Creep and creep fatigue interaction in new and service exposed P91 steel

Abstract

Power plant components that have been in operation for many years may have accumulated significant creep damage. Cyclic operations at high temperature lead to issues with interactive creep-fatigue failure of high temperature components. The creep-fatigue interaction may accelerate the failure and reduce the service life of power plant components. The aim of this research is to examine the effects of material’s service exposure, including prior creep damage, on subsequent creep-fatigue crack growth and low cycle fatigue behaviour. These effects which are important for safe component operation have been included in predicting the remnant life of high temperature material.

The material of interest is P91 steel, which is widely used in high temperature power plant components due to its high material performance. Tensile and uniaxial creep have been performed on the new and ex-service P91 steel at 620°C and 600°C, respectively to obtain the material properties. The result of uniaxial creep tests have been analysed and compared with available P91 data to examine the effect of long term exposure of P91 materials at high temperature in lower stress level.

Creep-fatigue crack growth testing has been performed on compact tension specimen at a range of temperature between 600°C to 625°C with hold time ranging from static to 600s to examine the CFCG behaviour. The CFCG results have been correlated with stress intensity factor range, ΔK and creep fracture mechanic parameter, C* and compared to the static creep, high temperature fatigue and CFCG test data available in the literature for P91 steel. The CFCG rate and the creep crack initiation (CCI) time have been compared to the NSW CCG model’s prediction. It is found that for low stress, low ductility and increase in constraint, the plane strain NSW model can conservatively bound the tests data at long terms which is more appropriate for components operational times. An interaction diagram based on a linear cumulative damage rule has been proposed to predict the creep-fatigue interaction results regardless of the degradation of the steel under ex-service condition. It is shown that the mean CFCG rates for ex-service steels are faster by a factor of 4 compared to the mean CCG data. The increase in cracking rate is directly related to the reduction in creep ductility which can occur both due to material degradation and under long term testing conditions. Fractography have revealed an intergranular ductile fracture surface for shorter term tests performed, which is an indication of the creep dominance for the creep- fatigue conditions.

Notched bar creep tests have been performed on new and ex-service material at 620°C and 600°C to examine the effect of multiaxial stress state on creep ductility. Finite element analysis coupled with a damage model were performed to evaluate the damage accumulation on the notched bar and predict the rupture life under multiaxial stress conditions. The finite element rupture life predictions based on the remaining creep ductility criteria were compared with short term experimental data and provide a basis to predict the long term behaviour. Metallographic and microstructural assessment on the notched bar have been performed to support the experimental findings.

Prior creep strain/damage has been introduced into a material by performing interrupted uniaxial creep testing. The uniaxial creep tests were interrupted at various levels of creep strain. In order to examine the influence prior creep strain/damage on tensile deformation, a series of tensile test have been performed on prior creep specimens. In this work, room temperature tensile test have been performed. The result of these tests have been analysed and compared with thermally aged specimen and the one without prior creep strain. It has been shown that prior creep strain reduces the 0.2% proof stress. Low cycle fatigue (LCF) tests have been performed on the specimen with and without prior creep strain at various strain ranges to examine the effects of prior creep strain/damage on the fatigue behaviour. It is shown that the stress amplitude for material with prior creep strain is lower than the material without prior creep strain which indicate that the material with prior creep strain reduces its strength by means of material degradation during the creep test. The result from these LCF tests are compared and analysed to provide a basis for the fatigue life prediction.

For the future works, further LCF tests on prior creep strain specimen and CFCG tests need to be performed to confirm the observed trend. Instead of prior creep strain, the influence of prior cyclic loading may be investigated and subsequent test may be performed. Numerical modelling of the creep damage process to predict uniaxial and multiaxial failure under cyclic loading need to be performed and enhanced by taking into account the actual material properties of the damaged material .This model can also be used to show its relevance to component failure under creep/fatigue conditions.