Equipment: Hydrogen Reformer Process Gas Heating coil with dissimilar weld between the low alloy 1.25Cr 1.5Mo) & high alloy (18Cr  8Ni, SS 304H)

Results Conclusion:As the above values of  PEAK_act ~ PEAK_req , It was found that given the operating conditions involving internal pressure & radial thermal gradient, and applying a Fatigue Strength Reduction Factor of 4 due to poor weld conditions, the existing service witnessed by the Coils C-102B has caused thermal fatigue, with peak stresses of about 108ksi, thus causing fatigue failure in 50 cycles (Start-up & shutdown cycles).

Problem: The Hydrogen Reformer Process Gas Heating coil had developed circumferential cracks in the HAZ zone of the butt welds between the coil tube and the stub pipe connecting the header. The following were the materials involved during the welding. 18in€� Header is low alloy (1.25Cr-0.5Mo), 3in€� Stub pipe connecting the header is low alloy (1.25Cr-0.5Mo), à 3in€� Coil tube connecting the stub pipe is high alloy (18Cr  8Ni, SS 304H). The weld between the low alloy (1.25Cr0.5Mo) & high alloy (18Cr 8Ni, SS 304H) was made using the Ni-Alloy (UNS-N07718, Inconel) filler material. It is suspected that the progressive and localized structural damage that occurred at the HAZ location of the dissimilar weld is due to it subjected to cyclic loading resulting from start-up & shut-down cycles of the Reformer itself.

Analysis: A steady state thermal analysis was completed to simulate the thermal gradient across the dissimilar weld. A static linear elastic structural stress analysis was done to simulate the peak stress intensities. For the detailed assessment of stresses, linearized stress intensity distributions has been determined with the aid of the FEM program along the path shown below. Herein the membrane, bending and peak stresses were separated from the total stresses along this linearized path (Stress Classification Line)

Benefits: Understanding the failure mechanism enabled to develop a mitigation strategy of upgrading the metallurgy of the coils and thus avoiding any future failures.