99 Diseases of Pressure Equipment: Spheroidization

Spheroidization is a rather technical term that describes a metallurgical aging phenomena that results in loss of mechanical and creep strength. It occurs when carbon and low alloy steels are exposed to temperatures in the range of 850F - 1400F (440C - 760C) where carbide phases (the strengthening element of steels) become unstable and begin to agglomerate, which then results in the loss of strength. At the upper end of that temperature range, spheroidization can occur within hours, while at the lower end, it may take years; so it is clearly another of our time- temperature degradation phenomena.

Spheroidization is not much of threat to our pressure equipment, except in some unusual circumstances. Usually the loss of strength is relatively minor, but under some high temperature conditions can cause a 30% reduction in strength. However, that loss of strength usually results in some reduction in design margin, which can sometimes be acceptable for continued safe operation with appropriate fitness-for-service analysis. If it’s not acceptable, then derating is usually the result. The unusual conditions where spheroidization could be a threat involve high stress intensification factors, high applied stresses, or in combination with other degradation or flaws.

Equipment that is susceptible to spheroidization include most vessels or piping operating in the susceptible temperature range or that might be inadvertently exposed to high temperatures because of operating malfunctions. Such equipment includes furnace tubes, reactor/ regeneration equipment in FCCU’s, cat reformers, cokers and hydroprocess reactors that might be exposed to uncontrolled exothermic conditions (i.e.temperature runaways). Some steels are more susceptible than others, including fine-grained steels vs course-grained, aluminum- killed steels vs siliconkilled, and normalized steels vs annealed.

Though the likelihood of spheroidization damage can be detected with standard hardness testing looking for characteristic softening, spheroidization can only be confirmed by taking samples for laboratory metallography or performing field replication metallography for observation under microscopes.

If you experience abnormally high temperatures in your process equipment, do you inspect for spheroidization and assess the impact with fitness-for-service analysis where need be?