A variety of metallurgical considerations influence selection of tube materials. Those described below can be controlled by proper selection of tube material. They may be difficult or impossible to control when the wrong tube material has been used.
Graphitization. This can occur on carbon steel and carbon-½ molybdenum steels, due to the carbon agglomerating in the form of graphite nodules or flakes upon prolonged high temperature exposure. The effect is to make the material brittle, and sensitive to failure from mechanical or thermal shock.
Graphitization is a significant problem only in welds and weld heat- affected zones. It sets lower temperature limits for tubes of carbon and carbon-moly steels, but only when they have welds within the firebox. The temperature limit for carbon steel is 800°F, and 850°F for carbon-moly.
Hydrogen Attack. Hydrogen attack failure can be catastrophic. It can occur at high temperatures when hydrogen partial pressure exceeds 100 psi: the hydrogen reacts with carbon in the steel, converting it to methane, which causes either surface decarbonization or internal fissures. Alloys containing chromium or molybdenum have improved resistance to hydrogen attack.
See the Corrosion Prevention Manual for a discussion of this phenomenon, and also API Publication 941, which defines temperature limits for various materials. Take great care in selecting tube materials for hydrogen service.
Sensitization. This occurs when austenitic stainless steels are exposed to high temperatures for prolonged periods of time. The chromium and the carbon in the alloy chemically combine, migrate to the grain boundaries, and precipitate there as chromium carbide.
This phenomenon is not especially damaging in itself. However, tubes in this condition (called “sensitized”) will be susceptible to polythionic acid stress corrosion cracking or intergranular corrosion when the furnace is taken out of service, allowed to cool, and exposed to moist conditions. It is best prevented through use of stabilized grades of stainless steel (Types 321 or 347). See the Corrosion Prevention Manual section on intergranular corrosion for a completed discussion of this phenomenon.
Carburization. This occurs when deposited carbon or coke diffuses into the metal and reacts with some of the alloy constituents to form metal carbides. It is relatively rare, occurring only in very hot services (approaching 1500°F), like ethylene plants. Fortunately, carburization is relatively rare because changes in tube material or operating conditions are not practical answers for most plants. Recent evidence indicates that diffusion aluminizing may prove to be a solution to the problem.