CYCLIC OXIDATION PLUS CARBURIZATION OF HEAT-RESISTANT ALLOYS

Nine heat-resistant alloys were exposed to cycles of oxidation plus ca rburization. Each cycle consisted of 4 h of oxidation in air at 700 de grees C followed by 16 h of carburization in hydrogen gas (H-2) + prop ylene (C3H6) at 950 degrees C. The alloys were tested for 1, 3, and 5 cycles, and invariably were found to have undergone internal carburiza tion. The rate of this process was measured with respect to cumulative carburization time, and the kinetics were found to be parabolic. Data from isothermal carburization of the same alloys were compared with r ates found in the cyclic process. In general, the internal carburizati on produced by the 3- and 5-cycle runs was greater than that resulting from isothermal carburization. This was a result of a ''metal dusting '' attack of the alloys and increased carbon activities in their surfa ce regions.