Investigation Of Microstructure, Properties And Fine Structure Of 800MPa High Strength Steel Joints By GMAW

High strength low alloy (HSLA) steels have been widely used in engineering machinery, energy, bridges, vehicles and vessels industries etc., due to their excellent strength and toughness, reasonable economy and high allowable design stresses. But the welding of high strength steel is complicated. Major problems may be encountered in joint during welding, including cold cracking, mismatching of toughness and strength, poor performances of fusion zone and heat affected zone (HAZ), and so on, which become limitations in extending the application of HSLA steel and improving production efficiency. In this paper, HSLA steels Q550 and Q690 used in hydraulic support of coal mine were welded by gas metal arc welding (GMAW) processes without pre-heating. The microstructure, fine structure, cracks and mechanical properties of welded joints were investigated to reveal the relation of welding parameters, alloying elements, microstructure and fine structure of joints, propagation of cracks and performance of joints.Oblique y-groove and straight Y-groove cracking tests of butt joints were conducted to research the influences of welding wire and welding heat input on the cracking ratio of Q550, Q550+Q690 and Q690 joints. Results indicated that with the increase of strength of welding wires and welding heat input, the cracking sensitivities of joints increased. The mechanical properties of Q550 and Q550+Q690 welded joints were studied via tensile test and Charpy V-notch (CVN) impact test. Tensile samples ruptured at the weld metal in the joints fabricated using ER50-6 welding wire, stating that weld metal was the weakest zone in the joints. The tensile strength of joints produced with ER60-G welding wire was close to that of base metal with the failure location at the fusion zone or HAZ. CVN impact test results showed the toughness was the best in the HAZ for Q550 and Q690 steel joints. Weld metal and fusion zone in joints produced with ER60-G welding wire exhibited better toughness than that fabricated using ER50-6 welding wire.The microstructure, fine structure and composition of inclusions in weld metal were investigated by means of optical microscope, scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectroscopy (EDS). The effects of welding heat input, alloying elements and inclusions on the microstructure of weld metal were analyzed. With the increase of concentration of alloying elements in welding wires, the content of proeutectoid ferrite (PF) decreased, and that of acicular ferrite (AF) increase firstly, then decreased gradually. When the content of alloying elements became higher, the formation of bainite was promoted. For the weld metal with small alloying elements (ER50-6 welding wire), the content of AF in weld metal was not sensitive to welding heat input since the prior austenite grain boundaries were almost fully covered by PF. With the increase of alloying elements in welding wire (ER60-G), welding heat input had important influences on the microstructure of weld metal. Small heat input (14kJ/cm) generated bainite dominated microstructure in weld metal, resulting in poor impact toughness.Both the composition and size of inclusions are the deciding factors for the nucleation of AF. Relatively large inclusions with the diameter of about 0.5-0.8μm were much more effective in promoting the nucleation of AF and refining microstructure within austenite grain than other small inclusions. Compared with single phase inclusions, those comprised of several different phases were easy to stimulate several AF plates with different directions leading to higher nucleation frequency of AF. The result of TEM analysis revealed high density dislocations inside of AF laths. The lath like bainitic ferrite and retained austenite had been observed by TEM. The crystallographic orientation between a and y conformed to K-S relationship.We also investigated the influence of welding heat input on microstructure and impact toughness of the heat affected zone (HAZ) in Q690 steel joints. With the increase of welding heat input from 14kJ/cm to 20kJ/cm, the impact toughness in the HAZ had the tendency to increase at first and then decrease. The optimum impact properties of HAZ were obtained with about 16kJ/cm heat input. When the welding heat input was raised to a suitable value, the formation of lower bainite ahead of the transformation from austenite to martensite could refine microstructure within austenite, providing more high angle lath boundaries, which played a very important role in enhancing impact toughness in HAZ of Q690 steel joints. Lower bainite was obtained with carbides adopting a single crystallographic variant in lower bainitic ferrite at about 16kJ/cm heat input. When higher heat input was employed, carbide particles paralleled to the habit planes of the bainitic ferrite were formed, which were detrimental to impact toughness of Q690 steel joints.Based on the investigation of microstructure and fine structure in Q550+Q690 steel joints, the propagation of cracks and fracture surface of Charpy impact specimens were studied via SEM. Cracks initiated from the root fusion zone of Q690 steel side and propagated along the fusion zone. Some cracks would turn from fusion zone to weld metal along the laths of PF or bainite. The mode of crack propagation was:nucleation of micro-crack→crack tip blunting→crack tearing→nucleation of new micro-crack. When the tip of crack encountered inclusion, more crack paths would be initiated from the interface between inclusion and acicular ferrite plates.The fibrous zone in fracture surface of weld metal fabricated with ER50-6 and ER60-G welding wires performed dimpled transgranular type of failures with the mechanism of micro-void coalescence. The analysis had proved that acicular ferrite structure region corresponded to relatively large dimples while grain boundary ferrite structure corresponded to small dimples. Radical region in the fractured surface of weld metal from ER50-6 joint was flat with river patterns, while that of weld metal produced using ER60-G welding wire exhibited tortuous crack propagation path and rough cleavage planes. The propagation of cracks in fusion zone and weld metal exhibited excellent cracking resistance of AF. Fibrous zone in fractured surfaces of impact specimens in Q690 steel HAZ with mixed microstructure of lath martensite and lower bainite was characterized by dominant elongated dimples. Complex river patterns and cleavage steps were observed within cleavage facets in the radical zone. But slip bands were shown in the fibrous zone in the HAZ fabricated at higher heat input. Coarsen carbides in upper bainite held back the formation of dimples. The radical zone was composed of small cleavage facets due to more crack initiation sites provided by carbides.This paper systematically researches the microstructure characteristics and mechanical properties of welded joints without preheating of HSLA steels Q550 and Q690 used in hydraulic support for coal mine. The propagation of cracks and fracture mechanisms in Q550+Q690 steel joints were also studied. The relations of welding parameters, alloying elements, microstructure and mechanical properties including tensile strength and impact toughness were established. The results provides experimental and theoretical basis for improving the application of high strength low alloy steel and promoting the efficiency of welding.