793 Carburization Metal Dusting

Besides oxygen attack, high-temperature alloys are frequently subjected to attack by carbon. Gaseous environments generated by many high-temperature industrial processes, particularly in the petrochemical/refinery industries, in the conversion of fossil fuels and in certain heat treatment operations, frequently contain gases with carbon activities of up to 1. In other cases, such as in ammonia or methanol synthesis, carbon activities can be much higher than 1. The degradation of metallic systems in carburizing environments can take two forms, namely carburization and metal dusting (sometimes referred to as catastrophic carburization). Due to the very low solubility of carbon in nickel, materials with high nickel content are considered beneficial for imparting carburization resistance. Alloys high in chromium, aluminum, and silicon may form protective oxide layers, which prevents the ingress of carbonaceous corrosive species thus providing improved resistance. However, if alternating exposure to carburizing and oxidizing environments is experienced, the precipitated carbides are converted to oxides, and the liberated CO widens the grain boundaries thus loosening the oxide layer, thereby causing accelerated deterioration.

The higher nickel plus chromium coupled with high aluminum content of alloy 602CA results in lowest weight gain in the temperature range tested as shown in Table 7.17. The reason for improved carburization behavior is due to the formation of an alumina sublayer rather than via the nickel content alone as exhibited by the oxidation data in Table 7.15 at 1200°C for alloy 602CA and alloy 601 and Table 7.16 data at 2100°F (1148°C). A recent study by Brill and Agarwal [44] examines the carburization behavior of various nickel and iron base alloys in the temperature range of 550-1200°C. The results from this study also confirm the excellent carburization resistance of alloy 602CA. Another study by Brill [45] shows the mathematical relationship between the effects of the alloying elements in an alloy with the carburization resistance. Brill introduced a constant

TABLE 7.17 Cyclic Carburization Behavior in CH4/H2 Environment (Ac = 0.8) in Temperature Range 750-1000°C

Alloy

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