fatigue properties of a sae 4340 steel coated with a nimet

Ž .Surface and Coatings Technology 133]134 2000 561]571

Fatigue properties of a SAE 4340 steel coated with a NimetHP autocatalytic nickel deposit

C. Guzmana, N. Dıaza, J.A. Berrıosb, A. Pertuza, E.S. Puchi Cabrerab,U´ ´ ´aSchool of Mechanical Engineering, Faculty of Engineering, Uni ersidad Central de Venezuela, Apartado Postal 47885,

Los Chaguaramos, Caracas 1045, VenezuelabSchool of Metallurgical Engineering and Materials Science, Faculty of Engineering, Uni ersidad Central de Venezuela,

Apartado Postal 47885, Los Chaguaramos, Caracas 1045, Venezuela

Abstract

Ž . WThe influence of a commercial electroless Ni]P EN deposit known as Nimet’s HP autocatalytic nickel , on the fatigueproperties of a quenched and tempered SAE 4340 steel, has been investigated. Such an EN deposit had a thickness of

Ž .approximately 10 mm, a P content of approximately 12 wt.% and it was evaluated in two different conditions: a as-deposited;Ž . Ž .and b deposited and post-heat treated PHT at 723 K for 1 h, the condition in which the deposit showed its maximum hardness.

It has been determined that the application of such a coating to the steel substrate gives rise to a significant reduction of thefatigue life and fatigue limit, in comparison with the uncoated material, which is more marked in the PHT condition. Thereductions in fatigue life have been quantified in terms of the computed values of the Basquin parameters of the materials testedunder different conditions. Thus, it has been shown that the fatigue life of the steel can be reduced up to 93% in the as-depositedcondition and up to 97% in the as-deposited and PHT condition. The fatigue limit can also be reduced between 12 and 23%depending upon the condition of the coating. From the microscopic point of view, it has been observed that the fatigue fractureof the substrate-coating composite initiates in the deposit and that it remains well adhered to the substrate during fatigue testingsince interfacial cracks have been very rarely observed. Such adhesion characteristics enhance the transference of the early cracksnucleated towards the substrate steel, a belief that is supported by the analysis of the fracture surfaces of the samples tested atdifferent stress levels. Q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Electroless nickel; Fatigue; 4340 steel

1. Introduction

EN deposits represent an important group of metal-lic coatings that are employed on a wide range ofsubstrates to provide protection against corrosion andabrasive wear. Such platings have found different appli-cations in a variety of fields like electrical and electron-ics, oil and gas, machinery, aerospace, motor, chemical,foodstuffs, textile and printing industries. Currently,their use continues to increase due to a number of

U Corresponding author. Tel.: q58-2-6628-927; fax: q58-2-7539-017.

Ž .E-mail address: [email protected] E.S. Puchi Cabrera .

advantages that these coatings present, in comparisonw xwith other plating methods 1 , which include better

corrosion and chemical resistance, improved ductility,and a harder deposit, especially after heat treatment,and the ability to achieve extremely uniform thicknesswithout costly post-machining or grinding, even on

w xparts with complex configurations 2 . However, protec-tion against corrosion and wear can be gained at theexpense of a decrease in other important propertiessuch as fatigue life and fatigue limit, which could be ofutmost importance in some of the applications pointedout above.

Regarding previous studies carried out on the effectof EN deposits on the fatigue properties of high

Žstrength steels ultimate tensile strength of the order of

0257-8972r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 8 9 8 - 7

( )C. Guzman et al. r Surface and Coatings Technology 133]134 2000 561]571´562

. w x1200 MPa , Wu and coworkers 3 conducted an inves-tigation on the fatigue resistance of a 30CrMo steelŽ .0.30 C, 1.09 Cr and 0.24 Mo oil quenched from 1143K and tempered at 893 K for 3 h. In this investigation,the source of Ni ions was NiSO with a pH of 4.5 and4the deposit was PHT a 473 K for 1.5 h. The authorsreported a reduction in the fatigue limit of approxi-mately 39% for the plated substrate and a reduction of20% when the substrate was previously shot peenedbefore plating. Also, it was indicated that the fatiguecracks initiated at the interface between the coatingand the substrate, and that in the deposit some of thecracks were parallel to the stress axis. The low fatiguestrength of the coating was found to be responsible forthe decrease in the fatigue limit of the plated steel.

w xOn the other hand, Zhang et al. 4 also carried outthree-point bending fatigue tests on a 30CrMo steelcoated with an EN deposit of 43 mm thickness and 9.5wt.% P. In this study some of the samples were shotpeened before plating and some of the deposited speci-mens were PHT at 2008, 4008 and 6008C. The residualstresses in the coatings were determined by means ofthe bent strip method. For all the conditions investi-gated it was observed that such stresses remained com-pressive after annealing, but decreased with increasingannealing temperature. Also, shot peening before plat-ing was observed to increase the compressive residualstress within the coatings. Regarding the influence ofEN deposits on the fatigue limit of the material, it wasdetermined that such coatings reduced this property incomparison with the unplated substrate. The decreasein fatigue strength was observed to be less marked forthe shot peened specimens but became significantlyhigher as the PHT temperature increased. In relationto the fractographic analysis of the plated samples, itwas reported that without the application of shot peen-ing, the fatigue cracks initiated at the surface of thespecimens, leading to the fatigue failure of the coating.On the contrary, when the samples were shot peenedprior to the coating deposition, the crack initiation siteswere displaced to the coating]substrate interface. The

w xwork conducted by Zhang et al. 4 concluded that thefatigue properties of this material, when it is coatedwith EN deposits, depends primarily on the fatigueresistance of the coating itself.

w xMore recently, Garces et al. 5 conducted an investi-´gation in order to study the fatigue life of a quenchedand tempered AISI 4340 steel in three different condi-

Ž . Ž .tions: a uncoated; b coated with an EN deposit of aP content of approximately 12]14 wt.%, as-deposited;

Ž .and c as-deposited, followed by a two-step PHT: 473K for 1 h plus 673 K for 1 h. The results of this workindicated that plating the base steel with this kind ofdeposit leads to a significant reduction of the fatiguelife of the material, particularly if the deposit wassubjected to a subsequent PHT. Such a reduction was

quantified by determining the Basquin parameters fromthe fatigue life curves obtained for the uncoated,coated, and coated and PHT substrate. Accordingly, itwas shown that the fatigue life of the base steel couldbe reduced by 78% in the as-deposited condition andby 92% after a subsequent PHT. The microscopicobservation of the fracture surfaces of the samplesconducted in this investigation indicated that the fa-tigue process was initiated at the surface of the depositand, subsequently, transferred to the substrate, withthe assistance of the metallic bonding established atthe substrate]deposit interface. Such a belief was sup-ported by the observation of some continuity of thefracture features between the coating and the substrate

w xunder low alternating stresses. Garces et al. 5 re-´ported that in their study, the bonding between the ENdeposit and the base steel was observed to be ratherpoor, a fact that was supported not only by the pres-ence of extensive secondary cracking along the coat-ing]substrate interface after fatigue testing but also bythe observation of the complete separation of the de-posit from the substrate during tensile testing. Such abehavior was believed to be related to the significantdifference that existed between the elastic and plasticproperties of the EN deposit and the base steel. Thestudy also concluded that the slight degree of metallicbonding that remained after the first stage of fatiguetesting seemed to be enough to allow the passage ofthe fatigue cracks, previously nucleated in the deposit,into the substrate, and therefore, that the EN depositacted as a surface crack source or surface notch whichdecreased the fatigue life of the coated material byreducing the crack nucleation stage.

Thus, the present investigation has been conductedin order to study further both the fatigue life andfatigue limit of a SAE 4340 steel which has been oilquenched and tempered prior to plating at industrialscale with an EN deposit of 10 mm in thickness and a Pcontent of approximately 12 wt.%, known commerciallyas Nimet’s HP autocatalytic NickelW , which is believedto have better adhesion properties than the EN coating

w xemployed in the previous investigation 5 . However, itis important to mention that although the work con-

w x w xducted by Wu et al. 3 and Zhang et al. 4 showed thatshot peening the substrate material prior to coatingdecreased the loss in fatigue limit, in order to carry outa reliable comparison of the present results with those

w xreported by Garces et al. 5 , the specimens had also to´be coated without any prior shot peening.

2. Experimental techniques

The present study has been carried out with samplesof a SAE 4340 steel with the following compositionŽ .wt.% : 0.41 C, 0.79 Mn, 0.24 Si, 0.79 Cr, 0.23 Mo and

( )C. Guzman et al. r Surface and Coatings Technology 133]134 2000 561]571´ 563

1.73 Ni. The alloy was already provided in the quenchedand tempered condition. This material is widely em-ployed in the manufacture of automotive crankshaftsand rear axle shafts, aircraft crankshafts, connectingrods, propeller hubs, gears, drive shafts, landing gearparts and heavy duty parts of rock drills. The materialwas provided as bars of approximately 16 mm diameterand 6 m length. Such bars were cut to pieces ofdifferent lengths: nine pieces of approximately 120 mmfor machining tensile specimens; 102 pieces of approxi-mately 102 mm for machining the fatigue samples; andseven pieces of approximately 10 mm for characterizingthe deposit and to conduct hardness tests. Both thetensile and fatigue specimens had a gage diameter of6.35 mm and shoulder diameter of 12.7 mm. However,the tensile samples had a gage length of 25.4 mm and afillet radius of 12.3 mm, according to the ASTM stan-dard A-370. The fatigue specimens had a curved gagelength of 38.1 mm along the cord, machined following acontinuous radius of 58.73 mm. The specimens weremachined in several steps, with a continuous reductionin the depth of cut of the material. The turning opera-tion was conducted on a horizontal turret lath at lowspeeds in order to minimize the introduction of resid-ual stresses at the surface of the samples. Finally, thespecimens were ground with successive SiC papers grit100]1200 in order to eliminate the circumferentialnotches and scratches, and polished mechanically to a‘mirror-like’ finish. The surface roughness within thegage length of the samples was maintained below ap-proximately 0.2 mm.

All the samples were plated at Nimet Industries Inc.Ž .South Bend, Indiana, USA . The deposit applied isknown commercially as Nimet’s HP AutocatalyticNickelW , with a phosphorous content of 12 wt.% and athickness of approximately 10 mm, which was cor-roborated by means of the ball cratering techniqueŽ . Ž .Calotest, CSEM and image analysis LECO 500 . Inorder to determine the optimum PHT temperature anumber of heat treatments were conducted at tempera-tures of 673, 723, 773, 823 and 873 K for 1 h in anargon atmosphere. Subsequently, hardness measure-ments were conducted on the cross section of theshortest cylindrical samples employed for this purpose.A total of 10 microhardness measurements were car-ried out on each specimen according to the ASTMstandards B-578 and E-384. Such measurements were

Žconducted employing a Knoop indenter Shimadzu,.Japan with a load of 50 g applied during 10 s. As

discussed later, this evaluation determined that thehighest hardness of this deposit is achieved at a tem-perature of 723 K. Such a PHT has a negligible effecton the mechanical properties of the substrate steelwhich, after quenching, is usually tempered at a muchhigher temperature. Thus, three tensile and 24 fatiguespecimens were PHT at this temperature for 1 h in an

argon atmosphere, employing a heating rate of 473Krh and allowing the samples to cool within the fur-nace after the treatment.

The chemical analysis of the plating both in theas-deposited and PHT conditions was determined by

Ž .means of SEM techniques Hitachi S-2400, Japan withEDS facilities. The observations were conducted at aconstant potential of 20 kV. Tensile tests were carriedout on a computer-controlled servohydraulic machineŽ .Instron 8502, USA at a cross-head speed of 3mmrmin. At least three samples were employed forcharacterizing the monotonic mechanical properties ofboth the coated and uncoated substrate. Fatigue testswere carried out under rotating bending conditionsŽ .Rsy1 , employing a Fatigue Dynamics RBF-200

Žequipment, at a frequency of 50 Hz 3000 revolutions.per minute . All the tests were carried out in air at

Ž .room temperature 238C . In this type of test, thespecimen is subjected to a dead-weight load whilebearings permit rotation. At the mid point of thecircular test section surface, the material is subjectedto a sinusoidal stress amplitude from tension on the topto compression on the bottom with each rotation. Thebending moment applied to the specimens was de-termined as a function of the alternating stress and thediameter of the sample by means of the simple rela-tionship:

p 3M s s d , N mmB max32

where M represents the bending moment in N mm,Bs the maximum alternating stress in MPa and d themaxspecimen diameter in mm. Thus, the uncoated sub-strate was tested at alternating stresses of 612, 650, 688and 726 MPa, which corresponded to 59, 63, 66 and70% of the yield stress of the material. For the samplesin the as-deposited conditions, the fatigue tests wereconducted at stress levels of 536, 574, 612 and 650MPa, which corresponded to 52, 55, 59 and 63% of theyield stress of the coated substrate.

For the coated and PHT samples the tests wereconducted at 498, 536, 574 and 612 MPa, correspondingto 44, 52, 55 and 59% of the yield stress of the basesteel in this condition. A total of 24 samples wereemployed for evaluating the fatigue properties of theuncoated substrate, 24 for the coated material and 24for the coated and PHT substrate, which fulfills thenumber of specimens required in S]N testing for reli-

Žability data according to the ASTM standard 739 12]24.samples . Thus, the testing procedure followed in the

present work allowed a replication greater than 80%. Itis important to point out that in order to make possiblea meaningful comparison of the fatigue life of theuncoated, coated and coated and PHT specimens, allthe samples were mechanically prepared in order to


Table 1Main parameters and experimental conditions involved in the present work, together with those employed in previous research previouslyreferred to

Authors Substrate Deposit PHTcharacteristics

w x ŽWu et al. 3 Quenched and tempered 30 Cr Mo steel 0.30 C, 43 mm thick 473 K for 1.5 h.1.09 Cr and 0.24 Mo . Samples with and without and 9.5 wt.% P

shot peening before plating.

w x ŽZhang et al. 4 Quenched and tempered 30 Cr Mo steel 0.30 C, 43 mm thick 473, 673 and 873 K.1.09 Cr and 0.24 Mo . Samples with and without and 9.5 wt.% P for 1 h

shot peening before plating.

w x ŽGarces et al. 5 Quenched and tempered AISI 4340 steel 0.34 C, 24 mm thick, Some of the samples´.1.50 Cr, 1.50 Ni and 0.20 Mo . Samples were not 12]14 wt% P were PHT at 473 K

shot peened prior to coating. for 1 hq673 K for 1 h

ŽPresent work Quenched and tempered AISI 4340 steel 0.41 C, 10 mm thick, Some of the samples.0.79 Cr, 1.73 Ni and 0.23 Mo . Samples were not 12 wt.% P were PHT at 723 K

shot peened prior to coating. for 1 h

have similar mirror-like polished surfaces before test-ing. The fatigue limit of the coated and uncoatedspecimens was determined by means of the staircasemethod employing a step of 7 MPa and 10 samples foreach condition. According to the ASTM standard E-468,infinite life was specified at a number of 5=106 cycles.

The fracture surfaces of the samples were closelyexamined by means of SEM techniques, particularly inrelation to the site initiation of fatigue cracks and thedifferent stages of their subsequent propagation.

Table 1 presents a summary of main parameters andtesting conditions involved in the present work, togetherwith those relevant to the previous work cited above.

3. Experimental results and discussion

3.1. Characteristics of the deposit

As expected, the typical microstructure of the sub-

Ž .Fig. 1. SEM view of the interface between the EN coating D andŽ .substrate S previous to fatigue testing. The deposit seems to be

uniform and has apparently satisfactory adhesion characteristics dueto the absence of visible cracks along the interface.

strate evaluated on the scanning electron microscoperevealed the presence of a large number of relativelycoarse martensite plates together with carbides, visibleas small particles which constitute a typical temperedmartensite structure. Fig. 1 shows a view of the inter-face between the EN coating and substrate previous tofatigue testing, illustrating the deposition of an appar-ently uniform coating with satisfactory adhesion char-acteristics, which was corroborated by the evaluation ofthe coated material during tensile testing up to theyield stress and also by the observation of the fracturesurfaces of the specimens after fatigue testing.

It has already been mentioned that the coating thick-ness was evaluated by means of the ball crateringtechnique and also SEM observations, which showed amean value of approximately 10 mm. As shown in Fig.2, the EDS analyses conducted on the deposit allowedto determine a P content of approximately 12 wt.%.

It is widely accepted that the residual stresses in the

Fig. 2. Typical EDS spectrum for the EN deposits involved in thepresent work.


deposit play a fundamental role in the fatigue behaviorof any coated material and that such stresses are inti-mately associated, among other variables, with thechemical composition of the deposit. Regarding ENplatings, an early investigation conducted by Parker

w xand Shah 6 , particularly employing samples of 1090steel as substrate, allowed us to conclude that theincrease in the P content beyond approximately 10wt.% gave rise to a change in the residual stress pat-tern, from tensile to compressive stresses and that for aP content of 12.4 wt.% the compressive residual stressescould achieve a magnitude of up to approximately 60MPa. Thus, it would be expected that the EN depositsof the Nimet’s HP autocatalytic NickelW employed inthe present investigation were also under compressive

w xresidual stresses. The findings of Wu et al. 3 andw xZhang et al. 4 would also support this view. These

authors reported compressive residual stresses of theorder of 45 MPa in the EN coatings deposited andsubsequently PHT at 673 K for 1 h, and 10 MPa in theEN coatings deposited and subsequently PHT at 873 Kfor 1 h, even though the P content of such deposits was

Ž .lower 9.5 wt.% than that present in the coated speci-mens employed in this study.

3.2. E¨aluation of mechanical properties

In the as-deposited condition, the EN Nimet’s HPautocatalytic NickelW deposit presented a hardness ofapproximately 488"78 KHN . However, as it was50mentioned earlier, in order to determine the optimumPHT temperature at which maximum hardness is at-tained, a number of heat treatments were conducted inthe temperature range of 673]873 K maintaining thespecimens for 1 h. Fig. 3 illustrates the change inhardness with PHT temperature that was determinedfrom these experiments, which indicates that a maxi-mum hardness of approximately 1356"82 KHN is50obtained after a PHT at 723 K. As it has been reported

w xby Riedel 1 , heat treatment of EN deposits fromapproximately 553 to approximately 873 K is an excel-lent means of improving abrasion resistance and othertribological characteristics, and that by treating suchdeposits at approximately 673 K for 1 h their hardnesswill typically increase from 500]600 up to 1000]1100VHN. Such changes in hardness are intimately associ-ated with the modifications undergone by both theatomic structure and microstructure of the deposit as aresult of solid state diffusion. It has been well docu-

w xmented in the literature 1 that both the microcrys-talline and the amorphous deposits undergo a crystalgrowth process which results in a mixture of relativelycoarse-grain metallic nickel together with intermetallicphases such as Ni P, Ni P and Ni P . The early work2 3 5 2

w xof Kreye et al. 7 has shown that the microstructure ofthe deposits with a P content of approximately 12 wt.%,

Fig. 3. Change in Knoop microhardness of the plated specimens withpost-heat treatment temperature.

PHT at 673 K for 1 h is composed of two phases: 80%of the tetragonal Ni P and 20% of metallic nickel.3

In order to evaluate if the EN Nimet’s HP autocat-alytic NickelW deposit had any influence on the mono-tonic mechanical properties of the compositecoating]substrate material, a number of tensile testswere conducted with samples in the uncoated andcoated conditions. The substrate material had a yieldstress of approximately 1037"43 MPa and an ultimate

Ž .tensile strength UTS of approximately 1143"30 MPa.In the as-deposited condition the coated samplesshowed somewhat lower mechanical properties: yieldstress of 980"32 MPa and UTS of 1077"26 MPa,whereas in the as-deposited and PHT condition theyield stress was observed to increase slightly to 1010"19 MPa and the UTS remained constant at a value of1077"18 MPa. Thus, it can be stated that the depositsplated onto the substrate employed in the presentstudy did not give rise to any significant change eitherin yield stress or in UTS of the base steel, which is notsurprising since in the present case the thickness ofsuch deposits is so small that its effect is negligible. In

w xcontrast to the results reported by Garces et al. 5 , in´the present investigation it was observed that duringboth tensile testing up to the yield stress and fatiguetesting in the whole stress amplitude interval, the de-posits remained well adhered to the substrate indicat-ing that the EN Nimet’s HP autocatalytic NickelW

deposit has a much better adhesion to the substratesteel which is not affected by the difference in mechan-


Table 2Ž . Ž .Mean number of cycles to failure N vs. stress amplitude S for thef

uncoated specimens

Ž .Stress MPa

612 650 688 726

216 900 77 600 68 800 43 000233 900 84 800 69 600 44 200283 300 104 100 72 300 46 800398 400 136 400 110 300 57 500424 100 142 000 120 500 70 300732 000 177 500 130 200 70 700

Mean 381 433 120 400 95 283 55 417S.D. 174 868 34 995 25 722 11 645

Ž .ical properties elastic and plastic between the platingand the base material.

In relation to the fatigue tests conducted in order toevaluate the fatigue life of both the coated and un-coated samples, the determination of the monotonicmechanical properties of the material allowed to es-tablish a stress amplitude range of 612]726 MPa forthe substrate which corresponded to a fraction of theyield stress of approximately 0.59]0.70. The coatedsamples in the as-deposited condition and after thecorresponding PHT at 723 K were tested at a fractionof the yield stress ranging between approximately0.55]0.66 and 0.49]0.61, respectively. The data showing

Ž .the number of cycles prior to fracture N as a func-fŽ .tion of the alternating stress applied to the material S

for the uncoated, coated as-deposited and coated andPHT specimens, are presented in Tables 2]4, whereasthe data concerning the determination of the fatiguelimit of the materials under different conditions arepresented in Tables 5]7.

The results obtained, regarding the fatigue proper-ties of the coated and uncoated materials, have beenplotted in Fig. 4 in which it can be observed that ateach alternating stress level for the uncoated, coatedas-deposited and coated and PHT specimens, six testswere carried out. As mentioned before, these condi-tions allowed the fulfillment of the reliability conditions


coated specimens in the as-deposited condition

Ž .Stress MPa

536 574 612 650

159 200 89 400 47 000 26 900185 800 96 700 66 300 34 800192 800 97 000 69 300 41 800197 500 98 300 71 400 46 500210 500 103 400 72 500 49 800259 800 125 000 80 100 51 900

Mean 200 933 101 633 67 767 41 950S.D. 30 565 11 227 10 194 8746


coated specimens in the PHT condition

Ž .Stress MPa

498 536 574 612

138 700 76 100 41 500 20 100143 000 87 300 47 500 27 000156 800 90 100 50 100 27 700169 800 106 100 55 700 28 500188 600 112 300 57 200 31 100217 400 114 900 59 300 31 400

Mean 169 050 97 800 51 883 27 633S.D. 27 296 14 213 6165 3744

prescribed in the ASTM standard E-739. In agreementwith the previous work conducted on similar substratesw x3]5 , this figure shows that plating an EN Nimet’s HPautocatalytic NickelW deposit even of this thicknessonto the substrate steel, significantly decreases thefatigue life of the material in relation to the uncoatedsubstrate, even though if the coating is in the as-de-posited condition, a state in which the maximum com-pressive residual stresses would be expected.

In the as-deposited condition, at elevated alternatingŽ .stress levels 726 MPa the curve obtained for the

plated samples indicates a reduction in fatigue life, incomparison with the uncoated substrate, of approxi-

Ž .mately 63%, whereas at low stresses 612 MPa thesamples present a reduction of approximately 93%.However, for the coated and PHT specimens the situa-tion is even worse since at 726 MPa the fatigue life isreduced by 86%, whereas at 612 MPa it is reduced by97%. As far as the reduction in fatigue limit is concern,for the samples in the as-deposited condition the re-duction was of approximately 12% whereas for thePHT specimens it achieved approximately 23%. Fig. 5illustrates the change in the percentage of reduction infatigue life for the as-deposited and PHT samples, incomparison with the uncoated specimens, as a functionof the alternating stress. As expected, at low stresses

Table 5Experimental results for determining the fatigue limit of the sub-strate material

Sample Alternating NumberŽ .stress MPa of cycles

1 574 5 000 0002 581 5 000 0003 588 5 000 0004 595 484 3005 588 5 000 0006 595 552 2007 588 5 000 0008 595 466 4009 588 5 000 000

10 595 1 086 900


Table 6Experimental results for determining the fatigue limit of the coatedsamples in the as-deposited condition


1 498 5 000 0002 505 5 000 0003 512 5 000 0004 519 263 9005 512 5 000 0006 519 483 5007 512 5 000 0008 519 5 000 0009 526 520 300

10 519 5 000 000

the reduction in fatigue life for both conditions is muchmore significant, with the trend to decrease as thestress increases. However, the rate of decrease is higherfor the samples in the as-deposited condition than forthe PHT specimens. These results, are in agreement

w xwith those reported by Garces et al. 5 who found´somewhat smaller reductions in fatigue life by coatingthe same substrate with a different EN deposit with athickness of 24 mm. These researchers determined thatin the as-deposited condition, at alternating stresses ofthe order of 663 MPa, the plated samples underwent areduction in fatigue life of approximately 49.4%,whereas at stresses of the order of 590 MPa, thereduction was approximately 77.7%. Also, for the coatedand PHT specimens the situation was even worse sinceat 663 MPa the fatigue life was reduced by 74.8%,whereas at 590 MPa it was reduced by 91.7%. Thepresent results also corroborate those reported by Wu

w x w xand co-workers 3 and also by Zhang et al. 4 regard-ing the decrease in the fatigue limit of the 30CrMosteel when plated with EN deposits and PHT at differ-ent temperatures for different periods. According tothese authors, a PHT for 1 h at 673 K gives rise to adecrease of 52% in the fatigue limit of the material,

Table 7Experimental results for determining the fatigue limit of the coatedsamples in the PHT condition


1 460 5 000 0002 467 222 3003 460 494 1004 453 624 3005 446 5 000 0006 453 5 000 0007 460 5 000 0008 467 298 7009 460 456 100

10 453 5 000 000

Ž .Fig. 4. Mean number of cycles prior to fracture N as function offŽ .the alternating stress applied to the material S for the uncoated,

coated as-deposited and coated and PHT specimens.

which initially was reported to be of the order of 750MPa.

The linear relationship between the alternating stressand the number of cycles to failure in a double lo-garithmic scale indicates the validity of the simpleparametric expression of the form earlier proposed by

w xBasquin 8 for the description of this type of data:

SsAN mf

where A and m represent constants that depend onboth material properties and testing conditions.

Fig. 5. Change in the percentage of reduction in fatigue life for theas-deposited and PHT samples, with the alternating stress.


A represents the fatigue strength coefficient of thematerial and m the fatigue exponent. Table 8 summa-rizes the values of the parameters A and m for thethree set of data represented in Fig. 4. The appropriatedetermination of such parameters, particularly for thecomposite coating]substrate material, is of upmost im-portance both for the evaluation of the fatigue perfor-mance and for design purposes under considerations ofhigh cycle fatigue of any component made of this steelthat could be EN coated with Nimet’s HP autocatalyticNickelW either for improving some of its properties,such as corrosion and wear resistance, or achieving therequired dimensions in order for the part to fulfillproperly its role in service.

3.3. E¨aluation of the fracture surfaces of the samples

A number of specimens tested under different alter-nating stress conditions were closely examined after

Table 8Parameters involved in the Basquin relationship for the conditionstested

Ž .Condition A MPa m

Substrate 1605.2 0.075As-deposited 2161.1 0.114Deposited and PHT 1847.2 0.108

failure by SEM in order to study, in more detail, themicrostructural characteristics of the crack initiationsites, as well as the microstructural changes that takeplace both in the coating and substrate during thesubsequent propagation of such cracks, leading eventu-ally to the final fracture of the samples. For example,Fig. 6a illustrates a typical photomicrograph of thegeneral view of the fracture surface of a sample in theas-deposited condition, tested at 650 MPa. As it can be

Ž .Fig. 6. a General view of the fracture surface of a sample in the as-deposited condition, tested at 650 MPa. Such a surface reveals the presenceŽ .of a number of fracture steps FST , indicating that fracture has occurred as a consequence of the propagation of several cracks initiated from the

Ž . Ž .surface of the specimen. The arrow indicates the origin O of the dominant crack. b Detailed view of the initiation site of the dominant crackŽ . Ž . Ž .pointed out in a . Secondary cracks SC that run parallel to the substrate]deposit interface are clearly visible. Fatigue striations FS in some

areas of the substrate near the interface are also observed.


Ž .Fig. 7. a General fracture surface of a specimen also in the as-de-posited condition, tested at 536 MPa. The crack initiation site,identified with the arrow, is clearly defined by a number of radial

Ž .lines that propagate from such a point. b Magnified view of theŽ .crack initiation site. The coating has been identified as D and the

Ž .substrate as S . The integrity of the substrate]deposit interface afterfracture can be observed.

appreciated, the fracture surface is not flat, revealingthe presence of a number of fracture steps, whichindicates that fracture has occurred as a consequenceof the propagation of several cracks that initiated fromthe surface of the specimen. The arrow on the pho-tomicrograph indicates the site initiation of the maincrack. On the other hand, Fig. 6b represents a compos-ite photomicrograph of the area where the main crackstarted.

Some small secondary cracks that run parallel to thesubstrate]deposit interface are also clearly visible, aswell as fatigue striations in some areas of the substratenear the interface. In general, after fracture, despitethe elevated alternating stress applied to the samples,the deposit is observed to remain well adhered to thesubstrate. Fig. 7a shows the fracture surface of a speci-

men also in the as-deposited condition, tested at 536MPa. In this case, the crack origin is clearly defined asthe site where the radial lines that propagate along thefracture surface converge. Such microstructural detailsare depicted more clearly in Fig. 7b where the integrityof both the deposit and the substrate]deposit interfaceafter fracture can be observed. Fig. 8a,b represents amore detailed description of Fig. 7b where it can beclearly observed that the fatigue cracks initiated withinthe EN deposit and propagated towards the substrate.This belief is supported by the continuity of the frac-ture features between the deposit and substrate whichis also seen in both pictures.

In relation to the samples in the PHT condition, Fig.9a illustrates a typical fracture surface when the speci-mens are tested at 498 MPa. In this particular case, itwas possible to identify the initiation sites of two domi-nant cracks which during propagation merged into asingle one, giving rise to the presence of a step on thefracture surface. Fig. 9b represents a detailed view ofthe initiation site identified as A in Fig. 9a where, asbefore, it can be observed that the crack was nucleated

Ž .Fig. 8. a Detailed description of area A in Fig. 7b. Fatigue cracksŽ .FC have nucleated within the EN deposit and propagate towards

Ž .the substrate. b Detailed description of area B in Fig. 7b. TheŽ . Ž .coating has been identified as D and the substrate as S . Some

continuity of the fracture features between the deposit and substrateare observed. The integrity of the substrate]deposit interface is seento be preserved after fracture.


within the EN deposit and that the integrity of thesubstrate]deposit interface has been preserved. Fi-nally, Fig. 10a shows the general fracture surface of asample, also in the PHT condition, tested at 612 MPawhere several steps were formed as a consequence ofthe simultaneous propagation of a number of cracksnucleated at the surface of the specimen. The detailedanalysis of the site identified as A, shown in Fig. 10b,revealed again that such cracks were nucleated withinthe EN deposit and that their propagation towards thesubstrate is enhanced by the preservation of the bond-ing at the interface.

All these observations lead to the conclusion thateven though the deposit plated in this investigation, inthe as-deposited condition, would be under compres-sive residual stresses due to its elevated P content, ithas a lower fatigue strength than the substrate, whichleads to the nucleation of fatigue cracks within it, priorto the nucleation of fatigue cracks at the substrate]de-posit interface. Once such cracks have been nucleated,

Ž .Fig. 9. a Typical fracture surface of a sample in the PHT conditiontested at an alternating stress of 498 MPa. The arrows point out theinitiation sites of two dominant cracks. Propagation of such cracks

Ž .has given rise to the presence of two steps FST visible on theŽ .fracture surface. b Detailed view of the site initiation identified as

Ž . Ž .A in a . The main crack was nucleated within the EN deposit D .

Ž .Fig. 10. a General fracture surface of a sample, also in the PHTŽ .condition, tested at 612 MPa. Several steps FST are observed as a

Ž .consequence of the simultaneous propagation of different cracks. bŽ . Ž .Detailed analysis of site A shown in a . Fatigue cracks FC have

Ž .been nucleated within the deposit D and their propagation towardsŽ .the substrate S is enhanced by the preservation of the bonding at

the interface.

due to the preservation of the bonding at the interface,they propagate towards the substrate giving rise to asignificant reduction in the fatigue properties of thesteel. Thus, in agreement with the findings reported by

w x WGarces et al. 5 , the Nimet’s HP autocatalytic Nickel´deposit employed in the present investigation, acts as asource of fatigue cracks whose propagation is enhancedby the apparently good bonding that exists between thesubstrate and the deposit. The largest reduction infatigue properties of the PHT samples in comparisonwith the as-deposited specimens, would be consistentwith a decrease in the compressive residual stresseswithin the deposit, as it has been reported by Zhang et

w xal. 3 .In summary, plating a high fatigue strength substrate

with a deposit of lower fatigue properties could lead toa reduction in the fatigue properties of the compositematerial, particularly if a good bonding exists at the


substrate]deposit interface which preserves its in-tegrity. On the contrary, the fatigue properties of thecomposite material could be improved if the depositperformed better than the substrate under these condi-tions.

4. Conclusions

The fatigue properties of a SAE 4340 steel could beseverely decreased by plating it with an EN Nimet’s HPAutocatalytic NickelW deposit, even of 10 mm thick-ness. In the as-deposited condition, the decrease infatigue life could reach between 63]93%, dependingupon the alternating stress applied to the material,whereas the decrease in fatigue limit could be of 12%.In the PHT condition, the reduction in fatigue lifecould achieve 86]97% and that of fatigue limit couldbe of 23%. In agreement with previous findings, it isbelieved that such a reduction in fatigue propertiestakes place due to the early nucleation of fatiguecracks within the deposit which are subsequently prop-agated towards the substrate. Crack propagation isenhanced by the integrity of the substrate]deposit in-terface which is preserved by the good bonding betweenthe two materials. Therefore, the use of EN platings,such as the one investigated in the present work, toimprove the corrosion and wear properties of highstrength steels, could lead to a severe reduction in thefatigue properties of the composite material since thelow strength deposit would act as a surface notch,decreasing the time required for the nucleation of thefatigue cracks. The reduction in fatigue life has beenfound to be less marked for the samples in the as-de-posited condition than those PHT. Also, the decreasein fatigue limit for both conditions seems to be lesssevere than that in fatigue life. The mechanical design

of structural components and parts made of this steeland coated with EN deposits of this kind, in order toavoid a potential fatigue failure under high cycle fa-tigue conditions, should take into account the fatiguecurves derived for the plated materials in the as-de-posited and PHT conditions rather than that obtainedfor the unplated steel. However, such curves are ex-pected to vary with the particular type of EN deposit,coating thickness and PHT applied after deposition.

Acknowledgements

This investigation has been conducted with the fi-nancial support of the Venezuelan National Council

Ž .for Scientific and Technological Research CONICITthrough the project LAB-97000644 and the Scientificand Humanistic Development Council of the Central

Ž .University of Venezuela CDCH-UCV through theproject 08-17-4595-2000. J.A. Berrıos is deeply grateful´to the School of Mechanical Engineering, Faculty ofEngineering and Architecture of the University of ElSalvador.

References

w x1 W. Riedel, Electroless Nickel Plating, ASM International, Met-als Park, Ohio, USA, 1991, pp. 48]181.

w x Ž . Ž .2 J. Abbott, Adv. Mater. Proc. 150 6 1996 21]23.w x Ž .3 Y.Y. Wu, Y.Z. Zhang, M. Yao, Plating Surf. Finish. 1995

83]85.w x Ž .4 Y.Z. Zhang, Y.Y. Wu, M. Yao, J. Matter. Sci. Lett. 15 1996

1364]1366.w x5 Y. Garces, H. Sanchez, J. Berrıos, A. Pertuz, J. Chitty, H.´ ´ ´

Ž .Hintermann, E.S. Puchi, Thin Solid Films 355r356 1999487]493.

w x Ž .6 K. Parker, H. Shah, Plating 3 1971 230.w x Ž .7 H. Kreye, H. Muller, T. Petzel, Galvanotechnik 77 1986 561.¨w x Ž . Ž .8 O.H. Basquin, Proc. ASTM 10 2 1910 625.

fatigue properties of a sae 4340 steel coated with a nimet

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