Numerical and experimental investigation of the mesoscale fracture behaviour of quenched steels

Abstract

During heat treatment processes, especially during quenching, cracks may form because of the presence of high thermal and mechanical stresses and strains. Notwithstanding the fact that increasingly detailed modelling for heat treatment is being performed (considering, i.a., grain size, creep and transformation plasticity), homogeneous microstructures are still normally assumed. Chemical and hence structural inhomogeneities are not commonly explicitly considered, which is especially accentuated in the case of real parts simulation because of the resulting numerical problem's size. Intensive quenching on a cylindrical specimen of 100Cr6 (SAE) steel is proposed here to experimentally investigate the microcrack generation. A finite element based model is proposed to numerically evaluate the fracture behaviour in a two‐step simulation. First, by solving the quenching problem in direct correspondance with the experimental test performed, and second, by studying the mesoscale response taking into account the influence of second phase particles in a representative volume element based approach. The maximum principal stress criterion is used to trigger the fracture by means of the extended finite element method at the mesoscale. The trend to form cracks in the surface region, experimentally observed, has been well captured by the model. The influence of carbides sizes and content on the mesoscale fracture response has been numerically analysed as well. A good agreement has been reached between the simulations and the experimental results, exhibiting the potential of the introduced approach to be used as a failure prediction methodology.

Thermodynamically consistent multiscale formulation of a thermo-mechanical problem with phase transformations

“…An additive strain rate decomposition is widely assumed in the literature for modeling a thermo-elastic-plastic process with phase transformations within a small strains framework [28,29] ε =ε e +ε th +ε pt +ε p (8)…”

“…Various classification schemes, based on thermodynamics, microstructure or mechanism, have been discussed from a practical and fundamental point of view by Ågren [41]. For example, it is normal in the heat treating community [28,29,42,43] to split the transformations into two different groups: (i) the diffusive transformations, commonly assessed with the Johnson-Mehl-Avrami model [44,45], and (ii) the non-diffusive transformations, dealing with the martensitic transformation, following Koistinen-Marburger model [46].…”