Investigations on the crack formation and propagation in the dissimilar pipe welds involving L360QS and N08825

• The paper investigated reasons why crack initiated and propagated.
• Crack initiated in N08825 alloy and crack propagated towards the direction of weld.
• NbC on grain boundaries weakened the strength of grain boundaries,and increased hot cracking susceptibility.
• Small angle grain boundaries within weld promoted crack propagation.

In this study, the causes of crack in the L360QS/N08825 bimetallic composite pipe welded joint were experimentally investigated. Microstructure and precipitates within crack site were studied by metallographic examination, scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). A characterization method based on the electron backscatter diffraction (EBSD) technique has been developed to analyze the features of grain and microstructure within crack site. The results show that the crack initiated in the side of N08825 alloy and it propagated towards the direction of weld. SEM and EDS results show that welding crack mainly propagated along columnar grain boundaries and NbC on grain boundaries weakened the strength of grain boundaries, which promoted crack propagation. The grain boundaries in weld were small angle grain boundaries. Small angle grain boundaries provided favorable orientation of dislocation slip, which also promoted crack propagation. In addition, a larger localized initial stress was found within a region close to crack, which resulted in localized plastic deformation and then promoted crack propagation.