WO1991000929A1

WO1991000929A1 - Process for case-hardening roller bearing components of low-alloy nickel steel

Info

Publication number: WO1991000929A1
Application number: PCT/EP1989/000787
Authority: WO
Original assignee: Aga Ab
Priority date: 1989-07-07
Filing date: 1989-07-07
Publication date: 1991-01-24
Also published as: EP0480924A1

Abstract

In a process for case-hardening roller bearing components of low-alloy nickel steel, the roller bearing components are carburised under a dissociating gas, cooled, heated to hardening temperature, austenitised in the carburising layer and finally quenched. To prevent the embrittlement of the roller bearing components through dissociated hydrogen from the carburising gas, they are cooled from the carburising temperature to about 300°C under a moving inert gas and then maitained at this temperature so that adequately dissociated hydrogen is released from the carburised layer of the components. The subsequent heating of the roller bearing components to hardening temperature or intermediate annealing temperature is preferably performed directly after their maintenance at 300°C.

Description

Process for case hardening of rolling elements made of low-alloy, nickel-containing steel

The present invention relates to a method for case hardening rolling element elements made of low-alloy, nickel-containing steel according to the preamble of claim 1.

Highly stressed roller elements, e.g. B. large Wälzlαgerrings for rolling mills are often made of a low-alloy, nickel-containing steel, which is surface hardened in the application process. In this case, an intermediate annealing is sometimes necessary after the carburizing and before the carburizing layer of the roller bearing elements has been austenitized in order to homogenize the steel structure and to reduce internal stresses.

In a known method for case hardening of rolling elements of the type mentioned, the rolling bearing elements are slowly cooled to room temperature after carburizing in propane gas, so that the hydrogen absorbed during carburization by dissociation of the propane gas in the carburizing layer ¬ partly from the carburizing layer to the outside can diffuse out (US Pat. No. 3,737,204). However, this cooling must not take place too slowly, because otherwise grain boundary carbides are precipitated in the structure of the carburizing layer. Such an excretion would change the hardness structure and the rolling fatigue life of the rolling element, e.g. B. Wälzlαgerrings reduce considerably. In the known method, some of the atomic and molecular hydrogen remains in the carburizing layer. In the case of steels containing nickel, this hydrogen causes embrittlement of the hardness structure of the roller elements, which can lead to dangerous crack formation.

In contrast, the present invention is based on the task of creating a method for case hardening of rolling element elements, in particular large rolling element rings, from nickel-containing, low-alloy steels of the type specified, in which embrittlement of the hardness structure of the rolling element elements is largely caused by the dissociated hydrogen of the carburizing gas is avoided. The process should also be economically applicable.

This object is achieved according to the characterizing part of claim 1.

With the method according to the invention it is achieved that the roller bearing elements after carburizing in an inert gas, for. B. nitrogen, cool. The inert gas acts as a protective gas, so that the rolling bearing elements do not undergo any oxidation and decarburization on their surfaces. Due to the movement of the inert gas, the roller elements cool down quickly enough so that the precipitation of grain boundary carbides is just prevented. The cooling also takes place in a certain cooling time, so that part of the hydrogen of the carburizing agent taken up in the carburizing layer during carburizing can diffuse out of the carburizing layer.

In this way, then only a relatively short time at about 300 ^β C needs to be held to have diffuse out to the still verblie¬ surrounded in the carburizing rest of the hydrogen.

Accordingly, an extraordinarily short heat treatment time is required to remove the hydrogen from the carburizing layer, so that the method according to the invention can be used particularly economically with little expenditure of time and energy.

Further advantageous measures of the method according to the invention are characterized in the subclaims. With the measure according to claim 2 it is achieved that when the roller bearing elements are kept at 300 ° C., the remainder of the dissociated hydrogen remaining after cooling from the carburizing heat diffuses out of the carburizing layer of the roller bearing elements to such an extent that the risk of crack formation on the Rolling bearing elements is turned away.

With the measure according to claim 3, there is a small need for heating energy for carrying out the method according to the invention, because when the bearing elements are heated to the hardening temperature, heating from 300 ° C. to the hardening temperature is required.

The measure according to claim 4 has the effect that the steel structure of the roller bearing elements is made stress-free and homogenized. This is particularly important in the case of large rolling bearing rings, because after cooling from the carburizing heat they can have considerable residual stresses.

Since this heat treatment does not cool to room temperature (20 ° C), the heating energy requirement is also relatively small.

A further heating energy saving is achieved with the additional measure according to claim 5.

With the measure according to claim 6 it is achieved that after carburizing and before hardening (austenite isizing the carburizing layer), the relatively soft rolling bearing elements can still be machined.

The measure according to claim 7 results in an economically justifiable heat treatment time.

The measure according to claim 8 provides the advantage that all treatment stages from carburizing to intermediate annealing take place in one and the same furnace and under nitrogen as a protective gas.

The measure according to claim 9 indicates the possibility of also performing the hardening in the furnace used for carburizing, so that results in an extremely economical heat treatment of the rolling bearing elements in an automated series or mass production.

Finally, with the measure according to claim 10, a steel available on the market, e.g. B. 17 NiCrMo 14 used. This steel is particularly suitable for the production of large rolling bearing rings.

In the following, the method according to the invention for case hardening of rolling element elements made of low-alloy, nickel-containing steel is explained in more detail with reference to the drawings, see illustration of inventive method sequences in FIGS. 1, 2 and 3.

For the production of Wälzlαgerelemente, z. B. large Wälzlαgerrings, a steel of the grade 17 NiCrMo 14 is expediently used, but the reduced values of S <usual for rolling bearing steels. 0.035% and P <0.035%. The other analysis values (%) of this steel are:

Remainder iron and melting impurities

The roller bearing rings made from this steel are carburized at around 970 ° C. For this purpose, these are placed individually or in groups in a carburizing furnace which works with propane gas (C_H.) As carburizing agent and heated to 970 ° C. The roller rings are then 10 to 20 hours to produce a sufficiently thick carburizing layer Held at 970 ° C. in the reeling furnace, the propane gas penetrating into the surface of the roller rings and producing carbon and hydrogen as decomposition products in a manner known per se.

In the further heat treatment, the procedure is as follows

that the rolling bearings are in the carburizing furnace from the carburizing heat are cooled to about 300 ° C. in agitated nitrogen, so that the precipitation of grain boundary carbides in the structure of the steel is avoided and, moreover, sufficient cooling time remains so that part of the dissociated hydrogen diffuses out of the carburizing layer,

that the rolling bearing rings are then kept in the carburizing furnace at about 300 ° C. with a holding time of at least 1 hour in a nitrogen atmosphere, so that a further part of the dissociated hydrogen escapes from the carburizing layer and

that the roller bearing elements are heated to a hardness temperature of about 805 ° C in the carburizing furnace immediately after being held at 300 ° C, austenitized at this hardness temperature and then quenched in air, oil or salt to achieve a surface hardness of 60 to 64 HRC (Fig. 1) .

With large rolling bearing rings, it is advisable to have an intermediate annealing after cooling and holding at 300 ° C. The bearing rings are then heated after holding at about 300 ° C directly from this temperature to about 610 ° C and at 610 C ^β to homogenize the microstructure and to intermediately Spαnnungsfreimαchung. This intermediate annealing is best done with a holding time of 8 hours. The intermediate annealing temperature can then be directly heated to the hardening temperature and hardened (FIG. 2). The heating to H "tetemperatur and curing the ^/ Välzlαgerringe can in accordance with heated carburizing erfol¬ gen so that the often heavy bearing rings during the entire treatment Wärme¬ not, for the carburizing furnace to another oven. B. incandescent ¬ oven or curing oven , must be transported.

If intermediate processing is necessary, the roller bearing rings can also be slowly cooled in nitrogen or air after the intermediate annealing from 610 ° C. to room temperature (20 °) (FIG. 3). Further dissociated hydrogen is released from the carburizing layer. The time to cool to room temperature (20 ° C) is about 1 hour. After the intermediate processing, the roller bearing rings are brought from room temperature to hardening temperature, hardened and quenched in air, oil or salt.

After hardening, tempering is usually also provided so that the roller bearing elements are given a martensitic structure which gives the surface (roller bearing surface) a hardness of 58 to 62 HRC.

The roller elements are finally ground and, if necessary, honed or polished on their tread.

Instead of propane gas as the carburizing agent, another G Gaass ,, zz .. BB .. MMeetthhaann ((CCHH ..)) ,, which releases carbon and hydrogen when carburizing can be used.

Claims

. Process for case hardening of rolling elements, in particular large rolling rings, made of low-alloy, nickel-containing steel, in which the rolling bearing elements for producing a carburizing layer at about 970 ° C. in a carburizing furnace in a carburizing gas, e.g. B. propane gas (C -), held with dissociative carbon and hydrogen, cooled, warmed to hardening temperature and austenitized in the carburizing layer and finally quenched to achieve a surface hardness of 60 to 64 HRC, characterized thereby,

- That the rolling bearing elements are cooled from the carburizing tip to about 300 ° C. in moving inert gas, so that the elimination of grain boundary carbides in the steel structure is avoided and part of the dissociated hydrogen of the carburizing gas diffuses out of the carburizing layer during cooling and

- That the roller bearing elements are then kept at this temperature in this inert gas, so that a further part of the dissociated hydrogen escapes from the carburizing layer.

2. The method according to claim ^" i, characterized in that the holding takes place at about 300 ° C with a holding time of at least 1 hour.

3. The method according to claim 1 or 2, characterized in that is heated immediately after holding at 300 ° C from this temperature to the hardening temperature of about 850 ° C.

4. The method according to claim 1 or 2, characterized in that immediately after holding to about 300 ° C heated from this temperature to about 610 ° C and is annealed at this temperature.

5. The method according to claim 4, characterized in that immediately after the intermediate annealing from 610 ° C to a hardening temperature of about 805 ° C and austenitized at this temperature.

6. The method according to claim 4, characterized in that after intermediate annealing from 610 ° C to room temperature (20 ° C) is slowly cooled in air.

7. The method according to claim 4, 5 or 6, characterized in that the intermediate annealing takes place with a holding time of up to 8 hours.

8. The method according to any one of the preceding claims 4 to 7, characterized in that the cooling after carburizing, holding at 300 ° C, heating to 610 ° C and intermediate annealing at 610 ° C in nitrogen as an inert gas in the nitrogen atmosphere baren, correspondingly heatable carburizing furnace.

9. The method according to claim 8, characterized in that the heating to hardening temperature and the austenitizing of the rolling bearing elements also take place in the correspondingly heatable carburizing furnace.

10. The method according to any one of the preceding claims, characterized gekenn¬ characterized in that a steel is used with the following analysis values (%):

0 0, 15 - 0, 20

Ni 3.25-3.75

Cr 1.30-1.60

Mon 0.15-0.25

Mn 0.40-0.70

S max. 0, 035

P max. 0, 035

Remainder iron and wear-related impurities.