Three austenitic stainless steels (AISI 316, AISI 316LN, and 253MA) have been investigated at 600°C with respect to fatigue and creep-fatigue behavior. Transmission electron microscopy showed that cross-slip and climb of dislocations are inhibited in AISI 316LN and 253MA due to the presence of nitrogen in solid solution. This results in a planar slip character and a concomitant increase in fatigue strength. However, during creep-fatigue deformation, nitrogen has an adverse effect on strength. This can be explained as an enhanced interaction between creep and fatigue due to the generation of high stresses in the grain boundary regions, caused by inhibited recovery.
A phenomenological model of the interaction between creep and fatigue in Type 304 stainless steel at elevated temperatures is presented. The model is based on a crack-growth equation and an equation governing cavity growth, expressed in terms of current plastic strain and plastic strain rate. Failure is assumed to occur when a proposed interaction equation is satisfied. Various parameters of the equations can be obtained by correlation with continuously cycling fatigue and monotonic creep-rupture test data, without the use of any hold-time fatigue tests. Effects of various wave shapes such as tensile, compressive, and symmetrical hold on the low-cycle fatigue life can be computed by integrating the damage-rate equations along the appropriate loading path. Microstructural evidence in support of the proposed model is also discussed.
Strain-controlled continuous fatigue and creep-fatigue experiments are reported for two types of 316 steel tested at 600°C (1112°F). It is shown that, although the continuous fatigue properties of the two materials are very similar, the one containing a controlled amount of nitrogen exhibits a better creep-fatigue resistance than the other alloy. Detailed measurements of intergranular damage made either on the fracture surfaces or in the bulk of creep-fatigue specimens indicate that the susceptibility of the materials to the effect of tensile hold times can be related to their propensity to intergranular cracking. A stress relaxation-propagation reduction factor per cycle correlation is proposed in order to account for the detrimental effect of tensile hold times on the fatigue life. This correlation relies upon experimental results on austenitic stainless steels published in the literature. It is shown that the proposed approach, derived largely from the quantitative measurement of intergranular damage, holds some promise for predictive purposes.
Creep, fatigue and creep-fatigue tests with five and ten minutes hold time were conducted using smooth and modified keyhole compact tension specimens. Measurements were made of the crack initiation and propagation of a 2 1/4Cr-1Mo steel in the normalized and tempered condition and a 304 stainless steel in the annealed condition at temperature of 565$spcirc$C and 650$spcirc$C, respectively, in air. Stress redistribution times under creep loading were computed to establish the governing stress field for crack initiation and crack growth using the creep isochronous curves and an analytical solution. Smooth specimen test results were used to predict the crack initiation life under fatigue and creep-fatigue loading using the local strain approach and to predict the total life under creep loading using the reference stress approach. Microstructural analysis was carried out to identify the failure mechanisms. It was found that crack initiation occupied a major portion of the failure life under creep and creep-fatigue loading and that stress redistribution times were two orders of magnitude lower than the crack initiation times. Both materials were identified as creep-ductile, with reference stress-controlled initiation and growth behavior. Creep failure mechanism predominates after short hold times ($>$5 minutes). Creep cavitation was the failure mechanism in 304 stainless steel while exhaustion of matrix ductility with very little creep damage was the dominant failure node for 2 1/4Cr-1Mo steel under creep and creep-fatigue loading.
