US20140143992A1

US20140143992A1 - Methods for treating a cast iron workpiece

Info

Publication number: US20140143992A1
Application number: US14/159,809
Authority: US
Inventors: Xiaochuan Xiong; Jeff Wang
Original assignee: GM Global Technology Operations LLC
Current assignee: GM Global Technology Operations LLC
Priority date: 2012-11-29
Filing date: 2014-01-21
Publication date: 2014-05-29
Also published as: DE112012007182T5; US20150292053A1; US20140149053A1; WO2014082238A1; CN104797732B; CN104797732A

Abstract

Examples of methods for treating a cast iron workpiece are disclosed herein. In one example of the method, a nanocrystallized microstructure at a finish surface of the cast iron workpiece is roller burnished. The roller burnishing reduces roughness of the nanocrystallized microstructure. In another example of the method, a machined, finish surface of the cast iron workpiece is deformed by rubbing the machined, finish surface against a blunt tool to form a nanocrystallized microstructure at the machined, finish surface. The machined, finish surface is cooled simultaneously with the deforming to promote nanocrystallization of the machined, finish surface.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of International Application Serial No. PCT/CN2012/085510, filed Nov. 29, 2012.

TECHNICAL FIELD

The present disclosure relates generally to methods for treating a cast iron workpiece.

BACKGROUND

Cast iron materials may be used in applications where resistance to surface wear from friction is desirable. Untreated cast iron materials generally tend to corrode when exposed to the environments in which they are used. Some surface treatments, e.g., painting, tend to wear off quickly and/or may be deleterious to proper functioning of the cast iron materials.

SUMMARY

In one example of the methods disclosed herein, a nanocrystallized surface layer at a finish surface of a cast iron workpiece is roller burnished. The roller burnishing reduces roughness of the nanocrystallized surface layer. In another example of the methods disclosed herein, a machined, finish surface of a cast iron workpiece is deformed by rubbing the machined, finish surface against a blunt tool to form a nanocrystallized surface layer at the machined, finish surface. The machined, finish surface is cooled simultaneously with the deforming to promote nanocrystallization of the machined, finish surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of examples of the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear.

FIG. 1 is a perspective view of a disc brake assembly in an example of the present disclosure;

FIG. 2 is a side view of a drum brake assembly in an example of the present disclosure;

FIG. 3 is a schematic perspective view showing an example of a workpiece and a tool operating thereon;

FIG. 3A is an enlarged cross-sectional schematic view of a portion of the workpiece and tool of FIG. 3, showing the tool forming the nanocrystallized surface layer;

FIG. 4 illustrates an example of a method for treating a cast iron workpiece involving cooling;

FIGS. 5A through 5C together schematically illustrate an example of a method for treating a cast iron workpiece involving roller burnishing;

FIG. 6 is an enlarged cross-sectional, schematic view showing an example of the workpiece in a nitrocarburizing environment;

FIG. 6A is a scanning electron microscope (SEM) image, similar to the view of FIG. 6, but showing an example of an actual workpiece depicting the substrate and the refined microstructure in the surface layer;

FIG. 6B is a schematic depiction of a section view showing an example of a compound layer after ferritic nitrocarburization at a microscopic enlargement;

FIG. 7 is a perspective view of a brake disc in an example of the present disclosure;

FIG. 8 is a perspective view of a brake drum in an example of the present disclosure;

FIG. 9 is a perspective view showing the inside of the brake drum depicted in FIG. 8;

FIG. 10 is a perspective view of a drum-in-hat rotational member;

FIG. 11 is a cross-sectional view of the drum-in-hat rotational member depicted in FIG. 10;

FIG. 12 is a perspective view of a shaft in an example of the present disclosure;

FIG. 13 is a perspective view of an engine block cylinder liner in an example of the present disclosure;

FIG. 14 is a perspective view showing the inside of the brake drum depicted in FIG. 8 with a schematic view of a tool operating thereon; and

FIGS. 15A and 15B are flow diagrams depicting various examples of the methods according to the present disclosure.

DETAILED DESCRIPTION

Examples of the present disclosure advantageously provide methods for treating a cast iron workpiece. In the examples disclosed herein, a surface nanocrystallization process is performed to generate a nanocrystallized microstructure at a finish surface of the cast iron workpiece. This nanocrystallized microstructure contributes to faster or more energy efficient ferritic nitrocarburizing (FNC) treatments of the cast iron workpiece. In particular, the nanocrystallized microstructure accelerates/facilitates diffusion of nitrogen atoms and carbon atoms therethrough.
As used herein, the term “finish surface” refers to the surface of the cast iron workpiece that has been exposed to machining. Also as used herein, the term “nanocrystallized microstructure” refers to the finish surface after it has been exposed to nanocrystallization, i.e., the severe plastic deformation process that is very localized to the surface and near (i.e., up to a depth of a few tens of microns) the surface of the cast iron workpiece. The nanocrystallized microstructure has a refined microstructure which has smaller grains (e.g., from about 5 nm to ≦2000 nm) than the finish surface (having a grain size >2000 nm).
Examples of the method disclosed herein also involve cooling to enhance the surface nanocrystallization process, roller burnishing to reduce surface roughness resulting from the surface nanocrystallization process, or combinations of cooling and roller burnishing. As such, the methods according to examples of the present disclosure include treating the machined, finish surface during surface nanocrystallization and/or treating the nanocrystallized microstructure after surface nanocrystallization has been performed.
In the examples disclosed herein, the surface nanocrystallization may be accomplished, e.g., by deforming the machined, finish surface against a blunt tool. Nitrocarburizing may be accomplished using accelerated diffusion of nitrogen and carbon atoms through the nanocrystallized surface layer. Surface nanocrystallization and nitrocarburization form a substantially rust-free and high wear/fatigue resistant surface on the cast iron components/workpieces.
It is to be understood that in examples of the present disclosure, the deformation against the blunt tool (i.e., nanocrystallization) is severe, plastic deformation local to the location of contact between the blunt tool and the workpiece. The deformation occurs substantially without forming chips and without removing material in the process of deformation. Further, the local deformation of examples of the present disclosure is distinct from global deformation that would occur in wire drawing or sheet-metal rolling. Although the deformation of the present disclosure occurs in the vicinity of the blunt tool, a large surface of a workpiece may be nanocrystallized by systematically applying the blunt tool to the entire surface. In an example, a cylindrical surface may be nanocrystallized by rotating the cylinder while moving the blunt tool along the cylindrical axis. In the example, the blunt tool would take a spiral path over the entire surface of the cylinder. It is to be further understood that more than one pass may be made over the finish surface with the blunt tool. In an example, four passes are made over the finish surface with the blunt tool.
Conventional Ferritic NitroCarburizing (FNC) usually takes about 5 to 6 hours at about 570° C. to obtain a 10 micron thick hard layer on the surface of metallic parts (for example, brake rotors) for better wear, fatigue and corrosion resistance. In contrast, examples of the method of the present disclosure may advantageously reduce FNC time down to about 1 to 2 hours to achieve the same hard layer thickness and therefore considerably reduce the processing energy cost.
Referring first to FIG. 15A, examples of the method 100 of the present disclosure are depicted. Each of the examples shown in FIG. 15A includes casting a cast iron (e.g., grey cast iron, nodular cast iron, etc.) workpiece, as shown at reference numeral 102; stress relieving the cast iron workpiece, as shown at reference numeral 104; and machining the workpiece to provide a finish surface thereon, as shown at reference numeral 106.
After the machining performed in step 106, one example of the method 100 then involves the steps shown along the path labeled 1, which include deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against a blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numeral 108; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110.
After machining at step 106, another example of the method 100 then involves the steps shown along the path labeled 2, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110.
After machining at step 106, still another example of the method 100 then involves the steps shown along the path labeled 2′, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; and nitrocarburizing the workpiece for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., as shown at reference numeral 110. This example of the method ends after the nitrocarburizing step.
Referring now to FIG. 15B, other examples of a method 100′ of the present disclosure are depicted. Each of the examples shown in FIG. 15B includes casting a cast iron workpiece, as shown at reference numeral 102 and machining the workpiece to provide a finish surface thereon, as shown at reference numeral 106.
After machining at step 106, an example of the method 100′ then involves the steps shown along the path labeled 3, which include deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against a blunt tool (described further herein), thereby forming a nanocrystallized microstructure at the surface, as shown at reference numeral 108; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′.
After machining at step 106, another example of the method 100′ then involves the steps shown along the path labeled 4, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; roller burnishing the nanocrystallized microstructure, as shown at reference numeral 114; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′.
After machining at step 106, still another example of the method 100′ then involves the steps shown along the path labeled 4′, which include simultaneously cooling and deforming the finish surface of the workpiece by rubbing (e.g., by rotating) the finish surface against the blunt tool (described further herein), thereby forming the nanocrystallized microstructure at the surface, as shown at reference numerals 108 and 112; and nitrocarburizing the workpiece for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C., as shown at reference numeral 110′. This example of the method ends after the nitrocarburizing step.
In each of the examples above of the present methods, the FNC renders the nanocrystallized microstructure into i) a friction surface (described further below at reference numerals 46, 46′), or ii) a corrosion-resistant surface (e.g., reference numerals 86, 86′ in FIGS. 6B and 12). As used herein, it is to be understood that a “friction” surface may also be a corrosion-resistant surface (in addition to being wear- and fatigue-resistant); however, a “corrosion-resistant” surface is not necessarily a friction surface. It is to be further understood that, in an example, the “corrosion-resistant” surface may be a free (non-contact) surface.
This formation of the nanocrystallized microstructure prior to FNC allows a higher diffusion rate of nitrogen and carbon into the cast iron workpiece, which leads to a considerably more efficient FNC process.
Without being bound to any theory, it is believed that at least the following three aspects are improved with methods of the present disclosure: 1. at conventional FNC temperatures (e.g., method 100), the FNC processing time may be reduced down to about 1 hour to 2 hours (from the conventional 5 to 6 hours); 2. alternatively (e.g., method 100′), FNC may be performed at a low temperature at which conventional FNC cannot thermodynamically create a hard nitride layer. This low temperature treatment may lead to a better dimensional stability, thereby eliminating the need for a stress relief step (i.e., step 104) in some instances; and 3. the surface nanocrystallized microstructure may itself contribute to better wear and fatigue performance of the workpiece. In addition, it is believed that the addition of cooling during deformation may enhance the effectiveness of nanocrystallization by slowing down or suppressing dynamic recrystallization, where deformed grain structures grow and become undesirably coarse. Still further, it is believed that a surface burnishing step may be performed to reduce surface roughness that may result from the nanocrystallization. Decreasing the surface roughness may improve the performance of the cast iron workpiece.
In the examples disclosed herein, machining 106 may be accomplished by, for example, turning, milling, sand blasting, grit blasting, grinding, and combinations thereof.
Referring now to FIG. 3, an example of a workpiece (e.g., a rotational member/brake rotor 12, 39) and a hard, blunt tool 80 operating thereon is shown (this takes place after the machining of the cast iron workpiece to create the finish surface). The workpiece is depicted rotating about an axis while the tool 80 including a pellet 82 (made, e.g., from an iron-tungsten alloy, silicon carbide, boron nitride, titanium nitride, diamond, hardened tool steel, or the like) is in contact with the finish surface. It is to be understood that, according to examples of the present disclosure, the workpiece, the tool 80, 80′ (see FIG. 14 for tool 80′), or both may be rotating while the tool 80, 80′ is operating on the workpiece. Still further, in examples of the present disclosure, neither the tool 80, 80′ nor the workpiece may be rotating, but rather the tool 80, 80′ may be moving, e.g., transversely forward and backward while the workpiece is translated longitudinally, or vice versa. It is to be yet further understood that other methods of bringing the tool 80, 80′ into deforming rubbing contact with the workpiece are contemplated as being with the purview of the present disclosure.
The tool 80, 80′ applies a deforming force to the machined, finish surface of the workpiece. In an example, the blunt tool 80, 80′ may be advanced by rotating a leadscrew that controls the advancement of the blunt tool 80, 80′ into a rotating finish surface of the workpiece by about 0.03 mm beyond first contact between the rotating workpiece and the blunt tool 80, 80′. It is to be understood that advancing the blunt tool 80, 80′ by about 0.03 mm does not necessarily create penetration of 0.03 mm in the workpiece because of elastic deformation of the workpiece, the pellet 82, and the holding fixture of the blunt tool 80, 80′. Further, the pellet 82 is not sharp and does not cut the finish surface. Blunt tool 80 reorganizes the crystal structure of the finish surface substantially without removing material therefrom. It is to be understood that the deformation of the finish surface may not be visible to the naked eye. However, a change in the reflective properties of the finish surface may be observable to the naked eye.
In an example, the tool 80, 80′ may cause pellet 82 to vibrate relative to the workpiece (as indicated by the double sided arrow V shown in phantom in FIG. 3). The vibration may be accomplished at ultrasonic frequencies (e.g., about 10,000 Hz to about 100,000 Hz).
FIG. 3A is an enlarged schematic view showing the pellet 82 of the tool 80 forming a nanocrystallized microstructure 70 at the surface of the workpiece substrate 84. It is to be understood that the pellet 82 may be a sphere, a spherical cap, a roller, a parabolic shape, an elliptical shape, or any shape that makes a local indentation on the finish surface and creates heavy deformation when operating on the workpiece (e.g., by rotating the workpiece thereagainst).
In examples of the present disclosure, cooling may be performed simultaneously with the deformation of the workpiece surface ( reference numerals 108 and 112 in FIGS. 15A and 15B). An example of the cooling is shown schematically in FIG. 4. In general, a coolant may be applied to the tool and/or the workpiece while deformation is being performed. It is to be understood that the heat transfer properties of the coolant may improve tool life and nanocrystallization characteristics (e.g., by reducing or eliminating dynamic recrystallization), however, lubrication may have deleterious effects in some instances. Examples of suitable coolants are liquid coolants (e.g., deionized water, mineral oil, high-flash point kerosene, liquid nitrogen, or lubricants such as WD-40® (WD-40 Company)) or gas coolants (e.g., air (e.g., compressed air), argon, helium, carbon dioxide gas, and nitrogen gas, which generally do not have high lubricity but have good heat transfer characteristics. Some of these examples may also be available as mists or aerosols. Any of the coolants may be chilled (e.g., water, gases, etc.).
Examples of the liquid coolant may also have phase changing material particles suspended therein to further enhance the cooling effect. An example of the phase changing material particles includes micro-encapsulated particles, such as sodium sulfate (Na₂SO₄.10H₂O) particles or lauric acid particles. The volume fraction of added phase changing material particles may range from about 1% to about 30%.
The coolant may have a temperature ranging from about −100° C. to about 25° C. In one example, the temperature of the coolant is about 0° C.
Rapid cooling may be performed via any suitable method. In an example, the coolant is in the form of a cooling gas jet that is directed toward the surface where deformation is taking place.
FIG. 4 illustrates one example of the rapid cooling, which utilizes a vortex tube 14. Generally, the vortex tube 14 includes a tube 76 having a specially designed chamber (a swirl chamber 16) and a conical nozzle 18. A pressurized gas 48 is injected into the swirl chamber 16 at an angle tangent to the surface of the swirl chamber 16, and is accelerated to a high rate of rotation. The combination of the pressure and the internal shape of the swirl chamber 16 accelerates the gas 48 to the high rate of rotation, which may be, e.g., over 1,000,000 rpm. The gas 48 is split into two streams, one of which gives kinetic energy to the other. The gas 48 separates into an outer vortex of a hot gas/airflow 48′ and an inner vortex of a cold gas/airflow 48″. The conical nozzle 18 at one end 78 ₂of the tube 76 allows the outer vortex of compressed gas (i.e., the hot gas/airflow 48′) alone to exit from that end 78 ₂of the tube 76. The cooler airflow 48″ is forced to return to another end 78 ₁of the tube 76 in the inner vortex, which is within the outer vortex of the hot gas/airflow 48′. The cooler gas/airflow 48″ exits the tube 76 at the end 78 ₁opposite the conical nozzle 18. The vortex tube 14 may be strategically positioned to achieve the desired cooling of the workpiece during deformation.
In the example shown in FIG. 4, the temperature of the cold gas/airflow 48″ may depend, at least in part, on the pressure of the compressed air. Generally, the higher the pressure, the lower the temperature.
In any of the examples involving cooling, the cooling rate may vary depending, at least in part, on the workpiece and tooling setup, the material of the workpiece (e.g., its thermal conductivity), the feeding rate (i.e., how fast the workpiece is deformed), and the cooling conditions.
It is to be understood that the vortex tube 14 (or other cooling device) may be separate from the blunt tool 80 (as shown in FIG. 4), or it may be integrated with the blunt tool 80 (as shown in phantom in FIG. 4).
The deformation or deformation and cooling of the machined, finish surface promotes the nanocrystallization of that surface, resulting in the formation of the nanocrystallized microstructure 70. It is to be understood that the nanocrystallized microstructure 70 (see FIG. 3A) may have any suitable thickness. However, in an example of the present disclosure, the thickness of nanocrystallized microstructure 70 ranges from about 3 μm to about 15 μm. In a further example, the thickness of nanocrystallized microstructure 70 is about 8 μm. As mentioned above, it is believed that the nanocrystallized microstructure 70 is better prepared (e.g., compared to a surface that is not nanocrystallized) for FNC treatments.
In some examples of the method disclosed herein, it may be desirable to surface burnish 114 the nanocrystallized microstructure 70. An example of the method utilizing surface burnishing 114 will now be described in reference to FIGS. 5A through 5C.
Referring now to FIG. 5A, the machining 106 results in the finish surface 69, which has a surface roughness that is relatively uniform. For example, the finish surface 69 has a regular pattern with similarly sized peaks and valleys. However, as shown in FIG. 5B, after nanocrystallization 108 (performed with or without cooling 112), the nanocrystallized microstructure 70 may exhibit a relatively random and non-uniform roughness that is undesirable for at least some applications. More particularly, the nanocrystallization process 108 (performed alone or in combination with cooling 112) may result in the formation of non-uniform features 88. The formation of non-uniform features 88 may be due, at least in part, to the severe cold working of the cast iron workpiece.
As shown in FIG. 5C, roller burnishing 114 may be performed on the nanocrystallized microstructure 70 to remove the non-uniform features 88. Roller burnishing 114 may be accomplished by pressing a cylindrical (or spherical) roller 90 against the nanocrystallized microstructure 70. The roller burnishing process obtains a very smooth surface finish by plasticizing the outermost layer of the nanocrystallized microstructure 70. During roller burnishing, the rolling force at the contact point between the nanocrystallized microstructure 70 and the roller 90 generates contact stresses in the edge of the workpiece. If these stresses are higher than the yield strength of the workpiece material, the workpiece material at the nanocrystallized microstructure 70 begins to flow. It is believed that during roller burnishing, the peaks of the non-uniform features 88 are pressed down, and the flowing workpiece material is moved into another position along the nanocrystallized microstructure 70. As the roller 90 moves across the surface of the nanocrystallized microstructure 70, the elastically deformed workpiece material springs back and pushes the now plastically deformed workpiece material into compression. The roller 90 also prevents the plasticized material from flowing against the direction of the roller 90. This process may be continued across the entire nanocrystallized microstructure 70 to achieve a relatively smooth surface.
In an example, roller burnishing reduces the roughness of the nanocrystallized microstructure 70 from Ra=1 μm to 5 μm to Ra <0.1 μm. It is to be understood that the reduction in surface roughness may depend, at least in part, on the material of the workpiece and the roller burnishing conditions that are used.
The roller 90 may be any commercially available cylindrical or spherical roller available from ECOROLL AG/ECOROLL Corp. In an example, the radius of the roller 90 ranges from about 1 mm to about 200 mm. In another example, the radius of the roller ranges from about 5 mm to about 10 mm. The roller 90 generally has a smooth surface, which is polished and has a surface roughness, Rz, less than 1 μm.
Roller burnishing has a relatively short cycle time. In an example, the cycle time ranges from about 10 seconds to about 120 seconds.
In the examples disclosed herein, after nanocrystallization (with or without active cooling), or after nanocrystallization (with or without active cooling) and surface burnishing, the method(s) involve nitrocarburizing ( reference numerals 110 and 110′).
FIG. 6 is an enlarged cross-sectional schematic view showing an example of the workpiece in a nitrocarburizing environment. In particular, the nanocrystallized microstructure 70 is depicted. It is to be understood that the nanocrystallized microstructure 70 shown in FIG. 6 may or may not have been exposed to the surface burnishing process disclosed herein. The nanocrystallized microstructure 70, due, e.g., to a large number of grain boundaries, accelerates/facilitates diffusion of the nitrogen and carbon therethrough, toward the base material substrate 84 during the FNC process(es) 110, 110′. The FNC process(es) will be described in more detail below.
FIG. 6A is a scanning electron microscope (SEM) image showing an example of an actual workpiece depicting the substrate and the refined microstructure in the surface layer. The refined microstructure in this image may be the nanocrystallized surface layer 70, which is believed to have a thickness of about 8 microns. A scale indicator is provided in FIG. 6A to facilitate estimation of relative sizes.
Examples of the methods of the present disclosure are relatively simple to execute and can be applied to many workpieces (one example of which is a component with axial symmetry that can be rotated during metal work, e.g., components having a disc shape or round bar shape). FIG. 12 depicts a cast iron shaft produced according to an example of the present disclosure. The cast-iron shaft 37 has a corrosion-resistant surface 86′ formed according to an example of the present disclosure. FIG. 13 depicts a cast iron engine block cylinder liner produced according to an example of the present disclosure. The cast iron engine block cylinder liner 35 has an internal surface 87 (formed according to an example of the present disclosure) that resists wear from friction by piston rings and resists corrosion.
An example of a cast iron workpiece is a rotational member of a vehicle brake. A brake 10 (shown in FIG. 2) is an energy conversion system used to retard, stop, or hold a vehicle. While a vehicle in general may include spacecraft, aircraft, and ground vehicles, in this disclosure, a brake 10 is used to retard, stop, or hold a wheeled vehicle with respect to the ground. More specifically, as disclosed herein, a brake 10 is configured to retard, stop, or hold at least one wheel of a wheeled vehicle. The ground may be improved by paving.
A vehicle brake 10 may be a disc brake 20, drum brake 50, and combinations thereof. FIG. 1 depicts an example of a vehicle brake, in particular, a disc brake 20. In a disc brake 20, a rotational member 12 is typically removably attached to a wheel (not shown) at a wheel hub 40 by a plurality of wheel studs 24 cooperatively engaged with lug nuts (not shown). The rotational member 12 in a disc brake 20 may be known as a brake disc (or rotor) 39. The rotor 39 may include vent slots 38 to improve cooling and increase the stiffness of the brake disc 39. When hydraulic fluid is pressurized in a brake hose 34, a piston (not shown) inside a piston housing 32 of a caliper 28, causes the caliper 28 to squeeze the brake disc 39 between brake pads 36, thereby engaging the disc brake 20. The brake pads 36 may include a friction material that contacts a friction surface 46 of the brake disc 39 when the disc brake 20 is engaged. If the wheel is rotating at the time the disc brake 20 is engaged, kinetic energy of the moving vehicle is converted to heat by friction between the brake pads 36 and the brake disc 39. Some of the heat energy may temporarily raise the temperature of the brake disc 39, but over time, the heat is dissipated to the atmosphere surrounding the vehicle. FIG. 7 illustrates a perspective view of the rotational member 12/brake disc 39, in which the vent slots 38 and the friction surface 46 are depicted.
Referring now to FIG. 2, an example of a drum brake 50 is shown. The rotational member 12′ is a brake drum 56 (see also FIGS. 9 and 10). The brake drum 56 is removably fastened to a wheel (not shown). The brake drum 56 may include fins 68 to improve cooling and increase the stiffness of the brake drum 56. When hydraulic fluid is pressurized in a wheel cylinder 52, a piston 54 causes the brake shoes 62 to press a brake lining 66 against the brake drum 56, thereby engaging the drum brake 50. It is to be understood that the brake lining 66 is a friction material. Alternatively, a drum brake 50 may be engaged mechanically by actuating an emergency brake lever 64 via an emergency brake cable 58. The emergency brake lever 64 causes the shoes 62 to press the brake lining 66 against the brake drum 56. If the wheel is rotating at the time the drum brake 50 is engaged, kinetic energy of the moving vehicle is converted to heat by friction between the brake lining 66 and the brake drum 56. Some of the heat energy may temporarily raise the temperature of the brake drum 56, but over time, the heat is dissipated to the atmosphere surrounding the vehicle.
FIG. 8 shows a perspective view of a brake drum 56 in an example of a rotational member 12′. FIG. 9 is a rotated perspective view of the brake drum 56 shown in FIG. 8, showing an inside view of the brake drum 56. The friction surface 46′ is visible in FIG. 9. In examples of the present disclosure, the workpiece may be nanocrystallized in a process similar to the process disclosed herein with respect to FIG. 3 and FIG. 3A. As shown in FIG. 14, an example of the blunt tool 80′ may have a right angle configuration to provide access to the friction surface 46′ on an internal wall of the brake drum 56. When the friction surface 46′ is cylindrical as in the brake drum example depicted in FIGS. 8 and 9, the blunt tool 80′ (see FIG. 14) is advanced into the finish surface by moving the blunt tool 80′ radially outward and engaging the pellet 82 with the finish surface and transforming the finish surface into the nanocrystallized microstructure 70 (not shown in FIG. 14) and then into friction surface 46′ after ferritic nitrocarburization (FNC). In the example of FIG. 14, if cooling is utilized during formation of the nanocrystallized microstructure 70, it may be desirable that the cooling device (not shown) be integrated with the blunt tool 80′.
As shown in FIGS. 8, 9, and 14, examples of a brake drum 56 may include fins 68.
It is to be understood that a disc brake 20 may be combined with a drum brake 50. As shown in FIGS. 10 and 11, a drum-in-hat rotational member 12″ may be included in such a combination. In a drum-in-hat type brake, small brakeshoes may be mechanically/cable actuated as an emergency brake, while the flange portion acts as a typical disc brake.
The rotational member 12, 12′, 12″ includes the friction surface 46, 46′ that is engaged by a friction material of the brake pad 36 or the brake shoe 62. As a brake is engaged to retard a vehicle, mechanical wear and heat may cause small amounts of both the friction material and the rotational member 12, 12′, 12″ to wear away. It may be possible to reduce the rate of wear of the rotational member 12, 12′, 12″ or the friction material by reducing the coefficient of friction between the two, but a lower coefficient of friction may make the brake 10 less effective at retarding the vehicle.
In cast iron, corrosion is mainly the formation of iron oxides. Iron oxides are porous, fragile and easy to scale off. Further, corrosion on a friction surface may be non-uniform, thereby deleteriously affecting the brake performance and useful life. Thus, corrosion may lead to undesirably rapid wear of the friction surface 46, 46′ and the corresponding friction material.
Ferritic nitrocarburization produces a friction surface 46, 46′ that resists corrosion and wear. In examples of the present disclosure, ferritic nitrocarburization is used to render the nanocrystallized microstructure 70 (which may have been exposed to surface burnishing) into a compound layer 70′ on the rotational member 12, 12′, 12″ of the workpiece (e.g., brake 10). In an example, rotational member 12, 12′, 12″ has a compound layer 70′ disposed at the friction surface 46, 46′ or the corrosion- resistant surface 86, 86′. The compound layer 70′ may have an exposed surface in contact with an atmosphere, for example, air. An example of the compound layer 70′ resulting from nitrocarburization is shown in FIG. 6B.
As depicted in FIG. 6B, compound layer 70′ may include an oxide layer 72 having Fe₃O₄disposed at the exposed surface (friction surface 46, corrosion-resistant surface 86). In a number of variations the compound layer 70′ may have a thickness ranging from 5 to 30 microns. An iron nitride layer 74, may include epsilon Fe₃N iron nitride and gamma prime Fe₄N iron nitride, may be generally subjacent the oxide layer 72 and may contain a majority of epsilon Fe₃N iron nitride. Further, the oxide layer 72 may have a thickness 73 ranging from about 5% to about 50% of a thickness 75 of the compound layer 70′. As shown in FIG. 6B, a diffusion layer 77 is subjacent the iron nitride layer 74 and is a transition between the iron nitride layer 74 and a portion of the workpiece (e.g., rotational member 12, 12′, 12″, not shown in FIG. 6B) that is beyond the reach of ferritic nitrocarburization. Non-limiting variations of salt bath ferritic nitrocarburization processes can be found in U.S. Patent App. Pub. No.: 2013/0000787A1.
In an example, a ferritically nitrocarburized rotational member 12, 12′, 12″ having a friction surface 46, 46′ formed by methods of the present disclosure exhibits a friction material wear of less than 0.4 mm per 1000 stops at about 350° C. An experiment using the test procedure in Surface Vehicle Recommended Practice J2707, Issued February 2005 by SAE International may be conducted. An Akebono NS265 Non Asbestos Organic (NAO) friction material may be used in the experiment.
The workpiece/ rotational member 12, 12′, 12″ may be made from cast iron. The friction surface 46, 46′ may exhibit a hardness of between about 56 HRC and about 64 HRC. Hardness is directly related to wear resistance.
In the examples of the methods 100, 100′ disclosed herein, it is to be understood that nitrocarburizing 110, 110′ includes a gas nitrocarburizing process, a plasma nitrocarburizing process, or a salt bath nitrocarburizing process. The salt bath nitrocarburizing process may include immersing at least the friction surface 46, 46′ of the rotational member 12, 12′, 12″ into a nitrocarburizing salt bath, and then immersing at least the friction surface 46, 46′ of the rotational member 12, 12′, 12″ into an oxidizing salt bath.
It is to be understood that the rotational member 12, 12′, 12″ may include a brake disc 39, a brake drum 56, or a combination thereof.
Overall, examples of the present methods 100, 100′ may improve corrosion resistance similarly to FNC methods performed without first forming nanocrystallized surface layer 70. In summary, examples of the method of the present disclosure may reduce FNC cycle time by a factor of about 5 to 10 (e.g., reduced from about 5 to 6 hours to about 1 to 2 hours at 570° C.). Alternately, examples may enable low temperature FNC (reduced from 570° C. to about 400° C.-450° C.) to reduce part distortion. Examples of the present disclosure further produce workpieces with improved wear/fatigue resistance and corrosion resistance. Increased productivity is achievable compared to other surface nanocrystallization processes. For example, nanocrystallization by shot peening may require about 36 seconds per square centimeter. In sharp contrast, examples of the method disclosed herein may take about 2 seconds per square centimeter. Some examples of the method 100, 100′ also produce a desirably smooth surface.
Numerical data have been presented herein in a range format. It is to be understood that this range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a time period ranging from about 5 hours to about 10 hours should be interpreted to include not only the explicitly recited limits of about 5 hours to about 10 hours, but also to include individual amounts such as 5.5 hours, 7 hours, 8.25 hours, etc., and sub-ranges such as 8 hours to 9 hours, etc. Furthermore, when “about” is utilized to describe a value, this is meant to encompass minor variations (up to +/−10%) from the stated value.
In describing and claiming the examples disclosed herein, the singular forms “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise.
Additionally, reference throughout the specification to “one example”, “another example”, “an example”, and so forth, means that a particular element (e.g., feature, structure, and/or characteristic) described in connection with the example is included in at least one example described herein, and may or may not be present in other examples. In addition, it is to be understood that the described elements for any example may be combined in any suitable manner in the various examples unless the context clearly dictates otherwise.
While several examples have been described in detail, it will be apparent to those skilled in the art that the disclosed examples may be modified. Therefore, the foregoing description is to be considered non-limiting.

Claims

1. A method for treating a cast iron workpiece, the method comprising roller burnishing a nanocrystallized microstructure at a finish surface of the cast iron workpiece, thereby reducing roughness of the nanocrystallized microstructure.

2. The method as defined in claim 1 wherein the roller burnishing is accomplished with a cylindrical shaped tool or a spherical shaped tool, the cylindrical or spherical shaped tool having a polished surface with a surface roughness Rz <1 μm.

3. The method as defined in claim 2 wherein the roller burnishing is accomplished for a time ranging from about 10 seconds to about 120 seconds.

4. The method as defined in claim 1 wherein prior to the roller burnishing, the method further comprises:

machining the cast iron workpiece to provide the finish surface thereon; and

deforming the finish surface of the workpiece by rubbing the finish surface against a blunt tool, thereby forming the nanocrystallized microstructure at the finish surface.

5. The method as defined in claim 4 wherein after the deforming and after the roller burnishing, the method further comprises:

nitrocarburizing the cast iron workpiece, the nanocrystallized microstructure accelerating diffusion of nitrogen atoms and carbon atoms therethrough, the nitrocarburizing taking place:

i) if the cast iron workpiece is stress relieved, for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or

ii) if the cast iron workpiece is not stress relieved, for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C.

6. The method as defined in claim 4 wherein machining is accomplished by a process selected from turning, milling, sand blasting, grit blasting, grinding, and combinations thereof.

7. The method as defined in claim 1 wherein the cast iron workpiece is selected from a rotational member of a vehicle brake, a shaft, or an engine block cylinder liner.

8. A method for treating a cast iron workpiece, the method comprising:

deforming a machined, finish surface of the cast iron workpiece by rubbing the machined, finish surface against a blunt tool, thereby forming a nanocrystallized microstructure at the machined, finish surface; and

cooling the machined, finish surface simultaneously with the deforming, thereby promoting nanocrystallization of the machined, finish surface.

9. The method as defined in claim 8 wherein the cooling is accomplished by applying a cooling gas jet to the machined, finish surface.

10. The method as defined in claim 9 wherein the cooling gas jet is at a temperature ranging from about −100° C. to about 25° C.

11. The method as defined in claim 8 wherein the cooling is accomplished by exposing the machined, finish surface to a coolant.

12. The method as defined in claim 11 wherein the coolant includes phase changing material particles to enhance the cooling.

13. The method as defined in claim 8 wherein a cooling device to perform the cooling is integrated with the blunt tool.

14. The method as defined in claim 8, further comprising:

i) if the workpiece is stress relieved, for a period of time ranging from about 1 hour to about 2 hours at a temperature ranging from about 550° C. to about 570° C., or

ii) if the workpiece is not stress relieved, for a period of time ranging from about 5 hours to about 10 hours at a temperature ranging from about 370° C. to about 450° C.,

thereby rendering the nanocrystallized microstructure into i) a friction surface, or ii) a corrosion-resistant surface by the nitrocarburizing.