The fracture resistance and crack propagation path of L415/N08825 bimetallic composite pipe welded joint

Abstract

The microstructure, fracture resistance and crack propagation path in the L415/N08825 bimetallic composite pipe welded joint has been carried out using the single-edge notched bend (SENB) specimens. The microstructure in the welded seam is dendritic austenite. There are high-density dislocations in the weld, and ellipsoidal NbC and TiC precipitates are distributed on the dendritic austenite matrix. There is element diffusion between the weld and base metals of the interface, and the concentration distribution of main elements at the interface presents a steep concentration gradient. The J-resistance (J-R) curves of the heat-affected zone (HAZ) and welded seam of the experimental welded joint were obtained, reflecting the HAZ of the L415/N08825 bimetallic composite pipe welded joint has a better fracture resistance than the welded seam. The crack propagation path is approximately straight in all specimens, and both regions extend along grain boundaries. The fracture mode in the HAZ is a ductile fracture. The crack is close to deflect to the interface and is greatly affected by the strength and toughness mismatch of the weld metal (WM) and L415. While, the main crack growth direction and the weld grain orientation have always led to the low fracture resistance of the WM.