Mismatch effect on fatigue crack propagation limit curves of S690QL, S960QL and S960TM type base materials and their gas metal arc welded joints

Nowadays, one of the basic trends in the industry is the environmental impact reduction, in other words the weight decreasing of structural elements and structures, which can be approached by applying different high strength steels. In case of different steel structures, the main manufacturing and joining technology is the welding, the conventional and advanced methods of fusion and pressure welding processes. Beside the weight decreasing, the reliability and safety requirements according to steel structures have significant grown. During the welding process, the welded parts are affected with heat-effect and mechanical loads, which result in inhomogeneous welded joint. The inhomogeneity of the welded joints appears both in microstructural (local) and in geometrical (both local and global) aspects. The changes in microstructure and geometry appear in deflections (basically acceptable), or rather in failures (basically unacceptable); and these influence both the behaviour and the loadability of welded joints. Discontinuities in base materials and their welded joints have especially high danger in case of cyclic loading conditions, which are typical for different structures and structural elements (e.g. bridges, vehicles). There are different standards and prescriptions containing fatigue crack propagation limit curves and rules for the prediction of the crack growth; simple and two-stage crack growth relationships can be found in the literature, most frequently based on the Paris-Erdogan law. The paper summarizes and presents the results according to our fatigue crack growth investigations on Weldox 700E and Weldox 960E quenched and tempered (Q+T) and on Alform 960M thermomechanically treated (TM) high strength steel base materials and their gas metal arc welded joints. The mismatch effect has also been studied; matched, overmatched, undermatched and matched/overmatched (mixed-matched) welded joints were investigated. 15 mm thick plates were used for the investigations, statistical aspects were applied both for presenting the possible crack locations in the real plates, as well as for processing the measured data. Furthermore, the results will be compared with each other, and fatigue crack propagation limit curves will be derived using simple crack growth relationship.

Mismatch effect on fatigue crack propagation limit curves of GMAW joints made of S960QL and S960TM type base materials

Welded structures cannot be produced without imperfections, cracks or crack like defects. Among the structural steels, 960 MPa strength category represents a reliable application possibility. Consumables are also available, but the behaviour of mismatch types under cyclic loading condition is not yet clear. In order to know the fatigue crack propagation resistance of 960 MPa strength category steels and their gas metal arc welded joints fatigue crack growth tests were performed. The tests results were analysed and fatigue crack propagation limit curves were determined.

Fatigue Crack Propagation Limit Curves for S690QL and S960M High Strength Steels and their Welded Joints

The objective of the paper is to present the newest results of our complex research work. In order to determination and comparison of the fatigue resistance, fatigue crack growth tests were performed on different grades of S690QL quenched and tempered, and S960TM thermomechanically rolled high strength steels.15 mmand30 mmthick base materials were used for our investigations. Welded joints were made from these base materials, using gas metal arc welding with matching, overmatching, and undermatching filler metals. In the paper, the performance of the welding experiments will be presented, especially with the difficulties of the filler material selection; along with the results of the fatigue crack growth examinations executed on the base materials and its welded joints. Statistical aspects were applied both for the presenting of the possible locations of the cracks in the base materials and the welded joints and for the processing of the measured data. Furthermore, the results will be compared with each other, and the possibility of derivation of fatigue crack propagation limit curves will be referred.