| Hydraulic supports, which are the core facilities of fully mechanized coal mining equipments, advance toward a large-scale, intelligent and efficient stage to fulfill the market. In recent years the strength of the steels has been upgrading continuously to seek a balance between high safety performance and light-weighting of hydraulic supports. In this case, much effort must be put into solving the problems that are prone to appearing during the welding of high strength steels, such as cold crack, brittleness and softening of heat affected zone (HAZ) and so on. In this paper, Q890/Q960 high strength steels used in hydraulic supports were welded by gas shielding arc welding (GMAW). The cold cracks, microstructure, and matching for strength and toughness of welded joints were studied to reveal the impact of welding materials and welding parameters on the microstructure and mechanical properties of the joints.Straight Y-type groove test was conducted to analyze the effect of different welding wires and heat input on crack sensitivity of Q890/Q960 welded joints by GMAW. The results indicated that preheating was essential to weld Q890/Q960 high strength steels, when the MK-GHS-80 or MKGHS-90 welding wires were used. When the preheating temperature was between 100℃ and 150℃, welding heat input should be controlled in the range of 12kJ/cm to 20kJ/cm to make the crack ratio less than 20%. Fine acicular ferrite and bainite in the weld metal were beneficial to reduce the crack sensitivity.The fracture behavior of Q890/Q960 welded joints was studied via tensile test and Charpy V-notch impact test. Meanwhile the fracture appearance and microstructure of the weld metal were observed by means of optical microscope and scanning electron microscope. When single MK-GHS-80, MK-GHS-90, or combining welding wires were used, the failure location of tensile specimens was at the fusion zone or HAZ on the side of Q890 steel. The fracture appearance, with the characteristics of ductile fracture, was blade-shaped or cup and corn-shaped. The Charpy V-notch impact test results showed that the toughness of all the welded joints were good enough to apply. However the weld metal in the joints produced with single wire exhibited better toughness than that produced with combining wires. The fracture mechanism of the weld metal in impact specimens was dimple+quasi-cleavage fracture. In the weld metal of joints with single wire, microstructure with poor toughness, such as lath martensite and bainite, was separated evenly by acicular ferrite and lower bainite, which leaded to small quasi-cleavage unit and tortuous crack propagation path. While the quasi-cleavage unit in the weld metal of joints with combining wires was large because massive ferrite with poor toughness distributed more intensively.The microstructure of HAZ undergoing welding thermal cycle once was analyzed to reveal the influence of welding heat input on performance of HAZ. The results showed that the types of the microstructure on both sides of Q890 and Q960 steels were same. And the microstructure from fusion zone to base metal followed by lath martensite+bainiteâ†' lath martensite+bainite+ferrite+a few carbidesâ†'tempered sorbite+bainite+ferrite +carbides. When welding heat input increased, not only the grain in the coarse grained region grew seriously, but also the amount of granular bainite accrued. If welding heat input was in the range of 12kJ/cm to 20kJ/cm, the soft zone would be too narrow to reduce the overall performance of welded joints.The effect of multi-pass welding thermal cycle on the performance of HAZ was also investigated via metallurgical analysis. As for the HAZ of cap layer, the pre-heated effect coming form backing weld weakened hardening tendency of coarse grained region. At the same time to the HAZ of backing layer, the content of bainite increased because of the post-heated effect coming from cap weld. The metal in the coincidence region of two layers experienced solid phase transformation twice. The grains were refined in the region where the second peak temperature was higher than Ac3. When the second peak temperature was in the range of Tt to Ac3, the region formed more M-A constituents and carbides. Small welding heat input of cap weld helped to improve the mechanical properties of HAZ, which ranged from 6kJ/cm to 10kJ/cm. |
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