Leo Vega

SEMINAR ON LEAK REDUCTION

Bolt Failure Analysis

Topics of Discussion

• Elements of a Failure Analysis• Metallurgical Characterization Techniques• Types of Damage Mechanisms • Case Studies• Summary

Typical Elements of a Failure Analysis

• History of Component• Visual Examination• Photographic

Documentation• Sample Selection• Collect Deposits • Sectioning

• Fractography• Metallography• SEM/EDS/XRD• Chemical Analysis• Mechanical Testing• NDE

Metallurgical Characterization Techniques

• Visual Examination• Metallography• Fractography• Scanning Electron Microscopy (SEM) and

Energy Dispersive X-ray Spectroscopy (EDS)• Hardness Testing• Non-destructive Evaluation (NDE)

Visual Examination

Fractography

Flat fracture

Shear lip

Microstructural EvaluationMetallography

• To assess material condition and features

SEM/EDS and XRD AnalysisDeposit Analysis

1000 ×100 µm

keV109876543210

Cou

nts

3,500

3,000

2,500

2,000

1,500

1,000

500 C

O

Si Cl Ca Mn

Fe

Fe

Fe

Ni

Ni

Ni CuCu

Cu

Hardness Testing

Rockwell tests on a failed shank

110

Hardness on Failed Shaft

No. Hardness, HRC

1 35

2 36

3 30

4 31

5 31

6 29

7 31

8 27

9 26

10 19

Micro Hardness Testing

Microhardness Values of Sample in Longitudinal Orientation

0

100

200

300

400

500

600

700

0.0030.050.0080.010.0130.0150.0180.020.0230.0250.0280.03

Distance from Edge of Hole, inches

Kno

op H

ardn

ess

Drilled Hole

Laser Cut Hole

NDE

Wet magnetic particle Inspectionof cracked bolt

Types of Damage Mechanisms

• Fatigue Fractures• Wear• Welding Defects• Corrosion• Casting Flaws• Liquid Metal

Embrittlement

• Selective Dissolution • Manufacturing/Metal

Working Defects• Quality Assurance/

Quality Control (QA/QC)

Fatigue FracturesMechanical Failures

Beach marks (macroscopic features)

Fatigue Striations (microscopic features)

WearGear reduction box

Evidence of a surface treatment

Absence of a surface treatment

Corrosion

Compressor Bi-metallic oil seal

Pitting and subsequent cracking – caused by water in zinc-type antiwear additives operating above 140 deg F

Liquid Metal Embrittlement

• SEM/EDS of Crack on Worn Shaft

PbSn, Sb

Ni alloy

Selective Dissolution• Dealuminification in

NiAl Bronze –selective attack of a phase

Manufacturing/Metal Working Defects – QA/QC Inspections

• Inspection at supplier’s facility

• Incoming material inspection

• In-process inspection

• Finished material inspection

Pipe Mill Defect -QA/QC

Extrusion Laps

Case Studies

• Case Study #1Failure of Compressor Cylinder Flange/Bolts (Fatigue)

• Case Study #2Failure of Riser Fasteners (Hydrogen Embrittlement/Intergranular Cracking)

• Case Study #3A fun one; Failed Spinal Screws (Fatigue)

Case Study #1 - Compressor Station “Hick-up” PN 113175

Case Study #1

• Fractured bolts and flange at compressor cylinder

Case Study #1 – “The Line Up”

SEM AnalysisFatigue Striations

Cut Threads

Case Study #1

276283283

• Knoop micro hardness impressions at thread root.

• Cut threads are evident.

Case Study #1 - Conclusions

• Fasteners failed by High Cycle Fatigue• Material and strength levels were adequate• Increase frequency of inspections• Bolts with cut threads should be considered as

replacements.

Case Study #2• Fasteners in bolted flange connection operating

in a seawater environment

Case Study #2 - FractographyIntergranular Fracture

Case Study #2 - Metallography

Intergranular cracking

Case Study #2 – Hardness Testing

• Micro hardness tests on banded structure

Case Study #2

• Failure due to Hydrogen Embrittlement• Intergranular fracture mode• Improper heat treatment revealed by hardness

variations in a banded structure, which may have contributed to the failure.

Case Study #3

• Fractured Spinal Screw

• Stage I – Crack Initiation• Stage II – Crack

Propagation• Stage III - Overload

Stage I Stage II

Stage III

What to do (or not do)!

SUMMARY• A variety of characterization

techniques are used in a metallurgical evaluation to characterize various damage mechanisms.

• Some projects require limited scope based on budget constraints.

• Some require multi-disciplined engineering support and extensive evaluations

• Each technique provides specific information that allows the investigator to assess the results and their contribution to the cause.

• Metallurgical Failure Analysis serves numerous clients in areas such as:

Failure Analysis and PreventionThird party review, audits, and oversight of inspectionsOn site metallurgical evaluations.Forensic Investigations

Metallurgical Failure Analysis

Leo VegaContact Info:281-897-1045 direct832-868-7449 cell