Structural characterisation, residual stress determination and degree of sensitisation of duplex stainless steel welds
Creators: name: Gideon Abdullah, Mohammed
Publisher: name: RMIT University
Year: 2024

description: Welding of duplex stainless steel pipeline material for the oil and gas industry is now common practice. To date, research has been conducted primarily on the parent material and heat affected zones in terms of its susceptibility to various forms of corrosion. However, there has been little research conducted on the degree of sensitisation of the various successive weld layers, namely the root, fill and cap layers. <br><br>The focus of this research study was to: (i) provide an in-depth microstructural analysis of the various weld passes, (ii) study the mechanical properties of the weld regions; (iii) determine degree of sensitisation of the various weld passes; and (iv) investigate the residual stress levels within the various regions/ phases of the welds. <br><br>Four test conditions were prepared using manual Gas Tungsten Arc Welding with 'V' and 'U' bevel configuration. Structural analysis consisted of (i) optical microscopy, scanning electron microscopy and magnetic force microscopy; (ii) ferrite determination using Magna-Gauge, Fischer Ferrite-scope and Point Count method. Mechanical testing consisted of Vickers hardness measurements, Charpy impact studies and transverse tensile testing. The degree of sensitisation was determined by three test methods: a modified ASTM A262, ASTM A923 and a modified Double Loop Electrochemical Potentiodynamic Reactivation (DL-EPR) test. Residual stress levels were determined using two neutron diffraction techniques: a reactor source and a time of flight spallation source. <br><br>Microstructure observed by optical microscopy and magnetic force microscopy shows the formation of both fine and coarse structures within the weld metal. There was no evidence of secondary austenite, being present in any of the weld metal conditions examined. In addition, no detrimental intermetallic phases or carbides were present. The DL-EPR test results revealed that the fill layer regions for all four conditions and the base material showed the highest values for Ir/Ia and Qr/Qa. All four test conditions passed the ASTM A262 and A923 qualitative type tests, even under restricted and modified conditions. <br><br>Residual stress measurements by neutron diffraction conducted at Lucas Heights Hi-Flux Reactor revealed that the ferrite phase stress was tensile in the heat affected zones and weld, and appeared to be balanced by a local compressive austenite phase stresses in the normal and transverse directions. <br><br>Residual stress measurements by neutron diffraction conducted at Los Alamos Nuclear Science Centre revealed that in the hoop direction, ferrite (211) and austenite (311) exhibit tensile strains in the weld. In the axial and radial direction, the strains for both phases were more compressive. <br><br>Correlations between the degree of sensitization and microstructural changes / ferrite content were observed. Higher degrees of sensitization (Ir/Ia and Qr/Qa) were associated with reduced ferrite (increased austenite) content. Correlations between the stresses generated, the evolved microstructures and degree of sensitization were evident. Stresses within the cap region were generally shown to be of a tensile nature in the transverse and longitudinal direction. In summary, the study has shown that correlations exist between the weld microstructure, susceptibility to sensitisation and levels / distribution of internal stresses within the weld regions.<br>