Graphitization of steels in elevated-temperature service

Prolonged exposure of carbon and low alloy steel components to temperatures exceeding 800 degreesF (427 degreesC) can result in several kinds of mater ial microstructural deterioration; for example, creep cavitation, carbide c oarsening and/or spheroidization, and, less commonly, graphitization. Graph itization generally results from the decomposition of pearlite (iron + iron carbide) into the equilibrium structure of iron + graphite and can severel y embrittle the steel when the graphite particles or nodules form in a plan ar, continuous manner. Graphitization has resulted in the premature failure of pressure boundary components, including high energy piping and boiler t ubes. Failure due to graphitization continues to be of concern in long-term aged carbon and carbon-molybdenum steels, both in weldments and in base me tal, where, as recently reported, prior deformation or cold work could acce lerate the graphitization process. This paper describes the characteristics and kinetics of graphitization, reviews pertinent laboratory and field exp erience, and summarizes time-temperature service regimes within which graph itization can be anticipated.