Session: MF-25-01 High Strength Steels for Pressure Vessels and Piping Applications

Paper Number: 101501

101501 - Validation of Deformation in Crystal Plasticity When Modelling 316H Stainless Steel for Use in Pressure Vessels 

The safety-critical nature of many high temperature/high pressure power plants means that understanding the behaviour of materials used within its key components remains a priority. Despite continuous development of design (ASMEIII and RCC-MR) and assessment (R5) codes for predicting creep damage, they continue to use estimates of deformation and damage, which while conservative have safety margins which are difficult to quantify. To further improve upon these codes, and incorporate more robust probabilistic methods, modelling the mesoscale interactions, and variability, which has been shown to significantly influence the deformation and damage distribution at the microstructural level, should be performed using length scale appropriate simulations such as the crystal plasticity finite element (CPFE) technique. CPFE allows for the efficient simulation of phenomena seen at the mesoscale such as strain localisation and micro-scale damage such as creep which are governing aspects in the materials micromechanical behaviour.

In this work to validate deformation predicted through a crystal plasticity model (based on the work of Kalidindi et al. [1] and Agius et al. [2]) experimental tensile stress relaxation tests with conditions representative of those seen in actual plants (3% strain, 550^oC) are performed on 316H stainless steel. To ensure the CPFE model captures the high temperature environment, a creep element is incorporated into the deformation using a method suggested by Agius et al. [2]. This method additively decomposes the flow rule into two power laws, one for plasticity and one for creep, allowing independent control of both physical processes. The actual microstructure from the test is examined to identify localised micro-mechanical deformation hot spots indicated by areas of high intra-granular elastic strain. These intra-granular elastic strain profiles can be quantified using cross correlation of high-resolution electron back scatter diffraction (HR-EBSD) patterns to a sensitivity of ±1x10^-4 strain. HR-EBSD also allows for kernel average misorientation and geometrically necessary dislocations to be estimated and used as a ‘measure’ of plastic deformation within the material. Both the elastic and plastic deformation observed is then related to relevant microstructural properties that are implicit in using crystal plasticity, such as grain boundary misorientation, local misorientations and grain geometry.  Crystal plasticity modelling is then used with a statistically robust representative volume element (RVE) under representative boundary conditions to those seen experimentally. The simulated local stress and strain distributions seen at an intragranular level are related to surrounding microstructural properties and these are compared with the experimentally measured distributions.

Critically, the deformation is linked to local stresses - the capability of the model to accurately predict these can improve our understanding of the accuracy of the material model and in doing so help improve CPFE’s efficacy in informing current design and assessment codes.

 

[1] Kalidindi, C. Bronkhorst, L. Anand, Crystallographic texture evolution in bulk deformation processing of fcc metals, Journal of the Mechanics and Physics of Solids 40 (3) (1992) 537–569.
doi: https://doi.org/10.1016/0022-5096(92)80003-9

[2] D. Agius, A. A. Mamun, C. A. Simpson, C. Truman, Y. Wang, M. Mostafavi, D. Knowles,
Microstructure-informed, predictive crystal plasticity finite element model of fatigue-dwells, Computational Materials Science 183 (2020) 109823. doi: https://doi.org/10.1016/j.commatsci.2020.109823.

 

Presenting Author: Edward Horton University of Bristol

Presenting Author Biography: I am a PhD student at the University of Bristol in the Solid Mechanics Research Group. My area of interest is developing and using a crystal plasticity finite element model created in house.
I am currently validating the deformation in this model to strains comparable to those seen in high temperature power plants.

Authors:

Edward W. Horton University of Bristol
Julio C. Spadotto University of Manchester
Albert D. Smith University of Manchester
Jack M. Donoghue University of Manchester
David Knowles University of Bristol
Brian Connolly University of Manchester
Ed J. Pickering University of Manchester
Mahmoud Mostafavi University of Bristol