Abstract
Hydrogen trapping and micro-cracking of a low alloy steel during delayed fracture were studied utilizing two types of piezo-electric sensors. A rod-shaped small sensor with a 2.5mm diameter (called as a pinducer) was tapped into the near front of a notch in a CT specimen of low alloy steel. This sensor measured the change of hydrogen gas pressure in a circular cone of 7.3×10-10m3 ahead of the pinducer. Another sensor is a conventional resonant-type AE sensor with a center frequency of 0.45MHz. This sensor monitored AEs produced by micro-crack generations. The pinducer monitored only the fast hydrogen gas pressure evolutions before the crack generation. Gas pressure in a small void of 10-17m3 is estimated to reach 107MPa. After long hydrogen charging time, AEs from both the gas pressure evolution and crack generation were simultaneously monitored, suggesting a rapid gas transfer from open micro-cracks to the circular cone. Though the quantitative evaluation of hydrogen gas evolution rate in micro voids is still difficult due to the finite sensing area of the pinducer, feasibility of fast and high hydrogen gas pressure evolutions was suggested.
