Creep-Fatigue Interactions in a 9 Pct Cr-1 Pct Mo Martensitic Steel: Part I. Mechanical Test Results

Abstract

Creep-fatigue (CF) tests are carried out on a modified 9 pct Cr-1 pct Mo (P91) steel at 550°C. These CF tests are strain controlled during the cyclic part of the stress-strain hysteresis loop and then load controlled when the stress is maintained at its maximum value, to produce a prescribed value of the creep strain before cyclic deformation is reversed under strain-controlled conditions. The observed cyclic softening implies that the applied creep stress continuously decreases with the number of cycles. However, the minimum creep rates measured at the end of the holding periods do not decrease when the applied stress decreases. The minimum creep rates measured at the end of these tests can be hundreds of times faster than those observed for the asreceived material. This acceleration of creep rates can be to the microstructural coarsening and to the decrease of the dislocation density observed after fatigue and CF loadings. Cyclic creep tests consisting of very long holding periods interrupted by unloading/reloading are also carried out. These results suggest that cyclic loadings affect the creep lifetime and flow behavior only if a plastic strain is applied during cycling. Creep tests carried out on a material cyclically prestrained and fatigue tests carried out on a material previously deformed in creep confirm that the deterioration of the mechanical properties is much faster in fatigue and CF compared to creep.

Creep-Fatigue Interactions in a 9 Pct Cr-1 Pct Mo Martensitic Steel: Part II. Microstructural Evolutions

“…Such tests are reported in the first part of this study. [25] Creep-fatigue tests can thus be used as ''accelerated'' tests. Because the microstructure observed after CF tests is comparable to that observed after several years of in-service conditions (mainly creep loading), testing samples prestrained in CF allows estimation of the actual mechanical properties of the steels after in-service loadings.…”

“…[25] B. Observation Techniques Several specimens are observed using different techniques in order to characterize the microstructural evolutions at different scales.…”

“…The first part of this study [25] showed that, under fatigue and creep-fatigue (CF) loadings, a very fast deterioration of the creep properties of 9 to 12 pct Cr steels is observed. Even though fatigue damage might be partly responsible for this deterioration, previous studies on other materials [26,27] showed that microstructural changes occurring during fatigue tests can also affect the creep properties.…”

High-temperature mechanical properties improvement on modified 9Cr–1Mo martensitic steel through thermomechanical treatments

“…Since these components will be subjected to high-temperature cyclic loadings, data about creep-fatigue properties are required in design procedures. Recent studies on commercial Grade 91 (''G91" which is a modified 9Cr-1Mo steel) showed that cyclic loadings coupled to high-temperature creep loadings lead to a fast and strong cyclic softening effect, which deteriorates the mechanical properties such as the creep strength [3][4][5][6][7]. This phenomenon was shown to be mainly due to a coarsening of the microstructure, in particular the decrease of the dislocation density, and the coalescence of precipitates.…”

High temperature fatigue: behaviour of three typical classes of structural materials

“…The value of D c which is specific to the test conditions has been experimentally determined for tensile stress 18 Virgo steel ( (8) where (da/dN) CF is the rate of crack propagation in 'pure fatigue', i.e. without hold time.…”