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The assessment of ball bearing steels of modified chemical composition
273
Wear, 46 (1978) 273 - 279 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
THE ASSESSMENT OF BALL CHEMICAL COMPOSITION*
BEARING
STEELS
OF MODIFIED
D. SCOTT and J. BLACKWELL National
Engineering
Laboratory,
East Kilbride,
Glasgow (Scotland)
(Received July 5,1977)
Summary With the development of cleaner ball bearing steels of improved properties and lower inclusion content, carbide content, an inherent feature of these steels, carbide size and carbide segregation have become of importance as large nondeforming carbides can fracture to initiate rolling contact fatigue and premature failure. A lower carbon and chromium type of steel, with increased silicon and manganese contents to provide matrix strengthening and to maintain hardness, offers a means of reducing carbide content and suppressing deleterious structural features such as carbide segregation, carbide stringers and carbide network. Under conditions of accelerated service simulation testing the steels of modified composition compared favourably in rolling contact fatigue resistance with steels of conventional compositions. The increased silicon content may also control deleterious interstitial nitrogen content by forming fine silicon nitride precipitates which were identified in microstructures produced by the optimum heat treatment procedure.
Introduction Technological progress makes more arduous the conditions under which many rolling mechanisms are required to operate and creates an ever increasing demand for improved materials of construction. Conventional En31 or 52100 (1.0% C, 1.5% Cr) type ball bearing steels have proved to be highly successful ball and roller bearing materials. It has been found that the steelmaking process can influence material properties [ 1 - 31. Vacuum treatments and other re-refining processes are beneficial [ 1 -61. The nature of non-metallic inclusions as well as their numbers, size and distribution can affect performance [ 1 -31. With the development of cleaner steels [ 4 - 61 of
*Paper presented at the International Conference on the Wear of Materials, St. Louis, MO., U.S.A., 26 - 28 April, 1977.
274
improved physical properties and lower inclusion content, carbide content, an inherent feature of En31 steel, carbide size and carbide segregation have become of importance as large non-deforming carbides can fracture to become a source of rolling contact fatigue initiation and premature failure. A lower carbon and chromium type of steel with increased silicon and manganese contents to provide matrix strengthening and to maintain hardness offers a method of reducing the carbide content and suppressing deleterious structural features such as carbide segregation, carbide stringers and carbide network. Samples of such modifiedcomposition ball bearing steels were assessed for rolling contact fatigue resistance by comparative testing with currently used ball bearing steels.
Method
of test
The ultimate assessment of materials is performance in practice, but as full scale testing is a long and expensive process accelerated service simulation testing has been used to screen materials. Rolling contact fatigue tests were carried out using National Engineering Laboratory (NEL) rolling fourball machines described elsewhere [ 7 - 121. With these machines, a 0.5 in diameter upper test ball held in a chuck drives three similar balls in a lower race, thus simulating the rolling and sliding experienced in angular contact ball bearings. The upper test ball takes the place of the inner race in a conventional ball bearing. The criterion of the test is the time taken from the start of the test till the appearance of the first failure pit in the upper test specimen. To test materials not available in ball form the upper ball may be replaced by a conical [ 131 or a ball-ended specimen [ 141 readily manufactured from a small amount of material. With a ball-ended specimen the kinematics of the system are essentially unaltered from the rolling four-ball setup which allows direct comparison between new materials and conventional balls. Use of a simple heater allows tests to be carried out at elevated temperature [ 151 . Although the loads used in the rolling four-ball test are in excess of those in practice, being chosen to reduce the time of test to a convenient magnitude, use of lower loads has confirmed the inverse cube relationship linking life with load of rolling bearings [ 12, 161. Also, in many instances good correlation between accelerated test results and practice has been found [ll, 16 - 181.
Results at ambient
temperature
Ball-ended specimens manufactured from bars of modified composition bearing steel (0.77% C, 1.0% Cr, 2.1% Si and 1.9% Mn) were heat treated according to the steel manufacturers’ recommendations. As material hardness [ 1,3,19] and thus heat treatment have been found to be important
recommended, O.Q. 810 ‘C, double recommended, O.Q. 820 OC, double recommended, O.Q. 820 ‘C, double recommended, O.Q. 840 ‘C, double ‘C, double temper 190 “C
lubricant, NEL reference mineral oil.
-
NEL reference balls, acid open hearth, En31 steel (0.97 C, 1.48 Cr, 0.37 Si, 0.35 Mn)
Load, 600 kg; speed, 1500 rev min-‘;
-
Manufacturers Manufacturers Manufacturers Manufacturers NEL O.Q. 820
Modified composition ball bearing steel (0.77 C, 1.0 Cr, 2.1 Si, 1.9 Mn)
NEL reference balls, basic electric arc, En31 (1.0% C, Cr) steel (0.99 C, 1.41 Cr, 0.28 Si, 0.33 Mn)
Heat treatment
Specimens
Summarized results of comparative rolling contact fatigue tests on ball bearing steels
TABLE 1
temper temper temper temper
130 130 150 150
“C “C “C “C
870
148
84
18 17 18 16 270
740 790 760 760 740
860
Mean life (min)
Hardness (HVlO)
276
some specimens were heat treated according to the heat treatment determined in NEL to give generally the maximum rolling contact fatigue life. Summarized results of rolling contact fatigue tests under conditions of total axial load of 600 kg (nominal maximum Hertzian stress of 500 tonf inW2) lubricated with NEL reference mineral oil are given in Table 1. For comparative purposes the results of similar tests on NEL reference En31 steel balls are also given. The mean life was determined from up to 30 tests depending upon the number of specimens available. From the results, under the conditions of test the rolling contact fatigue life of ball bearing steel of modified composition when heat treated according to NEL recommended practice was superior in rolling contact fatigue resistance to reference ball bearing steels considered to be typical of currently available conventional En31 steel bearing balls. The NEL heat treated modified composition steel had a fine uniform structure free from massive carbides and carbide network (Fig. 1) and the specimens failed by a single small pit in the bearing track (Fig. 2) in a similar manner to the reference En31 steel balls.
Fig. 1. Fine, uniform structure of modified magnification 525X. Fig. 2. Single failure pit. Magnification
composition
halt bearing steel (etched
Nitai).
56~.
Results at elevated temperature It has been shown that there is a trend towards improved rolling contact fatigue life with reduction of nitrogen content [20,21]. With high speed tool steels it has also been found that rolling contact fatigue resistance was improved when tested at 200 “C [ 22,231. This was particularly so with materials manufactured by the powder route [23,24] . From electron microscope examination together with microprobe analysis, X-ray energy analysis and electron diffraction f21,23] , it appears that under the conditions of elevated temperature heavily loaded test at 200 “C metahographic change occurs; a finely dispersed phase is precipitated which preliminary investigation by the extraction replica technique has indicated is a nitride, probably
277
silicon nitride. This would indicate a reduction in interstitial nitrogen content and may account for the enhanced performance at 200 “C compared with that at ambient temperature. The modified-composition ball bearing steel contains 2.1% silicon. The solubility of nitrogen in iron is decreased by the addition of silicon [ 251. Also, in the presence of silicon the activity coefficient of nitrogen increases with ageing at 250 “C and the driving force for precipitation is increased, resulting in a finer dispersion of nitrides [ 251. As the rolling contact fatigue resistance of En31 steel is considerably reduced at 200 “C compared with that at room temperature [15] it appeared useful to assess the modified composition material at 200 “C to determine whether silicon content affected nitride precipitation and thus reduced the deleterious effect of interstitial nitrogen. Summarized results of rolling contact fatigue tests at 200 “C under a total axial load of 400 kg (nominal maximum Hertzian stress of 415 tonf ine2) lubricated with a die&r-type lubricant currently in use in aero-engines are given in Table 2. The mean life was determined from up to 20 tests depending upon the number of specimens available. TABLE 2 Summarized results of comparative rolling contact fatigue tests on ball bearing steels at 200 ’ C Specimens
Modified composition ball bearing steel (0.77 C, 1.0 CT, 2.1 Si, 1.5 Mn)
Mean life (min)
121
NEL reference balls (En31 B.E.A.)
26
NEL reference balls (En31 A.O.H.)
27
Stock En31 steel balls, manufacturer A
25
Stock En31 steel balls, manufacturer B
32
Load, 400 kg; speed, 1500 rev min-’ ; lubricant, diester currently used in aeroengines.
From the results, under the conditions of test the rolling contact fatigue life of ball bearing steel of modified composition, heat treated according to NEL recommended practice, was superior in rolling contact fatigue resistance at 200 “C to reference En31 ball bearing steels considered to be typical of that currently available. Fine silicon nitride precipitates were identified in the microstructure of the modified composition steel produced by NEL heat treatment procedure. The concentration of the finely dispersed precipitated phase increased after testing at 200 “C!.
278
Conclusions Conventional ball bearing steels of modified composition to reduce deleterious carbide structural effects whilst maintaining desirable properties by matrix strengthening compare favourably in rolling contact fatigue resistance with steels of conventional analysis. The increased silicon content to counteract reduced carbon effects by matrix strengthening also controls deleterious interstitial nitrogen content by forming fine silicon nitride precipitates which were identified in microstructures produced by NEL heat treatment procedure and by ageing at elevated temperature testing.
Acknowledgments The paper is published by permission of the Director of the National Engineering Laboratory. It is Crown Copyright and reproduced by permission of the Controller of Her Britannic Majesty’s Stationery Office. References 1 D. Scott, Ball bearing steels - factors influencing their performance. In Low Alloy Steels, Proc. BISRA-IS1 Conf. on Low Alloy Steels, Iron and Steel Institute, London, 1968, pp. 203 - 209. 2 D. Scott and J. Blackwell, The effect of the steelmaking process on the life of rolling contact bearings, 2nd Lubrication and Wear Conv., Proc. Inst. Mech. Eng., London, 178 (3N) (1964) 81- 88. 3 D. Scott, The effect of material properties, lubricant and environment on rolling contact fatigue life, Symp. on Fatigue in Rolling Contact, Proc. Inst. Mech. Eng., London (1963) 103 - 115. 4 D. Scott and J. Blackwell, Steel refining as an aid to improved ball bearing life, 6th Lubrication and Wear Conv., Proc. Inst. Mech. Eng., London, 182 (3N) (1968) 239 242. 5 D. Scott, The effect of steelmaking vacuum melting and casting techniques on the life of rolling bearings, Vacuum, 19(4) (1969) 167 - 169. 6 D. Scott, Comparative rolling contact fatigue tests on En31 ball bearing steels of recent manufacture, Proc. IS1 Conf. on Tribology in Steelworks, Publication 125, Iron and Steel Institute, London, 1969, pp. 122 - 126. 7 D. Scott and J. Blackwell, An accelerated test for the study of materials under rolling contact, Proc. Inst. Mech. Eng., London, 178 (3N) (1964) 63 - 71. 8 D. Scott and J. Blackwell, NEL rolling contact fatigue tests - accelerated service simulation tests for lubricants and materials for rolling elements, Wear, 17 (1971) 323 - 337. 9 D. Scott, The NEL rolling four ball machine, NEL Rep. 557, Jan. 1974. 10 D. Scott and J. Blackwell, NEL rolling four ball test machines -their role in the laboratory’s work. In R. Tourret and E. P. Wright (eds.), Proc. Int. Symp. on Rolling Contact Fatigue - Performance Testing of Lubricants, Institute of Petroleum, London, Chap. 6, pp. 83 - 96. 11 F. T. Barwell and D. Scott, The effect of lubricant on the pitting failure of ball bearings, Engineering, 182 (4713) (1956) 9 - 12. 12 D. Scott, Study of the effect of lubricant on the pitting failure of balls, Proc. Inst. Mech. Eng. Conf. on Lubrication and Wear, Institution of Mechanical Engineers, London, 1957, pp. 463 - 468.
279 13 A. A. Milne and D. Scott, Studies in pitting -the cone 3 ball machine, Wear, 1 (1958) 347 - 348. 14 D. Scott, Studies in rolling surface fatigue - the four ball machine with a ball ended specimen, Wear, 5 (1962) 69 - 71. 15 D. Scott, Lubricants at higher temperatures - assessing the effects on ball bearing failures, Engineering, 185 (4811) (1958) 660 - 663. 16 D. Scott, Lubricant effects on rolling contact fatigue -a brief review, Proc. Int. Symp. on Rolling Contact Fatigue - Performance Testing of Lubricants, Institute of Petroleum, London, in the press. 17 M. D. Bezborodko and G. S. Krivoshein, An investigation of pitting with the four ball machine. In Friction and Wear in Machinery, Akad Nauk SSSR, Moscow, 1962, Collection 16, No. 5. 18 D. Scott, The cooperative rolling contact fatigue research project of IRG (OECD), to be published. 19 D. Scott and J. Blackwell, Study of the effect of material combination and hardness in rolling contact, Proc. Inst. Mech. Eng., London, 180 (3K) (1966) 32 - 37. 20 D. Scott and P. J. McCullagh, Hardness changes in rolling contact - their significance in ball bearing steel performance, Wear, 24 (1) (1973) 119 - 126. 21 D. Scott and P. J. McCullagh, The role of interstitial nitrogen in rolling contact fatigue, Wear, 25 (2) (1973) 339 - 344. 22 D. Scott, Rolling elements for elevated temperatures, Proc. Symp. on Lubrication in Hostile Environments, Proc. Inst. Mech. Eng., London, 183 (3L) (1969) 9 - 17. 23 D. Scott, New materials for rolling mechanisms, Proc. 2nd Israel Conf. on Tribology, 1976, in the press. 24 D. Scott, The assessment of high speed tool steel rolling bearings manufactured by the powder metallurgy process, Proc. 4th Eur. Symp. on Powder Metallurgy, Grenoble, Sociite Francaise de Metallurgic, 1975, Vol. 3, paper 4/9, pp. 1 - 6. 25 D. H. Jack and K. H. Jack, Carbides and nitrides in steel, Mater. Sci. Eng., 11 (1973) 1 - 27.
Wear, 46 (1978) 273 - 279 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
THE ASSESSMENT OF BALL CHEMICAL COMPOSITION*
BEARING
STEELS
OF MODIFIED
D. SCOTT and J. BLACKWELL National
Engineering
Laboratory,
East Kilbride,
Glasgow (Scotland)
(Received July 5,1977)
Summary With the development of cleaner ball bearing steels of improved properties and lower inclusion content, carbide content, an inherent feature of these steels, carbide size and carbide segregation have become of importance as large nondeforming carbides can fracture to initiate rolling contact fatigue and premature failure. A lower carbon and chromium type of steel, with increased silicon and manganese contents to provide matrix strengthening and to maintain hardness, offers a means of reducing carbide content and suppressing deleterious structural features such as carbide segregation, carbide stringers and carbide network. Under conditions of accelerated service simulation testing the steels of modified composition compared favourably in rolling contact fatigue resistance with steels of conventional compositions. The increased silicon content may also control deleterious interstitial nitrogen content by forming fine silicon nitride precipitates which were identified in microstructures produced by the optimum heat treatment procedure.
Introduction Technological progress makes more arduous the conditions under which many rolling mechanisms are required to operate and creates an ever increasing demand for improved materials of construction. Conventional En31 or 52100 (1.0% C, 1.5% Cr) type ball bearing steels have proved to be highly successful ball and roller bearing materials. It has been found that the steelmaking process can influence material properties [ 1 - 31. Vacuum treatments and other re-refining processes are beneficial [ 1 -61. The nature of non-metallic inclusions as well as their numbers, size and distribution can affect performance [ 1 -31. With the development of cleaner steels [ 4 - 61 of
*Paper presented at the International Conference on the Wear of Materials, St. Louis, MO., U.S.A., 26 - 28 April, 1977.
274
improved physical properties and lower inclusion content, carbide content, an inherent feature of En31 steel, carbide size and carbide segregation have become of importance as large non-deforming carbides can fracture to become a source of rolling contact fatigue initiation and premature failure. A lower carbon and chromium type of steel with increased silicon and manganese contents to provide matrix strengthening and to maintain hardness offers a method of reducing the carbide content and suppressing deleterious structural features such as carbide segregation, carbide stringers and carbide network. Samples of such modifiedcomposition ball bearing steels were assessed for rolling contact fatigue resistance by comparative testing with currently used ball bearing steels.
Method
of test
The ultimate assessment of materials is performance in practice, but as full scale testing is a long and expensive process accelerated service simulation testing has been used to screen materials. Rolling contact fatigue tests were carried out using National Engineering Laboratory (NEL) rolling fourball machines described elsewhere [ 7 - 121. With these machines, a 0.5 in diameter upper test ball held in a chuck drives three similar balls in a lower race, thus simulating the rolling and sliding experienced in angular contact ball bearings. The upper test ball takes the place of the inner race in a conventional ball bearing. The criterion of the test is the time taken from the start of the test till the appearance of the first failure pit in the upper test specimen. To test materials not available in ball form the upper ball may be replaced by a conical [ 131 or a ball-ended specimen [ 141 readily manufactured from a small amount of material. With a ball-ended specimen the kinematics of the system are essentially unaltered from the rolling four-ball setup which allows direct comparison between new materials and conventional balls. Use of a simple heater allows tests to be carried out at elevated temperature [ 151 . Although the loads used in the rolling four-ball test are in excess of those in practice, being chosen to reduce the time of test to a convenient magnitude, use of lower loads has confirmed the inverse cube relationship linking life with load of rolling bearings [ 12, 161. Also, in many instances good correlation between accelerated test results and practice has been found [ll, 16 - 181.
Results at ambient
temperature
Ball-ended specimens manufactured from bars of modified composition bearing steel (0.77% C, 1.0% Cr, 2.1% Si and 1.9% Mn) were heat treated according to the steel manufacturers’ recommendations. As material hardness [ 1,3,19] and thus heat treatment have been found to be important
recommended, O.Q. 810 ‘C, double recommended, O.Q. 820 OC, double recommended, O.Q. 820 ‘C, double recommended, O.Q. 840 ‘C, double ‘C, double temper 190 “C
lubricant, NEL reference mineral oil.
-
NEL reference balls, acid open hearth, En31 steel (0.97 C, 1.48 Cr, 0.37 Si, 0.35 Mn)
Load, 600 kg; speed, 1500 rev min-‘;
-
Manufacturers Manufacturers Manufacturers Manufacturers NEL O.Q. 820
Modified composition ball bearing steel (0.77 C, 1.0 Cr, 2.1 Si, 1.9 Mn)
NEL reference balls, basic electric arc, En31 (1.0% C, Cr) steel (0.99 C, 1.41 Cr, 0.28 Si, 0.33 Mn)
Heat treatment
Specimens
Summarized results of comparative rolling contact fatigue tests on ball bearing steels
TABLE 1
temper temper temper temper
130 130 150 150
“C “C “C “C
870
148
84
18 17 18 16 270
740 790 760 760 740
860
Mean life (min)
Hardness (HVlO)
276
some specimens were heat treated according to the heat treatment determined in NEL to give generally the maximum rolling contact fatigue life. Summarized results of rolling contact fatigue tests under conditions of total axial load of 600 kg (nominal maximum Hertzian stress of 500 tonf inW2) lubricated with NEL reference mineral oil are given in Table 1. For comparative purposes the results of similar tests on NEL reference En31 steel balls are also given. The mean life was determined from up to 30 tests depending upon the number of specimens available. From the results, under the conditions of test the rolling contact fatigue life of ball bearing steel of modified composition when heat treated according to NEL recommended practice was superior in rolling contact fatigue resistance to reference ball bearing steels considered to be typical of currently available conventional En31 steel bearing balls. The NEL heat treated modified composition steel had a fine uniform structure free from massive carbides and carbide network (Fig. 1) and the specimens failed by a single small pit in the bearing track (Fig. 2) in a similar manner to the reference En31 steel balls.
Fig. 1. Fine, uniform structure of modified magnification 525X. Fig. 2. Single failure pit. Magnification
composition
halt bearing steel (etched
Nitai).
56~.
Results at elevated temperature It has been shown that there is a trend towards improved rolling contact fatigue life with reduction of nitrogen content [20,21]. With high speed tool steels it has also been found that rolling contact fatigue resistance was improved when tested at 200 “C [ 22,231. This was particularly so with materials manufactured by the powder route [23,24] . From electron microscope examination together with microprobe analysis, X-ray energy analysis and electron diffraction f21,23] , it appears that under the conditions of elevated temperature heavily loaded test at 200 “C metahographic change occurs; a finely dispersed phase is precipitated which preliminary investigation by the extraction replica technique has indicated is a nitride, probably
277
silicon nitride. This would indicate a reduction in interstitial nitrogen content and may account for the enhanced performance at 200 “C compared with that at ambient temperature. The modified-composition ball bearing steel contains 2.1% silicon. The solubility of nitrogen in iron is decreased by the addition of silicon [ 251. Also, in the presence of silicon the activity coefficient of nitrogen increases with ageing at 250 “C and the driving force for precipitation is increased, resulting in a finer dispersion of nitrides [ 251. As the rolling contact fatigue resistance of En31 steel is considerably reduced at 200 “C compared with that at room temperature [15] it appeared useful to assess the modified composition material at 200 “C to determine whether silicon content affected nitride precipitation and thus reduced the deleterious effect of interstitial nitrogen. Summarized results of rolling contact fatigue tests at 200 “C under a total axial load of 400 kg (nominal maximum Hertzian stress of 415 tonf ine2) lubricated with a die&r-type lubricant currently in use in aero-engines are given in Table 2. The mean life was determined from up to 20 tests depending upon the number of specimens available. TABLE 2 Summarized results of comparative rolling contact fatigue tests on ball bearing steels at 200 ’ C Specimens
Modified composition ball bearing steel (0.77 C, 1.0 CT, 2.1 Si, 1.5 Mn)
Mean life (min)
121
NEL reference balls (En31 B.E.A.)
26
NEL reference balls (En31 A.O.H.)
27
Stock En31 steel balls, manufacturer A
25
Stock En31 steel balls, manufacturer B
32
Load, 400 kg; speed, 1500 rev min-’ ; lubricant, diester currently used in aeroengines.
From the results, under the conditions of test the rolling contact fatigue life of ball bearing steel of modified composition, heat treated according to NEL recommended practice, was superior in rolling contact fatigue resistance at 200 “C to reference En31 ball bearing steels considered to be typical of that currently available. Fine silicon nitride precipitates were identified in the microstructure of the modified composition steel produced by NEL heat treatment procedure. The concentration of the finely dispersed precipitated phase increased after testing at 200 “C!.
278
Conclusions Conventional ball bearing steels of modified composition to reduce deleterious carbide structural effects whilst maintaining desirable properties by matrix strengthening compare favourably in rolling contact fatigue resistance with steels of conventional analysis. The increased silicon content to counteract reduced carbon effects by matrix strengthening also controls deleterious interstitial nitrogen content by forming fine silicon nitride precipitates which were identified in microstructures produced by NEL heat treatment procedure and by ageing at elevated temperature testing.
Acknowledgments The paper is published by permission of the Director of the National Engineering Laboratory. It is Crown Copyright and reproduced by permission of the Controller of Her Britannic Majesty’s Stationery Office. References 1 D. Scott, Ball bearing steels - factors influencing their performance. In Low Alloy Steels, Proc. BISRA-IS1 Conf. on Low Alloy Steels, Iron and Steel Institute, London, 1968, pp. 203 - 209. 2 D. Scott and J. Blackwell, The effect of the steelmaking process on the life of rolling contact bearings, 2nd Lubrication and Wear Conv., Proc. Inst. Mech. Eng., London, 178 (3N) (1964) 81- 88. 3 D. Scott, The effect of material properties, lubricant and environment on rolling contact fatigue life, Symp. on Fatigue in Rolling Contact, Proc. Inst. Mech. Eng., London (1963) 103 - 115. 4 D. Scott and J. Blackwell, Steel refining as an aid to improved ball bearing life, 6th Lubrication and Wear Conv., Proc. Inst. Mech. Eng., London, 182 (3N) (1968) 239 242. 5 D. Scott, The effect of steelmaking vacuum melting and casting techniques on the life of rolling bearings, Vacuum, 19(4) (1969) 167 - 169. 6 D. Scott, Comparative rolling contact fatigue tests on En31 ball bearing steels of recent manufacture, Proc. IS1 Conf. on Tribology in Steelworks, Publication 125, Iron and Steel Institute, London, 1969, pp. 122 - 126. 7 D. Scott and J. Blackwell, An accelerated test for the study of materials under rolling contact, Proc. Inst. Mech. Eng., London, 178 (3N) (1964) 63 - 71. 8 D. Scott and J. Blackwell, NEL rolling contact fatigue tests - accelerated service simulation tests for lubricants and materials for rolling elements, Wear, 17 (1971) 323 - 337. 9 D. Scott, The NEL rolling four ball machine, NEL Rep. 557, Jan. 1974. 10 D. Scott and J. Blackwell, NEL rolling four ball test machines -their role in the laboratory’s work. In R. Tourret and E. P. Wright (eds.), Proc. Int. Symp. on Rolling Contact Fatigue - Performance Testing of Lubricants, Institute of Petroleum, London, Chap. 6, pp. 83 - 96. 11 F. T. Barwell and D. Scott, The effect of lubricant on the pitting failure of ball bearings, Engineering, 182 (4713) (1956) 9 - 12. 12 D. Scott, Study of the effect of lubricant on the pitting failure of balls, Proc. Inst. Mech. Eng. Conf. on Lubrication and Wear, Institution of Mechanical Engineers, London, 1957, pp. 463 - 468.
279 13 A. A. Milne and D. Scott, Studies in pitting -the cone 3 ball machine, Wear, 1 (1958) 347 - 348. 14 D. Scott, Studies in rolling surface fatigue - the four ball machine with a ball ended specimen, Wear, 5 (1962) 69 - 71. 15 D. Scott, Lubricants at higher temperatures - assessing the effects on ball bearing failures, Engineering, 185 (4811) (1958) 660 - 663. 16 D. Scott, Lubricant effects on rolling contact fatigue -a brief review, Proc. Int. Symp. on Rolling Contact Fatigue - Performance Testing of Lubricants, Institute of Petroleum, London, in the press. 17 M. D. Bezborodko and G. S. Krivoshein, An investigation of pitting with the four ball machine. In Friction and Wear in Machinery, Akad Nauk SSSR, Moscow, 1962, Collection 16, No. 5. 18 D. Scott, The cooperative rolling contact fatigue research project of IRG (OECD), to be published. 19 D. Scott and J. Blackwell, Study of the effect of material combination and hardness in rolling contact, Proc. Inst. Mech. Eng., London, 180 (3K) (1966) 32 - 37. 20 D. Scott and P. J. McCullagh, Hardness changes in rolling contact - their significance in ball bearing steel performance, Wear, 24 (1) (1973) 119 - 126. 21 D. Scott and P. J. McCullagh, The role of interstitial nitrogen in rolling contact fatigue, Wear, 25 (2) (1973) 339 - 344. 22 D. Scott, Rolling elements for elevated temperatures, Proc. Symp. on Lubrication in Hostile Environments, Proc. Inst. Mech. Eng., London, 183 (3L) (1969) 9 - 17. 23 D. Scott, New materials for rolling mechanisms, Proc. 2nd Israel Conf. on Tribology, 1976, in the press. 24 D. Scott, The assessment of high speed tool steel rolling bearings manufactured by the powder metallurgy process, Proc. 4th Eur. Symp. on Powder Metallurgy, Grenoble, Sociite Francaise de Metallurgic, 1975, Vol. 3, paper 4/9, pp. 1 - 6. 25 D. H. Jack and K. H. Jack, Carbides and nitrides in steel, Mater. Sci. Eng., 11 (1973) 1 - 27.