Carburization performance of Incoloy 800HT in CH4/H2 gas mixtures

Carburization performance of Incoloy 800HT has been studied after cyclic and isothermal exposures to CH4/H2 carburizing gas mixtures at high temperatures for 500 h. At 800 °C in 2% CH4/H2, Incoloy 800HT suffered external oxidation and carburization, the external continuous layer of reaction products consists primarily of Cr7C3, Mn1.5Cr1.5O4, and FeCr2O4 with Fe(Cr, Al)2O4 as a minor phase. At 1100 °C in 10% CH4/H2, external carburization did not occur likely due to high carbon dissolution in the alloy substrate at this temperature. A thermodynamic analysis indicated that 1000 °C was an approximate critical temperature, below which the environment should result in mixed oxidizing/carburizing behavior, while above this temperature reducing carburizing behavior should occur. The experimental results approximately agree with the thermodynamic analysis. Metal dusting was not observed under highly carburizing conditions (aC>1). The size and morphology of Cr-rich phases (or Cr-carbides) are both temperature- and time-dependent, while the external continuity is more temperature-dependent rather than time-dependent.