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Effect of specimen preparation on contact fatigue wear resistance of austempered ductile cast iron
Wear 263 (2007) 663–668
Short communication
Effect of specimen preparation on contact fatigue wear resistance of austempered ductile cast iron C. Brunetti, M.V. Leite, G. Pintaude ∗ Postgraduate Program in Mechanical and Materials Engineering, Contact and Surface Laboratory, Federal University of Technology – Paran´a, Av. Sete de Setembro, 3165 Curitiba/PR, Zip code: 80230-901, Brazil Received 1 September 2006; received in revised form 10 January 2007; accepted 16 January 2007 Available online 23 May 2007
Abstract Austempered ductile iron (ADI) has been tested as a wear resistance material for applications where the applied loads are very high and cyclic, such as gears. This paper analyzed the endurance lifetime of this material in wear testing equipment that applies contact fatigue stresses in a lubricated ball-on-flat system. The tests were performed at 3.0 or 3.7 GPa of maximum Hertz pressure, using ISO 46 lubricant at 85 ◦ C, until spalling occurrence. The effect of specimen preparation was studied for ground or polished specimens. The worn surfaces were characterized by means of an optical and electron microscope and by the difference between unworn and worn surface profiles. It was found that the graphite nodules were exposed at the surface in two ways: cracked and partially exposed. These morphologies were not found in polished specimens, but the wear process in all test conditions produces them. The low endurance lifetime observed in ground specimen was explained based on these defects, which were present before the tests. The slope of Weibull curves was related to the width of the worn track. For polished specimens the slope was smaller than that observed for ground conditions. Thus, any manual process for preparation can be considered as forbidden in order to obtain better results. © 2007 Elsevier B.V. All rights reserved. Keywords: Contact fatigue; Austempered ductile iron; Surface roughness; Endurance lifetime
1. Introduction A mechanical system is usually composed of elements that are in contact and under loading. Wear can take place as a result of this contact after a certain period of time. A particular mode of wear is that caused by contact fatigue, which occurs in components subjected to cyclic pressures such as gears. This mode of wear is the main cause of failure in these kind of components [1]. During the design of these components the reduction in severity of operational conditions is desired. A way to reduce the contact fatigue wear is the appropriate materials selection. Usually, ultra cleaned and hardened steels are used in components subjected to high contact pressures. Nevertheless, nowadays components such as gears have been made using steel with high level of alloying elements, following heat treatment. Another
∗
Corresponding author. Tel.: +55 41 3310 4660; fax: +55 41 3310 4660. E-mail address: [email protected] (G. Pintaude).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.078
possibility is the use of cast irons, especially the austempered ductile iron—ADI. Although the graphite nodules could be act as stress raisers, ADI has shown a satisfactory performance when subjected to contact fatigue wear [2]. The surface roughness has a huge impact on endurance lifetime of components subject to contact fatigue [3]. The austempered ductile iron has been used in some researches [4–6]. This paper analyzes the effect of specimen preparation on endurance lifetime of austempered ductile iron, tested in a lubricated ball-on-flat system. 2. Experimental procedure Austempered ductile iron (ADI) was subjected to contact fatigue wear and its chemical composition is presented in Table 1. The bars with 95 mm of diameter and 45 mm of thickness were produced through continuous casting process. These bars were austenitized at 910 ◦ C for 1.5 h followed by austempering in a salt bath at 290 ◦ C for 2 h. The metallographic characterization was performed by following the ASTM A247
664
C. Brunetti et al. / Wear 263 (2007) 663–668
Fig. 1. As-received ADI microstructure, revealed by optical microscope: (a) graphite nodules distribution and (b) bainitic matrix.
Table 1 Chemical composition of tested material, austempered ductile iron (mass%) C
Si
Mn
P
S
Cr
Cu
Mo
Mg
Ceq
3.71
2.54
0.18
0.065
0.01
0.031
0.72
0.186
0.038
4.56
Ceq = Equivalent carbon.
Standard [7] procedures. Fig. 1 presents the as-received ADI microstructure revealed by the optical microscope. The volumetric fraction of graphite nodules is 10.0 ± 0.6% and their average diameter is 23 ± 10 m. The bulk hardness of ADI is 362 ± 12 HB2.5/187.5 and its bainitic matrix has 508 ± 28 HV0.05 . The bars were turned in order to manufacture specimens with washer geometry having the following dimensions: 55 mm of external diameter, 30 mm of internal diameter and 5.5 mm of thickness. Two initial surface conditions were produced: ground (G) and polished (P). The ground surfaces were obtained in two different plane grinding machines, both using an aluminum oxide grinding wheel, grade AA-100 G5 VF8 (particle size with 0.16 mm of average diameter). The processes were differentiated by the
controlling of the longitudinal feed: in one of them, this parameter was not controlled, while in the other it was 7.9 m/min. Also, in both processes the transverse feed was not controlled. The other parameters used in grinding were the same for both machines: an in-feed of 10 m, a total depth of cut of 50 m and a speed of 30 m/s. The average roughness of ground specimens is around 0.04 m. After grinding process, the polishing was performed manually with 1 m diamond grains, using metallography technique. The average roughness of polished surface is about 0.01 m. The rolling contact fatigue (RCF) tests were performed in ball-on-flat equipment, projected and constructed by Leite [8]. Fig. 2 presents a schematic view of the used equipment. 1.2 l of lubricant oil ISO 46 are circulated through the chamber where specimens are located. The tests were always started after the oil temperature reached 85 ◦ C, and this temperature is maintained by electric resistance during all period of tests. The employed velocity of 1700 rpm allowed a load frequency of 0.5 × 105 cycles/h. The counter-body was an AISI 52100 sphere with 5/16 in. of diameter and it was received with 0.01 m of average roughness. The contact pressure – 3.0 or 3.7 GPa – is achieved by means of dead-weights applied under a cantilever; in order that an axial load is employing on three balls placed
Fig. 2. Schematic view of the rolling contact fatigue test rig.
C. Brunetti et al. / Wear 263 (2007) 663–668
in a cage of a 52206 thrust bearing. The design of the ball-onflat system allows a load application limited to 150 kg of mass, which results in 1.75 kN of maximum axial force, considering the action of the cantilever. In order to obtain 3.0 GPa of contact pressure 59 kg of mass were applied, while for 3.7 GPa test condition, 119 kg of mass were disposed as dead-weights. Each test condition was identified by the manufacturing process (grinding or polishing) and by the contact pressure (3.0 or 3.7 GPa). For example, P3.7 means that the test was conducted with polished specimens under a 3.7 GPa of contact pressure. The number of cycles to the failure was associated to sound changes, perceived by the operator. After all tests a spalling mechanism was detected. The fatigue life was evaluated through the Weibull distribution of two parameters [9]. Each testing condition corresponded to five specimens of ADI. The worn surfaces were analyzed using optical and scanning electron microscopes. The depth and width of worn tracks were determined through surface roughness measurements. The removal of graphite nodules was determined by metallographic counting before and after RCF tests, using areas of 0.05 mm2 , outside and inside the worn tracks. For polished specimens, in each counting, it was identified that the empty nodules were caused by the removal of the graphite. For ground specimens, due to the smaller amount of removed nodules, the counting was performed considering the kinds of morphologies produced by the grinding process, classified in this paper as cracked subsurface nodules (CSN) and partial subsurface nodules (PSN). All average values of graphite nodules counting corresponded to one hundred measurements for each studied region. The surface roughness was determined in SURTRONIC 25+ equipment. The evaluation length was 4 mm. The stylus diamond was perpendicularly disposed to the worn surface. The resulting profiles were analyzed in the TALY PROFILE software −3.1.10 version, to calculate the Rpk roughness parameter. For each roughness profile, the depth and width of worn track was determined, in four positions, each one separated by 90◦ . The average values for each position corresponded to fifteen measurements. 3. Results and discussion Fig. 3 presents the failure probability as a function of the load cycles. From Fig. 3 it is possible to extract the results presented in Table 2, which summarizes the L10 , L50 and average values of contact fatigue life. Table 2 shows that the endurance lifetime increases as the contact pressure was reduced, i.e., η value of P3.7 condition is smaller than those observed for P3.0 or G3.0 tests. More than that, the lifetime of ground specimens is smaller
665
Fig. 3. Failure probability as a function of load cycles for ADI tested in the following conditions: () P3.7, () G3.0 and () P3.0.
than the observed value for polished ones, tested at the same contact pressure (3.0 GPa). The reduction shown in Table 2 for endurance lifetime due to the initial surface roughness was 1.89. Dommarco and co-workers [4,10] observed reductions in the same order of magnitude −1.79 – when they compared ground and polished specimens of ADI in RCF tests. The observed difference for the endurance lifetime is due to the modification on the contact stress distribution. The rough contact causes local disturbances on the Hertzian pressure distribution, producing stationary pressure spikes [3,11]. This effect makes easy the nucleation of cracks, which is responsible by up to 85% of the total life of a component subjected to the contact fatigue [11]. The morphology of graphite nodules exposed at the surface was very different, when one compared the ground to polished specimens. Fig. 4 presents these aspects before RCF tests. Fig. 4 shows that in the ground specimen there were two morphologies of graphite nodules. These morphologies were classified as cracked subsurface nodules and partial subsurface nodules. All nodules in the Fig. 4a are positioned in the subsurface, because the grinding process gives rise to a metallic layer over them. On the other hand, some points presented in Fig. 4b could be classified as PSN, and it was not observed the CSN morphology in polished specimens. Probably, the metallographic preparation eliminated the CSN morphology. For ground specimen, the number of occurrences of CSN and PSN was determined and the results are presented in Table 3. Table 3 shows that the amount of PSN morphology is smaller before RCF tests than that observed inside the worn track, since the CSN/PSN ratio decreases after wear process. This result is an indicative that the deformed and cracked layer over the nodules was removed during tests and a large amount of nodules became
Table 2 Rolling contact fatigue tests results Test condition
L10 (cycles × 106 )
L50 (cycles × 106 )
η Medium life
β Weibull slope
r2
P3.7 G3.0 P3.0
0.05 1.02 1.70
0.13 1.48 2.78
0.16 1.60 3.03
2.03 4.94 3.91
0.94 0.93 0.93
666
C. Brunetti et al. / Wear 263 (2007) 663–668
Fig. 4. Surfaces before RCF tests, revealed by optical microscope: (a) ground (G) and (b) polished (P). CSN: cracked subsurface nodules. PSN: partially subsurface nodules. Table 3 Number of occurrences of cracked (CSN) and partially subsurface nodules (PSN), inside and outside the worn track, for ground specimens Morphology
CSN PSN
Outside the worn track (563 occurrences)
Inside the worn track (485 occurrences)
Occurrences
Occurrences
374 189
CSN/PSN ratio 1.98
253 232
CSN/PSN ratio 1.09
exposed as PSN morphology. Fig. 5 shows the worn surface of ground specimen. For polished specimens, it was verified a larger removal of graphite nodules inside the worn track. For instance, the specimen tested in P3.0 condition presented 12% of holes before RCF tests and 37% of them inside the worn track. An example of this kind of occurrence is presented in Fig. 6. Moreover, the CSN and PSN morphologies are also verified after RCF tests at worn surfaces of polished specimens, as shown in Fig. 7.
Fig. 5. Worn surface of ground specimen (G3.0 test condition) revealed by scanning electron microscope (SEM). CSN: cracked subsurface nodules. PSN: partially subsurface nodules.
Fig. 8 presents two cross-sections where the spalling mechanism was observed, for the specimens G3.0 e P3.0, respectively. Fig. 8 shows that sub-superficial nodules have a decisive role on the nucleation and propagation of the cracks. In both cases, the cracks were initiated in these nodules and they propagate to the surface direction. One can conclude that the higher surface roughness and the identified morphologies in ground specimen, cracked subsurface and partial subsurface nodules are responsible by the low endurance life time observed in G3.0 the test condition, because they act as “natural” defects and stress intensification on the surface and subsurface. Another important value presented in Table 2 is the slope of the Weibull curve, β. Larger values of β correspond to smaller deviations. In this study, the P3.7 condition was the one with smallest value for the β slope. In this study the width of worn track was selected as the parameter to evaluate the deviations that occurred in the ballon-flat system and the effect of specimen preparation on these results. It is important to remind that the specimens tested under P3.7 condition were ground using a poorly controlled process.
Fig. 6. Worn surface of polished specimen (P3.0 test condition), revealed by optical microscope, showing the removal of graphite nodules and holes exposed at surface. RD is the rolling direction.
C. Brunetti et al. / Wear 263 (2007) 663–668
667
Fig. 7. Worn surface of polished specimen, revealed by SEM. Defects introduced by wear process: (a) cracked sub superficial nodule and (b) partially subsurface nodule.
Fig. 8. Cross-section of the tested specimens with a spalling occurrence: (a) ground specimen (G3.0 test condition) and (b) polished specimen (P3.0 test condition). RD: rolling direction.
Table 4 presents the width of worn track as a function of measurement position. The reference of positions was taken considering a small hole, produced in specimens in order to set them inside the equipment. Thus, 0◦ corresponds to the position of this small hole. Table 4 shows that the standard deviations are the smallest for ground specimens, for any position. This result has relations with those presented in Table 2 for β values, which are the highest for ground specimens. Moreover, Table 4 shows the noticeable difference between width values for polished specimens, which reaches up to 17%. For ground specimens (G3.0 condition) the maximum difference between any two positions is about 8%.
These results confirm the assertive made based on the standard deviations presented in Table 4, i.e., when any kind of manual process is applied on the surface, the slope of failure probability can be dramatically reduced. Finally, Table 5 presents an example of surface roughness measurements, extracted from roughness profiles used to calculate the width of the worn tracks. Table 5 shows that the increase on the peaks fraction was up to twice, for the polished specimens, comparing outside and inside the worn track. This increase was expected, since the initial surface condition is limited to the first cycles [12] and the trend is an increase of surface roughness during the wear process. On the other hand, the ground specimens presented a
Table 4 Width of the worn track and its standard deviation (%) for each measurement position
Table 5 Rpk roughness parameter values for each testing condition, outside and inside the worn track
Test condition
Test condition
P3.7 G3.0 P3.0
Width of the worn track (m) and standard deviation (%) 0◦
90◦
180◦
270◦
726 ± 4 551 ± 6 605 ± 7
756 ± 7 517 ± 5 517 ± 7
644 ± 7 508 ± 4 534 ± 6
628 ± 8 540 ± 3 620 ± 10
P3.7 G3.0 P3.0
Rpk parameter (m) Outside wear track
Inside wear track
0.03 0.03 0.02
0.04 0.02 0.03
668
C. Brunetti et al. / Wear 263 (2007) 663–668
reduction in Rpk values after RCF tests. It is remarkable that the values of Rpk before and after RCF tests are not very different, considering two levels of contact pressure. 4. Conclusions Tests performed with austempered ductile iron under a lubricated ball-on-flat system allows for the following conclusion: • The specimen preparation has a pronounced effect on endurance lifetime. A poorly controlled grinding process, or even a manual polishing, is not recommended in order to obtain low values of deviation. • Grinding process exposed subsurface graphite nodules with two morphologies: cracked and partial. These morphologies act as stress raisers, reducing the endurance lifetime. • Endurance lifetime of ground specimens is smaller than that observed for polished ones. The reduction found in this study agrees with literature data. Acknowledgments C. Brunetti and M.V. Leite are very grateful to CAPES by the financial scholarship. The authors acknowledge to Araucaria Foundation for the financial support through project 018/04 and to FUNDICAO TUPY LTDA by the supplying of ductile iron specimens.
References [1] P.J. Blau, Rolling contact wear, in: P.J. Blau (Ed.), Friction, Lubrication and Wear Technology, ASM Handbook, 18, ASM International, Materials Park, 1992, pp. 481–493. [2] A.G. Fuller, Austempered ductile irons—present applications, Mater. Des. 6 (1985) 127–130. [3] T.H. Kim, A.V. Olver, Stress history in rolling-sliding contact of rough surfaces, Tribol. Int. 31 (1998) 727–736. [4] R.C. Dommarco, P.C. Bastias, H.A. Dall’O, G.T. Hahn, C.A. Rubin, Rolling contact fatigue resistance of austempered ductile iron, Wear 221 (1998) 69–74. [5] R.C. Dommarco, J.D. Salvande, Contact fatigue resistance of austempered and partially chilled ductile iron, Wear 254 (2003) 230–236. [6] L. Magalh˜aes, J. Seabra, C. S´a, Experimental observations of contact fatigue crack mechanisms for Austempered Ductile Iron (ADI) discs, Wear 246 (2000) 134–148. [7] ASTM A247-67, Standard test method for evaluating the microstructure of graphite in iron castings, In: Annual Book of ASTM Standards, Vol. 01, 02, Am. Soc. Testing Mater., Philadelphia, 1998. [8] M.V. Leite, Analysis of the Wear Mechanisms in Contact Fatigue, Case Study: Austempered Ductile Iron, Dissertation, Federal University of Technology, Paran´a, Curitiba-Brazil, p. 90, 2005 (in Portuguese). [9] C. Lipson, J.N. Sheth, Statistical Design and Analysis of Engineering Experiments, McGraw-Hill, New York, 1973, p. 518. [10] J. Salvande, R.C. Dommarco, Estudio de la resitencia a la fatiga de contacto de fundici´on nodular, in: J.E.P. Ipi˜na (Ed.), IV Coloquio Latinoamericano de Fractura y Fatiga, 2000, pp. 421–427. [11] P.C. Bastias, G.T. Hahn, C.A. Rubin, V. Gupta, X. Leng, Analysis of rolling contact spall life in 440C bearing steel, Wear 171 (1994) 169–178. [12] Y. Choi, C.R. Liu, Rolling contact fatigue life of finish hard machined surfaces. Part 2. Experimental verification, Wear 261 (2006) 492–499.
Short communication
Effect of specimen preparation on contact fatigue wear resistance of austempered ductile cast iron C. Brunetti, M.V. Leite, G. Pintaude ∗ Postgraduate Program in Mechanical and Materials Engineering, Contact and Surface Laboratory, Federal University of Technology – Paran´a, Av. Sete de Setembro, 3165 Curitiba/PR, Zip code: 80230-901, Brazil Received 1 September 2006; received in revised form 10 January 2007; accepted 16 January 2007 Available online 23 May 2007
Abstract Austempered ductile iron (ADI) has been tested as a wear resistance material for applications where the applied loads are very high and cyclic, such as gears. This paper analyzed the endurance lifetime of this material in wear testing equipment that applies contact fatigue stresses in a lubricated ball-on-flat system. The tests were performed at 3.0 or 3.7 GPa of maximum Hertz pressure, using ISO 46 lubricant at 85 ◦ C, until spalling occurrence. The effect of specimen preparation was studied for ground or polished specimens. The worn surfaces were characterized by means of an optical and electron microscope and by the difference between unworn and worn surface profiles. It was found that the graphite nodules were exposed at the surface in two ways: cracked and partially exposed. These morphologies were not found in polished specimens, but the wear process in all test conditions produces them. The low endurance lifetime observed in ground specimen was explained based on these defects, which were present before the tests. The slope of Weibull curves was related to the width of the worn track. For polished specimens the slope was smaller than that observed for ground conditions. Thus, any manual process for preparation can be considered as forbidden in order to obtain better results. © 2007 Elsevier B.V. All rights reserved. Keywords: Contact fatigue; Austempered ductile iron; Surface roughness; Endurance lifetime
1. Introduction A mechanical system is usually composed of elements that are in contact and under loading. Wear can take place as a result of this contact after a certain period of time. A particular mode of wear is that caused by contact fatigue, which occurs in components subjected to cyclic pressures such as gears. This mode of wear is the main cause of failure in these kind of components [1]. During the design of these components the reduction in severity of operational conditions is desired. A way to reduce the contact fatigue wear is the appropriate materials selection. Usually, ultra cleaned and hardened steels are used in components subjected to high contact pressures. Nevertheless, nowadays components such as gears have been made using steel with high level of alloying elements, following heat treatment. Another
∗
Corresponding author. Tel.: +55 41 3310 4660; fax: +55 41 3310 4660. E-mail address: [email protected] (G. Pintaude).
0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.078
possibility is the use of cast irons, especially the austempered ductile iron—ADI. Although the graphite nodules could be act as stress raisers, ADI has shown a satisfactory performance when subjected to contact fatigue wear [2]. The surface roughness has a huge impact on endurance lifetime of components subject to contact fatigue [3]. The austempered ductile iron has been used in some researches [4–6]. This paper analyzes the effect of specimen preparation on endurance lifetime of austempered ductile iron, tested in a lubricated ball-on-flat system. 2. Experimental procedure Austempered ductile iron (ADI) was subjected to contact fatigue wear and its chemical composition is presented in Table 1. The bars with 95 mm of diameter and 45 mm of thickness were produced through continuous casting process. These bars were austenitized at 910 ◦ C for 1.5 h followed by austempering in a salt bath at 290 ◦ C for 2 h. The metallographic characterization was performed by following the ASTM A247
664
C. Brunetti et al. / Wear 263 (2007) 663–668
Fig. 1. As-received ADI microstructure, revealed by optical microscope: (a) graphite nodules distribution and (b) bainitic matrix.
Table 1 Chemical composition of tested material, austempered ductile iron (mass%) C
Si
Mn
P
S
Cr
Cu
Mo
Mg
Ceq
3.71
2.54
0.18
0.065
0.01
0.031
0.72
0.186
0.038
4.56
Ceq = Equivalent carbon.
Standard [7] procedures. Fig. 1 presents the as-received ADI microstructure revealed by the optical microscope. The volumetric fraction of graphite nodules is 10.0 ± 0.6% and their average diameter is 23 ± 10 m. The bulk hardness of ADI is 362 ± 12 HB2.5/187.5 and its bainitic matrix has 508 ± 28 HV0.05 . The bars were turned in order to manufacture specimens with washer geometry having the following dimensions: 55 mm of external diameter, 30 mm of internal diameter and 5.5 mm of thickness. Two initial surface conditions were produced: ground (G) and polished (P). The ground surfaces were obtained in two different plane grinding machines, both using an aluminum oxide grinding wheel, grade AA-100 G5 VF8 (particle size with 0.16 mm of average diameter). The processes were differentiated by the
controlling of the longitudinal feed: in one of them, this parameter was not controlled, while in the other it was 7.9 m/min. Also, in both processes the transverse feed was not controlled. The other parameters used in grinding were the same for both machines: an in-feed of 10 m, a total depth of cut of 50 m and a speed of 30 m/s. The average roughness of ground specimens is around 0.04 m. After grinding process, the polishing was performed manually with 1 m diamond grains, using metallography technique. The average roughness of polished surface is about 0.01 m. The rolling contact fatigue (RCF) tests were performed in ball-on-flat equipment, projected and constructed by Leite [8]. Fig. 2 presents a schematic view of the used equipment. 1.2 l of lubricant oil ISO 46 are circulated through the chamber where specimens are located. The tests were always started after the oil temperature reached 85 ◦ C, and this temperature is maintained by electric resistance during all period of tests. The employed velocity of 1700 rpm allowed a load frequency of 0.5 × 105 cycles/h. The counter-body was an AISI 52100 sphere with 5/16 in. of diameter and it was received with 0.01 m of average roughness. The contact pressure – 3.0 or 3.7 GPa – is achieved by means of dead-weights applied under a cantilever; in order that an axial load is employing on three balls placed
Fig. 2. Schematic view of the rolling contact fatigue test rig.
C. Brunetti et al. / Wear 263 (2007) 663–668
in a cage of a 52206 thrust bearing. The design of the ball-onflat system allows a load application limited to 150 kg of mass, which results in 1.75 kN of maximum axial force, considering the action of the cantilever. In order to obtain 3.0 GPa of contact pressure 59 kg of mass were applied, while for 3.7 GPa test condition, 119 kg of mass were disposed as dead-weights. Each test condition was identified by the manufacturing process (grinding or polishing) and by the contact pressure (3.0 or 3.7 GPa). For example, P3.7 means that the test was conducted with polished specimens under a 3.7 GPa of contact pressure. The number of cycles to the failure was associated to sound changes, perceived by the operator. After all tests a spalling mechanism was detected. The fatigue life was evaluated through the Weibull distribution of two parameters [9]. Each testing condition corresponded to five specimens of ADI. The worn surfaces were analyzed using optical and scanning electron microscopes. The depth and width of worn tracks were determined through surface roughness measurements. The removal of graphite nodules was determined by metallographic counting before and after RCF tests, using areas of 0.05 mm2 , outside and inside the worn tracks. For polished specimens, in each counting, it was identified that the empty nodules were caused by the removal of the graphite. For ground specimens, due to the smaller amount of removed nodules, the counting was performed considering the kinds of morphologies produced by the grinding process, classified in this paper as cracked subsurface nodules (CSN) and partial subsurface nodules (PSN). All average values of graphite nodules counting corresponded to one hundred measurements for each studied region. The surface roughness was determined in SURTRONIC 25+ equipment. The evaluation length was 4 mm. The stylus diamond was perpendicularly disposed to the worn surface. The resulting profiles were analyzed in the TALY PROFILE software −3.1.10 version, to calculate the Rpk roughness parameter. For each roughness profile, the depth and width of worn track was determined, in four positions, each one separated by 90◦ . The average values for each position corresponded to fifteen measurements. 3. Results and discussion Fig. 3 presents the failure probability as a function of the load cycles. From Fig. 3 it is possible to extract the results presented in Table 2, which summarizes the L10 , L50 and average values of contact fatigue life. Table 2 shows that the endurance lifetime increases as the contact pressure was reduced, i.e., η value of P3.7 condition is smaller than those observed for P3.0 or G3.0 tests. More than that, the lifetime of ground specimens is smaller
665
Fig. 3. Failure probability as a function of load cycles for ADI tested in the following conditions: () P3.7, () G3.0 and () P3.0.
than the observed value for polished ones, tested at the same contact pressure (3.0 GPa). The reduction shown in Table 2 for endurance lifetime due to the initial surface roughness was 1.89. Dommarco and co-workers [4,10] observed reductions in the same order of magnitude −1.79 – when they compared ground and polished specimens of ADI in RCF tests. The observed difference for the endurance lifetime is due to the modification on the contact stress distribution. The rough contact causes local disturbances on the Hertzian pressure distribution, producing stationary pressure spikes [3,11]. This effect makes easy the nucleation of cracks, which is responsible by up to 85% of the total life of a component subjected to the contact fatigue [11]. The morphology of graphite nodules exposed at the surface was very different, when one compared the ground to polished specimens. Fig. 4 presents these aspects before RCF tests. Fig. 4 shows that in the ground specimen there were two morphologies of graphite nodules. These morphologies were classified as cracked subsurface nodules and partial subsurface nodules. All nodules in the Fig. 4a are positioned in the subsurface, because the grinding process gives rise to a metallic layer over them. On the other hand, some points presented in Fig. 4b could be classified as PSN, and it was not observed the CSN morphology in polished specimens. Probably, the metallographic preparation eliminated the CSN morphology. For ground specimen, the number of occurrences of CSN and PSN was determined and the results are presented in Table 3. Table 3 shows that the amount of PSN morphology is smaller before RCF tests than that observed inside the worn track, since the CSN/PSN ratio decreases after wear process. This result is an indicative that the deformed and cracked layer over the nodules was removed during tests and a large amount of nodules became
Table 2 Rolling contact fatigue tests results Test condition
L10 (cycles × 106 )
L50 (cycles × 106 )
η Medium life
β Weibull slope
r2
P3.7 G3.0 P3.0
0.05 1.02 1.70
0.13 1.48 2.78
0.16 1.60 3.03
2.03 4.94 3.91
0.94 0.93 0.93
666
C. Brunetti et al. / Wear 263 (2007) 663–668
Fig. 4. Surfaces before RCF tests, revealed by optical microscope: (a) ground (G) and (b) polished (P). CSN: cracked subsurface nodules. PSN: partially subsurface nodules. Table 3 Number of occurrences of cracked (CSN) and partially subsurface nodules (PSN), inside and outside the worn track, for ground specimens Morphology
CSN PSN
Outside the worn track (563 occurrences)
Inside the worn track (485 occurrences)
Occurrences
Occurrences
374 189
CSN/PSN ratio 1.98
253 232
CSN/PSN ratio 1.09
exposed as PSN morphology. Fig. 5 shows the worn surface of ground specimen. For polished specimens, it was verified a larger removal of graphite nodules inside the worn track. For instance, the specimen tested in P3.0 condition presented 12% of holes before RCF tests and 37% of them inside the worn track. An example of this kind of occurrence is presented in Fig. 6. Moreover, the CSN and PSN morphologies are also verified after RCF tests at worn surfaces of polished specimens, as shown in Fig. 7.
Fig. 5. Worn surface of ground specimen (G3.0 test condition) revealed by scanning electron microscope (SEM). CSN: cracked subsurface nodules. PSN: partially subsurface nodules.
Fig. 8 presents two cross-sections where the spalling mechanism was observed, for the specimens G3.0 e P3.0, respectively. Fig. 8 shows that sub-superficial nodules have a decisive role on the nucleation and propagation of the cracks. In both cases, the cracks were initiated in these nodules and they propagate to the surface direction. One can conclude that the higher surface roughness and the identified morphologies in ground specimen, cracked subsurface and partial subsurface nodules are responsible by the low endurance life time observed in G3.0 the test condition, because they act as “natural” defects and stress intensification on the surface and subsurface. Another important value presented in Table 2 is the slope of the Weibull curve, β. Larger values of β correspond to smaller deviations. In this study, the P3.7 condition was the one with smallest value for the β slope. In this study the width of worn track was selected as the parameter to evaluate the deviations that occurred in the ballon-flat system and the effect of specimen preparation on these results. It is important to remind that the specimens tested under P3.7 condition were ground using a poorly controlled process.
Fig. 6. Worn surface of polished specimen (P3.0 test condition), revealed by optical microscope, showing the removal of graphite nodules and holes exposed at surface. RD is the rolling direction.
C. Brunetti et al. / Wear 263 (2007) 663–668
667
Fig. 7. Worn surface of polished specimen, revealed by SEM. Defects introduced by wear process: (a) cracked sub superficial nodule and (b) partially subsurface nodule.
Fig. 8. Cross-section of the tested specimens with a spalling occurrence: (a) ground specimen (G3.0 test condition) and (b) polished specimen (P3.0 test condition). RD: rolling direction.
Table 4 presents the width of worn track as a function of measurement position. The reference of positions was taken considering a small hole, produced in specimens in order to set them inside the equipment. Thus, 0◦ corresponds to the position of this small hole. Table 4 shows that the standard deviations are the smallest for ground specimens, for any position. This result has relations with those presented in Table 2 for β values, which are the highest for ground specimens. Moreover, Table 4 shows the noticeable difference between width values for polished specimens, which reaches up to 17%. For ground specimens (G3.0 condition) the maximum difference between any two positions is about 8%.
These results confirm the assertive made based on the standard deviations presented in Table 4, i.e., when any kind of manual process is applied on the surface, the slope of failure probability can be dramatically reduced. Finally, Table 5 presents an example of surface roughness measurements, extracted from roughness profiles used to calculate the width of the worn tracks. Table 5 shows that the increase on the peaks fraction was up to twice, for the polished specimens, comparing outside and inside the worn track. This increase was expected, since the initial surface condition is limited to the first cycles [12] and the trend is an increase of surface roughness during the wear process. On the other hand, the ground specimens presented a
Table 4 Width of the worn track and its standard deviation (%) for each measurement position
Table 5 Rpk roughness parameter values for each testing condition, outside and inside the worn track
Test condition
Test condition
P3.7 G3.0 P3.0
Width of the worn track (m) and standard deviation (%) 0◦
90◦
180◦
270◦
726 ± 4 551 ± 6 605 ± 7
756 ± 7 517 ± 5 517 ± 7
644 ± 7 508 ± 4 534 ± 6
628 ± 8 540 ± 3 620 ± 10
P3.7 G3.0 P3.0
Rpk parameter (m) Outside wear track
Inside wear track
0.03 0.03 0.02
0.04 0.02 0.03
668
C. Brunetti et al. / Wear 263 (2007) 663–668
reduction in Rpk values after RCF tests. It is remarkable that the values of Rpk before and after RCF tests are not very different, considering two levels of contact pressure. 4. Conclusions Tests performed with austempered ductile iron under a lubricated ball-on-flat system allows for the following conclusion: • The specimen preparation has a pronounced effect on endurance lifetime. A poorly controlled grinding process, or even a manual polishing, is not recommended in order to obtain low values of deviation. • Grinding process exposed subsurface graphite nodules with two morphologies: cracked and partial. These morphologies act as stress raisers, reducing the endurance lifetime. • Endurance lifetime of ground specimens is smaller than that observed for polished ones. The reduction found in this study agrees with literature data. Acknowledgments C. Brunetti and M.V. Leite are very grateful to CAPES by the financial scholarship. The authors acknowledge to Araucaria Foundation for the financial support through project 018/04 and to FUNDICAO TUPY LTDA by the supplying of ductile iron specimens.
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