99 Diseases of Pressure Equipment: Decarburization

Decarburization is the antithesis of carburization and rarely results in equipment failure. However, surface decarburization is often a sign that something more serious is going on, ie high temperature hydrogen attack (HTHA), which is well covered in API RP 941, Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants. HTHA will be covered separately in another article in this series where I can devote more space to it. Decarburization can also be indicative of improper post weld heat treatment and help identify equipment that is fire damaged, as hardness testing will reveal a softening of the surface, indicative of loss of carbon (iron structure without carbon is relatively soft compared to steels with carbides in their structure). So, for purposes of RBI, decarburization as a high temperature degradation phenomena is somewhat of a non-issue.