Effect of Residual Tensile Stresses on Threshold Level for Fatigue Crack Propagation in Welded Joints of SM50B Steel

The fatigue crack propagation rate and the stress intensity threshold level were measured on welded joints which were made by three kinds of welding processes, manual arc welding with covered electrode, gas metal arc welding, and submarged arc welding. The specimens were 200 mm wide center notched type. The material was a SM50B steel 20 mm in thickness. The fatigue crack propagation properties of the weld metal and the heat affected zone of these welded joints were found to be quite similar to each other and inferior to that of the base metal. The stress intensity threshold level of welded joints was about one fourth of that of the base metal. It is assumed that the inferiority of welded joints in crack propagation properties may be caused from the tensile residual stresses distributed in the middle part of the specimens. The removal of the tensile residual stresses by postweld heat treatment could improve welded joints in the fatigue crack propagation properties. The fatigue crack propagation properties and the stress intensity threshold level of welded joints after the postweld heat treatment are similar to those of the base metal.