Bolt Failure Analysis
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Transcript of Bolt Failure Analysis
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