10 shown in Figure 8. The elongated inclusions not only influenced pit nucleation and pit growth [18], but also affected pit-to-crack Figure 8. Tiny cracks in the inclusion ahead of the blunt crack. transition and crack propagation, if the elongated inclusions were parallel to crack growth direction. However, if the inclusions were almost hemispherical, they would only affect the pit formation and growth, even tiny hydrogen induced cracks but these were very rare. If input fugacity of hydrogen was high enough, recombination at internal sites such as non-metallic inclusions would be prevailing, causing blistering or cracking (hydrogen-induced cracking), but this case could not happen in nearneutral pH condition. The hydrogen that diffused into the steel could also be trapped on some phase interfaces ahead of the blunt cracks, such as Figure 9, and caused tiny hydrogen related cracks to initiate. These hydrogen trapped phase interfaces were also perpendicular to the external stress applied. These hydrogen induced cracks were also rare, but they contributed to the blunt crack growth. Under appropriate conditions, the blunt cracks or pits would continue to grow and link to the nucleated hydrogen induced tiny cracks. In the end, these hydrogen produced clusters of tiny cracks would have been eaten away by the further growth of the mother blunt cracks or pits by further strain deformation facilitated dissolution. Therefore, the overall apparent crack growth process seemed that there was only stress assisted dissolution growth, engendered by galvanic stress corrosion cells, which concealed hydrogen related tiny crack initiation, although the latter was rare and only could occur under very stringent conditions. It could be considered that the blunt crack growth was associated with the bursts of dissolution of aligned non-metallic inclusions with the same orientation as the blunt cracks, and rare hydrogen induced tiny cracks while the rate-controlling periods seemed “non-propagation” which were concerned with the stress facilitated dissolution to form blunt cracks again that established the conditions for further crack bursts, probably because of dissolution of further non-metallic inclusions or even more rare hydrogen caused clusters of tiny cracks, or severe stress facilitated dissolution with higher stress concentrations. The crucial role of stress or strain was continued, resulting in much further dissolution, and when the total blunt cracks reached a critical size, approximately around 0.5 to 0.6 mm deep, the stress integrity factors could be sufficient, and the plastic zones ahead of the blunt
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