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Rolling contact fatigue tests of 52100 bearing steel using a modified NASA ball test rig
Wear, 98 (1984)
199
- 210
ROLLING CONTACT USING A MODIFIED
199
FATIGUE TESTS OF 52100 NASA BALL TEST RIG
BEARING
STEEL
C. A. STICKELS Ford
Motor
(Received
Company, September
2000 14,1984;
Rotunda
Drive,
accepted
Dearborn,
November
MI 48121-2053
(U.S.A.)
1, 1984)
Summary A new test was devised in order to obtain a better measure of the rolling contact fatigue lives of experimental materials. This test employs a NASA five-ball fatigue test rig, modified to accept a conical-ended test specimen in place of the upper ball. With this equipment, the fatigue lives of 52100 bearing steel given three different heat treatments were determined. While the hardnesses of all three test lots were nearly the same, the two lots characterized by high retained austenite content and fine prior austenite grains were significantly superior in fatigue resistance to steel heat treated in the conventional manner.
1. Introduction In earlier experiments [l, 21 devised to identify heat treatments for enhancing the resistance to rolling contact fatigue of 52100 bearing steel, fatigue lives of samples were measured using an RCF test rig of the type described by Barnburger [3]. In this machine a cylindrical test piece 9.53 mm (0.375 in) in diameter is loaded by two large rollers, 190 mm (7.5 in) in diameter, made of M-50 steel. Because many tests, typically eight, can be run on a single simply prepared sample, this test is attractive for evaluating experimental materials. Its primary disadvantage is that the specimen life is found to be a function of the surface finish on the large rollers. Over a period of time, as specimens are run on the same rollers, lives tend to increase as the rollers become more highly polished. There is probably also some flattening of the rollers along the line of contact. To circumvent this effect it has been our practice (1) to “condition” freshly ground rollers by running several tests on “standard” pieces until lives are obtained which are in reasonable agreement with earlier tests on the standards and then (2) to run tests alternately among the lots of steel samples making up a test batch. With this procedure valid comparisons of fatigue life between lots of steel within a batch that are tested over the same time period can be made. However, valid comparisons cannot be made between the lives of 0043-1648/84/$3.00
@ Elsevier
Sequoia/Printed
in The Netherlands
200
samples from two different batches, i.e. samples that were not tested alternately over the same time period. To overcome some of the sho~comings of the RCF test described above, a NASA five-ball tester [4] was modified to accept a test specimen with conical ends in place of the upper ball. A similar test using a modified four-ball lubricant tester has been described by Colcord and Kildsig [ 51. The test piece is run against four bearing balls which are replaced after each test. Since balls are manufactured in large batches, and since little variation is expected in the size, surface finish and mechanical properties of the balls within a batch, it is anticipated that by obtaining a suitably large number of balls from a single batch many hundreds of tests can be run without concern for systematic changes in the surface finish of the balls. Furthermore, because balls have more rotational freedom than rollers, a well-defined wear track does not develop on the balls so there is little chance of progressive flattening at the point of contact as a test proceeds. The NASA test rig is dead weight loaded while the RCF test rig is loaded by tightening a turnbuckle connecting the two rollers. If plastic deformation of the test piece occurs during a test, the load on the specimen does not change in the NASA test rig but some load relaxation is possible in the RCF test rig. This is not likely to be a significant difference in the tests, however. From our measurements it is estimated that the stiffness of the load relaxation due to plastic RCF rig is about 0.23 kg pm -‘; therefore, deformation of more than 1% is unlikely. The modified ball test rig was used to measure the rolling contact fatigue lives of three sets of 52100 steel samples given different heat treatments. One of these sets was spheroidize-annealed steel, austenitized at 840 “C, oil quenched and tempered; this is the conventional heat treatment for bearing steel and serves as a baseline. The other two sets were treated initially to develop a pearlitic microstructure, then austenitized at 900 ‘C, oil quenched and tempered. Previous work [ 1, 2 J had shown that the latter type of heat treatment improves rolling contact fatigue life.
2. Experimental
procedure
SAE 52100 bearing steel was obtained in the form of 19.5 mm (0.77 in) diameter spheroidize-annealed bar. The composition of the steel is given in Table 1. Test specimens (Fig. 1) were rough machined, heat treated and then ground to final dimensions. Eight specimens were given each of the heat treatments listed in Table 2. Specimens were enclosed in Vycor capsules, back filled with a partial pressure of argon, for all the high temperature treatments except as noted. Treatment A in Table 2 is similar to the usual commercial practice for heat treating bearing components. The aim in treatment B is to first dissolve all the carbides in austenite and then to convert the austenite to fine pearlite. Short time austenitization of the pearlite at 900 “C takes more carbon into
201 TABLE
1
Steel composition C Mn P s CU Ni
in weight per cent 1.09 0.36 0.015 0.020 0.13 0.07
Cr V MO Al Sn Ti, Zr, Nb, Pb
1.36 < 0.005 0.01 0.010 < 0.010 < 0.005
Fig. 1. Rolling contact fatigue specimen. The specimen slips into a collet (diametral clearance, about 0.13 mm) as shown in Fig. 2. A set screw, which abuts against a flat ground on the specimen, is used to retain the specimen in the collet during handling. Grooves near the conical ends are used to extract the specimen from the collet.
solution than would occur during conventional practice, leaving the remaining carbon in the form of finely divided carbides [l]. Treatment C is similar to treatment B except that the first austenitizing temperature is not sufficiently high to dissolve all the carbides. Treatments B and C have the effect of simultaneously increasing the retained austenite content while reducing the prior austenite grain size. The undissolved carbides remaining after treatment B are derived from pearlite and should be fine and well dispersed. After treatment C, however, a duplex carbide size distribution is expected; coarse carbides, which were undissolved at 955 “C, will be present as well as fine carbides derived from pearlite. The motivation for comparing treatments B and C is practical. Heat treatments are more costly, and control of surface carbon content is more difficult, the higher the treatment temperature. Dropping the initial treatment temperature from 1040 to 955 “C would, therefore, have commercial advantages. Fatigue life was measured at a nominal hertzian stress of 5026 MPa (729 klbf in-*) in machines which were originally designed for testing bearing balls (NASA five-ball fatigue tester [ 41). The specimen makes contact at an angle of 30” with four 13.5 mm (17/32 in) diameter grade 25 balls which are held in position in a bearing race by a bronze retainer (Fig. 2). Stauffer Jet II synthetic aviation lubricant (MIL-L-23699C) was sprayed as a mist onto the balls during testing. One test was run on each conical end of the specimen. Five specimens from each lot were reground on the conical ends (removing material to a depth of at least 0.4 mm) for additional tests. The balls used for all the tests were from a single batch and have the same nominal diameter within approximately 1 pm. After each test the four balls were replaced. Bearing races and retainers were replaced after four to six tests, depending on a visual estimate of whether or not they had
202 TABLE
2
Heat treatments
Lot
and microstructural
properties
Heat treatment
Average hardness WRCI
Per cent retained austenite
Austenife grain size (ASTM
number)
A
30 min at 840 “C, quench in 55 “C oil, temper for 1 h at 175 “C
63.9
11
8.5
B
30 min at 1040 ‘C, quench in salt at 650 ‘C, break capsule; hold 30 min, air cool; 15 min at 900 “C in salt, quench in 55 “C oil, temper for 1 h at 175 “C
63.6
24
11.5
- 12
C
30 min at 955 ‘C, quench in salt at 650 “C, break capsule, hold for 30 min, air coof; 15 min at 900 “C in salt, quench in 55 “C oil, temper for I h at 175 “C
63.1
25
11.5
- 12
-9
.3 -4
Fig. 2. Geometrical loaded against four is not shown.
arrangement during testing. The specimen bails 3 which ride in a 30” angular contact
1, held in a collet 2, is race 4. The ball retainer
sustained damage. An accelerometer mounted on the lever arm through which the load was applied was used to detect the onset of spalling. For a majority of the tests the spall which ended the test was less than 1 mm in diameter; the largest spa11for which data were used was about 3 mm in its greatest dimension. The prior austenite grain size was measured after etching in an aqueous solution of picric acid and HCI [6]. The retained austenite content of samples was determined by X-ray diffraction.
3. Results 16 tests were run on the eight samples from each lot, then five samples from each lot were re-ground and retested. The measured lives on
203
I
I
2
5 LIFE,
Fig.
I111111
3. Comparison
I IO
I
IllIlL
20
50
100
I-IRS. BEFORE REGRIND
between
the measured
fatigue
lives of specimens
before
and after
re-grind.
each wear surface were compared before and after re-grinding (Fig. 3) but no correlation was found between these values. The surface finish on the conical ends of six specimens from each lot was measured using a Gould Surfanalyzer employing a stylus with a trip radius of 13 pm (0.0005 in). The majority of the specimens had an arithmetic average (AA) roughness of 0.2 - 0.38 I.trn (8 - 15 pin), with a total range of 0.1 - 0.53 pm. Although all specimens were finish ground at the same time with the same procedures, specimens from lot A had an AA finish which was, on average, 0.13 pm coarser than specimens from lot B. The finish on specimens from lot C was similar to that from lot B but slightly coarser on average. The AA surface finish measured after re-grinding was, on average, 0.04 I.trn greater than the original finish. The rolling contact fatigue lives of the three lots of specimens are shown in Figs. 4 - 6 on Weibull plots. 26 tests are recorded for lots A and C and 25 for lot B. Failure of the lubricant supply caused one test on lot B to be discarded. The estimated lives for 10% and 50% failure rates and the 90% confidence limits on these values are summarized in Table 3. After testing, one specimen from each lot was sectioned for microstructural examination and hardness measurements. Values for hardness, prior austenite grain size and retained austenite content are given in Table 2 and representative scanning electron photomicrographs are shown in Fig. 7. The microstructure of lot B was not entirely free of carbides greater than 1 pm in diameter as was intended. This is a consequence of alloy segregation; the large carbides remaining are in bands high in alloy content. Large carbides were more numerous in lot C than in lot B as expected, and all the carbides were relatively large in lot A. In both lot B and lot C there are discontinuous films of proeutectoid carbide at some prior austenite grain
204
2
4
6810 LIFE,
20
40
60601
3
HRS
Fig. 4. Cumulative frequency of fatigue lives of specimens testing represents 1.89 X lo6 stress cycles.
95
from lot A. Each hour of
205
. 95
-
90
-
60
-
70
-
60
-
50
a k
30-
it IO
-
5-
.7
I
4
2
6810 LIFE,
20
40
6080100
HRS
Fig. 6. Cumulative frequency of fatigue testing represents 1.89 x lo6 stress cycles.
TABLE
from
lot C. Each
hour
of
3
Rolling contact Lot
lives of specimens
fatigue
life” (millions
Life for 10% faihes Lower
Median
of stress cycles) Life for 50% failures
Upper
Lower
Median
Upper
Weibull slope
Normalized B50 life
A
0.61
1.69
4.68
7.09
11.18
17.65
1.00
0.47
B
1.68
4.30
11.02
15.61
23.77
36.21
1.10
1.00
C
1.58
4.57
13.26
20.44
32.90
52.97
0.96
1.38
“Computed
using the methods
described
in ref. 7.
boundaries. These films were probably produced during the salt quench. A transformation temperature lower than 650 “C and/or more rapid quenching would tend to minimize formation of these films. From tapered sections at the wear track of specimens from each lot that had fatigue lives of 18 - 19 h, it was noted that the depth of the wear track was about 5 pm and a dark-etching region extended about 0.3’7 - 0.40 mm below the wear track for all specimens.
(a)
Fig. 7. Scanning electron (b) lot B and (c) lot C.
photomicrographs
4. Discussion
4.1. S~~~~stical
statistical procedures were employed to test whether or not these are correct. Using the ratio tests described by Johnson [7] it can be shown that the mean lives of samples from lots A and B are different at a 99% confidence level. The BlO lives of lots A and B, however, can only be said to differ at an 84% level of The mean and BlO ratio tests applied to lots A and C give the same results. The mean lives of samples from lots B and C can be said to differ at a 90% level of but the B10 lives of these lots differ at a confidence level of only 5355, indicating that no difference exists in these values.
207
McCool [S] has developed a procedure for estimating Weibull parameters based on the method of maximum likelihood. Using this approach one can show that, at better than a 90% level of confidence, the three sets of data have the same Weibull slope (PI,, in McCool’s notation, is 0.913). McCool proposes two tests to determine whether or not the Weibull scale parameters ?& differ. Applying the first of these, the shape parameter ratio test, to the data yields a value of 1.049 which, after comparing with the critical values listed in Table 5 of ref. 8, allows one to reject the hypothesis that the scale parameters of lots A, B and C are identical at a confidence level of 90%. The second test, the likelihood ratio test, allows one to reject the same hypothesis at a confidence level of 95%. The “smallest significant ratio” test is applied to decide which of the lots differ appreciably. The results of this test show (1) at all percentiles, lots A and C are different at a 95% confidence level, (2) lots A and B just fail to meet the criterion for difference at a 90% level of confidence and (3) lots B and C fail to meet the criterion for difference at a 90% confidence level by a wide margin. The likelihood ratio test applied to lots A and B alone indicates that they differ at a 90% confidence level. In summary, the lives of samples from lots A and C differ enough so that with a high degree of confidence we can be certain that they represent two populations. For lots B and C, only Johnson’s mean life ratio test suggests that the lots differ. The other tests indicate that the differences between B and C are not significant. Finally, the tests for significant difference between lots A and B give mixed results. On balance, we conclude that lots A and B differ, but the statistical confidence attached to the difference is less than that associated with the difference between lots A and C. 4.2. Comparison The results obtained earlier each of the lots
with RCF tests summarized in Tables 2 and 3 can be compared with results with the RCF test rig [2,9] (Table 4). 16 tests were run on in Table 4. Lots Al, Bl, Cl and Dl were run as one batch
TABLE Fatigue
stress cycles)
Lot
BfO life
Al
2.02 3.12
Cl Dl
6.12 5.98
1.81 2.44
F2 G2
350
4.14 4.78
2.49 3.59
8.05
life
We&u11
Normalized B50
2.56
0.67 1.02
1.77 2.56
0.63 0.86
2.82 2.37
1.15 1.68
208 TABLE
5
Heat treatments
Lot
given to RCF specimens
Pretreatment
Austenitizing treatment
Temper
Per cent retained austenite
Hardness WRC)
Al
None
30 min, 843 “C
lh,182”C
9
62.1
Bl
4 h at 1093 ‘C, salt quench at 650 ‘C, 30 min
30 min, 843 “C
lh,182”C
12
63.6
Cl
30 min at 1040 “C, salt quench at 650 ‘C, 30 min
30 min, 843 “C
1 h, 182 “C
14
63.5
Dl
As Cl, then spheroidize anneal
30 min, 843 “C
lh,182”C
6
61.2
E2
30 min at 1040 “C, salt quench at 650 “C, 30 min
30 min, 815 “C
1 h, 177 “C
10
63.2
F2
Same as E2
30 min, 843 “C
1 h, 177 “C
12
64.2
G2
Same as E2
30 min, 870 “C
1 h, 177 “C
16.5
64.5
H2
Same as E2
30 min, 900 “C
1 h, 177 “C
20
63.5
and lots E2, F2, G2 and H2 were part of a second batch. The nominal hertzian stress was also 5026 MPa (729 klbf in2) in these tests. The heat treatments given each lot are summarized in Table 5. The Weibull slopes are higher for the RCF tests than for the tests with angular contact specimens. Trends in B50 life as a function of the processing changes are reasonably systematic, but trends in BlO life are not. From the heat treatments given to the first batch, we expect lots Al and Dl to be identical and lots Bl and Cl to be nearly identical, which is confirmed by the B50 values but is uncertain based on BlO values. The second batch of data (E2 - H2) shows a progressive decrease in Weibull slope as the B50 life increases and, as a consequence, the trend in BlO life is much less clear. (Table 1 in ref. 3 shows, to a more marked degree, the tendency for Weibull slope to decrease as B50 life increases in this test. The Weibull slope in five-ball fatigue test results, in contrast, does not appear to drift systematically with B50 life; see Table 3 in ref. 4.) The statistical uncertainty in B10 values is, of course, much greater than that in the B50 values. From the processing we expect the lives of lots Cl and F2 to be nearly the same because there is only a minor difference in tempering temperature. The ratio of the lives of specimens H2 and Al should be very nearly the same as the ratio of the lives of lots B and A in Table 3. If we normalize the data in Table 4, assuming that lots F2 and Cl are identical, the ratio of B50 lives of lots Al and H2 is 0.40; this compares with 0.47 for the ratio of the B50 lives of lots A and B from Table 3. Thus at the B50 level the two types of tests are reasonably consistent. By a similar normalization procedure the ratio of the BlO lives of lots Al and H2 is 0.67, while the ratio of the BlO lives of lots A and B is 0.39. In view of the differences in the
209
two types of tests and the scatter inherent in rolling contact fatigue testing, the degree of consistency between the two sets of test results is about as good as can be expected. 4.3. Effects of heat treatment The results of both the modified ball test and the RCF test show that the nature of the microstructure affects fatigue life even if the hardness is effectively constant. Heat treatments designed to produce dispersions of fine primary carbides [l] produce as a consequence higher levels of retained austenite and small prior austenite grains. Higher levels of retained austenite, provided that they can be achieved without an appreciable loss in hardness, appear to enhance rolling contact fatigue life. These factors account for the relatively low fatigue life of specimens from lot A compared with the other two lots (Table 3). Specimens from lots B and C are similar in the amount of retained austenite, the prior austenite grain size and hardness. The main microstructural difference between these samples is the greater number of large carbides in lot C. Since the rolling contact fatigue lives of lots B and C do not differ significantly, one must conclude that carbide size per se does not have a major effect on fatigue life. The individual effects of retained austenite content and prior austenite grain size on fatigue life are difficult to isolate. The sequence E2 - H2 of RCF specimens, all of which should have been fine grained (grain size was not measured, however) indicate that the B50 life doubles with a doubling of the retained austenite content. Whether or not this would also be true if the grain size was coarser has yet to be determined.
5. Summarizing
remarks
and conclusions
(1) A NASA five-ball test rig was modified to run conical-ended test pieces in order to measure the rolling contact fatigue life of experimental materials. Results of tests on three lots of 52100 steel with different heat treatments are consistent with test results obtained previously on an RCF test rig. (2) Test results using angular contact specimens have a broader dispersion (lower Weibull slope) than results obtained in RCF testing. However, the Weibull slope is nearly the same for all test lots, while lots tested on the RCF test rig often show a decreasing slope with increasing B50 life. (3) Specimens processed to obtain fine dispersions of primary carbides, then austenitized at higher than normal temperatures (900 compared with 843 “C) to increase the retained austenite content, have improved fatigue lives. (4) In the carbide-refining pretreatment it is not necessary to dissolve all the carbide in order to enhance the fatigue life.
210
Acknowledgments The author would like to thank Dr. L. L. Ting for reading the manuscript and for helpful discussions during the course of this work and Mr. W. G. Sosnitza for carrying out the contact fatigue experiments.
References 1 C. A. Stickels, Metall. Trans., 5 (1974) 865 - 874. 2 C. A. Stickels, A. T. Anderson and A. M. Janotik, U.S. Patent 4,023,988, May 17, 1977. 3 E. N. Bamberger and J. C. Clark, Roiling Contact Fatigue Testing of Bearing Steels, ASTM Spec. Tech. Publ. 771, 1982, pp. 85 - 106 (ASTM, Philadelphia, PA). 4 E. B. Zaretsky, R. J. Parker and W. J. Anderson, Rolling Contact Fatigue Testing of Bearing Steels, ASTM Spec. Tech. Publ. 771, 1982, pp. 5 - 45 (ASTM, Philadelphia, PA). 5 P. L. Colcord and J. R. Kildsig, Met. Eng. Q., 10 (1) (1970) 40 - 44. 6 J. v. d. Sanden, SKF Engineering and Research Centre, Nieuwegein, personal communication, 1984. 7 L. G. Johnson, The Statistical Treatment of Fatigue Experiments, Elsevier, New York, 1964. 8 J. I. McCool, Rolling Contact Fatigue Testing of Bearing Steels, ASTM Spec. Tech. Publ. 771, 1982, pp. 293 - 319 (ASTM, Philadelphia, PA). 9 C. A. Stickels and A. M. Janotik, unpublished results, 1975, 1976.
199
- 210
ROLLING CONTACT USING A MODIFIED
199
FATIGUE TESTS OF 52100 NASA BALL TEST RIG
BEARING
STEEL
C. A. STICKELS Ford
Motor
(Received
Company, September
2000 14,1984;
Rotunda
Drive,
accepted
Dearborn,
November
MI 48121-2053
(U.S.A.)
1, 1984)
Summary A new test was devised in order to obtain a better measure of the rolling contact fatigue lives of experimental materials. This test employs a NASA five-ball fatigue test rig, modified to accept a conical-ended test specimen in place of the upper ball. With this equipment, the fatigue lives of 52100 bearing steel given three different heat treatments were determined. While the hardnesses of all three test lots were nearly the same, the two lots characterized by high retained austenite content and fine prior austenite grains were significantly superior in fatigue resistance to steel heat treated in the conventional manner.
1. Introduction In earlier experiments [l, 21 devised to identify heat treatments for enhancing the resistance to rolling contact fatigue of 52100 bearing steel, fatigue lives of samples were measured using an RCF test rig of the type described by Barnburger [3]. In this machine a cylindrical test piece 9.53 mm (0.375 in) in diameter is loaded by two large rollers, 190 mm (7.5 in) in diameter, made of M-50 steel. Because many tests, typically eight, can be run on a single simply prepared sample, this test is attractive for evaluating experimental materials. Its primary disadvantage is that the specimen life is found to be a function of the surface finish on the large rollers. Over a period of time, as specimens are run on the same rollers, lives tend to increase as the rollers become more highly polished. There is probably also some flattening of the rollers along the line of contact. To circumvent this effect it has been our practice (1) to “condition” freshly ground rollers by running several tests on “standard” pieces until lives are obtained which are in reasonable agreement with earlier tests on the standards and then (2) to run tests alternately among the lots of steel samples making up a test batch. With this procedure valid comparisons of fatigue life between lots of steel within a batch that are tested over the same time period can be made. However, valid comparisons cannot be made between the lives of 0043-1648/84/$3.00
@ Elsevier
Sequoia/Printed
in The Netherlands
200
samples from two different batches, i.e. samples that were not tested alternately over the same time period. To overcome some of the sho~comings of the RCF test described above, a NASA five-ball tester [4] was modified to accept a test specimen with conical ends in place of the upper ball. A similar test using a modified four-ball lubricant tester has been described by Colcord and Kildsig [ 51. The test piece is run against four bearing balls which are replaced after each test. Since balls are manufactured in large batches, and since little variation is expected in the size, surface finish and mechanical properties of the balls within a batch, it is anticipated that by obtaining a suitably large number of balls from a single batch many hundreds of tests can be run without concern for systematic changes in the surface finish of the balls. Furthermore, because balls have more rotational freedom than rollers, a well-defined wear track does not develop on the balls so there is little chance of progressive flattening at the point of contact as a test proceeds. The NASA test rig is dead weight loaded while the RCF test rig is loaded by tightening a turnbuckle connecting the two rollers. If plastic deformation of the test piece occurs during a test, the load on the specimen does not change in the NASA test rig but some load relaxation is possible in the RCF test rig. This is not likely to be a significant difference in the tests, however. From our measurements it is estimated that the stiffness of the load relaxation due to plastic RCF rig is about 0.23 kg pm -‘; therefore, deformation of more than 1% is unlikely. The modified ball test rig was used to measure the rolling contact fatigue lives of three sets of 52100 steel samples given different heat treatments. One of these sets was spheroidize-annealed steel, austenitized at 840 “C, oil quenched and tempered; this is the conventional heat treatment for bearing steel and serves as a baseline. The other two sets were treated initially to develop a pearlitic microstructure, then austenitized at 900 ‘C, oil quenched and tempered. Previous work [ 1, 2 J had shown that the latter type of heat treatment improves rolling contact fatigue life.
2. Experimental
procedure
SAE 52100 bearing steel was obtained in the form of 19.5 mm (0.77 in) diameter spheroidize-annealed bar. The composition of the steel is given in Table 1. Test specimens (Fig. 1) were rough machined, heat treated and then ground to final dimensions. Eight specimens were given each of the heat treatments listed in Table 2. Specimens were enclosed in Vycor capsules, back filled with a partial pressure of argon, for all the high temperature treatments except as noted. Treatment A in Table 2 is similar to the usual commercial practice for heat treating bearing components. The aim in treatment B is to first dissolve all the carbides in austenite and then to convert the austenite to fine pearlite. Short time austenitization of the pearlite at 900 “C takes more carbon into
201 TABLE
1
Steel composition C Mn P s CU Ni
in weight per cent 1.09 0.36 0.015 0.020 0.13 0.07
Cr V MO Al Sn Ti, Zr, Nb, Pb
1.36 < 0.005 0.01 0.010 < 0.010 < 0.005
Fig. 1. Rolling contact fatigue specimen. The specimen slips into a collet (diametral clearance, about 0.13 mm) as shown in Fig. 2. A set screw, which abuts against a flat ground on the specimen, is used to retain the specimen in the collet during handling. Grooves near the conical ends are used to extract the specimen from the collet.
solution than would occur during conventional practice, leaving the remaining carbon in the form of finely divided carbides [l]. Treatment C is similar to treatment B except that the first austenitizing temperature is not sufficiently high to dissolve all the carbides. Treatments B and C have the effect of simultaneously increasing the retained austenite content while reducing the prior austenite grain size. The undissolved carbides remaining after treatment B are derived from pearlite and should be fine and well dispersed. After treatment C, however, a duplex carbide size distribution is expected; coarse carbides, which were undissolved at 955 “C, will be present as well as fine carbides derived from pearlite. The motivation for comparing treatments B and C is practical. Heat treatments are more costly, and control of surface carbon content is more difficult, the higher the treatment temperature. Dropping the initial treatment temperature from 1040 to 955 “C would, therefore, have commercial advantages. Fatigue life was measured at a nominal hertzian stress of 5026 MPa (729 klbf in-*) in machines which were originally designed for testing bearing balls (NASA five-ball fatigue tester [ 41). The specimen makes contact at an angle of 30” with four 13.5 mm (17/32 in) diameter grade 25 balls which are held in position in a bearing race by a bronze retainer (Fig. 2). Stauffer Jet II synthetic aviation lubricant (MIL-L-23699C) was sprayed as a mist onto the balls during testing. One test was run on each conical end of the specimen. Five specimens from each lot were reground on the conical ends (removing material to a depth of at least 0.4 mm) for additional tests. The balls used for all the tests were from a single batch and have the same nominal diameter within approximately 1 pm. After each test the four balls were replaced. Bearing races and retainers were replaced after four to six tests, depending on a visual estimate of whether or not they had
202 TABLE
2
Heat treatments
Lot
and microstructural
properties
Heat treatment
Average hardness WRCI
Per cent retained austenite
Austenife grain size (ASTM
number)
A
30 min at 840 “C, quench in 55 “C oil, temper for 1 h at 175 “C
63.9
11
8.5
B
30 min at 1040 ‘C, quench in salt at 650 ‘C, break capsule; hold 30 min, air cool; 15 min at 900 “C in salt, quench in 55 “C oil, temper for 1 h at 175 “C
63.6
24
11.5
- 12
C
30 min at 955 ‘C, quench in salt at 650 “C, break capsule, hold for 30 min, air coof; 15 min at 900 “C in salt, quench in 55 “C oil, temper for I h at 175 “C
63.1
25
11.5
- 12
-9
.3 -4
Fig. 2. Geometrical loaded against four is not shown.
arrangement during testing. The specimen bails 3 which ride in a 30” angular contact
1, held in a collet 2, is race 4. The ball retainer
sustained damage. An accelerometer mounted on the lever arm through which the load was applied was used to detect the onset of spalling. For a majority of the tests the spall which ended the test was less than 1 mm in diameter; the largest spa11for which data were used was about 3 mm in its greatest dimension. The prior austenite grain size was measured after etching in an aqueous solution of picric acid and HCI [6]. The retained austenite content of samples was determined by X-ray diffraction.
3. Results 16 tests were run on the eight samples from each lot, then five samples from each lot were re-ground and retested. The measured lives on
203
I
I
2
5 LIFE,
Fig.
I111111
3. Comparison
I IO
I
IllIlL
20
50
100
I-IRS. BEFORE REGRIND
between
the measured
fatigue
lives of specimens
before
and after
re-grind.
each wear surface were compared before and after re-grinding (Fig. 3) but no correlation was found between these values. The surface finish on the conical ends of six specimens from each lot was measured using a Gould Surfanalyzer employing a stylus with a trip radius of 13 pm (0.0005 in). The majority of the specimens had an arithmetic average (AA) roughness of 0.2 - 0.38 I.trn (8 - 15 pin), with a total range of 0.1 - 0.53 pm. Although all specimens were finish ground at the same time with the same procedures, specimens from lot A had an AA finish which was, on average, 0.13 pm coarser than specimens from lot B. The finish on specimens from lot C was similar to that from lot B but slightly coarser on average. The AA surface finish measured after re-grinding was, on average, 0.04 I.trn greater than the original finish. The rolling contact fatigue lives of the three lots of specimens are shown in Figs. 4 - 6 on Weibull plots. 26 tests are recorded for lots A and C and 25 for lot B. Failure of the lubricant supply caused one test on lot B to be discarded. The estimated lives for 10% and 50% failure rates and the 90% confidence limits on these values are summarized in Table 3. After testing, one specimen from each lot was sectioned for microstructural examination and hardness measurements. Values for hardness, prior austenite grain size and retained austenite content are given in Table 2 and representative scanning electron photomicrographs are shown in Fig. 7. The microstructure of lot B was not entirely free of carbides greater than 1 pm in diameter as was intended. This is a consequence of alloy segregation; the large carbides remaining are in bands high in alloy content. Large carbides were more numerous in lot C than in lot B as expected, and all the carbides were relatively large in lot A. In both lot B and lot C there are discontinuous films of proeutectoid carbide at some prior austenite grain
204
2
4
6810 LIFE,
20
40
60601
3
HRS
Fig. 4. Cumulative frequency of fatigue lives of specimens testing represents 1.89 X lo6 stress cycles.
95
from lot A. Each hour of
205
. 95
-
90
-
60
-
70
-
60
-
50
a k
30-
it IO
-
5-
.7
I
4
2
6810 LIFE,
20
40
6080100
HRS
Fig. 6. Cumulative frequency of fatigue testing represents 1.89 x lo6 stress cycles.
TABLE
from
lot C. Each
hour
of
3
Rolling contact Lot
lives of specimens
fatigue
life” (millions
Life for 10% faihes Lower
Median
of stress cycles) Life for 50% failures
Upper
Lower
Median
Upper
Weibull slope
Normalized B50 life
A
0.61
1.69
4.68
7.09
11.18
17.65
1.00
0.47
B
1.68
4.30
11.02
15.61
23.77
36.21
1.10
1.00
C
1.58
4.57
13.26
20.44
32.90
52.97
0.96
1.38
“Computed
using the methods
described
in ref. 7.
boundaries. These films were probably produced during the salt quench. A transformation temperature lower than 650 “C and/or more rapid quenching would tend to minimize formation of these films. From tapered sections at the wear track of specimens from each lot that had fatigue lives of 18 - 19 h, it was noted that the depth of the wear track was about 5 pm and a dark-etching region extended about 0.3’7 - 0.40 mm below the wear track for all specimens.
(a)
Fig. 7. Scanning electron (b) lot B and (c) lot C.
photomicrographs
4. Discussion
4.1. S~~~~stical
statistical procedures were employed to test whether or not these are correct. Using the ratio tests described by Johnson [7] it can be shown that the mean lives of samples from lots A and B are different at a 99% confidence level. The BlO lives of lots A and B, however, can only be said to differ at an 84% level of The mean and BlO ratio tests applied to lots A and C give the same results. The mean lives of samples from lots B and C can be said to differ at a 90% level of but the B10 lives of these lots differ at a confidence level of only 5355, indicating that no difference exists in these values.
207
McCool [S] has developed a procedure for estimating Weibull parameters based on the method of maximum likelihood. Using this approach one can show that, at better than a 90% level of confidence, the three sets of data have the same Weibull slope (PI,, in McCool’s notation, is 0.913). McCool proposes two tests to determine whether or not the Weibull scale parameters ?& differ. Applying the first of these, the shape parameter ratio test, to the data yields a value of 1.049 which, after comparing with the critical values listed in Table 5 of ref. 8, allows one to reject the hypothesis that the scale parameters of lots A, B and C are identical at a confidence level of 90%. The second test, the likelihood ratio test, allows one to reject the same hypothesis at a confidence level of 95%. The “smallest significant ratio” test is applied to decide which of the lots differ appreciably. The results of this test show (1) at all percentiles, lots A and C are different at a 95% confidence level, (2) lots A and B just fail to meet the criterion for difference at a 90% level of confidence and (3) lots B and C fail to meet the criterion for difference at a 90% confidence level by a wide margin. The likelihood ratio test applied to lots A and B alone indicates that they differ at a 90% confidence level. In summary, the lives of samples from lots A and C differ enough so that with a high degree of confidence we can be certain that they represent two populations. For lots B and C, only Johnson’s mean life ratio test suggests that the lots differ. The other tests indicate that the differences between B and C are not significant. Finally, the tests for significant difference between lots A and B give mixed results. On balance, we conclude that lots A and B differ, but the statistical confidence attached to the difference is less than that associated with the difference between lots A and C. 4.2. Comparison The results obtained earlier each of the lots
with RCF tests summarized in Tables 2 and 3 can be compared with results with the RCF test rig [2,9] (Table 4). 16 tests were run on in Table 4. Lots Al, Bl, Cl and Dl were run as one batch
TABLE Fatigue
stress cycles)
Lot
BfO life
Al
2.02 3.12
Cl Dl
6.12 5.98
1.81 2.44
F2 G2
350
4.14 4.78
2.49 3.59
8.05
life
We&u11
Normalized B50
2.56
0.67 1.02
1.77 2.56
0.63 0.86
2.82 2.37
1.15 1.68
208 TABLE
5
Heat treatments
Lot
given to RCF specimens
Pretreatment
Austenitizing treatment
Temper
Per cent retained austenite
Hardness WRC)
Al
None
30 min, 843 “C
lh,182”C
9
62.1
Bl
4 h at 1093 ‘C, salt quench at 650 ‘C, 30 min
30 min, 843 “C
lh,182”C
12
63.6
Cl
30 min at 1040 “C, salt quench at 650 ‘C, 30 min
30 min, 843 “C
1 h, 182 “C
14
63.5
Dl
As Cl, then spheroidize anneal
30 min, 843 “C
lh,182”C
6
61.2
E2
30 min at 1040 “C, salt quench at 650 “C, 30 min
30 min, 815 “C
1 h, 177 “C
10
63.2
F2
Same as E2
30 min, 843 “C
1 h, 177 “C
12
64.2
G2
Same as E2
30 min, 870 “C
1 h, 177 “C
16.5
64.5
H2
Same as E2
30 min, 900 “C
1 h, 177 “C
20
63.5
and lots E2, F2, G2 and H2 were part of a second batch. The nominal hertzian stress was also 5026 MPa (729 klbf in2) in these tests. The heat treatments given each lot are summarized in Table 5. The Weibull slopes are higher for the RCF tests than for the tests with angular contact specimens. Trends in B50 life as a function of the processing changes are reasonably systematic, but trends in BlO life are not. From the heat treatments given to the first batch, we expect lots Al and Dl to be identical and lots Bl and Cl to be nearly identical, which is confirmed by the B50 values but is uncertain based on BlO values. The second batch of data (E2 - H2) shows a progressive decrease in Weibull slope as the B50 life increases and, as a consequence, the trend in BlO life is much less clear. (Table 1 in ref. 3 shows, to a more marked degree, the tendency for Weibull slope to decrease as B50 life increases in this test. The Weibull slope in five-ball fatigue test results, in contrast, does not appear to drift systematically with B50 life; see Table 3 in ref. 4.) The statistical uncertainty in B10 values is, of course, much greater than that in the B50 values. From the processing we expect the lives of lots Cl and F2 to be nearly the same because there is only a minor difference in tempering temperature. The ratio of the lives of specimens H2 and Al should be very nearly the same as the ratio of the lives of lots B and A in Table 3. If we normalize the data in Table 4, assuming that lots F2 and Cl are identical, the ratio of B50 lives of lots Al and H2 is 0.40; this compares with 0.47 for the ratio of the B50 lives of lots A and B from Table 3. Thus at the B50 level the two types of tests are reasonably consistent. By a similar normalization procedure the ratio of the BlO lives of lots Al and H2 is 0.67, while the ratio of the BlO lives of lots A and B is 0.39. In view of the differences in the
209
two types of tests and the scatter inherent in rolling contact fatigue testing, the degree of consistency between the two sets of test results is about as good as can be expected. 4.3. Effects of heat treatment The results of both the modified ball test and the RCF test show that the nature of the microstructure affects fatigue life even if the hardness is effectively constant. Heat treatments designed to produce dispersions of fine primary carbides [l] produce as a consequence higher levels of retained austenite and small prior austenite grains. Higher levels of retained austenite, provided that they can be achieved without an appreciable loss in hardness, appear to enhance rolling contact fatigue life. These factors account for the relatively low fatigue life of specimens from lot A compared with the other two lots (Table 3). Specimens from lots B and C are similar in the amount of retained austenite, the prior austenite grain size and hardness. The main microstructural difference between these samples is the greater number of large carbides in lot C. Since the rolling contact fatigue lives of lots B and C do not differ significantly, one must conclude that carbide size per se does not have a major effect on fatigue life. The individual effects of retained austenite content and prior austenite grain size on fatigue life are difficult to isolate. The sequence E2 - H2 of RCF specimens, all of which should have been fine grained (grain size was not measured, however) indicate that the B50 life doubles with a doubling of the retained austenite content. Whether or not this would also be true if the grain size was coarser has yet to be determined.
5. Summarizing
remarks
and conclusions
(1) A NASA five-ball test rig was modified to run conical-ended test pieces in order to measure the rolling contact fatigue life of experimental materials. Results of tests on three lots of 52100 steel with different heat treatments are consistent with test results obtained previously on an RCF test rig. (2) Test results using angular contact specimens have a broader dispersion (lower Weibull slope) than results obtained in RCF testing. However, the Weibull slope is nearly the same for all test lots, while lots tested on the RCF test rig often show a decreasing slope with increasing B50 life. (3) Specimens processed to obtain fine dispersions of primary carbides, then austenitized at higher than normal temperatures (900 compared with 843 “C) to increase the retained austenite content, have improved fatigue lives. (4) In the carbide-refining pretreatment it is not necessary to dissolve all the carbide in order to enhance the fatigue life.
210
Acknowledgments The author would like to thank Dr. L. L. Ting for reading the manuscript and for helpful discussions during the course of this work and Mr. W. G. Sosnitza for carrying out the contact fatigue experiments.
References 1 C. A. Stickels, Metall. Trans., 5 (1974) 865 - 874. 2 C. A. Stickels, A. T. Anderson and A. M. Janotik, U.S. Patent 4,023,988, May 17, 1977. 3 E. N. Bamberger and J. C. Clark, Roiling Contact Fatigue Testing of Bearing Steels, ASTM Spec. Tech. Publ. 771, 1982, pp. 85 - 106 (ASTM, Philadelphia, PA). 4 E. B. Zaretsky, R. J. Parker and W. J. Anderson, Rolling Contact Fatigue Testing of Bearing Steels, ASTM Spec. Tech. Publ. 771, 1982, pp. 5 - 45 (ASTM, Philadelphia, PA). 5 P. L. Colcord and J. R. Kildsig, Met. Eng. Q., 10 (1) (1970) 40 - 44. 6 J. v. d. Sanden, SKF Engineering and Research Centre, Nieuwegein, personal communication, 1984. 7 L. G. Johnson, The Statistical Treatment of Fatigue Experiments, Elsevier, New York, 1964. 8 J. I. McCool, Rolling Contact Fatigue Testing of Bearing Steels, ASTM Spec. Tech. Publ. 771, 1982, pp. 293 - 319 (ASTM, Philadelphia, PA). 9 C. A. Stickels and A. M. Janotik, unpublished results, 1975, 1976.