Objective As renewable, low-cost, and pollution-free energy, hydroelectric power has become crucial in the world energy structure due to increasing tensions with traditional nonrenewable energy. Hydropower stations in China have been rapidly developing in recent years. To meet the needs of hydropower stations, hydropower steel has gradually developed from 600 MPa and 800 MPa to 1000 MPa for a further reduction of the weight and wall thickness of pressure steel tubes, improvement in the welding performance of welded joints, and a reduction of the comprehensive cost of the project. Even with continuous improvements in high-strength steel, problems such as welding cracks, heat-affected zone softening, and welding pores are still prone to occur during the welding process, which impedes the popularization and application of high-strength steel. Traditional welding methods have a high welding heat input that result in a wider welding heat-affected zone and large residual stress after welding, which significantly diminishes the mechanical properties of the joints. However, the laser-MAG hybrid method is appropriate for high-strength steel welding due to low heat input, large weld depth-to-width ratio, minimal post-weld deformation, and low residual stress. Therefore, it is important to study the properties of laser-MAG hybrid welding joint of B950CF high-strength steel for its application in the field of hydropower.
Methods In this study, pulsed laser-MAG hybrid and laser-MAG hybrid welding are used for the butt welding of B950CF high-strength steel for better comprehensive mechanical properties. The welding equipment consisted of a TRUMPF LASER TruDisk 10002 fiber laser and a Fronius welding machine. The laser-guided hybrid welding method was implemented in this study. In addition, a Y-shaped groove was utilized, and the butt gap was 1.2 mm. The optimal process parameters for the welding methods were obtained through a single factor control variable method. The microstructure and fatigue fracture morphology of the welded joints was observed through a SEM QUANTA FEG250 scanning electron microscope. The hardness and tensile of the joints were tested by a Vickers hardness tester and a electronic universal tensile testing machine. The fatigue test was completed on a QGB-100 microcomputer controlled high-frequency fatigue testing machine. The stress ratio R(σmin/σmax) was 0.1, and the number of cycles was set to 10 7.
Results and Discussions This study indicates that both welding methods obtain joints with good forming performance. However, the addition of a pulsed laser results in the decrease of the width of weld seam and heat-affected zone (Fig. 4). Both welded joints consist of a weld zone, coarse-grain heat-affected zone, fine-grain heat-affected zone, and non-full transformation zone. The weld microstructure of the methods comprises lath martensite, bainite, and small quantities of residual austenite. Specifically, the coarse-grain zone microstructure is lath martensite, the fine-grain zone microstructure consists of fine martensite and bainite, and the microstructure of non-full transformation zone consists of bainite with small quantities of martensite. Using a pulsed laser, the line energy is minimal, the cooling speed is fast, the residence time of the weld and the heat-affected zone at high temperatures is short, and the growth of crystal grains is inhibited. Furthermore, the pulsed laser has a stirring effect on the molten pool, which increases the rate of heat dissipation of the metal in the molten pool. This creates a uniform molten pool temperature and further inhibits the growth of crystal grains, causing a finer crystal grain than that of the continuous laser, thereby improving the performance of the welded joint (Figs. 6 and 7). Additionally, the average fatigue S-N curve of pulsed laser-MAG hybrid welded joints is higher than that of laser-MAG hybrid welded joints, with a better fatigue life under all stress levels (Fig. 10). These advantages when utilizing the pulsed laser-MAG hybrid welding method occur because the fatigue source pores and the stress concentration around the pores are small, so the crack initiation life is increased. Further, there are more interlaced bainite laths in the weld zone, which hinders crack propagation and improves crack propagation life.
Conclusions In this study, the comparison of the microstructure and properties of the welded joints obtained by two welding methods provided the following interpretations. Compared with laser-MAG hybrid welding, pulsed laser-MAG hybrid welding has no clear difference in microstructure except for smaller crystal grains. The average hardness of the weld zone is 1.24 and 1.18 times higher than the base metal hardness (320 HV), with the highest value appearing in the fine-grain HAZ. The fracture positions of the tensile specimens for the two joints are in the base material. The average fatigue strength (Nf=10 7) of the pulsed laser-MAG hybrid welding joints is 311 MPa that is 22 MPa or approximately 7.6% higher than the laser-MAG hybrid welding joints (289 MPa). The small fatigue source pores and disorderly distribution of the weld zone's microstructure increase the fatigue crack initiation life and crack propagation life, respectively. These factors are the primary reason that the fatigue life of the smooth joint from pulsed laser-MAG hybrid welding of B950CF high-strength steel is higher than that of laser-MAG hybrid welding. In conclusion, it is observed that pulsed laser-MAG hybrid welding can improve the performance of the welded joint for B950CF high-strength steel.
