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.