Defects in Materials
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Transcript of Defects in Materials
Defects in MaterialsOrigin , Nature & Significance
Scope• Introduction
• Discontinuities in materials: Origin & NatureInherent - Processing - Service
• Assessment of Flaw significance
• Flaw Acceptance Standards
– Workmanship vs Fitness – for – Purpose
• NDT methods for characterizing severe flaws
• Case Studies• Conclusion
Discontinuity
FlawDefect
Interruption in Normal Physical Structure/metallurgical
- Keyways, Grooves, Holes present by design
Discontinuity with undesirable connotation
- Slag, Porosity, Lamination
Flaw which makes component unfit for service
- Cracks, Lack of Fusion in Weld
Living with Flaws
A: Flaws which will not grow at all during service
B: Flaw which will not grow to critical size during lifetime
C: Flaw which will grow to critical size in next few inspection intervals
D: Flaw size will grow to critical size before next inspection
Flaw A & B: Not significant
Flaw C & D: Significant
Role of NDT
Characterization & Classification of Flaws
Safety: C & D not placed as A & B
Economy: A & B not placed as C & D
Components of Flaw Characterization:
Flaw CharacteristicsFlaw Characteristics
DetectionDetection
Flaw GeometrySize, Shape , Orientation
Flaw GeometrySize, Shape , Orientation
NatureNature LocationLocation
Metallurgical Characteristics Controlling Material Properties
Material PropertiesMaterial
Properties
Chemical CompositionChemical Composition
Crystal StructureCrystal Structure
MicrostructureMicrostructure DislocationDensity
DislocationDensity
Heat Treatment: Commonly employed Processing Treatment to modify Metallurgical CharacteristicsHeat Treatment: Commonly employed Processing Treatment to modify Metallurgical Characteristics
Classification of Engineering Products
Based on Manufacturing Route
I Castings
Melting - Pouring into Mould Cavity - Solidification
II Powder Metallurgy Products
Powder Preparation - Pressing into Mould Cavity - Sintering
III Wrought Products
Cast ingot - Mechanical Working - Machining - Welding - Heat Treatment
Defects in Materials
Origin & Characteristics
I Inherent Discontinuities
* Melting, Casting & Solidification
II Processing Discontinuities
* Mechanical Working (Hot/Cold)
- Forging, Rolling, Extrusion, Forming
* Welding
* Heat Treatment
III Service Discontinuities
* Fatigue, SCC, Creep
Discontinuities are not necessarily Defects
Casting vs. Wrought Products
* Physical Discontinuities
* Chemical Inhomogeneities
* Microstructural Non uniformities
Lower Quality Factor or Higher Safety Factor for Castings as compared to Wrought Products
Weld ~ Mini Casting
Quality Requirements
Metallurgical NDT Dimensional
• Chemical Composition
• Microstructure
• Mechanical Properties
• Corrosion Properties
Freedom from Unacceptable Flaws affecting Structural Integrity
Stress
- Magnitude
- Concentration
- Distribution
Defects in MaterialsOrigin, Nature & Significance
• Casting• Forging• Rolling• Heat Treatment• Welding
Casting Defects
• International Committee of Foundry Technical Association
• 111 casting defects in 7 categories
- Metallic Projections - Cavities - Discontinuities - Defects (Surface)
- Incomplete Casting - Incorrect Dimensions
- Inclusion or Structural Anomalies
Defects in Casting
* Gas Defects – Blow Holes, Porosities
* Shrinkage Cavity – Piping
* Non-Metallic Inclusion – Exogenous, Indigenous
* Chemical Inhomogeneities – Segregation
* Contraction Defects (stress) – Hot Tears, Cold Cracks
* Shaping Faults – Misrun, Cold Shuts
Defects in Ingots
Casting Defects: Shrinkage Cavity• A depression or an internal
void in a casting that results from volume contraction during solidification
Level A Superheated liquid metal filled to the top pf the mouldLevel B Liquid shrinks on cooling to freezing temperatureArea C During L to S contraction, further redn. in volume
Localized near top of the ingot (Freezes last)Distance D Solid metal pulls away from mould wall as it contracts
Casting Defects: Blow Holes
• Balloon shaped cavities at or below the surface of the casting
• Cause- High moisture or organic content of sand mould gives
excessive steam or CO - Low permeability of mould due to excess clay content or
excessive ramming
Casting Defects: Porosity
• Very small gas holes uniformly dispersed through the entire casting
• Gas solubility more in liquid than solid
• Microporosity: Gas trapped within growing dendrites
• To reduce gas in the liquid metal - Keep minimum superheat
- Vacuum melting or Vacuum degassing- Inert gas bubbling
M.P.
Liquid
Solid.
SS
SL
Gas Solubility
Inclusions• Non-metallic or metallic phases in a metallic matrix
Two Types
Exogenous
- Derived from external causes
- Slag, entrapped mould material and refractory
- Macroscopic
- Non-uniform distribution
Indigenous
- Inherent in molten metal treatment
- Sulphides, Nitrides, Oxides (Al2O3, SiO2)
- Microscopic
- Uniformly distribution
Iron Making: Reduction Process, Fe2O3 CO/C Fe
Steel Making: Oxidation – C,S, P etc
High O in steel: Porosity, De-oxidation by Al, Si: Inclusions (Al2O3, SiO2)
Inclusion Rating in Steel• Four types of non-metallic inclusions: Thin & Heavy
• Type A: Sulphide
• Type B: Alumina
• Type C: Silicate
• Type D: Globular Oxide
Casting Defects: Hot Tears• Caused by unequal shrinking of light and heavy sections of a
casting as the metal cools
Casting Defects: Cold Shuts• Forms when molten metal meets the already solidified or relatively cold
metal
• Can also be formed by lack of fusion between the two intercepting surfaces of molten metal at different temperatures
Mechanical Working• Involves Plastic Deformation: Change in shape and size but no
volume change
• Hot working (Temp > Tre) & Cold Working (Temp < Tre)
• Tungsten – Working at 1000 C: Cold Working• Lead – Working at RT: Hot Working
Process Hot Working Cold WorkingForging √Rolling √ √Extrusion √ √Drawing √
Forging Defects
• Bursts
• Laps
• Hydrogen Flakes
• Flow Lines
Forging Defects: Burst• Rupture caused by forging at improper temperature
• Can be internal or external
Forging Defects: Lap• It is a discontinuity caused by folding of metal in a thin plate on the
surface of the forging
• Improper matching of mating surfaces of the two forging dies
Forging Defects: Hydrogen Flakes
• Randomly oriented internal thermal cracks in steel resulting from critical combination of stress and hydrogen content
• On an etched surface, they appear as short discontinuous cracks
Forging Defects: Improper Flow Lines• Patterns that reveal how the grain structure follows the direction of
working in forging• Flow lines refer to direction of inclusion deformation during forging.
Flow lines should not cut the surface to avoid failures due to high HCF.
• These are revealed macro-etching• Optimizing grain flow orientation maximizes mech. properties
Rolling Defects
• Laminations
• Stringers
• Seams
Rolling Defects: Laminations• Defects with separation or weakness generally aligned parallel to the
worked surface of the metal
• May be a result of pipe, blister, seams, inclusions or segregations elongated and made directional by working
Rolling Defects: Stringers• Longer and thinner configuration of non-metallic inclusions aligned in the
direction of working
• Commonly the term is associated with oxide or sulphide inclusions in metals
Rolling Defects: Seams• Un-welded fold or lap that appears as a crack
• Results from a defect obtained in casting or working
• Always open to surface
Heat Treatment
• Heating and Cooling operations applied to metals and alloys in solid state to obtain desired properties
• Purpose- Mechanical properties: Strength Ductility Toughness- Corrosion Resistance
- Dimensional Stability: Residual stress
• Heat Treatment Cycle: Min. Three Steps
• Metallurgical characteristics controlling properties- Chemical Composition - Dislocation Density- Microstructure - Texture
HS
C
Heat Treatment of Steels
• Annealing
• Normalizing
• Hardening
• Tempering
• Martempering
• Austempering
Cold Work – Anneal Cycle
Cold WorkingCold Working RecoveryRecovery RecrystallizationRecrystallization Grain GrowthGrain Growth
Defects in Heat Treatment
• Coarse Grain
• Quench Cracks
• Undesirable phase: Sensitization in Austenitic SS
• Embrittlement: Temper Embrittlement & Thermal Ageing Embrittlement
Quench Cracks• Cracks formed in steel as a result of tensile stresses produced during
hardening HT (Austenite to Martensite Transformation)
Prevention: Increase hardenibility by alloying
Adopt Martempering or Austempering heat treatment
Grain Size
• N = 2 n-1 , N: number of grains in an area of in2 at 100 X n: ASTM grain size number
• Effect of Grain Size on Material Property - Hall-Petch Equation: σy = σi + k d -1/2
- Cottrell Equation: σf.K.d1/2 = β. μ. γ
- Grain size ………. Strength Toughness DBTT
ASTM GS No. Grain Size (μm)
2 160
4 80
5 56
6 40
8 20
Coarse
Fine
Sensitization in Austenitic Stainless Steels
• Sensitization refers to Chromium Carbide precipitation with concomitant depletion of Cr to less than 12% making Austenitic Stainless Steel susceptible to IGC/IGSCC attack
• Sensitization is likely during Solution HT or Welding when SS is exposed to 450 – 800oC
Cr < 12%
Matrix Cr ≈ 18%
Cr23C6
(95% Cr) T
t
Non-sensitized
Sensitized
TTS Diagram
Intergranular Attack (IGC) attack in Austenitic Stainless Steels
• IGC attack refers to localized corrosion along the grain boundaries
• Sensitized stainless steel prone to IGC/IGSCC attack when exposed to corrosive environment
• ASTM Standard A262 Practice A to Practice E for
detecting IGC susceptibility
• Severity of IGC attack expressed as depth of IGC
HAZ HAZ
SS 304 SS 321
Defects in Fusion Welds
• IIW Atlas: 83 weld discontinuities
• 6 Broad classes
- Cracks- Cavities- Solid Inclusions- Lack of fusion & Penetration- Imperfect Shape- Miscellaneous
Significance of FlawsSignificance of Flaws
• Flaw type vs. Severity
• Critical flaw size
• Flaw location vs. Severity
• Flaw tolerance of materials
• Interaction between adjacent flaws
• NDT methods for detecting severe flaws
• Basis of flaw acceptance in codes
Types of Flaws:
• Planar Flaws (2D)– Lack of Fusion , Cracks
• Volumetric Flaws (3D)– Porosity , Inclusions
Ductile vs Brittle Fracture
Strengthσf
σoq.σo
TemperatureT.T T.TN
BFDF
Notched flow stress
Un-notched flow stress
q =
Plastic Constraint Factor
Temperature
Loading Rate
Triaxial Stress
Neutron Irradiation
Flow StressBrittle Fracture
Effects of Notch:• Notch increases tendency to brittle fracture
– By producing high local stress– By introducing triaxial tensile stress– By producing high local strain rate– By producing high local strain hardening & cracking
• Notch: Definite depth & root radius
vs
• Crack: Only depth & vanishingly small radius
Critical Flaw Size:
• Stress Concentration Factor (SCF)
σmax = σ nom [1+2 sqrt.(D/ρ)]
ρ 0, σmax Infinity
• Stress Intensity Factor (SIF: KI )
KI = σ . sqrt(πa)
• Critical SIF= Fracture Toughness (KIC)
• When KI > KIC Catastrophic Brittle Fracture
Fracture Toughness & Allowable Flaw Size
Stress σ
Flaw Sizeac
KIC = σ x (П . a)1/2
Material Property Stress Flaw Size
Fracture Toughness KIC
Significance of Flaws in Performance of Engineering Components
Allowable flaw size aa with adequate safety margin
Designer needs an assurance that at no stage during the service life of component there is any flaw of the order of ac
Stress σ
Flaw Sizeac
KIC
aa
Role of NDT is to detect flaw of size aa
Important Flaw Characteristics for Fitness-For-Service Assessment
Location Surface Flaw
Sub-surface Flaw
• Surface flaw more severe as compared to sub-surface flaw because of higher stress intensity associated with it
• Code provides guidelines on classification of flaw in to surface or sub- surface, if it lies just below the surface of the component
Important Flaw Characteristics for Fitness-For-Service Assessment
Size
Surface Flaw
Sub-surface Flaw
• Flaw size should never exceed the critical flaw size decided by the operating stress & fracture toughness, with adequate safety margin
• It refers to through wall dimension & length for crack like defects and area for laminations
• Most important flaw characteristics and also most difficult to predict accurately by conventional NDT methods
KIC = σ . (π. a) 1/2
a2a
Important Flaw Characteristics for Fitness-For-Service Assessment
Orientation Hoop Stress = PD / 2.T
Axial Stress = PD / 4.T
• Flaw perpendicular to maximum tensile stress more severe than the one oriented parallel to it
• For internally pressurized pipe, axial flaw more severe than circumferential flaw
Important Flaw Characteristics for Fitness-For-Service Assessment
Shape
• Flaws with sharp tip like crack more severe than flaws with smooth surface like porosity
• Small root radius leads to higher stress intensity
Important Flaw Characteristics for Fitness-For-Service Assessment
Proximity
• If two flaws are very close, they influence the stress intensity associated with each other
• Two flaws shall be separated by the length of the longest flaws, or else they shall be considered together as a singe flaw including the sound region in between
Important Flaw Characteristics for Fitness-For-Service Assessment
Nature
• Weld Flaw
- Operator: Porosity, lack of fusion, slag inclusion, undercut
- Metallurgical origin: Cold crack, Hot crack, laminar tearing
• Planar or Volumetric Flaw
• Planar flaw more severe than volumetric flaw
Flaw Tolerance of Materials
KIC KIC’ KIC’ KIC>>Stress σ
Flaw Size
aC’aC
• FCC Crystal structure and fine grain size makes material more tolerant to flaws
• Effect of welding and environment needs consideration
• FCC Crystal structure and fine grain size makes material more tolerant to flaws
• Effect of welding and environment needs consideration
Strengthening Mechanisms
• Solid Solution Hardening
- Substitutional : σ α √Cs
- Interstitial : σ α √Ci
• Grain Refinement : σy = σi + k d -1/2
• Precipitation Hardening : τ = Gb / l
• Composite : σc = σfVf + (1 – Vf) σm
• Strength Ductility Toughness Flaw Tolerance
• Fine grain size: Strength Toughness
NDT Methods for Characterizing Severe FlawsNDT Methods for Characterizing Severe Flaws
Important Flaw Characteristics for Fitness-For-Service Assessment
Nature • Planar Flaws severe than Volumetric
Location • Surface or Sub-surface
Size • Length/ Area/ Though-thickness dimension
Shape • Smooth or Irregular or Sharp
Orientation • Flaw orientation w.r.t. Principal Stress
A good NDT should have high reliability for harmful flaws and provide maximum information on above flaw characteristics
Proximity • Proximity of a flaw to other flaw(s)
NDT Methods for different Products
Product Surface Volumetric
Casting LPT / MPT RT
Forging LPT / MPT UT
Plates - UT
Tubes - ECT / UT
Welds LPT / MPT RT / UT
Radiography vs Ultrasonic Testing
Characteristics RT UT
Presence Miss Planar Flaws (even large)
Miss Globular Flaws (small)
Location (lateral) Very Good Good Enough
Location (Depth) No Information Good Enough
Size, Shape & Orientation
Very Good Very Poor
Type Very Good Very Poor
Leak Before Break
KIC = σ x √ π.ac
al
ac: Critical crack length
al : Crack length at leakage
If al < ac , then Leak Before Break Criteria is satisfied
If al > ac , then Break Before Leak
ac
Engineering Critical Assessment:ECA- Codes and Guides
• API RP 579 (2000): Recommended Practice for Fitness-for Service Assessment
• BS 7910 (1999): Guidance on Methods for Assessing the acceptability of flaws in metallic structure
• IIW / IIS-SST-1957-90: IIW Guidance on Assessment of Fitness-for-purpose of welded structure
• ASME B&PV Code Sec. XI: In-service Inspection of Nuclear Components
API RP 579 Assessment Approach
Kr = KI / KMAT
SIF KI
Stress Analysis
Flaw Size
KMAT
Stress Analysis
Flaw Size Reference Stress σref
Lr = σref / σYS σYS
Basis of Flaw Acceptance Standard in Codes
• Good Workmanship
• Proven Service Experience
• Capability of NDT Method
(Radiography)
Radiographic Examination of Welded Joints
Unacceptable Indications
• Crack or incomplete fusion or penetration
• Slag inclusion of length greater than– ¼ “ for t up to ¾ “– 1/3 t for t from ¾” to 2-1/4”– ¾” for t over 2-1/4”
• Rounded Indications of diameter greater than
- ¼ t or 5/32” (whichever is smaller) for t < 2”
- 3/8” for t > 2”
Economic Consequences of Arbitrary Acceptance Standards
• Alyeska Pipeline
• Audit of Radiographs: 4000 defects
• Cost of repair: $ 52 M
• One repair of weld in a river crossing: $ 2.5 M
• 3 such defects accepted without repair
• Analysis shoed none of the 4000 defects was affecting integrity based on FFS
A survey of Repairs (UK) on welds of Pressure Vessel
• 84% for Slag Inclusions
• 3% for Porosity
• 13% for Planar Defects
• Repair welds are made under conditions of high restraint and there is a risk that a harmless, but readily detectable defect, such as slag inclusion, will be replaced by a potential harmful crack, which is less easy to detect
Limitations of Codes
• Specified Limits of defect size: Arbitrary
• Material’s Flaw Tolerance: Not Considered
• Defect Location vs. Severity: Not Considered
• Different NDT Methods during different stages of inspection
- RT during fabrication (IMI)
- UT during PSI and ISI
Two-Tier Approach for Flaw Acceptance
Workmanship Standard (aw)
• Existing Std in code
• Quality Control Purpose
• Flaw < aw – Accept
• Flaw > aw – Assess for FFS
Fitness-for- Service (FFS) Standard (af)
• Based on Fracture Mech.
• For Repair/Reject Assessment
• Flaw < af – Accept
• Flaw > af – Repair or Reject
Case Studies
Spiral Cracks
Failure of Rocket Motor Casing during Hydro Test
• Material : Maraging Steel (Grade 250)
• Dimension : 6.6 mm dia. X 1.25 cm wt
• Proof Pressure : 63.4 Kg/cm2
• Failure Pressure : 38 Kg/cm2
• Yield Strength : 16800 Kg/cm2
• Membrane Stress at Failure : 7000 Kg/cm2
» Failure Originated in a weld defect
Failed rocket motor casing pieces laid out in proper relation to each other:
Evaluation of a Defect in Circumferential Weld of Pressure Vessel
• Material : Low Alloy Steel (125 mm thick)
• Tests:
Mechanical Properties (TT & IT), Satisfactory
Radiography: Using 6 MeV LINAC – 2% sensitivity, satisfactory
Hydro Test: 10 C to 1.5 times designed stress, satisfactory
Evaluation of a Defect in Circumferential Weld of Pressure Vessel
• UT for generating base-line data for ISI
185 mm long and 0.3-2mm wide defect at an angle to and 3 mm below the outer surface in circumferential weld
• Defect: Slag entrapment, treated as crack for Frac. Mech Ass.
• For Design and Proof Stress Conditions Safety Factor of more than 3 available, Defect accepted for service
Evaluation of a Defect in Circumferential Weld of Pressure Vessel
Sensitization Induced Corrosion Failures
• SCC 13.1%• Pitting 7.9%• IGC 5.6%• General 15.2%• Others 13.4%
• SCC 13.1%• Pitting 7.9%• IGC 5.6%• General 15.2%• Others 13.4%
Failure of Stainless Steel Equipment(685 cases, DU PONT, 1968-1971)
Failure of Stainless Steel Equipment(685 cases, DU PONT, 1968-1971)
Mechanical (44.8%)Mechanical (44.8%) Corrosion (55.2%)Corrosion (55.2%)
Oxalic Acid Etch Test: Practice A• Screening Test• Specimen is electro-etched for 1,5 min at 1A/cm2
• Microscopic examination
Step
Dual
Ditch
Sensitization Induced Corrosion Failures: IGSCC
Structural Integrity
StructuralIntegrity
Assessment
Flaw Characteristics
Material PropertiesStress
Location, Nature, Geometry
Residual & AppliedMicrostructure & Mechanical Prop.
Size, Shape, Orientation
Application of NDT during Component’s Lifetime
In-Manufacture Inspection (IMI)
- To detect processing discontinuities
Pre-Service Inspection (PSI)
- To collect base line data for future inspections
In-Service Inspection (ISI)
- To monitor growth of existing flaws and initiation and growth of service-induced flaws
Cradle
Grave
Structural
Integrity
Flaw Characteristics
- Type
- Location - Geometry (size, shape, orient.)
Stress
- Applied - Residual
Material Prop.
- YS, KIC
Effectiveness ofProcedure
Competence of Operators
Capability ofEquipments
NDT Results
Engineering Critical
Assessment
Importance of Basic Metallurgy for NDT Professionals
• No engineering structure is free from flaws. Flaw tolerance depends on metallurgical characteristics
• Flaw characteristics (Location, Size, Shape, Orientation & Nature depends on nature of Material Processing
• Knowing Flaw Characteristics helps in
- Selection of appropriate NDT method
- Selection of test parameters
- Interpretation of Relevant / False indications
Misconceptions regarding NDTNo defects found & reported
Means –No defects in the componentNo NDT technique capable of
detecting all defects (Uncertainty in detection)
Defects measured as 5mm means that defect actually is 5mm
Uncertainty in Defect sizing by NDT
If NDT reports defect growth or non- growth then this is actually the case
Comparison of two sizing has their own errors
100% Inspection Coverage Not necessarily 100% of component inspected
Hard copy results can’t lie Hard copy results only as good as tech. & data used to produce them.
NDT as per National /Int. standard always appropriate.
Standards only relevant to specific circumstances & include knowledge at
the time of development.NDT reqs. need to be checked against the Std. to see its
relevance to particular situation.