| The carburization behaviors of three Fe-based alloys (310 Stainless Steel, Incoloy 800HT and Haynes 556) have been investigated under varying high temperatures and different ratios of methane/hydrogen gas mixtures. The study involved cyclic and isothermal exposures of the above alloys to different carburizing conditions and microstructural characterization of selected exposed samples using advanced analytical techniques (scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy). Thermodynamic analysis indicates that the gas mixtures (2% and 10%CH4/H2 with oxygen impurity less than 100ppm) are mixed oxidizing/carburizing below approximately 1000°C while reducing carburizing for chromium in excess of 1000°C, where 1000°C is a boundary condition. Kinetic and microstructural results indicate that, for the three Fe-based alloys, the external continuous layers formed at lower temperatures (<1000°C) consist primarily of different oxides and carbides which are responsible for the improved protection, however at higher temperatures (>1000°C) the external scales consist of carbides or metallic Cr-rich phases which are discontinuous and non-protective leading to increased weight gain. 310SS suffers both internal and external carburization attack at 800°C in 2%CH4/H2, while it experiences external carburization at 1100°C in 10%CH4/H2. The presence of Co in Haynes 556 is beneficial for the improvement of carburization resistance. Nickel and Cobalt play an important role in combating carburization especially when an external continuous layer is not formed at elevated temperatures. Under certain exposure conditions, despite the carbon activity exceeds unity metal dusting has not been experienced during this study. |
