Study on the effect of microstructural changes on hydrogen embrittlement of a pearlitic steel

Abstract

Pearlitic steels are widely used in structural applications due to their high mechanical strength and economic viability. From a metallurgical standpoint, pearlitic and bainitic transformations directly influence the mechanical properties, electrochemical behavior, and susceptibility to hydrogen embrittlement. In this work, the influence of coarse pearlite, fine pearlite, and bainite microstructures on the hydrogen embrittlement of a eutectoid pearlitic steel (0.75%C), exposed to a 3.5% NaCl aqueous solution, was investigated. The microstructures were obtained through different heat treatment routes and were microstructurally characterized. The experimental evaluation involved hardness tests, electrochemical tests including open circuit potential, potentiodynamic polarization, and electrochemical impedance spectroscopy, as well as tensile tests after hydrogen charging and permeation tests using a Devanathan cell. The results indicated that microstructural refinement promoted an increase in mechanical strength but increased susceptibility to hydrogen embrittlement. The bainitic microstructure exhibited higher hydrogen solubility associated with the presence of retained austenite, favoring localized hydrogen accumulation and resulting in severe embrittlement, intensified by deformation-induced martensitic transformation. Despite the lower diffusivity, this condition showed the greatest degradation of mechanical properties. Fine pearlite exhibited intermediate behavior, while coarse pearlite was the least susceptible to embrittlement. Fractographic analyses allowed the identification of the acting embrittlement mechanisms, indicating that hydrogen facilitates phase separation, with ferrite/cementite interfaces acting as preferential sites for hydrogen accumulation and crack nucleation in pearlitic microstructures. In the bainitic microstructure, a more brittle fracture behavior was observed, with cracks associated with phase interfaces, characteristic of the hydrogen-enhanced decohesion mechanism (HEDE). The electrochemical results evidenced that microstructural modifications interfere with the kinetics of corrosion processes, influencing hydrogen generation and ingress into the material, which contributes to the differences observed in embrittlement susceptibility among the analyzed microstructures.