Physically-based methods for improved high temperature creep performance of 9Cr steels and welds: nano-, micro- and macro-scale studies

Ó Murchú, Cathal
The reduction of emissions from fossil fuel power plants is essential to minimising the environmental impact of power generation and this can be achieved by higher temperature, with more efficient combustion of the fuel. This requires the development, testing and improved understanding of materials capable of resisting the creep deformation that occurs during long-term operation under high temperature and pressure conditions. 9Cr tempered martensitic steels have excellent creep resistant properties due in part to their hierarchical microstructure stabilised by multiple precipitate types. To obtain the maximum life from a 9Cr component, plant operators require accurate modelling and assessment methods to predict the remaining service life of a given component. Thus, the thesis is concerned with the development of experimental small scale testing methodologies for component remnant creep life measurements and, the novel numerical modelling of microstructure evolution during high temperature creep. This numerical model utilises a physically-based continuum damage mechanics creep model, incorporating the mechanisms of (i) primary hardening, (ii) multi-precipitate type coarsening and (iii) cavitation damage. A novel multi-precipitate type term is developed, incorporating the thermal- and strain-induced coarsening behaviour of M23C6 and MX precipitates. The model is validated for current generation P91 materials at power plant conditions and accurately predicts the effect of altered composition on the creep strain behaviour. The production of a creep resistant microstructure in 9Cr components requires careful heat treatment to generate the precipitate strengthened hierarchical microstructure. As such, the effects of standard 9Cr, and elevated temperature heat treatments, on the microstructure and mechanical properties of the next-generation material MarBN are also investigated. This enables recommendations on the optimum heat treatment for MarBN components and these are also presented. The enhanced creep strain model is implemented in a finite element creep subroutine to simulate the behaviour of welded tensile and piping components and this accurately predicts the behaviour of P91 welded tensile specimens from laboratory to plant conditions. Literature data for precipitate and microstructural features, for the weld metal and the heat affected zones, are combined in a novel parameter identification approach to extract the necessary material terms for the model. Weld strength reduction factors are calculated, indicating that welded tensile results are conservative when compared to component level modelling. Small scale testing of in-service component materials, sampled without the requirement to take plants offline, play an important role in estimating the remaining life of a component. A small punch creep (SPC) test is one such method and the material requirement here is very small in comparison to standard tensile testing - the specimen being 7 mm in diameter and 0.5 mm thick. It is shown here that this approach produces reliable and repeatable test data which, importantly, is in agreement with the published tensile test data. SPC tests of the un-aged and thermally aged P91 specimens are presented. The aged specimens demonstrate significantly reduced creep lives with little to no warning of failure occurring prior to disk rupture. Unaged specimens were considerably more ductile with accelerating displacement rates observed prior to failure. The multi-precipitate type creep subroutine predicts the punch minimum displacement rate trend in agreement with the experimental results. In terms of the heat treatment of the next generation MarBN, normalisation was conducted at the (i) standard 9Cr temperature and (ii) a recommended elevated temperature for MarBN. The higher temperature normalisation is significantly different from the current standard for 9Cr materials. A softened region, observed in the sample normalised at the lower temperature, is investigated via micro-mechanical testing and electron microscopy. Lath size effects are correlated with the hardness data, indicating that decarburization potentially leads to softening after lower temperature normalisation. The higher temperature normalisation resulted in finer laths, which are indicative of a stronger material with a higher yield stress, it also prevents the formation of creep weak precipitates (BN) and promotes uniform mechanical properties. This work emphasises the requirement for elevated temperature normalisation of MarBN for component manufacturing. For plant operators employing 9Cr precipitate strengthened steels, there now exists a multi-precipitate type based model with enhanced prediction of creep lives of welded and plain components using measured microstructural features. Coupled with the small punch creep test, the remaining life assessment of in service components can now be performed from minimal material samples.
NUI Galway
Publisher DOI
Attribution-NonCommercial-NoDerivs 3.0 Ireland