Rolling bearing with long service life and high wear resistance

In a rolling bearing in which at least one member of an inner race, outer race and rolling elements is made of an alloy steel containing 0.2 to 1.0 wt % of C, 3.0 to 14.0 wt % of Cr and 0.8 to 3.0 wt % of V, the balance being Fe, the member being carburized or carbonitrided, followed by hardening and tempering to secure that the surface carbon concentration C% and the concentrations Cr%+V% of Cr and V satisfy a relationship of C%.ltoreq.0.13 (Cr%+V%)+1.10 and that carbides existing on a surface of the member have an area fraction of 15 to 50%, the rolling bearing has an extending service life even if it is used under lubrication in the presence of foreign matter and exhibits superior wear resistance to be hard to wear even if there occurs a breakage of oil film.

BACKGROUND OF THE INVENTION 
The present invention relates to a rolling bearing for use in vehicles, 
agricultural machines, construction machines, steel making machines or the 
like. More specifically, the present invention relates to a rolling 
bearing for applications in which long service life and high wear 
resistance are required. 
Rolling bearings are used under extremely severe conditions such that they 
are subjected to repeated shearing stress under high contact pressure. In 
order to withstand the applied shearing stress to thereby secure the 
necessary rolling fatigue life, conventional rolling bearings have been 
made of a high-carbon chromium bearing steel (SUJ 2), which is hardened 
and tempered to provide the hardness of H.sub.R C 58 to 64. 
Case hardening steels (e.g. SCR 420, SCM 420, SAE 4320H, and the like) have 
also been used in rolling bearings. To secure the required service life, 
those steels are carburized or carbonitrided, followed by hardening and 
tempering so as to have the surface hardness of HRC 58 to 64 and the core 
hardness of H.sub.R C 30 to 48. 
Recently, since vehicles with higher speed performance, lighter weight and 
better fuel economy, as well as steel making facilities with minimum 
maintenance are desired, the conditions for the use of bearings has become 
extremely severe. Under the circumstances, several problems have surfaced, 
such as the flaking of the bearing surface due to the damage caused by 
foreign matter, contamination, debris and the like which enter with a 
lubricant, progressive wear due to insufficient lubrication, and the like. 
On the other hand, there is an increasing demand for extending the service 
life of bearings under severe conditions. 
To deal with this situation, for example, Unexamined Japanese Patent 
Publication No. Hei. 2-277764 proposed a method in which a high chromium 
steel is carburized or carbonitrided to precipitate fine-grained carbides 
in a bearing surface layer and the amount of retained austenite is limited 
in an appropriate range to assure a longer life under lubrication in the 
presence of foreign matter. This method, however, has not given entire 
consideration to the wear due to insufficient lubrication. 
SUMMARY OF THE INVENTION 
The present invention aims at providing a rolling bearing that not only has 
an extended service life under lubrication in the presence of foreign 
matter but also exhibits superior wear resistance. 
The present invention can be attained by providing a rolling bearing with 
an inner race, an outer race and a plurality of rolling elements, in which 
at least one member of the inner race, the outer race and the rolling 
elements is made of an alloy steel containing: 0.2 to 1.0 wt % of C; 3.0 
to 14.0 wt % of Cr; 0.8 to 3.0 wt % of V; at most 3.0 wt % of Mo; and the 
balance being Fe, the member being carburized or carbonitrided, followed 
by hardening and tempering to satisfy a relationship of: C%&lt;0.13 
(Cr%+V%)+1.10 and to satisfy that carbides existing on a surface of the 
member have an area fraction of 15 to 50%, where C% is a surface carbon 
concentration and Cr%+V% is concentrations of Cr and V.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Description will be given below in detail of an embodiment of a rolling 
bearing according to the present invention with reference to the 
accompanying drawings. 
It is conventionally known to improve the wear resistance of bearing 
materials by adding large amounts of carbide-forming elements such as Cr, 
Mo and V so that carbides existing in the bearing's surface layer are 
precipitated in large quantities. This method is based on the idea of 
providing better wear resistance by increasing the amount of carbides to 
such an extent that the service life is not adversely affected. On the 
other hand, the present inventors aim at the carbide structure and the 
amount of carbides existing in the bearing's surface layer and studied the 
relationships between the carbide structure and the amount of carbides 
existing in the bearing's surface layer, and between the service life and 
the wear resistance of bearings. As a result, the inventors found out that 
the addition of V to have VC-type carbides precipitated was extremely 
effective in improving the wear resistance of bearings. 
Further, the inventors found out that when the concentrations of Cr and V 
satisfied a specified relationship with the concentration of C, extremely 
fine-grained and hard VC-type carbides were precipitated while preventing 
coarse and brittle M.sub.3 C-type carbides from precipitating. In 
addition, by limiting the area fraction of carbides to be within a 
specified range, a rolling bearing could be produced that had not only 
superior wear resistance but also an extending service life. The present 
invention has been accomplished on the basis of these findings. 
There various functions of the elements contained in the rolling bearing of 
the present invention, as well as the criticality of the content of each 
element, are described below. 
C: 0.2 to 1.0 wt % 
Carbon is the essential element for forming carbides and transforming the 
base metal to martensite to increase the hardness after hardening and 
tempering. 
The carbon content is set to at least 0.2 wt % in order to prevent 
reticulate coarse carbides from precipitating at grain boundaries as a 
mixed structure of an austenitic phase and unsolved carbides during 
carburizing or carbonitriding. The upper limit of the carbon content is 
set to 1.0 wt % because the toughness of the core of the bearing is 
deteriorated if carbon is contained in amounts greater than 1.0 wt %. 
Cr: 3.0 to 14.0 wt % 
Chromium improves hardenability and functions to strengthen the base metal. 
Additionally, if Cr is added in a large amount, the carbides produced by 
carburizing or carbonitriding are transformed to M.sub.7 C.sub.3 type 
having such a property as to make the carbides hard and slow in grain 
growth to prevent them from becoming coarse, so that the addition of Cr 
contributes to the improvement of the rolling fatigue strength. 
The lower limit of the chromium content is set to 3.0 wt % in order to 
strengthen the base metal while securing to prevent coarse M.sub.3 C-type 
carbides from being produced. The upper limit of the chromium content is 
set to 14.0 wt % because if the upper limit is exceeded, macro carbides 
are produced in a stage of raw materials and stress concentration occurs 
around the carbides to deteriorate the service life. 
V: 0.8 to 3.0 wt % 
Vanadium is an element which is effective to increase the resisting 
property for temper softening and to produce extremely fine-grained and 
hard VC carbides, so that the addition of V contributes to the improvement 
of wear resistance and life property. 
The lower limit of the vanadium content is set to 0.8 wt % because the 
addition effect of vanadium is not sufficiently exhibited if it is added 
in less than 0.8 wt %. The upper limit of the vanadium content is set to 
3.0 wt % because the excess addition of vanadium not only deteriorates 
grindability but also increases in cost. 
Mo: .ltoreq.3.0 wt % 
Molybdenum is a selective element which is effective to increase the 
resisting property for temper softening, similarly to vanadium. The 
addition of Mo, which is similar to chromium, also serves to improve the 
rolling fatigue strength by preventing the coarse-graining of carbides 
which are precipitated as the result of carburizing or carbonitriding. 
The upper limit of the Mo content is set to 3.0 wt % because excess 
addition of Mo not only deteriorates the plastic workability but also 
becomes higher in cost. 
C%.ltoreq.0.13 (Cr%+V%)+1.10 
The surface carbon concentration attained by carburizing or carbonitriding 
satisfies the following relation with the concentrations of Cr and V: 
EQU C%.ltoreq.0.13 (Cr%+V%)+1.10 
and the area fraction of the carbides is set to a range of 15 to 50%. 
If the surface carbon concentration satisfies the relation of 
C%.ltoreq.0.13(Cr%+V%)+1.10, extremely fine-grained and hard VC-type 
carbides are precipitated, so that the precipitation strengthening 
achieves a remarkable improvement in wear resistance. In contrast, if the 
surface carbon concentration indicates the relation of 
C%&gt;0.13(Cr%+V%)+1.10, the coarse and brittle M.sub.3 C-type carbides are 
precipitated, so that the improvement in wear resistance becomes 
insufficient. 
If the area fraction of the carbides exceeds 15%, the improvement in wear 
resistance is remarkable. However, if the area fraction of the carbides 
exceeds 50%, the raw materials containing the carbides are extremely 
deteriorated in mechanical strength. 
EXAMPLE 
Examples of the present invention will now be described with reference to 
the accompanying drawings. 
FIG. 1 is a front view of an embodiment of the rolling bearing 1 of the 
present invention. The rolling bearing 1 includes an outer race 2, an 
inner race 3, rolling elements 4 and a cage 5. 
Various alloy compositions of raw materials used for the rolling bearing 1 
are indicated in Table 1. 
TABLE 1 
______________________________________ 
No. C Cr V Mo Remarks 
______________________________________ 
1 0.3 3.2 1.4 -- Invention 
2 0.4 7.8 1.5 -- Invention 
3 0.4 13.8 1.5 -- Invention 
4 0.7 5.1 0.9 -- Invention 
5 0.8 5.3 2.6 -- Invention 
6 0.8 5.0 2.3 2.4 Invention 
7 0.5 5.3 0.5 -- Comparison 
8 0.5 1.8 2.1 -- Comparison 
9 1.0 1.5 -- -- Convention 
10 0.2 1.0 -- -- Convention 
______________________________________ 
Alloy composition Nos. 1 to 6 are alloy steels according to the present 
invention. Alloy composition No. 7 is an alloy steel not within the scope 
of the present invention in terms of the V content. Alloy composition No. 
8 is an alloy steel not within the scope of the present invention in terms 
of the Cr content. Alloy composition No. 9 (JIS SUJ 2) and No. 10 (JIS SCR 
420) are alloy steels that free of vanadium and also out of the scope of 
the present invention in terms of the Cr contents. 
Comparative tests for confirming the effect of the present invention were 
conducted as shown in Table 2. Test Specimens A to P were formed of the 
respective alloy steels having the compositions shown in Table 1 to be 
employed in each tests as described hereinafter. The thus prepared test 
specimens were subjected to heat treatments such as carburizing, hardening 
and the like. The heat treatments were performed under the following 
conditions. 
Test Specimens A to F according to the present invention and Test Specimens 
G to N serving as comparative examples were subjected to the carburizing 
which was performed in a carburizing gas atmosphere at 920.degree. to 
950.degree. C. for 4 to 8 hours, followed by hardening and subsequently 
tempering at 180.degree. C. for 2 hours. Further, in the case where the 
carbonitriding was performed, a carbonitriding temperature was set to 
840.degree. to 870.degree. C. and ammonia gas 3 to 5% was added in the 
carburizing gas atmosphere. The other carbonitriding conditions were 
similar to the carburizing and hardening treatment. 
Test Specimen O formed of steel species SUJ 2 was subjected to the 
hardening at 840.degree. C., followed by tempering at 180.degree. C. for 2 
hours. 
Test Specimen P formed of steel species SCR 420 was subjected to the 
carburizing at 930.degree. C. for 4 hours, followed by secondary hardening 
at 860.degree. C. and tempering at 180.degree. C. for 2 hours. 
The test specimens subjected by the necessary heat treatment were evaluated 
for heat treatments quality (in terms of the surface carbon concentration 
and the area fraction of carbides). In addition, they were subjected to 
wear tests and life tests. The results were then compared. 
The term "surface carbon concentration" as used herein means the average 
carbon concentration (wt %) in a range from a surface serving as an 
rolling surface to a depth which is equivalent to 2% of the diameter of 
the rolling elements. The term "the area fraction of carbides" indicates 
the proportion measured in the surface. 
The wear tests were conducted with a two-cylinder wear tester as shown in 
FIGS. 2A and 2B. Test specimens S were mounted on a pair of cylinders 10 
vertically opposing to each other, which were held in mutual contact and 
rotated contrary at low speed under a load P that applies from above of 
the wear tester. The wear rates (mg/m) relative to both specimens S were 
measured to obtain the average value. In order to test for wear resistance 
under insufficient lubrication, the rotating specimens were supplied with 
a lubricant 11 which was so low in viscosity that an oil film was easy to 
break. 
The specific conditions for the wear test were as follows. 
______________________________________ 
Load 100 kgf 
Contact pressure 100 kgf/mm.sup.2 
Rotational speed 10 rpm 
Slip ratio 30% 
Lubricant #10 spindle oil 
Oil temperature 60.degree. C. 
______________________________________ 
The life test was conducted with a thrust rolling life tester under 
lubrication in the presence of foreign matter. The thrust rolling life 
tester is disclosed in "Guide for Special Steel (1st ed.)" edited by the 
Institute of Electric Steel, RIKO-GAKUSHA, 1965, chap.10, p.21. 
The specific conditions for the life test were as follows: 
______________________________________ 
Load 655 kgf 
Contact pressure 500 kgf/mm.sup.2 
Rotational speed 1,000 rpm 
Lubricant #68 turbine oil 
Foreign matter Steel powder (SUS 420 J2); 
hardness H.sub.R C 52; particle 
size, 80 to 160 .mu.m 
Concentration of 300 ppm 
foreign matter 
______________________________________ 
The results of the wear and life test are shown in Table 2. The wear rates 
(mg/m) shown in Table 2 were obtained by using the two-cylinder wear 
tester with two-cylinder test specimens having dimensions of 30 mm in 
outer diameter, 16 mm in inner diameter, and 7 mm in width, in addition, 
the balls (outer diameter of 3/8 inch) which were used in the 
above-described thrust rolling life test as the rolling elements. The wear 
rates (mg/m) were obtained in the surface carbon concentration (wt %) 
providing the two-cylinder test specimens with the average carbon 
concentration in a range from a surface to a depth which was equivalent to 
2% of the outer diameter of the ball. 
TABLE 2 
__________________________________________________________________________ 
Area 
Surface carbon 
fraction of 
Wear 
Test concentration 
carbides 
Heat rate 
L.sub.10 life 
Specimen 
Alloy 
(wt %) (%) treatment 
(mg/m) 
(.times.10.sup.6 cycles) 
Remarks 
__________________________________________________________________________ 
A 1 1.5 20 carburizing 
0.18 
17.8 Invention 
and 
hardening 
B 2 1.8 32 carburizing 
0.17 
20.2 Invention 
and 
hardening 
C 3 2.9 42 carburizing 
0.16 
21.1 Invention 
and 
hardening 
D 4 1.6 27 carburizing 
0.19 
16.4 Invention 
and 
hardening 
E 5 2.0 31 carburizing 
0.12 
18.9 Invention 
and 
hardening 
F 6 1.9 30 carburizing 
0.13 
23.2 Invention 
and 
hardening 
G 1 1.9 28 carburizing 
0.33 
11.2 Comparison 
and 
hardening 
H 3 3.3 48 carburizing 
0.32 
12.4 Comparison 
and 
hardening 
I 4 2.2 39 carburizing 
0.31 
11.0 Comparison 
and 
hardening 
J 5 2.7 44 carburizing 
0.28 
8.7 Comparison 
and 
hardening 
K 4 0.9 9 carburizing 
0.36 
7.2 Comparison 
and 
hardening 
L 5 1.0 10 carburizing 
0.31 
9.1 Comparison 
and 
hardening 
M 7 1.6 25 carburizing 
0.24 
15.2 Comparison 
and 
hardening 
N 8 1.3 23 carburizing 
0.18 
10.8 Comparison 
and 
hardening 
O 9 1.0 8 through 
0.42 
2.1 Convention 
hardening 
P 10 0.9 4 carburizing 
0.43 
6.5 Convention 
and 
hardening 
__________________________________________________________________________ 
As is apparent from Table 2, Test Specimens A to E according to the present 
invention had at least twice the wear resistance and service life of Test 
Specimens O and P i.e., the conventional examples. Test Specimen F, which 
additionally had 2.4 wt % Mo, has a longer life than Test Specimens A to 
E. 
Test Specimens G to J were comparative examples having excessively high 
concentrations of C. Since they had coarse and brittle M.sub.3 C-type 
carbides precipitated on the surface, the improvement in wear resistance 
therein was not sufficient. Test Specimens K and L were comparative 
examples having excessively small carbide contents. Also, the improvement 
in wear resistance was not sufficient. 
Test Specimen M had vanadium V added in an amount less than the lower limit 
specified by the present invention. Evidently, the improvement in wear 
resistance was not sufficient. Test Specimen N had chromium Cr added in an 
amount less than the lower limit specified by the present invention. 
Evidently, the improvement of service life property was not sufficient in 
this example. 
FIG. 3 shows the relationship between the area fraction of carbides and the 
wear rate. When the area fraction of carbides exceeded 15%, the 
improvement in wear resistance was remarkable. As is also apparent from 
FIG. 3, the wear resistance was extremely superior when the relation of 
0%.ltoreq.0.13(Cr%+V%)+1.10 was satisfied. 
FIG. 4 shows the relationship between the concentrations of Cr+V and C 
existing in the surface layer, relative to the carbide structure. The area 
under the straight line expressed by C=0.13(Cr+V)+1.10 refers to the range 
of examples according to the present invention. The area above the 
straight line refers to the range of comparative examples. 
Additionally, in order to check the mechanical strength of Alloy 
Composition Nos. 1 and 5, ring-shaped test pieces (inner diameter 13 mm; 
outer diameter 20 mm; width, 8 mm) were made of those compositions and 
subjected to crushing tests. The thus prepared test pieces were carburized 
at 920.degree. to 950.degree. C. for 4 to 8 hours, followed by hardening 
and tempering at 180.degree. C. for 2 hours, and grinding to the specified 
dimensions. The test pieces had varying area fractions of carbides on the 
surface to conduct the crushing tests. 
The crushing tests were conducted at a ram speed of 0.5 mm/min, using an 
autograph made by Shimadzu Corp. The test results are shown in FIG. 5. 
Evidently, the crushing load abruptly deteriorated when the area fraction 
of carbides exceeded 50%. 
The above-described examples refer to the case where the present invention 
is applied to the inner race of a rolling bearing. The present invention 
is applicable, either individually or wholly, to the other components of a 
rolling bearing such as the outer race and rolling elements. In the case 
where the present invention is applied to the rolling elements, the 
surface carbon concentration is defined by the average carbon 
concentration (wt %) in a range from a surface of the rolling element to a 
depth which is equivalent to 2% of the diameter of the rolling element. 
As for the heat treatment, the present invention is also applicable to the 
rolling bearing subjected to carbonitriding as well as carburizing. 
As described above, the rolling bearing of the present invention is 
characterized in that at least one of the inner race, outer race and 
rolling elements is made of an alloy steel containing 0.2 to 1.0 wt % of 
C, 3.0 to 14.0 wt % of Cr and 0.8 to 3.0 wt % of V, with the balance being 
Fe and other alloy elements, the alloy steel being carburized or 
carbonitrided and subsequently hardened and tempered to secure that the 
surface carbon concentration and the concentrations of Cr and V satisfy 
the following relation: 
EQU C%.ltoreq.0.13 (Cr%+V%)+1.10 
and that the carbides have an area fraction of 15 to 50%. 
Because of these essential features, extremely fine-grained and hard 
VC-type carbides are precipitated in the steel structure while coarse and 
brittle M.sub.3 C-type carbides are prevented from precipitating; thus, 
the rolling load concentrating at the edges of indentations which form on 
the respective surfaces of the inner and outer races and the rolling 
elements in a rolling bearing as the result of repeated contact with the 
foreign matter in a lubricant is effectively mitigated to prevent the 
occurrence of microcracks and achieve a remarkable improvement in the wear 
resistance of the bearing. Hence, the rolling bearing of the present 
invention is long-lived even if it is used under lubrication in the 
presence of foreign matter and, additionally, it features superior wear 
resistance.