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Title: Developing material models for use in finite element predictions of residual stresses in ferritic steel welds
Author: O'Meara, Nicholas
ISNI:       0000 0004 6423 4084
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2016
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Nuclear reactor pressure vessels are constructed by welding low alloy steel forgings together. Welding processes can leave residual stresses which affect the RPV's resistance to fracture. Welding also induces microstructural changes and these changes have a number of associated consequences, including inelastic strains and altering mechanical properties. The extent to which these microstructural changes influence residual stress evolution during welding is not fully understood. The aim of this project is to characterise the microstructural and mechanical response of SA-508 Gr.3 Cl.1 pressure vessel steel to thermal cycles and develop representative models that can be used to determine how these effects influence stress predictions. There is insufficient materials data to inform the models used to predict how phase transformations influence residual stresses. Using the recently developed Gleeble thermo-mechanical simulator, previously unmeasured data characterising the response of the material to weld-like thermal cycles was generated. Variations in the kinetics of austenite formation and decomposition were investigated using dilatometry. It was found that when the steel is subjected to multiple thermal cycles that exceed the austenisation temperature, the behaviour during the first thermal cycle is different to that of subsequent cycles. In the subsequent thermal cycles, two observations were made: 1) the austenite formation rate increases on heating, and 2) for a given cooling rate, the austenite will decompose at lower temperatures into harder phases. It is explained how these changes in behaviour can affect the residual stress distribution in this thesis. Bainitic, austenitic and martensitic samples were generated. The stress-strain behaviour of these phases is presented and has been used to inform mechanical constitutive models. Finite element simulations of autogenous edge welded beams have shown how microstructural changes can affect the residual stress predictions. The extent of the transformed region of the HAZ and the yield stress of the material surrounding this region influences the location and magnitude of the peak tensile residual stress after a weld pass. Changes in mechanical properties induced by tempering bainitic and martensitic samples were quantified experimentally. The reductions in yield stress in bainite and martensite during short tempering heat treatments were found to be significant. A new approach to integrate the observed tempering behaviour into existing models is presented. The data and models presented in this thesis can provide guidance to structural integrity engineers and help produce more accurate and less conservative residual stress predictions for use in structural integrity assessments.
Supervisor: Withers, Philip Sponsor: Not available
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Residual stress ; Transformation plasticity ; Modelling ; SA508 steel ; Dilatometry ; Bainite ; Martensite