Corrosion-fatigue initiation processes in a maraging steel · The steel used in this work is a...

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Corrosion-fatigue initiation processes in a maraging steel R. A. Cottis Z. Husain The fatigue-crack initiation behaviour of an 1 8% Ni maraging steel has been examined in both air and 3 5% sodium chloride solution. Cracks were found to initiate almost exclusively at iron silicate inclusions, in both air and sodium chloride solution, although cracks initiated considerably more easily in the corrosive environment. The crack-initiation behaviour in air is consistent with debonding mechanisms, and it is proposed that debonding of inclusions by selective corrosion of the metal/inclusion interface could be responsible, at least in part, for the deleterious effect of the sodium chloride solution. MT/776D © 1982 The Metals Society. The authors are in the Corrosion and Protection Centre, University of Manchester Institute of Science and Technology. It is well known that fatigue cracks begin at the surface in most metals. Considerable work has been documented on the initiation of fatigue damage from. persistent slip bands’3 and from extrusions and/or intrusions.46 Studies to date have shown that persistent slip bands develop in bursts of slip in the saturated state of hardness, when a relatively uniform array of obstacles, dipoles, and multi- poles has been established. The persistent slip bands thus formed are found to contain micro-embryonic cracks,2 created either by the interaction of dislocations with point defects vacancies3 or by loss of crystal coherency due to accumulation of defects. The macrocrack results from a linkage of these embryonic cracks. The extrusions, thin ribbons of metal extruding from the surface, are frequently associated with intrusions, equivalent to a crack a few micro- metres deep.5 They are formed by an interaction between screw and edge dislocations in the slip and glide planes.6 However, the extrusions and/or intrusions which them- selves form on the slip bands are considered less important for nucleation than the slip bands. Localized regions of stress concentration and relatively low plasticity in the surface, such as at inclusion/matrix interfaces, may act as nucleation sites.71° The crack- nucleation characteristics of inclusions play an important role in influencing fatigue failure, and it has been reported that surface inclusions are more harmful than subsurface ones. The number, size, and type of inclusions have been found to be of importance in determining their effects on the fatigue characteristics. The orientation of inclusions with respect to the direction of applied stress and the ratio of elastic moduli of inclusion and matrix also greatly influ ence the nucleation behaviour. Work on the effect of inclu sions on Al alloys8 and steels9 has shown that crack nuclea tion occurs exclusively on surface inclusions and, although the inclusion/matrix interface is associated with large slip and higher dislocation density in some cases, the overall impression is that the crack nucleates by interfacial debonding.1° In general, the enhancement of fatigue damage in corro sive environments is considered to be caused by effects on nucleation and growth of cracks. The nucleation is enhanced by pitting leading to cracking,11 by preferential dissolution of deformed areas which are more anodic to the matrix,12 or by the rupture of the protective oxide film,’3 exposing fresh metal to the environment and leading to its dissolution and crack formation. Differences between the corrosion potentials of inclusions and the matrix may also lead to the preferential corrosion of one or the other, and to cracking.14 In this paper the results of an investigation into fatigue crack initiation in a maraging steel are presented, particular emphasis being given to the effect of a corrosive environ ment on the initiation process. Experimental MATERIAL The steel used in this work is a maraging steel to DTD specification 5212. The composition and mechanical prop- erties are given in Tables 1 and 2, respectively. The material was supplied in the form of cold-rolled bar, and was subse quently cut into cantilever-bend specimens with the major axis of the specimen parallel to the rolling direction. Before testing, the specimens were polished by hand to a 0 25 cm diamond finish, to give reproducible initiation behaviour and to facilitate optical and electron-optical examination. TEST CONFIGURATION AND PROCEDURE An Avery dynamic bend fatigue test machine was used with constant stress-amplitude loading at a frequency of 14Hz. Tests were performed in laboratory air and in aerated 3 5% sodium chloride solution. The tests with sodium chloride were carried out with a silicone rubber cell fitted over the specimen mounting points. S-N curves were determined for test durations in the range 1$-1O cycles. Further work concentrated on the development of cracks in the early stages of the fatigue test. For this purpose optical and scanning electron microscopy were used to examine the specimen surface. The SEM employed was an ISIDS 130, which permitted the non- destructive examination of complete fatigue specimens on its lower stage. The SEM was also used to examine the original surface and the fracture surface of failed speci mens. Electron-probe microanalysis was employed to determine inclusion chemistry. The dislocation structure of thin foils taken parallel to the specimen surface was studied by conventional transmission electron microscopy. Quantitative surveys of the proportion of inclusions with which fatigue cracks were associated were performed with the aid of a Quantimet 720. This was used to give unbiased automatic stepping across the specimen surface, although the analysis was performed manually. 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% proof strength, Reduction in Hardness, MNm’ Elongation, % area, % HV 1000 930 16 81 325 104 Metals Technology March-April 1982 Vol.9 T

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Page 1: Corrosion-fatigue initiation processes in a maraging steel · The steel used in this work is a maraging steel to DTD specification 5212. The composition and mechanical prop-erties

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

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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

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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

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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.

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