Study On Strength Mismatch Of Q550+Q690 Dissimilar Steels Joints

In order to enhance development of efficient and productive thick seams mining, minery posed high requirements to the design and fabrication of hydraulic support, especially the welding technology of high strength low alloy steels. The steels with higher strength often experiences welding cracking and low strength after welding. This problem may be difficult to overcome for steels with strength of 800 MPa during welding. Preheating treatment is often undertaken to prevent weldments from cracking, while the complex processes may increase cost. At the same time, appropriate mismatching can effectively reduce consumption of high strength steels and weld wires. Therefore, this paper aimed to investigate strength mismatching of welds without preheating treatment. This work with great theory and practical value will promote advancement of coal mine machinery and welding of high strength steels.Q550+Q690 high strength steels used in hydraulic support were welded without preheating by gas shielding arc welding process. The cracking sensitivity and strength mismatch were investigated. The propagation of cracking, effect of welding parameters and welding wires on microstructure and mechanical properties and strength lose of heat affected zone were analyzed.Straight Y-type groove test was conducted to compare the crack ratio of different matched welds and analyze effect of heat input on crack ratio. The results showed heat input should be controlled in the range of 12-18kJ/cm for overmatched or under matched welded joints. By the means of optical microscope and scanning electronic microscope(SEM) the formation and propagation of cracking were analyzed. Crack started in root fusion zone of Q690 steel side and then propagated in zigzag route. The dense inclusions in weld metal accelerated the propagation and can act as the origin of secondary crack.Results of tensile test and impact test revealed there was no big margin between the strength of undermatched ER50-6 weld joints and that of MK·G60-1. But the ductility and toughness of the latter were better than those of the former. The SEM fractography indicated that fracture model of ER50-6 weld metal was dimples+ cleavage while that of MK·G60-1 was dimples+quasi-cleavage. When welding wire was employed, too high heat input resulted in obvious cleavage step in radical zone of fusion zone of Q690 steel side. The river pattern appeared in large cleavage facet and the corresponding toughness was lower. Due to large and deep dimples the toughness of fusion zone of Q690 steel side of MK·G60-1 joints was higher than that of ER50-6.In weld metal of undermatched welded joints (MK·G60 and MK-G60-1) the alloying elements Mo, Cu, Cr and Ti can effectively suppressed the formation of primary ferrite and side plate ferrite while can promote the formation of acicular ferrite. The microstructure of overmatched MK·GHS70 weld metal consisted of main bainite, acicular ferrite and some primary ferrite.Nucleation of acicular ferrite was studied by SEM, transmission electron microscope (TEM) and energy dispersive spectrum (EDS). Results indicated that the inclusions composed of Ti, Mn, (Ti, Mn) oxides and Mn, Cu sulfides became nucleation sites of primary acicular ferrite. Size of inclusions was 0.4～0.8μm and space between them was over 3μm. Secondary acicular ferrites were nucleated on some pre-existing ferrite laths. The retained austenite between ferrite laths can promote crack resistance of weld metal.The microstructure and fractography were analyzed by SEM and TEM. Results showed that the wide-angle grain boundary between martensite and bainite can effectively prevent the rapid growth of crack and enhance the impact toughness of heat affected zone (HAZ). At the same time, this wide-angle grain boundary can become the obstacles of crack propagation and then resulted in cleaves fan-shaped patterns. The morphology, content, distribution and size of martensite-austenite constituents were greatly influenced by heat input. High heat input will cause coarse lath and large cleavage facet, even cleavage step. The corresponding toughness was low.