Method for producing highly corrosion-resistant and wear-resistant cast iron brake discs

Abstract

The invention relates to a method for producing a mechanically and preferably machined cast iron or grey cast iron surface with improved wear and corrosion resistance, in particular on brake discs, characterized in that a water jet treatment carried out on said surface is usually carried out according to the so-called fluid injection process, said water jet treatment being adjusted so as to completely or at least partially remove voids opened by said machining and containing graphite inclusions surrounded by said basic structures, so that the height of said graphite inclusions, if present, is below the outer surface of said basic structures surrounding said voids, after which a diffusion layer is applied by nitrocarburizing and an oxide layer is applied on said diffusion layer.

Description

[Technical Field]

[0001] The present invention relates to a method for producing mechanically, preferably machined, cast iron or grey cast iron surfaces, in particular on brake discs, with increased wear and corrosion resistance, according to the generic part of claim 1.

[0002] Furthermore, the invention relates to a specific use of a specifically tailored water jet process as defined in the general part of claim 8 and to a special brake disc as defined in the general part of claim 9. [Background technology]

[0003] Modifying the surface condition of cast iron components, such as brake discs, can potentially combine the advantageous properties of cast iron as a core material (e.g., "castability," availability, high thermal conductivity, sufficient stability at high temperatures) with advanced coating properties (e.g., improving the corrosion and wear resistance of the component).

[0004] A real technical requirement in the brake disc market, beyond those already known, is to provide a permanently corrosion-resistant component while reducing the particulate emissions into the environment that are unavoidable due to the braking process and the wear of the substrates that make up the brake discs, since in electric vehicles the main source of particulate emissions is no longer the engine but the brakes.

[0005] One of the state-of-the-art solutions for improving the wear resistance of brake discs is the so-called "iDisc®" product. The iDisc® is a brake disc from a well-known German automotive supplier. The friction surface of the iDisc® is coated (usually by spray coating) with a tungsten carbide-based layer as a topcoat on the braking surface. However, this solution is very expensive and does not provide fully satisfactory corrosion resistance, at least in the long term.

[0006] The main reason for this is the high thermal load placed on the brake disc during the braking process, which leads to cracks in the coating due to differences in the thermal expansion coefficients of the materials used (e.g., cast iron and the top coating). These cracks become the initiation points for corrosion, which then progresses to "underside corrosion" of the substrate and ultimately leads to delamination of the coating.

[0007] Alternative solutions include, for example, surface modification processes (instead of providing a coating), which include nitriding, carbonitriding, or carbonitriding and oxidation of the substrate by diffusing nitrogen (N) and / or carbon (C) and / or oxygen (O) into the substrate. Such processes are called, for example, gas nitrocarburizing (GNC) processes, ferritic nitrocarburizing (FNC) processes, or simply nitriding processes. These processes result in substrate materials with the same or even improved properties (by improving wear and corrosion resistance). The advantage of these processes is that they do not lead to the deposition of an areal coating layer, which can lead to delamination.

[0008] For this reason, these nitriding, carbonitriding or carbonitriding and oxidation processes are options for improving the wear as well as the corrosion resistance of brake discs.

[0009] The significantly different drive cycles of modern electric vehicles increase the demand for improved corrosion resistance. In modern electric vehicles, whether hybrid electric or fully electric, brake discs are braked much less in dry conditions in humid urban traffic than in traditional internal combustion engine vehicles, because in electric vehicles the majority of the braking force is provided by regeneration (i.e., by the electric motor itself) and the brakes are rarely used.

[0010] To date, this need has not been met by the above-mentioned nitriding, carbonitriding or carbonitriding processes and oxidation processes alone.

[0011] However, corrosion-inhibiting paints or "coatings" (e.g. UV paints, Zn paints or Zn / Al paints), although they perform well under these new conditions (e.g. 120 h in the standard DIN EN ISO 9227 salt spray test), wear easily within a few braking steps and therefore do not provide a corrosion-free braking surface.

[0012] (Object of the invention) It is an object of the present invention to provide cast iron surfaces, and in particular grey cast iron surfaces (especially parts of brake discs), which have improved corrosion resistance. Summary of the Invention [Problem to be solved by the invention]

[0013] According to the invention, this object is met by the following method for producing a mechanically (preferably machined) cast iron or grey cast iron surface with improved wear and corrosion resistance, in particular for brake discs.

[0014] The inventors have discovered that the specific water jet treatment of the cast or grey cast iron surface, which is usually carried out by the so-called fluid injection process (known per se), allows a significant improvement in corrosion resistance if a special adjustment of the water jet treatment is selected. According to the invention, the adjustment of the water jet completely or at least partially removes (partially = reduces) the graphite inclusions present in the voids of the basic cast metal structure opened by the machining, i.e., the graphite no longer reaches or appears on the surface. Typically, the graphite inclusions are present in the form of graphite lamellae or graphite balls.

[0015] Hereinafter, a diffusion layer is applied by nitrocarburizing and an oxide layer is applied on top of this diffusion layer, both of which are previously known in the state of the art.

[0016] The keys to success that the inventors have realized are:

[0017] The presence of graphite inclusions in the voids of the basic cast metal structure opened by the machining process will cause corrosion insofar as they extend directly into the region of the diffusion zone created by the encounter between the nitrocarburized and the underlying unaffected basic casting material.

[0018] Although more detailed investigations are ongoing, it is speculated that unfavorable electrochemical constellations occur within this triple contact zone, resulting in rapid corrosion (as in a localized electrical element in the broader sense).

[0019] The inventors have discovered that corrosion initiation occurs much more slowly when the cut voids in the casting surface contain no or very little graphite, so that the graphite level in each void (the boundary of the graphite "bulk") is further below the diffusion layer created by carbonitriding (when viewed from the direction of the component core).

[0020] Such "further down" characteristics can be considered at least sufficiently satisfied when the complete number of voids, or at least the upper fourth or, more preferably, the upper third of these voids, are essentially free of natural graphite loading. Experience has shown that in this case, the presence of graphite is kept sufficiently far away from the diffusion layer and surface that the corrosion process is significantly reduced by delaying and slowing its initiation.

[0021] Two technical effects were used here.

[0022] For larger voids (i.e., voids with a relatively large gap width), sufficient removal of the graphite originally contained in the voids also leads to the formation of a diffusion layer within the voids during nitrocarburizing, which extends down the sidewalls of the voids to the bottom, i.e., to the extent that the diffusion layer extends down, corrosion protection is also provided at the sidewalls of the voids.

[0023] The inventors have discovered that if the diffusion layer can reach deep enough into the void, the corrosion can be significantly delayed. The inventors have recognized that it is important for the diffusion layer to reach a depth below the region where diffusion begins at the outer surface of the brake disc surrounding the void (e.g., starting from the actual friction surface of the disc brake). This is only possible to the extent that the void is not filled with graphite.

[0024] In the case of smaller voids (i.e., voids with a relatively narrow gap width), another effect is added. During nitrocarburizing, as already mentioned, material also diffuses into the surfaces forming the side walls of the voids. As a result, the material in the diffusion zone expands to a certain extent. As a result, voids with a narrow gap width, like all voids, become increasingly narrow. However, in the case of essentially narrow voids, this has the effect of almost closing them, which decisively impedes or slows down liquid penetration, thereby leading to the initiation of the corrosion process.

[0025] These two mechanisms thus result in a decisive slowing of corrosion. However, this can only be achieved by emptying the voids (deeper and deeper than the graphite originally present in the voids). Superficial removal of the graphite from the voids is not useful, since corrosion would then rapidly diffuse from these voids into the surrounding environment and begin its destructive action.

[0026] The pulsed water jet process itself is already state of the art. A pulsed water jet is known from EP 2 741 862 B1, the disclosure of which is incorporated herein by reference.

[0027] What is not presently known is that the pulsed water jet process, with proper parameter settings, is a tool that allows for highly effective and selective removal of graphite from machined or sandblasted voids in cast iron surfaces, with essentially no detrimental effect on the surrounding underlying cast metal structure. Furthermore, it is not presently known or expected that removing more than an insignificant amount of graphite from open voids would result in significant corrosion resistance improvements.

[0028] At this stage, the process of the present invention, as applied to brake discs, can be briefly restated to a certain extent as follows:

[0029] First, casting and preferably fine-tuning of the cast iron brake disc is carried out, in certain cases preferably using lamellar cast iron (also called grey cast iron), to obtain the finished product with the correct dimensions and geometry.

[0030] Next, a pulsed water jet treatment is preferably performed, especially on corrosion-related surfaces. At this point, it is important to understand that the removal or opening of graphite-containing voids can be achieved not only by machining or lathing, but also by sandblasting. Therefore, it can be useful to perform this water jet treatment on the braking surfaces, inner and outer peripheries, or ventilation channels. This significantly reduces the graphite lamellae that form on these surfaces, improving further nitrocarburizing performance.

[0031] Therefore, gas and / or plasma nitrocarburizing, along with post-oxidation, leads to improved mechanical and corrosion performance.

[0032] Hereinafter, the finishing processes of the brake discs may be carried out: marking and / or labelling, balancing, dimensions and quality control.

[0033] While it is not possible to teach a general set of parameters that can be used to initiate the pulsed water jet process, it may be possible to teach the nature of the cast ferrous metal surface being treated. How to adjust the set of parameters to obtain the effects of the present invention for each individual casting surface must necessarily be discovered individually through some simple application testing and subsequent analysis of the test results.

[0034] The main parameters to be adjusted include pulse frequency (in the range of 10 kHz to 50 kHz, preferably about 20 kHz), water jet pressure (550 to 800 bar, preferably 600 to 700 bar, with a preferred distance to the surface of 30 to 70 mm), speed (500 to 1200 mm / s) and offset (2 to 10 mm).

[0035] In the specific example of surface activation prior to the gas nitrocarburizing process, some specific parameters have been found to be more relevant: for example, the nozzle diameter, typically < or ≦ 3 mm, and the opening angle, typically 0-45°, play a key role, directly influencing the overall water flow (which affects the treatment effectiveness).

[0036] The pulsed water jet process has the objective, and is adjusted accordingly, to ideally not erode any pearlite and / or alpha ferrite metal particles and only erode the graphite (also called "carbon agglomerates") naturally present in the voids (in the form of lamellae and / or globules and / or mixed vermicular structures).

[0037] For this process, the surface needs to be as smooth as possible, with ideal or essentially no impact on roughness (Ra and Rz) (excluding the space vacated by graphite removal).

[0038] The reason for the low roughness is to provide the required coefficient of friction (as in state of the art solutions), and indeed to get the tribology right a combination of adhesion and wear is used and large surface contact is required (without micro-peaks (high roughness) which adversely affect the adhesive component of the friction).

[0039] Furthermore, in the case of a coating layer, the adhesive phenomenon is favorable, which leads to a longer product life.

[0040] The use of an abrasive layer obtained by an ultra-hard pad material would only lead to a downgrading of the wear of both components (disc and pad) and would not represent a significant improvement over the state of the art.

[0041] The spaces obtained from the graphite described above are partially closed during gas and / or plasma nitriding and / or carbonitriding (eg, IONIT G Ox) to further smooth the surface.

[0042] For nitrocarburizing and subsequent oxidation, the process detailed in EP 0 753 599 B1 is preferably used, which is incorporated herein by reference, and the process taught therein is called IONIT OX.

[0043] The objective of the patent is to obtain (looking from the free surface towards the core) the following by infiltration of N and C atoms into the surface and the addition of post-oxidation:

[0044] -An oxide layer (Fe3O4) that provides higher corrosion resistance, - a white layer composed of gamma' and epsilon Fe-N particles, which has good corrosion resistance and a very high hardness (300-450 HV5 compared to a typical range of 200 HV5 for cast iron), and - A diffusion layer whose hardness is at least 50 HV5 points higher than that of the core material.

[0045] Gamma and epsilon nitrides have a wider microstructure than alpha ferrite, resulting in a smaller increase in surface area, which helps seal the spaces provided by graphite as discussed above.

[0046] Optional possibilities for further improvement

[0047] It is advantageous to subject the casting surface to plasma cleaning before commencing nitrocarburizing, so that the diffusion layer growth achieved by nitrocarburizing is maximized and defect-free.

[0048] Ideally, the oxide layer is produced after plasma treatment (preferably in the form of plasma activation) on the diffusion layer produced by nitrocarburizing. Such sputter cleaning of the gas nitrided surface optimizes the crystallization conditions for achieving a coherent and finely structured Fe3O4 oxide layer. It also compensates for the loss of N and C during cooling. The result is an ε-nitride.

[0049] Ultrasonic assistance of the water jet process is highly advantageous. The superposition of additional pulsed energy from the sound waves at the ultrasonic distance makes it much easier to loosen the graphite embedded in the ablated void. As a result, graphite removal from significantly deeper within the void is possible.

[0050] This is due to the presence of cavitation bubbles that form when ultrasonic waves are superimposed on the outlet orifice or nozzle of the water jet. These cavitation bubbles merge with the water jet and impinge on the brake disc surface, causing them to implode due to the well-known destructive effect of cavitation.

[0051] However, this destructive effect is less pronounced on the peripheral surface of the base material, especially if the water jet is applied at an angle, because the water jet does not act long enough to damage the base material. With graphite, the situation is different: graphite deposits can be fragmented at very high speeds by the implosion of cavitation bubbles and then expelled by the water jet.

[0052] It is highly preferred if the water jet is directed / blasted along a non-rectangular angle relative to the surface to be treated, in other words the water jet is not emitted perpendicularly (perfectly perpendicular and preferably essentially perpendicularly) to the surface to be treated.

[0053] For example, if a friction surface needs to be treated, the main direction of the water jet is arranged at an angle to said friction surface so that the water jet impinges on said friction surface (at least primarily or even substantially at this angle), with an ideal angle being around 45° to at least 50° to around 60°, instead of 90° + / - tolerance.

[0054] In this way, a more powerful water jet can be used, which, if it strikes the brake disc surface from the front (i.e. along a vertical line rising from the surface to be treated), will generate a large kinetic impact, which will have a negative effect on the surface quality and in particular the roughness of the metal substrate of the brake surface.

[0055] The more powerful the water jet (even when applied with a blast angle as described above), the more effective it will be at removing material from the voids.

[0056] Further possibilities for modification of the present invention, further suggestions for the functional aspects of the present invention, and preferred technical effects according to the present invention are disclosed in the following description of preferred embodiments. [Brief explanation of the drawings]

[0057] [Figure 1]The details arising from the application of the teachings of the present invention are shown in Figure 1. For this purpose, Figure 1 shows a grey cast iron surface (1) with three parts A, B and C. [Figure 2] Figures 2A-2D show the brake disc (Figure 2A) and a cross-section (Figure 2B) of this disc with the plates open after coating with IONIT OX. In Figure 2C), the lamellae are partially disrupted after heat treatment and subsequent IONIT OX coating. The cross-section in Figure 2D) shows the lamellae disruption and lamella-free areas after water-jet treatment and subsequent IONIT OX treatment. [Figure 3] Figures 3A-3D show the results and substrates after 48 hours of exposure to a saltwater environment. Figure 3A) shows IONIT OX without pretreatment. Figure 3B) shows thermal pretreatment before IONIT OX. Figure 3C) shows water-jet pretreatment before IONIT OX. Figure 3D) shows the same substrate as in Figure 3C (i.e., water-jet pretreatment before IONIT OX after 240 hours of salt spray testing, remaining visually largely corrosion-free). [Figure 4] A table is shown. [Figure 5] 1 shows a typical configuration of a water jet activation process prior to application of the IONIT OX process according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0058] DESCRIPTION OF THE PREFERRED EMBODIMENTS In a preferred embodiment, the iron-based component to which the present invention is applied is a cast iron brake disc.

[0059] The brake disc is first subjected to mechanical fine-tuning to reach the appropriate disc thickness variation (DTV), flatness and lateral runout (LRO) as known from the state of the art. These primary mechanical finishing methods make it possible to reduce the chatter and judder of the brake disc during operation (one of the main causes of brake disc failure).

[0060] Thereafter, treatment using pulsed water jet techniques as detailed above is carried out, particularly in areas of braking surfaces or other surfaces that have been machined after casting.

[0061] Starting from the broader parameter ranges mentioned above, the following preferred values ​​were selected as follows for determining the parameters used in this particular case:

[0062] a pressure of approximately 550-650 bar, a distance of at least approximately 30 mm between the water jet nozzle and the target surface of the brake disc, and a nozzle with a circular opening having a nominal diameter of approximately 1.6 mm to 2.2 mm, the nozzle extending outwardly therefrom at a core angle of approximately 20°.

[0063] Adjustments to the above parameters must be made by testing to suit the individual substrate characteristics (i.e., relative to cast iron composition, hardness, particle distribution, and overall brake disc geometry). These tests were completed once "microscopic" photographs showed sufficient graphite removal from the ablation voids (in accordance with the teachings of this invention), while other measurements showed no deterioration or more than unrelated deterioration of the surrounding surface structure (roughness).

[0064] It is worth mentioning at this point that the expected overall roughness (especially for brake discs) should be Ra<5 μm, preferably Ra<3 μm and Rz<12 μm, preferably Rz<10 μm.

[0065] Figure 1 shows the details arising from the application of the teachings of the present invention. For this purpose, Figure 1 shows a grey cast iron surface (1) with three parts A, B and C.

[0066] On the far left (A), it can be seen that in the pre-processed state of the substrate (1), there are cut (open) graphite lamellae (11) due to the previous machining of the surface, which may be preferably understood as the brake disc friction surface, while the graphite lamellae (10) are present deeper in the substrate and remain unchanged through the machining process.

[0067] The middle part (B) shows some, but still insufficient, thermal decarburization or (for the purposes of this invention) insufficient cleaning with a softer, but not dangerous, water jet (i.e., a water jet that is not strong enough to damage the base material surface surrounding the open voids (23) (even when properly directed relative to the surface) and not strong enough to allow deeper removal (24) of the internal graphite (21) from the voids). If this were the only step performed before the nitrocarburizing process, the corrosion protection would be insufficient, since under the first emergency braking thermal load the graphite filling the open voids would (at the latest) burn out. The "bare" sidewalls of the non-nitrocarburized voids would then remain open and begin to corrode in the area immediately adjacent to the friction surface of the brake disc.

[0068] In the right-hand portion (C), the lamellae have been completely (32) or partially (31) removed by applying the process of the present invention. The side walls (33) of the voids are free after graphite removal, leaving more than one-quarter or one-third of the void depth (34). Due to this, these free side walls of larger / wider voids (as shown on the right) can be provided with a protective diffusion layer extending downward along the voids. Additionally or alternatively, smaller voids that only have limited access can be further closed (33) due to material expansion by diffusion, making it more difficult for moisture to penetrate.

[0069] From the table shown in FIG. 4, it can be seen that the present invention is extremely useful for experts who are familiar with the commonly used values ​​for comparison, as these values ​​are always related to each other in corrosion behavior and other important parameters.

[0070] In the leftmost column (A), data are shown for a solution that has been implemented by the applicants for research purposes, but which is not in accordance with the present invention. In this solution, grey cast iron brake discs have already been cleaned with a pulsed water jet. However, in the past, in order to avoid damaging the surrounding surface, the parameters of the water jet have not been adjusted to obtain a water jet that is sharp enough to remove significant amounts of graphite from the voids. These discs withstood a similar salt spray test for about 10 hours before visible corrosion appeared on the surface.

[0071] In the right column (B) the data for the solution according to the invention are shown.

[0072] Within the scope of this solution, gray cast iron brake discs were specially treated with a pulsed water jet adjusted according to the invention. The water jet was sharp enough to risk causing undesirable damage to the brake disc surface (if proper care was not taken). The water jet cut through most of the voids to a depth of more than one-quarter. Thus, the effects described above for the entry zone can occur in the voids. As a result, the durability of the brake discs in a standard salt spray test was significantly improved. Visible corrosion only occurred after more than 300 hours.

[0073] Moving to the right side of the table in Figure 4 (cast iron, no paint), we next see a listing for a regular grey cast iron disc as has been the state of the art for several decades.

[0074] If we look further to the right of the table in Figure 4 (cast iron, painted), we see a typical grey cast iron disc, but this grey cast iron disc is coated with a modern spray-on protective lacquer based on zinc. As can be seen, such a protective coating can achieve a lot in terms of corrosion. However, a crucial disadvantage is that the protective coating on the actual friction surface wears away very quickly during everyday braking.

[0075] In the description of the grey cast iron discs in the last column (FNC) in the table in FIG. 4, oxidation after nitrocarburizing according to the invention has been omitted.

[0076] Figures 2A-2D show the brake disc (Figure 2A) and a cross-section (Figure 2B) of this disc with the plates open after coating with IONIT OX. In Figure 2C), the lamellae are partially disrupted after heat treatment and subsequent IONIT OX coating. The cross-section in Figure 2D) shows the lamellae disruption and lamella-free areas after water-jet treatment and subsequent IONIT OX treatment.

[0077] IONIT OX is a diffusion layer that is produced by nitrocarburizing followed by plasma treatment and oxide coating as taught in the above patents.

[0078] Figures 3A-3D show the results and substrates after 48 hours of exposure to a saltwater environment. Figure 3A) shows IONIT OX without pretreatment. Figure 3B) shows thermal pretreatment before IONIT OX. Figure 3C) shows water-jet pretreatment before IONIT OX. Figure 3D) shows the same substrate as in Figure 3C (i.e., water-jet pretreatment before IONIT OX after 240 hours of salt spray testing, remaining visually largely corrosion-free).

[0079] Figure 5 shows a typical configuration of a water-jet activation process prior to application of the IONIT OX process according to the present invention. It includes a substrate to be treated, represented by a brake disc (1), a water-jet gun (2), and a nozzle (3). Here, the water-jet gun (2) is positioned at a specific nozzle-to-substrate distance (d) relative to the brake disc surface and tilted at a specific angle so that the axis of the water-jet gun and the surface of the brake disc form an angle (a). The water jet is represented by (4) in the figure. During water-jet surface activation, the brake disc rotates at a specific rotational speed (v) simultaneously with the water-jet gun on two axes in a plane parallel to the brake disc surface. This allows for treatment of the entire brake disc surface.

[0080] After the pulsed water jet process for lamellar erosion, the brake disc undergoes a heat treatment process at a temperature of approximately 500°C to 590°C (preferably 570°C to 580°C), followed by a nitrocarburizing process in a controlled atmosphere (typically at a pressure approaching atmospheric pressure of approximately 1030 mbar) involving exposure to gases such as ammonia, nitrogen, and carbon dioxide. The gas flow is adapted depending on the weight of the cast iron substrate and the brake disc component. The nitrocarburizing process is suitable for iron-based materials, as it forms an overall harder material, Fe-NC, on the exposed surfaces of the component.

[0081] The component is then cooled to a lower temperature of about 500°C and can be subjected to an optional plasma activation process at operating pressure (less than 2 mbar, preferably 1-2 mbar) or directly to a further optional oxidation process, which is described in more detail in US5679411A, while the overall process including the latter further oxidation process is more commonly known as gas nitrocarburizing and oxidation or GNC OX.

[0082] This optional plasma-activated sputtering allows for further surface cleaning, and the sputtering ions generated during this process also create lattice defects on the surface, contributing to the final denser oxide layer after the oxidation process. The resulting nitrocarburized layer or diffusion zone is at least 15 μm thick, and the oxide layer is at least 2 μm thick. The additional optional thin oxide layer of magnetite (Fe3O4) is a continuous, closed layer formed on the entire component surface, improving the corrosion resistance of the component.

Claims

1. 1. A method for forming a machined cast iron or grey cast iron surface on a brake disc to improve its wear and corrosion resistance, characterized in that a water jet treatment applied to the surface is carried out according to a fluid injection process and is adjusted to completely remove at least the upper quarter of the voids opened by the machining, containing graphite inclusions surrounded by a basic cast iron structure, so that the height of the graphite inclusions is below the outer surface of the basic cast iron structure surrounding the voids, and then a diffusion layer is applied by nitrocarburizing and an oxide layer is applied on the diffusion layer.

2. 2. A method for producing brake discs according to claim 1, characterized in that plasma cleaning of the casting surface is carried out before nitrocarburizing.

3. 3. A method for producing a brake disc according to claim 1 or 2, characterized in that the oxide layer is produced after plasma treatment in the form of plasma activation is carried out on the diffusion layer produced by nitrocarburizing.

4. A method for manufacturing a brake disc according to any one of claims 1 to 3, characterized in that the water jet treatment is ultrasonically assisted.

5. Method for manufacturing a brake disc according to any one of claims 1 to 4, characterized in that the water jet is directed / blasted along a non-rectangular angle relative to the surface to be treated.

6. 6. The method according to claim 1, wherein the carbonitriding and oxidation processes are controlled by one or more of the following parameters: heating time, holding time, temperature during the carbonitriding phase, subsequent cooling time and temperature reached after the cooling time has elapsed, and subsequent oxidation time and temperature held or driven during this.

7. 1. Use of a correspondingly adjusted fluid injection process in a brake disc for completely or partially removing from machined open voids containing graphite inclusions surrounded by a basic cast iron structure, whereby the height of the graphite inclusions, if present, is below the outer surface of the basic cast iron structure surrounding the voids, whereby at least the upper quarter of the voids is completely freed from the graphite within the voids, preparing the brake disc for nitrocarburizing and subsequent oxidation.

8. 1. A method for manufacturing a brake disc made of cast iron, characterized in that the brake disc has voids opened by prior machining at least on the friction surface, at least the upper quarter of the voids is completely freed from the graphite in the voids, and the brake disc is nitrocarburized and oxidized.

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