Ultrasonic Measurement of Mechanical Properties of SA-516-70 Steel in Wet H₂S Services

Hydrogen-induced cracking (HIC) is a common problem occurring on pressure equipment made of SA-517-70 carbon steel in wet H2S services. The damage appears predominantly as blisters and laminar cracks, which may be linked in the through-wall direction to form stepwise cracking. The conventional approach used for crack inspection and fitness-for-assessment decision relies on finding and sizing the biggest through-wall crack. It does not work well for HIC assessment because there are usually too many cracks to be sized individually. Moreover, the through-wall links are often difficult to confirm by regular ultrasonic shear-wave techniques since the sound path may be blocked laminar cracks. This paper presents an alternative method using Crack Indication Density (CID) as the parameter to measure. This is a parameter can be easily extracted from ultrasonic C-scan data. Using empirical relations between CID and mechanical properties, one can then nondestructively determine the mechanical properties of the damaged steel and thus the integrity of the damaged equipment. The formula to calculate CID and examples of measured data are discussed in this paper.