Publication InformationFarragher, TP, Scully, S, O Dowd, NP, Leen, SB (2013) 'Development of life assessment procedures for power plant headers operated under flexible loading scenarios'. International Journal Of Fatigue, 49 :50-61. PublisherElsevier ScienceDirect 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 Abstract A finite element methodology for thermomechanical fatigue analysis of a subcritical power plant outlet header under realistic loading conditions is presented. The methodology consists of (i) a transient heat transfer model, (ii) a sequential anisothermal cyclic viscoplastic model and (iii) a multiaxial, criticalplane implementation of the Ostergren fatigue indicator parameter. The methodology permits identification of the local thermomechanical stress-strain response at critical locations and prediction of fatigue life and cracking orientation for complex transient, anisothermal, cyclic elastic-plastic-creep material behaviour. Measured plant data, in the form of steam and pipe temperature transients and steam pressure data, are employed to identify heat transfer constants and validate the predicted thermal response, with particular attention given to plant start-up and attemperation effects. The predictions indicate out-of-phase temperature-strain response at the header inside surface and in-phase response on the outside surface. Cooling transients are predicted to control damage and crack initiation at the inner bore, whereas heating transients are predicted to have a more damaging effect at weld locations. A representative test cycle is presented, which is shown to capture the salient thermo-mechanical cyclic damage of the realistic cycle. The predicted results correlate well with industrial experience in terms of crack (initiation) orientation, location and life. IntroductionThere is a need to understand the performance of candidate power plant materials for higher temperatures and pressures. 9-12% Cr steels are an example of such candidate materials, owing to their excellent mechanical properties, and steels such as P91 and P92 are already being utilised in power generation plant [1]. There is an increasing trend towards shifting of energy supply from conventional fossil fuel power plant to sustainable energies, e.g. wind power. To allow for the variable nature of renewable energy supply, there is a requirement to operate existing plant, designed for 'base-load' operation, with relatively infrequent shutdown and start-up, in 'load-following' mode, characterised by increased frequency of shutdown and start-up. This issue has increased the importance of design and assessment of materials and components for high temperature, pressurised plant with respect to thermomechanical fatigue (TMF) and creep-fatigue failure.Current header design does not consider cyclic thermal stresses and load cycling that can lead to creep-f...

This paper is concerned with the development of a methodology for thermomechanical analysis of high temperature, steam-pressurized P91 pipes in electrical power generation plant under realistic (measured) temperature and pressure cycles. In particular, these data encompass key thermal events, such as “load-following” temperature variations and sudden, significant fluctuations in steam temperatures associated with attemperation events and “trips” (sudden plant shut-down), likely to induce thermomechanical fatigue damage. An anisothermal elastic-plastic-creep material model for cyclic behavior of P91 is employed in the transient finite element (FE) model to predict the stress–strain-temperature cycles and the associated strain-rates. The results permit characterization of the behavior of pressurized P91 pipes for identification of the thermomechanical loading histories relevant to such components, for realistic, customized testing. This type of capability is relevant to design and analysis with respect to the evolving nature of power plant operating cycles, e.g., associated with more flexible operation of fossil fuel plant.

The finite element (FE) implementation of a hyperbolic sine unified cyclic viscoplasticity model is presented. The hyperbolic sine flow rule facilitates the identification of strain-rate independent material parameters for high temperature applications. This is important for the thermo-mechanical fatigue of power plant where a significant stress range is experienced during operational cycles and at stress concentration features, such as welds and branch connections. The material model is successfully applied to the characterisation of the high temperature low cycle fatigue behaviour of a service-aged P91 material, including isotropic (cyclic) softening and non-linear kinematic hardening effects, across a range of temperatures and strain-rates.KEYWORDS: service-aged P91, strain-rate independence, unified viscoplasticity, high temperature low cycle fatigue, material Jacobian. INTRODUCTIONGrade P91 steel, a 9Cr martensitic steel developed at Oakridge National Laboratory (ORNL) [1], is used extensively in fossil-fuel based power plant due to its high creep strength and low coefficient of thermal expansion. However, modern and next generation power plants are subjected to thermomechanical fatigue (TMF) due to a rapid rise in the number of start-up cycles to accommodate renewable sources of energy. This results in premature failure of plant components, including plant header and piping systems. Thus, there is a greater requirement to be able to characterise the advanced materials of modern and next generation plants under realistic loading conditions and accurately predict failure at the component level.The high strength of such 9Cr steels may be attributed to its complex microstructure, consisting of prior austenite grains, packets and blocks in a hierarchical format [2]. The blocks contain long martensitic laths and more equi-axed subgrains [2,3]. The martensitic laths contain MX type precipitates which resist motion of mobile dislocations and M 23 C 6 carbonitrides are dispersed along boundaries to increase the resistance of the material to creep deformation [4]. Although 9Cr steels have a high creep strength, fatigue loading results in a coarsening of this microstructure, leading to the cyclic softening phenomenon observed in such materials [2,3,5] and to a reduction in creep strength.

The high temperature low cycle fatigue behaviour of P91 weld metal (WM) and weld joints (cross-weld) is presented. Strain-controlled tests have been carried out at 400 o C and 500 o C. The cyclic behaviour of the weld material (WM) and cross-weld (CW) specimens are compared with previously published base material (BM) tests.The weld material is shown to give a significantly harder and stiffer stress-strain response than both the base material and the cross-weld material. The cross-weld tests exhibited a cyclic stress-strain response which was similar to that of the base material. All specimen types exhibited cyclic softening but the degree of softening exhibited by the cross-weld specimens was lower than that of the base material and all-weld tests. Finite element models of the base metal, weld metal and cross-weld test specimens are developed and employed for identification of the cyclic viscoplasticity material parameters. Heat affected zone (HAZ) cracking was observed for the crossweld tests.

This paper is concerned with the development of a methodology for thermo-mechanical analysis of high temperature, steam-pressurised P91 pipes in electrical power generation plant under realistic (measured) temperature and pressure cycles. In particular, these data encompass key thermal events, such as ‘load-following’ temperature variations and sudden, significant fluctuations in steam temperatures associated with attemperation events and ‘trips’ (sudden plant shut-down), likely to induce thermo-mechanical fatigue damage. An anisothermal elastic-plastic-creep material model for cyclic behaviour of P91 is employed in the transient FE model to predict the stress-strain-temperature cycles and the associated strain-rates. The results permit characterisation of the behaviour of pressurised P91 pipes for identification of the thermo-mechanical loading histories relevant to such components, for realistic, customised testing; this type of capability is relevant to design and analysis with respect to the evolving nature of power plant operating cycles, e.g. associated with more flexible use of fossil fuel plant to complement renewable energy sources.