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Eingeladener Vortrag
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Today’s light weight design trends lead to a growing application of high-strength structural steels (yield strength ≥ 690 MPa). The mechanical properties of the weld and the component safety have to meet the increased requirements of these steel grades. However, high residual stresses in welded components are detrimental to their safety and integrity. Analyses concerning weld stresses in high-strength steels welded under component related restraint conditions revealed that heat control significantly affects global and local stresses. This occurs especially in highly restrained joints due to superimposing local and global stresses and may cause crack-critical stress-levels. In this study weld tests were performed with plates of high-strength steel in a special test facility. The experimental setup allowed transferring defined restraint conditions to the test welds similarly to real components. Temperature and reaction forces due to restraint were observed online while welding and cooling of multilayer-component MAG-welds. Mobile X-ray diffraction was used for local stress determination in the weld seam areas of the restrained specimens. It was found that interpass temperature has a major influence on the local and global welding forces and stresses. Thus, among the analysed results especially transverse residual stresses of the heat affected zone were strongly affected.
Beside quenched and tempered (QT) high strength steels advanced technologies in steel manufacturing provide steels produced by the thermo-mechanical controlled process (TMCP) with yield strength of 960 MPa. These steels differ in the carbon and micro-alloying element content. With variation of heat control TIG-welded dummy seams on both steel types were performed. Analyses concerning microstructure and residual stress evolution due to welding showed typical stress distributions according to common concepts. Yet, the TMCP-steel shows higher residual stresses than the QT-steel.
Today high-strength structural steels (yield strength ≥ 960 MPa) are increasingly applied. Therefore, weldments have to achieve equal strength. Yet, high residual stresses in those welds diminish the components safety. Especially high restraint intensities can lead to crack-critical stress-levels. A special 2-MN-test facility allowed online-measurements of global reaction forces under defined restraint conditions during welding and cooling of multilayer-component MAG-welds. Local residual stresses were measured via X-ray diffraction before and after relief of the restraint. Local and global stresses were highly affected by heat control.
Correlating welding reaction stresses and weld process conditons for high-strength steel S960QL
(2014)
As a result of current trends towards lightweight design, a growing amount of high-strength steels with yield strengths above 690 MPa is applied. In comparison to the weld process of lower-strength steels, small working ranges have to be achieved with respect to a special microstructure and high yield ratio. However, the sustainable and economic application of these steels depends on the loading capacity and the safety of welds when designing weld constructions. For these demands, a precise knowledge of welding stress level and distribution is essential. Therefore, the present study is concerned with the interaction between heat control (interpass temperature and heat input) and local as well as global stresses in high-strength steel welds. Specimens were multirun welded under defined restraint conditions in a special test facility (controlled tensile weldability (CTW) test) to consider global restraint. For the comparison concerning local residual stresses, free shrinkage test welds were performed as well. The evaluation shows a significant influence of the interpass temperature on the global reaction forces. Furthermore, increased heat input and high interpass temperatures cause higher tensile residual stresses. This occurred in the weld area of both free shrinkage test specimen and CTW test specimen.