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New materials for rolling mechanisms
71
Wear,43 (1977) 71 -87 62 Elsevier Sequoia S.A., Lausanne - Printed in the Nethertands
NEW MATERIALS
FOR ROLLING
MECHANISMS*
D. SCOTT National
Engineering
Laborutory,
East Kilbride,
Glusgow
(Gt. Britain)
(Received December 3,1976)
Summary 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. Using a simple accelerated service-simulation test, new materials and recently developed surface treatments have been assessed under conditions of elevated temperature, high speed and unlubricated rolling contact. Methods of in situ lubrication by sacrificial cage materials have been explored. Lightweight and hollow rolling elements manufact~ed by electrodeposition have been developed. modifications in steel manufactu~ng methods to avoid carbide segregation, and steels of reduced carbon content to limit deleterious carbide structural effects have been investigated. Some of the work of the National Engineering Laboratory (NEL) in this field is briefly reviewed.
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. The ultimate assessment of such materials is performance in practice but, as full-scale testing is a long and expensive process, accelerated lace-simulation testing has been used to screen materials developed for elevated tempe~ture, high speed and unlubricated conditions of rolling contact and to screen sacrificial cage materials for the in situ lubrication of otherwise unlubricated rolling elements. Some of the work of the National Engineering Laboratory (NEL) in this field is briefly reviewed.
*Paper presented at the 2nd Israel Tribology Conference, 1976.
Haifa, October 20 - 22,
72
Method of test Rolling contact fatigue tests and rolling contact wear tests were carried out using NEL rolling four-ball machines described elsewhere [ 1 - 31. With these machines an upper test ball of diameter 0.5 in, which is held in a chuck, drives three similar balls in a lower race, thus simulating the rolling and sliding that is experienced in angular contact ball bearings. The upper test ball takes the place of the inner race in a conventional ball bearing. To test materials not available in ball form the upper ball may be replaced by a ball-ended specimen [4] which is readily manufactured from a small amount of material; the kinematics of the system are essentially unaltered from the rolling four-ball set-up to allow direct comparison of new materials and conventional balls. A simple induction heater enables tests to be carried out at elevated temperatures [5, 61. Using bath lubrication, the criterion of lubricated tests is the time taken from the start of the test till the appearance of the first failure pit on the upper test specimen. Under conditions of dry running and lubrication by a sacrificial cage material, wear is the test criterion. This is assessed by weighing the top specimen before and after a specific test time.
Conventional
ball bearing materials
Conventional ball bearing steels of the En31 type (1.0% C, 1.5% Cr) have proved to be highly successful ball and roller materials. It has been found that the steelmaking process can influence material properties and the initiation of failure [ 7 - 9 ] . Vacuum treatments and other re-refining processes are beneficial [7 - 121. The nature of non-metallic inclusions as well as their number, size and distribution can affect performance [l, 7, 81. With the development of cleaner steels [lo - 121 of improved physical properties and lower inclusion content, carbide content (an inherent feature of En31 steel), carbide size and carbide segregation have become of importance, since 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. Ball-ended specimens manufactured from bars of modified composition ball bearing steel (0.77 C, 1.0 Cr, 2.1 Si, 1.9 Mn) were heat-treated according to the steel manufacturers’ recommendations. As hardness [ 1, 91 and thus heat treatment and structure have been found to be important, some specimens were heat-treated according to the method that is found in NEL generally to give the maximum rolling contact fatigue life. Summarized results of rolling contact fatigue tests under conditions of a total axial load of 600 kg (nominal maximum Hertzian stress of 500 ton inF2) lubricated
73
Fig. 1. Fine, uniform 408 x).
structure
of modified
composition
ball bearing
steel (magnification
with NEL reference mineral oil are given in Table 1. For comparative purposes the results of similar tests on NEL reference 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 the ball bearing steel of modified composition, when heattreated according to NEL recommended practice, compared favourably with that of reference ball bearing steels considered to be typical of currently available conventional bearing balls. The NEL heat-treated steel of modified composition had a fine uniform structure (Fig. 1) and the specimens failed by a single small pit in the bearing track in a similar manner to the reference balls,
Elevated tempe~t~e
materials
En31 ball bearing steels have temperature and environmental limitations which render them unsuitable for some of the more arduous conditions of rolling contact created by technological progress. Eiigh speed tool steels, which have higher tempering temperatures, superior hot hardness and oxidation resistance and which are less susceptible to deleterious lubricant effects than En31 steels, are currently used [6, 7,13,14]. The ingot size of such steels can be impo~ant because large ingots, which are commerci~ly desirable for modern production methods, contain carbide-segregated steel which is deleterious to rolling contact fatigue resistance [ 151. One method that is potentially attractive for the elimination of carbide segregation and the production of a homogeneous structure is to manufact~e the high speed tool steel via the powder route [163. Ball-ended specimens were machined from bars of high speed tool steel of the M2 type (6-5-4-2; W, MO, Cr, V) which were produced from
740 790 760 760 740
870
“C “C ‘C “C
-
NEL reference balls, acid open hearth, En31 steel (0.97 C, 1.48 Cr, 0.37 Si, 0w.*_?_ 15 Mnj _.___,
130 130 150 150 860
temper temper temper temper
-
reference balls, eiectric arc, (1.0% C, Cr) (0.99 C, 1.41 Cr, Si, 0.33 Mn)
double double double double
NEL basic En31 steel 0.28
recommended, O.Q. 810 ‘C, recommended, O.Q. 820 ‘C, recommended, O.Q. 820 ‘6, recommended, O.Q. 840 “C, “C, double temper 190 “C
Manufacturers’ Manufacturers’ Manufacturers’ Manufacturers’ NEL, O.Q. 820
Modified composition ball bearing steet (0.77 C, 1.0 Cr, 2.1 Si, 1.9 Mn)
Hardness (HVIO)
148
a4
18 17 18 16 222
Mean life (min)
with a load of 600 kg, a speed of
Heat treatment
steels,
Specimen
Summarized results of comparative rolling contact fatigue tests on ball bearing 1500 rev min-“ and using NEL reference mineral oil as lubricant -.
TABLE 1
75
various types of powder by a range of currently used techniques by wellknown international manufacturers. The materials may be considered to be typical of the best that are currently available by the manufacturers’ approved methods. The specimens were heat-treated according to the manufacturers’ recommended specifications and were finished to a degree of dimensional accuracy and surface finish comparable with those of commercial best quality balls. For comparative purposes tests were also carried out on M2 high speed tool steel balls from batches commercially available. Details of the materials tested are given in Table 2; those manufactured by the powder route are designated PM. The lubricant was a diester of the type currently in use in aeroengines. Summarized results of tests under a total axial load of 600 kg at ambient temperature and at 200 “C are given in Table 2. The mean life was determined from up to 20 tests, depending upon the number of specimens available. From the results, under the conditions of test the PM1 material was the best of those manufactured from powder, followed by the PM2 material. Both were generally comparable with conventional materials at 200 “C and were vastly superior to the other materials produced from powder. All materials produced by the powder route were inferior to conventional materials at ambient temperature. Better results were obtained with all materials at 200 “C than at ambient temperature. The variation in results from batch to batch of M2 balls confirms previous findings. There is a trend towards improved rolling contact fatigue life with reduction of the nitrogen content [17, 181. PM materials 3 and 4 were high in gas content (Table 2), which may explain their poorer performance. PM1 material, which was the most successful of the materials produced from powder, had the lowest gas content of the materials tested and is comparable in this respect with vacuum remelted material. Specimens of PM1 and PM2 materials were sound and homogeneous but with a pronounced grain structure (Fig. 2) and they had a good surface texture, comparable with that of commercially produced balls. Specimens of PM3 and PM4 material contained surface defects (Fig. 3), microporosity and inclusions (Fig. 4) PM3 material had a coarse acicular structure (Fig. 5) and PM4 material contained areas of banding caused by carbide segregation (Fig. 6). From electron microscope examination together with microprobe analysis, X-ray energy analysis and electron diffraction [ 161, it appears that under the conditions of the elevated temperature heavily loaded test at 200 “C, metallo~aphic change occurs; a finely dispersed phase is precipitated, which preliminary investigation by the extraction replica technique has indicated is a nitride, probably 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.
remelted
canned
Stock (USA) vacuum
Gas atomised annealed
Produced process
Water atomised, as-extruded
Rotating electrode asextruded
Balls M50
PM2
PM3
PM4
and
produced
-
fully pre-alloyed
-
steel
steel
by the Stora Kopparburg
powder,
remelted
PM1
(UK) vacuum
Stock
822 a22 790
Preheat 850 T, A.C. 1220 T!, triple temper 550 “C Preheat 850 “C, A.C. 1150 ‘C, triple temper 550 “C Preheat 850 ‘C, A.C. 1220 =C, triple temper 550 “C
818
805
Preheat 850 ‘C, A.C. 1220 ‘C, triple temper 500 “C
890
-
-
18
18
105
1470
-
-
-
-
Balls (18-4-l)
steel
-
remelted
Stock (USA) vacuum
-
Balls M2 (batch 3)
steel
02
80
165
400
-
-
-
-
-
90
N2
Gas content (ppm)
-
remelted
Stock (USA) vacuum
Balls M2 (batch 2)
-
742
-
remelted
Stock (USA) vacuum
Balls M2 (batch 1)
steel
Hardness (HVlO)
Heat treatment
tests on high speed tool steel materials
Description
fatigue
Specimen
of rolling contact
results
Summarized
TABLE 2
6
8
14
-
-
-
-
-
6
H2
5
6
14
35
72
95
-
-
‘72
At ambient temperature
Mean life (min)
.-
15
12
95
110
111
105
198
72
121
At 200 “C
2
Fig. 2. Structure of the PM1 material (magnification 1224
x).
Fig. 3. Microporosity and inclusions in the surface of PM3 material (magnification 68
X).
e
.
L
i
.*
3. ..* 1
.I *ye
. t
:
; -!J
;’ ., -
8 .t
= ;
.
Fig. 4. Porosity in the PM3 material (magnification 68
x).
Fig. 5. Acicular structure of the PM3 material (magnification
1020
x).
Lightweight rolling elements for high speed applications Rolling bearings in current commercial aircraft turbine engines operate in a speed range of 2 X lo6 DN (bearing more in millimetres times shaft speed in revolutions per minute). Trends in design indicate that future engines may require bearings to operate as high as (3 - 4) X 10” DN [19, 201. At such high speeds analyses predict a significant reduction in rolling contact fatigue life owing to the high centrifugal forces developed at the outer race contact [21]. The use of low mass balls either spherically hollow [22, 231 or cylindrically drilled [24 - 261 is one approach to reduce stresses [27]. Earlier work [ 28 J has shown that materials free from deleterious inclusions electrolytically applied on lightweight easily produced cheap substrates
Fig. 6. Banded
structure
of the PM4 material
(magnification
1020
x).
appear to be an attractive alternative to hollow rolling elements. Under arduous conditions of rolling contact, iron and iron-476 nickel alloy casehardened and heat-treated to 700 - 800 HV compared favourably with conventional rolling bearing materials. Plastic material has also been used as a substrate which was removed by heat treatment to provide hollow rolling elements. Simultaneous electrodeposition of iron-4% Ni and deposition of hard carbides produced hollow specimens with high speed tool steel characteristics [29, 301 . The performance of various electrodeposited materials was assessed and compared with that of conventional ball bearing materials. For comparative purposes all materials were given the same heat treatment, i.e. they were carburized at 950 “C and air cooled, quenched from 820 “C and double tempered for 1 h at 150 “C. Details of the materials and summarized rolling contact fatigue test results are given in Table 3. From the test results, hollow specimens composed of electrodeposited material were comparable with or slightly superior in rolling contact fatigue resistance to conventional En31 through hardening ball bearing steel, which is typical of the basic electric arc material used for the manufacture of bearing balls, and with case-hardened En39 steel, which is typical of that used for roller bearings. The combination of material compatibility and hardness in rolling contact has been shown to be important [l, 9, 311. Double tempering at 190 ‘C!, the highest tempering temperature, to produce the optimum hardness and reduce the deleterious retained austenite content has been found to give the best results. Some specimens, heat-treated using the higher tempering temperature of 190 “C, were tested under the same conditions as the previous batches of specimens. Summarized results are given in Table 4. Tempering at 190 “C significantly improved the rolling contact fatigue life of electrodeposited hollow specimens. The electrodeposited material was superior to the conventional En31 steel.
Tempered at 150 “C Tempered at 190 “C Case-hardened and tempered at 150 “C Case-hardened and tempered at 190 “C
Standard basic electric arc En31 steel
Fe-4% Ni electrodeposit on C steel
Condition
Specimen
820 770 700 770
Hardness (HVlO)
95 297 120 372
Mean life (min)
Summarized results of rolling contact fatigue tests on specimens tempered at various temperatures under a total axial load 400 kg and lubricated with NEL reference mineral oil
TABLE 4
a20 750 750 700 740 775 775 760
Heat-treated (H.T.) Case-hardened and Case-hardened and Case-hardened and Case-hardened and Case-hardened and Case-hardened and Case-hardened and
Standard basic electric arc En31 steel En39 air melted steel En39 vacuum melted steel Fe-4% Ni electrodeposit on C steel Fe-4% Ni electrodeposit on 50% porous iron sinter Fe-4% Ni electrodeposited on titanium Fe-4% Ni electrodeposited hollow specimen Fe-4% Ni electrodeposit with codeposited hard carbides on C steel H.T. H.T. H.T. H.T. H.T. H.T. H.T.
Hardness (HVlO)
Condition
Specimen
28 20 37 38 20 29 35 39
Mean life (min)
Summarized results of rolling contact fatigue tests on various specimens under a total axial load of 600 kg lubricated with NEL reference mineral oil (standard heat treatment)
TABLE 3
80
Fig. 7. Section tion of Fe-4%
through the hollow ball end of a specimen Ni on a plastic former (magnification 4.2
Fig. 8. Typical
failure pit in a tested
Fig. 9. Section through fication 45 X )_ Fig. 10. Structure
a typical
of heat-treated
manufactured
Fe--4% Ni electrodeposit
surface
pit in a tested
electrodeposited
(magnification
Fe-4%
Fe-4%
by electrodeposi-
x ).
42
Ni electrodeposit
Ni (magnification
x ).
(magni405
x ).
The electrodeposited iron-4% nickel alloy, containing codeposited hard carbides, was heat-treated using a high speed tool steel heat treatment, soaked at 850 ‘C, heated to 1250 “C and air cooled followed by tempering at 500 “C. The resultant material was essentially hard martensite. Thus the material has air-hardening characteristics similar to those of conventional high speed tool steels. It had a similar rolling contact fatigue life to the standard heat-treated material. Selected failed specimens were microscopically and metallographically examined. Figure 7 shows a sectioned electrodeposited hollow specimen and Fig. 8 shows a typical single failure pit in such a specimen. Figure 9 shows a section through such a pit and Fig. 10 shows the material to be sound, metallographically satisfactory and free from deleterious non-metallic inclusions.
81
Materials for unlubricated
and in situ cage lubricated
rolling contact
Conventional liquid and semi-solid grease lubricants, besides having elevated temperature limitations, also have vapour pressures higher than some en~ronmen~ and are consequently lost by evaporation. Thus, in such hostile environments rolling mechanisms are required to operate unlubricated or with types of lubricants not affected by the environment ]5,6]. Lamellar solid film lubricants can lubricate rolling mechanisms under conditions where conventional lubricants are ineffective owing to inaccessibility, hostile environments, temperature limitations or excessive loading, but replenishment is difficult [6, 32 - 351. Lubrication of rolling bearings from a sacrificial cage material offers an attractive solution to some of these problems [ 36 - 411. Under conditions of unlubricated rolling contact, failure does not occur by the usual pitting mechanism but as a result of wear which causes noise and vibration in excess of an acceptable level. Comparative rolling contact wear tests have therefore been carried out on a time basis (30 min). Wearresistant materials and surface coatings have been assessed under conditions of unlubricated rolling contact [42 - 491 at elevated temperature and lubricated in situ by cage materials. Some summa~zed results are given in Table 5. Under conditions of unlubricated rolling contact, fine-grained tungsten carbide and hot-pressed silicon nitride performed the best of the materials tested. These balls ran smoothly and retained their smooth surface appearance. Some wear was experienced with hot-pressed silicon nitride at 280 “C; tungsten carbide performed better at this temperature. Of the many surface treatments assessed to date, only soft nitriding which provides a low friction surface that is rich in oxygen ions appeared to be satisfactory under the test conditions. The use of suitable cage materials reduced the wear of otherwise unlubricated materials and thus effected in situ lubrication with little sacrificial wear of the cage material. PTFE-based material, suitably reinforced with glass or carbon fibres and with molybdenum disulphide, was the most effective. Cages of these materials could be used for many tests before eventually failing by fracture. Some polyim~de-based cages fractured prematurely during test. Wear of hot-pressed silicon nitride appeared to be by surface smearing (Fig. 11) and erosion (Fig. 12). Wear of tungsten carbide was by fine pitting or erosion (Fig. 13). Use of a sacrificial lubricating cage produced a smooth bearing surface (Fig. 14). The rubbed surface of PTFE-based cage materials appeared fairly smooth, as if PTFE and molybdenum disulphide had been smeared on the surface. The rubbed surface of polyimide-bred cage materials generally appeared rough, as if pieces of polyimide had been plucked from the surface exposing carbon fibres. The virgin material reinforced with glass fibre and molybdenum disulphide appeared to be solid and of uniform structure but, after being subjected to rubbing action during the test, voids appeared in the material usually
26.0 0.2
No cage Glass fibre and MO& reinforced PTFE No cage Glass fibre and MoSz reinforced PTFE Carbon fibre and MoSz reinforced PTFE Carbon fibre and MoS2 reinforced polyimide* No cage Glass fibre and MoSz reinforced PTFE No cage Glass fibre and MO& reinforced PTFE
Ml0 high speed tool steel
Hot-pressed silicon nitride
Tungsten carbide (commercial)
F = cages fractured. *Results only on cages which did not fracture.
Soft nitrided M50 High speed tool steel
No cage Glass fibre and MoSz reinforced PTFE
Tungsten carbide (0.005 in) coated En31 steel No cage
Fine grain tungsten carbide
30.0 0.3 0.3 0.4
No cage Glass fibre and MoSz reinforced PTFE Carbon fibre and MoSz reinforced PTFE Carbon fibre and MoSz reinforced polyimide*
8.3 0.3
70
7.0 Nit Nil Nil
0.6 0.1 0.1 0.1
0.4
-
Nil -
Nil -
Nil 0.4 0.4 -
Nil -
Nil 0.3 0.8 .-
-
11.7 1.5
-
7.8 1.0 1.0 0.4
46.0 3.8 10.5 10.5
19.8 2.5
59.0 3.5 3.4 8.0
Top ball
Top ball
En31 steel
At 280 “C
At ambient temperature Cage
Mean loss of weight (mg)
Cage material
Test material
Comparative rolling wear tests under a total axial load of 100 kg at 1500 rev min-’
TABLE 5
0.3
-
-
0.6
0.3
0.4
0.4 F F
-
-
0.4 0.5 30.0
-
Cage
[icj
Fig. 11. Scanning electron micrograph showing evidence ticles and of sliding contact (magnification 180 x ). Fig. 12. Scanning electron micrograph showing silicon nitride (magnification $00 X )~
Fig. 13. Fine erosion-type Fig. 14. Smooth 60 x ).
polished
pitting
of tungsten
appearance
damage
carbide
of the bearing
of bonding by erosion
(magnification track
in silicon
and smearing to the bearing
66
of partrack of
x ).
nitride
(magnification
in areas devoid of fibres (Fig. 15). Cracking and fracture appeared to initiate in such areas. The virgin polyimide-based cage material was not homogeneous (Fig. 16). Some areas were depleted of carbon fibres and these areas generally contained a concentration of large particles of molybdenum disulphide (Fig. 17). The inhomogeneous structure seems to be the cause of the large variation in performance of cages in this material.
Conclusions Conventional ball bearing steels of a composition that is modified to reduce deleterious carbide structural effects whilst maintaining properties by
Fig. 15. Porosity, cracking and subsurface reinforced PTFE (magnification 96 X ). Fig. 16. Inhomogeneous 30 x ).
structure
deformation
in rubbed
of a polyimide-based
Fig. 17. Concentration of MO& in a carbon cage material (magnification 78 X ).
fibre depleted
glass fibre and MoSz
cage material
(magnification
region of the polyimide-based
matrix strengthening compare favourably in rolling contact fatigue resistance with steels of conventional analysis. The increased silicon content to provide matrix strengthening may also control deleterious interstitial nitrogen content by forming fine silicon nitride precipitates; these were identified in microstructures produced by the NEL heat-treatment procedure. Sound homogeneous material produced by the powder route with uniform fine carbide distribution and a gas content comparable with conventionally cast material seems potentially suitable for rolling bearings. Interstitial nitrogen content appears to have a significant effect on rolling contact fatigue resistance, and improved elevated temperature performance may be due to the metallographic structural changes brought about by the test conditions, the interstitial nitrogen content being reduced by a precipitation process. Heat-treatable electrodeposited materials, free from deleterious nonmetallic inclusions and having rolling contact fatigue resistances comparable
85
with currently used materials, are potentially suitable for the production of lightweight rolling elements and hollow rolling elements and races for high speed applications. Hot-pressed silicon nitride and sintered tungsten carbides appear potentially attractive for the arduous conditions of unlubricated rolling contact. In situ lubrication of such materials can be effected by reinforced polymeric composite cage materials to reduce wear with very little loss of the sacrificial cage material.
Acknowledgments The paper is published by permission of the Director of the National Engineering Laboratory. It is Crown cop~ight and is reproduced by permission of the Controller of Her Britannic Majesty’s Stationery Office.
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86 14 D. Scott, Effect of environment on bearing performance, Symp. on Corrosion and Its Prevention in Motor Vehicles, Proc. Inst. Mech. Eng., London, Part 35, 182 (1968) 116 - 123. 15 D. Scott and J. Blackwell, The effect of some manufacturing variables on the performance of high speed tool steel bearings, Wear, 18 (1971) 19 - 28. 16 D. Scott and J. Blackwell, A preliminary assessment of high speed tool steels manufactured from powder for rolling bearing materials, Wear, 34 (1974) 149 - 158. 17 D. Scott and P. J. McCullagh, Hardness changes in rolling contact -their significance in ball bearing steel performance, Wear, 24 (1) (1973) 119 - 126. 18 D. Scott and P. J. McCullagh, The role of nitrogen content on the rolling contact fatigue performance of En31 ball bearing steels, Wear, 25 (3) (1973) 339 - 344. 19 H. H. Coe, H. W. Scribbe and R. J. Parker, NASA Tech. Note, NASA TN-D-5800, 1970. 20 P. F. Brown, Bearings and dampers for advanced jet engines, SAE paper 700318 1970. 21 A. B. Jones, The life of high speed ball bearings, Trans. ASME, 74 (5) (1952) 695 703. 22 H. H. Coe, R. J. Parker and H. W. Scribbe, Performance of 75 millimeter-bore bearings using electron-beam welded hollow balls with a diameter ratio of 1.26, NASA Tech. Note, NASA TN-D-7869,1975. 23 H. H. Coe, R. J. Parker and H. W. Scribbe, Evaluation of electron-beam welded hollow balls for high speed ball bearings, J. Lubr. Technol., 93 (1) (1971) 47 - 59. 24 H. H. Coe, H. W. Scribbe and W. J. Anderson, Evaluation of cylindrically hollow (drilled) balls in ball bearings at DN values to 2.1 million, NASA Tech. Note, NASA TN-D-7007,1971. 25 P. W. Holmes, Evaluation of drilled ball bearings at DN values to three million I variable oil flow tests, NASA Contract. Rep., NASA CR-2004, 1972. 26 P. W. Holmes, Evaluation of drilled ball bearings at DN values to three million II experimental skid study and endurance tests, NASA Contract. Rep., NASA CR-2005, 1972. 27 T. A. Harris, On the effectiveness of hollow balls in high speed thrust bearings, ASLE Trans., 11 (4) (1968) 290 - 294. 28 D. Scott and P. J. McCullagh, Hardenable electrodeposited coatings for rolling bearings - a preliminary assessment, Electrodeposition Surf. Treat., 1 (1) (1972) 21- 31. 29 D. Scott, P. J. McCullagh and B. J. Hands, Lightweight rolling elements manufactured by electrodeposition, Trans. Inst. Met. Finish., 32 (3) (1975) 309 - 314. 30 D. Scott and P. J. McCullagh, Lightweight rolling elements by electrodeposition. In T. Sakurai (ed.), Proc. ASLE/JSLE Int. Lubrication Conf., Tokyo, 1975, Elsevier, Amsterdam, 1976, pp. 484 - 491. 31 D. Scott and J. Blackwell, Study of the effect of material combination and hardness in rolling contact fatigue, Proc. 4th Lubrication and Wear Convention, Proc. Inst. Mech. Eng., London, Part 3K, 180 (1966) 32 - 37. 32 D. Scott and D. T. Jamieson, Molybdenum disulphide as a lubricant additive in rolling contact -a study of compatibility with other lubricants, J. Inst. Pet., London, 48 (460) (1962) 91- 104. 33 D. Scott, A study of solid lubricants for use in rolling bearings, Proe. ASLE Cod’. on Solid Lubricants, Denver, Colo., 1971, pp. 32 - 40. 34 D. Scott, A study of solid lubricants for roiling bearings, Wear, 21 (2) (1972) 155 165. 35 D. Scott and J. Blackwell, The assessment of recently developed lubricants for rolling elements, Lubr. Eng., 29 (3) (1973) 99 - 106. 36 D. Scott and J. Blackwell, Sacrificial cage materials for the in situ lubrication of rolling bearings, Proc. 1st European Space Symp. on Tribology, Frascati, Italy, 1975, European Space Agency, Noordwijk, SPIII, pp. 175 - 183.
87
37
38 39
40
41 42 43 44 45 46
47 48
49
D. Scott and P. J. McCullagh, Solid lubricant cage materials for the in situ lubrication of rolling mechanisms, Proc. Int. Solid Lubrication Symp., Tokyo, JSLE, Tokyo, 1975, pp. 49 - 50. D. Scott, Polymeric cage materials for the in situ lubrication of rolling bearings, Colloques Int. du CNRS, No 233, Polymeres et Lubrication, 1974, pp. 433 - 439. D. Scott and G. H. Mills, Polymeric composite materials for the lubrication of rolling bearings. In Advances in Polymer Friction and Wear, Plenum Press, New York, 1974. pp. 441 - 450. D. Scott and G. H. Mills, A scanning electron microscopical study of fibre reinforced polymeric cage materials for rolling bearings, Proc. Electron Microscopy Conf., 19’72, Institute of Physics, London. D. Scott and G. H. Mills, A scanning electron microscopical study of fibre reinforced polymeric cage materials for rolling bearings, Polymer, 14 (1973) 130 - 132. D. Scott, Factors influencing the performance of sintered carbides under arduous conditions of rolling contact, Wear, 32 (3) (1975) 309 - 314. D. Scott and J. Blackwell, Hot-pressed silicon nitride as a rolling bearing material a preliminary assessment, Wear, 24 (I) (1973) 61 - 67. D. Scott, J. Blackwell and P. J. McCullagh, Silicon nitride as a roiling bearing material -a preliminary assessment, Wear, 17 (2) (1971) 72 - 86. D. Scott, Materials for elevated temperature rolling bearings. In Groupement pour I’Avancement de la Mecanique Industrielle, Journees d’Etude sur l’usure, Paris, 1970. D. Scott, A study of hard non-ferrous materials for arduous conditions of rolling contact. In Proc. Symp. on Recent Developments in Non-Ferrous Metal Technology, NML Jamshedpur, India, Vol. 3, 1968, pp. 279 - 285. D. Scott, Hard materials, Proc. Int. Conf. on Lubrication and Wear, Proc. Inst. Mech. Eng., London, Part 3A, 182 (1967) 325 - 341. D. Scott and J. Blackwell, Study of sintered carbides and surface treated materials for unlubricated and elevated temperature rolling elements, 5th Lubrication and Wear Convention, Proc. Inst. Mech. Eng., London, Part 30,181 (1967) 77 - 84. D. Scott and J. Blackwell, Study of some materials for unlubricated and elevated temperature rolling elements, Rev. Metall. (Paris), March (1966) 257 - 264.
Wear,43 (1977) 71 -87 62 Elsevier Sequoia S.A., Lausanne - Printed in the Nethertands
NEW MATERIALS
FOR ROLLING
MECHANISMS*
D. SCOTT National
Engineering
Laborutory,
East Kilbride,
Glusgow
(Gt. Britain)
(Received December 3,1976)
Summary 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. Using a simple accelerated service-simulation test, new materials and recently developed surface treatments have been assessed under conditions of elevated temperature, high speed and unlubricated rolling contact. Methods of in situ lubrication by sacrificial cage materials have been explored. Lightweight and hollow rolling elements manufact~ed by electrodeposition have been developed. modifications in steel manufactu~ng methods to avoid carbide segregation, and steels of reduced carbon content to limit deleterious carbide structural effects have been investigated. Some of the work of the National Engineering Laboratory (NEL) in this field is briefly reviewed.
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. The ultimate assessment of such materials is performance in practice but, as full-scale testing is a long and expensive process, accelerated lace-simulation testing has been used to screen materials developed for elevated tempe~ture, high speed and unlubricated conditions of rolling contact and to screen sacrificial cage materials for the in situ lubrication of otherwise unlubricated rolling elements. Some of the work of the National Engineering Laboratory (NEL) in this field is briefly reviewed.
*Paper presented at the 2nd Israel Tribology Conference, 1976.
Haifa, October 20 - 22,
72
Method of test Rolling contact fatigue tests and rolling contact wear tests were carried out using NEL rolling four-ball machines described elsewhere [ 1 - 31. With these machines an upper test ball of diameter 0.5 in, which is held in a chuck, drives three similar balls in a lower race, thus simulating the rolling and sliding that is experienced in angular contact ball bearings. The upper test ball takes the place of the inner race in a conventional ball bearing. To test materials not available in ball form the upper ball may be replaced by a ball-ended specimen [4] which is readily manufactured from a small amount of material; the kinematics of the system are essentially unaltered from the rolling four-ball set-up to allow direct comparison of new materials and conventional balls. A simple induction heater enables tests to be carried out at elevated temperatures [5, 61. Using bath lubrication, the criterion of lubricated tests is the time taken from the start of the test till the appearance of the first failure pit on the upper test specimen. Under conditions of dry running and lubrication by a sacrificial cage material, wear is the test criterion. This is assessed by weighing the top specimen before and after a specific test time.
Conventional
ball bearing materials
Conventional ball bearing steels of the En31 type (1.0% C, 1.5% Cr) have proved to be highly successful ball and roller materials. It has been found that the steelmaking process can influence material properties and the initiation of failure [ 7 - 9 ] . Vacuum treatments and other re-refining processes are beneficial [7 - 121. The nature of non-metallic inclusions as well as their number, size and distribution can affect performance [l, 7, 81. With the development of cleaner steels [lo - 121 of improved physical properties and lower inclusion content, carbide content (an inherent feature of En31 steel), carbide size and carbide segregation have become of importance, since 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. Ball-ended specimens manufactured from bars of modified composition ball bearing steel (0.77 C, 1.0 Cr, 2.1 Si, 1.9 Mn) were heat-treated according to the steel manufacturers’ recommendations. As hardness [ 1, 91 and thus heat treatment and structure have been found to be important, some specimens were heat-treated according to the method that is found in NEL generally to give the maximum rolling contact fatigue life. Summarized results of rolling contact fatigue tests under conditions of a total axial load of 600 kg (nominal maximum Hertzian stress of 500 ton inF2) lubricated
73
Fig. 1. Fine, uniform 408 x).
structure
of modified
composition
ball bearing
steel (magnification
with NEL reference mineral oil are given in Table 1. For comparative purposes the results of similar tests on NEL reference 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 the ball bearing steel of modified composition, when heattreated according to NEL recommended practice, compared favourably with that of reference ball bearing steels considered to be typical of currently available conventional bearing balls. The NEL heat-treated steel of modified composition had a fine uniform structure (Fig. 1) and the specimens failed by a single small pit in the bearing track in a similar manner to the reference balls,
Elevated tempe~t~e
materials
En31 ball bearing steels have temperature and environmental limitations which render them unsuitable for some of the more arduous conditions of rolling contact created by technological progress. Eiigh speed tool steels, which have higher tempering temperatures, superior hot hardness and oxidation resistance and which are less susceptible to deleterious lubricant effects than En31 steels, are currently used [6, 7,13,14]. The ingot size of such steels can be impo~ant because large ingots, which are commerci~ly desirable for modern production methods, contain carbide-segregated steel which is deleterious to rolling contact fatigue resistance [ 151. One method that is potentially attractive for the elimination of carbide segregation and the production of a homogeneous structure is to manufact~e the high speed tool steel via the powder route [163. Ball-ended specimens were machined from bars of high speed tool steel of the M2 type (6-5-4-2; W, MO, Cr, V) which were produced from
740 790 760 760 740
870
“C “C ‘C “C
-
NEL reference balls, acid open hearth, En31 steel (0.97 C, 1.48 Cr, 0.37 Si, 0w.*_?_ 15 Mnj _.___,
130 130 150 150 860
temper temper temper temper
-
reference balls, eiectric arc, (1.0% C, Cr) (0.99 C, 1.41 Cr, Si, 0.33 Mn)
double double double double
NEL basic En31 steel 0.28
recommended, O.Q. 810 ‘C, recommended, O.Q. 820 ‘C, recommended, O.Q. 820 ‘6, recommended, O.Q. 840 “C, “C, double temper 190 “C
Manufacturers’ Manufacturers’ Manufacturers’ Manufacturers’ NEL, O.Q. 820
Modified composition ball bearing steet (0.77 C, 1.0 Cr, 2.1 Si, 1.9 Mn)
Hardness (HVIO)
148
a4
18 17 18 16 222
Mean life (min)
with a load of 600 kg, a speed of
Heat treatment
steels,
Specimen
Summarized results of comparative rolling contact fatigue tests on ball bearing 1500 rev min-“ and using NEL reference mineral oil as lubricant -.
TABLE 1
75
various types of powder by a range of currently used techniques by wellknown international manufacturers. The materials may be considered to be typical of the best that are currently available by the manufacturers’ approved methods. The specimens were heat-treated according to the manufacturers’ recommended specifications and were finished to a degree of dimensional accuracy and surface finish comparable with those of commercial best quality balls. For comparative purposes tests were also carried out on M2 high speed tool steel balls from batches commercially available. Details of the materials tested are given in Table 2; those manufactured by the powder route are designated PM. The lubricant was a diester of the type currently in use in aeroengines. Summarized results of tests under a total axial load of 600 kg at ambient temperature and at 200 “C are given in Table 2. The mean life was determined from up to 20 tests, depending upon the number of specimens available. From the results, under the conditions of test the PM1 material was the best of those manufactured from powder, followed by the PM2 material. Both were generally comparable with conventional materials at 200 “C and were vastly superior to the other materials produced from powder. All materials produced by the powder route were inferior to conventional materials at ambient temperature. Better results were obtained with all materials at 200 “C than at ambient temperature. The variation in results from batch to batch of M2 balls confirms previous findings. There is a trend towards improved rolling contact fatigue life with reduction of the nitrogen content [17, 181. PM materials 3 and 4 were high in gas content (Table 2), which may explain their poorer performance. PM1 material, which was the most successful of the materials produced from powder, had the lowest gas content of the materials tested and is comparable in this respect with vacuum remelted material. Specimens of PM1 and PM2 materials were sound and homogeneous but with a pronounced grain structure (Fig. 2) and they had a good surface texture, comparable with that of commercially produced balls. Specimens of PM3 and PM4 material contained surface defects (Fig. 3), microporosity and inclusions (Fig. 4) PM3 material had a coarse acicular structure (Fig. 5) and PM4 material contained areas of banding caused by carbide segregation (Fig. 6). From electron microscope examination together with microprobe analysis, X-ray energy analysis and electron diffraction [ 161, it appears that under the conditions of the elevated temperature heavily loaded test at 200 “C, metallo~aphic change occurs; a finely dispersed phase is precipitated, which preliminary investigation by the extraction replica technique has indicated is a nitride, probably 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.
remelted
canned
Stock (USA) vacuum
Gas atomised annealed
Produced process
Water atomised, as-extruded
Rotating electrode asextruded
Balls M50
PM2
PM3
PM4
and
produced
-
fully pre-alloyed
-
steel
steel
by the Stora Kopparburg
powder,
remelted
PM1
(UK) vacuum
Stock
822 a22 790
Preheat 850 T, A.C. 1220 T!, triple temper 550 “C Preheat 850 “C, A.C. 1150 ‘C, triple temper 550 “C Preheat 850 ‘C, A.C. 1220 =C, triple temper 550 “C
818
805
Preheat 850 ‘C, A.C. 1220 ‘C, triple temper 500 “C
890
-
-
18
18
105
1470
-
-
-
-
Balls (18-4-l)
steel
-
remelted
Stock (USA) vacuum
-
Balls M2 (batch 3)
steel
02
80
165
400
-
-
-
-
-
90
N2
Gas content (ppm)
-
remelted
Stock (USA) vacuum
Balls M2 (batch 2)
-
742
-
remelted
Stock (USA) vacuum
Balls M2 (batch 1)
steel
Hardness (HVlO)
Heat treatment
tests on high speed tool steel materials
Description
fatigue
Specimen
of rolling contact
results
Summarized
TABLE 2
6
8
14
-
-
-
-
-
6
H2
5
6
14
35
72
95
-
-
‘72
At ambient temperature
Mean life (min)
.-
15
12
95
110
111
105
198
72
121
At 200 “C
2
Fig. 2. Structure of the PM1 material (magnification 1224
x).
Fig. 3. Microporosity and inclusions in the surface of PM3 material (magnification 68
X).
e
.
L
i
.*
3. ..* 1
.I *ye
. t
:
; -!J
;’ ., -
8 .t
= ;
.
Fig. 4. Porosity in the PM3 material (magnification 68
x).
Fig. 5. Acicular structure of the PM3 material (magnification
1020
x).
Lightweight rolling elements for high speed applications Rolling bearings in current commercial aircraft turbine engines operate in a speed range of 2 X lo6 DN (bearing more in millimetres times shaft speed in revolutions per minute). Trends in design indicate that future engines may require bearings to operate as high as (3 - 4) X 10” DN [19, 201. At such high speeds analyses predict a significant reduction in rolling contact fatigue life owing to the high centrifugal forces developed at the outer race contact [21]. The use of low mass balls either spherically hollow [22, 231 or cylindrically drilled [24 - 261 is one approach to reduce stresses [27]. Earlier work [ 28 J has shown that materials free from deleterious inclusions electrolytically applied on lightweight easily produced cheap substrates
Fig. 6. Banded
structure
of the PM4 material
(magnification
1020
x).
appear to be an attractive alternative to hollow rolling elements. Under arduous conditions of rolling contact, iron and iron-476 nickel alloy casehardened and heat-treated to 700 - 800 HV compared favourably with conventional rolling bearing materials. Plastic material has also been used as a substrate which was removed by heat treatment to provide hollow rolling elements. Simultaneous electrodeposition of iron-4% Ni and deposition of hard carbides produced hollow specimens with high speed tool steel characteristics [29, 301 . The performance of various electrodeposited materials was assessed and compared with that of conventional ball bearing materials. For comparative purposes all materials were given the same heat treatment, i.e. they were carburized at 950 “C and air cooled, quenched from 820 “C and double tempered for 1 h at 150 “C. Details of the materials and summarized rolling contact fatigue test results are given in Table 3. From the test results, hollow specimens composed of electrodeposited material were comparable with or slightly superior in rolling contact fatigue resistance to conventional En31 through hardening ball bearing steel, which is typical of the basic electric arc material used for the manufacture of bearing balls, and with case-hardened En39 steel, which is typical of that used for roller bearings. The combination of material compatibility and hardness in rolling contact has been shown to be important [l, 9, 311. Double tempering at 190 ‘C!, the highest tempering temperature, to produce the optimum hardness and reduce the deleterious retained austenite content has been found to give the best results. Some specimens, heat-treated using the higher tempering temperature of 190 “C, were tested under the same conditions as the previous batches of specimens. Summarized results are given in Table 4. Tempering at 190 “C significantly improved the rolling contact fatigue life of electrodeposited hollow specimens. The electrodeposited material was superior to the conventional En31 steel.
Tempered at 150 “C Tempered at 190 “C Case-hardened and tempered at 150 “C Case-hardened and tempered at 190 “C
Standard basic electric arc En31 steel
Fe-4% Ni electrodeposit on C steel
Condition
Specimen
820 770 700 770
Hardness (HVlO)
95 297 120 372
Mean life (min)
Summarized results of rolling contact fatigue tests on specimens tempered at various temperatures under a total axial load 400 kg and lubricated with NEL reference mineral oil
TABLE 4
a20 750 750 700 740 775 775 760
Heat-treated (H.T.) Case-hardened and Case-hardened and Case-hardened and Case-hardened and Case-hardened and Case-hardened and Case-hardened and
Standard basic electric arc En31 steel En39 air melted steel En39 vacuum melted steel Fe-4% Ni electrodeposit on C steel Fe-4% Ni electrodeposit on 50% porous iron sinter Fe-4% Ni electrodeposited on titanium Fe-4% Ni electrodeposited hollow specimen Fe-4% Ni electrodeposit with codeposited hard carbides on C steel H.T. H.T. H.T. H.T. H.T. H.T. H.T.
Hardness (HVlO)
Condition
Specimen
28 20 37 38 20 29 35 39
Mean life (min)
Summarized results of rolling contact fatigue tests on various specimens under a total axial load of 600 kg lubricated with NEL reference mineral oil (standard heat treatment)
TABLE 3
80
Fig. 7. Section tion of Fe-4%
through the hollow ball end of a specimen Ni on a plastic former (magnification 4.2
Fig. 8. Typical
failure pit in a tested
Fig. 9. Section through fication 45 X )_ Fig. 10. Structure
a typical
of heat-treated
manufactured
Fe--4% Ni electrodeposit
surface
pit in a tested
electrodeposited
(magnification
Fe-4%
Fe-4%
by electrodeposi-
x ).
42
Ni electrodeposit
Ni (magnification
x ).
(magni405
x ).
The electrodeposited iron-4% nickel alloy, containing codeposited hard carbides, was heat-treated using a high speed tool steel heat treatment, soaked at 850 ‘C, heated to 1250 “C and air cooled followed by tempering at 500 “C. The resultant material was essentially hard martensite. Thus the material has air-hardening characteristics similar to those of conventional high speed tool steels. It had a similar rolling contact fatigue life to the standard heat-treated material. Selected failed specimens were microscopically and metallographically examined. Figure 7 shows a sectioned electrodeposited hollow specimen and Fig. 8 shows a typical single failure pit in such a specimen. Figure 9 shows a section through such a pit and Fig. 10 shows the material to be sound, metallographically satisfactory and free from deleterious non-metallic inclusions.
81
Materials for unlubricated
and in situ cage lubricated
rolling contact
Conventional liquid and semi-solid grease lubricants, besides having elevated temperature limitations, also have vapour pressures higher than some en~ronmen~ and are consequently lost by evaporation. Thus, in such hostile environments rolling mechanisms are required to operate unlubricated or with types of lubricants not affected by the environment ]5,6]. Lamellar solid film lubricants can lubricate rolling mechanisms under conditions where conventional lubricants are ineffective owing to inaccessibility, hostile environments, temperature limitations or excessive loading, but replenishment is difficult [6, 32 - 351. Lubrication of rolling bearings from a sacrificial cage material offers an attractive solution to some of these problems [ 36 - 411. Under conditions of unlubricated rolling contact, failure does not occur by the usual pitting mechanism but as a result of wear which causes noise and vibration in excess of an acceptable level. Comparative rolling contact wear tests have therefore been carried out on a time basis (30 min). Wearresistant materials and surface coatings have been assessed under conditions of unlubricated rolling contact [42 - 491 at elevated temperature and lubricated in situ by cage materials. Some summa~zed results are given in Table 5. Under conditions of unlubricated rolling contact, fine-grained tungsten carbide and hot-pressed silicon nitride performed the best of the materials tested. These balls ran smoothly and retained their smooth surface appearance. Some wear was experienced with hot-pressed silicon nitride at 280 “C; tungsten carbide performed better at this temperature. Of the many surface treatments assessed to date, only soft nitriding which provides a low friction surface that is rich in oxygen ions appeared to be satisfactory under the test conditions. The use of suitable cage materials reduced the wear of otherwise unlubricated materials and thus effected in situ lubrication with little sacrificial wear of the cage material. PTFE-based material, suitably reinforced with glass or carbon fibres and with molybdenum disulphide, was the most effective. Cages of these materials could be used for many tests before eventually failing by fracture. Some polyim~de-based cages fractured prematurely during test. Wear of hot-pressed silicon nitride appeared to be by surface smearing (Fig. 11) and erosion (Fig. 12). Wear of tungsten carbide was by fine pitting or erosion (Fig. 13). Use of a sacrificial lubricating cage produced a smooth bearing surface (Fig. 14). The rubbed surface of PTFE-based cage materials appeared fairly smooth, as if PTFE and molybdenum disulphide had been smeared on the surface. The rubbed surface of polyimide-bred cage materials generally appeared rough, as if pieces of polyimide had been plucked from the surface exposing carbon fibres. The virgin material reinforced with glass fibre and molybdenum disulphide appeared to be solid and of uniform structure but, after being subjected to rubbing action during the test, voids appeared in the material usually
26.0 0.2
No cage Glass fibre and MO& reinforced PTFE No cage Glass fibre and MoSz reinforced PTFE Carbon fibre and MoSz reinforced PTFE Carbon fibre and MoS2 reinforced polyimide* No cage Glass fibre and MoSz reinforced PTFE No cage Glass fibre and MO& reinforced PTFE
Ml0 high speed tool steel
Hot-pressed silicon nitride
Tungsten carbide (commercial)
F = cages fractured. *Results only on cages which did not fracture.
Soft nitrided M50 High speed tool steel
No cage Glass fibre and MoSz reinforced PTFE
Tungsten carbide (0.005 in) coated En31 steel No cage
Fine grain tungsten carbide
30.0 0.3 0.3 0.4
No cage Glass fibre and MoSz reinforced PTFE Carbon fibre and MoSz reinforced PTFE Carbon fibre and MoSz reinforced polyimide*
8.3 0.3
70
7.0 Nit Nil Nil
0.6 0.1 0.1 0.1
0.4
-
Nil -
Nil -
Nil 0.4 0.4 -
Nil -
Nil 0.3 0.8 .-
-
11.7 1.5
-
7.8 1.0 1.0 0.4
46.0 3.8 10.5 10.5
19.8 2.5
59.0 3.5 3.4 8.0
Top ball
Top ball
En31 steel
At 280 “C
At ambient temperature Cage
Mean loss of weight (mg)
Cage material
Test material
Comparative rolling wear tests under a total axial load of 100 kg at 1500 rev min-’
TABLE 5
0.3
-
-
0.6
0.3
0.4
0.4 F F
-
-
0.4 0.5 30.0
-
Cage
[icj
Fig. 11. Scanning electron micrograph showing evidence ticles and of sliding contact (magnification 180 x ). Fig. 12. Scanning electron micrograph showing silicon nitride (magnification $00 X )~
Fig. 13. Fine erosion-type Fig. 14. Smooth 60 x ).
polished
pitting
of tungsten
appearance
damage
carbide
of the bearing
of bonding by erosion
(magnification track
in silicon
and smearing to the bearing
66
of partrack of
x ).
nitride
(magnification
in areas devoid of fibres (Fig. 15). Cracking and fracture appeared to initiate in such areas. The virgin polyimide-based cage material was not homogeneous (Fig. 16). Some areas were depleted of carbon fibres and these areas generally contained a concentration of large particles of molybdenum disulphide (Fig. 17). The inhomogeneous structure seems to be the cause of the large variation in performance of cages in this material.
Conclusions Conventional ball bearing steels of a composition that is modified to reduce deleterious carbide structural effects whilst maintaining properties by
Fig. 15. Porosity, cracking and subsurface reinforced PTFE (magnification 96 X ). Fig. 16. Inhomogeneous 30 x ).
structure
deformation
in rubbed
of a polyimide-based
Fig. 17. Concentration of MO& in a carbon cage material (magnification 78 X ).
fibre depleted
glass fibre and MoSz
cage material
(magnification
region of the polyimide-based
matrix strengthening compare favourably in rolling contact fatigue resistance with steels of conventional analysis. The increased silicon content to provide matrix strengthening may also control deleterious interstitial nitrogen content by forming fine silicon nitride precipitates; these were identified in microstructures produced by the NEL heat-treatment procedure. Sound homogeneous material produced by the powder route with uniform fine carbide distribution and a gas content comparable with conventionally cast material seems potentially suitable for rolling bearings. Interstitial nitrogen content appears to have a significant effect on rolling contact fatigue resistance, and improved elevated temperature performance may be due to the metallographic structural changes brought about by the test conditions, the interstitial nitrogen content being reduced by a precipitation process. Heat-treatable electrodeposited materials, free from deleterious nonmetallic inclusions and having rolling contact fatigue resistances comparable
85
with currently used materials, are potentially suitable for the production of lightweight rolling elements and hollow rolling elements and races for high speed applications. Hot-pressed silicon nitride and sintered tungsten carbides appear potentially attractive for the arduous conditions of unlubricated rolling contact. In situ lubrication of such materials can be effected by reinforced polymeric composite cage materials to reduce wear with very little loss of the sacrificial cage material.
Acknowledgments The paper is published by permission of the Director of the National Engineering Laboratory. It is Crown cop~ight and is reproduced by permission of the Controller of Her Britannic Majesty’s Stationery Office.
References 1 D. Scott and J. Blackwell, An accelerated test for the study of materials under rolling contact, Proc. Inst. Mech. Eng., London, Part 3N, 178 (1964) 63. 2 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. 3 F. T. Barwell and D. Scott, The effect of lubricant on the pitting failure of ball bearings, Engineering (London), 182 (1956) 9 - 12. 4 D. Scott, Studies in rolling surface fatigue -the four-bat1 machine with a ball-ended upper specimen, Wear, 5 (1962) 69 - 71. 5 D. Scott, Lubricants at higher temperatures: assessing the effects on ball bearing failures, Engineering (London), 185 (1958) 660 - 668. 6 D. Scott, Rolling elements for elevated temperatures, Symp. on Lubrication in Hostile Environments, Proc. Inst. Mech. Eng., London, 183 (34) (1969) 9 - 17. 7 D. Scott, Ball bearing steels - factors influencing their performance. In Low Alloy Steels, Proc. BISRA-ISI Conf. on Low Alloy Steels, London, Iron and Steel Institute, 1968, pp. 203 - 209. 8 D. Scott and J. Blackwell, The effect of the steelmaking process on the life of rolling contact bearings, 2nd Lubrication and Wear Convention, Proc. Inst. Mech. Eng., London, Part 3N, 178 (1964) 81 - 88. 9 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. 10 D. Scott and J. Blaekwell, Steel refining as an aid to improved ball bearing life, 6th Lubrication and Wear Convention, Proc. Inst. Mech. Eng., London, Part 3N, 182 (1968) 239 - 242. 11 D. Scott, The effect of steelmaking, vacuum melting and casting techniques on the life of rolling bearings, Vacuum, 19 (4) (1969) 167 - 169. 12 D. Scott, Comparative rolling contact fatigue tests on En31 ball bearing steels of recent manufacture, Proc. IS1 Conf. on Tribology in Steelworks, Iron and Steel Institute Publ. 125, 1969, pp. 122 - 126. 13 D. Scott and J. Blackwell, Study of the effects of some elevated temperature lubricants on materials for rolling elements, 5th Lubrication and Wear Convention, Proc. Inst. Mech. Eng., London, Part 30,181 (1967) 77 - 84.
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D. Scott and P. J. McCullagh, Solid lubricant cage materials for the in situ lubrication of rolling mechanisms, Proc. Int. Solid Lubrication Symp., Tokyo, JSLE, Tokyo, 1975, pp. 49 - 50. D. Scott, Polymeric cage materials for the in situ lubrication of rolling bearings, Colloques Int. du CNRS, No 233, Polymeres et Lubrication, 1974, pp. 433 - 439. D. Scott and G. H. Mills, Polymeric composite materials for the lubrication of rolling bearings. In Advances in Polymer Friction and Wear, Plenum Press, New York, 1974. pp. 441 - 450. D. Scott and G. H. Mills, A scanning electron microscopical study of fibre reinforced polymeric cage materials for rolling bearings, Proc. Electron Microscopy Conf., 19’72, Institute of Physics, London. D. Scott and G. H. Mills, A scanning electron microscopical study of fibre reinforced polymeric cage materials for rolling bearings, Polymer, 14 (1973) 130 - 132. D. Scott, Factors influencing the performance of sintered carbides under arduous conditions of rolling contact, Wear, 32 (3) (1975) 309 - 314. D. Scott and J. Blackwell, Hot-pressed silicon nitride as a rolling bearing material a preliminary assessment, Wear, 24 (I) (1973) 61 - 67. D. Scott, J. Blackwell and P. J. McCullagh, Silicon nitride as a roiling bearing material -a preliminary assessment, Wear, 17 (2) (1971) 72 - 86. D. Scott, Materials for elevated temperature rolling bearings. In Groupement pour I’Avancement de la Mecanique Industrielle, Journees d’Etude sur l’usure, Paris, 1970. D. Scott, A study of hard non-ferrous materials for arduous conditions of rolling contact. In Proc. Symp. on Recent Developments in Non-Ferrous Metal Technology, NML Jamshedpur, India, Vol. 3, 1968, pp. 279 - 285. D. Scott, Hard materials, Proc. Int. Conf. on Lubrication and Wear, Proc. Inst. Mech. Eng., London, Part 3A, 182 (1967) 325 - 341. D. Scott and J. Blackwell, Study of sintered carbides and surface treated materials for unlubricated and elevated temperature rolling elements, 5th Lubrication and Wear Convention, Proc. Inst. Mech. Eng., London, Part 30,181 (1967) 77 - 84. D. Scott and J. Blackwell, Study of some materials for unlubricated and elevated temperature rolling elements, Rev. Metall. (Paris), March (1966) 257 - 264.