Dependence of manufacturing parameters on the performance quality of welded joints made of direct quenched ultra-high-strength steel
Skriko, Tuomas (2018-09-29)
Lappeenranta University of Technology
Acta Universitatis Lappeenrantaensis
School of Energy Systems
School of Energy Systems, Konetekniikka
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The use of ultra-high-strength steels (UHSS) has increased in the engineering industry during past decades especially in the fields of welded structural applications where products are required to possess a specific combination of high load-carrying capacity, structural durability and energy efficiency. However, current material standards and welding codes or recommendations do not fully apply UHSS grades and recognize the special characteristics of direct quenched UHSS, which set substantial demands for design and manufacturing. This thesis discusses the performance quality of direct quenched UHSS weld joints, which determine the functional quality of components, structures, and eventually, the final product. The aim is to concretize the essential factors of performance quality and establish a method to recognize the quality level of a UHSS weld joint in terms of static and fatigue strength and deformation capacity. The research comprises a relationship between workshop manufacturing operations and the final properties of direct quenched UHSS weldments by means of theoretical review, experimental testing and finite element analyses. The results of the research show the effect of geometry, microstructure and residual stresses on the performance quality of welded joints made of direct quenched UHSS. Furthermore, appropriate manufacturing parameters are found to result in higher performance quality compared to available standards and recommendations. Future research is needed to develop analysing tools for UHSS weldments with high performance quality and to extend the perspective of the performance quality of a welded joint to larger entities, such as components, structures and products.
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