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|Thermomechanical factors influencing weld metal hydrogen assisted cold cracking
|Kurji, Rahim Nazmudin
|School of Mechanical Engineering
|Hydrogen Assisted Cold Cracking is a phenomena that manifests itself in weldments when a critical hydrogen concentration is trapped within a susceptible microstructure and subjected to a threshold level of stress and poses a significant threat to pipeline girth weld integrity, in particular the root pass of girth welds. This thesis explores the thermomechanical factors influencing the formation of Weld Metal Hydrogen Assisted Cold Cracks (WMHACC) in High Strength Low Alloy line pipe steel when welded with cellulosic electrodes using the Manual Metal Arc Welding process. The overarching objective of this body of work is to delineate a safe boundary across which the characteristics of weld metal samples can be defined to enhance the understanding of the factors which influence the formation of weld metal hydrogen cracks. The significance of achieving this objective is the improved ability to predict the onset of the phenomenon, consequently facilitating the development of strategies, which can be assimilated by industry to minimise the presence of cold cracks increasing the safety and reliably of pipeline girth welds. To address the overarching objective and address the limitations identified though the literature review, in particular the limited number of studies focusing on welding in the Australian context, a three-part experimental and analytical program was developed and undertaken. Development of an Enhanced Weldability Test. To facilitate deposition of tests welds under high restraint in a range of heat inputs which reflected the desired test window, an enhanced weldability test, the MWIC, was designed and commissioned. The test’s geometry and characteristics were based on the well-established Welding Institute of Canada Weldability test, but was enhanced to allow for deposition of welds at very low heat inputs, of which no published cracking data exits. Additionally, the test was modified to allow for the extraction of critical welding data, facilitating its use as a research tool. Delineation of Safe Welding Envelope. Using the MWIC test, deposition envelopes were created for thick (20mm) and thin (10mm) sections of API 5L X70 line pipe steel welded with E6010 electrodes under high and low restraint. Welds were deposited at low heat inputs (<1kJ/mm) over a range of preheats and examined under magnification to establish the critical/ threshold preheats above which no cracking was observed. Characterisation of weld metal samples across the derived cracking boundary. The weldability test samples generated from weldability testing were characterised using a range of macroscopic, microstructural, and micromechanical techniques to establish the interrelationship between cracking and thermomechanical parameters. The influence of heat input on bead eccentricity was established and its consequent effect on the welds susceptibly to hydrogen cracking was proposed. The potential interrelationship between solidification and hydrogen cracks was discussed in relation to restraint levels. The inefficiency of the currently accepted hardness threshold of 350HV used to predict the onset of HAZ-HACC as a proxy for Weld Metal Hydrogen Assisted Cold Cracking susceptibly was established for all test cases. Additionally, the use of micromechanical properties as a proxy, in particular the use of the ratio of E/H was introduced as a means of quantifying susceptibly to WMHACC.
|Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2017.
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