Corrosion-fatigueinitiation processesin a maraging steel

R. A. CottisZ. Husain

The fatigue-crackinitiation behaviourof an 1 8% Ni maragingsteelhas beenexaminedin both air and3 5%sodiumchloridesolution. Crackswerefoundtoinitiate almostexclusivelyat iron silicate inclusions,in both air andsodiumchloridesolution, although cracksinitiated considerablymore easily in the corrosiveenvironment.The crack-initiation behaviourin air is consistentwith debondingmechanisms,andit isproposedthatdebondingofinclusionsby selectivecorrosionof the metal/inclusioninterfacecould be responsible,at least in part, for thedeleteriouseffectof thesodiumchloride solution. MT/776D

© 1982 The Metals Society. The authors are in the Corrosion and ProtectionCentre, Universityof ManchesterInstitute of Scienceand Technology.

It is well known that fatiguecracksbeginat the surfaceinmostmetals.Considerablework hasbeendocumentedonthe initiation of fatigue damage from. persistent slipbands’3 andfrom extrusionsand/orintrusions.46Studiesto datehave shownthat persistentslip bandsdevelopinburstsof slip in the saturatedstate of hardness,when arelatively uniform array of obstacles,dipoles, and multi-poleshasbeenestablished.The persistentslip bandsthusformed are found to contain micro-embryonic cracks,2createdeither by the interactionof dislocationswith pointdefectsvacancies3or by loss of crystalcoherencydue toaccumulationof defects.The macrocrackresultsfrom alinkage of these embryonic cracks. The extrusions, thinribbonsof metalextrudingfrom the surface,arefrequentlyassociatedwith intrusions,equivalentto acrackafewmicro-metresdeep.5They areformedby an interactionbetweenscrewand edgedislocationsin the slip and glide planes.6However, the extrusionsand/or intrusions which them-selvesform on the slip bandsareconsideredless importantfor nucleationthan the slip bands.

Localized regionsof stressconcentrationandrelativelylow plasticity in the surface,such as at inclusion/matrixinterfaces, may act as nucleation sites.71° The crack-nucleationcharacteristicsof inclusionsplay an importantrole in influencingfatiguefailure,andit hasbeenreportedthat surfaceinclusionsare more harmful than subsurfaceones.The number,size, andtype of inclusionshavebeenfound to be of importancein determiningtheir effectsonthe fatigue characteristics.The orientationof inclusionswith respectto thedirectionof appliedstressandthe ratioof elasticmoduli of inclusion andmatrix also greatlyinfluencethe nucleationbehaviour.Work on the effectof inclusionson Al alloys8andsteels9hasshownthatcracknucleation occursexclusivelyon surfaceinclusionsand,althoughthe inclusion/matrix interfaceis associatedwith largeslipand higher dislocationdensity in somecases, the overallimpression is that the crack nucleates by interfacialdebonding.1°

In general,theenhancementof fatiguedamagein corrosive environmentsis consideredto be causedby effectsonnucleation and growth of cracks. The nucleation isenhancedby pitting leadingto cracking,11 by preferentialdissolutionof deformedareaswhicharemoreanodicto thematrix,12 or by the ruptureof the protectiveoxide film,’3exposingfreshmetalto the environmentandleadingto itsdissolution andcrack formation. Differencesbetweenthecorrosionpotentialsof inclusionsandthe matrix may alsoleadto thepreferentialcorrosionof oneor theother,andtocracking.14

In this paperthe resultsof an investigationinto fatiguecrackinitiation in amaragingsteelarepresented,particularemphasisbeinggiven to the effect of a corrosiveenvironment on the initiation process.

Experimental

MATERIALThe steel used in this work is a maragingsteel to DTDspecification5212. Thecompositionandmechanicalprop-ertiesaregivenin Tables1 and2, respectively.Thematerialwassuppliedin the form of cold-rolledbar, andwassubsequently cut into cantilever-bendspecimenswith themajoraxisof the specimenparallelto therolling direction.Beforetesting,the specimenswerepolishedby handto a 0 25cmdiamondfinish, to give reproducibleinitiation behaviourandto facilitate optical andelectron-opticalexamination.

TEST CONFIGURATION AND PROCEDUREAn Avery dynamicbendfatiguetestmachinewasusedwithconstantstress-amplitudeloadingat a frequencyof 14Hz.Testswereperformedin laboratoryair andin aerated3 5%sodiumchloride solution. The testswith sodiumchloridewerecarriedout with a siliconerubbercell fitted overthespecimenmountingpoints.

S-N curves were determinedfor test durations in therange 1$-1O cycles. Further work concentratedon thedevelopmentof cracksin theearlystagesof thefatiguetest.For this purposeoptical andscanningelectronmicroscopywere used to examinethe specimensurface. The SEMemployedwas an ISIDS 130, which permitted the non-destructiveexaminationof completefatiguespecimensonits lower stage.The SEM wasalso used to examinetheoriginal surface and the fracture surfaceof failed specimens. Electron-probemicroanalysis was employed todetermineinclusion chemistry.

The dislocationstructureof thin foils takenparallelto thespecimensurfacewasstudiedby conventionaltransmissionelectronmicroscopy.

Quantitativesurveysof theproportionof inclusionswithwhich fatiguecrackswereassociatedwereperformedwiththe aidof a Quantimet720.This wasusedto give unbiasedautomaticsteppingacrossthe specimensurface,althoughthe analysiswasperformedmanually.

Table I Composition of steel DTD 5212, wt-%

C Mo Ni Co Fe

016 15 173 7.7 Bal.

Table 2 Mechanical properties of steel DTD 5212

UTS,MNm’

02% proofstrength, Reduction in Hardness,MNm’ Elongation, % area, % HV

1000 930 16 81 325

104 Metals Technology March-April 1982 Vol.9

T

TheS-Ncurvesobtainedareshownin Fig. 1 . Theseresultsareconsistentwith manyothersobtainedfor similar mater-ials andconditions,thecorrosiveenvironmentreducingthestressamplituderequiredfor a givenfatiguelifetime. Thiseffect is relativelysmall for high-strain,low-cycletests,andbecomesmore significantfor lower stressesandlongertestdurations.The 1 0 cycle endurancelimit is reducedfrom410 to 120MNm by the corrosiveenvironment.Examinationof the specimensurfaceat variousstagesofthefatiguetestrevealedthat crackinitiation in both airandsodium chloride solution is associatedalmost exclusivelywith surfaceinclusions.Typical casesareshownin Figs.2-4.Similarly, examinationof fracturedspecimenssuggeststhatcrackshaveinitiated from inclusionsFig.5.

The averageof four typical inclusionanalysesis given inTable 3. Theseresults imply that the inclusionsare ironsilicatesof the generalformula FeOSiO2. Preliminaryexaminationssuggestedthatspecimenstestedin acorrosiveenvironmentandremovedafter, say, 5% of theexpectedfatigue life had a greaterproportion of inclusions withassociatedcracksthan had specimenstestedat the samestressin air eventhoughthe specimenstestedin airwouldhavebeenexposedto agreaternumberof cycles,sincetheir

Cottis and Husain Corrosion-fatigue initiation processes 105

I S-N curves

Results

3 Microcrack associated with inclusion in sodiumchloride solution; high stress, IO cycles

2 Microcrack associated with inclusion in air4 Microcrack associated with inclusion in sodiumchloride solution; low stress, 1O cycles

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106 Cottis and Husain Corrosion-fatigue initiation processes

in air and in sodium chloride solution lie on roughly thesamecurve. As the test durationincreasesor the stressdecreases,thenumberof inclusionswith associatedfatiguecrackspresentat 5% of the expectedlife decreases.

Typical subsurfacedislocationstructuresare shown inFig.8. No significantdifferencesin cell structurehavebeendetectedbetweensamplestestedin air andin 3 5%sodiumchloride solution.

Discussion

b enlargement of area indicated in a; c enlargement of area indicated in b

5 Fracture surface showing inclusion at crack nucleation site

expectedlife was greater. Consequently,some quantitativework hasbeenperformedin whichtheproportionofthe surface inclusions with an associatedfatigue-cracknucleushasbeenmeasuredat 5%oftheexpectedlifetime ata rangeof stresslevels.

The resultsof this work areshownin Fig.6. For a givenstressit can be seenthat testsin the corrosiveenvironmentrevealabout70% moreinclusionswith cracksthan do testsperformedin air. In contrast,if theproportionof inclusionswith cracksis plotted as a function of the expectedtestdurationseeFig.7, it can be seenthat the resultsfor tests

Table 3 Composition of inclusions, wt-% average offour typical analyses

Fe 0 Si Co Ni Mn

555 221 172 31 21 00

It is clear from Figs.2-4that fatigue-crackinitiation in thismaterial is associatedalmostexclusivelywith surfaceinclusions,both for testsin air andin sodiumchloride solution.This is confirmed by the large proportionsof inclusionsfound to have cracksassociatedwith them after 5%of thelife.

Thegenerallyacceptedmechanismof crackinitiation atinclusionsinvolves debondingor decohesionof the inclusion/matrixinterface.’0In steelsthis interfaceis incoherent.Owing to differencesin elasticmoduli, elasticstressescon-centratearoundthe particle, andlocal yielding will occur.This progressivelyincreasesthestresson theinterfaceuntildecohesionoccurs.

STRESS AMPLITUDE , MN m2

6 Percentage of inclusions associated with cracks V.

stress amplitude

CYCLES TO FAILURE

7 Percentage of inclusions associated with cracks v.expected number of cycles to failure

Metals Technology March-April 1982 Vol.9

Cottis and Husain Corrosion-fatigue initiation processes 107

The observationthat the proportionof inclusionswithassociatedcracksfalls as the stressamplitude is reducedisconsistentwith this mechanism.At thelower stresses,fewerinclusionswill give sufficientstressconcentrationto exceedthe yield strengthof the material,hencefewer crackswillinitiate.

A simple model of this processmay be developedbyconsideringthe relationshipbetweenthe stressconcentration at an inclusion andthe yield strengthof the material.Thestressconcentrationresultingfrom theinclusionwillbeafunctionofits size,shape,andtheelasticmoduli ofthetwomaterials.Two specialcaseswhich give boundaryconditions for the analysisare:

i a hole effectively an inclusion of zero elasticmodulus

ii an inclusion of the same elastic modulus as themetallic matrix.

Thestress-concentrationfactor for casei may be takentobe given by the relationship

0iocaiTmeanC* 1

wherecroca is the local stressat anypoint, mean S the netsectionstress,and C is a geometricconstant.

In caseii therewill clearly beno stressconcentrationasa resultof the inclusion, hence

Ulocal= °mean

Assuming a linear interpolationbetweenthesetwo casesleadsto the relationship

ffiocai_ffmean[C11i/1rn+ i]

5p.rnF- -1

9 Inclusion after 24 x I O cycles in sodium chloridesolution, showing cavity formation and cracking atinterface; I inclusion, M matrix, C cavity

whereE, is the elasticmodulusof theinclusion, andEmtheelasticmodulusof the metal.

For the silicate inclusion presentin the material beingstudied, the ratio Ei/Em 5 approximately2/3, hence

0iocaiffmeanC/3+ 1

The value of C can be estimatedfor the morphologyofinclusions presentin this steel by assumingthat the airfatiguelimit correspondsto the local stressbeingequaltothe yield stress.Thematerialbeingstudieddoesnot showawell definedyield point, but a reasonableapproximationisthat the fatigue limit is o-/2. Hence

orff=ff/2C/3+ 1

C=3A possibleaction of the corrosiveenvironmentis that iteffectivelycausesdebondingof the inclusion by corrosionof the metal/inclusioninterface.This will reducetheeffective modulusof the inclusion to zero, hence

1oca1mean’ 1

or { 40’mean for sodiumchloride solution

- 2Omean for air

This very simple analysisshowsthat ‘corrosive debonding’of the metal/inclusioninterfaceis sufficientto accountfor ahalvingof thestressrequiredfor afatiguecrackto initiate ina given numberof cycles.

The valueof the stress-concentrationfactorobtainedfora debondedinclusion appearsreasonablefor thatexpectedat a near-sphericalhole, while the value obtainedfor abondedinclusion is in generalagreementwith the work ofGoodier.15

It is clearfrom Fig. 1 that thenumberof cyclesto failure inair and in sodium chloride solution doesnot match thisprediction.However, thesecurvesaredominatedby prop-agationtime, cracksinvariablybeingobservedat 5%of theexpectedlife. At the relatively high frequencyused forthesetests,crackgrowthratesareexpectedto be essentiallyunaffectedby the corrosiveenvironment,hencethe prop-agationcomponentof thefailure time will be controlledbythestressamplitude,andwill be thesamefor testsin air andin sodium chloride solution. Only at the lower stresses,

l.

a in air; b in sodium chloride solution

8 Dislocation substructure near surface of specimens

Metals Technology March-April 1982 Vol.9

108 Cottis and Husain Corrosion-fatigue initiation processes

wherelocal stressesin air approachandfall belowtheyieldstress,doesthedifferencebetweentestsin air andin sodiumchloride solution becomesignificant.

At 10 cycles, the failure stressin air is about3 4 timesthat in sodiumchloridesolution. This is somewhatgreaterthan is predictedby a simple debondinganalysis,andmdi-catesthat other effectsare also significant in this region.Figure 9 shows the developmentof corrosionaroundaninclusion, which could enhancethe stressconcentration,after2 . 5x 1 0" cycles.Thevariousformsof strain-enhanceddissolutioncould also be playing a part in this process.

Conclusions

1 . The S-Ncurves for the steelusedin this investigationshowan enhancementof damagein 3 5%sodiumchloridesolution when comparedto air, theendurancelimit beingreducedfrom 410 to 1 20MN m2.

2 . Nucleationis observedexclusivelyat inclusionsbothin air and in sodium chloride solution, the proportionofinclusions with associatedcracks being 70% higher insodium chloride thanin air.

3. Theresultsobtainedfor testsin airareconsistentwiththeestablishedinclusion-debondingmechanism.

4. Debondingof inclusionsby a corrosiveenvironment

hasbeenobserved,andcan accountfor a reductionof thestressrequiredfor crack initiation by a factor of 2.

References

1. N. THOMPSON, N. J. WADSWORTH, and N. LOUAT: Philos. Mag., 1956, 1,113.

2. M. HEMPEL: ‘Proc. mt. conf. fatigueof metals’, held in Londonand NewYork 1956, 543; London, Institutionof MechanicalEngineers.

3. A. 5EEGER: Colloquium on ‘Fatigue of metals’,Stuttgart,1964.4. i. j. E. FORSYTH: Nature, 1953,171, 172.5. A. H. CO1TRELL and D. HULL: Proc. R. Soc.,1957, A242, 211.6. A. J. KENNEDY: ‘Processesofcreepand fatiguein metals’; 1962, London,

Edinburgh, Oliverand Boyd.7. F. B. STULEN, H. N. CUMMINGS, and w. c. 5CHULTE: ‘Proc. mt. conf. fatigue

of metals’, held in Londonand New York 1956, 439; London,Institution of MechanicalEngineers.

8. j. c. GROSSKREUTZ and G. G. SHAW: ‘Proc. 2nd. mt. conf. fracture’,ed. P. L. Pratt,620; 1969, London,Chapmanand Hall.

9. w. E. DUCKWORTH and a. INEs0N: ‘Clean steel’,87; 1963,London,TheIron and Steel Institute.

10. T. Y. SHIH and T. AR/do: Trans. Iron SteelInst. Jpn,1973, 13, 11.11. D. J. MCADAM, JR and G. w. GEIL: Proc. ASTM, 1928, 41, 696.12. U. R. EVANS: ‘The corrosionand oxidation of metals’; 1960,London,

Edward Arnold.1 3. c. LAIRD and D. J. DUQUETrE: ‘Corrosion fatigue: chemistry, mechanics

and microstructure’,eds. 0. Devereuxetal.,88; 1972,Houston,Tex.,National Associationof Corrosion Engineers.

14. . H. PAYNE and R. W. STAEHLE: ‘Corrosion fatigue: chemistry, mechanicsand microstructure’,eds. 0. Devereuxet al, 211; 1972, Houston,Tex., National Association of Corrosion Engineers.

15. N. J. GOODIER: J. Eng. Power Trans. ASMEA, 1933, 55, 39.

Metals Technology March-April 1982 Vol.9

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