Functionally graded materials for marine risers by additive manufacturing for high-temperature applications: Experimental investigations

In an offshore environment, marine risers are subjected to high environmental loads caused due to axial tension, wave, and current action. The hydrocarbons flowing through these marine risers put the marine riser under a continuous threat of corrosion. In this research, duplex stainless steel, a highly corrosion-resistant alloy, is functionally graded with carbon-manganese steel and the inner layer, providing structural and functional properties to the newly developed material. An experimental investigation on using functionally graded material for a marine riser application is carried out, testing its strength and durability. The mechanical properties and durability aspects are compared with that of the conventional X52 Carbon Manganese steel, a common candidate for marine risers. Wire Arc Additive Manufacturing (WAAM) is used to fabricate the FGM in the lab scale. WAAM is economical and possesses high deposition rates. ASTM E8 tensile test specimens are cut out from the FGM build to determine the mechanical properties. Based on the experimental studies carried out, it is seen that FGM shows about 11% and 25% enhanced yield strength and ultimate strength, respectively. The yield strength and elongation at a high temperature of the FGM material is close to that of X52 at room temperature. In contrast, the ultimate strength is higher, which proves its suitability for high-temperature applications. It also possesses improved durability due to the innate corrosion resistance of duplex stainless steel. The proposed FGM is seen as an effective alternate candidate for marine risers with enhanced structural properties and durability characteristics.