Effect of Large Shear Deformation on Rail Steels and Pure Metals

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$dissertation global and local fracture properties of metal matrix ...$

1 1 Introduction the origin <strong>of</strong> cracks, too. Figure 1.1 depicts two micrographs <strong>of</strong> the gauge corner <strong>of</strong> a rail after service. The deformation layer with cracks inside is well pronounced and also a WEL can be seen. Generally it is assumed that if the rate <strong>of</strong> crack growth is larger than the wear rate, these cracks can grow to a critical length and result in spalling damage or transverse rail fracture as can be seen in Figure 1.2 ∗ . This becomes especially important for the modern rail steels, that are much more wear restistant, but also have a somewhat lower fracture toughness. Therefore, Figure 1.2: Damages <strong>of</strong> rail tracks. (a) Head check cracking on the gauge corner (b) Transverse rail cracking from a head check crack. (c) Spalling damage on the gauge corner in a curve track. it is <strong>of</strong> utmost importance to be able to predict the occurrence and growth <strong>of</strong> cracks for different rail steels and different loading conditions. One possibility to do this are field tests20 or tests on a testing rig for rails. These tests are very time consuming and cost-intensive. Hence, great affords are made to simulate the rail-wheel contacts in order to be able to study the influence <strong>of</strong> different geometries or to develop tools to predict the occurrence <strong>of</strong> critical cracks. 8, 21–23 At the moment such calculations suffer especially from two shortcomings: Firstly, there exists almost no fundamental knowledge <strong>of</strong> the microstructural changes <strong>of</strong> the material due to large shear deformation. Secondly, virtually nothing is known about the mechanical properties <strong>of</strong> this deformed material as a function <strong>of</strong> the shear deformation, especially in terms <strong>of</strong> fracture toughness and anisotropy. 2 ∗ taken from a presentation held by Jonas Rinsberg at the CHARMEC Meeting, 14.-18.01.2004, Leoben
1.2 Objectives The aim <strong>of</strong> this work is to understand the processes occurring in pearlitic and bainitic steels used as rail steels during severe plastic shear deformation and to relate these to the resulting changes in the mechanical properties. Together with steels, the behavior <strong>of</strong> pure metals shall be investigated in order to gain fundamental insight in the processes during severe plastic deformation. The investigated materials comprise the coarse-pearlitic steel 900A, the finepearlitic steel 350 LHT (or HSH-S), the bainitic steel Dobain 430, pure iron, pure nickel and pure copper. Figure 1.3 depicts the initial microstructure <strong>of</strong> the investigated rail steels. The composition <strong>of</strong> the investigated steels is given in Table 1.1. C Si Mn Cr Pmax Smax 900A 0,76 0,35 1,0 0,014 0,017 0,04 350 LHT 0,78 0,46 1,18 0,23 0,012 0,015 Dobain 430 0,70-0,82 0,4-1,0 0,7-1,1 0,4-0,7 0,02 0,02 Table 1.1: Composition <strong>of</strong> the used materials In the last years, in the scientific field <strong>of</strong> severe plastic deformation many methods were developed to severely plastically deform samples to strains not reachable with conventional processes, see for example the proceedings <strong>of</strong> the many conferences held in this area. 24–26 In the present work, some <strong>of</strong> this methods are applied to the investigated materials to produce specimens with a defined deformation for further investigations. With samples produced by High Pressure Torsion (HPT), the microstructural evolution and the mechanical strength as a function <strong>of</strong> the monotonic shear strain are determined. In order to evaluate the influence <strong>of</strong> cyclic severe plastic deformation a new method <strong>of</strong> SPD, Cyclic High Pressure Torsion (CHPT), was developed and applied to different materials. Investigation methods for characterizing the microstructure comprised scanning electron microscopy (SEM) using secondary (SE) electrons or backscattered electrons (BSE) for depicting or obtaining orientation image maps (OIM), transmission electron microscopy (TEM), analytical transmission electron microscopy, ion microscopy and different X-ray diffraction techniques. The mechanical strength was determined by means <strong>of</strong> microhardness, in-situ measurement <strong>of</strong> the torque and subsize tensile tests. To determine the fracture toughness and the crack growth properties, samples <strong>of</strong> the rail steel 900A were deformed by Equal Channel Angular Pressing (ECAP) using Route A. With these samples, also the resulting anisotropy (in terms <strong>of</strong> fracture toughness, crack growth and mechanical strength) was studied. To investigate the formation <strong>of</strong> WEL, combinations <strong>of</strong> severe plastic deformation and different heat treatments were performed for the rail steel 900A. 3 1
Page 1 and 2: University of Leob
Page 3: ASTRID, LÄTITIA
Page 7 and 8: Danksagung Die hier vorliegende Arb
Page 9 and 10: Summary The investigation o
Page 11 and 12: Kurzfassung Die Untersuchung der Au
Page 13 and 14: Sobald jemand in einer Sache Meiste
Page 15 and 16: Contents 1 Introduction 1 1.1 Motiv
Page 17 and 18: Contents E.3.2 Mechanical strengths
Page 19: 1.1 Motivation 1 Introduction Today
Page 23 and 24: 2 Methods of Sever
Page 25 and 26: 2.2 Equal Channel Angular Pressing
Page 27 and 28: 2.2 Equal Channel Angular Pressing
Page 29 and 30: 3 Results and Discussion In the fol
Page 31 and 32: 3.1 Structural Evolution of
Page 45 and 46: 3.3 Mechanical Properties In contra
Page 47 and 48: 3.3 Mechanical Properties Figure 3.
Page 49 and 50: 3.3 Mechanical Properties Figure 3.
Page 51 and 52: 3.3 Mechanical Properties can be se
Page 53 and 54: 4 Conclusions The methods o
Page 55 and 56: [1] G. Baumann, H.-J. Fecht, and S.
Page 57 and 58: Bibliography [36] T. Hebesberger, H
Page 59 and 60: 5 List of appended
Page 61 and 62: A Structural Refinement of<
Page 63 and 64: A.1 Introduction A.1 Introduction K
Page 65 and 66: A.3 Results Figure A.2: Micrograph
Page 67 and 68: A.4 Discussion Figure A.3: Microgra
Page 69 and 70: Bibliography to paper A [1] W. Lojk
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B Structural Changes of</st
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B.1 Introduction B.1 Introduction D
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B.3 Results and Discussion after a
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Figure B.5: 2Θ scans of</s
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B.4 Conclusion B.4 Conclusion • T
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Bibliography to paper B [1] G. Baum
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C Strain Hardening during High Pres
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C.1 Introduction C.1 Introduction R
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C.3 Results With this equation it i
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C.4 Discussion Figure C.4: SEM micr
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Bibliography to paper C [1] T. Hebe
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D Formation of sur
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D.1 Introduction D.1 Introduction T
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D.3 Results D.3.1 Monotonic <strong
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D.3 Results Figure D.4: SEM microgr
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D.4 Discussion Figure D.6: SEM micr
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occurring on the surface of
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Bibliography to paper D [1] Y. K. C
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E High Pressure Torsion of<
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E.1 Introduction E.1 Introduction T
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E.3 Results Figure E.2: SEM microgr
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E.3.2 Mechanical strengths E.4 Disc
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Bibliography to paper E [1] L. Wang
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F TEM Investigation of</str
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F.1 Introduction F.1 Introduction C
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F.4 Discussion see Figure F.2b. The
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Figure F.3: In-situ measured torque
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F.4 Discussion Figure F.6: Comparis
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F.6 Appendix • Electron energy-lo
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[1] M. Zelin. Acta Mater., 50(17):4
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G Fracture Processes in Severe Plas
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G.2 Introduction G.2 Introduction T
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G.4 Results toughness test. It can
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G.5 Discussion Figure G.3: (a) Load
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G.5.2 Fracture G.5 Discussion Fract
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Bibliography to paper G [1] J. D. E
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H Cyclic High Pressure Torsion <str
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H.1 Introduction H.1 Introduction I
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Figure H.2: Sample preparation for
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H.3 Results With increasing ∆ɛ a
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H.3 Results Figure H.7: SEM microgr
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H.3 Results between 2°and 15°, th
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H.4 Discussion comparable to ECAP R
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H.5 Conclusions bon steels as a fun
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H.5 Conclusions • The steady stat
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Bibliography to paper H [1] Y. T. Z
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Bibliography to paper H [33] C. Buq
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I Structural Evolution during Cycli
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I.1 Introduction I.1 Introduction T
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I.3 Results Figure I.2: SEM microgr
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I.4 Discussion be seen that saturat
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I.5 Conclusions structure size is a
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[1] H. Mughrabi. Mater. Sci. Eng.,
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Effect of Large Shear Deformation on Rail Steels and Pure Metals

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