EP0627019B1 - Process for the thermochemical-heat treatment of case-hardened steels - Google Patents

Description

The invention relates to a method for thermochemical-thermal Treatment of case-hardened 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 undergoes a martensitic hardening.

One such method is carbonitriding to treat a Workpiece in the austenitic state with the purpose of enrichment the surface layer with carbon and with nitrogen, whereby both elements are then in the austenite in solid solution. In connection this treatment generally takes place immediately Quenching to achieve hardening. By carbonitriding the surface hardness and wear resistance of the construction parts improved (technology of heat treatment of steel, p. 169 ff, VEB German publishing house for basic materials industry, Leipzig 1986).

The workpieces treated by this method have improved Properties with regard to wear resistance, the however for components that are subject to high tribological stresses such as the contact surfaces of tappets in the valve train of an internal combustion engine are not sufficient in every application. Beyond that due to the required shape accuracy in carbonitrided parts Grinding of these components necessary, so that in the course of this cutting Shaping the highly enriched, wear-resistant outermost surface layer is at least partially ground away.

Another method of increasing wear resistance is that Nitro carburizing. This is a thermochemical process for enriching the Surface layer of a workpiece with nitrogen and carbon with formation a connection layer, a below the connection layer especially forms nitrogen-enriched diffusion layer. requirement for The functionality of nitro-carburized parts is next to the existence this sufficiently thick connection layer enriched with nitrogen and carbon a corresponding supporting effect of the diffusion layer the naturally more or less brittle connection layer.

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

Another process for the thermochemical-thermal treatment of ferrous materials, its marginal zones enriched with carbon and nitrogen and subsequently subjected to a martensitic hardening is from FR-A-22 83 244 known. This is done in such a way that the parts to be treated carbonitrided, quenched in oil, nitro carburized, quenched and tempered be cooled down again.

The object of the present invention is a thermochemical-thermal To create treatment methods that are highly stressed tribologically Components ensure sufficient wear resistance.

This object is achieved by the characterizing part of claim 1 several successive process steps.

- A first process step consists of carbonitriding at a temperature from 780 to 1,050 ° C, with carburization and Embroidery of 0.4 to 0.9 weight percent 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 is a corresponding has high solvency for both carbon and nitrogen. The enrichment of the diffusion elements nitrogen and carbon has to be carried out so that their solubility in austenite is not exceeded, d. H. the coal potential in the atmosphere is according to the S-E line in the Coordinate iron carbon diagram. The same applies to the nitrogen supply according to the iron-nitrogen state diagram. The hold time during carbonitriding, which lasts one to four hours can be based on the desired hardening depth, the The upper limit can be one millimeter. The chemical is reached Composition of the edge zone by diffusion of carbon and Nitrogen at the temperatures mentioned in a known manner Use a working gas that has both carbon-emitting components and also contains nitrogen-donating components.

• An das Karbonitrieren schließt sich als dritter Verfahrensschritt eine Wärmebehandlung an, im Zuge derer der Werkstoff bei 520 bis 650°, d. h. 20 bis 40° C über der nachfolgenden Nitrocarburiertemperatur, angelassen wird. Die Aufheizgeschwindigkeit liegt dabei zwischen 10 bis 30° pro Minute und die Haltezeit beträgt etwa 1 bis 2 Stunden. Nach dem Anlassen schließt sich als vierter Verfahrensschritt eine Abkühlung auf Raumtemperatur an, wobei die Abkühlgeschwindigkeit so gewählt wird, daß durch die Abkühlung keine neuen Spannungen im Bauteil erzeugt werden. Durch das Anlassen bei einer Temperatur von 20 bis 40° C über der Nitrokarburiertemperatur wird erreicht, daß sich der durch das Karbonitrieren im Randbereich des Werkstückes eingestellte Gefügezustand beim nachfolgenden Nitrokarburieren durch Temperatureinflüsse nicht mehr verändert. Da jede Änderung des Gefügezustandes mit einer Volumenvergrößerung bzw. -verkleinerung verbunden ist, wird eine derartige Volumenänderung beim nachfolgenden Nitrokarburieren ausgeschlossen. Darüberhinaus wird der beim vorhergehenden Karbonitrieren mit nachfolgender Abkühlung mit inneren Spannungen eingefrorene Ungleichgewichtszustand in ein bei der Nitrokarburiertemperatur im Gleichgewicht befindliches Gefüge umgewandelt. Der Abbau von inneren Spannungen beim Anlassen ist ebenfalls mit Maß- und Formänderungen des Werkstückes verbunden.
• Nach der Anlaßbehandlung werden die durch die vorhergehenden Behandlungsstufen Karbonitrieren und Anlassen eingetretenen Form- und Maßänderungen der Teile durch einen spangebenden Formgebungsprozeß als fünften Verfahrensschritt korrigiert, um die zu nitrokarburierenden Teile auf das Fertigteilendmaß zu bringen. Gegebenenfalls ist dabei ein durch die Stickstoff- und Kohlenstoffaufnahme beim Nitrokarburieren eintretendes Volumenwachstum maßlich zu berücksichtigen.
• Nach der spanenden Formgebung schließt sich als sechster Schritt des erfindungsgemäßen Verfahrens das Nitrokarburieren an. Ziel ist der Aufbau einer bis zu 20µm dicken, geschlossenen Verbindungsschicht. Hierzu werden die geschliffenen Teile bei Temperaturen von 500 bis 620° C 60 bis 150 Minuten lang behandelt. Das Nitrokarburieren erfolgt im Gasgemisch aus Ammoniak, Kohlendioxid, Stickstoff und Endo- oder Exogas. Die Abkühlung des Nitriergutes als letzter Schritt des Verfahrens kann unter Schutzgas im Ofen oder durch Abschreckung in Öl erfolgen.

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 temper resistance of said case hardening steel against case hardening.

• The third step of the carbonitriding is followed by a heat treatment in which the material is tempered at 520 to 650 ° C, 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 by 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 area of the workpiece no longer changes during the subsequent nitro carburizing due to temperature influences. Since every change in the microstructure is associated with an increase or decrease in volume, such a change in volume during subsequent nitrocarburization is excluded. In addition, the imbalance state frozen during the previous carbonitriding with subsequent cooling with internal stresses is converted into a structure which is 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 carbonitriding and tempering treatment steps are corrected by a metal-cutting shaping process as the 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 machining, the nitro carburizing follows as the sixth step of the method according to the invention. The aim is to build a closed connection layer up to 20 µm 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 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 high enough to grow at a sufficiently high rate build the connection layer. Beyond that comes there is no additional structural transformation in this temperature range in the embroidered edge area, so that a quenching and with it associated dimensional and shape changes can be dispensed with.

In a development of the invention according to claim 3, 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 is case hardening 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 takes place with a holding time of 1 to 4 hours. The subsequent process steps remain the same as described in the characterizing part of claim 1.

The inventive method for thermochemical-thermal Treatment will give the material high wear resistance and Load-bearing capacity awarded because the one below the connection layer and this supporting diffusion layer significantly improved Supportive effect, so that even with the highest tribological loads the connection layer is not caused by plastic deformation of the underlying diffusion layer can be damaged.

Figur 1

die einzelnen Verfahrensschritte des erfindungsgemäßen Verfahrens in Abhängigkeit von Zeit und Temperatur;

Figur 2

einen Querschnitt im Bereich der Funktionsfläche.

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

Figure 1

the individual process steps of the process according to the invention as a function of time and temperature;

Figure 2

a cross section in the area of the functional surface.

In the phase labeled 1, the carbonitriding takes place at one Temperature from 780 to 1050 ° C. Depending on the desired hardening depth carburizing and nitriding take place within 1 to 4 hours the edge zone. This phase is followed by a phase 2, in which the Microstructure at a temperature significantly below the martensite starting point of the Edge zone is quenched. In a third phase, the material Annealed at 20 to 40 ° C above the nitro carburizing temperature for 1 to 2 hours. In a fourth phase, the material is Protective gas atmosphere cooled before in as shown in dashed lines in a fifth phase the parts of an exciting molding process be subjected to bring them to their final dimension. In A sixth phase takes place within 60 to 150 minutes Build a 2 to 20 µm, preferably 6 to 12 µm thick, closed Link layer. The ground parts are used for this Temperatures of 500 to 620 ° C in a gas mixture of ammonia, Treated carbon dioxide, nitrogen and endogas or exogas. After that the last phase 7 closes the cooling of the nitro carburized material under protective gas in the oven or by quenching in oil or aqueous Media. Places that are not subject to wear can be chipping be reworked.

Figure 2 shows schematically the layer structure of the edge zone one behind part treating the method according to the invention. To the outside Compound layer 8, which consists of ∈-nitrides, Y'-nitrides, carbides and carbonitrides, the diffusion layer 9 follows, which consists of nitrides, carbides, carbonitrides and ferrite. The Size relationships between connection layer and diffusion layer are such that the thickness of the connection layer is up to 20 µm while the diffusion layer has a thickness of several tenths Can have millimeters. Connects to the diffusion layer 9 the starting material 10.

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

List of reference numbers

1

Carbonitriding or case hardening

2nd

cooling down

3rd

Start

4th

cooling down

5

cutting shape

6

Nitro carburizing

7

cooling down

8th

Link layer

9

Diffusion layer

10th

Starting material

Claims (4)

1. A method of thermochemical heat treatment of case hardening steels in which an edge region of a workpiece, in particular cup tappets, rolling bearing parts, transmission and clutch elements, is enriched with carbon and nitrogen and then subjected to martensitic hardening, characterised by

   a first step of carbonitriding (1) at a temperature of 780 to 1050° C whereby the edge region is carburised to 0.4 to 0.9 percent by weight of carbon and nitrided to 0.1 to 0.8 percent by weight of nitrogen with a holding time of of 1 to 4 hours,

   a second step of quenching (2) to a temperature well below the initial martensite point of the edge region,

   a third step of tempering (3) at a temperature higher than a nitrocarburising temperature by 20 to 40° C at a heating rate of 10 to 30° C per minute and a holding time of 1 to 2 hours,

   a fourth step of cooling (4) to ambient temperature,

   a fifth step of shaping the workpieces by machining (5),

   a sixth step of nitrocarburising (6) in a gas mixture of ammonia, carbon dioxide, nitrogen and endothermic or exothermic gas at a temperature of 500 to 620° C with a holding time of 60 to 150 minutes, and

   a last step of cooling to ambient temperature.
2. A method of thermochemical heat treatment of case hardening steels according to Claim 1, wherein nitrocarburising (6) is performed at a temperature of 530 to 570° C.
3. A method of thermochemical heat treatment of case hardening steels according to Claim 1, wherein nitrocarburising is performed in plasma or in a salt bath.
4. A method of thermochemical heat treatment of case hardening steels in which an edge region of a workpiece, in particular cup tappets, rolling bearing parts, transmission and clutch elements, is enriched with carbon and nitrogen and then subjected to martensitic hardening, characterised by

   a first step of carburising (1) at a temperature of 780 to 1050° C whereby the edge region is carburised to 0.4 to 0.9 percent by weight of carbon with a holding time of of 1 to 4 hours,

   a second step of quenching (2) to a temperature well below the initial martensite point of the edge region,

   a third step of tempering (3) at a temperature higher than a nitrocarburising temperature by 20 to 40° C at a heating rate of 10 to 30° C per minute and a holding time of 1 to 2 hours,

   a fourth step of cooling (4) to ambient temperature,

   a fifth step of shaping the workpieces by machining (5),

   a sixth step of nitrocarburising (6) in a gas mixture of ammonia, carbon dioxide, nitrogen and endothermic or exothermic gas at a temperature of 500 to 620° C with a holding time of 60 to 150 minutes, and

   a last step of cooling to ambient temperature.

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