US20070240791A1

US20070240791A1 - Rolling, sliding part and process for producing same

Info

Publication number: US20070240791A1
Application number: US11/764,426
Authority: US
Inventors: Katsuhiko Kizawa
Original assignee: JTEKT Corp
Current assignee: JTEKT Corp
Priority date: 2001-11-14
Filing date: 2007-06-18
Publication date: 2007-10-18
Also published as: US20030102054A1; EP1312694B1; EP1312694A3; EP1312694A2

Abstract

A rolling, sliding part comprises a bearing steel, is subjected to a carburizing treatment and has a surface layer from a surface the part to a depth to which a maximum shear stress acts. The surface layer is 1.0 to 1.6 wt. % in total carbon content, has a matrix containing 0.6 to 1.0 wt. % of solid-solution carbon, and contains a precipitating carbide in an amount of 5 to 20% in area ratio, the carbide being up to 3 μm in particle size. The rolling or sliding part is improved in wear resistance, usable with a lubricant free of or less susceptible to impression by extraneous matter present in the lubricant, and less susceptible to compressive stress due to the extraneous matter, with the development of cracks suppressed.

Description

BACKGROUND OF THE INVENTION

The present invention relates to rolling, sliding parts and a process for producing the same, and more particularly to races of antifriction bearings and rolling parts for use as rolling elements of such bearings, or sliding parts suitable for use in sliding bearings, as adapted for use with a lubricant containing extraneous matter, and to a process for producing such parts.
To give a prolonged life to antifriction bearings for use with a lubricant containing extraneous matter becoming mixed therewith, it is required to increase the surface hardness of the races and rolling elements of the bearing, for example, to 62 to 67 in Rockwell C hardness (hereinafter referred to as “HRC”), to precipitate a predetermined amount of fine carbide as dispersed in the surface layers of the raceway surfaces and the rolling surfaces of the rolling elements, to adjust the amount of retained austenite in the surface layers of the raceway surfaces and the rolling surfaces of the rolling elements, for example, to a predetermined value of 20 to 40 vol. %, and to impart a predetermined compressive residual stress, for example, of at least 100 MPa to the surface layers of the raceway surfaces and the rolling surfaces of the rolling elements. The increase in the surface hardness is intended to prevent impression by extraneous matter, and the dispersion of fine carbide precipitate is intended to give improved wear resistance, ensure dispersion strengthening of the material by precipitating a hard second phase of carbide and the like in steel, increased yield strength and higher deformation resistance and improved toughness. The retained austenite as adjusted in amount to the predetermined value is intended to reduce the compressive stress attributable to the extraneous matter and suppress the development of cracks due to extraneous matter. The predetermined compressive residual stress to be given is intended to inhibit the development of cracks due to the extraneous matter.
Conventionally used as rolling, sliding parts fulfilling the above requirements are those produced by preparing blanks of case-hardened steel in a specified shape and carburizing or carbonitriding the blanks.
However, case-hardened steel to be used for bearings is small in amount, is not produced in large quantities, is therefore costly as a material, and further requires an expensive heat treatment for carburizing or carbonitriding. The steel accordingly has the problem of increasing the overall manufacturing cost of rolling, sliding parts.
It appears useful to produce rolling, sliding parts by preparing blanks of JIS SUJ2 or like bearing steel in a specified form and carburizing or carbonitriding the blanks, whereas it is then impossible to afford an increased surface hardness and form a finely divided carbide at the same time. For example, if the steel JIS SUJ2 which originally has a high carbon content and contains carbide is carburized to give an increased surface hardness, the existing carbide further grows to form large particles of carbide. Consequently, the product has the problem of exhibiting a shortened life when used with a lubricant containing extraneous matter.

DISCLOSURE OF THE INVENTION

An object of the present invention is to overcome the above problems and to provide a rolling, sliding part and a process for producing the same, the rolling, sliding part being improved in wear resistance, usable with a lubricant free of or less susceptible to impression by extraneous matter present in the lubricant, and less susceptible to compressive stress due to the extraneous matter, with the development of cracks suppressed.
The term “rolling, sliding part” refers to a part which is brought into contact with other part, the term “contact” involving pure rolling contact, pure sliding contact and both rolling contact and sliding contact.
The present invention provides a rolling, sliding part comprising a bearing steel, subjected to a carburizing treatment and characterized in that a surface layer of the part from a surface thereof to a depth to which a maximum shear stress acts is 1.0 to 1.6 wt. % in total carbon content, the surface layer having a matrix containing 0.6 to 1.0 wt. % of solid-solution carbon, the surface layer containing a precipitating carbide in an amount of 5 to 20% in area ratio, the carbide being up to 3 μm in particle size.
The rolling, sliding part of the present invention has an increased surface hardness, is free from or less susceptible to impression by the extraneous matter contained in the lubricant, is improved in wear resistance, and is therefore usable for affording antifriction bearings of prolonged life. The part is made of bearing steels (High carbon chromium bearing steels) which are produced in large quantities, accordingly low in material cost and consequently low in overall manufacturing cost. Especially JIS SUJ2 is available by mass production unlike other bearing steels, so that use of this steel results in a very low material cost and is preferred.
The present invention provides an antifriction bearing which comprises an inner and an outer race and rolling elements, and one of the inner and outer races and the rolling member comprises the rolling, sliding part of the invention described. The inner race of the antifriction bearing is particularly liable to break because the contact between the raceway surface thereof and the rolling elements is contact between projecting or bulging portions which produces an increased compressive stress. In the case where the rolling, sliding part of the invention is used for the inner race of an antifriction bearing, the inner race can be prevented from breaking, giving a prolonged life to the antifriction bearing.
The present invention provides a first process for producing a rolling, sliding part characterized in that a worked blank of the part prepared from a bearing steel in a predetermined form is carburized by being heated at 840 to 870° C. for at least 3 hours in a carburizing atmosphere having a carbon potential of at least 1.2% and thereafter quenched to thereby form a surface layer of the part from a surface thereof to a depth to which a maximum shear stress acts, the surface layer being 1.0 to 1.6 wt. % in total carbon content and having a matrix containing 0.6 to 1.0 wt. % of solid-solution carbon, the surface layer containing a precipitating carbide in an amount of 5 to 15% in area ratio, the carbide being up to 3 μm in particle size.
The first process for producing the rolling, sliding part uses a bearing part blank made of a bearing steel which is generally used for bearings and is therefore low in material cost. The blank is carburized at a temperature of 840 to 870° C. once and thereafter quenched, and can therefore be heat-treated at a low cost. The bearing part is therefore low in total manufacturing cost. Among bearing steels, JIS SUJ2 is available especially by mass production, so that use of this steel results in a very low material cost and is preferred.
The present invention provides a second process for producing a rolling, sliding part characterized in that a worked blank of the part prepared from a bearing steel in a predetermined form is heated at 930 to 970° C. for at least 1 hour in an atmosphere having a carbon potential of 0.9 to 1.1% to thereby dissolve the existing carbide in a matrix, then quenched, subsequently carburized by being heated at 840 to 870° C. for at least 3 hours in an atmosphere having a carbon potential of at least 1.2% and thereafter quenched to thereby form a surface layer of the part from a surface thereof to a depth to which a maximum shear stress acts, the surface layer being 1.0 to 1.6 wt. % in total carbon content and having a matrix containing 0.6 to 1.0 wt. % of solid-solution carbon, the surface layer containing a precipitating carbide in an amount of 10 to 20% in area ratio, the carbide being up to 2 μm in particle size.
According to the second process of the invention for producing the rolling, sliding part, the blank is treated to dissolve the existing carbide in the matrix and thereafter carburized, so that finely particulate carbide can be caused to separate out again from the nuclei of the solid-solution carbide in the matrix. This prevents fatigue cracking and assures toughness, giving a prolonged life to the rolling, sliding part. Furthermore, the use of the bearing steel which is available by mass production results in a lower material cost. As a result, the bearing part obtained is low in total manufacturing cost. Among bearing steels, JIS SUJ2 in particular is available by mass production, so that use of this steel entails a very low material cost and is preferred.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing heat treatment condition 1.
FIG. 2 is a diagram showing heat treatment condition 2.
FIG. 3 is a diagram showing heat treatment condition 3.
FIG. 4 is a diagram showing heat treatment condition 4.
FIG. 5 is a diagram showing heat treatment condition 5.
FIG. 6 is a diagram showing heat treatment condition 6.
FIG. 7 is a diagram showing heat treatment condition 7.
FIG. 8 is a diagram showing heat treatment condition 8.
FIG. 9 is a diagram showing heat treatment condition 9.
FIG. 10 is a diagram showing heat treatment condition 10.
FIG. 11 is a diagram showing heat treatment condition 11.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The values relating to the rolling, sliding part of the invention are limited for the following reasons.
Total Carbon Content of Surface Layer
The total carbon content is limited to 1.0 to 1.6 wt. % because if the content is over the upper limit, the carbide becomes very coarse and can not be finely divided. The lower limit is inevitably determined because a bearing steel such as JIS SUJ2 is used as the base material.
Solid-Solution C Content of Surface Layer Matrix
The solid-solution carbon content is limited to 0.6 to 1.0 wt. % because amounts less than the lower limit fail to afford the desired surface hardness, permitting the extraneous matter in the lubricant to produce impressions on the part, and because if the upper limit is exceeded, the amount of fine carbide in the surface layer becomes smaller than 5% in area ratio to entail lower wear resistance.
Amount of Fine Carbide in Surface Layer
The amount of fine carbide is limited to 5 to 20% in area ratio because amounts less than the lower limit result in impaired wear resistance, and because if the upper limit is exceeded, coarse carbide particles are formed, serving as starting points of fatigue cracks and giving a shortened life to the rolling, sliding part.
Particle Size of Fine Carbide in Surface Layer
The particle size is limited to not smaller than 3 μm because if greater than 3 μm, the carbide provides starting points of fatigue cracks like nonmetallic inclusions, failing to ensure toughness.
In producing the rolling, sliding part of the invention, it is desired that the carburizing temperature be 840 to 870° C. The temperature is then lower than the heating temperature conventionally used for carburizing or carbonitriding steel for case hardening to result in a lower heat treatment cost, hence a lower total manufacturing cost.
With the rolling, sliding part of the invention, the amount of carbide may be 5 to 15% in area ratio. The upper limit for the amount of carbide is 15% for the following reason. If the amount of the carbide is to be increased to more than 15% in area ratio, the blank needs to be carburized for a longer period of time. This results in an increased heat treatment cost, making the total manufacturing cost higher than when the area ratio is up to 15%.
With the rolling, sliding part of the invention, the amount of carbide may be 10 to 20% in area ratio, and the particle size thereof may be up to 2 μm. The lower limit for the amount of the carbide is 10% in area ratio because if less than 10%, the amount of carbide particles of micron order and submicron order become insufficient to result in the likelihood that the effect to improve the rolling fatigue life will not be available. The carbide of micron order is effective for preventing formation of a slip band that will cause rolling fatigue, while the carbide of submicron order is effective to diffuse the slip band although having no effect to prevent formation of the slip band. The carbide is up to 2 μm in particle size because if greater than 2 μm, carbide particles will provide starting points of fatigue cracks like nonmetallic inclusions, possibly failing to ensure toughness.
With the rolling, sliding part of the invention, the surface layer may have a surface hardness of 62 to 67 in Rockwell C hardness, contain 20 to 40 vol. % of retained austenite and have a compressive residual stress of at least 100 MPa. When the surface layer has a surface hardness of 62 to 67 in HRC, the part is prevented from becoming impressed by the extraneous matter present in the lubricant, whereas such an effect is not available if the hardness is less than 62 in HRC. Although there is a need to give an increased solid-solution carbon content to the matrix of the surface layer in order to give the surface layer a surface hardness greater than 67 in HRC, the amount of fine carbide in the surface layer then reduces, shortening the life of the part as used in the lubricant containing extraneous matter. Further when the surface layer contains 20 to 40 vol. % of retained austenite, it is possible to reduce the compressive stress of the surface layer attributable to the presence of the extraneous matter in the lubricant and to suppress the development of cracks, consequently giving a further prolonged life to antifriction bearings comprising the rolling, sliding part. This effect is nevertheless unavailable if the amount of retained austenite is less than 20 vol. %, whereas if the amount is over 40 vol. %, the surface layer fails to have a surface hardness of at least 62 in HRC. When the surface layer has a compressive residual stress of at least 100 MPa, the development of cracks can be inhibited, consequently giving a further extended life to antifriction bearings comprising the rolling, sliding part.
The carburizing treatment included in the first process of the invention for producing rolling, sliding parts involves numerical values which are limited for the following reasons.
Incidentally, the same reasons as given above for the rolling, sliding part described can be explained for limiting the total carbon content of the surface layer, the amount of solid-solution carbon in the matrix of the surface layer, the lower limit for the amount of carbide in the surface layer, the particle size of the carbide in the surface layer, and the upper limit for the amount of carbide in the surface layer.
Carbon Potential of Carburizing Atmosphere
The carbon potential is limited to not smaller than 1.2% because if the value is less than 1.2%, the bearing steel which is about 1 wt. % in carbon content then remains almost free of carburizing, with the result that the surface layer fails to have the desired hardness and the desired carbide area ratio, and that a finely divided carbide is not available.
Carburizing Temperature
The temperature is limited to 840 to 870° because temperatures lower than the lower limit fail to effect the desired carburizing treatment described with reference to the carbon potential, whereas if the temperature is in excess of the upper limit, the surface layer has excessively large grain sizes and permits precipitation of large carbide particles to entail an impaired strength. Stated more specifically, the yield strength is in proportion to the −½ power of the grain size, so that excessively great grain sizes lead to a lower strength.
Carburizing Time
The time is limited to at least 3 hours because an insufficient carburizing depth will result if the time is less than 3 hours.
In the first process of the invention for producing rolling, sliding parts, the carbon potential is preferably 1.2 to 1.4% since if the carbon potential is in excess of 1.4%, there arises the problem that a large quantity of soot will be produced.
Further in the first process of the invention for producing rolling, it is desired that the heating time be 3.5 to 5 hours because if the heating time is more than 5 hours, a higher heat treatment cost will result along with the problem that vary large carbide particles will be formed.
The heat treatment included in the second process of the invention for producing rolling, sliding parts involves numerical values which are limited for the following reasons.
Incidentally, the same reasons as given above for the rolling, sliding part described can be explained for limiting the total carbon content of the surface layer, the amount of solid-solution carbon in the matrix of the surface layer, the amount of carbide in the surface layer, and the particle size of the carbide in the surface layer.
Step of Causing Existing Carbide to Dissolve in Matrix
The atmosphere used for this step is limited to 0.9 to 1.1% in carbon potential to prevent the carburizing and decarbonization of the bearing part. If the potential is in excess of 1.1%, the bearing steel becomes carburized which is about 1 wt. % in carbon content, while the steel becomes decarbonized if the potential is less than 0.9%.
The heating temperature for this step is limited to 930 to 970° C. because at temperatures of lower than 930° C., the carbide present as a second phase after spheroidizing will not form a sufficient solid solution in the matrix, and further because at temperatures in excess of 970° C., the steel is likely to develop a quenching crack.
The heating time for this step is limited to at least one hour because if the time is less than 1 hour, the carbide present as the second phase after spheroidizing fails to form a sufficient solid solution in the matrix.
Carburizing Step
The carbon potential is limited to not smaller than 1.2% because if the value is less than 1.2%, the bearing steel which is about 1 wt. % in carbon content then remains almost free of carburizing, with the result that the surface layer fails to have the desired hardness and the desired carbide area ratio, and that a finely divided carbide is not available. The upper limit for the carbon potential is preferably 1.4% in order to preclude production of a large quantity of soot.
The heating temperature for this step is limited to 840 to 870° because temperatures lower than the lower limit fail to effect the desired carburizing treatment described with reference to the carbon potential, whereas if the temperature is in excess of the upper limit, the surface layer has excessively large grain sizes and permits precipitation of large carbide particles to entail an impaired strength. Stated more specifically, the yield strength is in proportion to the −½ power of the grain size, so that excessively great grain sizes lead to a lower strength.
The heating time for this step is limited to at least 3 hours because the required carburizing depth will not be available if the time is less than 3 hours.
With the second process of the invention for producing rolling, sliding parts, it is desired that the amount of carbide in the surface layer obtained by the carburizing treatment be 13 to 16%.
The reason is that if the amount of carbide is at least 13% in area ratio, this further increases the foregoing effect of the carbide of micron order to prevent formation of the slip band and the above-mentioned effect of the carbide of submicron order to diffuse the slip band which effects have been described with reference to the rolling, sliding part already described, resulting in an improved rolling fatigue life. In view of the cost, it is reasonable that the amount of carbide resulting from gas carburizing be up to 16% in area ratio.
In the case of the rolling, sliding parts of the invention and the two processes of the invention for producing such rolling, sliding parts, the surface layer from the surface to a depth to which a maximum shear stress acts is made to have the state described above for the following reason. Within the range wherein the maximum shear stress acts to cause internally initiated separation, the part is given the foregoing values with respect to the total carbon content, solid-solution carbon content of the matrix and carbon content thereof and is thereby improved in strength, whereby the contemplated object is fulfilled. With rolling, sliding parts typical of which are common antifriction bearing parts, the depth to which the maximum shear stress acts is in the range of from the surface to a depth of 0.5 mm.

EXAMPLES

Specific examples of the invention are given below along with comparative examples.

Examples 1-3 and Comparative Examples 1-4

Two kinds of steels were prepared as listed in Table 1 and used to make seven kinds of inner race blanks for use in antifriction bearings, type number 6206.

TABLE 1

Composition (wt. %)

Steel type Fe C Si Mn Ni Cr Mo

A(SUJ2) Bal. 1.01 0.24 0.36 0.04 1.46 0.01

B(case- Bal. 0.20 0.20 0.80 0.05 0.85 0.01

hardened

steel)
The inner race blanks were then heat-treated under the conditions shown in FIGS. 1 to 5 to produce inner races (Examples 1-3 and Comparative Examples 1-4).
The heat treatment condition 1 shown in FIG. 1 is such that the blank was heated at 850° C. for 3 hours in an atmosphere having a carbon potential of 1.3% and thereafter quenched to 80° C. in oil.
The heat treatment condition 2 shown in FIG. 2 is such that the blank was heated at 850° C. for 0.7 hour in an atmosphere having a carbon potential of 0.6% and thereafter quenched to 80° C. in oil.
The heat treatment condition 3 shown in FIG. 3 is such that the blank was heated at 930° C. for 5 hours in an atmosphere having a carbon potential of 0.8%, subsequently heated at 850° C. for 0.7 hour in an atmosphere with a carbon potential of 0.8% and thereafter quenched to 80° C. in oil.
The heat treatment condition 4 shown in FIG. 4 is such that the blank was heated at 930° C. for 5 hours in an atmosphere having a carbon potential of 1.3%, subsequently heated at 850° C. for 0.7 hour in an atmosphere with a carbon potential of 1.1% and thereafter quenched to 80° C. in oil.
The heat treatment condition 5 shown in FIG. 5 is such that the blank was heated at 930° C. for 5 hours in an atmosphere having a carbon potential of 1.3%, then quenched to 80° C. in oil, subsequently heated at 850° C. for 3.5 hours in an atmosphere with a carbon potential of 1.3% and thereafter quenched to 80° C. in oil.
Although not shown in the diagrams, the blank in each of the foregoing heat treatments was finally heated at 160° C. for 2 hour for tempering.
Table 2 shows the steels, heat treatment conditions and heat treatment costs of the inner races of Examples 1-3 and Comparative Examples 1-4 thus produced. In Table 2, the heat treatment condition 1A corresponds to the heat treatment condition 1 wherein the heating time only was changed to 5 hours, and the heat treatment condition 1B corresponds to the heat treatment condition 1 wherein the heating time only was changed to 3.5 hours. The heat treatment costs are represented by the numerals 1 to 5 in the order of increasing cost.

TABLE 2

Heat treatment

Type of steel Condition Cost

Example 1 A 1 2

Example 2 A 1A 2

Example 3 A 1B 2

Comp. Ex. 1 A 2 1

Comp. Ex. 2 B 3 3

Comp. Ex. 3 B 4 4

Comp. Ex. 4 B 5 5

Table 3 shows the properties or particulars of the inner races of Examples 1-3 and Comparative Examples 1-4 with respect to the hardness (HRC) of the raceway surface, total carbon content of the outermost surface of the raceway, solid-solution carbon content of matrix of the raceway outermost surface, amount (area ratio) of precipitating carbide on the raceway outermost surface, maximum particle size of precipitating carbide on the raceway outermost surface, amount (amount γ_R) of retained austenite at the position with a depth of 50 μm from the surface, and compressive residual stress at the position with a depth of 50 μm from the surface.

TABLE 3


Surface	Total C	Solid-soln. C
hardness	content	content	Carbide area
(HRC)	(wt. %)	(wt. %)	ratio (%)

Example 1	65.2	1.32	0.80	9.6
Example 2	65.0	1.58	0.83	14
Example 3	64.8	1.40	0.81	11
Comp. Ex. 1	61.5	1.02	0.60	5.6
Comp. Ex. 2	62.1	0.85	0.72	2.3
Comp. Ex. 3	63.8	1.07	0.83	3
Comp. Ex. 4	65.4	1.29	0.75	9.3

Max.		Compressive
particle		residual
size (μm)	γ_R(vol. %)	stress (MPa)	L₁₀life (h)

Example 1	1.3	23	150	27.9
Example 2	2.8	32	178	28.4
Example 3	2.1	28	134	33.2
Comp. Ex. 1	1.1	7	30	4.1
Comp. Ex. 2	0.3	25	200	10.3
Comp. Ex. 3	0.5	26	255	15.6
Comp. Ex. 4	1.5	31	280	26.8

Evaluation Test 1

Each of the inner races of Examples 1-3 and Comparative Examples 1-4, and an outer race and balls which were made from JIS SUJ2 and subjected to a usual carbonitriding treatment were assembled into a ball bearing, type number 6206C3. The ball bearings thus obtained were subjected to a life test using a lubricant containing extraneous matter becoming mixed therewith. The test conditions are given in Table 4.

	TABLE 4


	Tester	KS radial life tester
	Radial load	Fr = 9000N/set
	Speed of rotation	2500 rpm
	Extraneous matter	0.06 wt. % of high-speed steel powder
		(730 Hv in hardness, 100-150 μm
		in particle size)
	Lubrication	turbine oil #68, oil bath (aerated)
	Oil temperature	spontaneous rise in temp. (about
		100° C.)
	Calculated life	67.8 h
	Test method	2 bearings × 5 sudden death tests
		(evaluation of life of inner race)

Incidentally the tester shown in Table 4 is adapted to test two ball bearings at the same time, and the radial load in Table 4 means the radial load of one ball bearing.
Table 3 also shows the result of the life test.
L₁₀life listed in Table 3 was determined by setting two ball bearings having the same inner races on the tester, testing the bearings to measure the time taken for one of the ball bearings to break, repeating this test five times, and calculating the average of the time measurements.
The results described above indicate that the products of Examples 1-3 according to the invention are especially low in material cost because JIS SUJ2 which is the largest of the bearing steels in the amount manufactured is used. Further the products of the invention are also low in heat treatment cost and give a prolonged life to bearings when used for the bearings.

Examples 4 and Comparative Examples 5-9

The two kinds of steels listed in Table 1 were used to make six kinds of inner race blanks for use in antifriction bearings, type number 6206.
The inner race blanks were then heat-treated under the conditions shown in FIGS. 6 to 11 to produce inner races (Example 4 and Comparative Examples 5-9).
The heat treatment condition 6 shown in FIG. 6 is such that the blank was heated at 950° C. for 2 hours in an atmosphere having a carbon potential of 1.1%, then quenched to 80° C. in oil, subsequently heated at 850° C. for 3.5 hours in an atmosphere with a carbon potential of 1.3% and thereafter quenched to 80° C. in oil.
The heat treatment condition 7 shown in FIG. 7 is such that the blank was heated at 830° C. for 0.7 hour in an atmosphere having a carbon potential of 0.6% and thereafter quenched to 80° C. in oil.
The heat treatment condition 8 shown in FIG. 8 is such that the blank was heated at 850° C. for 3.5 hours in an atmosphere having a carbon potential of 1.3% and thereafter quenched to 80° C. in oil.
The heat treatment condition 9 shown in FIG. 9 is such that the blank was heated at 930° C. for 5 hours in an atmosphere having a carbon potential of 0.8%, subsequently heated at 850° C. for 0.7 hour in an atmosphere with a carbon potential of 0.8% and thereafter quenched to 80° C. in oil.
The heat treatment condition 10 shown in FIG. 10 is such that the blank was heated at 930° C. for 5 hours in an atmosphere having a carbon potential of 1.3%, subsequently heated at 850° C. for 0.7 hour in an atmosphere with a carbon potential of 1.1% and thereafter quenched to 80° C. in oil.
The heat treatment condition 11 shown in FIG. 11 is such that the blank was heated at 930° C. for 5 hours in an atmosphere having a carbon potential of 1.3%, then quenched to 80° C. in oil, subsequently heated at 850° C. for 3.5 hours in an atmosphere with a carbon potential of 1.3% and thereafter quenched to 80° C. in oil.
Although not shown in the diagrams, the blank in each of the foregoing heat treatments was finally heated at 160° C. for 2 hour for tempering.
Table 5 shows the steels, heat treatment conditions and heat treatment costs of the inner races of Example 4 and Comparative Examples 5-9 thus produced. The heat treatment costs are represented by the numerals 1 to 4 in the order of increasing cost.

TABLE 5

Heat treatment

Type of steel Condition Cost

Example 4 A 6 3

Comp. Ex. 5 A 7 1

Comp. Ex. 6 A 8 2

Comp. Ex. 7 B 9 3

Comp. Ex. 8 B 10 3

Comp. Ex. 9 B 11 4

Table 6 shows the properties or particulars of the inner races of Example 4 and Comparative Examples 5-9 with respect to the hardness (HRC) of the raceway surface, total carbon content of the outermost surface of the raceway, solid-solution carbon content of matrix of the raceway outermost surface, amount (area ratio) of precipitating carbide on the raceway outermost surface, maximum particle size of precipitating carbide on the raceway outermost surface, amount (amount γ_R) of retained austenite at the position with a depth of 50 μm from the surface, and compressive residual stress at the position with a depth of 50 μm from the surface.

TABLE 6


Surface	Total C	Solid-soln. C
hardness	content	content	Carbide area
(HRC)	(wt. %)	(wt. %)	ratio (%)

Example 4	65.0	1.45	0.81	15.5
Comp. Ex. 5	61.5	1.01	0.65	5.6
Comp. Ex. 6	65.2	1.39	0.82	9.6
Comp. Ex. 7	62.1	0.86	0.67	2.3
Comp. Ex. 8	63.8	1.03	0.82	3
Comp. Ex. 9	65.4	1.38	0.85	9.3

Example 4	1.55	25	175	41.9
Comp. Ex. 5	1.68	7	30	4.1
Comp. Ex. 6	2.17	23	150	27.9
Comp. Ex. 7	0.3	25	200	10.3
Comp. Ex. 8	0.5	26	255	15.6
Comp. Ex. 9	1.5	31	280	26.8

Evaluation Test 2

Each of the inner races of Example 4 and Comparative Examples 5-9, and an outer race and balls which were made from JIS SUJ2 and subjected to a usual carbonitriding treatment were assembled into a ball bearing, type number 6206C3. The ball bearings thus obtained were test for life in the same manner as Evaluation Test 1 using a lubricant containing extraneous matter becoming mixed therewith.
Table 6 also shows the result of the life test.
L₁₀life listed in Table 6 was determined by setting two ball bearings having the same inner races on the tester, testing the bearings to measure the time taken for one of the ball bearings to break, repeating this test five times, and calculating the average of the time measurements.
The results described above indicate that the product of Example 4 according to the invention is especially low in material cost because JIS SUJ2 which is the largest of the bearing steels in the amount manufactured is used. Further the product of the invention gives a prolonged life to bearings when used for the bearings.

Claims

1. A process for producing a rolling, sliding part, comprising carburizing a worked blank of the part, prepared from a high carbon chromium bearing JIS SUJ 2 steel, in a predetermined form, by a single carburizing step of heating the steel at 840 to 870° C., for at least 3 hours, in a carburizing atmosphere having a carbon potential of at least 1.2% as the only carburizing step of the process, and thereafter quenching, to thereby form a surface layer of the part, from a surface thereof, to a depth to which a maximum shear stress acts, the surface layer being 1.0 to 1.6 wt. % in total carbon content, and having a matrix containing 0.6 to 1.0 wt. % of solid-solution carbon, the surface layer contains 20 to 40 vol. % of retained austenite, and the surface layer containing a precipitating carbide, in an amount of 5 to 15% in area ratio, the carbide being up to 3 μm in particle size, and further, the surface layer has a surface hardness of 62 to 67 in Rockwell C hardness and has a compressive residual stress of 100 to 178 MPa.

2. The process of claim 1, wherein the single carburizing step of heating the steel at 840 to 870° C., for at least 3 hours, in a carburizing atmosphere having a carbon potential of at least 1.2% permits adjusting area ratio and particle size of the carbide to be precipitated in the surface layer and amount of retained austenite in the surface layer.

3. The process of claim 1, wherein the single carburizing step is performed for a period of from 3.5 to 5 hours.