A Unified Viscoplastic Model for High Temperature Low Cycle Fatigue of Service-Aged P91 Steel

Abstract

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.

A physically-based constitutive model for high temperature microstructural degradation under cyclic deformation

“…Sauzay and co-workers [15,16] have established that this effect is primarily related to the loss of the lath microstructure and a concomitant reduction in dislocation density. Barrett et al [17,18] showed that a significant strain-rate effect occurs for P91 at 600 °C, although the effect is negligible for temperatures of 500 °C and less. This strain-rate effect has also been observed in other 9Cr martensiticferritic steels, such as P92 [11] and MarBN [19], at temperatures in excess of 550 °C.…”

The effect of inclusions on the high‐temperature low‐cycle fatigue performance of cast MarBN: Experimental characterisation and computational modelling

“…The constitutive behaviour of the material is defined using a unified cyclic viscoplastic material model with combined non‐linear isotropic and kinematic hardening, as described in more detail by Barrett et al This model has been implemented in an implicit UMAT subroutine in the commercial finite element code Abaqus and modified in this work to include life prediction and fatigue damage effects. A flowchart of the constitutive damage model is presented in Figure .…”

Microscale deformation of a tempered martensite ferritic steel: Modelling and experimental study of grain and sub-grain interactions

“…More details on these experimental tests are provided in [52]. Data from a typical room temperature tensile test are shown in figure 5, taken from the same header [53] as the miniature three point bend specimen. The material model used within the continuum level FE analysis is also shown in the figure.…”