EP0347568A2 - Method of making wear-resisting castings - Google Patents

Abstract

In order to obtain castings of improved wear resistance and simultaneously improved tensile strengths and elongation characteristics in a process for the production of wear-resistant castings from a cast-iron alloy, the castings, which contain ledeburite in the peripheral zones and graphite depositions, being annealed at an austenitising temperature and subsequently quenched, it is proposed that the quenching take place after the casting matrix has been substantially converted into austenite by the annealing, but while the carbides of the alloy are still essentially undecomposed. <IMAGE>

Description

Process for producing wear-resistant castings from a cast iron alloy using castings with ledeburitic edge zones or surface areas and graphite deposits in further component areas, the castings being subjected to a heat treatment in order to convert the matrix of the ledeburitic edge zones or surface areas into bainite.

Such a method is known, for example, from the review article by P. Peppler "Shell die casting - properties and application" in "Konstruieren + Gießen" (1979, page 12 ff.). In the production of cast parts subject to wear, the hard shell casting process or the TIG remelting process (TIG = tungsten inert gas torch) is often used, a ledeburitic structure is usually produced in certain edge zones or surface areas, in which the carbides are in a pearlitic Matrix are embedded.

However, today's requirements for the strength of the matrix of ledeburite often exceed the given properties of pearlite. On the other hand, it is known from the literature that a carbide-free, graphitic cast iron component with a predominantly bainitic structure has a significantly higher strength compared to a component with a pearlitic structure. This one graphitic cast iron components, however, lack the necessary wear resistance.

Proceeding from this, the object of the invention is to propose a method with which castings with improved wear resistance and at the same time improved tensile strength and elongation characteristics can be obtained.

This object is achieved according to the invention in the method described at the outset by annealing the castings at an austenitizing temperature and quenching them to an intermediate stage temperature at which the casting matrix is essentially converted to austenite and the carbides of the alloy are still essentially are undecomposed.

Surprisingly, it was found that the cast components treated according to the invention in the ledeburitic edge zones with a bainitic matrix have a significantly improved wear resistance, in particular with regard to rolling fatigue, since the wear-inhibiting effect of the carbides in combination with the bainitic matrix results in a higher fatigue strength.

It is very important in heat treatment to preserve the carbides of the ledeburitic areas and in no way to decompose them in the heat treatment. Decomposition of the carbides would result in graphite deposits in the ledeburite areas, which would at least partially nullify the wear-inhibiting effect of the carbides.

This procedure ensures that at least surface areas, if not edge zones of the castings, have a ledeburitic structure in which iron carbide and / or mixed carbides contribute significantly to the wear resistance of the Surface contribute, while at the same time by embedding the carbides in a now bainitic matrix, a very good strength of the component and a high wear resistance, in particular rolling fatigue strength, is guaranteed.

Although a whole series of processes for treating gray cast iron parts are known (see, for example, DE-OS 28 53 870), in which the components are annealed at an austenitizing temperature, these processes cannot be applied to the heat treatment of castings due to the fundamentally different cast matrix designs with ledeburitic marginal zones if the advantages of the ledeburitic marginal zone are to be retained.

The method according to the invention is particularly suitable for treating castings which are cast using the hard shell casting method and thereby have ledeburitic edge zones or surface areas, the matrix in these areas, however, being predominantly formed from pearlite.

Castings that are conventionally cast, i.e. graphitically solidified components or so-called gray cast iron parts, in which, however, surface areas or edge zones were remelted into a ledeburitic structure by means of high-energy radiation, such as that of a tungsten inert gas burner, a laser or an electron beam.

The optimal holding time for the austenitizing temperature depends, on the one hand, to some extent on the alloying constituents of the cast iron alloy, the decay rate of the carbide crystals being able to be reduced, for example, with certain element additions, and, on the other hand, on the preselected austenitizing temperature itself. A holding time for a given Austenitization temperature at which the conversion to austenite takes place at least to about 80% The conversion is preferably carried out almost completely.

The second essential criterion of the method according to the invention is the maintenance of the carbide crystals. According to special variants of the method according to the invention, these should be retained at least in the order of magnitude of approximately 80% when the casting matrix is converted into austenite, with at least 90% of the carbides still being in crystalline form at the end of the holding time at the austenitizing temperature in a preferred procedure.

A process control in which more than 95% of the carbides are still present in crystalline form in the casting matrix is most preferred.

The preheating process is advantageously preceded by annealing at the austenitizing temperature, in which the casting is heated to a temperature of approximately 300 to 700.degree. The temperature in this preheating or preheating process is chosen so that the casting structure does not change substantially and, on the other hand, the austenitizing temperature can be reached relatively quickly. Preferably, a largely uniform heating of the cast part should be achieved over the entire cross-section while maintaining the preheating temperature.

As a result, the holding time at the austenitizing temperature can be kept very short when the casting matrix is almost completely converted to austenite, which in turn essentially completely retains the carbide content. The holding times for such processes are between 3 and 10 minutes, depending on the individual alloy components and the austenitizing temperature.

The holding times at the austenitizing temperature can ins in particular, be kept relatively short if the temperature is set in the range from 550 to 650 ° C. during the preheating process. This preheating temperature range is optimal because, on the one hand, at a temperature of up to 650 ° C, no decomposition processes of the carbide crystals in the casting matrix can certainly take place and, on the other hand, the entire casting is preheated to close to the austenitizing temperature. The subsequent heating to the austenitizing temperature, at which the conversion of pearlite to austenite takes place, has the consequence that the interior of the casting is also heated to the austenitizing temperature during the holding time. In addition to these advantages described, the preheating process preceding the annealing at the austenitizing temperature has the further advantage that the cast part to be treated does not warp due to uneven temperature distribution in the cast part.

In the subsequent annealing of the castings at the austenitizing temperature, a temperature range of 800 to 960 ° C is preferred. At the lower limit of this temperature range, a somewhat longer holding time at the austenitizing temperature will of course be necessary than at the upper limit of the specified range.

The austenitizing temperature should be maintained for at least 3 minutes to a maximum of 10 minutes.

The austenitizing temperature is preferably kept for only 5 to 7 minutes.

Following the austenitization, the components are preferably quenched in a warm bath, which allows a targeted generation of the bainite matrix. Oil baths, salt baths or sand fluidized beds are used as warm baths, as is known from another context.

The warm bath temperature is preferably selected in the range from approximately 220 to 450 ° C. Below 220 ° C, martensite is increasingly obtained on cooling, which has a negative effect on the properties of the castings. Sufficient hardening of the cast part is not achieved above 450 ° C.

The duration of treatment in the warm bath is between 0.1 and 4 hours. The lower limit of 0.1 hours results from the fact that in the case of shorter periods of time there is no longer sufficient conversion into bainite. The upper limit for the treatment time of 4 hours results from the fact that there the loss of bainitic properties of the matrix begins, i.e. the already formed bainitic areas are then subject to significant transformation processes.

In order to achieve defined properties of the cast part, the temperature in the hot bath is preferably kept constant, i.e. the temperature is regulated to a value of approx. ± 20 ° C.

Alternatively, the temperature of the hot bath may be lower in a first period after the castings have been introduced into the hot bath than in the remaining part of the treatment time. This temperature difference is preferably between approximately 30 and 100 ° C.

In particular, there are two cases in which a constant temperature control during the hot bath treatment is assumed and in which the temperature of the hot bath is initially lower. This procedure is recommended if the weight ratio of bath content and castings to be introduced is relatively small, ie if the hot bath has a comparatively small heat capacity compared to the heat capacity of the castings to be introduced at the same time. If the bath temperature is initially regulated to a lower temperature value under these conditions, then he If the hot castings are introduced, the bath is not heated above the temperature value desired for the treatment in the warm bath.

Regardless of the ratio of the heat capacities of the bath content to the cast parts to be introduced, an initially lower temperature value of the hot bath can be used to form more crystallization centers in the cast part due to the greater cooling during quenching, so that a finer cast structure results. The subsequent increase in temperature to the actual heat bath treatment value is therefore carried out in order to accelerate the desired conversion of austenite into bainite. The holding time at the hot bath treatment temperature can thus be significantly reduced.

The same idea of the invention is embodied in a procedure which is completely different from the previous process, namely in that in a process for the production of castings with ledeburitic marginal zone regions by adding alloying constituents to the alloy prior to casting, which change the time / temperature behavior in such a way that that in an essentially unchanged casting and cooling process, ledeburitic surface areas or edge zones are obtained whose casting matrix essentially comprises bainite.

With this method, the subsequent heat treatment of the cast part is therefore omitted, so that drastic savings in terms of production times and also production costs can be achieved, particularly in the cast parts that can be used without post-processing. Nevertheless, in comparison to the previously described methods, identical components are obtained in their structure, which therefore also have the same positive properties as extremely high wear resistance with simultaneously improved tensile strength and elongation characteristics.

In both process variants, particularly good results are achieved if a cast alloy is used which contains the elements chromium, vanadium and tungsten as alloy components individually or in combination, the proportion of each of these elements - if contained - 0.1 to 5% by weight. and the sum of the proportions, if they are in combination, is up to 10% by weight. The elements chromium, vanadium and tungsten can be used in particular to regulate the resistance to decomposition of the iron and / or mixed carbides in the austenitic phase.

As an alternative to this, cast alloys can be used which contain the elements boron, titanium, tellurium and bismuth as alloy components individually or in combination, the proportion of a single one of these elements - if present - being 0.01 to 0.2% by weight. These elements and the variation of their proportions can also be used to regulate the decay rate of the carbides in the austenitic phase or, in other words, to stabilize the iron carbide crystals at least partially or even completely.

Cast iron alloys, which contain copper, nickel and molybdenum as alloy components individually or in combination, each with a respective proportion of 0.1 to 8% by weight, if present, have also proven to be advantageous, in the case of a combination of these elements, the The sum of their shares can be up to 15% by weight.

All of the aforementioned elements can be used as an alloy component to specifically change the time and temperature-dependent conversion characteristics of the cast alloy or to influence the time / temperature behavior of the alloy in a targeted manner, as has already been explained above. As already mentioned, this not only opens up the possibility of direct casting with a component which is pronounced according to the invention come, but also in the post-treatment of the castings to a process management in which the conversion processes are so slow that small time differences in the holding times, especially in the austenitizing temperature and in the treatment in a warm bath, no longer lead to serious quality fluctuations. As a result, the treatment times of the components in the various process stages are lengthened in part, but the advantages obtained by the fact that higher quality standards can be maintained with fewer rejects predominate.

A further object of the invention is to propose wear-resistant cast parts made of a cast iron alloy with graphite precipitates and ledeburite parts, in which, in addition to excellent tensile strength and elongation values, there is improved rolling fatigue strength on surface areas.

This object is achieved in the wear-resistant castings from a cast iron alloy with ledeburitic edge zones or surface areas in that a predominantly bainitic matrix of ledeburite is provided, in which Fe₃C and / or mixed carbides are embedded.

Castings with such a structure use on the one hand the properties of the high-strength and wear-resistant bainite structure and supplement this additionally by the hardness of Fe₃C and / or the mixed carbides, which in combination lead to an unprecedented wear resistance in castings.

In addition to a predominantly bainitic structure, the castings according to the invention can also comprise austenite and / or martensite fractions without achieving significantly worse results with regard to the abrasion resistance.

In addition to the surface areas which are essentially graphite-free, other areas of the castings contain lamellar, vermicular and / or spherical graphite precipitates which, for example depending on the magnesium content, can be freely selected and determine properties other than the wear resistance of the castings.

Further advantageous designs of the castings can be found in the subclaims.

The castings according to the invention are preferably obtained by the processes described above, the castings in the aftertreatment process initially being in a processable state and being subjected to the process according to the invention only in one of the last phases of the production process.

The direct method described above, in which alloy components are added to the alloy in such a way that the time / temperature behavior of the alloy during the casting and solidification process is influenced in such a way that ledeburitic marginal zones are formed, the casting matrix of which essentially comprises bainite, is completely equivalent to this Manufacturing process, which is mainly used when the castings no longer have to be subjected to further processing. The alloy components, in particular components such as nickel, copper, molybdenum and / or tungsten, are used in a targeted manner to influence the phase transition characteristics of the alloy in such a way that the structure according to the invention is obtained in the edge zones of the cast part with unchanged, natural cooling behavior of the cast part. This means that subsequent heat treatment is no longer necessary and that the cooling process of the castings is in no way prolonged by the process according to the invention.

Because of their particularly high wear resistance, the castings according to the invention are used in particular as components for the valve control of internal combustion engines, as camshafts and their counter-rotors, e.g. Levers or plungers, trained and used.

However, this is by no means the only area of application for the castings according to the invention, since they can advantageously be used wherever castings are subject to abrasive wear. For example, only the use of such parts in mining and agricultural machinery should be mentioned here.

These and other advantages of the invention are explained in more detail below with the aid of the examples.

Example 1:

The casting to be produced in this example is a camshaft, which is first produced using the well-known hard chilled casting process. The camshaft already has marginal zone areas made from ledeburite, but in which the matrix for the carbides consists essentially of pearlite. In this hard shell casting process, a mold is used which, in the solidifying casting, enables such a high solidification front speed in certain areas that the cast iron melt solidifies there in the edge zone of the casting according to the metastable state diagram with ledeburitic structure, the carbide crystals of the ledeburite areas are embedded in a pearlite matrix here .

The subsequent heat treatment of this camshaft according to the invention is shown in a time / temperature diagram in the drawing. Thereafter, the castings are first heated to a temperature of about 600 ° C (P 1) and ge until the temperature compensation in the casting (P 2) at this temperature hold. This is followed by rapid heating to the austenitizing temperature (P 3) of approx. 900 ° C. After a holding time of approx. 7 minutes (P 4), the camshafts are quickly cooled to an intermediate stage tempering temperature of approx. 300 ° C (P 5) and held at this temperature (see dash-dotted curve) for approx. 2 1/2 hours (P 7). The short holding time at high temperatures ensures that the carbides in the casting, as they were produced in hard shell casting, remain essentially unchanged, while, on the other hand, the preheating process at approx. 600 ° C (P 1 - P 2) is the prerequisite for an essentially complete transformation to austenite during this short hold time.

At the end of the heat treatment (according to P 7), the camshaft is cooled in air to room temperature.

The figure represents a variant in the range from P 5 to P 6, that is to say in the process step of quenching, in which the temperature of the hot bath is initially selected approximately 50 ° C. below the hot bath temperature subsequently maintained. The advantages here are, on the one hand, the possibility of making better use of the bath contents during quenching, since a relatively small ratio of the heat capacity of the bath to the heat capacity of the cast parts to be introduced therein and thus also a relatively small bath volume can be selected. This not only entails lower system costs, but also reduces the energy costs of the system, since a significantly reduced bath content has to be brought to the appropriate post-treatment temperature or intermediate stage compensation temperature. It can also be achieved with this procedure that an increased formation of crystallization centers is caused, which results in a finer structure of the casting structure.

Example 2:

The component to be treated with the method according to the invention, here a camshaft, can likewise be produced instead of in the hard shell casting process by remelting a gray solidified edge zone by means of high-energy radiation, here for example a TIG torch. The following heat treatment corresponded to the procedure according to embodiment 1.

Example 3:

In this embodiment, a combination of alloying elements was added to the cast iron alloy, namely 1.2% nickel, 1% molybdenum and 0.7% copper. The camshaft was cast in a manner known per se using the hard chill casting process, without a special temperature control being forced upon the cooling and solidification of the melt. Through the targeted use of the alloy components, such a shift in the phase conversion curves in the continuous time / temperature diagram to longer conversion times was achieved that essentially ledeburit with a bainitic matrix was again obtained in the edge zones.

Example 4:

In this exemplary embodiment, camshafts were produced in accordance with Example 3, with the difference that instead of the alloy components nickel, molybdenum and copper, only a proportion of 2.5 to 3% by weight of tungsten was added to the cast iron alloy.

The limit terms of the according to Examples 1 to 4 camshafts and cam followers with the ledeburitic marginal zones with bainitic matrix are, depending on the stress conditions, up to 30% above the values that can be achieved for the same components with the same casting structure but pearlitic matrix in the ledeburitic marginal zones.

Claims (27)

1. A process for the production of wear-resistant castings from a cast iron alloy using castings with ledeburitic edge zones or surface areas and graphite deposits in further component areas, the castings being subjected to a heat treatment for converting the matrix of the ledeburitic edge zones or surface areas into bainite, characterized in that the castings annealed at an austenitizing temperature and quenched to an intermediate stage temperature at a time when the matrix is essentially converted to austenite and the carbides of the alloy are still essentially undecomposed. 2. The method according to claim 1, characterized in that the quenching is carried out by introducing the castings into a warm bath. 3. The method according to claim 2, characterized in that the temperature of the hot bath is 220 to 450 ° C. 4. The method according to any one of claims 2 or 3, characterized in that the castings are treated in the warm bath for about 0.1 to 4 hours. 5. The method according to claim 4, characterized in that the temperature of the hot bath is kept substantially constant. 6. The method according to claim 4, characterized in that the temperature of the hot bath is increased after the introduction of the components. 7. The method according to claim 6, characterized in that the temperature in the warm bath after the introduction of the components is increased by about 30 to 100 ° C. 8. The method according to one or more of the preceding claims, characterized in that the annealing of the castings is carried out at an austenitizing temperature of approximately 800 to 960 ° C. 9. The method according to claim 8, characterized in that the austenitizing temperature is maintained for about 3 to 10 minutes. 10. The method according to one or more of the preceding claims, characterized in that the annealing at the austenitizing temperature is preceded by a heating process in which the casting is first heated to a temperature of approximately 300 to 700 ° C. 11. The method according to claim 10, characterized in that the temperature is set during the preheating process in the range of about 550 to 650 ° C. 12. The method according to any one of claims 10 or 11, characterized in that the temperature is maintained during the preheating process until a substantially constant temperature has been established over the cross section of the casting. 13. The method according to one or more of the preceding claims, characterized in that the casting to be used is cast in the hard shell casting method, optionally using cooling iron. 14. The method according to one or more of claims 1 to 12, characterized in that surface areas or, if appropriate, entire edge zones of the cast part to be used are remelted by means of high-energy radiation, for example a tungsten inert gas burner, a laser or an electron beam, to form a ledeburitic structure . 15. A process for the production of castings with ledeburitic marginal zone areas, characterized by the addition of alloying components to the alloy prior to casting, which change the time / temperature behavior in such a way that ledeburitic surface areas or in an essentially unchanged casting and cooling process Edge zones are obtained whose casting matrix essentially comprises bainite. 16. The method according to one or more of claims 1 to 15, characterized in that a cast iron alloy is used which contains the elements chromium, vanadium and tungsten as alloy components individually or in combination, the proportion of the individual element in each case 0.1 to 5 % By weight and the sum of the proportions of the elements contained in the combination is up to 10% by weight. 17. The method according to one or more of claims 1 to 15, characterized in that a cast iron alloy is used which contains the elements boron, titanium, tellurium and bismuth individually or in combination, the proportion of each of the elements being 0.01 is up to 0.2% by weight. 18. The method according to one or more of the preceding claims, characterized in that a cast iron alloy is used which, as alloy components, contains copper, nickel and molybdenum individually or in combination with a respective proportion of 0.1 to 8% by weight, with one Combination of these elements the sum of their proportions is up to 15% by weight. 19. Wear-resistant cast parts from a cast iron alloy with ledeburitic edge zones or surface areas, characterized by a predominantly bainitic matrix of ledeburite, in which Fe₃C and / or mixed carbides are embedded. 20. Castings according to claim 19, characterized in that the matrix comprises austenite and / or martensite fractions in addition to the bainite fraction. 21. Castings according to claim 19 or 20, characterized in that the castings comprise areas with graphite precipitates, which are preferably lamellar, vermicular and / or spherical. 22. Castings according to one or more of claims 19 to 21, characterized in that the alloy, individually or in combination, chromium, vanadium and tungsten with contents of 0.1 to 5% by weight in each case, in combination up to a maximum of 10 % By weight. 23. Castings according to one or more of claims 19 to 21, characterized in that the alloy comprises boron, titanium, tellurium and bismuth individually or in combination with contents of 0.01 to 0.2% by weight. 24. Castings according to one or more of claims 19 to 23, characterized in that the alloy individually or in combination copper, nickel and molybdenum with contents of 0.1 to 8 wt.%, In combination with a sum of the contents up to 15% by weight. 25. Castings according to one or more of claims 19 to 24, characterized in that they are heat-treated according to one or more of claims 1 to 11. 26. Castings according to one or more of claims 19 to 24, characterized in that they are produced by direct, controlled cooling after casting, the time-dependent phase transformation characteristic of the casting alloys used preferably by components such as nickel, copper, molybdenum and / or Tungsten is set for longer conversion times. 27. Castings according to one or more of claims 19 to 26, characterized in that they are used as components for the valve control of internal combustion engines, in particular camshafts and their counter-rotors, such as e.g. Lever or plunger are formed.

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