Study On The Toughness Of High Strength Low Alloy Structural Steels And The Weld Joints

Impact toughness and fracture toughness of high strength low alloy structural steels SHT490C and SHT780C were investigated in different temperatures. And weld metal of SHT780C were also studied. The relationship between the two indices was analyzed. The microstructure and the fracture morphology of these materials were studied by optical microscope, micro-hardness tester, X-ray diffraction (XRD), energy dispersive spectrometer(EDS), transmission electron microscopy (TEM). Fracture mechanism and the relationship between the toughness and temperature were discussed.The microstructures of SHT490C were mainly acicular ferrite and pearlite, whose grains were fine and grow orientation is not obvious along the rolling direction. The microstructures of SHT780C were mainly low bainite, which was fine and the orientation along the rolling direction. The microstructure of SHT780C weld metal was mainly acicular boundary ferrite, side plate ferrite, granular bainite, and a few pearlite. The twin substructure was observed in SHT780C, and the indensity of dislocation was large. The alloy compounds distributed in the matrix dispersively. But the indensity in the weld metal was low, and the inclusion was exited in spherical form, the size is large. The alloy compound in base metal is mainly (Nb,Ti)(C,N), while FeMnTiO₄ and Ti₂O₃ in weld metal. Fe₄N and Fe₃C were appeared in both of them. Due to the different microstructures, there was great difference in microhardness. The weld had the minimum hardness, while the heat affected zone had the maximum but uneven hardness. The hardness of the base metal was between the weld and heat affected zone.According to the results of Charpy impact test can be drawn: the impact absorb energy of the three materials were decreasing when the temperature reduced; the toughness of SHT780C weld metal was lower than that of the base metal. The toughness of C direction was superior to that of L direction. With the temperature increasing, the difference of C and L the direction decreases. And the direction influence of SHT780C was more than that of SHT490C.The impact fracture morphologys of base metal and weld were observed. With the temperature reduced, the fracture morphologys changed from the ductile fracture to quasi-cleavage fracture. The inclusions appeard in the fracture surfaces were analysed by EDS. The alloy elements of inclusions in the weld metal were more than that in base metal. The toughness of weld metal was lower than the base metal, because FeMnTi and Ti oxide inclusions were formed in the welding process and materials were weakened in welding metallurgical process. The welding process had significant influence on the toughness of the materials.In the CTOD test,δc of the base metals and weld metal reached the peak in -40℃. The fracture toughness of weld metal was between that of the two directions of base metal. For SHT780C, the toughness of C direction was higher than that of L direction, while the direction of SHT490C affected by the directions was not obvious.δc of SHT490C was higher than that of SHT780C.The relationship ofδc(T) and A_KV(T+â–³T) was linear, the ratio ofδc(T)/A_KV(T+â–³T) of the same material changed little when the temperatures changed. But the ratio ofδc(T)/A_KV(T+â–³T) was not a constant, it varied with different materials.