This paper presents the high-temperature low-cycle fatigue (HTLCF) behavior of a precipitate strengthened 9Cr martensitic steel, MarBN, designed to provide enhanced creep strength and precipitate stability at high temperature. The strain-controlled test program addresses the cyclic effects of strain-rate and strain-range at 600 °C, as well as tensile stress-relaxation response. A recently developed unified cyclic viscoplastic material model is implemented to characterize the complex cyclic and relaxation plasticity response, including cyclic softening and kinematic hardening effects. The measured response is compared to that of P91 steel, a current power plant material, and shows enhanced cyclic strength relative to P91.

This paper is concerned with the high temperature low cycle fatigue behaviour of a new nanostrengthened martensitic-ferritic steel, MarBN. A range of strain-controlled, low cycle fatigue tests are presented on MarBN at 600 °C and 650 °C, and compared with previously published data for a current state-of-the-art material, P91 steel, including microstructural analysis of the fracture mechanisms. A modified Chaboche damage law, incorporating Coffin-Manson life prediction, is implemented within a hyperbolic sine unified cyclic viscoplastic constitutive model. Calibration and validation of the model with respect to the effects of strain-rate and strain-range is performed based on an optimisation procedure for identification of the material parameters. The cyclic viscoplasticity model with damage successfully predicts fatigue damage evolution and life in the cyclically-softening materials, MarBN and P91.

The changing face of fossil fuel power generation is such that next generation plants must be capable of operating under (i) flexible conditions to accommodate renewal sources of energy and (ii) higher steam pressures and temperatures to improve plant efficiency. These changes result in increased creep and fatigue degradation of plant components. The key limiting factor to achieving more efficient, flexible plant operation is the development of advanced materials capable of operating under such conditions. MarBN is a new precipitate strengthened 9Cr martensitic steel, with added boron and tungsten, designed to provide enhanced creep strength and precipitate stability at high temperature. Accurate characterisation of this material is necessary so that it can be used under flexible plant operating conditions with high temperature fatigue. This paper presents a combined work program of experimental testing and computational modelling on a cast MarBN material. To characterise and assess the fatigue performance of MarBN, an experimental program of high temperature low cycle fatigue (HTLCF) tests is conducted at a temperature of 600 °C. MarBN is found to give an increased stress range compared to previous P91 steel experiments, as well as considerable cyclic softening. To characterise the constitutive behaviour of the cast MarBN material, a recently developed unified cyclic viscoplastic material model is calibrated and validated across a range of strain-rates and strain-ranges, with good correlation achieved with the measured data throughout.

The presence of inclusions is a known source of crack initiation and component failure in cast materials. In this work, the role of inclusions is investigated via a combined program of high‐temperature low‐cycle fatigue testing and computational modelling of a tempered martensitic steel, MarBN. Microstructural analysis has shown that manufacturing‐induced oxide inclusions are a key source of fatigue crack initiation. A fully coupled, critical plane life prediction and damage model is implemented in a unified cyclic viscoplastic user‐material subroutine and applied to predict damage and micro‐crack initiation for inclusions. It is deduced that more careful control of the development of inclusions in the manufacturing process will provide enhanced material and component performance for highly flexible and ultrasupercritical plant conditions.

Manufacturing-induced defects are a key source of crack initiation and component failure under high temperature cyclic loading. In this work, 3D X-ray micro-computed tomography and microstructural analysis of manufacturing-induced defects is presented for forged and cast MarBN martensitic-ferritic steel, along with high temperature, low cycle fatigue testing, for assessment of the comparative effects of two manufacturing processes.Forging is found to significantly reduce the volume fraction and complexity of manufacturing defects, compared to the cast material, and as a result, approximately double fatigue life. A voxel-based finite element methodology for experimentallyidentified cast and forged manufacturing defects is presented, in conjunction with a multiaxial, critical-plane damage model, within a unified viscoplastic user-material subroutine. The effect of the complex morphologies of the manufacturing defects on high 2 temperature fatigue crack initiation is thus quantified, highlighting the relative effects of the two different manufacturing processes.