Abstract
A fractographic and numerical approach is presented to analyze hydrogen-plasticity interactions in pearlitic steel and to elucidate the main hydrogen transport mechanism in this material under triaxial stress states produced by notches. Fractographic analysis showed that the microdamage produced by the hydrogen was clearly detectable by scanning electron microscopy (SEM), through a specific microscopic topography associated with hydrogen effects (tearing topography surface or TTS). Numerical computations obtained by using an elastic-plastic finite element program gave the progressive spreading of the plastic zone, closely associated with the movement of dislocations. In the majority of cases, the plastic zone (PZ) clearly exceeds the hydrogen affected region (TTS) and has no relation with it. In some tests, however, the hydrogen-induced micro-damage surpasses the only region in which there is dislocation movement, and in this case the net macroscopic transport of hydrogen cannot be attributed to dislocation dragging, but only to a random-walk stress-assisted diffusion. Therefore, in spite of the fact that dislocational transport of hydrogen has been sufficiently demonstrated on a microscopic scale, it might not be a hydrogen embrittlement mechanism per se, detectable with loss of fracture load, on a macroscopic scale.
Highlights
In hydrogen assisted cracking (HAC) processes, a basic issue is to elucidate the main hydrogen transport mechanism
Results demonstrate that the tearing topography surface (TTS) depth has no relation with the active plastic zone dimension, i.e., with the size of the only region in which there is dislocation movement, so hydrogen transport cannot be attributed to dislocation dragging, but to random-walk lattice diffusion in which the hydrostatic stress field plays a relevant role
The rising load HAC test after fatigue precracking involves three phases, each of one characterised by its K-level and the active plastic zone (APZ) size: (i) initial phase associated with elastic unloading in the near-tip region and no plasticity; (ii) intermediate phase beginning at K ≈ 0.2Kmax and finishing at K = Kmax during which the APZ is the straindefined one; (iii) final phase K > Kmax during which the APZ is the stress-defined one
Summary
In hydrogen assisted cracking (HAC) processes, a basic issue is to elucidate the main hydrogen transport mechanism. The effect of fatigue precracking is beneficial for the HAC resistance, since FHAC is an increasing function of Kmax This may be caused by the cyclic plastic zone or the compressive residual stresses near the crack tip which is prestrained (or prestressed) by fatigue: the higher the cyclic load level, the more pronounced the prestraining/prestressing effect which delays the hydrogen entry and improves material performance in the hostile environment. The rising load HAC test after fatigue precracking involves three phases, each of one characterised by its K-level and the active plastic zone (APZ) size: (i) initial phase associated with elastic unloading in the near-tip region and no plasticity; (ii) intermediate phase beginning at K ≈ 0.2Kmax and finishing at K = Kmax during which the APZ is the straindefined one (reversed or cyclic plastic zone of depth x∆); (iii) final phase K > Kmax during which the APZ is the stress-defined one (forward or monotonic plastic zone of depth xY).
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