Fatigue Crack Propagation Properties of HT80 Steel Welded Joints Heat Treated for Relieving Residual Stresses

Abstract

The fatigue crack propagation rate and the fatigue threshold were measured on transverse butt welded joints. The center cracked type specimens were made from as-welded joints and heat treated joints. The heat treatment were performed at 630°C and 720°C. The residual stresses were tensile around the middle of specimen and compressive near the edges. The tensile residual stresses were about 200 MPa, 20 MPa and 5 MPa for as-welded and heat treated specimens at 630°C and 720°C, respectively. The fatigue crack propagation properties obtained at various stress ratios on as-welded and heat treated specimens at 630°C were similar to each other and inferior to those obtained at the stress ratio of 0 on the base metal specimens. The fatigue threshold for as-welded specimens was about a quarter of that for base metal specimens. These properties obtained at the stress ratio of 0 on the heat treated specimen at 720°C were similar to those for the base metal specimens. While, the properties obtained avoiding crack closure at the very high stress ratio on the heat treated specimen at 720°C coincided with those for as-welded specimens. The coincidence of these properties means that the critical stress ratio conditions at which crack tips are fully open during all range of loading are made around crack tips by welding tensile residual stresses of 200 and 20 MPa.