Carburization can occur when metals are exposed to carbonaceous material or carburizing environment at elevated temperatures. Carbon from the environment combines with carbide-forming elements such as Cr, Mo, Nb, W, Mo, Ti, and Fe in the alloy to form internal carbides. These carbides precipitate at the grain boundaries of the alloy or inside the grains. As a rule, carburization problems only occur for Cr-Mo alloys at temperatures above 1100°F, and 1500°F (815°C) for austenitic alloys. In refining operations, carburization damage is sometimes found in ferritic heater tubes in catalytic reformers and coker units. The effect of carburization is to reduce the ambient temperature ductility, toughness, and weldability of the alloy. Carburization also reduces oxidation resistance by tying up chromium in the form of stable chromium-rich carbides. The creep strength of the alloy may also be adversely affected as the result of the reduced ductility of the alloy due to the carbide precipitation within in the grains and at the grain boundary.
In petrochemical operations, carburization is typically found in austenitic heater tubes in ethylene pyrolysis and steam reformer furnaces where significant carburization can occur during decoking cycles. Carburization has been identified as the most frequent failure mechanism of ethylene furnace tubes. Experience has indicated that the severity of carburization damage in ethylene cracking is process dependent. Some important factors identified include the following:
- Steam dilution, which tends to decrease the rate of damage
- The use of lighter feeds versus heavier feeds, the former having a higher carbon potential
- The frequency and nature of decoking operations; decoking is thought to be a major contributor to carburization damage
Carburization causes the normally nonmagnetic wrought and cast heat-resistant alloys to become magnetic. Measurement devices range from simple hand-held magnets to advanced multi-frequency eddy current instruments. Carburization patterns can also reveal uneven temperature distributions that might otherwise have gone undetected. Most alloys tend to have more carburization penetration with increasing temperatures.
As in the case of oxidation and sulfidation, chromium is considered to impart the greatest resistance to carburization. Aluminum and silicon alloying additions can also contribute positively to resistance to carburization. It should be noted that the addition of aluminum or silicon to the heat-resistant alloys in quantities to develop full protection involves metallurgical trade-offs in strength, ductility, and/or weldability. Considering fabrication requirements and mechanical properties, viable alloys are generally restricted to about 2 percent for each element. Other approaches to reducing the potential for carburization damage includes reducing the carbon activity of the environment through lower temperatures and higher oxygen/sulfur partial pressures. Also, the addition of H2S in the process stream inhibits carburization in steam/gas cracking in olefin and thermal hydrodealkylation units
Originally, tubes in ethylene cracking furnaces were manufactured out of cast HK-40 alloy (Fe-25Cr-20Ni). Since the mid-1980s, more resistant HP alloys have been utilized, but carburization problems have not been eliminated as the result of more severe operating conditions in the form of higher temperatures. Some operators have implemented a 35Cr-45Ni cast alloy, with various additions, to combat these conditions. For short residence-time furnaces with small tubes, wrought alloys including HK4M and HPM, Alloy 803, Alloy 800H have been used.