Research On Forming Theories And Microstructure Evolution Of High Speed Railway Wheels

The demand of high-speed railway (HSR) wheels has increased dramatically with the rapiddevelopment of HSR transportation in recent years. So far, the situation that HSR wheels are alldependent on foreign imports has not been broken in China. The HSR wheels require high strengthand toughness, which lead to higher requirements in metallurgy, hot forming and heat treatmentprocesses. There are some key issuers in the theories, technologies of HSR wheels manufacturing thatmust be resolved. This thesis has studied in depth metal deformation and microstructure evolutionduring HSR wheel multi-stage forming processes supported by the “863” key project titled of“Research and development of high speed EMU wheels”. At last, a new heat treatment process wasdeveloped to improve the wheel fracture toughness. The main contributions of the present thesis indetails are as follows:(1) A quick prediction model for railway wheel final forging load was developed based on the upperbound method, by which the effect of process parameters on forging load was analyzed. The effects offillet radius of the rim external side on the load and metal flow in the rim were investigated. Thephenomenon of underfill in the conner of external end of the rim during final forging was explained.(2) Based on analytical method, the methods for determining the contact area of rolls and the ratioof unilateral reductions of web roll side and back roll side were obtained. It was found that theunilateral reduction of the web roll side is slightly larger than that of the back roll side and with theincreasing of the rim radius the proportion of the reduction in web rolls side increases during rolling.The bite condition and plastic penetration condition of the rim were also obtained, which providetheoretical guidance for rolling optimization.(3) The metal deformation during HSR wheel multi-forming processes, especially for the rollingprocess, were analyzed systematically by FEM. The main results are as follows:1) It was found thatthere are two difficult-deformation regions in the rim, one of which is the area near the middle of riminternal side and the other is the center region of the rim. The metal in the these regions subjects totensile stresses in circumferential and radial directions.2) The expanding of the radius of rim mainlytakes place in the two affected zones before and after the main deformation zone.3) During rolling,the web subject to radial and circumferential tensile stresses, the metal of the rim outside thedeformation zones subject to compressive stress in circumferential direction.4) The phenomenon ofthe web thinning during rolling process was identified and explained.5) The effects of feedings perround on rolling deformation were obtained.6) Research indicated that the local underfill in the wheel flange is one of the reasons for the wheel ellipsing. The main cause of the folding defect occurs in theinternal side of the rim corresponding to the positioin of severely underfilling in the flange subject tointensive stretching action of the metal near the defect during rolling. These results further deepen theunderstanding of metal deformation characteristics during wheel rolling and provide guidance tooptimizing rolling process and reducing rolling defects.(4) Based on Gleeble experiments, the mathematical models for the recrystallization and graingrowth of a HSR wheel steel were derived. The evolution of the austenite grain size during the HSRwheel multi-stage forming processes was simulated by integrating the microstructure evolutionmodels with the finite element model based on programming the user subroutines. The main resultsare as follows:1) In the primary forging, almost all the metal subjects to full dynamic recrystallization;In the final forging, dynamic recrystallization only take place mainly in center and down part of thehub, the web and the rim areas.2) The average grain size increased by20~30m with the initialtemperature elevated every30℃; The grain size increases for the rim center and decreases for themetal near surface when forging speed reduces.3) During wheel rolling process, only part of externalend of the rim subject to partial dynamic recrystallization, while static recrystallization occurs in otherdeformation regions.4) The grains for the metal near to surface were refined evidently, thedistribution depth of the fine grains in the external end of the rim is larger than that of the internal end.(5) A new heat treatment of railway wheel, which involving a pre-treatment and a final treatment,was developed to improve the microstructure morphologies and increase the fracture toughness of theHSR wheel rim. The main results are as follows:1) It was found that the grain size and its distributionafter pre-treatment are mainly dependant on the temperature, while the initial microstructure has littleeffect on it.2) It is appropriate that the temperature of the pre-treatment be in the range of840~880℃,while the temperature of the final treatment be about840℃.3) The pilot production of HSR wheelwas conducted by the process of870℃×2.5h pre-treatment+840℃×2.5h final treatment.Contracted to the conventional technology, the new heat treatment process improves the fracturetoughness significantly and makes all the mechanical properties meet the specifications of the HSRwheel. The technical conditions for production HRS wheel had passed the technical assessment andthe pilot products are preparing to service test.