Carburization behaviour of Haynes 556 exposed to CH₄/H₂ gas mixtures

Abstract

The carburization behaviour of Haynes 556 has been studied after cyclic and isothermal exposures to CH₄/H₂ carburizing gas mixtures at high temperatures for 500h exposures. A thermodynamic analysis indicated that 1000°C was an approximate critical temperature, below which the environment should result in mixed oxidizing/carburizing behaviour, while above this temperature reducing carburizing behaviour should occur. The experimental results agree with the thermodynamic analysis. Below 1000°C Haynes 556 suffered external carburization and oxidation, typically at 800°C in 2%CH₄/H₂ where the external reaction products comprised Cr₇C₃ and Co₃W₃C (major phases), and (Co,Mn)(Mn,Co)₂O₄ and TaO_x (minor phases). At temperatures in excess of 1000°C exclusive external carburization occurred, typically at 1100°C in 10%CH₄/H₂ resulting in the formation of Cr₇C₃ (major phase) and (Cr,Fe)₇C₃ (minor phase). Metal dusting was not experienced under highly carburizing conditions (a_c >1) in this study. The morphology of an outer carbide layer is both temperature- and time-dependent, while its continuity is more temperature-dependent rather than time-dependent.

Keywords:

• Haynes 556
• carburization
• reducing environment
• continuity