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

SLIDE 1

1

Improving the Fatigue Resistance of Thermite Railroad Rail Weldments

F. V. Lawrence

Y-R. Chen

J. P. Cyre

SLIDE 2

2

Outline

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

SLIDE 3

3

Metallic Fatigue

A.M. Zarembski – Bulletin 673, 1979, Volume 80 of AREA proceedings

ACELA

SLIDE 4

4

Rolling contact fatigue

Thermite Weld Rail

B H W

Railroad car wheel moving over rail causes fatigue to

ccur in both the rail

head and base.

SLIDE 5

5

Fatigue crack initiation sites

Internal Fatigue Crack Rail Head Rail Web Web-to-base Fillet Fatigue Crack at Weld Toe in Fillet Rail Base Fatigue Crack at Weld Toe in Base

! ≈ 40% of all service

failures are due to thermite field welds.

!

≈ 10% of all derailments are due to broken field welds.

SLIDE 6

6

Fatigue crack in rail head

Internal fatigue crack initiation in rail head

SLIDE 7

7

Fatigue crack in rail base

Cold Lap Site of crack initiation Limit of fatigue crack growth

SLIDE 8

8

Thermite weld service failures

0.0% 5.0% 10.0% 15.0% 20.0% 25.0% 30.0% 35.0% Base Web-base fillet Web Head-web fillet Head ?

Location

Record of 244 service failures on a Class 1 railroad involving thermite field welds.

SLIDE 9

9

Service failures or “markouts”?

! Most field-weld service failures

riginate 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!

SLIDE 10

10

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

ccurs at external stress concentration.

! Fatigue cracks in head are more

frequent but are generally removed. Crack initiation occurs at internal stress concentration.

SLIDE 11

11

Outline

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

SLIDE 12

12

Porosity types

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

SLIDE 13

13

Porosity initiates fatigue

Formation of shells in tangent track at interdendritic shrinkage porosity. Running surface.

Odario 1992

SLIDE 14

14

Interdendritic shrinkage porosity

SLIDE 15

15

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.

SLIDE 16

16

Squeeze welding

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

SLIDE 17

17

Weld longitudinal-sections

Standard Squeezed WM HAZ BM

Fry 1992

SLIDE 18

18

Laboratory test results

10 100 1000 104 105 106 107 108 Withee - Squeezed Liu - Squeezed/Vibrated Liu - Squeezed Liu - Vibrated Liu - Standard Withee - Vibrated Withee - Standard Maximum Stress, Smax (MPa) Fatigue Life, N f (cycles)

Fatigue behavior of small specimens taken from head of weld shows some improvement.

Withee 1998

87 mm 19 mm 9.27 mm R 208 mm 19 mm 6.35 mm

SLIDE 19

19

But distribution unchanged!

0.0 0.2 0.4 0.6 0.8 1.0 100 101 102 103 104 105 Standard Weld Squeezed Weld Vibrated Weld Cumulative Probability Pore Size, area ( µm2) size range of pores initiating failure imputed from SEM images

Pore size distribution unchanged!

Withee 1998

SLIDE 20

20

Largest pore size controls!

1.0 10.0 100.0 104 105 106 107 Standard Weld (C) Squeezed Weld (B) Vibrated Weld (D) Regression Analysis Initial Stress Intensity Factor, K o (MPa*m

1/2)

Fatigue Life, Nf (cycles) B1 C1 C5 D5 B4 B3 B2 1 3

Withee 1998

Single relation for all treatments depending only on pore size (and applied stress).

87 mm 19 mm 9.27 mm R 208 mm 19 mm 6.35 mm

SLIDE 21

21

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.

SLIDE 22

22

Theoretical study

p Σ(t)

(t)

Fry 1995

SLIDE 23

23

Stress history experienced

Stress MPa)

SLIDE 24

24

Fatigue occurs at critical depth

Fatigue damage parameter Worst depth Worst locations on pore Depth below running surface, Y (mm) No residual stress Considering residual stress

Fry 1995

SLIDE 25

25

Effects of pore shape?

2

0.5 1 1.5 2 2.5 3 3.5 0.5 1 1.5 2 2.5

2

4 6 8 10 15 20 25 30 40 50 60 72

λ

Y /λ Z

Ratio of pore's longitudinal and transverse axes, λX / λZ

FBY PCV FBX Sphere PCH FBZ PCT Detail Fracture Vertical Split Head Shelling RAHELS Predictions

Favors Detail fracture Favors vertical split-head Favors shelling

Fry 1995

SLIDE 26

26

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.

SLIDE 27

27

Central portion of weldment machined and ground flat to 12.7 mm thickness. Stepped penetrameter. specimen film

New measurement technique

Chen 2000

SLIDE 28

28

L1

Typical radiograph

!?! Difference in contrast due to micro-porosity (shrinkage porosity. Porosity not uniformly distributed!

Chen 2000

SLIDE 29

29

Radiographs of field welds

F1 F2 F3

SLIDE 30

30

0.52% 1.72% interface

Optical determination of porosity

SLIDE 31

31

25000 27000 29000 31000 33000 35000 37000 39000 500 1000 1500 2000 2500 3000 3500 4000 Distance

Measured changes in grey scale in photoshop. Penetrameter with 0.11 mm steps indicate at least 1% sensitivity

F1 BM WM 0% 0.9% porosity

Radiographic image density

SLIDE 32

32

0.2 0.4 0.6 0.8 1 1.2 1.4 B-3 B-4 A-6 A-7 A-8 A-1 L-1 B-1 A-2 A-3 Porosity (%)

Porosity in 10 thermite welds

Average porosity in 10 “markouts” varies considerably!

Chen 2000

SLIDE 33

33

Developing detail fracture

Detail fracture in head of rail appears to be developing in association with an area with a high concentration

f shrinkage

porosity?

SLIDE 34

34

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.

SLIDE 35

35

Why?

A-1 A-2 A-3 A-4 A-8 A-9 A-10 B-1 L-short L-long 100mm

Apparently there are large variations in thermal conditions during thermite welding. Observed variations in melt-back (weld profile) on radiographs.

SLIDE 36

36

Outline

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

SLIDE 37

37

Thermite weld service failures

0.0% 5.0% 10.0% 15.0% 20.0% 25.0% 30.0% 35.0% Base Web-base fillet Web Head-web fillet Head ?

Location

Record of 244 service failures on a Class 1 railroad involving thermite welds.

SLIDE 38

38

Web-to-base fillet !?!

! Why does this happen ????? ! Answer:

! Residual stresses! ! Weld toe geometry!

! Flank angle. ! Cold laps.

SLIDE 39

39

Webster et al.

Residual stresses

Neutral Axis Critical locations:

Web-to base fillet
Rail base

Compression Tension

SLIDE 40

40

Weld toe flank angle

≈ 85˚ Flank Angle

SLIDE 41

41

Weld toe geometry

Toe Radius (r) Roughness (R) Flank Angle Weld Metal Base Metal

(θ)

Fatigue Severity = 1+0.27 tanθ0.25 t r       1+0.1054Su R

( )−1

Improve by:

Flank angle ↓


↑



↓

SLIDE 42

42

Current Orgo-thermit mold profiles

A-A B-B C-C D-D E-E

45 30

A-A

AA BB CC DD

Mold Rail and weld Measured profiles of Orgo-thermit molds

SLIDE 43

43

Modified Orgo-thermit mold profiles

Suggested modifications to Orgothermit molds

AA BB CC DD Modified Current

SLIDE 44

44

Nature of critical defects

0.0% 5.0% 10.0% 15.0% 20.0% 25.0% 30.0% 35.0% Cold Lap Slag Hot Tear Porosity Lack of Fusion Columnar Grains in Head Grind Burn Hot Pull-apart Inclusion in Head Sand burn in ?

Analysis of 244 service failures on a Class 1 railroad involving thermite welds.

SLIDE 45

45

Cold laps - Dimitrakis

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

SLIDE 46

46

Cold laps at thermite weld toe

Cold Lap Base Metal

SLIDE 47

47

Weld toe cold laps

Loading Direction D

Weld Metal Base Metal Heat Affected Zone

Weld Toe Location Without Cold-Lap Defect Curved Path Vertical Path θ r

φ

SLIDE 48

48

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%

SLIDE 49

49

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?

SLIDE 50

50

Variations in melt-back

A-1 A-2 A-3 A-4 A-8 A-9 A-10 B-1 L-short L-long 100mm

Melt back varies considerably in the location of the web-to base fillet

SLIDE 51

51

Melt back dimensions

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 6 5 4 3 2 1

Height of Rail (in.) Length of Melt Back (in.) Weld Sample #2 Weld Sample # 3 Weld Sample #1

SLIDE 52

52

Melt back at web-to-base fillet

10 20 30 40 50 26-30 31-35 36-40 41-45 46-50 51-55 56-60 61-65 66-70

Melt-back width (mm)

Collar width defined by mold

Bad! Good!

SLIDE 53

53

UIUC Experimental Program

UIUC modified 1” thermite weld molds are used with a 1.4” rail gap. Mold sealed at weld toe with refractory paste. And: Reduced flank angle! Standard 1” thermite weld: Large flank angle and cold laps.

Rail Rail Weld Metal Melt back

SLIDE 54

54

UIUC Experimental Program

Lutting paste from Railtech. Sealing paste from Railtech w/ Brazing Flux.

SLIDE 55

55

UIUC Experimental Program

Uni Ram Blu refractory paste. Leecote mold wash and Uni Ram Blu refractory paste.

SLIDE 56

56

Weld Fabrication 1 3 4 6 5 2

SLIDE 57

57

Fatigue testing

Standard 4-point bending test.

SLIDE 58

58

Modified Weld Specimen #28

Crack initiation points Limit of fatigue crack growth

SLIDE 59

59

Effect of modifications

Cold lap formation beyond sealing paste

SLIDE 60

60

Standard weldments

100 1,000 10,000 1,000 10,000 100,000 1,000,000 10,000,000

Cycles to Failure, N

f

Process A Process B Process C Process D TAMU 1 3

SLIDE 61

61

100 1,000 10,000 1,000 10,000 100,000 1,000,000 10,000,000

Cycles to Failure, N

f

Process A Process B Process C Process D Modified UIUC TAMU

UIUC experimental welds

1 3

SLIDE 62

62

100 1,000 10,000 1,000 10,000 100,000 1,000,000 10,000,000

Cycles to Failure, N

f

Process A Process B Process C Process D Modified UIUC TAMU

UIUC experimental welds

1 3

SLIDE 63

63

Summary - Head failures

! Head failures caused by internal

defects notably porosity and high concentration areas of porosity.

! Thermal conditions during

solidification may cause one weldment to be good and another to be bad?

SLIDE 64

64

Summary - Web-base failures

! Web and base failures aggravated by

severe external geometry and cold laps.

! Thermal conditions during

solidification play a role in web-base fatigue problems?

! Fatigue life can be increased by