section 5 - material properties for ffs assessments

Fitness-for-Service (FFS) Assessment based on API RP579

Section 5 - Material Properties fro FFS Assessment

Copyright 2005, TWI Ltd 1

Copyright © 2005, TWI Ltd World Centre for Materials Joining TechnologyWorld Centre for Materials Joining Technology

Appendix F -Material propertiesAppendix F -Material properties

•• Yield and tensile strengthYield and tensile strength•• Material physical propertiesMaterial physical properties•• Fracture toughnessFracture toughness•• Fatigue (Crack growth)Fatigue (Crack growth)•• Fatigue (Crack initiation, smooth bar Fatigue (Crack initiation, smooth bar

and welded joint)and welded joint)•• Creep (Remaining life and crack Creep (Remaining life and crack


Yield and tensile strengths (1/2)Yield and tensile strengths (1/2)

•• Measuring tensile properties of the component directly; using Measuring tensile properties of the component directly; using standard test proceduresstandard test procedures

•• Hardness test results for the estimation of the yield and tensilHardness test results for the estimation of the yield and tensile e strengths (Table F in Appendix F)strengths (Table F in Appendix F)

•• If service temperature is not the same as the test temperatures If service temperature is not the same as the test temperatures then then the values should be correctedthe values should be corrected

•• Mean values of the yield and tensile strengths can be approximatMean values of the yield and tensile strengths can be approximated ed using several equations given in Appendix F.using several equations given in Appendix F.

•• A method for computing yield and tensile strength as a function A method for computing yield and tensile strength as a function of of temperature for PRESSURE VESSEL, PIPING and TANKAGE materials temperature for PRESSURE VESSEL, PIPING and TANKAGE materials are given in Table F2 (Source: MPC Materials database).are given in Table F2 (Source: MPC Materials database).

•• For pipe and tube materials the method is based on the API RP530For pipe and tube materials the method is based on the API RP530guidance.guidance.

•• Values of yield and tensile strengths below the creep region forValues of yield and tensile strengths below the creep region forPRESSURE VESSLE, PIPING AND TANKAGE steels can be found in PRESSURE VESSLE, PIPING AND TANKAGE steels can be found in ASME Code, Section II, Part DASME Code, Section II, Part D

Estimates of material yield and tensile strength to be used in FEstimates of material yield and tensile strength to be used in FFS FS can be obtained as follows:can be obtained as follows:





Yield and tensile strengths (2/2)Yield and tensile strengths (2/2)

•• Several relationships have been proposed which are Several relationships have been proposed which are summarised in Appendix F.summarised in Appendix F.

•• In the absence of material test report, the specified minimum In the absence of material test report, the specified minimum yield strength (SMYS) and the specified minimum tensile yield strength (SMYS) and the specified minimum tensile strength (SMTS) for the material (plate, pipe or weld metal) strength (SMTS) for the material (plate, pipe or weld metal) can be used to calculate flow stress.can be used to calculate flow stress.

•• For most assessments the following is recommended. For most assessments the following is recommended.

The flow stress can be thought of the effective yield strength oThe flow stress can be thought of the effective yield strength of f work hardened material. work hardened material. Estimates of material flow strength to be used in FFS can be Estimates of material flow strength to be used in FFS can be

obtained as follows:obtained as follows:

( )2σ+σ

=σ utsysf


Physical propertiesPhysical properties

•• Elastic ModulusElastic Modulus•• PoissonPoisson’’ss ratioratio•• Coefficient of thermal expansion (CTE)Coefficient of thermal expansion (CTE)•• Thermal conductivityThermal conductivity•• Thermal diffusivityThermal diffusivity•• DensityDensity





Fracture toughness (1/4)Fracture toughness (1/4)Resistance against crack initiation and growth. Resistance against crack initiation and growth.

It is expressed in terms of critical stress intensity It is expressed in terms of critical stress intensity factor (factor (KKICIC), the critical ), the critical JJ ((JJcritcrit..) and the critical ) and the critical crack tip opening (CTOD orcrack tip opening (CTOD or δδcritcrit).).

• It should come from the test data but it is not always possible.

• For most materials covered by API 579, it is possible to measure toughness only in terms of J and CTOD.

• Valid KIC data can only be measured for brittle materials or thick sections.


Correlation exist to correlate the Correlation exist to correlate the toughness parameters:toughness parameters:

2crit

JC v1EJ

=K

critfcrit δσm=J• m = 1.4 in the absence of more reliable

information

• Charpy results can be converted to fracture toughness by using correlation listed in Appendix F and used in the FFS

Fracture toughness (2/4)Fracture toughness (2/4)





•• Removal of A sample from A component Removal of A sample from A component currently in service for testingcurrently in service for testing

•• Removal of a sample from a retired component Removal of a sample from a retired component in a similar service and testingin a similar service and testing

•• Testing a plate that was welded at the time of Testing a plate that was welded at the time of fabrication.fabrication.

Fracture toughness tests should be carried out on the same material as the component in service



•• ASTM E 1820 covers fracture toughness tests.ASTM E 1820 covers fracture toughness tests.•• Minimum three specimens should be tested for Minimum three specimens should be tested for

each condition and temperature.each condition and temperature.•• For testing the weldments use BS 7448 Pt2 For testing the weldments use BS 7448 Pt2

procedure.procedure.•• Variation of the fracture toughness can be Variation of the fracture toughness can be

quantified by using master curve approach. quantified by using master curve approach. •• The fracture toughness results from the The fracture toughness results from the

conventional specimens are usually conventional specimens are usually underestimate fracture toughness as the underestimate fracture toughness as the specimens are highly constrained. It will cause specimens are highly constrained. It will cause conservatism in the predictions by the FFS conservatism in the predictions by the FFS procedures. procedures.






Charpy testing (1/2)Charpy testing (1/2)

Charpy tests could be done on standard Charpy tests could be done on standard versus subversus sub--size specimenssize specimens• Sub-size specimens

result lower absorbed energy compared with the standard samples tested at the same temperature.

• Fracture mode transition will occur at lower temperatures in sub-sized CVN specimens than in the standard ones due to the lowertriaxiality for the former.


Charpy testing (2/2)Charpy testing (2/2)

There exist correlation to calculate the amount of energy and temperature shift:

• For plate material used in the pressure vessels and piping and tankage correlation based on the ASTM A 370, for the energy and temperature shift.

• For pipe materials (e.g., API 5L):

– The expressions were determined from the statistical analysis data from tests on plain carbon and low alloy steels

– The expressions are not exact because Charpy data often exhibits scatter

– The expressions are not applicable for materials do not exhibit impact toughness behaviour typical of plain carbon and low alloy steels.





Lower bound toughness (1/3)Lower bound toughness (1/3)

Indexing procedure based on the reference temperature (Ref.: ASME boiler and pressure vessel code Section XI).


• Two curves were fitted to the huge data set from different heats of low-alloy pressure vessel steels as shown in below.

• KIC is the lower envelope to all the fracture toughness tests loaded at quasi-static rates

• KIR, also known as KIA curve, is a lower envelope to all data for all loading conditions.

• The reference curves are validated for carbon steel plates and weldments as well as several heats of 2 ¼ Cr-1 Mo steel.








Fracture toughness from CVNFracture toughness from CVN

•• Lower bound fracture toughness based Lower bound fracture toughness based on the on the reference temperaturereference temperature obtained obtained from CVN:from CVN:–– 20.3 Joules for carbon steels20.3 Joules for carbon steels–– 27 Joules for low27 Joules for low--alloy steelsalloy steels

•• Fracture toughness based on the master Fracture toughness based on the master curve or a reference transition curve or a reference transition temperature (for 27 Joules energy)temperature (for 27 Joules energy)

•• Correlations between KICCorrelations between KIC--CVN.CVN.





Fracture toughness for in-service degradation

Fracture toughness for in-service degradation

•• Hydrogen reduces fracture toughnessHydrogen reduces fracture toughness•• Temperature exposure can produce Temperature exposure can produce

embrittlement, such as:embrittlement, such as:–– Strain agingStrain aging–– AgingAging–– Temper embrittlementTemper embrittlement–– 88588500F temper embrittlementF temper embrittlement–– Sigma phase embrittlementSigma phase embrittlement

Recommend: Use lowest fracture toughnessRecommend: Use lowest fracture toughness

resulting from the service environmentresulting from the service environment


Fracture toughnessaustenitic stainless steels

Fracture toughnessaustenitic stainless steels

•• No ductileNo ductile--brittle transitionbrittle transition

•• High fracture toughness in the absence High fracture toughness in the absence of degradation due to exposureof degradation due to exposure

•• If no data is available take (No If no data is available take (No degradation):degradation):–– 220 MPa m for base material220 MPa m for base material–– 132 MPa m for weld material132 MPa m for weld material





Fracture toughness - Master curveFracture toughness - Master curve•• Scatter in the fracture toughness data Scatter in the fracture toughness data

and hence statistical analysis is needed.and hence statistical analysis is needed.

•• A particular material does not have a A particular material does not have a single value of toughness but a single value of toughness but a toughness distribution (ASTM E1921).toughness distribution (ASTM E1921).

•• Fracture toughness distribution is Fracture toughness distribution is defined by a threedefined by a three--parameter Weibull parameter Weibull distribution.distribution.


•• Shift in the ductile Shift in the ductile ––brittle transition brittle transition temperature.temperature.

•• For austenitic stainless steels, rate For austenitic stainless steels, rate effect consideration is not required.effect consideration is not required.

•• For upper shelf toughness the rate For upper shelf toughness the rate effects is not significant.effects is not significant.

•• Three methods in API to account for the Three methods in API to account for the effect of loading rate on toughness are effect of loading rate on toughness are provided.provided.

Fracture toughness effect of Fracture toughness effect of loading rateloading rate





Crack growth calculationsCrack growth calculations

•• Categories of crack growthCategories of crack growth–– Crack growth by FatigueCrack growth by Fatigue–– Crack growth by Stress corrosion Crack growth by Stress corrosion

cracking (SCC)cracking (SCC)–– Crack growth by Hydrogen assisted Crack growth by Hydrogen assisted

cracking (HAC)cracking (HAC)–– Crack growth by corrosion fatigueCrack growth by corrosion fatigue


Crack growth calculations - FatigueCrack growth calculations - Fatigue

•• Fatigue crack growth equations in Fatigue crack growth equations in API 579:API 579:–– Paris equationParis equation–– Walker equationWalker equation–– Bilinear equationBilinear equation–– Modified Forman equationModified Forman equation–– NASGRO equationNASGRO equation–– Collipriest Collipriest equationequation–– ASME section XI ferritic steel air and water ASME section XI ferritic steel air and water

equationequation–– ASME section XI Austenitic steel equations ASME section XI Austenitic steel equations

for air and water environmentsfor air and water environments






•• Fatigue crack growth data can be Fatigue crack growth data can be obtained from:obtained from:–– TestsTests–– Equations (Upper bound) for ferritic, Equations (Upper bound) for ferritic,

austenitic and austenitic and martensitic martensitic steels for steels for given values of fatigue thresholds.given values of fatigue thresholds.



•• Parameters for crack growth equations Parameters for crack growth equations are provided inare provided in–– BS 7910BS 7910–– API 579API 579–– ASME section XIASME section XI–– NASGRO documentationNASGRO documentation





Crack growth calculations - SCCCrack growth calculations - SCC

Crack growth equations relate crackCrack growth equations relate crackgrowth rate (growth rate (dada//dtdt) to the stress ) to the stress intensity factor (intensity factor (KK), the material, service), the material, serviceenvironment and time (environment and time (tt).).


Fatigue curvesFatigue curves

•• Are required to evaluate the remaining Are required to evaluate the remaining life of a component subject to cyclic life of a component subject to cyclic loading.loading.

•• Should include curves relevant to the Should include curves relevant to the environment.environment.

•• Are presented into two forms:Are presented into two forms:–– smooth bar test specimenssmooth bar test specimens–– test specimens include weld detailstest specimens include weld details





Fatigue curves on smooth bar specimensFatigue curves on smooth bar specimens


Fatigue curves on welded specimensFatigue curves on welded specimens





Creep analysis (Larson-Miller)Creep analysis (Larson-Miller)

•• LarsonLarson--Miller parameter for minimum Miller parameter for minimum and average creep rupture data.and average creep rupture data.

•• It combines time to rupture and It combines time to rupture and temperature into a single variable. temperature into a single variable.


Creep strain-rate data(OMEGA project)


•• Estimating the remaining life of a Estimating the remaining life of a component operating at high component operating at high temperature.temperature.

•• The project covers wide range of The project covers wide range of materials in the petrochemical industry.materials in the petrochemical industry.

•• A strainA strain--rate parameter and multirate parameter and multi--axial axial damage parameter (Omega) are used damage parameter (Omega) are used to:to:–– predict the rate of strain accumulationpredict the rate of strain accumulation–– creep damage accumulationcreep damage accumulation–– remaining time to failureremaining time to failure







•• The result of OMEGA can be used to The result of OMEGA can be used to determine secant modulus and tangent determine secant modulus and tangent modulus for structural stability in the modulus for structural stability in the creep regime.creep regime.


•• May be required to evaluate the May be required to evaluate the remaining life of a component operating remaining life of a component operating at high temperature.at high temperature.

•• Useful in evaluating the creep buckling Useful in evaluating the creep buckling of a component.of a component.

Creep data-Isochronous stress-strainCreep data-Isochronous stress-strain





Creep crack growth dataCreep crack growth data

•• May be used to evaluate the remaining May be used to evaluate the remaining life of a component operating at high life of a component operating at high temperature temperature containing a crackcontaining a crack..

•• The creep crack growth rate can be The creep crack growth rate can be correlated to creep fracture mechanics correlated to creep fracture mechanics parameter C*.parameter C*.

•• If OMEGA project methodology is used If OMEGA project methodology is used then all the required material then all the required material parameters can be computed from the parameters can be computed from the MPC Omega data coefficients. MPC Omega data coefficients.

section 5 - material properties for ffs assessments

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