Plain carbon steels that are in service at elevated temperatures (typically 400-450 °C) for prolonged periods of time (typically in excess of 20 years) may experience graphitization, often in the heat affected zone (HAZ). This study aimed to determine the effects of HAZ graphite on the mechanical properties of plain carbon steels as well as to characterize the graphite that was observed. Additionally, it was required to evaluate whether repair welding of graphitized material was viable. A combination of gas-tungsten arc welding and shielded metal arc welding was used, in association with various joint configurations. A number of extended heat treatments were performed. Transverse tensile samples did not always fracture through the HAZ graphite. Failure in the graphitized HAZ resulted in a decrease in reduction in area; no other tensile properties were affected by the presence of HAZ graphite. A prediction of the location of the HAZ graphite was developed using the Rosenthal equation. It was demonstrated that it is possible to perform repair welding on graphitized material using conventional welding procedures.