The influence of the cooling time t 8/5 has been examined as part of the ongoing AiF-FOSTA P1020 research project for the development of a new design approach for welded joints on highstrength steels. Four thermomechanically rolled and quenched steel grades were investigated. The temperature-time course of a MAG welding process was thermo-physically simulated in a quenching and deformation dilatometer to evaluate the material behaviour. Subsequently, the influence of the cooling time on the mechanical properties of welds was examined using a flat tensile test specimen with a centric hole. The results of the examinations form the scientific basis for a significant improvement of the current execution rules for welded joints between high-strength steels taking into account mismatching of base material and filler metal.

Im AiF‐FOSTA‐Forschungsprojekt P1020 [1] wurde ein neues Bemessungsmodell für Schweißverbindungen entwickelt. In diesem Zusammenhang wurde ein Kleinteilversuch an Flachzugproben konzipiert, mit dessen Hilfe verschiedene Einflüsse auf die Festigkeit und das Verformungsvermögen der Nähte untersucht wurden. Hierzu gehören solche aus dem Grund‐ und Schweißzusatzwerkstoff, der Abkühlgeschwindigkeit und der Anzahl der Schweißlagen. Im folgenden Beitrag werden das Bemessungsmodell und dessen Eingangsparameter beschrieben. Nach einer Charakterisierung der untersuchten Stähle und Schweißzusatzwerkstoffe wird der Kleinteilversuch zur Bestimmung der mechanischen Eigenschaften der Schweißnähte erläutert. Im Anschluss folgen Parameterstudien, im Rahmen derer wesentliche Einflussgrößen auf die Eigenschaften von Schweißnähten untersucht werden. Auf Basis statistischer Auswertungen werden Schweißnahtfestigkeiten in Abhängigkeit vom Zusatzwerkstoff und der Abkühlzeit t8/5 angegeben.

In the AiF‐FOSTA research project P1020 [1], a new design model for welded joints was developed. In this context, a small scale test on flat tensile specimens was designed, with the help of which various influences on the strength and ductility of the welds were investigated. Furthermore, extensive tests were carried out on overlap joints, cruciform joints with double fillet welds as well as partially and fully penetrated butt joints. This was done to calibrate the weld construction factor αw, which takes into account the influence of the type of joint on the load‐bearing capacity. In the following article, after a short summary of the current state of research, the design model, and the results from the parameter studies on flat tensile tests are described in more detail in [2–4]. Subsequently, the test program on welded joints and the calibration of the weld construction factor αw are presented. Finally, the results of the design model and the tests carried out are compared with test results from other research projects and the design model of prEN 1993‐1‐8 [5].

The changes in mechanical properties due to local energy input are important for welded joints. This affects both the base material in the area of the heat-affected zone and the resulting weld seam. Extensive investigations on the influence of the cooling time t8/5, which is representative of the energy input, were carried out on a newly developed test specimen for determining the mechanical properties of welds. From these investigations, functional relationships between the strength class of the filler metal, the t8/5 time and the mechanical properties of the weld seam were derived. In order to evaluate the influence of multi-layer welding, hardness measurements were carried out on individual weld beads at different levels of the welded joint. Furthermore, the relationship between the local hardness values and the integrally determined tensile strength was investigated.

Bild 4 Bemessungswerte der Streckgrenze für Hohlprofilknoten aus normal-und hochfesten Stahlsorten nach prEN 1993-1-8 [7]Design value of yield strength for hollow section joints made of normal and high-strength steel grades acc. to