Study of Graphitization in Carbon Steel Weldments for Remaining Life Assessment

Carbon steels and low-alloy steels are often used in various stages of the refining process in petrochemical industries and power plants where they are susceptible to graphitization after prolonged exposure at temperatures of 800°F (427°C) or above. Graphitization is a result of solid-state phase transformation of metastable iron carbide to form iron and graphite structure. The formation of graphite results in the loss of tensile strength, ductility, and creep strength, which may result in untimely catastrophic failure of the component. The current study focused on developing a further understanding of graphitization on ex-service welded carbon steel components, which were removed from service after 17 years at 800°F (427°C). The materials received were examined using metallography, hardness, creep-testing, bend tests, Gleeble thermal simulation, Cryo-cracking, and heat-treatment for further fundamental studies of graphitization. In the materials examined, graphitization was primarily observed in a narrow band just outside of the visible HAZ which corresponds to a weld thermal cycle just below the lower-critical transformation temperature (Ac₁). A hypothesis to support for the increased amount of graphitization observed in the base metal just outside of the HAZ was proposed. The materials examined also exhibited increased amount of graphitization with the increase of aluminum content. A hypothesis to support this increase in graphitization with aluminum content based on thermodynamic principles was also proposed in this study. Creep studies using accelerated stress and temperature conditions were performed to evaluate the remaining service life of the graphitized components. An accelerated heat-treatment studies were performed to obtain the time-temperature behavior of graphite nucleation and growth. A potential in situ inspections studies using both non-destructive (ultrasonic inspection) and destructive methods (core extraction) were studied. In situ studies showed possible viable approach towards the inspection of graphitized carbon steel components for run, repair or replace decisions. Repair welding performed on core extraction sites using friction hydro tapered pillar processing (FTHPP), showed viable approach.