A precipitate evolution-based continuum damage mechanics model of creep behaviour in welded 9Cr steel at high temperature

Abstract

A multiaxial, physically-based, continuum damage mechanics methodology for creep of welded 9Cr steels, is presented, incorporating a multiple precipitate type state variable which simulates the effects of strain-and temperature-induced coarsening kinematics. Precipitate volume fraction and initial diameter for carbide and carbo-nitride precipitate types are key microstructural variables controlling time to failure in the model. The heat affected zone material is simulated explicitly utilising measured microstructural data, allowing detailed investigation of failure mechanisms. Failure is shown to be controlled by a combination of microstructural degradation and Kachanov type damage for the formation and growth of creep cavities. Comparisons with experimental data demonstrate the accuracy of this model for P91 material.

Characterization of thermal aging effects on the creep performance of T91 using small punch creep testing

“…The MSR is strongly influenced by growth of precipitates, which accelerates the accumulation of creep strain leading to cavity‐induced creep failure. This dependence of MSR on microstructural mechanisms has previously been modeled by the authors for both the parent and cross‐weld T91 materials 7,15,21 …”

“…However, recent work by the authors, on the development of physically based, multiaxial, creep damage models, incorporating the effects of the evolution of different salient strengthening mechanisms, for example, temperature‐ and strain‐dependent precipitate coarsening, chemical composition, and high‐ and low‐angle grain boundaries, has demonstrated significant benefits for creep life prediction. For example, the ability to predict the detrimental effects of poor control of Al content on creep life of P91, 15 as well as the ability to predict the effect of welding on long‐term creep life, 7 and significantly improved extrapolation capability from accelerated testing. Future work by the authors, based on the present experimental methodology, will focus on application of such advanced physically based models to provide significant improvements vis‐à‐vis interpretation of SPCT (and tensile creep) test data and, hence, provide a basis for plant life assessment.…”

“…Cross‐weld tensile creep tests only provide bulk specimen results from the combined behavior of parent, weld, and HAZ materials, 6 that is, it is not generally possible to identify the creep behavior of individual zones from cross‐weld tests. Previously, numerical approaches have been applied to extract the generalized HAZ behavior utilizing a three material implementation of a physically based creep model in the finite element code ABAQUS 7 . Such an approach proved useful for identifying an approximate behavior of the HAZ material as a single homogeneous unit.…”

A review of thermal, microstructural and constitutive modelling of 9Cr steel for power plant applications: Towards a through-process model for structural integrity of welded connections

“…Physically-based creep models, which could be used for PWHT stressrelaxation, have also been proposed, e.g. [135] and [136], based on the Hayhurst model [121]:…”

A process-structure-property model for welding of 9Cr power plant components: The influence of welding process temperatures on in-service cyclic plasticity response

“…Numerous creep models, some examples of which are reviewed by Rouse et al [19], have been applied to 9Cr steel, e.g. [20,21], including physically-based creep models applied to CW specimens [22].…”