CRACK-GROWTH MORPHOLOGY AND MICROSTRUCTURAL CHANGES IN 316-STAINLESS-STEEL UNDER CREEP-FATIGUE CYCLING

A study into microstructural effects and crack growth behaviour of AIS I type 316 stainless steel under creep-fatigue conditions at 550 degre es C within the high strain ranges of 0.9-2.5%, including a 60 min hol d time, was undertaken on a high-temperature reverse-bending rig. Thro ughout the tests, surface cracks on both the tensile-hold and the comp ressive-hold sides were monitored by means of a plastic-strip replicat ion technique. Additional investigations were conducted on failed spec imens to examine the crack morphology in the depth direction, and to e xamine the function of oxidation; also to study changes of fracture su rface morphology, changes in dislocation structures and precipitate co nfigurations corresponding to the different strain ranges. These detai led analyses revealed that the predominantly intergranular long cracks on the tensile-hold side and transgranular short cracks on the compre ssive-hold side are dominant aspects of the investigation. The disloca tion structures under creep-fatigue conditions are strain-range depend ent, with a clearly defined cell structure at the higher strain ranges and dense dislocation tangles at lower strain ranges. The large reduc tion in creep-fatigue endurance can be attributed to early crack growt h and grain boundary cracking caused by stress relaxation, oxidation, precipitation and, most importantly, the coalescence of the many minor surface short cracks.