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  Fracture toughness of nanostructured railway wheels M.R. Zhang a,b, * , H.C. Gu a a State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China b Technical Center, Maanshan Iron and Steel Co. Ltd., Maanshan 243000, China a r t i c l e i n f o  Article history: Received 29 November 2007Received in revised form 21 July 2008Accepted 28 July 2008Available online 3 August 2008 Keywords: Fracture toughnessNanostructured materialsRailway wheelsBainitePearlite a b s t r a c t This paper describes nanostructured railway wheels made of Si–Mn–Mo–V low-carbonsteel through an advanced metallurgy process and fabrication technology. The microstruc-ture of the wheels, particularly in the rim portion, is composed of carbide-free bainite thatconsists of bainitic ferrite laths and retained austenite films along the lath boundaries. Thethickness of the laths andfilms is innanometer scale. For comparison, traditional pearlite–ferritewheelsteelsarealsoinvestigated. Testresultsshowthatcarbide-freebainitesteel issuperior to pearlite–ferrite steel not only in yield strength but also in fracture toughness.Theoretical explanation of these phenomena is also elucidated.   2008 Elsevier Ltd. All rights reserved. 1. Introduction Wheels are key components whose failure often results in catastrophic consequences in rail transportation. The majorfailure modes of railway wheels can be classified into five types: shelling, spalling, flat, rim cracking, and brittle fracture[1]. At present, most wheels are made of medium-high carbon grade steels with carbon content in the range of 0.45  0.80wt.%. Their microstructures are typically pearlite–ferrite. The yield strength, toughness, ductility, resistances tocontactfatigueandthermaldamageofthesesteelsinpearlite–ferriteconditionislow.Increasesintrainspeedandaxle-loadrequire that wheels have higher stability and reliability [2,3].Nanostructuredmaterials,withaveragestructuraldomainsizesbelow100nm,arebelievedtoexhibitamarkedimprove-ment in mechanical properties over their coarse-structured counterparts [4–8]. There is now strong evidence showing thatsevere plastic deformation (SPD) and controlled solid-state transformation are effective means to obtain nanostructuredmaterials. Railway wheels are huge in mass (weight: 330  450kg, and diameter:  U 840  U 1250mm) and complex in shape(seeFig. 1). It seems impossibletofabricatethe wheelsfromnano-scaleclusters orbysevereplastic deformation(SPD). Thispaperwillreportourachievementinproducingnanostructuredwheels(especiallyintherimportion)usinganadvancedmet-allurgyprocessandfabricationtechnologyatacostcomparabletocurrentcommercialwheels. Theexcellent combinationof yield strength and fracture toughness is associated with bainitic ferrite laths and retained austenite films both in nano-size. 2. Experimental  2.1. Materials A novel carbide-free bainite steel alloyed with silicon and manganese was designed and developed. Silicon can suppresstheprecipitationofcementiteduringbainitictransformation;manganeseincreasesthehardenabilityofthesteel,andthus,it 0013-7944/$ - see front matter    2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.engfracmech.2008.07.007 *  Corresponding author. Address: Technical Center, Maanshan Iron and Steel Co. Ltd., Maanshan 243000, Anhui, China. Tel.: +86 555 2887272; fax: +86555 2883612. E-mail address: (M.R. Zhang).Engineering Fracture Mechanics 75 (2008) 5113–5121 Contents lists available at ScienceDirect Engineering Fracture Mechanics journal homepage:  homogenizes the microstructure and properties of the wheel section. The micro-alloying elements vanadium and molybde-numarealsoaddedinordertogainfinelathsandeliminatetemperedembrittlement.Thesteelwassmeltwithintermediatefrequency induction furnace, and electroslag remelt into ingots with a diameter of   U 420mm and a weight of 920kg. Afterstress relief annealing and cutting, the round ingots were forged and rolled into the wheels of   U 840mm in diameter forwagon transportation. During heat treatment, the wheels were heated to 910  C for austenitization. After soaking 2h, slackquenching with water were conducted on the tread of rim section by programmed control to simulate the isothermal heattreatment. The alloy design and manufacture process of this steel are summarized in [9]. For comparison, traditionalpearlite–ferrite wheel steels CL60, a plain carbon steel with carbon content 0.60wt.%, were also investigated.ThechemicalcompositionsofthesteelsweremeasuredusinganARL4460OpticalEmissionSpectrometer.Theresultsareshown in Table 1. Nomenclature a  crack depth  A, m  constant parameters in Paris equation  Aku  impact toughness using standard  U  -notch specimens at room temperature CT   compact tension EL  elongation E   Young’s modulusd a /d N   crack growth rate K   stress-intensity factor K  IC   fracture toughness K  Q   SIF at applied load pop-in K  max  SIF at maximum applied load L  grain size R  stress ratio RA  reduction of area UTS   ultimate tensile strength r  * a fixed distance ahead a microcrack (by Knott) S  P   interlamellar spacing in pearlite D K   stress-intensity factor range r 0 ,  K   y  constant parameters in Hall  Petch relation r  f   cyclic flow stress r F   local fracture stress r  y , YS   yield strength Fig. 1.  The counter diagram of a wheel on the rail.5114  M.R. Zhang, H.C. Gu/Engineering Fracture Mechanics 75 (2008) 5113–5121   2.2. Microstructure The microstructure of specimens taken fromthe rim, web, and hub sections was observed with a Zeiss Axioskop 1  MAToptical microscope and a scanning electron microscope (SEM) Philips XL30+DX41. Because cracks usually nucleate andpropagate on the rim portion, in this paper, we will focus on the rim portion only.Fig.2showsthemicrostructureofCL60,inwhichpearliteandpro-eutectoidferritecanbeseen.Themorphologyofpearl-iteindetail,withalternatinglayersofcementite(Fe 3 C)andferriteformedinpearlitecoloniesarerevealed.Anopticalmicro-graph of carbide-free bainite is shown in Fig. 3. The detail of microstructure is unresolved under 1000   magnification.Samples of the novel bainitic steel were machined down to foils of   U 3mm diameter and 50 l m thickness, and then, thesefoils were electro-polished until perforation occurred using a twin jet electro-polisher. These foils were examined with atransmission electron microscope (TEM). Figs. 4–6 were taken with a field emission high-resolution transmission electronmicroscope (HRTEM) JEM-2100F. As shown in Fig. 4, the details of carbide-free bainite are revealed now, i.e., the slenderlaths of bainitic ferrite and the films of retained austenite along the lath boundaries. Fig. 5 shows a twin within the filmofretainedaustenite,andtheFeatomarrangementonthe(111)planeinretainedaustenitecanbeseeninFig.6.Thefeatureof thestructureisconsistent withthepreviousresearch[10,11]. Retainedausteniteinfilmmorphologyandnano-sizethick-ness has excellent thermal and mechanical stability [12].  Table 1 Actual chemical composition of the steels, wt.% Alloy C Si Mn Cr Mo V P SNovel steel 0.21 1.53 1.92 0.03 0.30 0.10 0.019 0.003CL60-a 0.62 0.76 0.70 0.19 0.0063 0.0034 0.023 0.019CL60-b 0.63 0.27 0.75 0.17 0.0013 0.0024 0.017 0.010 Fig. 2.  Pearlite and pro-eutectoid ferrite of CL60 etched by 4% nital. Fig. 3.  Optical micrograph of carbide-free bainite wheel etched by 4% nital. M.R. Zhang, H.C. Gu/Engineering Fracture Mechanics 75 (2008) 5113–5121  5115   2.3. Mechanical properties Tensile and impact specimens were taken from the rim portion of the wheels. Tensile specimens were tested with aWAW  Y500A material testing system according to the Chinese railway standard TB/T2817  1997. Impact toughness wasevaluated using 10mm   10mm   55mm Charpy U2-notch specimens broken at room temperature with a Zwick/RoellAmsler KRP 450 instrumented impactor. The Brinell hardness numbers (HB) were measured at a point 30mm under thetread with a Digital Brinell Testor 970/3000. The test results are summarizedin Table 2. The novel steel reflects an excellentcombinationof strength and toughness compared with CL60 wheel steel, especially in yield strength and impact toughness. Fig. 4.  TEM micrograph of carbide-free bainite wheel. Fig. 5.  A twin within the film of retained austenite. Fig. 6.  Two-dimensional lattice image of retained austenite.5116  M.R. Zhang, H.C. Gu/Engineering Fracture Mechanics 75 (2008) 5113–5121
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