Microstructure And Properties Of TRIP Steel And TIG Welded Joints

TRIP steels has been widely used in automobile manufacturing and other fields due to its good strength and toughness.However, the application of TRIP steels in oil&gas offshore platform and pipeline has been limited because of the scientific theory was not enough.TRIP590steel was taken as the experiment subject to study the weld performance and the properties of TRIP steel, TIG welding method and CHG-52, MC welding wire were adopted. The microstructure and properties of TRIP590and its weld joint were studied by using optical microscope, Vickers hardness, electrochemical station, SEM, direct-reading spectrometer and so on.The’results showed that the microstructure of TRIP590was composed of ferrite, bainite and retained austenite. The Ms of TRIP590was8.1℃, the tensile strength and yield strength were respectively390MPa and590MPa, and the elongation was28.3%, the hardness of TRIP590increased with the strain increasing with hardness of HV0.3330～360. The good strength and toughness of TRIP590attributed to the retained austenite to martensite transformation during the tensile process.The analysis of tensile fractures of the TRIP590weld joint showed that the fractures were all located in the base metal with the yield strength of420-45OMPa and tensile strength of540-550MPa. The study on the TRIP590weld joint under different heat inputs indicated that the elongation reached to the maximum when the heat input was57.6J/mm. Different welding materials had different effects on the microstructure and properties of the weld joint of TRIP590steel, when the weld wire was CHG-52T, the weld metal consisted of martensite, bainite and ferrite, and the HAZ contained ferrite, pearlite and some bainite; when the heat input was57.6J/mm, the microstructure was the most uniform and densest, the grain size of HAZ was the finest. When MC welding wire was adopted, the weld metal was ferrite. bainite and pearlite and the HAZ comprised bainite and ferrite. Furthermore, when the heat input was57.6J/mm. ferrite grain boundaries sliding occurred when in strain conditions, which split the grains and was beneficial to relieve the stress concentration. In the HAZ zone, the amount of ferrite and bainite increased after tensile test, the refinement of the grain sizes, as well as the increasing amount of bainite and ferrite with same orientation improved the strength and toughness of the weld joint.