Characterisation of metallurgical and mechanical properties of friction stir welded low alloy steels for marine applications

Student thesis: Doctoral Thesis

Abstract

Friction stir welding of steel presents numerous advantages across many industrial sectors compared to conventional fusion welding techniques. However, the fundamental knowledge of the process on steel remains relatively limited, hence industrial uptake of friction stir welding is practically non-existent. This thesis report on a large-scale microstructure and property evaluation of friction stir welded low alloy steel grade DH36 plate, commonly used in marine applications. The extensive study examined butt welds produced by a wide range of process parameters through microstructural characterisation, transverse tensile testing, Charpy impact testing and hardness measurements. A preliminary process parameter envelope has been developed and parameter sets established that deliver high integrity welds and enhance the process’s techno-economic competitiveness through a step change increase in the conventionally recognised welding traverse speed.In parallel, a comprehensive fatigue performance assessment of friction stir welded DH36 steel has been implemented. Original experimental procedures specific to friction stir welding have been put forward and the consequent exhaustive study examined the weld microstructure and hardness distribution in support of tensile and fatigue testing, also accounting for the effect of varying welding parameters. Steel friction stir welds exhibit fatigue lives well above the relevant international recommendations for fusion welding, irrespective of minor surface breaking flaws which have been identified. A detailed fracture surface analysis has concluded that surface breaking irregularities such as theses induced by the tool shoulder’s features on the weld top surface can be the dominant factor for crack initiation under fatigue loading.Friction stir welding is a solid state thermo-mechanical deformation process from which the plasticisation behaviour of the stirred material can be evaluated through the study of flow stress evolution. Novel flow stress data on DH36 steel were generated over a range of strain rates and temperatures by hot compression testing; these data and the subsequent innovative metallurgical examination of the tested samples will contribute to the scientific understanding of the process. The evolution of flow stress is found to be considerably affected by the test temperature and deformation rate; this relation has led to a number of significant observations which will inform future refinement of process parameters.
Date of Award1 Oct 2015
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorAlexander Galloway (Supervisor) & James Wood (Supervisor)

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