Nonmetallic Inclusions.pdf

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Role of Nonmetallic Inclusionsin Fatigue, Pitting, and Corrosion Fatigue

Y. Wang* and R. Akid**

ABSTRACT

A study was conducted of the effect of nonmetallic inclusionson the fatigue and corrosion fatigue resistance of a high-strength steel in 0.6 M sodium chloride (NaCl) solutions.Results indicated that angular calcium-aluminate inclusionsplayed an important role in introducing cracks during airfatigue cycling; however, sulfide inclusions appeared to bethe main contributors to sites for corrosion pits and subse-quent crack initiation. Conventional consideration based onthe stress intensification caused by such defects was insuffi-cient to describe the role of nonmetallic inclusions in fatiguecrack development in air and by corrosion pits during corro-sion fatigue. However, it was considered that the interactionbetween geometric discontinuities (i.e., nonmetallic inclusionsand cyclic loading) resulted in plasticity localization and,thus, facilitated crack development. Similarly, enhancementof localized dissolution resulting from plasticity localizationcontributed to corrosion pit development at nonmetallic inclu-sion sites, thus promoting early crack development when acorrosive environment was presented.

KEYWORDS: cracking, dissolution, fatigue, high-strengthsteel, inclusions, pitting, plasticity localization, sodiumchloride

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Submitted for publication January 1995; in revised form, July1995.

* SIRIUS, Department of Mechanical and Process Engineering,University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K.Present address: Department of Mechanical Engineering,University of Toronto, 5 Kings College Road, Toronto, Ont., M551A4, Canada.

** SIRIUS, Department of Mechanical and Process Engineering,University of Sheffield, Mappin Street, Sheffield, S1 3JD, U.K.

0010-9312/96/00002© 1996, NACE

INTRODUCTION

It is well known that the presence of geometricaldiscontinuities, such as nonmetallic inclusions andcorrosion pits, can have a deleterious effect on thefatigue and fracture resistance of a material. Thecrack nucleation characteristics of inclusions play animportant role in influencing fatigue failure, and ithas been reported that surface inclusions are moreharmful than subsurface ones.1-3 The number, size,and type of inclusions has been found to be of impor-tance in determining their effects on subsequentfatigue behavior. The orientation of inclusions withrespect to the orientation of the stress field and therelative deformability of inclusions with respect tothat of the matrix also greatly affect the crack nucle-ation behavior. The deleterious effect of inclusions onfatigue resistance of materials is related convention-ally to the localized stress/strain concentration atthe boundary of the inclusion and matrix, owing tothe mismatch of physical, thermal, mechanical, andchemical properties. Crack nucleation at inclusionsites also was believed by some investigators to beoperated by interfacial debonding mechanisms.1-2

Corrosion environments can enhance fatiguedamage by affecting crack initiation and propagation.The crack nucleation process often is facilitated bypitting that can lead to cracking, and corrosion pitsfrequently are observed to form at the sites previ-ously occupied by inclusions. Materials that show afatigue limit when tested in air will have their fatiguelimit reduced or even removed when a corrosive envi-ronment is introduced. In this case, the percentage of

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TABLE 1Chemical Composition of UNS G92600 Steel (wt%)

C Mn Si P S Cr Ni Mo

0.56 0.81 1.85 0.026 0.024 0.21 0.15 0.025

TABLE 2Mechanical Properties of UNS G92600 Steel

0.2% Proof UltimateStress Tensile Strength % Elongation Hardness

1,440 MPa 1,610 MPa 9.3 480 Hv

FIGURE 1. Fatigue endurance data for different environmentalconditions. Stress ratio (R ) = –1 for all tests. (Loading frequency was10 Hz for air tests at stress amplitudes below 750 MPa. All other testsin air and 0.6 M NaCl were conducted at 1 Hz. Arrow indicates anonfailure condition).

life spent during crack nucleation can be reducedremarkably.

The present work presents results of an investi-gation into the fatigue and corrosion fatigue behaviorof a high-strength steel in 0.6 M sodium chloride(NaCl) solution, with particular emphasis on the ef-fect of nonmetallic inclusions on the initiation andearly stages of propagation of fatigue cracks, as wellas on their effect on the process of pit developmentand subsequent crack initiation.

EXPERIMENTAL

Chemical composition and mechanical propertiesof the silico-manganese spring steel used (UNSG92600) are given in Tables 1 and 2, respectively.(1)

The microstructure of this high-strength steel wastempered martensite, obtained through a quench-and-temper heat treatment. The average prioraustenite grain size was 30 µm. Two main kinds ofinclusions were detected: heavily elongated sulfide-type inclusions distributed along the longitudinaldirection of the specimen and a smaller population ofangular-shaped inclusions that were identified ascalcium-aluminate inclusions.

Fully reversed load-control, push-pull tests werecarried out at room temperature in laboratory airand aqueous 0.6 M NaCl solution at different stressamplitudes for fatigue lifetimes covering three ordersof magnitude.

Shallow hour-glass profiled specimens were usedfor fatigue tests in both environments, thus ensuringthat cracks initiated and grew within the gaugelength but without incurring high stress or strainconcentrations at the minimum section. Corrosionfatigue tests were carried out at 1 Hz. For some airfatigue tests, a loading frequency of 10 Hz was usedwhen the applied stress amplitude approached thefatigue limit. The gauge section of the specimen waspolished to a 1-µm surface finish. Information re-garding surface crack development was obtained bytaking acetate replicas of the surface at regular inter-vals. Crack length was measured from the replicasusing an optical microscope with an associated im-age analysis system. The crack growth rate (CGR)was calculated using a simple secant method, andgraphs of CGR-vs-surface crack length were plottedfor individual cracks.

Metallographically polished samples in the un-stressed condition were immersed in the 0.6 M NaClsolution and were examined at regular intervals toinvestigate pit development. Scanning electron mi-croscopy (SEM) was used to examine the fracturesurface of failed specimens, and electron-probe mi-

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(1) UNS numbers are listed in Metals and Alloys in the UnifiedNumbering System, published by the Society of AutomotiveEngineers (SAE) and cosponsored by ASTM.

croanalysis (EPMA) was applied to determine inclu-sion chemistry.

RESULTS

Figure 1 shows the effect of stress amplitude onthe fatigue endurance of specimens exposed to airand immersed in 0.6 M NaCl solution. A fatigue limitin air was observed at a stress amplitude around650 MPa. The corrosive environment markedlyreduced the stress amplitude required to produce agiven fatigue lifetime. The influence of the environ-ment was relatively small for high-stress, low-cycletests but became more significant for lower stress-longer life tests. No fatigue limit was apparent forcorrosion fatigue testing even when the applied

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← Loading Direction →

N 600 3,000 4,200 8,808N/Nƒ 0.04 0.19 0.27 0.56

(b)

FIGURE 2. Early stages of fatigue crack development (observed from replica): (a) in air, stress amplitude ( s) = 750 MPa,frequency (f) = 10 Hz, R = –1, and Nf = 79,400; and (b) in 0.6 M NaCl solution, s = 750 MPa, f = 1 Hz, R = –1, and Nf = 15,710.

20 µm

N 0 20,010 35,000 55,144N/Nƒ 0 0.25 0.44 0.69

(a)10 µm

stress amplitude was reduced to less than one-thirdof the in-air fatigue limit.

Examination of replicas taken from specimensurfaces at various stages during fatigue testsrevealed that crack initiation in air and in NaCl solu-tion was associated with inclusions and corrosionpits, respectively. Typical cases for crack develop-ment in those two environments are shown inFigures 2(a) and (b). Both specimens were subjectedto identical loading conditions, at a stress amplitudeof 750 MPa. In air (Figure 2[a]), a rectangular inclu-sion ~ 15 µm long was found to have an orientationof ~ 45° to the loading axis (i.e., coincident with themaximum shear plane). Debonding at the interfacebetween the inclusion and matrix occurred veryearly, being observed on the first replica when thefraction of lifetime (N/Nf ) = 0.06. Subsequent cyclingresulted in the inclusion falling out, leaving a surfacecavity. Observation of the surface (not the notch

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root), however, did not show any significant changeuntil a crack was detected around 40% of the lifetime(Arrow, Figure 2[a]). This crack then developedquickly to propagate eventually in a direction perpen-dicular to the loading axis.

When tests were conducted in 0.6 M NaCl solu-tion, multiple cracks were observed. Figure 2(b)shows the dominant crack that led to the final failureat a stress amplitude of 750 MPa. In this instance,crack initiation was associated with a corrosion pit.Compared to the behavior in air, the initial stage ofcrack development was reduced markedly by thepresence of the corrosive environment, and thechemical and electrochemical reactions at thesolution-material interface effectively assisted thecrack initiation. As shown in Figure 2(b), a circularetched area was observed surrounding a small rect-angular zone, which might have been an inclusionsite previously. Little change in surface condition was

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FIGURE 3. Early stages of fatigue crack development in 0.6 M NaCl solution: s = 400 MPa, f = 1 Hz, R = –1, and Nf = 110,110(observed from replica).

44,970 55,003 62,2100.41 0.50 0.5720 µm

N 23,362 30,570 37,770N/Nƒ 0.21 0.28 0.34

noted until N/Nf = 0.19 (3,000 cycles), when corro-sion attack was observed around the edges of thissmall zone. Later, at N/Nf = 0.27, a crack clearlyemerged from the corroded area.

When the applied stress amplitude was muchless than the in-air fatigue limit, cracks still devel-oped but were proceeded by a prolonged stage ofcorrosion pitting, as shown in Figure 3 for the case ofcorrosion fatigue at a stress amplitude of 400 MPa.Because of the limitation of the experimental tech-nique used to obtain crack growth behavior, changesin the depth of the corrosion pit could not be mea-sured accurately from the replicas. In general, thesurface dimensions (i.e., pit diameter) of the corro-sion pit increased with continuing load cycling, asfresh metal surfaces were created and fresh solutionflowed in and out of the defect arising from crack


opening displacement. This increase in pit diameter,however, was not always obvious from surfacereplicas taken during the test (Figure 3). Here, pitgrowth rate was slow until a sharp increase of pitdiameter occurred at N/Nf = 0.41. The reason for thissudden burst of activity was not clear, although theformation of a crack at this point would have contrib-uted to an increase in the degree of metal dissolutionand ferrous ion hydrolysis with a subsequent de-crease in the local solution pH. Interestingly, thecrack sizes of this test and that at 750 MPa werealmost identical at ~ 55% of the lifetime despite thedifferent initiation times for the two loading condi-tions.

Further evidence of the interrelationship betweennonmetallic inclusions, corrosion processes, andcracking is shown in Figure 4. In each of the photo-

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N 19,995 23,624N/Nƒ 0.31 0.37

31,1410.49

49,6090.78

20 µm

FIGURE 4. Early stages of fatigue crack development in 0.6 M NaCl solution, maximum stress ( smax) = 800 MPa, f = 1 Hz,R = 0.1, and Nf = 63,869 (observed from replica).

42,4000.66

(a)

(b)

graphs, a reference point can be seen at the top leftcorner, in the form of a small circle. Localized corro-sion attack was observed to occur at N/Nf = 0.37, theinterface of the matrix and inclusion (Arrow, Figure4). This process of dissolution resulted in thedebonding and subsequent loss of the inclusion.Shortly after this, cracking appeared at the edges ofthe site previously occupied by the inclusion.

The above results indicated inclusions play animportant role both in crack initiation for air fatigueand, in corrosion fatigue conditions, promoting a pitfrom which cracking subsequently can occur. How-ever, detailed investigation showed the above effectswere strongly dependent upon the type of inclusion

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present. For air fatigue, cracks were associated ex-clusively with angular-shaped inclusions, as shownin Figure 2(a). X-ray spectra analysis at a crackinitiation site on the fracture surface of a testconducted in air (Figure 5) suggested that theseinclusions were of the calcium-aluminate type. How-ever, calcium-aluminate inclusions seemed to havelittle influence on corrosion pit development, asshown in Figure 6, where unstressed specimens wereimmersed fully in 0.6 M NaCl solution for periods upto 65 h. Here, elongated manganese sulfide (MnS)inclusions proved to be more likely sources for pro-viding sites for corrosion pit development than theircalcium-aluminate counterparts. Again, localized


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FIGURE 5. Fracture surface of a fatigue specimen tested in airshowing crack initiation associated with a surface inclusion (Arrow).

corrosion attack occurred at the interface betweenthe inclusion and matrix, followed by the generationof a macrocorrosion pit.

Although the vast majority of fatigue cracksoriginated at the free surfaces of components, em-bedded cracks also were found to be a source offatigue failure, especially when the stress amplitudeswere low and approached the fatigue limit. Figure 7is an example of a crack originated at an internalinclusion. In this case, surface replica analysisshowed no signs of surface cracking up to 98% offatigue life, implying that lifetime was controlled byinternal crack initiation and propagation. Analysis ofthe fracture surface revealed the presence of an in-ternal calcium-aluminate inclusion at the center of ashiny circular area. The inclusion size was of theorder of 40 µm, which was greater than that of theprior austenite grain size.

For corrosion fatigue, it was evident that bothinitial crack development and crack propagationstages were affected by the environment when com-pared with the data obtained from tests where thecorrosive environment was not involved. Figure 8(a)shows that, for identical loading conditions, the CGRincreased in the presence of the corrosive environ-ment, especially when the crack size was relativelysmall (i.e., < 100 µm). This increase in CGR causedby the presence of the corrosive environment dimin-ished as the crack length increased, as indicated byCGR curves of these two tests, which tended tomerge when the crack lengths approached 1 mm(0.039 in.). Apart from that, the fatigue life was re-duced markedly by the presence of the NaCl solution.Figure 8(b) further shows that, for the same stressingconditions, cracking in air was dominated by initia-tion and the early stages of growth for crack lengths< 100 µm, while in corrosion fatigue, the fraction oflifetime spent during crack initiation was reduced,and crack propagation was dominant.

As mentioned above, the applied stress levelinfluenced the overall fatigue lifetime and the num-ber of load cycles required for crack initiation in airand in NaCl solution. Figure 8(a) also shows that, forthe same exposure condition, CGR of corrosion fa-tigue cracks was affected by the applied stress level.In contrast with the effect of environment, the rela-tive difference in CGR at different applied stresslevels seemed less dependent upon crack size. Therelationship between the surface length of corrosionfatigue cracks and fraction of lifetime (N/Nf), pre-sented in Figure 8(b), appeared to be less influencedby the applied stress level.

DISCUSSION

Inclusion and Fatigue Cracking in AirGenerally, inclusions can affect fatigue proper-

ties in two ways. First, because of the different


deformabilities of the inclusion and matrix, thoseinclusions with low deformability can introducemicrocracks at the matrix-inclusion interface duringhot or cold working of the metal. These microcracks,which already are present in the material from thebeginning of service, then may cause direct propaga-tion of cracks leading to failure, and as a result,eliminate the initial stage of crack nucleation. Sec-ondly, if the microcrack does not exist prior to fatigueloading, inclusions still can affect the fatigue resis-tance strongly, mainly by facilitating the crackinitiation phase.

As summarized by Kiessling, for an inclusion tobe a potential source for fatigue failure, two maincriteria must be fulfilled: the inclusion should have acritical size, depending on the depth below the steelsurface; and the inclusion should have have a lowdeformability, related to its expansion coefficienttaken at the actual temperature during fatigue.3 Forsteels, “dangerous” inclusions include single-phasealumina (Al2O3), spinels, and calcium-aluminates> ~ 10 µm in size. The MnS types appeared to be theleast harmful. In the present investigation, two majortypes of inclusions, calcium-aluminates and the MnStype, were found in the steel. Cracks developed dur-ing air fatigue cycling were associated exclusivelywith angular- or spherical-shaped calcium-aluminateinclusions.

The detrimental effects of geometricaldiscontinuities upon the cracking resistance ofmaterials conventionally are related to their influencein creating regions of stress concentration. It isbelieved that, when inclusions or cavities are presentin the steel, the applied load is maintained only par-

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T = 0 T = 17 h T = 65 h

T = 0 T = 17 h T = 65 h10 µm

FIGURE 6. Micrograph of specimens before and after immersion for 17 h and 65 h in 0.6 M NaCl solution: (a) elongatedsulfide-type inclusion and (b) annular calcium-aluminate-type inclusion.

(a)

10 µm

(b)

tially by the inclusions or the cavities, and the stressis concentrated in the surrounding material.

However, it is difficult to explain the phenomenarelated to fatigue cracking by considering stressconcentration alone. For example, determination ofthe stress concentration factor (Kt) for an inclusion isbased upon the shape and the relative modulus ofelasticity with respect to the matrix and is notdependent upon size.4 For the most common shape(i.e., spherical), typical values of Kt for a cavity andrigid inclusion are 2.05 and 1.94, respectively. Thesevalues are lower than those previously cited,2

although a higher value of Kt might be expected forangular-shaped inclusions. It may be true in somecases, as suggested for angular-shaped inclusions,that local stresses exceed the macroscopic yieldstress of the material resulting in plastic deformationand crack nucleation. However, for most cases, thelocal stress found around an inclusion is lower thanthe yield stress, and yet, it is still observed widely

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that cracks develop on the application of fatiguecycling.

A similar argument also may be applied to thenotch effect upon fatigue strength.5 The fatigue notchfactor (fatigue strength reduction factor) usually isless than Kt (i.e., very sharp notches have less effecton fatigue strength than would be expected basedupon their theoretical Kt value). A recent study, usingfinite-element methods, revealed that the stressaround a spherical internal pore does not differsignificantly from that around a semispherical sur-face-breaking pore.6 These observations imply that,when stress concentration alone is considered, thesize and location of the inclusion has no affect onfatigue behavior. However, this is contrary to experi-mental observations.3,7-8 In the latter two studies, itwas observed that the lifetime of a specimen where acrack develops from an interior inclusion is signifi-cantly longer than the lifetime of a specimen in whichfailure occurs from a crack initiated at the surface.


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(c)(b)(a)

FIGURE 7. Fracture surface of fatigue specimen tested in air showing embedded cracking associated with an internalinclusion (s = 675 MPa, f = 10 Hz, and R = –1).

This may, for the most part, be attributed to a lack ofconstraint for a surface-generated crack.

When fracture mechanics is applied to assessthe fatigue problem, it is usual to consider an inclu-sion as a preliminary crack. Fatigue failure, however,can be caused by crack growth from inclusions thatare of a size much smaller than the critical size cal-culated from the threshold stress intensity factorrange.7-8 Interfacial debonding between inclusion andmatrix was believed by some investigators as themajor mechanism for fatigue crack initiation in air1

and under corrosion fatigue conditions.2 Suchdebonding, resulting in a cavity or pore on the sur-face, may be an important process for crackdevelopment, as most examples given in this studyhave involved such a stage. However, this processdoes not raise the stress concentration effectively,nor is it a necessary prerequisite stage for fatiguecrack development.9 As observed during the currentstudy even after total debonding and the formation ofa cavity on the surface, a considerable number ofload cycles still was required for a crack to emergefrom such cavities.

It cannot be disregarded that, under a number offorms of loading, inclusions and other forms of mate-rial geometrical discontinuities may act as stressraisers; however, it is through the interaction of in-clusions with cyclic loading (i.e., by facilitating theplasticity localization process) that the cracking re-sistance is affected when a material is under fatigueloading. The basic mechanism of fatigue crack devel-opment in model materials, such as single crystals,appears still to be operational in engineering materi-als where inclusions or other forms of surface


discontinuities are involved. It is understood fromprevious studies that early fatigue damage, whichmay include either cyclic hardening or softening,reaches a saturation state with the development ofhighly localized persistent slip bands (PSB).10 Thesebands generally are glissile, and their reverse slipaction supports most of the cyclic plastic strain inthe material. As pointed out by Miller, the criticalPSB will be aligned with the maximum applied shearstress direction and will be in a position that pro-duces the maximum amount of plasticity.11 Becauseof stress intensification effects, the presence of inclu-sions will result in heterogeneous deformation,particularly during the early stages of fatigue cycling,where local deformation is concentrated within thematerial surrounding inclusions. It also has beenreported that fatigue cracks that initiate near inclu-sions often seem to develop on slip bands thatimpinge on the inclusion.12 The pileup of matrix dis-locations at the inclusion may be responsible for thecracking of inclusions or interfacial debonding, withsubsequent development of a fatigue flaw within thematrix. Laird and Duquette suggested that the sud-den appearance of coarse slip lines around aninclusion is a result of the release of stored disloca-tions.13 Fine and Ritchie concluded for an Al 2024(UNS A92024) alloy and high-strength, low-alloy steelthat cracked inclusions or debonded regions betweeninclusions and matrix do not necessarily contributeto fatigue crack initiation; however, inclusions do aidslip-band development.12

The principle of plasticity localization aroundinclusions that leads to the formation of fatigue flawsalso is applicable when explaining the fact that most

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(a) (b)

FIGURE 8. Effect of environment and stress amplitude upon fatigue crack growth behavior: (a) CGR vs crack length and(b) crack length vs fraction of lifetime (N/Nf).

fatigue cracks are associated with surface inclusions.This is due to the ease of deformation which occurson the surface, as opposed to inside the material.However, this does not exclude the effect that interiorinclusions have on fatigue crack initiation. In mostcases where fatigue crack nucleation is associatedwith internal inclusions, the stress levels are rela-tively low or close to the fatigue limit.7-8 It also hasbeen found that the critical inclusion size for initiat-ing fatigue cracks increases with the distance fromthe free surface.3 When the applied stress is suffi-ciently low, the degree of surface deformation can benegligible, and it is the hydrostatic stress concen-trated around the internal inclusion that causesdamage and leads to cracking. In addition, for thepresent study, the sizes of the inclusions that causedinternal fatigue cracking were greater than the prioraustenite grain size. Such a defect size can contrib-ute greatly to eliminating the microstructuraldominated-crack growth regime. A bright circulararea with the inclusion at its center frequently wasobserved from the fracture surface, this so-called“white spot” has been reported to be attributed to thehydrogen content of inclusions.3 In this case, theinclusions served as small internal reservoirs ofhydrogen, allowing small amounts of hydrogen to bereleased into the steel each time the surroundingsteel was in tension. However, it also is possible thatthis “white spot” was caused by the repeated contactof the two crack surfaces during cyclic loading. Acertain number of loading cycles is required for acritical strain energy to be built up in the material

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around the inclusion, and the formation of a fatigueflaw results from the release of the stored energy.Once a crack is initiated, the constraint of materialaround the crack, compared with a surface crack,will limit the crack opening more effectively, therebyincreasing the contact or fretting of the crack sur-faces during the compression phase of the fatiguecycle. Furthermore, because the embedded crack isisolated from the outside atmosphere by the materialsurrounding the crack, it actually extends within avacuum environment, and thus, the CGR can be veryslow. This, in turn, further increases the chances ofcrack surfaces contacting. In this study, more than98% of lifetime was consumed by crack developmentinside the material for the case shown in Figure 7.Statistical studies on the fatigue life distribution alsoindicate that, under the same test conditions, ifcrack initiation is associated with an internal inclu-sion, a longer lifetime frequently can be expectedthan for the case where crack initiates at a surfaceinclusion.7-8

Inclusions, Corrosion Pitting,and Corrosion Fatigue Cracking

When a corrosive environment is present, theelectrochemical activity of the inclusions is of mostimportance in determining localized corrosion behav-ior that may result in the nucleation of corrosionpits. Current work has shown that the angular- orspherical-shaped calcium-aluminate inclusions,which play a major role in promoting cracking in airfatigue (Figures 2[a], 5, and 7), had less influence on


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corrosion pit development (Figure 6[b]). Howeverelongated sulfide types, which are the least harmfulin air fatigue, were the most important sites fornucleation of corrosion pits. Many theories have beenproposed for the effect of sulfide inclusions on theinitiation and propagation of pits in steels in chloridesolution. Such theories are reviewed by Kiessling,3

Jones,14 and Ray, et al.15 Despite the difference in thedetails of these theories, it generally is agreed thatthe sulfide inclusions or the immediate area sur-rounding the inclusions are anodic with respect tothe steel matrix and that the hydrogen sulfide (H2S)and HS– ions formed by dissolution of sulfides hasthe most deleterious effect on the development ofcorrosion pits. The H2S and HS– ions produced insolution can catalyze the anodic dissolution of ironfrom the matrix and poison the cathodic discharge ofhydrogen. Local acidification due to hydrolysis offerrous and ferric ions, in turn, enhances the disso-lution of sulfide inclusions, and accumulation of HS–

ions favors progressive attack, thus producingmicropits.

In this study, in some areas of the specimen, pitor crack density was higher than in other areas. Thismay have reflected the variation in inclusion densityat the specimen surface. On immersion in 0.6 MNaCl solution, without the application of stress, localattack first occurred in only a few places at the inter-face between the inclusion and matrix (Figure 6[a]).Macropits or cavities were generated at positionsoccupied by the inclusions. Such macropits resultfrom the formation, propagation, and coalescence ofmicropits at the interface between the inclusion andmatrix. It has been reported that, under the applica-tion of a cyclic stress, corrosion at the inclusion-matrix interfaces developed more rapidly than understress-free conditions.15 Furthermore, local corrosionattack at the interface between the inclusion andmatrix was observed before the formation ofmacropits. These results supported the theory pro-posed by Gainer and Wallwork.16

For corrosion fatigue tests, it also noted that aconsiderable number of loading cycles still is re-quired for a microcrack to be developed from a pit.Furthermore, during those loading cycles, the dimen-sions of the pits, at least in the case of surfacedimensions, do not show any significant change. Theresults of the current work cast some doubt on thetraditional consideration of the effect of corrosionpits upon the corrosion fatigue behavior.2,17-18 Asmentioned above, debonding of the inclusion-matrixinterface by preferential dissolution leaving a cavityin the surface did not increase the stress concentra-tion as significantly as might have been expected.2

In this respect, fracture mechanics was appliedto estimate the threshold conditions for fatiguecracks initiated from corrosion pits, where pits wereconsidered as sharp cracks.17-18 Calculated threshold


stress-intensity values generally agreed with experi-mental values obtained from specimens prepitted insolution but fatigued in air.17 However thresholdvalues obtained from specimens fatigued in solutionwere much lower than those estimated based on asharp crack.18 Congleton, et al., reported that fatiguecrack initiation time, for specimens tested undercorrosion fatigue conditions, is much less than thatfor specimens prepitted in solution but fatigued inair.19 Furthermore, the study conducted by Buxton,et al., showed that the predicted corrosion fatiguelives, based on calculations employing the thresholdstress intensity factor and a pit growth law, weregenerally greater than the experimental results,although better correlation was found at very lowapplied stress levels.20

The above findings imply that consideration ofstress intensification alone is not sufficient to explainthe deleterious effects of pits upon corrosion fatiguebehavior.

Parkins stated that, quite apart from their influ-ence on stress intensification, geometricaldiscontinuities, such as pits, fissures, or cracks, maybe important since they may act as sites for preferen-tial electrochemical activity.21 The electrochemicalchanges within these geometrical discontinuities,such as changes in solution composition, pH, andelectrochemical potential, may induce a local envi-ronment that is very different to the bulk solutionand, thereby, facilitate cracking.

As mentioned previously, cyclic loading facili-tates the pitting process, and corrosion pits, actingas geometrical discontinuities, subsequenyly canenhance the plasticity localization phase under theinteraction of the cyclic load and the corrosive envi-ronment. Slip bands are known to be regions of highdislocation density. High-strain energy fields, associ-ated with dislocations, favor dissolution at slipbands. During fatigue cycling, the to-and-fro motionof PSB provides a continuous source of fresh metal tothe environment for subsequent dissolution. Thisdissolution, in turn, results in the removal of barriersto further slip, effectively enhancing the localizationof both deformation and further dissolution. As aresult of these interactions, crack development isfacilitated from a corrosion pit. Furthermore, the H2Sand HS– ions formed by dissolution of sulfides in thesolution within corrosion pits or at crack tips maypoison hydrogen discharge, and hence, hydrogenembrittlement may be involved in the process ofcrack development. However, as indicated by recentwork, cathodic polarization can prolong the fatiguelifetime, especially the initiation phase.9 Therefore,hydrogen effects appear effective only when the crackis well established. The early stages of corrosion fa-tigue cracking of this high-strength steel within NaClsolution are believed to be dominated by localizeddissolution processes.

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CONCLUSIONS

❖ Nonmetallic inclusions played an important rolefor fatigue cracking of this high-strength steel, andcrack initiation via inclusions limited the fatigueresistance of the steel.❖ Angular-shaped calcium-aluminate-type inclu-sions contributed to crack initiation in air fatigue byenhancing the localization of plasticity around theinclusions. Models based upon a consideration ofstress intensification alone appeared insufficient toexplain the effect of nonmetallic inclusions on fatiguecracking, particularly where inclusion orientationmarkedly affects fatigue lifetime.❖ During corrosion fatigue, elongated sulfide-typeinclusions provided sites for corrosion pitting andsubsequent crack initiation. A stage of localized cor-rosion attack at the interface between inclusion andmatrix was involved in the pitting process irrespec-tive of whether the specimen was subjected to anapplied stress.❖ Corrosion fatigue crack initiation was the result ofthe interaction of corrosive environment and cyclicloading where a mutual synergistic effect of localizeddissolution and plasticity concentration facilitatedthe cracking process.

ACKNOWLEDGMENTS

The authors acknowledge the sponsorship byand a scholarship for Y. Wang from the Engineeringand Physical Sciences Research Council.

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REFERENCES

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Threshold Crack Growth,” in Fatigue and Microstructure,(Materials Park, OH: ASM, 1978), p. 245.

13. C. Laird, D.J. Duquette, Mechanism of Fatigue CrackNucleation, in Corrosion Fatigue (Houston, TX: NACE, 1972),p. 88.

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