Effect of microstructure evolution on the creep properties of a polycrystalline 316H austenitic stainless steel

Abstract

Creep deformation and failure is one of the most critical life limiting factors of structural components used at elevated temperatures, such as in nuclear power plants. Understanding of the mechanisms of creep in nuclear power plant steels, such as Type 316H austenitic stainless steels, is still incomplete. It has been observed that long-term creep curves of initially solution-treated (ST) 316H stainless steels exhibit multiple secondary stages at the operational temperature and stress range. This paper probes the internal mechanisms for this complex phenomenon by correlating and quantifying the evolution of microstructural state (dislocations, precipitation and solid solution elements) and its mechanistic influence on the material's creep properties. This is examined for the first time by a multi-scale self-consistent crystal plasticity framework combined with a simple classical phase transformation model and thermal solute strengthening model. The novel integrated model is capable of describing a broad range of physical processes, including dislocation multiplication (hardening) and climb-controlled recovery, precipitation nucleation, growth and coarsening (Ostwald Ripening) and thermal solute dragging. The mechanisms responsible for the observed multiple secondary stages in the creep curves of initially solution-treated 316H stainless steels are explained through the strengthening and softening effects associated with these processes.

Comparison of self-consistent and crystal plasticity FE approaches for modelling the high-temperature deformation of 316H austenitic stainless steel

“…Recently, Hu and Cocks [3,12] developed a CP model for the inelastic behaviour of FCC alloys, which takes into account the role of evolving precipitate structure and solute distribution, as well as dislocation structure on the crystal constitutive response. They employed this model in the selfconsistent framework of Budiansky and Wu [13] to determine the macroscopic response and development of internal residual stress state in a polycrystal containing grains with a random array of orientations.…”

“…The model developed in [18,23] was successfully employed to describe elastic-plastic monotonic, creep and stress relaxation in a range of polycrystalline materials, particularly austenitic stainless steels [9]. The CP self-consistent model developed by Hu and Cocks [3,12,14] and extended by Petkov et al [4] uses a physically-richer mechanistic deformation model compared with [9] to better capture the monotonic, creep, relaxation and cyclic deformation behavior of austenitic 316H stainless steel through the evolution of the dislocation structure within the grains.…”

“…The model from [41] describes the evolution of several types of dislocation structures, but adds complexities due to the large number of free parameters. The model established in [40], which is a physically-richer version of the dislocation-density based models developed by Kocks and co-workers [42,43] applied within a crystal plasticity framework, has provided the physical basis and motivation for development of the multi-scale dislocation network based model developed by Hu and Cocks [3,14] within their self-consistent modelling framework (see Section 2). The Hu and Cocks micromechanical model, in which the state variable is the mean spacing between obstacles to dislocation motion, has been extended recently by Petkov et al [4] and is also employed in the present study.…”

“…Another major objective of this study is to verify further the validity of the self-consistent models reviewed in Section 2. This will be achieved comparing the model predictions using the SCM [3,14], with those obtained from the crystal plasticity finite element method described in Section 3 and results from experimental studies.…”

“…At elevated temperatures, the life-limiting issues become the time-dependent inelastic deformation of components, such as creep, together with subsequent creep fracture due to steady accumulation of damage. Difficulties arise in predicting the deformation behaviour of the alloy at elevated temperatures during complex loading histories due to the heterogeneities in the intragranular residual stress field [2], the evolution of microstructural phases [3] and the way in which these influence the dislocation-obstacle interactions [4]. There is an extensive literature on the development of empirical and phenomenological models for creep deformation based on continuum plasticity theory [5,6].…”