Abstract
Failure of a number of Type 316 stainless steel tubes in a fired heater at a phenol solvent treating plant after ten years of service is outlined. Metallurgical investigations showed that carbide precipitation at the grain boundaries had occurred after this length of service at a temperature of about 750 degrees F. thus sensitizing the tubes to intergranular corrosion. Inspection of the remaining tubes in the heater by a Probolog survey and by fluorescent dye penetrant showed that approximately two-thirds of the tubes while in a susceptible state had not yet progressed to the point of developing cracks.
The possibility of salvaging these uncracked tubes for further use was investigated. Samples were heat treated at 1600 degrees F. for 24 hours. Microscopic examination of polished and etched specimens showed a definite agglomerization of the carbides at the grain boundaries tending to break up the network of precipitates. Tensile and bend tests on heat treated and welded specimens showed good results.
This heat. treatment was carried out on the uncracked tubes and they were re-installed in the heater. No further difficulty has been encountered after 24 months of service.
