JPH05331615A - Rolling bearing parts made of nonmagnetic steel - Google Patents

Abstract

PURPOSE:To produce a nonmagnetic roller bearing excellent in rolling life by subjecting roller bearing parts made of nonmagnetic steel with specific composition to electric discharge carburizing treatment and providing a surface hardened layer with specific surface hardness and also a specific magnetic permeability. CONSTITUTION:The roller bearing parts are formed by using a nonmagnetic Mn-Cr austenitic steel having a composition consisting of 0.1-0.7% C, <=1% Si, 12-20% Mn, <=5% Ni, 10-18% Cr, <=0.50% N, and the balance Fe with inevitable impurities. The parts are heated and held at about 950-1200 deg.C and subjected to electric discharge carburizing treatment in a hydrocarbon gas atmosphere of about 1-50mmHg for 2-3hr, followed by air cooling. By this method, the rolling bearing parts having a surface hardened layer with a surface hardness of >=HRC55 Rockwell C hardness and also having <=1.1% magnetic permeability can be obtained, and rolling life can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rolling bearing used in a strong magnetic field and relates to an improvement of a rolling bearing obtained by surface-carburizing and hardening a non-magnetic Mn-Cr austenitic steel.

[0002]

2. Description of the Related Art Rolling bearings used in a strong magnetic field, for example, rolling bearings for axle bearings used in the vicinity of superconducting magnets of linear motor cars, are
Since it rotates by cutting off the strong magnetic field from the superconducting magnet, if it is made of a ferromagnetic material such as martensitic steel, eddy currents will be generated in the rolling elements and bearing rings of the bearing, causing heat generation and increasing the temperature. Along with this, there is a risk that seizure will occur.

The bearing used in such a strong magnetic field needs to be non-magnetic. As non-magnetic steel, Cr-Ni
Although it seems that austenitic steels of 18 series (for example, 18Cr-8Ni series) can be used, it is expensive and a method effective for surface hardening treatment has not been put to practical use, and a martensite phase is generated during processing due to work deformation. Since it is magnetized, it is not used for bearing steel.

As a conventional austenitic steel that is stable at room temperature, there is a high carbon Mn-Cr non-magnetic steel, which can be hardened and used by subjecting it to solution treatment by adding V and aging treatment.

The present inventors have already performed a nitriding treatment on mechanical parts such as rolling bearing members made of high Mn-high Cr austenitic non-magnetic steel in advance, and then carbonitriding them to obtain nitrogen. A bearing having a surface-hardened layer formed of an enriched layer has been proposed (Japanese Patent Application No. 2-262268).

[0006]

In the formation of a bearing made of non-magnetic steel, first, the magnetic permeability is stable in the vicinity of 1 and the surface layer is preferably a surface hardened layer having a hardness of H _RC 55 or more. It is required to be 0.2 mm or more. In the above-mentioned conventional bearing obtained by age hardening Mn-Cr austenitic steel,
The surface hardness was only Rockwell C hardness H _{RC of} about 40 to 45, and it was still soft as a rolling bearing material, and the wear resistance of the rolling surface was insufficient.

With Mn-Cr austenitic steel, a surface hardened layer cannot be obtained by simply nitriding or carbonitriding in an atmospheric furnace such as a gas carburizing furnace, and a hardened layer of H _RC 55 or higher is obtained. Even if it is extremely thin, the hardened layer is removed by precision finish polishing of the rolling surface. It is also possible to form a TiN layer by irradiating nitrogen ions after precision processing. In this case, the surface hardness can be obtained, but the hardness of the lower surface layer required as a bearing is low, and the maximum of the lower surface of the rolling surface due to contact with rolling elements is low. Since the hardness from the depth at which shear stress acts to the depth position several times lower is low, the improvement of rolling life cannot be expected.

Therefore, the rolling life can be improved by making the hardened layer on the surface of the Mn-Cr non-magnetic steel a thick layer. Generally, when the Cr content is high, a passivation layer is formed on the surface. As a result, the carburizing / nitriding reaction is suppressed / inhibited, so a carburized layer thickness of 0.2 mm or more can be secured only after special pretreatment such as nitriding at low temperature before carbonitriding as described above. be able to. However, there is a problem in that two steps of nitriding treatment and carbonitriding treatment are required, and that the carbonitriding treatment requires a long time, resulting in lack of mass productivity.

The present invention is intended to provide a non-magnetic rolling bearing having a sufficient hardened layer thickness of Mn-Cr austenitic steel and having an excellent rolling life.

[0010]

The rolling bearing of the present invention comprises:
C 0.1 to 0.7%, Si 1% or less, Mn 12 to 20
%, Ni 5% or less, Cr 10-18%, N 0.50% or less, and formed by steel consisting of the balance Fe and unavoidable impurities, the surface of Rockwell C hardness H _RC 55 or more by discharge carburizing or discharge carbonitriding A non-magnetic rolling bearing part having a hardened surface layer and having a magnetic permeability of 1.1% or less.

[0011]

The non-magnetic bearing component of the present invention is an austenitic steel containing a large amount of Mn and Cr, and C0.
Even if it contains 1 to 0.7%, it becomes stable non-magnetic and maintains the magnetic permeability within the range of 1.0 to 1.1.

The C content is preferable because it strengthens the austenite structure, but if it exceeds 0.7%, the internal portion is hardened,
The workability is impaired, so 0.7% is preferable as the upper limit.

[0013] Mn is necessary as an austenite forming element, but if a large amount is added, work hardening is large and workability is impaired, so 20% is preferable as the upper limit.

Cr is necessary because it coexists with Mn to promote and stabilize demagnetization and to prevent the generation of work-induced martensite by adding a large amount, and forms and disperses carbides and nitrides in the carburized layer in the surface layer portion. Then, it is important as an element for hardening. Addition of a large amount of Cr is preferable because it enhances corrosion resistance and high temperature strength, but if it exceeds 20% Cr, toughness is impaired.

Ni is an austenite-forming element, but when added in a large amount, it has magnetism that causes work-induced martensite in use, and is expensive, so the upper limit is 5%.

Since N is an austenite stabilizing element, it is preferably added together with Mn, and Ni and Mn can be partially replaced at low cost.

An austenitic steel having the above composition is used to form a bearing ring or rolling element of a rolling bearing, and a carburized hard layer of high carbon is formed on the outer surface of the bearing ring by discharge carburizing or discharge carbonitriding. The amount of C in the carburized layer should be in the range of 1 to 2%. In the electric discharge carburizing process, a voltage is applied between the rolling bearing and the anode arranged in the vicinity of the rolling bearing, and when glow discharge is performed under a reduced pressure in a hydrocarbon gas atmosphere, excited hydrocarbons and their dissociated ions are generated. It collides with the surface of the component, is adsorbed, diffuses and moves to the surface layer, and forms a carbon concentrated layer.

During the discharge carburization process, the surface of the Mn-Cr steel is activated and the carburization is promoted because the passivation film and oxide layer on the surface are removed and cleaned by the collision of the ions with the surface. In particular, prior to discharge carburization, if the surface is cleaned by a discharge action by discharging in an inert atmosphere under reduced pressure (for example, in Ar gas), carburization in the subsequent carburization process proceeds rapidly.

The discharge carburization is performed by using the above composition steel in the range of 950 to 120.
When heated and held at 0 ° C. and carried out in a hydrocarbon gas atmosphere of 1 to 50 mmHg in the carburizing process, a carburized layer of 0.2 mm or more, preferably 0.4 mm or more can be easily obtained in a processing time of 2 to 3 hours.

After carburizing, the furnace is allowed to cool while the pressure is reduced. The high concentration of C in the carburized layer forms and hardens due to the formation of carbides and Cr and Mn in the steel during carburization and cooling, and the surface hardness is H _RC.
The hardened layer has a hardness of 55 or more, and the wear resistance of the raceway surface is improved. During discharge carburizing, by adding ammonia gas together with hydrocarbon gas, carburizing and nitriding can be performed at the same time.By this carbonitriding, the surface hardness is higher than that of carburizing alone and the high temperature strength is increased by nitriding. Therefore, the heat resistance of the raceway surface is improved.

[0021]

Example As shown in Table 1, the test materials are 0.2% C,
0.5% Si, 18.0% Mn, 0.03% P, 0.0
Outer diameter 12 by hot working from steel with 20% S, 15.0% Cr, 3% Ni, 0.25% N composition (material A)
A cylindrical test piece having a length of 12 mm and a length of 12 mm was formed and subjected to the following carburizing treatment.

[0022]

[Table 1]

[0023] It is charged and fixed in a resistance heating type vacuum furnace, and is heated by resistance heating at a reduced pressure of 0.1 mmHg or less for about 30 min at around 1050 ° C, then Ar gas is introduced, and first, a graphite counter electrode. A cleaning operation was performed for 30 minutes by discharging by discharging between and. Then 1
While adjusting the pressure to ˜2 mmHg, propane gas was introduced into the furnace to carry out carburization by continuing discharge for 3 hours, and then cooled in the furnace while keeping the pressure reduced.

The C content in the surface layer portion after discharge carburization was 1.5%. Further, the cross-sectional hardness distribution of the surface layer portion is as shown in FIG.
Vickers hardness H from the surface to a depth of 0.2 mm
It was v600 or more. FIG. 2 shows a micrograph of the surface layer portion after discharge carburization. It can be seen that the interior portion is an austenite single phase, whereas the carburized layer in the surface portion is in a state where pearlite-like layered carbide is densely precipitated.

The magnetic permeability of the test piece after discharge carburizing was measured and the magnetic permeability was found to be 1.08.

As comparative examples, steels having the composition of the material B in Table 2 were used to prepare similar test pieces, which were age-hardened after solution treatment and those which were carbonitrided after nitriding in a gas furnace. It was

For the age-hardened product, the test piece of the above material B was heated at 1150 ° C. for 60 minutes and then cooled with oil, and 700 ° C. for 4 hours.
It was heated and air-cooled. The carbonitrided product is obtained by nitriding in molten salt at 580 ° C. for 1 hour, followed by heating at 840 ° C. for 6 hours in a carbonitriding atmosphere in a gas furnace and allowing it to cool.

Using the test rollers of the examples and comparative examples, a rolling life test was conducted with a line contact type life tester. The test conditions are 12 mm in diameter and 12 mm in length for the sample shape.
For the (sub-curvature radius 480 mm), a roller with a diameter of 20 mm and a length of 20 mm is used as the mating sample, and the contact load is 4.9 kN (contact stress 2.94 GPa) and 6.86 k.
With two levels of N (3.29 GPa), lubrication is forced circulation oil of turbine 68 oil, and load speed is 20400c.
pm. The test results are summarized in Table 2.

[0029]

[Table 2]

In Table 1, the raw material A of the example has a lower carbon content than the raw material B of the comparative example, and a large amount of both Cr and Mn is added. It can be seen from Table 2 that the test piece subjected to the discharge carburizing treatment of the example has a longer rolling life under the low load condition than the test piece subjected to the nitriding-carbonitriding treatment of the comparative example. This result indicates that the bearing has a sufficiently practical life as a bearing used under a medium load for vehicles. Further, since the magnetic permeability after the rolling test does not change and is practically non-magnetic, it can be used for bearings used in a strong magnetic field.

[0031]

INDUSTRIAL APPLICABILITY The rolling bearing component of the non-magnetic steel of the present invention is made of Mn-Cr austenitic steel, is non-magnetic, has a high Cr content and is unlikely to undergo work-induced transformation, and has a carburized surface layer portion. The layer also becomes non-magnetic.

At least on the surface layer portion of the raceway surface of the rolling bearing component, a hardened layer having an equivalent thickness of surface hardness H _RC 55 or more is obtained by electric discharge carburization to obtain the rolling life of the raceway surface required for the rolling bearing. Be done.

Since it is discharge carburizing, carburizing can be performed in a short time, and mass productivity of bearing heat treatment is excellent.

[Brief description of drawings]

FIG. 1 is a diagram showing a hardness distribution in the vicinity of the surface of a cross section of a specimen subjected to discharge carburization.

FIG. 2 is a metallographic photograph (magnification: 100) of the vicinity of the surface of the cross section of the specimen subjected to the discharge carburizing treatment by an optical microscope.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location F16C 33/34 7403-3J 33/62 7403-3J

Claims (1)

[Claims] 1. C 0.1 to 0.7%, Si 1% or less, M
n12-20%, Ni5% or less, Cr10-18%, N
It is formed of steel containing 0.50% or less of balance Fe and unavoidable impurities, and is provided with a surface hardening layer having a surface hardness of Rockwell C hardness H _RC 55 or more by discharge carburizing or discharge carbonitriding. Rolling bearing parts made of non-magnetic steel with a magnetic susceptibility of 1.1% or less.