Monotonic Fracture Studies on Bi-metallic Pipe Weld Joints Having Circumferential Through-Wall Crack

Abstract

Bi-metallic welded joints are necessary in pressurized and boiling water reactors where heavy section low alloy carbon steel components are connected to stainless steel primary piping systems. The behaviour of such bi-metallic weld joints is very difficult to understand, and very limited experimental data are available in the literature on the fracture behaviour of such joints. It is essential to develop fracture assessment methodologies for these joints and validate them experimentally to ensure safe operation of power plants. In this background, fracture studies were carried out under monotonic bending on seven numbers of bi-metallic pipe weld joints of 324 mm nominal outer diameter having circumferential through-wall crack. The specimens were made of SA 508 Gr.3 Cl-1 low alloy steel (ferritic) pipe on one half and SA312 Type 304LN stainless steel (austenitic) pipe on the other half. The initial through thickness notch was located in the different regions of the weld joints such as weld centre, buttering, heat-affected zones and base metals (low alloys steel and stainless steel). Initial notch angle was either 60° or 90°. Prior to the fracture tests, fatigue pre-cracking was carried out to create sharp crack front. Subsequently, fracture tests were conducted under four-point bending and displacement control. The collapse load of the weld joint having crack in the buttering layer is 12% less compared to that having crack in the heat-affected zone. Increase in the crack angle present in the heat-affected zone from 60° to 90° reduced the collapse load of the weld joint by 19%.