Cyclic plasticity and low cycle fatigue damage characterisation of thermally simulated X100Q heat affected zone

Ronan J Devaney,Heiner Oesterlin,Sean B Leen,Padraic E O’Donoghue,G Hénaff

doi:10.1051/matecconf/201816503002

Abstract

This paper presents the cyclic plasticity and low cycle fatigue (LCF) damage characterisation of thermally simulated heat affected zone (HAZ) for API 5L X100Q weldments. Microstructures representative of the HAZ for two cooling rates are generated using a Gleeble thermomechanical simulator for manufacture of strain-controlled cyclic plasticity test specimens. The simulated HAZ specimens are subjected to a strain controlled test programme which examines the cyclic effects of strain-range and the tensile response at room temperature. A modified version of the Chaboche rate independent plasticity model, which accounts for early stage damage is implemented to characterise the cyclic plasticity response, including isotropic and kinematic hardening effects. The constitutive parameters are fitted to experimental data using an optimisation procedure developed within a MATLAB code. The measured response of the simulated HAZ specimens is compared to that of the X100Q parent material (PM), and the simulated HAZ is shown to share the early stage fatigue damage behaviour of the PM, but exhibits significantly a higher yield and cyclic strength.

Highlights

Steel catenary risers (SCRs) are pipelines used to conduct hydrocarbons or injection fluids between a production facility and a subsea wellhead
On either side of the homogeneous heat affected zone (HAZ) region, an interface zone (IF) exists in which there is a gradation in microstructure from those of the HAZ to those of the parent material (PM)
This requires further investigation, for example, using the through-process modelling methodology [6] under development within the present Science Foundation Ireland (SFI) project “Multiscale, through-process characterisation for nextgeneration welded connections” (MECHANNNICS), in order to quantitatively assess the effects of the gradation in microstructure in the IF zone

Summary

Introduction

Steel catenary risers (SCRs) are pipelines used to conduct hydrocarbons or injection fluids between a production facility and a subsea wellhead. SCRs are subjected to heave caused by floating production facilities, vortex induced vibrations and cyclic pressurisation, making fatigue performance a primary concern. Weight reduction is a key requirement for deep and ultra-deepwater SCRs. The use of generation high strength steels facilitates a reduction in riser weight and installation time through the use of lower wall thickness pipe sections. Welded connections are of particular concern for fatigue failure in SCRs [1]. The thermal cycles experienced during the welding process alter the microstructure and the constitutive and fatigue performance of the PM adjacent to the weld. The material and geometrical mismatch resultant from the welding process causes the welded connection to act as a fatigue hot-spot [2]

Methods

Results

Discussion

Conclusion