Microstructure and properties of investment casting coupler
The coupler is made of E-grade steel ZG25MnCrNiMo by investment casting. The overall dimensions are 594 mm × 370 mm × 350 mm, making it a large complex investment casting. The minimum cross-sectional area of the coupler’s longitudinal axis is 0.008 m2, and its geometric model and physical object are shown in the figure.
The structure of the coupler in the as-cast state mainly consists of pearlite and ferrite. Compared with the Level 1 standard in the “Cast Steel for Locomotive and Rolling Stock – Part 2: Metallographic Structure Inspection Atlas” (TB/T 2942.2-2018), the as-cast grain size and distribution morphology of the coupler meet the requirements of the standard. After normalizing preparatory heat treatment and quenching and tempering, the structure of the coupler casting is fine and uniform tempered sorbite, as shown in the figure. Under high-magnification SEM observation, many fine and uniform carbide particles are distributed on the ferrite matrix. Six samples were cut from each large planar area of the coupler casting before and after heat treatment for tensile testing, and the average data were obtained. The tensile strength of the casting in the as-cast state is 550-675 MPa, with an elongation rate of 1.5%-2.7%. After quenching and tempering, its tensile strength reaches 900-1450 MPa, with a yield strength of 920 MPa, and the elongation rate can reach up to 14.5%, which is significantly improved compared to the as-cast state. This is mainly due to the effective refinement of grain size and improvement of defects such as inclusions and segregation by normalizing + quenching and tempering heat treatment. Compared with the main mechanical performance parameters of ZG25MnCrNiMo steel in the railway industry standard (TB/T 2942-2015) (after heat treatment), the coupler in this study meets the requirements for use.
Observing the tensile fracture surface under an electron microscope, it was found that the as-cast sample did not exhibit significant necking, and the fracture surface was clearly cleaved, indicating brittle fracture. The microstructure of the fracture surface is shown in the figure; while the fracture surface of the sample after tempering treatment exhibits quasi-cleavage fracture characteristics, as shown in the figure, with a discontinuous expansion of the toughness, and there are many tear ribs on the quasi-cleavage small fracture surface. The fracture behavior is between cleavage fracture and toughness fracture, and there is a significant improvement in toughness compared to as-cast toughness.
Connection and stress of coupler
When the two couplers are coupled, the coupler head of one side is inserted into the coupler head hole of the other side, as shown in the figure. At this time, the inner side of the convex cone presses the other coupler tongue to rotate during the forward movement, which compresses the spring of the decoupling cylinder and rotates the coupler tongue by 40° in the counterclockwise direction. After the contact between the two coupler connecting surfaces, the inner side of the convex cone no longer presses the other coupler tongue. At this time, due to the action of the spring, the coupler tongue rotates clockwise to return to its original state, that is, in the locked position. The coupler is mainly used to connect carriages, so the direction of force is longitudinal. The part of the coupler that transfers force is mainly the coupler tongue. When the train starts, the coupler is mainly subjected to tensile force, and at this time, the circular arc surface of the coupler tongue is mainly stressed in the coupler assembly. When the train brakes, the coupler is subjected to compressive force When the coupler assembly is working, the forces are mainly applied on the curved surface of the coupler tongue and the connection port. Ignoring gravity, the forces applied on the coupler during operation are shown in the figure.
Coupler load
The tensile and compressive loads experienced by the coupler during start-up and braking are the highest. When the train starts, the coupler is subjected to tensile loads, with the tensile force reaching its maximum value instantaneously and then gradually decaying. When the train begins to run smoothly, the tensile force tends to stabilize. When the train brakes, the coupler is subjected to compressive loads, with the compressive force reaching its maximum value instantaneously and then gradually decaying until it reaches zero when the train stops. According to the requirements of “Technology of Metro Vehicle Couplers”, the coupler and coupler seat should meet a static pressure of not less than 1000 kN, a tensile force of not less than 800 kN, and a maximum tensile force of 600 kN when the vehicle is running smoothly. Therefore, the above-mentioned load data is used for static simulation analysis, with the aim of examining whether the strength of the coupler meets design requirements through simulation analysis of the extreme working load. The fatigue analysis uses an 8-level load spectrum prepared from measured coupler load-time history on Datian-Qinhuangdao Line as the coupler working condition loading. However, due to the fact that the Shibata-type abutment coupler is used for urban rail passenger lines, there is a difference in maximum load compared to freight trains on Datian-Qinhuangdao Line. Therefore, the maximum amplitude of load on Datian-Qinhuangdao Line has been corrected, that is, any amplitude exceeding 800 kN is taken as 800 kN. The fatigue simulation load spectrum on Datian-Qinhuangdao Line is shown as a pink broken line.
Simulation results and discussion of mechanical properties of couplers
The coupler is subjected to tensile and compressive loads of 600 kN, 800 kN, and 1000 kN, respectively, under steady operation, startup, and emergency braking conditions. The stress distribution of the coupler is shown in the figure. From the simulation results, it can be seen that under a static tensile load of 600 kN during steady operation, the maximum stress at the traction end of the coupler is 362.04 MPa, and the maximum deformation at the traction end is 2.41 mm
Under the starting condition, the maximum stress on the coupler is 482.72 MPa, and the maximum deformation is 3.21 mm, with the larger value appearing at the bottom of the coupler head. From the analysis of the force on the coupler in the figure, it can be seen that when the coupler is under tension, the force-bearing position is mainly the circular arc surface of the coupler tongue, as shown by the green arc in the figure, which is not at the center of the coupler head, resulting in uneven force on the coupler head and causing the maximum stress to appear at the bottom of the coupler head. According to the mechanical testing results of coupler materials, the average tensile limit of coupler materials after heat treatment is 1020 MPa, and the stress under normal operating conditions is far less than the yield limit of materials. The elongation after fracture of coupler materials after heat treatment is 14.5%, and the total length of coupler excluding coupler head is 430 mm. It can be calculated that the maximum deformable amount of coupler is 62.35 mm. From the simulation results in the figure, it can be seen that under starting condition, the total deformation of coupler is 3.21 mm, and under stable operation, the maximum total deformation is 2.41 mm, far from reaching the maximum deformable amount. Therefore, coupler can be safely used under normal operating conditions.
Conclusion
1) Scanning electron microscopy of investment casting coupler materials revealed uniform microstructure, fine grain size, and no significant casting defects; tensile testing
It is shown that the tensile strength of the as-cast state can reach up to 675 MPa, and the tensile strength after heat treatment can reach up to 1540 MPa, with an evaluation of up to 1020 MPa. The organization and performance meet the requirements of the coupler.
2) Under the three working conditions of starting, stable operation, and emergency braking, the stress and deformation of the coupler are far less than the allowable yield strength and deformation of the coupler, and the coupler meets the service requirements.
3) The simulation draws a sinusoidal load-fatigue life curve for the coupler, and the fatigue life of the coupler loading the Datong-Qinhuangdao Line load spectrum can reach up to 12.8 years. The fatigue life of Chongqing Rail Transit Line 3 under the 800 kN amplitude sinusoidal spectrum is 31.5 years, and the coupler meets the requirements for use. The simulation results can provide a basis for the maintenance of the coupler.