Studies of High Temperature Hydrogen Related Damage in Welded Carbon Steel Components used in Refineries

Five materials encompassing various grades of carbon steel were received from retired and ex-service petroleum refinery equipment for root-cause evaluation of High Temperature Hydrogen Attack (HTHA) damage at UTK. These materials were submitted by three contributors: Irving Oil Refinery (IOR #1110-2), Valero (#67 & #1252134), and Phillips 66 (#555-S1 & #555-S2) on behalf of American Petroleum Institute (API). These materials originated from various flange-to-pipe welded components that were exposed in hydrogen pressure environment at elevated temperatures (> 640°F and >70 psia).

A singular crack originating at the flange inside surface just outside of the visible HAZ was observed in IOR #1110-2 and Valero #67, respectively. Cracking in IOR #1110-2 was 1/4” long in the circumferential direction and ~3/8” deep (3/4 of the flange wall) in the sample received at UTK. A singular crack 5/8” long in the flange circumferential direction and 3/4 deep was found only in one quadrant in the flange base metal just outside of the visible HAZ. However, the cracks in these components (IOR #1110-2 and Valero #67) were different from traditionally accepted HTHA morphology, i.e. no decarburization, no residual methane, and no evidence of cavitation were observed adjacent to crack areas. Cracking in these two components showed a mixture of intergranular and transgranular cracking morphology. No evidence of cracking or microstructural damage was observed in Valero #1252134.

The Phillips 66 component, #555-S1 showed the evidence of the beginning stage of HTHA damage with grain boundary microfissures in the flange base metal just outside of the visible heat-affected zone (HAZ); no apparent decarburization was observed adjacent to microfissures. The presence of methane cavities were observed along pearlite/ferrite and ferrite/ferrite boundaries. A residual methane content of 28 ppm was measured in the damaged region of P66 #555-S1 (from the most damaged area). No evidence of fissuring or decarburization was observed in the second component received from P66 #555-S2. The results showed two of the components exhibited nontraditional form of HTHA damage, two components did not exhibit any damage, and one component from Phillips 66 exhibited traditional HTHA damage.