Graphitization of steels in elevated-temperature service

Prolonged exposure of carbon and low alloy steel components to temperatures exceeding 800 °F (427 °C) can result in several kinds of material microstructural deterioration; for example, creep cavitation, carbide coarsening and/or spheroidization, and, less commonly, graphitization. Graphitization generally results from the decomposition of pearlite (iron + iron carbide) into the equilibrium structure of iron + graphite and can severely embrittle the steel when the graphite particles or nodules form in a planar, continuous manner. Graphitization has resulted in the premature failure of pressure boundary components, including high energy piping and boiler tubes. Failure due to graphitization continues to be of concern in long-term aged carbon and carbon-molybdenum steels, both in weldments and in base metal, where, as recently reported, prior deformation or cold work could accelerate the graphitization process. This paper describes the characteristics and kinetics of graphitization, reviews pertinent laboratory and field experience, and summarizes time-temperature service regimes within which graphitization can be anticipated.