Case Study of a UNS N06693 Weld Tube Fracture

UNS N06693 (IN 693) is a Ni-base alloy developed for superior metal dusting resistance, which was chosen for a process gas transport application between a gasifier and syngas cooler in a transfer duct with operating temperatures up to 550°C. Fracture occurred in a 6.5mm thick single-V tube tubular seam weld after 10 years of operation and about two months after a weld modification was completed on an adjacent weld 40 mm away. Cracks initiated on the inner diameter (ID) and propagated radially and circumferentially to the outer diameter (OD) through the weld heat affected zone (HAZ) and fusion zone (FZ). Fractography indicated a predominantly intergranular/interdendritic fracture mode. The direct cause of failure was increase in transverse tensile residual stresses due to the weld modification in the adjacent weld at a distance of 40-mm away. The contributing causes to failure include: 1) complex thermal history via multiple direct and indirect PWHT thermal cycles contributing to complex microstructural evolution and residual stress redistribution, 2) process effects on ID wear, metal dusting, and tube thinning, and 3) higher than recommended weld heat input (>1.5 kJ/mm) contributing to excessive grain coarsening in the ID HAZ. Stress relaxation cracking (SRC), a thermomechanical mechanism for cracking, was the most likely fracture mechanism. SRC susceptible microstructures with a high microhardness (>400HV) were observed in the HAZ ID, as evidenced by needlelike Cr-rich α-phase, blocky, intergranular Cr-rich precipitates, and a high γ’ volume fraction, which simultaneously provide a creep resistant (low creep ductility) matrix and grain boundary embrittlement. Recommended strategies for improving microstructural stability and service performance will be discussed.