from a single source and propagates continuously from facet to facet [46]. However, according to [46], for the applied stress to achieve the value of peel resistance even in the absence of general plastic deformation, a small amount of local plastic deformation is required. In the region of brittle fracture, a plastic zone can also form before the front of a propagating main crack [46]; therefore, prior to the sample failure, there is certain plastic deformation of the material near the main crack sides.…”

Impact Fracture Surfaces as the Indicators of Structural Steel Post-Fire Susceptibility to Brittle Cracking

“…It has been acknowledged that those characteristics should determine the expected mechanical properties identified on these steels after cooling down in fire episodes. A conventional low alloy S355J2+N steel [36], representative for a whole group of steels exhibiting ferritic-pearlitic structure (including among others the Chinese X80 steel [37][38][39], and also many high strength steels [40]) was selected at the beginning. The results obtained for this steel were juxtaposed with the results characterizing the post-fire properties of selected stainless steels such as X20Cr13 steel-of martensitic structure [41], X6CrNiTi18-10 steel-of austenitic structure, and X2CrNiMoN22-5-3 duplex steel-of austenitic-ferritic structure [42][43][44].…”