FAILURE ANALYSIS OF WELDED STAINLESS STEEL TUBES OF REACTOR FEED HEATER IN OIL REFINERY

A vertical cylindrical reactor feed heater with helical tube coil in oil refinery was manufactured by welding ASTM 321 stainless steel tubes. The tubes were supplied to the company by different vendors at different times, the older being in stock for more than 40 years and the new was 3 years old. The new and old tubes were welded together by manual TIG welding technique using ASTM-347 stainless steel as welding rod. Water leakage at the weld joint was observed during hydrotesting after 30 minutes approximately due to cracking in old tube side of the weld. Failure investigation was conducted using dye penetration, chemical analysis, visual inspection, stereomicroscopy, light microscopy, microhardness testing, scanning electron microscopy and energy dispersive X-ray microanalysis techniques. The microstructure of both tubes was austenitic but with finer partially recrystallized grains in the old tube. Cracks were found to start at weld pool / HAZ and at welding defects such as pores and oxide inclusions in the weld pool and extended into the HAZ and base metal of the old tube. The SEM / EDS examination identified a small volume fraction of titanium nitrides and titanium carbonitrides having diameter of 3-5 µm in addition to larger volume fraction of finer submicron Cr carbide precipitates inside grains and on grain boundaries in the old tube. The change of microstructure of the old tube caused by aging led to increasing the brittleness of the material and made it susceptible to cracking.