Analysis of the Low Cycle Fatigue Behavior of DP980 Steel Gas Metal Arc Welded Joints

Abstract

Dual phase (DP) steels have high strength, while maintaining outstanding elongation capacities. This is possible using a well-controlled thermomechanical process that produces a perfect phase combination in the DP microstructures. However, automotive makers are required to weld the DP steels, which generates a soft zone in the microstructure. In this work, 1.6 mm-thick DP980 steel sheets were welded by gas metal arc welding process to analyze the response of the welded soft zone to cyclic loading conditions. Conducted macrographic and metallography analyses revealed good quality in the appearance of the welded joints, with a complete fusion of the DP980 joint and without the presence of discontinuities. Low cycle fatigue tests of the DP welded joints were conducted under a constant amplitude strain control mode. The welded joints experienced a fatigue life reduction with respect to the DP980 steel of ~16% at strain amplitudes of 0.2, 0.3, and 0.4%. For strain amplitudes larger than 0.6%, the fatigue life of the welded joint was reduced by 39%. Weld thermal cycles combined with metallography analysis indicated that a tempered process of the martensite during the welding was responsible for the soft-zone formation and the poor fatigue response.

Residual Stress Relaxation in the Laser Welded Structure after Low-Cycle Fatigue and Fatigue Life: Numerical Analysis and Neutron Diffraction Experiment

“…Microstructures consisting of a ferrite matrix and dispersed martensite islands are produced by the well-controlled thermo-mechanical process. Strength can be increased by adequately regulating the content, morphology, and distribution of martensite, while ductility can also be achieved, which meets the needs of engineering applications [23]. DP steel exhibits lower σ y (370 ± 40 Mpa), but the continuous yield behaviour contributes to larger elongation (23 ± 1%) and higher tensile strength (654 ± 14 Mpa) [24].…”

Influence of Overheating on High-Cycle Fatigue Characteristics of the Base Metal and Weld Metal of Low-Carbon Steel Welded Joints

“…This is especially true for CC vessel construction conducted in areas with increased seismic hazards. The fatigue characteristics of lowcarbon steels have been extensively studied [14][15][16][17][18][19][20][21]. In particular, the mechanisms of fatigue crack propagation [22], the relationship between the fatigue groove spacing and the fatigue crack propagation rate [23], and the effect of the chemical composition [24] have been studied.…”

“…In the case of a welded joint, another influencing factor appears, namely the welding method. In addition, researchers use different approaches to assess fatigue strength [14,18,25]. Therefore, designers often require fatigue test data for a particular material that take into account its operating conditions.…”

The Influence of Two-Jet Gas Shielding Parameters on the Structure and Microhardness of Steel 45 Joints during Consumable Electrode Welding

“…Reducing the time of the drop's molten metal and the weld pool being in the liquid phase and, at the same time, increasing the rate of their mixing can be achieved by using pulsed or dynamic effects, such as programmable controlled transfer of the electrode metal into the weld pool [6][7][8], controlled dynamic effect of the shielding gas jet on the processes in the welding zone [7,[9][10][11][12][13][14][15][16][17], etc.…”

“…The authors of works [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] researched the effect of changing shielding gas flows on the consumable electrode welding process and noted that an increase in the gas outflow rate improves the quality of the welding zone shielding, the formation of the weld, and the stability of the welding process.…”