METHOD TO PRODUCE CAST IRON BRAKE DISCS WITH HIGH CORROSION AND WEAR RESISTANCE

Abstract

Method for producing a mechanically and preferably machined cast iron or grey cast iron surface, in particular on a brake disc, with increased wear and corrosion resistance, characterized in that said surface is subjected to a water jet treatment—usually according to the so-called fluid jet process, which is adjusted so that it completely or at least partially clears the cavities opened by the machining, which contain a graphite inclusion surrounded by the basic structure, so that in the latter case the level of the graphite inclusion lies below the outer surface of the basic structure surrounding the cavity, whereupon a diffusion layer is applied by nitrocarburizing and an oxide layer is applied on the diffusion layer.

Claims

1. A method for producing a mechanically and machined cast iron or grey cast iron surface on a brake disc, resulting in increased wear and corrosion resistance, comprising: subjecting a surface of a cast iron or grey cast iron substrate to a water jet treatment, wherein a water jet is adjusted so that the water jet completely or at least partially clears cavities opened by the machining, which cavities contain a graphite inclusion surrounded by a basic structure, so that when the water jet at least partially clears the cavities, a level of the graphite inclusion lies below an outer surface of the basic structure surrounding the cavity, applying a diffusion layer by nitrocarburizing; and applying an oxide layer on the diffusion layer.

2. The method according to claim 1, comprising carrying out a plasma cleaning of said cast surface before nitrocarburizing.

3. The method according to claim 1, wherein the diffusion layer produced by nitrocarburization is subjected to a plasma treatment before the oxide layer is produced.

4. The method according to claim 1, wherein the water jet treatment is assisted by ultrasound.

5. The method of according to claim 1, wherein the water jet is directed/blasted along a non-rectangular angle to the surface to be treated.

6. The method according to claim 1, wherein the process of carbonitriding and preferably also that of oxidation is controlled inter alia or substantially by at least one of the parameters selected from the group consisting of: heating time, holding time, a temperature during the carbonitriding phase, subsequent cooling time and a temperature reached after the cooling time has elapsed, and subsequent oxidation time and a temperature held or driven during this time.

7. A method of using a tuned fluid jet process, comprising: using a water jet treatment to fully or partially clear a brake disc's cavities opened by machining, which cavities contain a graphite inclusion surrounded by a basic structure, so that when the water jet treatment at least partially clears the cavities, a level of the graphite inclusion lies below an outer surface of the basic structure surrounding the cavity, in order to prepare the brake disc for nitrocarburization.

8. A brake disc made from grey cast iron having at least at its friction surfaces cavities opened by prior machining and originally filled with graphite, wherein the cavities are fully or partially cleared from the graphite that was in it in a manner that does not cause deposit of a solid blasting material or ash within the cavities and the brake disc is nitrocarburized.

9. The method according to claim 3, wherein the plasma treatment is in the form of plasma activation.

10. The method according to claim 6, wherein the process of oxidation is also controlled inter alia or substantially by at least one of the parameters selected from the group consisting of: heating time, holding time, the temperature during the carbonitriding phase, the subsequent cooling time and the temperature reached after the cooling time has elapsed, and subsequent oxidation time and the temperature held or driven during this time.

11. The method according to claim 7, comprising using the water jet treatment to prepare the brake disc for nitrocarburization and subsequent oxidation.

12. The brake disc according to claim 8, wherein the brake disc is also oxidized.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] FIG. 1 shows results of the application of the process of the invention.

[0058] FIGS. 2A to 2D show a brake disc.

[0059] FIGS. 3A to 3D show the results and substrates after a 48-hour exposure in a salt spray environment.

[0060] FIG. 4 is a table providing results of the process of the invention in terms of corrosion behavior and other important parameters.

[0061] FIG. 5 shows the typical configuration for the water jet activation process prior to applying the IONIT OX process according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0062] In the preferred embodiment the iron-based component to which the invention is applied is a cast iron brake disc.

[0063] The brake disc is initially finely mechanically turned in order to reach the adequate Disc Thickness Variation (DTV), planarity and Lateral Runout (LRO) as known from the state-of-the-art. These primary mechanical finishing methods allow to reduce the chatter and judder of the brake disc during operation which are amongst the main cause of brake disc failures.

[0064] Afterwards it is treated with pulsed waterjet technology as explained in greater detail above, in particular in the area of its braking surfaces or other surfaces that are machined after casting.

[0065] Starting from the above-mentioned wider parameter ranges the following preferred values for the determining parameters used here, in this particular case, have been chosen as follows:

[0066] A pressure around 550 to 650 bar, a distance between water jet nozzle and target surface of the brake disc of at least around 30 mm, a nozzle with a circular opening having a nominal diameter of around 1.6 mm to 2.2 mm, extending outside from there with a cone angle of around 20°.

[0067] The above-mentioned parameters must be adjusted by tests according to the individual base material characteristics, i. e. in orientation to cast iron composition, hardness, grain distribution and overall brake disc geometry. The tests have been finished as soon as “microscopic” pictures showed that the cut cavities are sufficiently cleared from graphite according to what the invention teaches—while the other measurements have proven that there is not yet a worsening or more than an irrelevant worsening of the structure (roughness) of the surrounding surface.

[0068] At this point it has to be mentioned that the overall expected roughness should be—in particular for a for a brake disc—Ra<5 μm, preferably Ra<3 μm and Rz<12 μm, preferably Rz<10 μm.

[0069] FIG. 1 shows what happens by the application of the inventive teaching. For this purpose, FIG. 1 shows a grey cast iron surface (1) in three sections A, B, C:

[0070] On the far left (A), one sees the original raw state of the substrate (1) with the graphite lamellae (11) cut (opened) due to prior machining of the surface which may preferably be understood as a friction surface of a brake disc. The graphite lamellae (10) which are deeper in the substrate remain unchanged by the machining process.

[0071] The middle section (B) shows a slightly but still insufficient thermal decarburization or (for the purpose of the invention) an insufficient cleaning with a soft, non-dangerous water jet. i. e. a water jet not strong enough to potentially impair the base metal surface surrounding the opened cavities (23), even if not properly directed to the surface, and not strong enough to provide a deeper clearing (24) of the cavities from the graphite (21) in it. If one would only do this before the nitrocarburization process, then the corrosive protection would be insufficient. This is because under the heat load of the first emergency braking (at the latest) the graphite filling the open cavities would be burned out. Then the “naked” side walls of the cavity which have not been nitrocarburized, lied open and quickly began to corrode, in an area very close to the friction surface of the brake disc.

[0072] On the right section (C) the lamellas are fully (32) or partially removed (31) by the application of the inventive process. The side walls of the cavities (33) lie free after removal of the graphite along more than ¼ or better ⅓ of the depth of the cavities (34). Due to that these lying free sidewalls of a bigger/broader cavity (as shown on the right-hand side) can be provided with a protective diffusion layer extending down along the cavity. Additionally, or alternatively a slimmer cavity, having a narrow access only, will be additionally closed (33) due to material expansion by diffusion, so that it becomes difficult for humidity to intrude.

[0073] The table presented in FIG. 4 proves for the expert who is familiar with the values commonly used for comparison, as they are always the relevant ones, the extremely beneficial effect of the invention in terms of corrosion behavior and other important parameters.

[0074] The far-left column (A) contains the data of a solution that has been—for investigation purposes—practiced by the applicant so far, but which is not in accordance with the invention. In this solution, the grey cast iron brake disc has been already cleaned with a pulsating water jet. However, in the past, in view of the dogma that the surrounding surface must not be negatively affected, the parameters of the water jet have not yet been adjusted in such a way that the water jet was sufficiently sharp to remove a significant amount of graphite from the cavities. These discs withstood the familiar water salt spray test for about 10 hours until visible corrosion appeared on the surface.

[0075] In the column to the right (B) are the data of the solution according to the invention.

[0076] Within the scope of this solution, the grey cast iron brake disc is subjected to a special treatment with a pulsating water jet adjusted according to the invention. The water jet is so sharp that there is a risk that the surface of the brake disc will be undesirably negatively affected if it is not applied with appropriate care. The water jet has cleared most of the cut cavities to more than a quarter of their depth. Thus, the effect described above in the introduction could occur in the cavities. As a result, the endurance of the brake disc in the standard water salt spray test has improved dramatically. Visible corrosion only occurred after 300 hours and more.

[0077] If one moves to the right in the table according to FIG. 4 (CAST IRON, not painted), the next thing found is a description of a normal grey cast iron disc, as has been the state of the art for decades.

[0078] If one continues in the table according to FIG. 4 to the right (CAST IRON, painted), then one finds there the description of a normal grey cast iron disk, which is equipped however here now with a modern, sprayed protective lacquer finish on zinc basis. As one can see, such a protective coating can achieve quite a lot from the corrosion point of view. But the decisive disadvantage is that the protective coating on the actual friction surfaces is very quickly worn away during daily braking action.

[0079] In the last column of the table in FIG. 4 (FNC), one will find the description of a gray cast iron disc, where the later oxidation, according to the invention, after nitrocarburizing was omitted.

[0080] FIG. 2A to 2D show a brake disc FIG. 2A) and a cross section FIG. 2 B) of this disc with open plates after coating with IONIT OX. The figure shows in FIG. 2 C) partially interrupted lamellas after thermal treatment and subsequent IONIT OX coating. The figure in section FIG. 2 D) shows interrupted lamellas and lamella-free areas after water jet treatment and subsequent IONIT OX treatment.

[0081] IONIT OX is the diffusion layer created by nitrocarburization followed by plasma treatment and oxide coating, as taught by the above-mentioned patent.

[0082] FIGS. 3A to 3D show the results and substrates after an exposition of 48 hours in a salt spray environment: FIG. 3A) IONIT OX without pretreatment, FIG. 3 B) thermal pretreatment prior IONIT OX, FIG. 3 C) water jet pretreatment before IONIT OX and FIG. 3 D) shows the same substrate as in FIG. 3 C), namely water jet pretreatment prior IONIT OX after 240 hours of salt spray test and which remains visually still predominantly free of corrosion.

[0083] FIG. 5 shows the typical configuration for the water jet activation process prior applying the IONIT OX process according to the present invention. This includes the substrate to be processed, illustrated by a brake disc (1), the water jet gun (2) and the nozzle (3). Here the water jet gun (2) is placed at a certain nozzle-substrate distance (d) with respect to the surface of the brake disc and tilted at a certain angle, so that the axis of water jet gun and the plane of the surface of the brake disc forms the angle (a). The water jet is represented by (4) in the figure. During the surface activation by the water jet, the brake disc is rotated at a certain rotating speed (v) at the same time as the water jet gun in two axis which are in a plane that is parallel to the surface of the brake disc. This allows to process the whole surface of the brake disc.

[0084] After pulsed waterjet process for lamellae erosion, the brake disc is going through a heat treatment process at temperatures of approximately 500° C. to 590° C., preferably between 570° C. to 580° C. and is subsequently subjected to a nitrocarburization process in a controlled atmosphere, usually at a pressure close to the atmospheric pressure of about 1030 mbar, and exposed to gases such as ammonia, nitrogen and carbon dioxide. The respective gas flows are adapted depending on the cast iron base material and weight of the brake disc component. The nitrocarburization process is favorable for iron-based material as it forms a harder material of Fe—NC over the whole exposed surfaces of the component.

[0085] The component afterwards is cooled down at a lower temperature of about 500° C. where it can optionally go through a plasma activation process at work pressures below 2 mbar, preferably between 1 to 2 mbar or directly through the additional optional oxidation process. The optional plasma activation process is described more in detail elsewhere U.S. Pat. No. 5,679,411A whereas the whole process including the latter process of additional oxidation is better known as gas nitrocarburization and oxidation or GNC OX.

[0086] The optional plasma activation allows an additional cleaning of the surface by sputtering and also sputter-ions produced during this process create lattice defects on the surface which contribute to a final denser oxide layer after the oxidation process. The resulting nitrocarburizing layer or diffusion zone are at least 15 μm thick and the oxide layer at least 2 μm. The additional optional thin oxide layer of magnetite (Fe3O4) is a continuous and a closed layer which is produced over the whole component surface, allowing an improved corrosion resistant of the component.