EP0601451B1 - Process for hardness increasing and possibly for smoothing of work pieces and work pieces made by this process - Google Patents

Description

The present invention relates to a method for hardening and possibly smoothing machine components using a cause surface heating of the respective component Beam, e.g. a laser beam, an electron beam or the beam of an arc lamp.

In the prior art, there are various processes with machine components, those with laser beams or electron beams are known to have certain properties to achieve on the surfaces of the components.

For example, the so-called laser transformation hardening known. Here a laser is basically considered to be high energy Heat source used for martensite hardening without thereby melting the surface of the treated components. In other words, it becomes more similar to induction hardening Operation performed. Examples of such processes are U.S. Patent Nos. 4,304,978, 4,093,842 and 4,686,349, and U.S. Pat German patent 33 43 783 and the Metal Science Heat Treatment 20, No. 7/8, 1978, Pages 544-546.

As is well known, martensite hardening leads to a martensitic Structure that is very hard, but a needle-like one Has structure so that it is not necessarily the ideal hard Surface with regard to abrasive and adhesive wear represents. The martensitic structure is therefore for Components such as camshafts and rocker arms are not the ideal Structure.

DD-A-204 106 does not expressly say that the method mentioned there is martensite hardness. The specified hardness values (HV _0.05 ) are between 1500 and 1650. The surface to be hardened is sanded beforehand. The laser treatment converts both the steel (examples 1 and 2) and the cast iron (example 3) to the previously existing pearlite / ferrite structure without melting the surface. Graphite deposits in cast iron are not changed by laser treatment.

In the second writing "Metal Science and Heat Treatment No. 20 "is the influence of electron radiation on properties of different types of cast iron. The generated surface temperatures are expressly so controlled to undergo transformation hardening by martensite formation comes from the third and second paragraph from below on page 545 of the corresponding magazine on Expression comes. On page 544, however, it is mentioned that through Irradiation with electron cementite with a hardness of HV1100 in cementite a hardness of HV1250 was converted.

Werkstatt und Betrieb 123 (1990) November, No. 11, p.875-877 on martensite hardening using laser beams Carbon powder coating; if the layer thicknesses were too high, carbon diffusion and a remelting conversion to ledeburit was observed.

There is also the process of laser remelt hardening. Here is a melted by means of the laser beam Creates a layer on the surface of the component, which through rapid solidification into a hard ledeburitic Structure leads. Examples of this procedure are from German patents 34 18 555 and 36 26 799 known.

Another category of processes includes laser layer melting. In this process, one on one The graphite layer applied to the substrate is melted into the layer Solidify or cool the substrate themselves carbides. In other words, by melting of the graphite carbon introduced into the layer molten state dissolves, a mixed crystal forms and when cooling with atoms of the substrate Cr, W, V, Mn, Fe forms carbides in the form of granular or dendritic There are excretions, for example TiC (if the substrate is a Ti material) from the dissolved Graphite. An example of this procedure is the DE-OS 35 45 128.

In all of these cases, a high-power laser, often a CO ₂ laser, is used.

DE-OS 39 32 328 is a method for the rest Machining of surfaces subjected to friction in Internal combustion engines, especially the cylinder surfaces known from piston engines, the area honed and additional is subjected to a laser beam treatment. This Laser beam treatment after honing is preferred by a pulsed excimer laser carried out, this treatment a surface evaporation of micro grooves while maintaining the macro grooves (oil-producing honing grooves) without unwanted remelting caused. DE-OS 39 32 398 does not mention one targeted "skin" in the nanometer range (<1 µm), i.e. in the order of 0.001 µm. Due to the extreme quenching rates it is usually oversaturated or already already amorphous and therefore possibly hard.

In comparison to the known methods, the present one Invention based on the object, a method of provide the kind mentioned that the hardening and preferably also the simultaneous micro-smoothing of machine components, enables which either in the form of a Chilled cast part with ledeburitic structure or in the form of a Steel part with pearlitic structure are present, the Process is carried out so that a new structure is reached on the surface of the component, not only a hard and preferably also micro-smooth Surface offers, but also no post-processing requires, but a possible postprocessing for Special purposes is not excluded.

According to the invention, the features are used to achieve this object provided by claim 1. Preferred embodiments of the invention are in the claims 2-16 defined.

In contrast to the known laser transformation hardness, which has a martensitic structure on the surface of the Machine component is produced by the invention through a targeted choice of power density and duration the local warming up is almost closed Cementite surface instead of pearlite in steel and Pearlite areas created in chilled cast iron. The choice of Power density and duration of treatment will continue to be made so that after local warming up the surface a self-quenching of the surface layer by the ambient temperature and the inside prevailing temperatures of the component, wherein a regression to the original phase state not or only incompletely. This means, that it is in a pearlitic or ledeburitic Microstructure on the surface or on the surface layer to mix the cementite substance at the expense of stoichiometric structure comes. By treatment with the respective working beam (laser, electron beam or arc lamp) is a structural component locally dough or it melts while the other structural components remain in the solid state. Usually the structural component is which locally becomes dough or melts around the Areas between the large cementite slats and the Perlite. This is where the iron-carbon state diagram plays taking into account the imbalance ratios a crucial role.

The treatment should be carried out in this way, i.e. in front everything so short that there are none in the boundary layer Homogeneous, e.g. form austenitic mixed crystals can, always enough cementite germs in the surface layer and must be present in the substrate so that when quenching (self-deterrence or possibly with With the help of a cold jet) always cementite and not Austenite is formed.

The treatment can be carried out in this way, for example with a pulsed radiation source of high energy density, such as. with an excimer laser that a pronounced Evaporation (sublimation) and melting a thin one Surface skin occurs, resulting in a pronounced Micro-smoothing the surface leads.

In general, it makes sense to start with before treatment the beam of high energy density, the respective component to grind at least on the surface to be treated, but also hard shell cast surfaces can be treated with the method according to the invention can. Typical application examples for the present Processes are the generation of hard and possibly micro-smooth surfaces on camshafts or cam followers of internal combustion engines.

Typical values for the energy density used are in the range from 2 x 10 ³ to 5 x 10 ⁵ W / cm ² .

• sie ersetzt das mechanische Mikroglätten (Mikrofinishen) von Verschleißoberflächen auf Nockenwellen und Schlepphebeln,
• sie verringert den Einlaufverschleiß,
• die Behandlung dauert sehr kurz, typischerweise bis zu einer halben Minute pro Nockenwelle,
• die Methode läßt sich sehr gut in die Produktionslinie einfügen.

Information on typical exposure times or treatment times for the individual points on the surface can be found in the further subclaims or the examples. The invention therefore creates a non-contact, very fast method for simultaneously smoothing and hardening heterogeneous wear surfaces of metallic materials. Furthermore, the invention makes it possible to combine the micro-smoothing of ground wear surfaces with the hardening by cementite mixing on cast chilled castings with ledeburitic or steel with pearlitic structure in one operation. The formation of almost compact closed cementite surfaces instead of pearlite in steel and pearlite areas in chilled cast iron improves the adhesive wear resistance in particular. The invention also has the following advantages:

• it replaces mechanical micro-smoothing (microfinishing) of wear surfaces on camshafts and rocker arms,
• it reduces run-in wear,
• the treatment takes a very short time, typically up to half a minute per camshaft,
• the method can be inserted very well into the production line.

The invention deals with machine components that either in the form of a hard casting with ledeburitic Structure or in the form of a steel part with pearlitic Structures are present, with the special characteristic, that an almost closed cementite surface is present, the component having a surface hardness above 900 HV, preferably about 1100 HV and the cementite precipitation density in the interlamellar Areas from the surface towards Matrix is continuously decreasing. The component can for example a camshaft or a rocker arm be, but there are of course many other conceivable Machine components which are treated according to the invention can be.

Finally, it should be expressed that the treatment can basically be carried out in the air, at least with a laser beam or with an arc lamp, since there is little or no fear of oxidation processes. It may be useful to carry out the treatment with the selected jet type in a selected gas atmosphere in order to achieve special effects. For example, the treatment could be carried out in a nitrogen-containing or CO ₂ -containing atmosphere if nitriding or carburizing the surface of the workpiece is additionally desired.

The invention is explained in more detail below with reference to of four electron microscopic images and three Embodiments.

Fig. 1

eine Draufsicht auf eine umgeschmolzene Lauffläche einer Gußeisennockenwelle vor Anwendung der erfindungsgemäßen Behandlung (2580 mal vergrößert),

Fig. 2

die Oberfläche nach Fig. 1, jedoch nach zusätzlicher erfindungsgemäßer Behandlung mit einem Excimer-Laser (2040 mal vergrößert),

Fig. 3

eine Draufsicht auf ein eutektoidales Gefüge eines unbehandelten Stahls mit 0,8%C (1010 mal vergrößert und geätzt),

Fig. 4

das Gefüge aus Fig. 3 nach erfindungsgemäßer Laserbehandlung mit Schliff senkrecht zur behandelten Oberfläche bei 40000-facher Vergrößerung und geätzt.

The scanning electron microscopic images show:

Fig. 1

a plan view of a remelted tread of a cast iron camshaft before applying the treatment according to the invention (enlarged 2580 times),

Fig. 2

1, but after additional treatment according to the invention with an excimer laser (magnified 2040 times),

Fig. 3

a plan view of a eutectoidal structure of an untreated steel with 0.8% C (1010 times enlarged and etched),

Fig. 4

the structure of Fig. 3 after laser treatment according to the invention with grinding perpendicular to the treated surface at 40,000 times magnification and etched.

As already explained above, the invention is concerned with the surface treatment of machine components with heterogeneous (over-, under- or -eutectic) Casting structure, as shown in Fig. 1, or with over-, under- or -eutectoidal steel structure, such as shown in Fig. 3.

The scanning electron microscopic top view of FIG. 1 shows a TIG remelted tread of a Cast iron camshaft with an hypoeutectic structure made of cementite flakes and fine pearlite. The TIG remelting process represents a possible, but not mandatory pretreatment. In Fig. 1 the cementite lamellas are the large islands, while the pearlite areas the filigree structure exhibit. A qualitatively similar structure also delivers Chilled cast iron.

After additional treatment of the surface with an excimer laser with a pulse power density of, for example, 40 mJ / mm ² , 2 pulses, pulse duration 40 ns, a surface structure according to FIG. 2 is formed. From the structural components as in FIG. 1, ie from the cementite lamellae and fine Perlite has formed an almost closed layer of non-stoichiometric cementite in the surface layer. It is characteristic of this treatment that the surface layer is briefly heated up to the vicinity of the melting temperature (continuously or by repeated pulses) so that the carbon diffuses in the boundary layer from the cementite lamellae of the ledeburite into the soft, interlamellar ferrite areas. The holding time at this temperature is chosen so that there is no complete dissolution of existing phase components and formation of a homogeneous mixed crystal. As a result of the subsequent self-quenching of the surface layer, a regression to the original phase state cannot take place or can only take place incompletely. This creates an imbalance cementite with a higher volume than the original one. In the case of a pearlitic structure, the cementite substance "blends", as can clearly be seen in FIG. 2, at the expense of the stoichiometric structure, combined with the effect of hardening the surface layer up to 1100 HV.

3 shows a scanning electron microscope Inclusion of a steel eutectoid structure about 0.8% C. One sees ferrite as dark parts of the Matrix and cementite as light parts of the matrix in lamellar Arrangement. After treatment with the excimer laser, As for the cast iron sample of Fig. 1, a structure is created in the surface layer, as shown in Fig. 4. The original cementite slats, which are considered lighter Cores are still visible, have been in an edge layer of about 2 µm depth. Almost that whole edge layer probably does not consist of one balanced, partly granular excreted cementite. Here too, the surface layer has briefly warmed up up to near the melting temperature (continuously or by repeated pulses of the laser) a diffusion of carbon in an edge layer from the cementite flakes of pearlite into the soft interlamellar ferrite areas.

The temperature reached near the melting temperature must be chosen so that it is not a complete Dissolution of existing phase components and A homogeneous mixed crystal is formed. Here too prevents self-quenching following heat treatment a regression in the surface layer the original phase state. Likewise, in this example an imbalance with a cementite higher volume than the original.

Both the pearlitic and the ledeburitic Therefore there is a "blending" of the microstructures Cementite substance at the expense of the stoichiometric structure, combined with the effect of hardening the surface layer down to 1100 HV. In the cheapest Case it is treated with both types of metal on the Surface to form an even closed one non-stoichiometric layer of cementite come one significant improvement in abrasive and especially the adhesive wear resistance in the running-in phase and results in further operation.

It is also important for both types of metal that the local dough or melting of some, at the other structural components remain at the same time is possible in the solid state, so that the primary Workpiece surfaces are true to shape even after treatment are retained if the surface smoothness is low due to evaporation of a surface layer is disregarded. Because the treatment is so short, that no homogeneous, e.g. austenitic Mixed crystal can always be sufficient Cementite germs in the surface layer and in the substrate is present, which is always cementite when quenched and no residual austenite is formed. Will the boundary layer with a pulsed radiation source, preferably treated with an excimer laser, so is additional a strong evaporation (sublimation) and melting a thin surface skin. As a result of Evaporation and the surface tension of the melted Surface skin becomes an additional one in this case Micro smoothing (removal of the sanding marks, scaling and the sheet metal jacket) take place. With appropriate Setting the laser parameters will be a simultaneous smoothing of the surface and "blending" of cementite until the formation of a closed one Shift take place.

The invention therefore describes a method of non-contact Generation of thin wear layers, mainly by "mixing" the cementite surface. It will also improve pitting and Maturation resistance achieved. The invention includes at the same time smoothing the surface. This Mechanical microfinishing can be successful replace. The inlet wear is caused by the Smoothing greatly reduced. The procedure also has the special advantage that it is easily in existing production lines without much effort integrates.

In order to illustrate the practice of the invention in more detail, are now some concrete examples described:

example 1

The ground ledeburitic cam surface of a camshaft (NW) is cut with a CO ₂ laser in CW mode (continuous, non-pulsed laser beam) with a rectangular beam cross-section of size 2 x 10 or 1 x 20 mm ² by rotating the camshaft under the laser beam treated. The width of approx. 10 or 20 mm corresponds to the cam width of an NW with 4 or 2 valve technology. The surface temperature in the area of liquidus solidus from 1150 to 1250 ° C (pasty state of the surface layer) is monitored with known "on line" temperature measuring systems.

The power density is 5 x 10 ³ to 10 ⁵ W / cm ² . With the beam cross-section size mentioned, a laser power of 5 to 8 kW is required. The speed of rotation of the camshaft is determined from the dwell time of the laser beam on the cam surface. A dwell time (exposure time) of 0.3 to 10 s is required for a carbide layer thickness of 3 to 10 µm. If the treatment in pulse mode is carried out with a CO ₂ or Nd: YAG laser, at least 20% lower average power densities are required.

Example 2

The ledeburitic cam tread of a hard cast or surface layer remelted (TIG; laser, electron beam) Rocker arm is used for training purposes thin, but almost dense carbide Wear layer treated with the electron beam. The e-beam with 0.1 to 0.5 mm beam diameter grids the entire cam tread in a known manner one or more times. Repeated scanning of the Surface also becomes an almost constant, medium one Temperature of the surface above the liquidus does not rise, remain intact. For example becomes a deflection frequency of the e-beam from 100 to 500 Hz in the Y axis and a feed rate the rocker arm applied in the X-axis from 5 to 60 mm / s, depending on whether preheating the rocker arm has previously taken place or not. The in Example required power of the electron beam gun was 3 kW (60 V, 50 A).

The advantage of the electron beam is in this case in the high level of guidance and distractibility as well as in the local repeatability of the treatment. This allows you to do so without any special effort as required different carbide layers on one surface Thickness can be generated (customized layer thicknesses).

Example 3

Surface temperatures of 1250 to 1450 ° C are required to produce a thin carbide layer on steel surfaces with pearlitic or pearlitic-ferritic structure. A CO ₂ laser in CW operation (continuous beam) works with a laser power density in the range of 2 x 10 ⁴ to 5 x 10 ⁵ W / cm ² and in pulse mode with an average power density that is at least 20% lower.

The radiation exposure time is comparable to that is required for a ledeburitic structure (example 1). The liquidus solidus temperature is here higher, but the diffusion rate is also higher according to the temperature higher.

Characteristic of the carbide layer of the present Invention is that these are wear-resistant on existing Ledeburit or perlite is produced, the wear resistance is improved and that the cementite precipitation density in the interlamellar areas always from the surface towards the matrix decreases.

Through the examples made above and those mentioned there Parameter values are the differences from those in U.S. Patent No. 4,304,978 used parameter values clearly, i.e. it is not just a different endeavor before (as mentioned at the beginning, the US PS deals with Transformation hardness achieved through martensite formation ), but also perform those in U.S. Patent No. 4,304,978 parameter values mentioned are not random to the one in the present application aimed cementite mixing.

The power density of 1550 to 2480 W / cm ² described in U.S. Patent No. 4,304,978 contrasts with the value of 5,000 to 500,000 W / cm ² according to the present invention. The exposure times of 0.017 to 0.026 S mentioned in the US patent are also not comparable with the values of 0.1 to 10 s mentioned in the present application.

If the parameters of the pulsed excimer laser are taken into account, the present invention has much shorter exposure times of 4 pulses x 40ns = 160 ns = 0.00000016 s in front. The average power density, for example in the case of an Nd: YAG laser, with a beam cross section of 0.5 x 0.5 cm ² and an average power of 500 W is of the order of 20,000 W / cm ² and therefore in a completely different range as indicated in U.S. Patent No. 4,304,978.

Claims (16)

1. Method of hardening and optionally smoothing machine components by means of a beam which brings about a surface heating of the respective component, such as for example a laser beam, an electron beam or an arc, wherein the respective component, which is present either in the form of a chill cast part with ledeburitic structure or in the form of a steel part with perlitic structure, is treated with a high energy density and the surface layer is heated up to and into the region of the melting temperature, i.e. into the region of the liquidus-solidus, for a short period of time, either continuously or by multiple pulses, so that in a marginal layer a diffusion of the carbon takes place from the cementite lamella of the ledeburite and/or of the perlite into the soft ferrite regions between the lamella, with the power density and time duration of the treatment being so selected that with perlitic and ledeburitic structures an increase of the cementite volume occurs at the surface or in the layer close to the surface with a non-stochiometric structure, with the one structure component becoming locally pasty or fusing during the treatment while the other structure components remain in the solid state, with the holding time at the temperature in the vicinity of the melting temperature being so selected that a non-complete dissolution of existing phase components arises and no austenite is formed and sufficient cementite nuclei are present in the marginal layer and in the substrate so that during quenching cementite is once more always formed and not rest austenite, with the power density and also the time duration of the local heating being so selected that a cementite surface arises instead of the perlite in steel and instead of the perlite regions in chill cast iron.
2. Method in accordance with claim 1, characterised in that the treatment takes place with a pulsed beam source of high energy density such as for example with an excimer laser in order to additionally cause pronounced vaporisation (sublimation) and fusion of a thin surface layer.
3. Method in accordance with one of the preceding claims, characterised in that prior to the treatment with the beam of high power density the respective component is ground, at least at the surface to be treated, or cast skin hard.
4. Method in accordance with one of the preceding claims, characterised in that it is carried out on cam shafts or rocker arms.
5. Method in accordance with one of the preceding claims, characterised in that the power density preferably lies in the range of 5 x 10³ to 5 x 10⁵ W/cm².
6. Method in accordance with one of the preceding claims for the treatment of a cam shaft with a ledeburitic cam surface, characterised in that the treatment is carried out with a CO₂ laser in constant wave operation (continuous, non-pulsed laser beam) with a rectangular beam cross-section with a size in the range 3 mm x 5 mm to 25 mm x 10 mm, preferably 2 mm x 10 mm to 1 mm x 20 mm, with the cam shaft being turned during the treatment with the dwell time (exposure time) at each point of the treated surface lying in the range between 0.3 to 10 seconds in order to achieve a carbide layer thickness of 3 to 10 µm.
7. Method in accordance with claim 6, characterised in that the laser power lies in the range from 4 to 12 kW.
8. Method in accordance with one of the preceding claims, in particular in accordance with claim 6, characterised in that the surface temperature is kept in the region of the liquidus-solidus from 1150 to 1250°C (parily pasty state of the surface structure) and is preferably monitored by means of a temperature measuring system.
9. Method in accordance with claim 7 or claim 8, characterised in that the treatment is carried out in pulse operation instead of in constant wave operation, with either a CO₂ laser or an Nd:YAG laser being used, with the mean power density being at least 20 % lower than in constant wave operation.
10. Method in accordance with one of the preceding claims 1 to 5 for the treatment of a ledeburitic cam running surface of a chill cast or surface layer remelted rocker arm by means of an electron beam, with the electron beam having a circular cross-section with a diameter of 0.1 to 0.8 mm which scans the entire cam running surface in grid-like manner once or several times and produces an almost constant average temperature of the surface which does not go above the liquidus.
11. Method in accordance with one of the claims 1 to 5 for the treatment of a ledeburitic cam running surface of a chill cast or surface layer remelted rocker arm by means of an electron beam, with the electron beam having an almost rectangular cross-section which covers the entire cam running surface width or sliding bearing surface width and continuously or multiply swingingly treats the surface in the peripheral direction and thus produces an almost constant mean temperature of the surface which does not go above the liquidus or only increases above it in locally restricted part regions or insubstantially.
12. Method in accordance with claim 10 or 11, characterised in that the electron beam gun has a power of about 3 kW, for example 50 A at 60 V.
13. Method in accordance with claim 10, 11 or 12, characterised in that with alignment of the electron beam gun in the Z-axis the beam is deflected in a direction substantially perpendicular thereto, for example in the direction of the Y- or X-axis with a deflection frequency of 100 to 500 Hz over the width or length of the cam running surface of the rocker arm, with the cam running direction being advanced in the respective other X- or Y-axial direction obliquely to it at a speed of advance of 5 to 60 mm/sec depending on whether a preheating of the rocker arm has previously taken place.
14. Method in accordance with one of the claims 1 to 5, characterised in that, for production of thin carbide layers on steel surfaces with a perlitic or perlitic-ferritic structure, the surface is heated to a temperature in the range of 1250° to 1450°C which is produced with a CO₂ laser in constant wave operation (continuous beam) with a power density in the range of 5 x 10³ to 5 x 10⁵ W/cm² and in pulsed operation with an average power density which is about 20 % lower, with the time of action of the beam, i.e. the dwell time at each point of the treated surface amounting to 0.1 to 10 s.
15. Method in accordance with one of the preceding claims, characterised in that the cementite precipitation density in the regions between the lamella continuously decreases from the surface in the direction of the matrix.
16. Method in accordance with one of the preceding claims, characterised in that the treatment with the beam is carried out in a gas atmosphere, for example in a gas atmosphere which contains nitrogen or CO₂.

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