Fracture pecularities of prestressed steel reinforcement under hydrogenation in the model working medium
Journal Title: Вісник Тернопільського національного технічного університету - Year 2015, Vol 80, Issue 4
Abstract
Pecularities of fracture behaviour of progressively drawn pearlitic steels from the initial hot rolled bar (not cold drawn at all) to the final commercial product (prestressing steel wire) were investigated in air and under hydrogen charging in the Ca(OH)2 + NaCl solution with pH 12.5 which modelled a working medium of a steel reinforcement. The slightly drawn steels showed isotropic fracture behaviour and a smooth fracture surface at the macroscopic scale. As the cold drawing degree increases, steels fractography became more irregular with numerous elevations and valleys. The percentage of reduction in area ψ calculated after slow stran rate tests in air increases with the cold drawing degree. However, heavily drawn steels exhibit ψ decrease with cold drawing. This behaviour can be explained in function of the accumulated plastic strain εp and the colonies orientation inside the steels. In the hot rolled bar (not cold drawn at all) the orientation of pearlitic colonies (ferritic and cementite lamellas too) is random. As the cold drawn degree advances the pearlitic colonies are oriented progressively in the cold drawing direction (and ψ increase progressively too) until the steel reaches a certain value in its accumulated plastic strain. When further lamellas reorientation is not possible (almost all colonies have already been oriented), reduction in area begins to decrease, even with the increment of plastic strain. All the steels tested in air presented a symmetrical fracture surface in a macroscale (cup and cone fracture). On the contrary to the test in air, stable low values of reduction in area were obtained under hydrogenation conditions, revealing high sensitivity of the tested steel to hydrogen embrittlement which also manifested in plural microcracking near the fracture surfaces. However plasticity loss for the tested steel due to hydrogen was less in the case of the commercial high strength wire comparing to the as-received hot rolled bar. Surface cracks appeared because of facilitation of share processes in the surface layers by hydrogenating environment. Fracture in these cases always is initiated near the lateral surface which proves a crucial role of surface defects in the case of steel wires hydrogenation. High susceptibility of the tested structural steel to hydrogen embrittlement should be taken into account using electrochemical protection of reinforced concrete structures.
Authors and Affiliations
Myroslava Hredil
Keywords
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