WO1993017146A1

WO1993017146A1 - Process for the thermochemical-heat treatment of case-hardened steels

Info

Publication number: WO1993017146A1
Application number: PCT/EP1992/002951
Authority: WO
Inventors: Wenzel Bina; Dieter Eckert; Werner Kreiss
Original assignee: Ina Wälzlager Schaeffler Kg
Priority date: 1992-02-25
Filing date: 1992-12-18
Publication date: 1993-09-02
Also published as: DE4205647A1; EP0627019A1; DE4205647C2; DE59209268D1; EP0627019B1

Abstract

The invention relates to a process for the thermochemical-heat treatment of case-hardened steels in which an edge region of a workpiece, especially tappets, roller bearing components, drive and clutch components, can be enriched with carbon and nitrogen and subsequently subjected to martensitic hardening. In the process of the invention, carbon nitriding (1) is performed at temperature of 780 to 1050 °C with the case-hardening and nitriding of the edge region with 0.4 to 0.9 wt % carbon and 0.1 to 0.8 wt % nitrogen for a period of 1 to 4 hours, carbo-nitriding is followed by quenching (2) of a temperature well below the initial martensite point of the edge region, followed in turn by an annealing process (3) at a temperature of 20 to 40 °C above a nitrocarburising temperature at a heating rate of 10 to 30 °C per minute over a period of 1 to 2 hours and cooling (4) to room temperature, again followed by the material-removing shaping of the workpiece, after which finally nitrocarburing (6) takes place at a temperature of 500 to 600 °C for 60 to 150 minutes and cooling (7) to room temperature. The process of the invention gives the material high wear resistance and carrying capacity even under the heaviest tribologic stress.

Description

description

Process for the thermochemical-thermal treatment of case-hardening steels

The invention relates to a method for the thermochemical treatment of case-hardening steels, in which an edge zone of a workpiece, in particular cup tappets, roller bearing parts, gear and coupling elements, is enriched with carbon and nitrogen and then subjected to a martensitic hardening.

One such method is carbonitriding for treating a workpiece in the austenitic state with the purpose of enriching the surface layer with carbon and with nitrogen, both elements subsequently being in solid solution in the austenite. Following this treatment, quenching is generally carried out immediately in order to achieve hardening. Carbonitriding improves the surface hardness and wear resistance of the structural parts (technology of heat treatment of steel, p. 169 ff, VEB German publishing house for basic material industry, Leipzig 1986).

The workpieces treated according to this method have improved properties with regard to wear resistance, but these are not sufficient in every application for components which are subject to high tribological stress, such as the contact surfaces of tappets in the valve train of an internal combustion engine. In addition, because of the required shape accuracy in carbonitrided parts, these components have to be ground so that the highly enriched, wear-resistant outermost surface layer is at least partially ground away in the course of this cutting shaping.

Another method of increasing wear resistance is that Nitro carburizing. This is a thermochemical method for enriching the edge layer of a workpiece with nitrogen and carbon to form a connection layer, a diffusion layer enriched with nitrogen being formed above the connection layer. In addition to the presence of this sufficiently thick connection layer enriched with nitrogen and carbon, a prerequisite for the functionality of nitrocarburized parts is a corresponding supporting effect of the diffusion layer under the naturally more or less brittle connection layer.

The disadvantage of this method is that the diffusion layer under the connection layer tends to plastic deformation under high tribological stresses, such as foreign bodies in the oil circuit of an internal combustion engine, and as a result can damage the connection layer.

The object of the present invention is to create a thermochemical-thermal treatment method which ensures sufficient wear resistance in the case of components which are subject to high tribological stress.

This object is achieved according to the characterizing part of claim 1 by several successive method steps.

A first process step consists of carbonitriding at a temperature of 780 to 1050 ° C., with carburization and nitriding to 0.4 to 0.9 percent by weight of carbon and 0.1 to 0.8, preferably 0, 3 to 0.7 weight percent nitrogen is set. The high temperatures ensure that the austenite in the peripheral zone has a correspondingly high solvency for both carbon and nitrogen. The enrichment of the diffusion elements nitrogen and carbon has to be carried out in such a way that their solubility in austenite is not exceeded, ie the carbon potential in the atmosphere is to be adjusted in accordance with the SE line in the iron-carbon diagram. The same applies to the nitrogen supply according to the iron-nitrogen status diagram. The holding time during carbonitriding, which is one to four hours may depend on the desired hardening depth, the upper limit of which may be one millimeter. The chemical composition of the edge zone is achieved by diffusion of carbon and nitrogen at the temperatures mentioned in a known manner using a working gas which contains both carbon-donating components and nitrogen-donating components.

The second step in the process of carbonitriding is rapid subcooling of the hardened material by quenching in suitable media. The quenching should take place, for example in an oil bath, at temperatures well below the martensite starting point of the edge zone. As a result, the diffusion process of the iron companions nitrogen and carbon is interrupted and the cementite precipitation at the austenite grain boundaries is suppressed, and a structure is formed which is composed of martensite containing carbon and nitrogen and a residual austenite content of up to 50%. The surface hardness is between 65 and 55 Rockwell hardness. The aim of the simultaneous enrichment with carbon and nitrogen in the present case is to increase the tempering resistance of said case hardening steel against case hardening.

- The third step in the process of carbonitriding is followed by a heat treatment in which the material is tempered at 520 to 650 °, ie 20 to 40 ° C above the subsequent nitrocarburizing temperature. The heating rate is between 10 to 30 ° per minute and the holding time is about 1 to 2 hours. After tempering, the fourth process step is followed by cooling to room temperature, the cooling rate being selected so that no new stresses are generated in the component as a result of the cooling. Tempering at a temperature of 20 to 40 ° C. above the nitro carburizing temperature means that the structural state set by carbonitriding in the edge region of the workpiece no longer changes during the subsequent nitro carburizing due to temperature influences. Since any change in the microstructure is associated with an increase or decrease in volume, such a change in volume is excluded in the subsequent nitro-burying. In addition, the previous Carbonitriding with subsequent cooling with internal stresses frozen imbalance state converted into a structure in equilibrium at the nitro carburizing temperature. The reduction of internal stresses during tempering is also associated with dimensional and shape changes of the workpiece.

- After the tempering treatment, the changes in shape and size of the parts which have occurred as a result of the preceding treatment stages carbonitriding and tempering are corrected by a metal-cutting shaping process as a fifth process step in order to bring the parts to be nitro-carburized to the finished part dimension. If necessary, a volume growth due to the nitrogen and carbon uptake during nitro-carburizing has to be taken into account.

- After the cutting shaping, the nitro carburizing follows as the sixth step of the method according to the invention. The goal is to build a closed connection layer up to 20μm thick. For this purpose, the ground parts are treated at temperatures from 500 to 620 ° C for 60 to 150 minutes. Nitro carburizing takes place in a gas mixture of ammonia, carbon dioxide, nitrogen and endogas or exogas. The cooling of the nitrided material as the last step of the process can take place under protective gas in the furnace or by quenching in oil.

According to a preferred embodiment of the invention according to claim 2, the nitro carburizing is carried out at a temperature of 530 to 570 ° C. On the one hand, these temperatures are below the eutectoid temperature and, on the other hand, they are high enough to build up the connection layer with a sufficiently high growth rate. In addition, there is no additional structural transformation in the embroidered edge region in this temperature range, so that quenching and the associated changes in shape and form can be dispensed with.

In a development of the invention according to claim 3, the nitro carburizing can also be carried out in plasma or in a salt bath.

According to a further embodiment of the invention according to the preamble of independent claim 4, it is also possible that, instead of carbonitriding, case hardening takes place at a temperature of 780 to 1050 ° C. with a carburization of the peripheral zone of 0.4 to 0.9 percent by weight of carbon with a holding time of 1 to 4 hours . The subsequent method steps remain the same as described in the characterizing part of claim 1.

The process according to the invention for thermochemical-thermal treatment gives the material a high degree of wear resistance and load-bearing capacity, since the diffusion layer lying underneath the supporting layer has a significantly improved supporting effect, so that the connecting layer does not become plastically deformed even under the highest tribological stresses the underlying diffusion layer can be damaged.

The invention is explained in more detail using the following exemplary embodiment. Show it:

Figure 1 shows the individual process steps of the method according to the invention as a function of time and temperature;

Figure 2 shows a cross section in the region of the functional surface.

In the phase labeled 1, the carbonitriding takes place at a temperature of 780 to 1050 ° C. Depending on the desired hardening depth, the edge zone is carburized and nitrided within 1 to 4 hours. This phase is followed by a phase 2, in which the structure is quenched to a temperature well below the martensite starting point of the edge zone. In a third phase, the material is left at 20 to 40 ° C. above the nitro carburizing temperature for 1 to 2 hours. In a fourth phase, the material is cooled under an oven or protective gas atmosphere before, as shown in dashed lines, the parts are subjected to an exciting shaping process in a fifth phase in order to bring them to their finished part dimensions. In a sixth phase, a closed connecting layer 2 to 20 μm, preferably 6 to 12 μm thick is built up within 60 to 150 minutes. The ground parts are used for this Treated temperatures from 500 to 620 ° C in a gas mixture of ammonia, carbon dioxide, nitrogen and endogas or exogas. Then the last phase 7 is followed by cooling of the nitro carburized material under protective gas in the oven or by quenching in oil or aqueous media. Points that are not subject to wear and tear can be reworked.

FIG. 2 schematically shows the layer structure of the edge zone of a part which is treated by the method according to the invention. The diffusion layer 9, which consists of nitrides, carbides, carbonitrides and ferrite, adjoins the external connecting layer 8, which consists of G nitrides, Y ¹ nitrides, carbides and carbonitrides. The size relationships between the connection layer and the diffusion layer are such that the thickness of the connection layer is up to 20 μm, while the diffusion layer can have a thickness of several tenths of a millimeter. The starting material 10 adjoins the diffusion layer 9.

According to the invention, the supporting effect of the diffusion layer located under the connecting layer is significantly improved by carbonitriding and tempering compared to an only nitro-carburized part.

LIST OF REFERENCE NUMBERS

1 Carbonitriding or case hardening

2 cool down

3 start

4 Cooling 5 Machining

6 Nitro carburizing

7 Cool down

8 connection layer

9 diffusion layer 10 starting material

Claims

Protection claims

1. Method for the thermochemical-thermal treatment of case-hardening steels, in which an edge zone of a workpiece, in particular

Bucket tappets, roller bearing parts, gear and coupling elements, enriched with carbon and nitrogen and then subjected to a martensitic hardening, characterized in that, in a first process step, carbonitriding (1) at a temperature of 780 to 1050 ° C with carburizing and The edge zone is embroidered with 0.4 to 0.9 percent by weight of carbon and 0.1 to 0.8 percent by weight of nitrogen with a holding time of 1 to 4 hours, so that in a second process step of carbonitriding (1), quenching (2nd ) at a temperature well below the martensite starting point of the edge zone, that a third process step is a tempering process (3), which is carried out at a temperature of 20 to 40 ° C above a nitro carburized temperature, with a heating speed of 10 to 30 ° C per minute and a holding time of 1 to 2 hours, that cooling is the fourth process step (4) follows to room temperature and that the fifth process step is followed by machining (5) of the workpieces and that the sixth process step is nitrocarburization (6) in a gas mixture of ammonia, carbon dioxide, nitrogen and endogas or exogas at a temperature of 500 to 620 ° C with a holding time of 60 to 150 minutes follows, which is followed by a cooling down to room temperature.

2. Process for thermochemical thermal treatment of case-hardening steels according to claim 1, characterized in that the nitro-carburizing (6) takes place at a temperature of 530 to 570 ° C.

3.Procedure for thermochemical-thermal treatment of insert-- steels according to claim 1, characterized in that the nitro carburizing (6) takes place in the plasma or salt bath.

4. Process for the thermochemical-thermal treatment of case-hardening steels in which an edge zone of a workpiece, in particular cup tappets, roller bearing parts, gear and coupling elements, is enriched with carbon and then subjected to a martensitic hardening, characterized in that in a first Process step a case hardening (1) at a temperature of 780 to 1050 ° C. with a carburization of the edge zone of 0.4 to 0.9 percent by weight carbon with a holding time of 1 to 4 hours that the case hardening takes place in a second process step (1) a quenching (2) to a temperature well below the start point of the edge zone follows that a third process step is a tempering process (3) which is carried out at a temperature of 20 to 40 ° C. above a nitro carburizing temperature with a Heating speed of 10 to 30 ° C per minute and a holding time of 1 to 2 hours It is shown that the fourth process step is followed by cooling (4) to room temperature and that the fifth process step is followed by machining (5) of the workpieces and that the sixth process step is nitrocarburization (6) in a gas mixture of ammonia, carbon dioxide, Nitrogen and endogas or exogas at a temperature of 500 to 620 ° C followed by a holding time of 60 to 150 minutes, followed by cooling to room temperature as the last step in the process.