| Effective resistance is a key parameter of hydraulic support of coal mine, but with low strength steel to produce the big resistance of hydraulic support makes the product is overweight, not easy to loading, unloading and transportation. So constantly improve the strength of steel is an inevitable trend in the production of hydraulic support. However raising high strength low alloy steels strength may reduce welding reliability and cause crack, softening, brittle fracture and other issues. In this paper, the high strength low alloy steels Q690/Q890used in hydraulic support of coal welded joints by GMAW were the research object. The strong toughness of the high strength low alloy steels Q690/Q890welded joints with different welding parameters were investigated. The effect of the microstructure on the mechanical behaviors and the cracking sensitivity of the high strength low alloy steels Q690/Q890welded joints was analyzed, in order to obtain the best welding parameters and wire chemical composition.The results of straight Y-groove cracking tests of the high strength low alloy steels Q690/Q890butt joints indicated that the crack usually appeared in the root fusion zone of high strength steels Q890side. And with the increase of welding heat input,the cracking ratio of joints increased. The results of cracking ratio were under10%without preheating process no matter used the MK-GHS-60or MK·GHS-70wire in the welding heat input range of12-17kJ/cm. But with the welding heat input increasing, the increase of cracking ratio was small when used MK-GHS-70wire.The microstructure with different welding parameters and wires and the crack propagation in Q690/Q890welded joints were studied by means of metallographic microscope and scanning electron microscopy. And the relation of welding parameters, welding wire composition, microstructure and cracking resistance was established. After initiating, the cracks propagated along the fusion zone. The paths of cracks propagation were twists and turns. After a distance,the cracks propagation stopped or turned into weld stopped. The branches which appeared during the cracks propagation could turn into weld along brittle phase such as proeutectoid ferrite. The cracking resistance of acicular ferrite in weld was very good. And the inclusions in weld promoted heterogeneous nucleation of acicular ferrite. But if the size of inclusions was too large, it would be reduce the cracking resistance of joint. When MK-GHS-60wires was used the content of proeutectoid ferrite and ferrite side plate increased with the increasing of welding heat input. If the welding heat input was continue to increase, blocky ferrite would be find. Ni element that was added into MK-GHS-70wires limits the proeutectoid ferrite and ferrite side plate transformation, which increases the high temperature stability of the austenite. So the content of acicular ferrite in weld is very high at a large welding heat input when MK-GHS-70wires was used. The increase in welding heat input made the HAZ grain size of Q690/Q890joint increased, the content of bainite tissues in the heat affected zone increased, the size of bainite lath and grain bainite island also gradually became thick. With the increasing of welding heat input carbide precipitation in tempering zone increased, which made the degree "soften" of joint more worse. The match of strong and toughness of Q690/Q890multi-layer and multi-pass welding joint with different welding wires was investigated. This composite welding technology which combined MK-GHS-60with MK-GHS-70wires own an advantage of higher impact toughness and less cost. The strength and toughness of weld related to the content of acicular ferrite in weld. With the content of acicular ferrite in weld increasing, the strong and toughness of welded joint became better. The joint had a good strength and toughness when the microstructure in Q890side changed from the mixture of in heat affected zone side to AF+BL in weld side. |
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