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1Ludwig & Associates Engineering Ltd.
Metallurgical Failure Analysis:Techniques and Case Studies
Prepared By:Derrick Sarafinchan, M.Eng., P.Eng.
2Ludwig & Associates Engineering Ltd.
Failure Analysis Objectives Primary objective
! Identify the root cause for a particular failure
1. Metallurgical Evaluation
2. Root Cause Analysis
Failed Component(s) Failure Analysis Investigation Reporting: Examination Findings Failure Mechanism(s) Probable Root Cause
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Failed Component
Failure: A term which infers that a component or assembly is in a state of damage that renders it no longer capable of reliably performing its intended function.
Definitions
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Failed Component
! Prior Damage: damage that may have contributed to the failure
! Consequential Damage: damage that resulted due to continued operation post failure
! Mechanical/Environmental Damage: subjected to the failed component!Handling
!Cleaning
!Storage
Definitions
Secondary Damage: Inadvertent changes to a component that may or may not affect functionality.
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Fracture (full section) Cracking (partial section) Distortion (bending, elongation, plastic collapse) Corrosion (pitting, through wall perforation) Wear (material wastage)
Failed ComponentGeneral Failure Categories
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Failure Analysis Investigation
A. Examination and Testing WHAT physical evidence can be attained from the
failed component(s)?
B. Identify Failure Mechanism(s) HOW did the damage/failure occur, in metallurgical
terms?
1. Metallurgical Evaluation
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Metallurgical Evaluation
1. Characterize Failure Identify Failure Origin Characterize fracture textures
A. Examination and TestingPrimary Objectives:
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Fracture Examples
! Ductile shear lip Fracture Macro Textures:
Metallurgical EvaluationA. Examination and Testing
Primary Objectives:
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Metallurgical Evaluation
Fracture Examples Fracture Macro Textures:
!Chevron markings
1. Characterize Failure
A. Examination and Testing
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Fracture Examples Fracture Macro Textures:
! Ratchet markings! Beach markings 1
Metallurgical Evaluation
1. Characterize Failure
A. Examination and Testing
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Fracture Examples Fracture Micro Textures:
!Intergranular
Metallurgical Evaluation
1. Characterize Failure
A. Examination and Testing
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Fracture Examples Fracture Micro Textures:
! Transgranular ! Dimple (MVC)
Metallurgical Evaluation
1. Characterize Failure
A. Examination and Testing
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Fracture Examples Fracture Micro Textures:
! Transgranular ! Cleavage
Metallurgical Evaluation
1. Characterize Failure
A. Examination and Testing
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Fracture Examples Fracture Micro Textures:
! Transgranular ! Striated! Fibrous textured
Metallurgical Evaluation
1. Characterize Failure
A. Examination and Testing
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2. Characterize and categorize other forms of visual damage present. Secondary cracks along component or adjacent to fracture Anomalies associated with the region of initiation or otherwise
potentially contributory to the failure
Metallurgical EvaluationA. Examination and Testing
Primary Objectives:
3. Characterize the material. Microstructure Composition Mechanical Properties
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1. Visual examination and NDE (by eye)
2. Macroscopic examination (stereomicroscope) provides depth of field magnification to about 20X
3. Microscopic examination Fractures - examined by SEM to characterize fracture textures
(20X to 3000X magnification) Prepared cross section - optical microscopy (no depth of field
magnifications between 50 and 1000X)! Characterize microstructure and damage/fracture profiles
Metallurgical EvaluationA. Examination and Testing
Tools and Techniques:
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4. Chemical Analysis Generally for grade verification or impurity evaluation Examples:
!Stainless steel corrosion/oxidation resistance!Alloy steel hardenability, temper embrittlement resistance,
weldability
Metallurgical EvaluationA. Examination and Testing
Tools and Techniques:
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4. Chemical AnalysisTypical Methods
Bulk Analysis !Spark Emission!Wet Analysis
Microanalysis !SEM EDX
!XRF/XRD (non-metallic scales and friable substances)
Metallurgical EvaluationA. Examination and Testing
Tools and Techniques:
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5. Mechanical Testing Generally for purposes of grade determination/verification,
or estimation of component fracture properties
Examples:! Material strength and ductility! Fatigue properties! Fracture resistance
Metallurgical EvaluationA. Examination and Testing
Tools and Techniques:
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5. Mechanical TestingTypical Methods
Hardness testing (micro and macro) Tensile testing (yield, ultimate, and elongation) Charpy V-notch impact testing (temperature transition, value
conversion for Level 1 fracture mechanics assessment) Fatigue testing (axial or bending) Fracture toughness testing (CTOD)
Metallurgical EvaluationA. Examination and Testing
Tools and Techniques:
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Definition Specific series of events that describe both how the
damage was incurred and the resulting consequences.
Metallurgical Evaluation
Examples Metal fatigue Stress corrosion cracking Hydrogen embrittlement
Ductile or brittle fracture
High temperature creep, oxidation, sulphidation
B. Identification of Failure Mechanism(s)
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General Categories of Failure Root Cause
Metallurgical deficiencies Mechanical overload Service environment Manufacturing deficiency Remanufacturing/repair deficiency Design deficiency
Failure Analysis Investigation2. Root Cause Analysis
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Root Cause Analysis
Castings - Porosity (shrinkage)
Common Metallurgical Deficiencies
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Root Cause Analysis
Castings - Poor homogeneity
Common Metallurgical Deficiencies
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Root Cause Analysis
Castings - Microsegregation (Al-N embrittlement)
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3
Common Metallurgical Deficiencies
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Wrought Materials (rolled, forged, etc.) Inadequate heat treatment
! Inadequate case depth!Poor transformation
Poor chemical homogeneity
Root Cause AnalysisCommon Metallurgical Deficiencies
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Root Cause Analysis
Wrought Materials (rolled, forged, etc.) Poor material cleanliness (eg. sulphur, phosphorus,
tramp elements)
Common Metallurgical Deficiencies
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Root Cause Analysis
Wrought Materials (rolled, forged, etc.) Forging flaws (seams, laps)
Common Metallurgical Deficiencies
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Root Cause Analysis
Lack of fusion, lack of penetration Slag Insufficient preheat (elevated hardness, residual stresses) Elevated hydrogen Under matched consumable selection
Welds
Common Metallurgical Deficiencies
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Root Cause Analysis
Metallurgical deficiencies Remanufacturing/
repair deficiency Mechanical overload Service environment Manufacturing deficiency Design deficiency
Metallurgy determines cause
Metallurgy alone is not sufficient
General Categories
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1. Assess for mechanical overload relative to rated capacity.
2. Assess for a critical manufacturing deficiency relative to component design.
3. Examine component design for deficiencies relative to intended service conditions.
Root Cause Analysis
Example: Mechanical Failures
General Approach
Solving the Problem when the Metallurgy Appears Sound
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Attain background information regarding the failure and equipment
Understand the mechanics of the equipment and the operating conditions
1. Attain additional information and knowledge:
Root Cause Analysis
Example: Mechanical Failures
Detailed Approach
Solving the Problem when the Metallurgy Appears Sound
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2. Application of Metallurgical Findings
Root Cause Analysis
Example: Mechanical Failures
Detailed Approach
Solving the Problem when the Metallurgy Appears Sound
Failure mechanism
Failure origin
Direction and mode of loading
Condition of loading (single event or cyclic)
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3. Perform a component stress assessmentA. Metallurgical Approach
Estimate applied loading causing failure
Root Cause Analysis
Example: Mechanical Failures
Detailed Approach
Solving the Problem when the Metallurgy Appears Sound
! Ductile failure (yield, ultimate strength)
! Brittle failure from flaw precursor (fracture toughness)
! Fatigue failures (estimation of material endurance limit for conditions of loading based on microstructure, tensile strength, or fatigue test data)
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Estimate nominal service stresses and critical failure loads
3. Perform a component stress assessmentB. Mechanical Approach
Root Cause Analysis
Example: Mechanical Failures
Detailed Approach
Solving the Problem when the Metallurgy Appears Sound
! Closed form analysis (moment, force balance)
! Finite element analysis
! Instrumented strain gauge analysis
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1. Metallurgical Evaluation
Failure Analysis Investigation
! Failure mechanism! Material compliance and properties
2. Root Cause Analysis! Background information! Analytical analysis! Rationalization
Summary