Surface and Coatings Technology 133᎐134 Ž2000. 561᎐571
Fatigue properties of a SAE 4340 steel coated with a Nimet HP autocatalytic nickel deposit C. Guzman ´ a , N. Dıaz ´ a , J.A. Berrıos ´ b, A. Pertuz a , E.S. Puchi Cabrerab,U a
School of Mechanical Engineering, Faculty of Engineering, Uni¨ ersidad Central de Venezuela, Apartado Postal 47885, Los Chaguaramos, Caracas 1045, Venezuela b School of Metallurgical Engineering and Materials Science, Faculty of Engineering, Uni¨ ersidad Central de Venezuela, Apartado Postal 47885, Los Chaguaramos, Caracas 1045, Venezuela
Abstract The influence of a commercial electroless Ni᎐P ŽEN. deposit known as Nimet’s HP autocatalytic nickel 䊛 , on the fatigue properties of a quenched and tempered SAE 4340 steel, has been investigated. Such an EN deposit had a thickness of approximately 10 m, a P content of approximately 12 wt.% and it was evaluated in two different conditions: Ža. as-deposited; and Žb. deposited and post-heat treated ŽPHT. 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 the fatigue life and fatigue limit, in comparison with the uncoated material, which is more marked in the PHT condition. The reductions in fatigue life have been quantified in terms of the computed values of the Basquin parameters of the materials tested under different conditions. Thus, it has been shown that the fatigue life of the steel can be reduced up to 93% in the as-deposited condition 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 fracture of the substrate-coating composite initiates in the deposit and that it remains well adhered to the substrate during fatigue testing since interfacial cracks have been very rarely observed. Such adhesion characteristics enhance the transference of the early cracks nucleated towards the substrate steel, a belief that is supported by the analysis of the fracture surfaces of the samples tested at different stress levels. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Electroless nickel; Fatigue; 4340 steel
1. Introduction EN deposits represent an important group of metallic coatings that are employed on a wide range of substrates to provide protection against corrosion and abrasive wear. Such platings have found different applications in a variety of fields like electrical and electronics, oil and gas, machinery, aerospace, motor, chemical, foodstuffs, textile and printing industries. Currently, their use continues to increase due to a number of
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Corresponding author. Tel.: q58-2-6628-927; fax: q58-2-7539017. E-mail address:
[email protected] ŽE.S. Puchi Cabrera..
advantages that these coatings present, in comparison with other plating methods w1x, which include better corrosion and chemical resistance, improved ductility, and a harder deposit, especially after heat treatment, and the ability to achieve extremely uniform thickness without costly post-machining or grinding, even on parts with complex configurations w2x. However, protection against corrosion and wear can be gained at the expense of a decrease in other important properties such as fatigue life and fatigue limit, which could be of utmost importance in some of the applications pointed out above. Regarding previous studies carried out on the effect of EN deposits on the fatigue properties of high strength steels Žultimate tensile strength of the order of
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 9 8 - 7
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1200 MPa., Wu and coworkers w3x conducted an investigation on the fatigue resistance of a 30CrMo steel Ž0.30 C, 1.09 Cr and 0.24 Mo. oil quenched from 1143 K and tempered at 893 K for 3 h. In this investigation, the source of Ni ions was NiSO4 with a pH of 4.5 and the deposit was PHT a 473 K for 1.5 h. The authors reported 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. Also, it was indicated 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. On the other hand, Zhang et al. w4x also carried out three-point bending fatigue tests on a 30CrMo steel coated with an EN deposit of 43 m thickness and 9.5 wt.% P. In this study some of the samples were shot peened before plating and some of the deposited specimens were PHT at 200⬚, 400⬚ and 600⬚C. The residual stresses in the coatings were determined by means of the bent strip method. For all the conditions investigated it was observed that 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 influence 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 significantly 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, leading to the fatigue failure of the coating. On the contrary, when the samples were shot peened prior to the coating deposition, the crack initiation sites were displaced to the coating᎐substrate interface. The work conducted by Zhang et al. w4x concluded that the fatigue properties of this material, when it is coated with EN deposits, depends primarily on the fatigue resistance of the coating itself. More recently, Garces ´ et al. w5x conducted an investigation in order to study the fatigue life of a quenched and tempered AISI 4340 steel in three different conditions: Ža. uncoated; Žb. coated with an EN deposit of a P content of approximately 12᎐14 wt.%, as-deposited; and Žc. as-deposited, followed by a two-step PHT: 473 K for 1 h plus 673 K for 1 h. The results of this work indicated that plating the base steel with this kind of deposit leads to a significant reduction of the fatigue life of the material, particularly if the deposit was subjected to a subsequent PHT. Such a reduction was
quantified by determining the Basquin parameters from the fatigue life curves obtained for the uncoated, coated, and coated and PHT substrate. Accordingly, it was shown that the fatigue life of the base steel could be reduced by 78% in the as-deposited condition and by 92% after a subsequent PHT. The microscopic observation of the fracture surfaces of the samples conducted in this investigation indicated that the fatigue process was initiated at the surface of the deposit and, subsequently, transferred to the substrate, with the assistance of the metallic bonding established at the substrate ᎐deposit interface. Such a belief was supported by the observation of some continuity of the fracture features between the coating and the substrate under low alternating stresses. Garces ´ et al. w5x reported that in their study, the bonding between the EN deposit and the base steel was observed to be rather poor, a fact that was supported not only by the presence of extensive secondary cracking along the coating᎐substrate interface after fatigue testing but also by the observation of the complete separation of the deposit from the substrate during tensile testing. Such a behavior was believed to be related to the significant difference that existed between the elastic and plastic properties of the EN deposit and the base steel. The study also concluded that the slight degree of metallic bonding that remained after the first stage of fatigue testing seemed to be enough to allow the passage of the fatigue cracks, previously nucleated in the deposit, into the substrate, and therefore, that the EN deposit acted as a surface crack source or surface notch which decreased the fatigue life of the coated material by reducing the crack nucleation stage. Thus, the present investigation has been conducted in order to study further both the fatigue life and fatigue limit of a SAE 4340 steel which has been oil quenched and tempered prior to plating at industrial scale with an EN deposit of 10 m in thickness and a P content of approximately 12 wt.%, known commercially as Nimet’s HP autocatalytic Nickel 䊛 , which is believed to have better adhesion properties than the EN coating employed in the previous investigation w5x. However, it is important to mention that although the work conducted by Wu et al. w3x and Zhang et al. w4x showed that shot peening the substrate material prior to coating decreased the loss in fatigue limit, in order to carry out a reliable comparison of the present results with those reported by Garces ´ et al. w5x, the specimens had also to be coated without any prior shot peening.
2. Experimental techniques The present study has been carried out with samples of a SAE 4340 steel with the following composition Žwt.%.: 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
1.73 Ni. The alloy was already provided in the quenched and tempered condition. This material 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 different lengths: nine pieces of approximately 120 mm for machining tensile specimens; 102 pieces of approximately 102 mm for machining the fatigue samples; and seven pieces of approximately 10 mm for characterizing the deposit and to conduct hardness tests. Both the tensile and fatigue specimens had a gage diameter of 6.35 mm and shoulder diameter of 12.7 mm. However, the tensile samples had a gage length of 25.4 mm and a fillet radius of 12.3 mm, according to the ASTM standard A-370. The fatigue specimens had a curved gage length of 38.1 mm along the cord, machined following a continuous radius of 58.73 mm. The specimens were machined in several steps, with a continuous reduction in the depth of cut of the material. The turning operation was conducted on a horizontal turret lath at low speeds in order to minimize the introduction of residual stresses at the surface of the samples. Finally, the specimens were ground with successive SiC papers grit 100᎐1200 in order to eliminate the circumferential notches and scratches, and polished mechanically to a ‘mirror-like’ finish. The surface roughness within the gage length of the samples was maintained below approximately 0.2 m. All the samples were plated at Nimet Industries Inc. ŽSouth Bend, Indiana, USA.. The deposit applied is known commercially as Nimet’s HP Autocatalytic Nickel 䊛 , with a phosphorous content of 12 wt.% and a thickness of approximately 10 m, which was corroborated by means of the ball cratering technique ŽCalotest, CSEM. and image analysis ŽLECO 500.. In order to determine the optimum PHT temperature a number of heat treatments were conducted at temperatures of 673, 723, 773, 823 and 873 K for 1 h in an argon atmosphere. Subsequently, hardness measurements were conducted on the cross section of the shortest cylindrical samples employed for this purpose. A total of 10 microhardness measurements were carried out on each specimen according to the ASTM standards B-578 and E-384. Such measurements were conducted employing a Knoop indenter ŽShimadzu, Japan. with a load of 50 g applied during 10 s. As discussed later, this evaluation determined that the highest hardness of this deposit is achieved at a temperature of 723 K. Such a PHT has a negligible effect on the mechanical properties of the substrate steel which, after quenching, is usually tempered at a much higher temperature. Thus, three tensile and 24 fatigue specimens were PHT at this temperature for 1 h in an
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argon atmosphere, employing a heating rate of 473 Krh and allowing the samples to cool within the furnace after the treatment. The chemical analysis of the plating both in the as-deposited and PHT conditions was determined by means of SEM techniques ŽHitachi S-2400, Japan. with EDS facilities. The observations were conducted at a constant potential of 20 kV. Tensile tests were carried out on a computer-controlled servohydraulic machine ŽInstron 8502, USA. at a cross-head speed of 3 mmrmin. At least three samples were employed for characterizing the monotonic mechanical properties of both the coated and uncoated substrate. Fatigue tests were carried out under rotating bending conditions Ž R s y1., employing a Fatigue Dynamics RBF-200 equipment, at a frequency of 50 Hz Ž3000 revolutions per minute.. All the tests were carried out in air at room temperature Ž23⬚C.. In this type of test, the specimen is subjected to a dead-weight load while bearings permit rotation. At the mid point of the circular test section surface, the material is subjected to a sinusoidal stress amplitude from tension on the top to compression on the bottom with each rotation. The bending moment applied to the specimens was determined as a function of the alternating stress and the diameter of the sample by means of the simple relationship: MB s
d 3 , N mm 32 max
where M B represents the bending moment in N mm, max the maximum alternating stress in MPa and d the specimen diameter in mm. Thus, the uncoated substrate was tested at alternating stresses of 612, 650, 688 and 726 MPa, which corresponded to 59, 63, 66 and 70% of the yield stress of the material. For the samples in the as-deposited conditions, the fatigue tests were conducted at stress levels of 536, 574, 612 and 650 MPa, which corresponded to 52, 55, 59 and 63% of the yield stress of the coated substrate. For the coated and PHT samples the tests were conducted at 498, 536, 574 and 612 MPa, corresponding to 44, 52, 55 and 59% of the yield stress of the base steel in this condition. A total of 24 samples were employed for evaluating the fatigue properties of the uncoated substrate, 24 for the coated material and 24 for the coated and PHT substrate, which fulfills the number of specimens required in S᎐N testing for reliability data according to the ASTM standard 739 Ž12᎐24 samples.. Thus, the testing procedure followed in the present work allowed a replication greater than 80%. It is important to point out that in order to make possible a meaningful comparison of the fatigue life of the uncoated, coated and coated and PHT specimens, all the samples were mechanically prepared in order to
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Table 1 Main parameters and experimental conditions involved in the present work, together with those employed in previous research previously referred to Authors
Substrate
Deposit characteristics
PHT
Wu et al. w3x
Quenched and tempered 30 Cr Mo steel Ž0.30 C, 1.09 Cr and 0.24 Mo.. Samples with and without shot peening before plating.
43 m thick and 9.5 wt.% P
473 K for 1.5 h
Zhang et al. w4x
Quenched and tempered 30 Cr Mo steel Ž0.30 C, 1.09 Cr and 0.24 Mo.. Samples with and without shot peening before plating.
43 m thick and 9.5 wt.% P
473, 673 and 873 K for 1 h
Garces ´ et al. w5x
Quenched and tempered AISI 4340 steel Ž0.34 C, 1.50 Cr, 1.50 Ni and 0.20 Mo.. Samples were not shot peened prior to coating.
24 m thick, 12᎐14 wt% P
Some of the samples were PHT at 473 K for 1 h q 673 K for 1 h
Present work
Quenched and tempered AISI 4340 steel Ž0.41 C, 0.79 Cr, 1.73 Ni and 0.23 Mo.. Samples were not shot peened prior to coating.
10 m thick, 12 wt.% P
Some of the samples were PHT at 723 K for 1 h
have similar mirror-like polished surfaces before testing. The fatigue limit of the coated and uncoated specimens was determined by means of the staircase method employing a step of 7 MPa and 10 samples for each condition. According to the ASTM standard E-468, infinite life was specified at a number of 5 = 10 6 cycles. The fracture surfaces of the samples were closely examined by means of SEM techniques, particularly in relation to the site initiation of fatigue cracks and the different stages of their subsequent propagation. Table 1 presents a summary of main parameters and testing conditions involved in the present work, together with 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-
Fig. 1. SEM view of the interface between the EN coating ŽD. and substrate ŽS. previous to fatigue testing. The deposit seems to be uniform and has apparently satisfactory adhesion characteristics due to the absence of visible cracks along the interface.
strate evaluated on the scanning electron microscope revealed the presence of a large number of relatively coarse martensite plates together with carbides, visible as small particles which constitute a typical tempered martensite structure. Fig. 1 shows a view of the interface between the EN coating and substrate previous to fatigue testing, illustrating the deposition of an apparently uniform coating with satisfactory adhesion characteristics, which was corroborated by the evaluation of the coated material during tensile testing up to the yield stress and also by the observation of the fracture surfaces of the specimens after fatigue testing. It has already been mentioned that the coating thickness was evaluated by means of the ball cratering technique and also SEM observations, which showed a mean value of approximately 10 m. As shown in Fig. 2, the EDS analyses conducted on the deposit allowed to 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 the present work.
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deposit play a fundamental role in the fatigue behavior of any coated material and that such stresses are intimately associated, among other variables, with the chemical composition of the deposit. Regarding EN platings, an early investigation conducted by Parker and Shah w6x, particularly employing samples of 1090 steel as substrate, allowed us to conclude that the increase in the P content beyond approximately 10 wt.% gave rise to a change in the residual stress pattern, from tensile to compressive stresses and that for a P content of 12.4 wt.% the compressive residual stresses could achieve a magnitude of up to approximately 60 MPa. Thus, it would be expected that the EN deposits of the Nimet’s HP autocatalytic Nickel 䊛 employed in the present investigation were also under compressive residual stresses. The findings of Wu et al. w3x and Zhang et al. w4x would also support this view. These authors reported compressive residual stresses of the order of 45 MPa in the EN coatings deposited and subsequently PHT at 673 K for 1 h, and 10 MPa in the EN coatings deposited and subsequently PHT at 873 K for 1 h, even though the P content of such deposits was lower Ž9.5 wt.%. than that present in the coated specimens employed in this study. 3.2. E¨ aluation of mechanical properties In the as-deposited condition, the EN Nimet’s HP autocatalytic Nickel 䊛 deposit presented a hardness of approximately 488 " 78 KHN50 . However, as it was mentioned earlier, in order to determine the optimum PHT temperature at which maximum hardness is attained, a number of heat treatments were conducted in the temperature range of 673᎐873 K maintaining the specimens for 1 h. Fig. 3 illustrates the change in hardness with PHT temperature that was determined from these experiments, which indicates that a maximum hardness of approximately 1356 " 82 KHN50 is obtained after a PHT at 723 K. As it has been reported by Riedel w1x, heat treatment of EN deposits from approximately 553 to approximately 873 K is an excellent means of improving abrasion resistance and other tribological characteristics, and that by treating such deposits at approximately 673 K for 1 h their hardness will typically increase from 500᎐600 up to 1000᎐1100 VHN. Such changes in hardness are intimately associated with the modifications undergone by both the atomic structure and microstructure of the deposit as a result of solid state diffusion. It has been well documented in the literature w1x that both the microcrystalline and the amorphous deposits undergo a crystal growth process which results in a mixture of relatively coarse-grain metallic nickel together with intermetallic phases such as Ni 2 P, Ni 3 P and Ni 5 P2 . The early work of Kreye et al. w7x has shown that the microstructure of the deposits with a P content of approximately 12 wt.%,
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Fig. 3. Change in Knoop microhardness of the plated specimens with post-heat treatment temperature.
PHT at 673 K for 1 h is composed of two phases: 80% of the tetragonal Ni 3 P and 20% of metallic nickel. In order to evaluate if the EN Nimet’s HP autocatalytic Nickel 䊛 deposit had any influence on the monotonic mechanical properties of the composite coating᎐substrate material, a number of tensile tests were conducted with samples in the uncoated and coated conditions. The substrate material had a yield stress of approximately 1037 " 43 MPa and an ultimate tensile strength ŽUTS. of approximately 1143 " 30 MPa. In the as-deposited condition the coated samples showed somewhat lower mechanical properties: yield stress of 980 " 32 MPa and UTS of 1077 " 26 MPa, whereas in the as-deposited and PHT condition the yield stress was observed to increase slightly to 1010 " 19 MPa and the UTS remained constant at a value of 1077 " 18 MPa. Thus, it can be stated that the deposits plated onto the substrate employed in the present study did not give rise to any significant change either in yield stress or in UTS of the base steel, which is not surprising since in the present case the thickness of such deposits is so small that its effect is negligible. In contrast to the results reported by Garces ´ et al. w5x, in the present investigation it was observed that during both tensile testing up to the yield stress and fatigue testing in the whole stress amplitude interval, the deposits remained well adhered to the substrate indicating that the EN Nimet’s HP autocatalytic Nickel 䊛 deposit has a much better adhesion to the substrate steel which is not affected by the difference in mechan-
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Table 2 Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the uncoated specimens
Table 4 Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the coated specimens in the PHT condition
Stress ŽMPa.
Mean S.D.
Stress ŽMPa.
612
650
688
726
498
536
574
612
216 900 233 900 283 300 398 400 424 100 732 000 381 433 174 868
77 600 84 800 104 100 136 400 142 000 177 500 120 400 34 995
68 800 69 600 72 300 110 300 120 500 130 200 95 283 25 722
43 000 44 200 46 800 57 500 70 300 70 700 55 417 11 645
138 700 143 000 156 800 169 800 188 600 217 400 169 050 27 296
76 100 87 300 90 100 106 100 112 300 114 900 97 800 14 213
41 500 47 500 50 100 55 700 57 200 59 300 51 883 6165
20 100 27 000 27 700 28 500 31 100 31 400 27 633 3744
ical properties Želastic and plastic. between the plating and the base material. In relation to the fatigue tests conducted in order to evaluate 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 amplitude range of 612᎐726 MPa for the substrate which corresponded to a fraction of the yield stress of approximately 0.59᎐0.70. The coated samples in the as-deposited condition and after the corresponding PHT at 723 K were tested at a fraction of the yield stress ranging between approximately 0.55᎐0.66 and 0.49᎐0.61, respectively. The data showing the number of cycles prior to fracture Ž Nf . as a function of the alternating stress applied to the material Ž S . for the uncoated, coated as-deposited and coated and PHT specimens, are presented in Tables 2᎐4, whereas the data concerning the determination of the fatigue limit of the materials under different conditions are presented in Tables 5᎐7. The results obtained, regarding the fatigue properties of the coated and uncoated materials, have been plotted in Fig. 4 in which it can be observed that at each alternating stress level for the uncoated, coated as-deposited and coated and PHT specimens, six tests were carried out. As mentioned before, these conditions allowed the fulfillment of the reliability conditions Table 3 Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the coated specimens in the as-deposited condition
Mean S.D.
prescribed in the ASTM standard E-739. In agreement with the previous work conducted on similar substrates w3᎐5x, this figure shows that plating an EN Nimet’s HP autocatalytic Nickel 䊛 deposit even of this thickness onto the substrate steel, significantly decreases the fatigue life of the material in relation to the uncoated substrate, even though if the coating is in the as-deposited condition, a state in which the maximum compressive residual stresses would be expected. In the as-deposited condition, at elevated alternating stress levels Ž726 MPa. the curve obtained for the plated samples indicates a reduction in fatigue life, in comparison with the uncoated substrate, of approximately 63%, whereas at low stresses Ž612 MPa. the samples present a reduction of approximately 93%. However, for the coated and PHT specimens the situation is even worse since at 726 MPa the fatigue life is reduced by 86%, whereas at 612 MPa it is reduced by 97%. As far as the reduction in fatigue limit is concern, for the samples in the as-deposited condition the reduction was of approximately 12% whereas for the PHT specimens it achieved approximately 23%. Fig. 5 illustrates the change in the percentage of reduction in fatigue life for the as-deposited and PHT samples, in comparison with the uncoated specimens, as a function of the alternating stress. As expected, at low stresses Table 5 Experimental results for determining the fatigue limit of the substrate material Sample
Alternating stress ŽMPa.
Number of cycles
1 2 3 4 5 6 7 8 9 10
574 581 588 595 588 595 588 595 588 595
5 000 000 5 000 000 5 000 000 484 300 5 000 000 552 200 5 000 000 466 400 5 000 000 1 086 900
Stress ŽMPa.
Mean S.D.
536
574
612
650
159 200 185 800 192 800 197 500 210 500 259 800 200 933 30 565
89 400 96 700 97 000 98 300 103 400 125 000 101 633 11 227
47 000 66 300 69 300 71 400 72 500 80 100 67 767 10 194
26 900 34 800 41 800 46 500 49 800 51 900 41 950 8746
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Table 6 Experimental results for determining the fatigue limit of the coated samples in the as-deposited condition Sample
Alternating stress ŽMPa.
Number of cycles
1 2 3 4 5 6 7 8 9 10
498 505 512 519 512 519 512 519 526 519
5 000 000 5 000 000 5 000 000 263 900 5 000 000 483 500 5 000 000 5 000 000 520 300 5 000 000
the reduction in fatigue life for both conditions is much more significant, with the trend to decrease as the stress increases. However, the rate of decrease is higher for the samples in the as-deposited condition than for the PHT specimens. These results, are in agreement with those reported by Garces ´ et al. w5x who found somewhat smaller reductions in fatigue life by coating the same substrate with a different EN deposit with a thickness of 24 m. These researchers determined that in the as-deposited condition, at alternating stresses of the order of 663 MPa, the plated samples underwent a reduction in fatigue life of approximately 49.4%, whereas at stresses of the order of 590 MPa, the reduction was approximately 77.7%. Also, for the coated and PHT specimens the situation was even worse since at 663 MPa the fatigue life was reduced by 74.8%, whereas at 590 MPa it was reduced by 91.7%. The present results also corroborate those reported by Wu and co-workers w3x and also by Zhang et al. w4x 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 rise to a decrease of 52% in the fatigue limit of the material,
Fig. 4. Mean number of cycles prior to fracture Ž Nf . as function of the alternating stress applied to the material Ž S . for the uncoated, coated as-deposited and coated and PHT specimens.
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 of the form earlier proposed by Basquin w8x for the description of this type of data: S s ANfm where A and m represent constants that depend on both material properties and testing conditions.
Table 7 Experimental results for determining the fatigue limit of the coated samples in the PHT condition Sample
Alternating stress ŽMPa.
Number of cycles
1 2 3 4 5 6 7 8 9 10
460 467 460 453 446 453 460 467 460 453
5 000 000 222 300 494 100 624 300 5 000 000 5 000 000 5 000 000 298 700 456 100 5 000 000
Fig. 5. Change in the percentage of reduction in fatigue life for the as-deposited and PHT samples, with the alternating stress.
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A represents the fatigue strength coefficient of the material and m the fatigue exponent. Table 8 summarizes the values of the parameters A and m for the three set of data represented in Fig. 4. The appropriate determination of such parameters, particularly for the composite coating᎐substrate material, is of upmost importance both for the evaluation of the fatigue performance and for design purposes under considerations of high cycle fatigue of any component made of this steel that could be EN coated with Nimet’s HP autocatalytic Nickel 䊛 either for improving some of its properties, such as corrosion and wear resistance, or achieving the required dimensions in order for the part to fulfill properly its role in service. 3.3. E¨ aluation of the fracture surfaces of the samples A number of specimens tested under different alternating stress conditions were closely examined after
Table 8 Parameters involved in the Basquin relationship for the conditions tested Condition
A ŽMPa.
m
Substrate As-deposited Deposited and PHT
1605.2 2161.1 1847.2
0.075 0.114 0.108
failure by SEM in order to study, in more detail, the microstructural characteristics of the crack initiation sites, as well as the microstructural changes that take place both in the coating and substrate during the subsequent propagation of such cracks, leading eventually to the final fracture of the samples. For example, Fig. 6a illustrates a typical photomicrograph of the general view of the fracture surface of a sample in the as-deposited condition, tested at 650 MPa. As it can be
Fig. 6. Ža. General view of the fracture surface of a sample in the as-deposited condition, tested at 650 MPa. Such a surface reveals the presence of a number of fracture steps ŽFST., indicating that fracture has occurred as a consequence of the propagation of several cracks initiated from the surface of the specimen. The arrow indicates the origin ŽO. of the dominant crack. Žb. Detailed view of the initiation site of the dominant crack pointed out in Ža.. Secondary cracks ŽSC. that run parallel to the substrate ᎐deposit interface are clearly visible. Fatigue striations ŽFS. in some areas of the substrate near the interface are also observed.
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men also in the as-deposited condition, tested at 536 MPa. In this case, the crack origin is clearly defined as the site where the radial lines that propagate along the fracture surface converge. Such microstructural details are depicted more clearly in Fig. 7b where the integrity of both the deposit and the substrate ᎐deposit interface after fracture can be observed. Fig. 8a,b represents a more detailed description of Fig. 7b where it can be clearly observed that the fatigue cracks initiated within the EN deposit and propagated towards the substrate. This belief is supported by the continuity of the fracture features between the deposit and substrate which is also seen in both pictures. In relation to the samples in the PHT condition, Fig. 9a illustrates a typical fracture surface when the specimens are tested at 498 MPa. In this particular case, it was possible to identify the initiation sites of two dominant cracks which during propagation merged into a single one, giving rise to the presence of a step on the fracture surface. Fig. 9b represents a detailed view of the initiation site identified as A in Fig. 9a where, as before, it can be observed that the crack was nucleated
Fig. 7. Ža. General fracture surface of a specimen also in the as-deposited condition, tested at 536 MPa. The crack initiation site, identified with the arrow, is clearly defined by a number of radial lines that propagate from such a point. Žb. Magnified view of the crack initiation site. The coating has been identified as ŽD. and the substrate as ŽS.. The integrity of the substrate ᎐deposit interface after fracture can be observed.
appreciated, the fracture surface is not flat, revealing the presence of a number of fracture steps, which indicates that fracture has occurred as a consequence of the propagation of several cracks that initiated from the surface of the specimen. The arrow on the photomicrograph indicates the site initiation of the main crack. On the other hand, Fig. 6b represents a composite photomicrograph of the area where the main crack started. Some small secondary cracks that run parallel to the substrate ᎐deposit interface are also clearly visible, as well as fatigue striations in some areas of the substrate near the interface. In general, after fracture, despite the elevated alternating stress applied to the samples, the deposit is observed to remain well adhered to the substrate. Fig. 7a shows the fracture surface of a speci-
Fig. 8. Ža. Detailed description of area A in Fig. 7b. Fatigue cracks ŽFC. have nucleated within the EN deposit and propagate towards the substrate. Žb. Detailed description of area B in Fig. 7b. The coating has been identified as ŽD. and the substrate as ŽS.. Some continuity of the fracture features between the deposit and substrate are observed. The integrity of the substrate ᎐deposit interface is seen to be preserved after fracture.
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within the EN deposit and that the integrity of the substrate ᎐deposit interface has been preserved. Finally, Fig. 10a shows the general fracture surface of a sample, also in the PHT condition, tested at 612 MPa where several steps were formed as a consequence of the simultaneous propagation of a number of cracks nucleated at the surface of the specimen. The detailed analysis of the site identified as A, shown in Fig. 10b, revealed again that such cracks were nucleated within the EN deposit and that their propagation towards the substrate is enhanced by the preservation of the bonding at the interface. All these observations lead to the conclusion that even though the deposit plated in this investigation, in the as-deposited condition, would be under compressive residual stresses due to its elevated P content, it has a lower fatigue strength than the substrate, which leads to the nucleation of fatigue cracks within it, prior to the nucleation of fatigue cracks at the substrate ᎐deposit interface. Once such cracks have been nucleated,
Fig. 10. Ža. General fracture surface of a sample, also in the PHT condition, tested at 612 MPa. Several steps ŽFST. are observed as a consequence of the simultaneous propagation of different cracks. Žb. Detailed analysis of site A shown in Ža.. Fatigue cracks ŽFC. have been nucleated within the deposit ŽD. and their propagation towards the substrate ŽS. is enhanced by the preservation of the bonding at the interface.
Fig. 9. Ža. Typical fracture surface of a sample in the PHT condition tested at an alternating stress of 498 MPa. The arrows point out the initiation sites of two dominant cracks. Propagation of such cracks has given rise to the presence of two steps ŽFST. visible on the fracture surface. Žb. Detailed view of the site initiation identified as A in Ža.. The main crack was nucleated within the EN deposit ŽD..
due to the preservation of the bonding at the interface, they propagate towards the substrate giving rise to a significant reduction in the fatigue properties of the steel. Thus, in agreement with the findings reported by Garces ´ et al. w5x, the Nimet’s HP autocatalytic Nickel 䊛 deposit employed in the present investigation, acts as a source of fatigue cracks whose propagation is enhanced by the apparently good bonding that exists between the substrate and the deposit. The largest reduction in fatigue properties of the PHT samples in comparison with the as-deposited specimens, would be consistent with a decrease in the compressive residual stresses within the deposit, as it has been reported by Zhang et al. w3x. In summary, plating a high fatigue strength substrate with a deposit of lower fatigue properties could lead to a reduction in the fatigue properties of the composite material, particularly if a good bonding exists at the
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substrate ᎐deposit interface which preserves its integrity. On the contrary, the fatigue properties of the composite material could be improved if the deposit performed better than the substrate under these conditions.
4. Conclusions The fatigue properties of a SAE 4340 steel could be severely decreased by plating it with an EN Nimet’s HP Autocatalytic Nickel 䊛 deposit, even of 10 m thickness. In the as-deposited condition, the decrease in fatigue life could reach between 63᎐93%, depending upon 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 life could achieve 86᎐97% and that of fatigue limit could be of 23%. In agreement with previous findings, it is believed that such a reduction in fatigue properties takes place due to the early nucleation of fatigue cracks within the deposit which are subsequently propagated towards the substrate. Crack propagation is enhanced by the integrity of the substrate ᎐deposit interface which is preserved by the good bonding between the two materials. Therefore, the use of EN platings, such as the one investigated in the present work, to improve the corrosion and wear properties of high strength steels, could lead to a severe reduction in the fatigue properties of the composite material since the low strength deposit would act as a surface notch, decreasing the time required for the nucleation of the fatigue cracks. The reduction in fatigue life has been found to be less marked for the samples in the as-deposited condition than those PHT. Also, the decrease in fatigue limit for both conditions seems to be less severe than that in fatigue life. The mechanical design
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of structural components and parts made of this steel and coated with EN deposits of this kind, in order to avoid a potential fatigue failure under high cycle fatigue conditions, should take into account the fatigue curves derived for the plated materials in the as-deposited and PHT conditions rather than that obtained for the unplated steel. However, such curves are expected to vary with the particular type of EN deposit, coating thickness and PHT applied after deposition.
Acknowledgements This investigation has been conducted with the financial support of the Venezuelan National Council for Scientific and Technological Research ŽCONICIT. through the project LAB-97000644 and the Scientific and Humanistic Development Council of the Central University of Venezuela ŽCDCH-UCV. through the project 08-17-4595-2000. J.A. Berrıos ´ is deeply grateful to the School of Mechanical Engineering, Faculty of Engineering and Architecture of the University of El Salvador. References w1x W. Riedel, Electroless Nickel Plating, ASM International, Metals Park, Ohio, USA, 1991, pp. 48᎐181. w2x J. Abbott, Adv. Mater. Proc. 150 Ž6. Ž1996. 21᎐23. w3x Y.Y. Wu, Y.Z. Zhang, M. Yao, Plating Surf. Finish. Ž1995. 83᎐85. w4x Y.Z. Zhang, Y.Y. Wu, M. Yao, J. Matter. Sci. Lett. 15 Ž1996. 1364᎐1366. w5x Y. Garces, J. Berrıos, ´ H. Sanchez, ´ ´ A. Pertuz, J. Chitty, H. Hintermann, E.S. Puchi, Thin Solid Films 355r356 Ž1999. 487᎐493. w6x K. Parker, H. Shah, Plating 3 Ž1971. 230. w7x H. Kreye, H. Muller, T. Petzel, Galvanotechnik 77 Ž1986. 561. ¨ w8x O.H. Basquin, Proc. ASTM 10 Ž2. Ž1910. 625.
