Magnetic nondestructive evaluation using higher sensitive magnetic field sensor for thermally aged Inconel 600 alloy

Ni base alloy, Inconel 600, has high strength and high corrosion resistance, and is therefore widely used for steam generators tubing in power generation plant. During long-term plant operation at higher temperature, sensitization occurs along grain boundary, which will induce stress corrosion cracking. The alloy is paramagnetic basically; however, it transforms into ferromagnetic along grain boundaries when Cr depletion occurs. Since Cr depletion is attributed to sensitization of the material, nondestructive testing for sensitization using changes in magnetic properties will be possible. Here, Inconel 600 alloy was heat treated at 600 degrees C from 0 to 400 hours to occur sensitization. Then, the magnetic field sensor with relatively higher sensitivity scanned over the sensitized specimen. The change in the magnetic field near the surface of specimen is related to the degree of chromium carbides progressing sensitization. The obtained results suggest that scanning with magnetic field sensor is effective to assess sensitization due to heat treatment for Ni base alloy.