N. Gubeljak 1, J. Predan 1, Viktor ŠINKOVEC 2

Transcription

1 N. Gubeljak 1, J. Predan 1, Viktor ŠINKOVEC 2 1 -University of Maribor, Faculty of Mechanical Engineering, Smetanova 17, SI-2000 Maribor, Slovenia 2 -Slovenian Railways Ltd, Ljubljana, Slovenia 1

2 ontent Introduction Motivation and tasks Mechanical testing and Fracture toughness testing Fractographic analysis FE modeling of axels with semi-elliptical surface crack Analysis of results Conclusion 2

3 Introduction During regular inspection of axles the longitudinal cracks have been found! What is the reason? 3

4 Introduction Aim of present work: How danger is presence of longitudinal cracks? Find the reason for longitudinal cracking of axles? Figure out, if the longitudinal crack can turn crack path perpendicular to axle Provide suggestion for inspection in order to prevent failure of axle caoused by longitudinal cracks! 4

5 Material A1N in tempered and quenched condition, 30 years in use! Mechanical properties and fracture toughness are determine by using specimens with different oreintation of loading regarding to axle! 5

6 Material-Tensile testing Tensile test according to DIN B daimeter ø5 mm! 6

7 Material-Tensile testing It is not effect of specimen orientation to tensile properties! Stress, MPa ,02 0,04 0,06 0,08 0,1 0,12 0,14 0,16 0,18 0,2 0,22 0,24 0,26 0,28 0,3 0,32 0,34 0,36 Strain, - p01 p02 p03 p04 v01 v02 v03 v04 7

8 Material-Tensile testing It is not effect of specimen orientation to tensile properties!c # Spec. Yield stress R eh N/mm 2 UTS R m N/mm 2 elongation e max % Direction regarding to center line 1 P ,8 2 P ,2 Transversal 3 P ,6 4 P ,0 1 V ,4 2 V ,6 Longitudinal 3 V ,7 4 V ,7 Measured Charpy impact At room temp. +20 C ISO-V J/80mm 2 D-1 45,5 D-2 49,3 Longitudinal propagation D-3 44,1 Parallel to center line E-1 105,5 E-2 83,1 Transversal to center line E-3 78,0 Tensile properties does not corresponding to declare steel (34CrNiMo6) A1 8

9 Materials-Fracture behaviour testing 3PB Specimen have been used in order to determine fracture behavior of material regarding to different crack plane orientation) Specimens A from the surface to middle of axle. Specimens B-in longitudinal direction (same as detected cracks)! Specimens C-perpendicular to center line of axle (usually most dengerous orientation! 9

10 Materials-Fracture behaviour testing Fracture behavour test was performed according to ASTM 1820 standard and GKSS GTP-2004! 10

11 Materials-Fracture behaviour testing Experimentaly obtained CDF in term of CTOD(δ5) CTOD- 5, mm 4 3,6 3,2 2,8 2,4 2 1,6 1,2 0,8 0,4 Series of spec. B Series of spec. A Series of spec. C a1 a2 a3 b1 b2 c1 c2 c4 c Load, kn 11

12 Materials-Fracture behaviour testing The worst fracture behaviour shows specimens B-in longitudinal direction (same as detected cracks)! The best fracture behaviour shows specimens C- Perpendicular to center line of axle. 12

13 Fractographic analysis.1 Fractographic analysis of specimen A A from the surface to middle of axle 13

14 Fractographic analysis.2 Fractographic analysis of specimen B B-in longitudinal direction (same as detected cracks)! 14

15 Fractographic analysis.2 Fractographic analysis of specimen B B-in longitudinal direction (same as detected cracks), detail at x25 mag.! 15

16 Fractographic analysis.2 Fractographic analysis of specimen C Specimens C-perpendicular to center line of axle (usually most dengerous orientation! 16

17 Fractographic analysis.2 Fractographic analysis of specimen C Specimens C-perpendicular to center line of axle (usually most dengerous orientation, detail at x25 mag! 17

18 Inspection of axles Magnet-flux techniques Results of inspections on same axles in two years. Mark of axle and year of delivering Number of detected long. cracks inspection Number of detected long. cracks inspection Decision cracks (20mm, 18mm) More than 40 Not further use cracks (25, 27, 20, 25, 10) More than 50 No further use cracks (15, 18, 20, 30) 36 short surfce cracks Crack in critical area No further use cracks (20, 18, 25, 23) More than 40 Crack in critical area No further use No cracks No cracks Back in use 18

19 Inspection of longitudinal crack.2 Cut perpendicular to centre line of axle 19

20 Inspection of longitudinal cracks Part of specimen was bended in order to find plastification around longitudinal cracks a) Short but longitudinal crack under surfce of specimen b) Detail of crack 20

21 Inspection of longitudinal cracks Specimen with surface longitudinal crack (2c=5 mm and depth a=4 mm) at ege of specimen 21

22 FEM Analysis Aim: Find effect of bending stress to stress concentration in case of longitudinal crack with short perpendicular surface crack s extension) Find effect of combine loading (bending and torsion) to stress concentration Of longitudinal crack with short perpendicular surface crack s extension 22

23 FEM Analysis 2c=100 mm; a=5 mm; b=0, b=0.2 mm, b=2 mm! 23

24 FEM Analysis Used 3D FEM model for bending loading σ max =100 MPa 24

25 FEM Analysis 2c=100 mm; a=5 mm; b=0.2 mm, -maximum bending load σ max =100 MPa Stress concentration remain at longitudinal crack! 25

26 FEM Analysis 2c=100 mm; a=5 mm; b=2 mm, -maximum bending load σ max =100 MPa Stress concentration appear at perpendicular crack tip! 26

27 FEM Analysis Used 3D FEM model for combine loading (bend σ max =100 MPa + torsion τ=18 MPa) 27

28 FEM Analysis 2c=100 mm; a=5 mm; b=0, -bending load σ max =100 MPa + t=18 MPa 28

29 FEM Analysis 2c=100 mm; a=5 mm; b=0.2 mm, -bending load σ max =100 MPa + t=18 MPa Stress concentration remain at longitudinal crack! 29

30 FEM Analysis 2c=100 mm; a=5 mm; b=2 mm, -bending load σ max =100 MPa + τ=18 MPa Stress concentration appear at perpendicular crack tip! 30

31 FEM analysis Why longitudinal crack appear only on the surface? 31

32 Analysis of surface Analysis of surface of longitudinal crack 2c=300 mm and a=4 mm a) Crack wall close to surface b) Crack wall inside of material decarbonation with ferritical strips 32

33 Analysis of surface Analysis of surface of longitudinal crack 2c=300 mm and a=4 mm SEM with decarbonate area close to surface of crack wall a) at the crack tip and b) at the middle of crack 33

34 Analysis of surface Analysis of surface of longitudinal crack 2c=300 mm and a=4 mm SEM of cracked surface 34

35 Analysis of surface Analysis of surface of longitudinal crack 2c=300 mm and a=4 mm EDS analysis of decarbonated area shows mainly ferrit 35

36 Metallography analysis Ferrite perlite microstructure Longitudinal orientation 36

37 Metallography analysis Ferrite perlite microstructure MnS Longitudinal orientation 37

38 Metallography analysis Ferrite perlite microstructure MnS Longitudinal orientation 38

39 FEM analysis Axle is made by forging, qunched and temperate condition! Residual stresses shall be less than of equal +100 MPa (EN 13261:2003)! 39

40 FEM analysis Axle is made by forging, qunched and temperate condition! Forging for 1mm depth cause tensile stresses more than +450 MPa! 40

41 FEM analysis Residual stresses distribution after surface depth deformation 1mm! σ 11 0 Stress, MPa σ 33 σ 11 σ distance from the surface, mm Residual stresses are less than 100 MPa in radial direction! 41

42 FEM analysis Residual stresses distribution after surface depth deformation (cold deformation)! depth of reduction: 800 Residual stress σ33, MPa σ 33 h=0.5; path1 h=1.0; path distance from surface, mm Tensile residual stresses are more than 300 MPa in axial direction when axles are rolling! 42

43 FEM analysis Residual stresses distribution when volume of outer boundary material increasing for 15% regarding to core! Change from ferrite to martenzite causes compress residual stresses 43

44 FEM analysis Residual stresses distribution when volume of outer boundary material increasing for 15% regarding to core! Change from ferrite to martenzite causes compress residual stresses 44

45 FEM analysis of residual stresses Analysis of residual stresses when volume are 15% greather at the outer baundary than in the centre! 600 σ eq von Mises σ 2-princ =σ 33-axial 400 Residual stresses, MPa σ 1-princ =σ 22-radial σ 3-princ =σ 11-tang σ 11 σ 33 σ depth below surface, mm Inductive surface quenching of axle causes compress residual stresses 45

46 Conclusion The performed analysis show that longitudinal cracking is not consequence of loading or load hystory Longitudinal cracking can be consequence of forging process and material ageing process During service of axles the number and size of longitudinal cracks increasing. It should be necessary to define criteria for exlusion axles with longitudinal crack regarding to number of detected longitudinal cracks between two inspection 46

47 Conclusion Rolling is not appropriate process for axles fabrication! Forging produce tensile residual stresses at outer boundary of axles! Thermomechanical treatment should reduce transversal residual stresses! It seems that more appropriate is inductive quenching, tempering, turning and griding as final treatment for railway axles (suggested by Hirakawa)! Microstructural transformation to martenzite causes 15% increasing of material volume! It causes transversal residual stresses! 47