Prevention of Sensitization and Fracture Analysis of Keyhole Plasma Arc Welded AISI 304 HCu Tube Joints

Reheaters and superheaters of the Advanced Ultra Supercritical (AUSC) Boilers prefer AISI304 HCu. The steels are prone to sensitization on welding (550 to 900 °C), segregation of alloying elements, and wider bead profiles, resulting in the degradation of joint properties. Plasma Arc Welding by keyhole mode (KPAW) provided a better solution by autogenously joining 9 mm thick tube joints in a single pass. The KPAW joint has minimum bead geometry compared to Tungsten Inert Gas (TIG) welds. The strength of the KPAW joints (610 MPa) is 17% higher than the TIG joints (502 MPa) and close to the strength of the base material. The homogenous morphology of the KPAW joint with fine dendrites of δ-ferrite enhances the properties of welded joints. The cooling rate influences the finer nucleation of dendrites with minimal arm spacing of 12.4 µm per unit area. The estimated cooling rate for the KPAW joint is around 0.97 × 105 K/s, which results in an average dendritic length of 69 µm.