AE Monitoring of Hydrogen Embrittlement Crack in Maraging Steel

Abstract

AE (Acoustic Emission) result from initiation and propagation of cracks in materials. Then AE monitoring can detect the micro cracks. Maraging steels witch have superior strength are used as structural materials. However, these have hydrogen embrittlement problem under humid air. Hydrogen embrittlement resistance of maraging steels is usually estimated by visual testing (VT) which checks whether rupture or not in laboratory. However the testing method is batch style testing. Then it is difficult to estimate the exact rupture time through VT. Then the AE monitoring applied to detect hydrogen embrittlement of maraging steel. Hydrogen embrittlement crack accelerated by a droplet of NaCl solution that was set on the center of the maraging steel thin plate. In this case, the droplet was too small to determine the hydrogen embrittlement crack through the electrochemistry method.AE sensor also set on the plate and detected AE signals.AE signals were classified as to amplitude, frequency, and wave form. AE event rate was also focused. The surface crack propagation was also recorded by digital camera. Crack growth was confirmed from the pictures. AE event counts separate 3 regions. At first region, AE event counts detect before surface cracks were able to be observed. In this region, cracks may initiate under the droplet. At Second region, AE event counts were constantly increased. In this region it was observed that increment by crack growth was constant by digital camera. It shows that means the second region was the crack growth stage. At third region, AE event rate was increased twice. The second crack growth was observed in digital camera images. Then AE event rate reflected the number of cracks. There was good relation between crack propagation and AE event rate. AE event rates were difference between types of steels. The other steel which has high hydrogen embrittlement resistance tend to have low AE events rate.