Mixed sulfidation/carburization attack on several heat-resistant alloys at900 degrees C

A sulfidation/carburization study of seven commercial heat-resistant alloys was carried out at 900 degrees C in a H-2-25 vol.% CH4-14.8N(2)-4CO-0.6CO( 2)-0.6H(2)S atmosphere. The equilibrium partial pressures for oxygen (O-2) and sulfur (S-2) were 1.1 x 10(-22) and 4.1 x 10(-8) atm, respectively and the carbon activity for this system was unity. The time of exposure was 500 hr. Relatively thick, mixed sulfide scales were formed on all of the alloy s tested. In addition, internal carburization occurred in all of the alloys . Using metal loss (i.e., the reduction in sample thickness) plus internal attack (internal sulfidation plus internal carburization) as a performance criterion, an alloy with a nominal composition of Ni-29 wt.% Co-28Cr-2.75Si performed the best, showing 0.71 mm of attack. An alloy with a nominal com position of Fe-20 wt.% Ni-25Cr performed the worst, being totally consumed by the test (>3.18 mm of attack). Alloys containing relatively high amounts of silicon (>2.5%) showed a dramatic increase in their sulfidation resista nce compared to the other alloys containing lower silicon contents. The amo unt of iron present within a given material played a dominant role in the c arburization attack that occurred with as expected, high-iron alloys showin g significant internal carburization because of a high solubility and diffu sivity of carbon in the matrix. The importance of the various alloying elem ents with respect to sulfidation and carburization resistance is discussed.