Ratcheting Fatigue Failure of Welded Stainless Steel Pipe and Dislocation Microstructure

Abstract

Fatigue failures of welded piping joints are continued to be the majority of the failures in the nuclear power plants (NPPTs), resulting in unscheduled plant downtime and financial loss. This study investigates the influence of ratcheting on the fatigue failure of the welded piping joints. Many studies on the phenomenon of strain ratcheting have been conducted at the macroscale but not in the microscale. This study presents results on the microstructure evolution of the heat affected zone under cyclic loading. Four welded piping specimens are subjected to displacement-controlled cyclic loading with different number of cycles. Microstructural evolutions of the piping material coupons conditioned by various temperature cycles to simulate HAZ materials are also investigated. The coupons are subjected to three different types of fatigue loading cycles, uniaxial strain-controlled, biaxial ratcheting, and uniaxial force-controlled before the microstructural investigation. The stress response of welded pipe specimens showed initial cyclic hardening until 20 cycles followed by softening, and strain ratcheting was observed at the weld toe. Microstructure at the weld toe showed various dislocation types under different number of cycles. More cell and wall structures were formed with changing volume fraction of each type of substructure as the number of fatigue cycles increases. Dislocation density measured at weld toe initially increased and then decreased until failure, which was attributed to cyclic hardening and softening. More heterogeneous dislocations were noted near the weld toe in contrast to more planar slip away from the weld toe. Cyclic hardening and softening are observed during cyclic loading of coupons. Stress amplitude responses show decrease in amplitude with increase in heat-treatment temperature under uniaxial strain-controlled tests. Before fatigue, heterogeneous dislocation structures are not visible, but various dislocation structures are observed after fatigue cycles; these include planar dislocations, dislocation cells, ladder like structures, tangles, dislocation walls as well as stacking faults. Percent of cell structure increases as the planar slip and tangles decrease with increasing number of cycles in both unconditioned and 800ËšC heat-treated materials. However, high proportion of planar slip remains even though cell structure increases in the 1050ËšC heat-treatment coupons. Dislocation density increases during cyclic loading where stress amplitude initially increases followed by decrease with progressive cycles. Dislocation cell size was inversely proportional to cyclic hardening. Ratcheting strain was observed when positive mean stress applied to a coupon under uniaxial force-controlled cycles. Dislocation structures were similar for both ratcheting and non-ratcheting coupons. However, cell and wall structures were better defined in non-ratcheting specimen while higher dislocation density is noted in ratcheting specimens. Under biaxial ratcheting cycles, all specimens exhibited dislocation cell and wall structures. However, high proportion of dislocation wall structures are observed in 1050ËšC heat-treated specimens. Dislocation density decreased with increasing heat-treatment temperature, and cell size decreased when stress amplitude and dislocation density increased. This work clearly demonstrated that ratcheting resulted in lower dislocation density along with larger cell size.