An Updated Assessment of Graphitization of Steels in Elevated Temperature Service

Recent experience with damage and failure due to graphitization in electric power plant piping of carbon (C) and carbon-molybdenum (C-Mo) steel suggests that the previously developed time-temperature curves for graphitization prediction, first published over two decades ago, merit review. Recent data were combined with an exhaustive review of available literature. As with the earlier research, available experience data with reported approximate extent of graphitization and nominal service exposure conditions were analyzed for the predictions. When the data were combined, the database consisted of 281 data points. The data are in contrast to the roughly 40 points used in the prior research. The nature of the expanded C steel weldment database was such that the analysis could not effectively discriminate between all of the five graphitization levels used in previous research efforts. In this analysis, it was found that the level of graphitization as a function of time and temperature could be categorized into three broad risk ranges-defined as low, moderate, and significant, and that the curves delineating these ranges could be developed in a statistically conservative manner. These conservative time-temperature prediction curves are offered as an update to the previous time-temperature curves. Although the data for C-Mo steel base metal and weldments and on C steel base metal were inadequate for a full quantitative analysis, the experience with these materials cases is presented within the context of the C steel weldment risk curves and preliminary time-temperature conditions warranting concern for graphitization are offered. Finally, a partial validation of the risk curves is presented in examination of power plant piping that had operated for over 400,000 h.