The Research Of Bainite Thermite Welding Flux And Heat Treatment Process

This paper studied an bainite thermite flux and heat treatment process that is applicable for bainite rail with high strength and hardness in China, and analyzed the microstructure and mechanical performance of bainite thermite joints, complete the laying test of bainite thermite joints in Datong-Qinhuangdao line.Research on thermite flux shows that the manganese silicon ratio is too high will lead to the emergence of retained austenite in grains, so that the surface hardness of the joint is improved; the manganese silicon ratio is too low will lead to the emergence of ferrite at grain boundaries, so that the surface hardness of the joint is reduced. When the manganese silicon ratio is at 1.1~1.2, the microstructure of thermite joint consists of bainite + ferrite in a small amount + retained austenite in a small amount. With the increase of the content of Cr, ferrite in thermite joint disappears, retained austenite appears, and the surface hardness can be improved. The results of substitution of Mn for Cr shows that Cr has similar effect on bainite thermite joint compared with Mn. Adding Ni in flux can lead to the increase the amount of retained austenite in thermite joint, and surface hardness is improved. The comparison study of two plans of bainite thermite flux finds that the content of alloy elements at 0.20%~0.30%C, 1.50%~1.90%Mn, 1.40%~1.90%Si, 0.40%~0.60%Cr, 0.30%~0.50%Mo, after the joint is normalized, the microstructure of weld and fusion zone has good uniformity, surface hardness can reach 350~360HB, impact strength AKU is 24.25 J on average at normal temperature, static bending strength and tensile strength meet the requirements of TB / T 1632.3-2014.Study on heat treatment process of bainite thermite welding shows that heating + jetting air is not applicable to bainite thermite welding; the research develops the heat treatment process of heating + slow cooling for bainite thermite welding, and the devices that are easy to operate for the heat treatment. After heat treatment, the micro structure of joint in all sections consist of carbide free bainite + granular bainite, surface hardness reaches 340~350HB and tensile strength is 808.11 MPa on average, the microstructure of fusion zone is uniformed, static bending strength is 2504.83 kN on average.The formation of granular bainite in bainite thermite joint is related to the nonuniform distribution of Cr and Mo, and the mirohardness of granular bainite is 337.44 HV lower than that of carbide free bainite which is 400.92 HV on average. Bainite thermite joints resulted fracture in the fatigue test is due to the rail bottom slag inclusion. After using the wider of sand bottom mould the joints can complete the fatigue test. After fatigue test, there appears a small amount of acicular structure in thermite joints, but the joints were not affected during the fatigue test.Bainite and pearlite rails are welded by using bainite flux, after the heat treatment, the microstructure of fusion zone at bainite rail is uniformed, the grains are refined at pearlite rail, static bending strength of the joints can reach 1909.22 kN on average, surface hardness is 312 HB, tensile strength is 809.89 MPa, the width of softened zone at bainite rail is 12 mm, and the width of softened zone at pearlite rail is 6mm.The results of bainite thermite joints in Daqin line testing section shows that the bainite thermite joints are with good performance.