Flame Straightening And Analysis Of Microstructure And Mechanical Property On S355J2Structure Steel

Manufacturing technology of high-speed railway vehicles in our country is in thestage of strategic development period, and bogie manufacturing technology is one ofthe key technologies for high speed vehicles. Now bogie of vehicles in200~300km/h is using S355J2structure steel. Because bogie is welded steel structure, anddeformation of the local area or the whole is always appeared, so it is necessary tostraightened after welding. Besides, as bogie’s volume is enormous, and structure iscomplex, so flame straightening technology is used in usual in industry. To verifyoperation of flame straightening on S355J2steel, and confirm the best flame heatingtemperature, microstructure and mechanical property of S355J2structure steel areanalyzed and studied. At the same, according to the experiment analyzing influence ofstress relieving annealing to flame straightening zone, flame straightening processprogram on S355J2steel used bogie is ultimately determined.Two kinds of flame straightening experiments are preceded: thermal simulationtest is carried out by the box-type heating furnace with KSY SCR temperaturecontroller, and flame rectification test is using oxyacetylene torch, and temperature ismeasured by infrared thermometer. In experiment, S355J2steel is respectively heatedto600℃、650℃、700℃、750℃and800℃. Based on optical microscope and SEM,microstructure of S355J2after flame straightening is observed. Besides, based ontensile test, bending test, hardness test and impact test, mechanical property of S355J2after flame straightening is researched. Its main results are as follows:In thermal simulation test, microstructure of S355J2steel after flamestraightening below750℃has hardly changed, and is similar with the original. Whilemicrostructure of S355J2steel after flame straightening at800℃has changed slightly,and lamellar pearlite has changed partly into granular pearlite, and ferrite has littlechange. Though flexural strength, break elongation and impact energy of S355J2 could meet the required standards after flame straightening at800℃, the hardness andstrength of materials have decreased significantly, especially yield strength has notreached the requirements. When the heating temperature is700℃, microstructureand performance of materials maintain satisfactory. Considering the difference ofactual flame correction and simulation test, the best heating temperature of flamestraightening on S355J2structure steel is recommended finally730℃～750℃.Then, flame straightening zone of S355J2steel is respectively normalized andannealed. Based on observing microstructure, grain refining after normalization ismore obvious, but lamellar pearlite after annealing treatment distributed uniformly inthe organization, its morphology is more similar with parent material. Based on themechanical performance test, the hardness of normalizing and annealing treatmentafter flame straightening at800℃is basically same. However, the hardness ofnormalizing has decreased after flame straightening at600℃and700℃, while thehardness of annealing has increased contrarily, especially the hardness at600℃hasincreased17.1%. Consider comprehensively, flame straightening zone of S355J2steelis more suitable for subsequent annealing treatment, namely stress relieving annealinghas not adverse effect on the properties of flame straightening zone.