Repair welding of carbon steel pipe that has experienced partial graphitization during elevated temperature service

Abstract

Plain C-Mn steels such as SA-516 Grade 65 are used as steam lines to transport high temperature steam under pressure. 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 an undesirable microstructural change in the form of secondary graphitization. Secondary graphitization of carbon steels is classically defined as the decomposition of cementite into carbon (present as the graphite phase) and iron (ferrite). Many cases have been documented where the decomposition reaction occurs more readily in a region closely associated to welds. The resultant graphite is often referred to as Heat Affected Zone (HAZ) graphite. This study aimed to evaluate whether or not it would be possible to weld on graphitized material and if so, to recommend viable repair procedures. In addition to this, it was critical to study the effects of HAZ graphite on the mechanical properties of plain C-Mn steels after in-service exposure of approximately 35 years at 420°C. Additional work was also performed that supplemented the tests necessary to qualify a procedure qualification record (PQR) in the form of several prolonged heat treatments and detailed metallography. This was done in an effort to understand the process of graphitization and the subsequent effects that its formation in the HAZ would have on the mechanical properties of not only a single sample, but also on welded joints with varying degrees of graphitization. It was concluded that the HAZ graphite had no effect on the tensile strength, impact toughness, hardness or ductility as characterised by the total elongation. Several tensile tests failed through the graphite plane associated with the HAZ. The samples that failed through the graphitized HAZ displayed a tensile strength, yield strength and total elongation similar to material that failed in the base material (away from the graphitized HAZ). However, failure in the graphitized HAZ resulted in a decrease in reduction in area from 72% to 44%. In spite of the change in reduction in area, the total elongation of samples that exhibited failure in the graphitized heat affected zone was acceptable. It was concluded that it is possible to perform repair welding on graphitized material using conventional welding procedures. Based on a procedure that used GTAW to perform the root run, followed by SMAW to fill and cap the joint, two PQRs were produced for graphitized plain carbon steel welding. A number of repair techniques were considered, of which the most viable that was recommended was to wrap the graphitized joints that are at highest risk of failing with a reinforcing metal band and to fillet weld the band in place. It was concluded that the presence of HAZ graphite was unlikely to affect the capacity of the steam line to resist plastic collapse. Future work on the influence of the presence of heat affected zone graphite on other failure mechanisms (such as thermal fatigue, mechanical fatigue and creep) is recommended.