erry Wilks - CITGO Lemont
Chromium-molybdenum alloys have lower creep properties at welds, leading to creep cracking in high temperature vessels and piping at many refineries. Lemont Refinery has a fluidized bed catalytic cracking reactor made of UNS K12062 steel that had operated for 38 years in the 510-535°C temperature range when the first creep crack occurred in this vessel in 2006. Follow-up inspections of the reactor conducted in 2008 revealed small cracks developing in several of the other welds. To arrest further creep cracking a new weld repair technique was tested: external reinforcement of the weld with UNS N06625 weld overlay. When the unit came down for a scheduled shut-down in 2010, those repairs were cut out and tested, and it was discovered that the weld overlay had stopped creep crack propagation. In 2012, additional creep cracking occurred in plates a few inches from welds. Analysis of the vessel determined that a large nozzle was creating high stresses in a zone beyond welds when the vessel was rapidly heated during start-ups. In order to extend the life of the vessel until it could be replaced, all the vertical seams in the vessel were weld overlaid with UNS N06625 in 2012. In 2014 small creep cracks occurred in the weld overlay due to residual stresses, and on-line repairs were made following the repair guidelines in ASME PCC-2. This reactor was replaced in June of 2015.