| Pressurized smelting technology is of great significance for improving the quality of steel materials.It can improve the solubility of volatile elements in molten steel,increase the element yield to play the role of this element in purifying molten steel and micro-alloying,thus improving the microstructure and properties of steel.At atmospheric pressure,the melting point of magnesium is 650? and the boiling point is 1103?,which is lower than the temperature of molten steel.The physical state of magnesium is gas,and it is difficult for magnesium to dissolve in molten steel.In this study,three kinds of SKS51 steel with different magnesium contents were prepared by means of pressurized magnesium smelting technology.Taking magnesium content as the main variable,the microstructure and mechanical properties of SKS51 steel treated with pressurized magnesium at each heat treatment stage were explored.At the same time,three kinds of hot-rolled plates of SKS51 steel were butt-welded using a high-power fiber laser system,and the effects of magnesium content on the microstructure in the specimen as-welded and after various heat treatment stages of post-welding were examined.The effects of the welding process parameters on the surface morphology,microstructure and microhardness distribution of welded joints were characterized using the optical microscope,field emission scanning electron microscope,laser scanning confocal microscope Vickers microhardness tester and X-ray diffractometer.The optimized welding parameters were obtained.The effect of magnesium on microstructural transformation of SKS51 steel and its welded joints during the whole heat treatment process was also studied.The distribution of magnesium in the microzone of SKS51 steel welded joints with different magnesium contents was characterized using the secondary ion mass spectrometer.The impact test and dynamic fracture toughness test of SKS51 steel and its laser welded joints at different temperatures were carried out using impact testing machine,and the fracture morphology was characterized.The effect of magnesium on toughness and fracture mechanism of SKS51 special steel and its welded joints was examined in the study.It is found that magnesium could refine the grain size of pearlite and austenite,improve the spheroidizing effect of carbide and refine the tempered microstructure.When the pressurized magnesium content was 0.2%,the refining effect of pearlite and austenite was better,but there were more harmful inclusions.When the pressurized magnesium content was 0.046%,the spheroidizing effect of carbide and the degree of microstructure refinement of tempered microstructure were better.When the pressurized magnesium content was 0.046%,SKS51 steel could obtain high hardness at all stages of heat treatment.In terms of impact toughness of SKS51 steel,it is best when its pressurized magnesium content was 0.046%.At the same time,its impact toughness decreased gradually with the decrease of temperature.The optimized laser welding process parameters for SKS51 steel are laser power of 2.6 kW,welding speed of 2.0 m/min,defocusing amount of 0 mm and Ar shielding gas flow rate of 30 L/min.The welded joint consisted of fusion zone(FZ),heat affected zone(HAZ)and base metal zone(BM).The microstructure of FZ was martensite.The HAZ could be divided into coarse-grained HAZ(HAZ-1),fine-grained HAZ(HAZ-2),and were both martensite.The microstructure of BM was pearlite.There was a large amount of volatile magnesium in the FZ of SKS51 steel welded joints with 0.2%pressurized magnesium content,which showed that the magnesium content in the FZ was lower than that in the HAZ.SKS51 steel and its welded joints with 0.046%pressure magnesium content had good microhardness distribution in each heat treatment stage,and good impact toughness and dynamic fracture toughness,indicating that appropriate magnesium content could improve its toughness,but the effect of improving toughness gradually decreased with the decrease of temperature. |
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