Crack growth morphology and microstructural changes in 316 stainless steel under creep-fatigue cycling

A study into microstructural effects and crack growth behaviour of AISI type 316 stainless steel under creep-fatigue conditions at 550 degrees C within the high strain ranges of 0.9-2.5%, including a 60 min hold time, was undertaken on a high-temperature reverse-bending rig. Throughout the tests, surface cracks on both the tensile-hold and the compressive-hold sides were monitored by means of a plastic-strip replication technique. Additional investigations were conducted on failed specimens to examine the crack morphology in the depth direction, and to examine the function of oxidation; also to study changes of fracture surface morphology, changes in dislocation structures and precipitate configurations corresponding to the different strain ranges. These detailed analyses revealed that the predominantly intergranular long cracks on the tensile-hold side and transgranular short cracks on the compressive-hold side are dominant aspects of the investigation. The dislocation structures under creep-fatigue conditions are strain-range dependent, with a clearly defined cell structure at the higher strain ranges and dense dislocation tangles at lower strain ranges. The large reduction in creep-fatigue endurance can be attributed to early crack growth and grain boundary cracking caused by stress relaxation, oxidation, precipitation and, most importantly, the coalescence of the many minor surface short cracks.