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

Abstract

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.

Experimental and computational characterization of the effect of manufacturing-induced defects on high temperature, low-cycle fatigue for MarBN

“…Maruyama et al [49] have shown that tungsten also reduces the coarsening rate of M23C6 carbides, enhancing the effect of precipitate strengthening. Previous analysis of M23C6 carbide growth by O'Hara et al [33] indicated no significant change in carbide diameter as a result of HTLCF loading. However, the number of particles was found to almost triple.…”

“…Figure 4(c). Thus, the high initial yield strength of MarBN, prior to cyclic softening, has been attributed to thermally stable, boron-enriched M23C6 carbides along boundaries (Figure 5), providing barriers to dislocation motion, combined with enhanced tungsten solid solution strengthening and high dislocation density [27,33,47,48]. This is a key benefit of MarBN for application to higher temperatures and pressures, compared to current materials in steam header applications, and is particularly encouraging in terms of the potential for a rolled MarBN equivalent.…”

“…Thermal calibration of the oven was previously performed for MarBN up to 700 °C at NUI Galway using high temperature thermocouples [30]. A more detailed description of HTLCF and creep-fatigue (CF) testing for the present cast MarBN has previously been presented for both 600 °C [31,32] and 650 °C [33]; the latter also provides details on calibration and validation of material parameters for unified cyclic viscoplastic modelling described below.…”

“…Furthermore, in order to computationally characterise the expected detrimental effects of complex-morphology defects on fatigue damage using threedimensional modelling, a new multiaxial, critical-plane life prediction methodology is incorporated here within the latter user-material subroutine. Previous work by the authors examined the effects of idealised spherical manufacturing defects [33]. However, in this paper, attention is focussed on the key role of the complex geometry of manufacturing defects, as measured experimentally.…”

A miniaturized thin‐plate low cycle fatigue test method at elevated temperature

“…The cyclic softening behavior is also essential for the isotropic parameter identification (Q and b in Equation 8) [1][2][3] in Chaboche UVP model and damage parameters. 62 Thus, the measured peak stress evolution versus fatigue life (N) subjected to the sawtooth and dwell-type loadings is given in Figure 12. Similarly, significant cyclic softening is observed in all the MTP fatigue tests with different magnitude levels and rates, as the primary mechanism of cyclic softening in the investigated material is low angle boundary dislocation annihilation.…”