The interaction between low cycle fatigue and lamellar tearing in AISI 316L welded joints

Abstract

Experimental joints were manufactured by arc welding relatively thick plates of AISI 316L austenitic stainless steel. These joints simulated genuine joints employed in high-temperature nuclear plants. Mechanical testing of small specimens machined from the experimental joints and from base materials revealed the loss of ductility which is commonly encountered in thick welded plates when they are subjected to loading in the short transverse (through thickness) direction. Low cycle fatigue experiments, both at ambient temperature and at 550°C (1020°F) in air, were performed on base material and on welded specimens, after any surface roughness and geometrical notch effects had been removed by machining. In the case of specimens machined from a T-joint, it was observed that the strain was concentrated in the fusion zone, because of its lower strain hardening properties, but that the ruptures nucleated and grew in the short transverse direction. The fatigue life of the welded joint specimens was in all cases shorter than that of the base materials, but the life reduction was more marked at ambient temperature than at high temperature. Both phenomena can be explained with reference to the metallurgical microstructures. Observation made both before and after the tests revealed that inclusions of precipitated phases ( ie δ-ferrite and σ-phase) were responsible for a mechanism of interaction between lamellar tearing and low cycle fatigue, which is governed by the number, size and orientation of the inclusions.