Microscale modelling of the deformation of a martensitic steel using the Voronoi Tessellation method

peer-reviewedThe deformation of a martensitic steel (P91) at the microscale is investigated using the finite element method. The approach takes into account the hierarchical grain-packet-block microstructure of the steel as determined experimentally by electron backscatter diffraction (EBSD). The orientation relationship for P91 between the prior austenite grain (PAG) and the martensitic packet/block is determined and found to be consistent with the Kurdjumow-Sachs (K-S) relationship. This relationship is incorporated within a finite-element model to represent the material microstructure, using a representative volume element (RVE) generated by a modified centroidal Voronoi tesselation (VT) approach. A non-linear, rate dependent, finite strain crystal plasticity model is used to simulate the mechanical response of the material at the micro- and macro-level and the sensitivity of the results to the model assumptions is investigated. It is found that the global (macro) mechanical response predicted by the RVE generated using the modified VT model is in good agreement with that predicted by an RVE taken directly from the measured EBSD microstructure. The influence of block/packet/grain boundaries on the local (micro) deformation is examined and it is found that the microscale prediction obtained using the RVE based on the modified VT microstructure, with an appropriate choice of microstructural parameters, is consistent with that obtained using the measured EBSD map

Micromechanical finite element modelling of thermo-mechanical fatigue for P91 steels

In this paper, the cyclic plasticity and fatigue crack initiation behaviour of a tempered martensite ferritic steel under thermo-mechanical fatigue conditions is examined by means of micromechanical finite element modelling. The crystal plasticity-based model explicitly reflects the microstructure of the material, measured by electronic backscatter diffraction. The predicted cyclic thermo-mechanical response agrees well with experiments under both in-phase and out-of-phase conditions. A thermo-mechanical fatigue indicator parameter, with stress triaxiality and temperature taken into account, is developed to predict fatigue crack initiation. In the fatigue crack initiation simulation, the out-of-phase thermo-mechanical response is identified to be more dangerous than in-phase response, which is consistent with experimental failure data. It is shown that the behaviour of thermo-mechanical fatigue can be effectively predicted at the microstructural level and this can lead to a more accurate assessment procedure for power plant components

The role of hardness on condition monitoring and lifing for high temperature power plant structural risk management

In this work, the use of hardness data in a novel predictive lifing model is explored. This study provides for the first time large amounts of site hardness data acquired during successive outages on an ageing coal fired power plant and draws conclusions regarding interpretation of these data in accordance with current practice, which is included in a case study. A novel, phenomenological relationship between room temperature hardness and creep data, obtained by uniaxial creep and impression creep tests, has been found and used for an innovative lifing approach that includes hardness data in a creep damage model. The latter is discussed with a description of how it could be practically implemented and validated in-service

Correction: Zheng et al. Correlating Prior Austenite Grain Microstructure, Microscale Deformation and Fracture of Ultra-High Strength Martensitic Steels. Metals 2021, 11, 1013

In the original article [...

Correlating Prior Austenite Grain Microstructure, Microscale Deformation and Fracture of Ultra-High Strength Martensitic Steels

Herein, we correlate the prior austenite grain (PAG) microstructure to deformation and fracture mechanisms of an ultra-high strength martensitic steel. To this end, a low-carbon martensitic steel is subjected to five heat-treatments and the PAG microstructure in the material is reconstructed from the EBSD inverse pole figure maps of the martensitic microstructure. The deformation and fracture response of all heat-treated materials are characterized by in situ tension tests of dog-bone and single-edge notch specimens that allow us to capture both the macroscopic mechanical response and the evolution of microscopic strains via microscale digital image correlation. The experimental results, together with microstructure-based finite element analysis, are then used to elucidate the effect of the PAG microstructure on the mechanical response of the material. Our results show that the interaction between the heterogeneous deformation fields induced by the notch and the bimodal PAG size distribution leads to an increase in the propensity of shear deformation and degradation in the fracture response of the material with increasing heat-treatment temperature and time. Our results also suggest that achieving a unform distribution of fine grains is an effective way to enhance both the strength and fracture properties of this class of materials