Improving the Fatigue Resistance of Thermite Railroad Rail - PowerPoint PPT Presentation

Improving the Fatigue Resistance of Thermite Railroad Rail Weldments F. V. Lawrence Y-R. Chen J. P. Cyre 1

Outline ! Fatigue problems with thermite welds ! Improving the rail head ! Improving the rail web and base 2

Metallic Fatigue ACELA A.M. Zarembski – Bulletin 673, 1979, Volume 80 of AREA proceedings 3

Rolling contact fatigue Railroad car wheel moving over rail causes fatigue to occur in both the rail head and base. H Rail W B Thermite Weld 4

Fatigue crack initiation sites ! ≈ 40% of all service failures are due to Internal Fatigue Crack thermite field welds. Rail Head ≈ 10% of all ! derailments are due to broken field Rail Web welds. Web-to-base Fillet Fatigue Crack at Weld Toe in Fillet Rail Base Fatigue Crack at Weld Toe in Base 5

Fatigue crack in rail head Internal fatigue crack initiation in rail head 6

Fatigue crack in rail base Cold Lap Site of crack initiation Limit of fatigue crack growth 7

Thermite weld service failures 35.0% 30.0% 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% Base Web-base Web Head-web Head ? fillet fillet Location Record of 244 service failures on a Class 1 railroad involving thermite field welds. 8

Service failures or “markouts”? ! Most field-weld service failures originate at web or base. ! But defects detected and removed from the rail head before a service failure can occur (“markouts”) exceed service failures by 2:1! 9

Implications ! Fatigue cracks in the web and base are less frequent but are the principal cause of service failures since they are difficult to detect. Crack initiation occurs at external stress concentration. ! Fatigue cracks in head are more frequent but are generally removed. Crack initiation occurs at internal stress concentration. 10

Outline ! Fatigue problems with thermite welds ! Improving the rail head ! Improving the rail web and base 11

Porosity types Shrinkage Gas (Spherical) Thermite welds studied contain about 1.5% shrinkage porosity. 12

Porosity initiates fatigue Odario 1992 Running surface. Formation of shells in tangent track at interdendritic shrinkage porosity. 13

Interdendritic shrinkage porosity 14

Possible solution? ! Eliminate weld metal! (?) ! Developed a modified thermite welding process called “Squeeze Welding” in which ends of joint forced together to expel most of the thermite weld metal. 15

Squeeze welding Rail Ends Moved Together While Weld Expelled Imputities Metal Still Molten Force Force Final Weld Thickness Rail Cross-section 16

Weld longitudinal-sections Squeezed Standard WM HAZ BM Fry 1992 17

Laboratory test results Withee 1998 1000 Maximum Stress, S max (MPa) 100 Liu - Standard Withee - Standard Withee - Squeezed Liu - Squeezed Withee - Vibrated Liu - Vibrated Liu - Squeezed/Vibrated 10 10 4 10 5 10 6 10 7 10 8 Fatigue Life, N f (cycles) Fatigue behavior of small specimens taken from head of 6.35 mm 9.27 mm weld shows some improvement. 19 mm 19 mm R 208 mm 18 87 mm

But distribution unchanged! 1.0 0.8 Cumulative Probability size range of pores 0.6 initiating failure imputed from SEM images 0.4 Standard Weld 0.2 Squeezed Weld Vibrated Weld 0.0 10 0 10 1 10 2 10 3 10 4 10 5 Pore Size, area ( µ m 2 ) Withee 1998 Pore size distribution unchanged! 19

Largest pore size controls! 100.0 6.35 mm 9.27 mm 1/2 ) Initial Stress Intensity Factor, K o (MPa*m Standard Weld (C) 19 mm 19 mm R 208 mm Squeezed Weld (B) 87 mm Vibrated Weld (D) Regression Analysis B2 B1 10.0 B4 D5 B3 1 C5 C1 3 Withee 1998 1.0 10 4 10 5 10 6 10 7 Fatigue Life, N f (cycles) Single relation for all treatments depending only on pore size (and applied stress). 20

Implications ! Reducing the size of the largest pores and/or the volume of weld metal should increase in the (average) fatigue life. ! Largest pore per unit volume (porosity) and the volume of weld metal jointly determine the fatigue strength. 21

Theoretical study Σ (t) p (t) Fry 1995 22

Stress history experienced Stress MPa) 23

Fatigue occurs at critical depth Depth below running surface, Y (mm) No residual stress Considering residual stress Worst depth Worst locations on pore Fatigue damage parameter Fry 1995 24

Effects of pore shape? λ X / λ Z Ratio of pore's longitudinal and transverse axes, Favors vertical 0 0.5 1 1.5 2 2.5 split-head RAHELS Predictions 3.5 Shelling Z Y / λ Vertical Split Head 3 λ Detail Fracture FBY 2.5 2 4 Fry 1995 2 6 Favors PCV 8 1.5 Detail 10 fracture 15 1 20 25 30 PCT FBX 40 50 0.5 60 72 2 Favors 0 shelling Sphere FBZ PCH 25

Model predictions ! Critical depth for fatigue crack initiation ( ≈ 15mm) determined by wheel-contact-induced residual stresses. ! Model predicted that shelling, vertical split heads and detail fracture could all initiate at shrinkage pores depending upon the pore shape. 26

New measurement technique specimen film Central portion of weldment machined and ground flat to Stepped penetrameter. 12.7 mm thickness. Chen 2000 27

Typical radiograph L1 Difference in contrast due to micro-porosity (shrinkage porosity. !?! Porosity not uniformly distributed! Chen 2000 28

Radiographs of field welds F1 F2 F3 29

Optical determination of porosity 1.72% interface 0.52% 30

Radiographic image density 0% 0.9% porosity 39000 37000 35000 33000 31000 29000 27000 25000 0 500 1000 1500 2000 2500 3000 3500 4000 Distance Measured F1 changes in grey scale in WM BM photoshop. Penetrameter with 0.11 mm steps indicate at least 1% sensitivity 31

Porosity in 10 thermite welds Chen 2000 1.4 1.2 1 Porosity (%) 0.8 0.6 0.4 0.2 0 B-3 B-4 A-6 A-7 A-8 A-1 L-1 B-1 A-2 A-3 Average porosity in 10 “markouts” varies considerably! 32

Developing detail fracture Detail fracture in head of rail appears to be developing in association with an area with a high concentration of shrinkage porosity? 33

Conclusions ! Large variation in porosity from weld to weld. Porosity not uniformly distributed. ! Porosity clusters at weld centerline frequently seen. Fatigue cracks in head often associated with associated with porosity clusters. 34

Why? Apparently there are large variations in thermal conditions A-4 A-8 A-2 A-3 A-1 during thermite welding. Observed variations in melt-back (weld profile) on radiographs. A-9 A-10 B-1 L-short L-long 100mm 35

Outline ! Fatigue problems with thermite welds ! Improving the rail head ! Improving the rail web and base 36

Thermite weld service failures 35.0% 30.0% 25.0% 20.0% 15.0% 10.0% 5.0% 0.0% Base Web-base Web Head-web Head ? fillet fillet Location Record of 244 service failures on a Class 1 railroad involving thermite welds. 37

Web-to-base fillet !?! ! Why does this happen ????? ! Answer: ! Residual stresses! ! Weld toe geometry! ! Flank angle. ! Cold laps. 38

Residual stresses Compression Tension Neutral Axis Critical locations: • Web-to base fillet • Rail base Webster et al . 39

Weld toe flank angle ≈ 85 ˚ Flank Angle 40

Weld toe geometry Improve by: • Flank angle ↓ Toe Radius (r) • ���  ������ Flank Angle ↑ (θ) • Roughness (R) ���������  Weld Metal ↓ Base Metal   ( ) − 1 t Fatigue Severity = 1 + 0.27 tan θ 0.25  1 + 0.1054 S u R    r 41

Current Orgo-thermit mold profiles Measured profiles of AA Orgo-thermit molds BB A-A A-A CC B-B 30 45 D-D C-C E-E DD Mold Rail and weld 42

Modified Orgo-thermit mold profiles Modified Current AA Suggested modifications to BB Orgothermit molds CC DD 43

Nature of critical defects ? Sand burn in Inclusion in Head Hot Pull-apart Grind Burn Columnar Grains in Head Lack of Fusion Porosity Hot Tear Slag Cold Lap 0.0% 5.0% 10.0% 15.0% 20.0% 25.0% 30.0% 35.0% Analysis of 244 service failures on a Class 1 railroad involving thermite welds. 44

Cold laps - Dimitrakis Cold lap No cold lap Cold laps greatly reduce the fatigue life of a weldment 45

Cold laps at thermite weld toe Cold Lap Base Metal 46

Weld toe cold laps r φ θ D Weld Metal Weld Toe Location Without Cold-Lap Defect Heat Affected Zone Base Metal Loading Direction Vertical Path Curved Path 47

Effect of cold laps Condition Percentage of Fatigue Life Flank angle ( θ ) = 30Þ 100% Flank angle ( θ ) = 45Þ 56% Flank angle ( θ ) = 60Þ 44% Cold lap depth (D) = 0 100% Cold lap depth (D) = 1mm 20% Cold lap depth (D) = 2mm 15% 48

Causes of cold laps ! Gap between mold and rail in the critical web-to-base fillet area. ! Inadequate melt back causing incomplete fusion at the weld toe? 49

Variations in melt-back Melt back varies considerably A-4 A-8 A-2 A-3 A-1 in the location of the web-to base fillet A-9 A-10 B-1 L-short L-long 100mm 50

Melt back dimensions 0 1 Height of Rail (in.) 2 Weld Sample #2 3 Weld Sample # 3 4 Weld Sample #1 5 6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Length of Melt Back (in.) 51