Fatigue behavior of a quenched and tempered AISI 4340 steel coated withan electroless Ni-P deposit

Y. GarceÂsa, H. SaÂncheza, J. BerrõÂosb, A. Pertuza, J. Chittyc, H. Hintermannd, E.S. Puchib,*

aSchool of Mechanical Engineering, Faculty of Engineering, Central University of Venezuela, Apartado Postal 47885, Los Chaguaramos,

Caracas 1045, VenezuelabSchool of Metallurgical Engineering and Materials Science, Faculty of Engineering, Central University of Venezuela, Apartado Postal 47885, Los

Chaguaramos, Caracas 1045, VenezuelacDepartment of Applied Mathematics, Faculty of Engineering, Central University of Venezuela, Apartado Postal 47885, Los Chaguaramos,

Caracas 1045, VenezueladFaculty of Sciences, University of NeuchaÃtel, NeuchaÃtel, Switzerland

Abstract

The fatigue life of a quenched and tempered AISI 4340 steel has been evaluated in three different conditions: (a) uncoated, (b) coated with

an electroless Ni-P (EN) deposit of a P content of approximately 12±14wt.%, as-deposited and (c) as-deposited, followed by a two-step post-

heat treatment (PHT): 473 K for 1 h plus 673 K for 1 h. The results indicate that plating the base steel with this kind of deposit leads to a

signi®cant reduction of the fatigue life of the material, particularly if the deposit is subjected to a subsequent PHT. Such a reduction has been

quanti®ed by determining the Basquin parameters from the fatigue life curves obtained for the uncoated, coated, coated and PHT substrates.

It has been shown that the fatigue life of the base steel can be reduced by 78% in the as-deposited condition and a 92% after a subsequent

PHT. The microscopic observation of the fracture surfaces of the samples indicate that the fatigue process is initiated at the surface of the

deposit and, subsequently, transferred to the substrate, with the assistance of the metallic bonding established at the deposit-substrate

interface. This belief is supported by the observation of some continuity of the fracture features between the coating and the substrate

under low alternating stresses. In the present study, the bonding between the EN deposit and the base steel was observed to be rather poor.

Extensive secondary cracking along the coating-substrate interface after fatigue testing as well as the complete separation of the deposit from

the substrate during tensile testing support this view. Such a behavior is believed to be related to the signi®cant difference that exists between

the elastic and plastic properties of the EN deposit and the base steel. Nonetheless, the slight degree of metallic bonding that remains after the

®rst stage of fatigue testing seems to be enough to allow the passage of the fatigue cracks, prior nucleated in the deposit, into the substrate. It

is therefore concluded that, in the present case, the EN deposit acts as a surface crack source or surface notch which decreases the fatigue life

of the coated material by reducing the crack nucleation stage. q 1999 Published by Elsevier Science Ltd. All rights reserved.

Keywords: Fatigue behavior; Plain carbon steels; Electroless Ni-P deposits; Fatigue cracks

1. Introduction

The fatigue and corrosion-fatigue behavior induced by

EN deposits on plain carbon steels of different carbon

content have been extensively investigated in previous

studies [1±7]. For example, Riedel [1] has reported an

increase in the fatigue life of two St 52 and AISI 1055 steels

when these have been coated with an EN deposit of 12% P

and subsequently post-heat treated (PHT). Izumi et al. [2]

have also reported an increase of about 20% in the fatigue

life of medium strength steels (UTS of approximately 440±

750 MPa) when the EN coating is in the as-deposited condi-

tion. However, after a PHT these authors have reported a

decrease which has been attributed to the precipitation of

Ni3P particles.

Puchi et al. [3] have also reported an increase in fatigue

life of both AISI 1010 and 1045 steels which is more

marked as the mechanical strength of the substrate material

decreases. Therefore, those steels with tensile strengths of

the order of 250±440 MPa could experience an increase in

fatigue life when coated with EN deposits, depending upon

the coating thickness and the predominant residual stresses

in the deposit itself. BerrõÂos et al. [5] have investigated the

effect of the coating thickness of an EN deposit on the

fatigue behavior of an annealed AISI 1045 steel. In this

study, 7±37 mm-thick EN coatings were deposited and

PHT at two different temperatures.

The only coatings that behaved similarly to the uncoated

Thin Solid Films 355±356 (1999) 487±493

0040-6090/99/$ - see front matter q 1999 Published by Elsevier Science Ltd. All rights reserved.

PII: S0040-6090(99)00673-2

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* Corresponding author. Tel.: 158-2-662-8927; fax: 158-2-662-8927.

E-mail address: epuchi@reaciun.ve (E.S. Puchi)

substrate was the 7 mm deposit PHT at 473 K for 1 h,

whereas all the other samples displayed a reduction in fati-

gue life. Particularly, for thicknesses ranging between 17±

37 mm it has been suggested that tensile residual stresses in

the coatings contribute to the observed fatigue behavior and

that such residual stresses could be associated with a rela-

tively low P content, crystallization of amorphous Ni and

precipitation of Ni3P particles. According to Riedel [1], the

P content of the deposit depends of the pH of the solution

employed as source of Ni ions. More speci®cally, it depends

on the molar ratio Ni21:(H2PO2)2, since as the concentration

of hypophosphite increases, the pH decreases and the P

increases. For a pH less than 5, the EN deposit could have

a P content of 10% or higher.

The studies of Parker and Shah [8] indicate that if the P

content of the deposit is greater than approximately 11±

12%, the residual stresses would be of a compressive nature.

In relation to high strength steels employed as substrate, Wu

and coworkers [9] have conducted an investigation on the

fatigue resistance of a 30CrMo steel (0.30 C, 1.09 Cr and

0.24 Mo) oil quenched from 1143 K and tempered at 893 K

for 3 h. The source of Ni ions was NiSO4 with a pH of 4.5.

The deposit was PHT a 473 K for 1.5 h. These authors found

a reduction in the fatigue limit of approximately 39% for the

plated substrate and a reduction of 20% when the substrate

was previously shot peened before plating.

It was also reported that the fatigue cracks initiated at the

interface between the coating and the substrate, and that in

the deposit some of the cracks were parallel to the stress

axis. The low fatigue strength of the coating was found to be

responsible for the decrease in the fatigue limit of the plated

steel. Zhang et al. [10] have also carried out three-point

bending fatigue tests on a 30CrMo steel coated with an

EN deposit of 43 mm thickness and 9.5 wt.% P. In this

investigation some of the samples were shot peened before

plating and some of the deposited specimens were PHT at

200, 400 and 6008C.

The residual stresses in the coatings were determined by

means of the bent strip method and it was observed that for

all the conditions investigated such stresses remained

compressive after annealing, but decreased with increasing

annealing temperature. Also, shot peening before plating

was observed to increase the compressive residual stress

within the coatings. Regarding the in¯uence of EN deposits

on the fatigue limit of the material, it was determined that

such coatings reduced this property in comparison with the

unplated substrate.

The decrease in fatigue strength was observed to be less

marked for the shot peened specimens but became signi®-

cantly higher as the PHT temperature increased. In relation

to the fractographic analysis of the plated samples, it was

reported that without the application of shot peening, the

fatigue cracks initiated at the surface of the specimens, lead-

ing to the fatigue failure of the coating. On the contrary,

when the samples were shot peened previously to the coat-

ing deposition, the crack initiation sites were displaced to

the coating±substrate interface. The work conducted by

Zhang et al. [10] allowed the conclusion that the fatigue

properties of this material, when it is coated with EN depos-

its, depends primarily on the fatigue resistance of the coat-

ing itself.

Thus, the present investigation has been conducted in

order to study the fatigue behavior, above the fatigue

limit, of an AISI 4340 steel which has been oil quenched

and tempered prior to plating at industrial scale with an EN

deposit of 24 mm in thickness and a P content ranging

between 12 and 14%.

2. Experimental techniques

The present investigation has been carried out with

samples of an AISI 4340 steel with the following composi-

tion (%wt): 0.34 C, 0.50 Mn, 0.30 Si, 1.5 Cr, 0.20 Mo and

1.50 Ni. This alloy is widely employed in the manufacture

of automotive crankshafts and rear axle shafts, aircraft

crankshafts, connecting rods, propeller hubs, gears, drive

shafts, landing gear parts and heavy duty parts of rock drills.

The material was provided as bars of approximately 16 mm

diameter and 6 m length. Such bars were cut to pieces of

approximately 120 mm length for machining tensile speci-

mens and of 90 mm length for machining the fatigue

samples. Both type of specimens had a gage diameter of

6.35 mm, gage length of 12.7 mm, ®llet radius of 25.4

mm and shoulder diameter of 12.7 mm, according to the

ASTM standard E 606.

The alloy was already provided in the quenched and

tempered condition. The specimens were subsequently

ground with successive SiC papers grit 600±1200 and

polished mechanically. Fifty-six of these samples were

degreased in a 5% HCl solution at 348±353 K for 7 min,

rinsed again in distilled water, rinsed in a sodium bicarbo-

nate (100 g per liter) solution and rinsed in water. The

deposition was conducted industrially employing a bath

composed of 30 g/l nickel sulphate, 30 g/l sodium hypopho-

sphite, 35 g/l malic acid, 1.5 ppm lead sulphate, 10 g/l

succinic acid and a stabilizer.

During deposition the pH was maintained at approxi-

mately 5, at a mean temperature of about 358 K. The deposi-

tion rate was of approximately 12 mm/h and the process was

conducted for 2 h which allowed a thickness of about 24 mm

to be achieved. Such thicknesses were corroborated by

means of the ball cratering technique (Calotest, CSEM)

and image analysis (LECO 500). Twenty-four of the depos-

ited samples were PHT in an argon atmosphere following a

two-step process that involved an initial treatment at 473 K

for 1 h and a subsequent heating at 673 K also for 1 h. The

chemical analysis of such deposits was determined by

means of SEM techniques (Hitachi S-2400) with EDS facil-

ities.

The observations were conducted at a constant potential

of 20 kV. Tensile tests were carried out on a computer-

Y. GarceÂs et al. / Thin Solid Films 355±356 (1999) 487±493488

controlled servohydraulic machine (Instron 8502) at a cross

head speed of 10 mm/min. At least three samples were

employed for characterizing the monotonic mechanical

properties of both the coated and uncoated substrate. Fati-

gue tests were carried out under rotating bending conditions

(Fatigue Dynamics, RBF-200) at a frequency of 50 Hz and

alternating stresses of 590, 611, 634 and 663 MPa, for the

uncoated substrate and the specimens in the as-deposited

condition, which corresponds to 80, 83, 86 and 90% of the

yield stress of the unplated substrate.

For the coated and PHT samples the tests were conducted

at 442, 516, 590 and 663 MPa, corresponding to 60, 70, 80

and 90% of the base steel. A total of 40 samples were

employed for evaluating the fatigue properties of the

uncoated substrate, 28 for the coated material and 28 for

the coated and PHT samples, which exceeds the minimum

number of specimens required in S-N testing for reliability

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%. In order to make

possible a meaningful comparison of the fatigue life of the

coated and uncoated specimens, all the samples were

mechanically prepared in order to have similar polished

surfaces before testing.

The fracture surfaces of the samples were examined by

means of SEM techniques, particularly in relation to the

initiation of fatigue cracks and the different stages of their

propagation.

3. Results and discussion

3.1. Characteristics of the deposit

Fig. 1 illustrates the typical microstructure of the

substrate evaluated on the scanning electron microscope.

The presence of a large number of relatively coarse marten-

site plates together with carbides, visible as small particles

which constitute a typical tempered martensite structure,

can be observed. On the other hand, Fig. 2 shows a view

of the interface between the EN coating and substrate prior

to fatigue testing, illustrating the deposition of an apparently

uniform coating with satisfactory adhesion characteristics.

The evaluation of the coated material during tensile testing

and the observation of the fracture surfaces of the specimens

after fatigue testing however, would indicate otherwise.

As it has already been mentioned, the coating thickness

was corroborated by means of the ball cratering technique,

optical microscopy and scanning electron microscopy,

which allowed to determine a mean value of approximately

24 mm. As shown in Fig. 3, the EDS analyses conducted on

the deposit allowed to determine that the P content ranged

between 12 and 14wt.%. As has already been pointed out,

Parker and Shah [8] conducted an investigation concerning

the effect of the P content of an EN deposit on the residual

stresses within the coating. Accordingly, for a P content of

the order of that present in the samples under examination,

the residual stresses in the coatings are expected to be of a

compressive nature.

This fact would be in agreement with the ®ndings of Wu

et al. [9] and Zhang et al. [10], who also reported compres-

sive residual stresses of the order of 80 MPa in the coatings

deposited and subsequently PHT at 473 K for 1 h, even

though the P content of such deposits was lower

(9.5wt.%) than that contained in the coated specimens

employed in the present study, which also were in the as-

deposited condition.

3.2. Evaluation of mechanical properties

In order to evaluate if this particular deposit had any

in¯uence on the monotonic mechanical properties of the

composite coating-substrate material, a number of tensile

Y. GarceÂs et al. / Thin Solid Films 355±356 (1999) 487±493 489

Fig. 1. SEM photomicrograph illustrating a typical microstructure of the

substrate. A large number of relatively coarse martensite plates (M) and

carbide particles, visible as small white particles, can be observed.

Fig. 2. SEM view of the interface between the EN coating (D) and substrate

(S) previous to fatigue testing. The deposit seems to be uniform and has

apparently satisfactory adhesion characteristics due to the absence of visi-

ble cracks along the interface.

tests were conducted with samples in the uncoated and

coated conditions. The deposits plated onto the substrate

employed in the present study did not show any signi®cant

change either in yield stress or in the UTS of both the coated

and uncoated base steel. The mean yield stress was found to

be approximately 737 MPa, whereas the UTS was found to

be approximately 1287 MPa.

During testing of the coated samples, the deposits were

observed to tear off signi®cantly from the substrate, indicat-

ing a poor adhesion of such coatings to the steel, which

could worsen as a result of the difference in mechanical

properties (elastic and plastic) between coating and

substrate. The fact that the monotonic mechanical properties

are observed to remain virtually unchanged after the appli-

cation of the EN coating is not surprising since even if the

plated deposits contributed somewhat to the tensile mechan-

ical properties of the composite material, in the present case

the thickness of such deposits is so small that its effect

would be negligible.

In relation to the fatigue tests conducted in order to eval-

uate the fatigue life of both the coated and uncoated

samples, the determination of the monotonic mechanical

properties of the material allowed to establish a stress ampli-

tude range of 590±663 MPa for the substrate and coated as-

deposited samples, which corresponded to a fraction of the

yield stress of approximately 0.80±0.90. The coated and

PHT specimens were tested in the stress range of 442±663

MPa, that is to say, 0.60±0.90 of the yield stress. The data

showing the mean number of cycles prior to fracture (Nf) in

terms of the alternating stress applied to the material (S) for

the uncoated, coated as-deposited and coated and PHT

specimens, are presented in Table 1.

The results obtained have been plotted in Fig. 4 in which

it can be observed that at each alternating stress level for

both the coated as-deposited and coated and PHT materials,

at least ®ve tests were carried out, whereas the fatigue prop-

erties of the uncoated substrate were evaluated employing at

least eight samples at each stress. As mentioned before,

these conditions allowed the ful®llment of the reliability

conditions prescribed in the ASTM standard E 739. The

most important aspect highlighted in Fig. 4 is the fact that

plating an EN deposit of these characteristics onto the

substrate steel signi®cantly decreases the fatigue life of

the material in relation to the uncoated substrate, even if

the coating is in the as-deposited condition, a state in

which the maximum compressive stresses would be

expected.

In the as-deposited condition, at elevated alternating

stress levels (663 MPa) the curve obtained for the plated

samples indicates a reduction in fatigue life, in comparison

to the uncoated substrate, of approximately 49.4%, whereas

at low stresses (590 MPa) the samples present a reduction of

approximately 77.7%. However, for the coated and PHT

specimens the situation is even worse since at 663 MPa

the fatigue life is reduced by 74.8%, whereas at 590 MPa

it is reduced by 91.7%. These results, in a sense, corroborate

those obtained by Wu and co-workers [9] and also by Zhang

et al. [10] regarding the decrease in the fatigue limit of the

30CrMo steel when plated with EN deposits and PHT at

different temperatures for different periods.

According to these authors, a PHT for 1 h at 673 K gives

Y. GarceÂs et al. / Thin Solid Films 355±356 (1999) 487±493490

Fig. 3. Typical EDS spectrum for the EN deposits involved in the present

work.

Table 1

Mean number of cycles to failure (Nf) vs. stress amplitude (S) for the

uncoated and coated specimens

S (Mpa) Substrate As-deposited Deposited and PHT

442 39520 ^ 8698

516 33780 ^ 8611

590 278538 ^ 144864 62160 ^ 10999 22980 ^ 5151

611 169913 ^ 36699 49900 ^ 6321

634 95100 ^ 33011 40860 ^ 14830

663 65125 ^ 6752 32960 ^ 3128 16400 ^ 3883

Fig. 4. Mean number of cycles prior to fracture (Nf) as function of the

alternating stress applied to the material (S) for the uncoated, coated as-

deposited and coated and PHT specimens.

rise to a decrease of 52% in the fatigue limit of the material,

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

The linear relationship between the alternating stress and

the number of cycles to failure in a double logarithmic scale

indicates the validity of the simple parametric expression

earlier proposed by Basquin [11] for the description of this

type of data, of the form

S � AN2mf �1�

where A and m represent constants that depend on both

material properties and testing conditions. A represents the

fatigue strength coef®cient of the material and m the fatigue

exponent. Table 2 summarizes the values of the parameters

A and m for the three set of data represented in Fig. 4. The

appropriate determination of the Basquin parameters, parti-

cularly for the composite coating-substrate material, is

important for the evaluation of the fatigue performance of

any component made of this steel that could be EN-coated

either for improving some of its properties, such as corro-

sion and wear resistance, or achieving the required dimen-

sions in order for the part to ful®ll properly its role in

service.

3.3. Evaluation of the fracture surfaces of the samples

The specimens tested at 590 and 660 MPa were examined

after failure by SEM in order to study more closely the sites

of crack initiation and their microstructural features, as well

as the microstructural changes that take place in general

during the subsequent propagation of such cracks, leading

eventually to the ®nal fracture of the samples. As an exam-

ple, Fig. 5a,b shows two photomicrographs of typical crack

initiation sites for samples tested under these conditions,

respectively. In both cases, localized areas where the cracks

nucleated, can be clearly observed. These constitute the

focal points of a number of radial lines that propagate

along the fracture surfaces.

In Fig. 5a (sample tested at 590 MPa), it would seem that

the crack nucleation site was associated with an isolated

surface defect of a blister type and that the deposit-substrate

interface was not disturbed to a signi®cant extent. For the

sample tested at 663 MPa (Fig. 5b), severe secondary crack-

ing along the deposit±substrate interface can be observed,

possibly as a result of both poor bonding between them and

the application of higher tensile stresses during each loading

cycle. The severity of such cracking leads to the complete

separation of the coating from the substrate in some areas of

the cross section of the sample.

Fig. 6a,b corresponds to a magni®ed view of the coating±

substrate interface of the specimen shown in Fig. 5a, parti-

cularly at the site where the main crack started to propagate

towards the substrate. The deposit is observed to remain

relatively sound, although some secondary cracking can

be seen specially at some areas of the interface. Also, it is

possible to observe clearly some continuity of the fracture

marks between the deposit and substrate, despite the rela-

tively poor adhesion between them. This last feature indi-

cates that the fatigue failure probably started at the surface

of the deposit and propagated towards the base steel.

Fig. 7, on the other hand, illustrates a magni®ed picture of

the crack nucleation site of Fig. 5b. It clearly shows the

severe cracking of the deposit and its separation from the

substrate on both sides of the point where cracking of the

substrate is believed to have started. Here, the metallic

Y. GarceÂs et al. / Thin Solid Films 355±356 (1999) 487±493 491

Table 2

Parameters involved in the Basquin relationship for the conditions tested

Condition A (MPa) m

Substrate 1555.6 2 0.077

As-deposited 4479.7 2 0.184

Deposited and PHT 44354.1 2 0.431

Fig. 5. Typical crack initiation sites for samples tested at (a) 590 and (b) 663 MPa, respectively. At high alternating stresses (b), the deposit (D) has been

severely detached from the substrate (S). The radial markings on both pictures indicate the origin of the fatigue failure.

bonding between deposit and substrate is apparently main-

tained to some degree, which again indicates that the possi-

ble failure mechanism should be the passage of fatigue

cracks from the coating to the substrate. Therefore, cracks

would be formed at the surface of the deposit, propagate

through it leading to its fracture in some localized areas and

®nally transferred to the substrate in a region in which the

metallic bonding between the coating and the substrate is

still preserved.

Thus, even though the deposit would be under compres-

sive residual stresses due to its P content, its lower mechan-

ical strength in comparison with the substrate material leads

to the its prior failure with the consequent transfer of the

propagating cracks to the substrate. Hence, the coating actu-

ally operates as a fatigue crack source or surface stress

concentration which becomes very effective even if the

adherence of the deposit to the substrate is rather poor, as

in the present case. This mechanism would explain the

Y. GarceÂs et al. / Thin Solid Films 355±356 (1999) 487±493492

Fig. 6. (a) Magni®ed view of the coating-substrate interface of the specimen shown in Fig. 5a. (b) Magni®ed view of the coating-substrate interface of the

specimen shown in (a). The coating has been identi®ed as (D) and the substrate as (S). Continuity of the fracture features between the coating and substrate can

be clearly observed, as well as some secondary cracking along the interface.

Fig. 7. Magni®ed picture of the crack nucleation site of Fig. 5b. Severe cracking (C) of the deposit (D) and its separation from the substrate (S) can be clearly

observed. The main crack has propagated from the deposit into the substrate.

reduction in fatigue life observed for the coated specimens

in comparison with the uncoated samples tested at the same

alternating stresses.

These features are consistent with the previous ®ndings of

Zhang et al. [10] and Pertuz and co-workers [7], who were

able to observe fatigue marks in EN deposits plated onto

different steels as substrates. Such observations support the

view that if the mechanical strength of the substrate is

greater than that of the EN deposit or similar to it, the coat-

ing is bound to undergo fatigue failure before the substrate

and to transfer the fatigue cracks to it, giving rise to a reduc-

tion in its fatigue strength. If, however the fatigue strength

of the deposit is higher than that of the substrate, it is possi-

ble to observe an improvement in its fatigue performance, as

it has been reported by Puchi and co-workers [3] for an EN

deposit on AISI 1010 steel.

4. Conclusions

Plating of a quenched and tempered AISI 4340 with EN

deposits leads to a signi®cant reduction in the fatigue life of

the material. Such a reduction, at a stress amplitude of 590

MPa, can achieve up to approximately 78% if the coating is

in the as-deposited condition and 92% after a PHT such as

the one explored in the present work. Thus, according to the

present results, the reduction in fatigue life for this material

when coated with this kind of deposits is much more signif-

icant than previously reported. It has been shown that such a

decrease in fatigue performance occurs as a result of the

passage of fatigue cracks form the coating to the substrate,

a process which is believed to be assisted by the metallic

bonding established between them. The continuity of certain

fracture features between coating and substrate observed

from the analysis of some fracture surfaces supports this

view.

In the present case, due to the signi®cant difference that

exists between the elastic and plastic properties of coating

and substrate, the adherence of the deposit to the base steel

is rather poor. This conclusion is supported by the observa-

tion of extensive secondary cracking along the deposit±

substrate interface after fatigue testing and the actual

separation of the coating from the base steel during tensile

testing. Nevertheless, such degree of adherence is found to

be enough to allow the transfer of cracks from the coating to

the substrate.

Thus, it is concluded that due to the relatively lower

fatigue properties of the EN coating in comparison with

the substrate, the deposit actually operates as a source of

surface fatigue cracks, i.e. as a surface notch capable of

imparting a signi®cant reduction to the fatigue properties

of the substrate by reducing the time required for the nuclea-

tion of cracks. Coating of strong and tough substrates with

weaker deposits in order to impart wear and/or corrosion

resistance can thus lead to the fatigue failure of the compo-

nent prior to expected. Therefore, mechanical design under

conditions of high cycle fatigue should be based on the

Basquin parameters of the fatigue life curve of the coated

material rather than that determined for the uncoated

substrate.

Acknowledgements

This investigation has been conducted with the ®nancial

support of the Venezuelan National Council for Scienti®c

and Technological Research (CONICIT) through the project

LAB-97000644. J.A. BerrõÂos is deeply grateful to the Orga-

nization of the American States for the ®nancial support

received through the Multinational Material Project. He is

also grateful to the School of Mechanical Engineering of the

University of El Salvador.

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