阅读说明：本技术 铁质工件和制造方法 (Iron workpiece and method of manufacture ) 是由 湛弘义 王建锋 M·L·霍利 M·T·里费 于 2018-09-03 设计创作，主要内容包括：一种制造铁质旋转构件的方法,包括以下步骤：以足够的进给速率车削摩擦表面的第一部分,以在摩擦表面的第一部分上提供第一变形层；以足够的进给速率精细车削摩擦表面的第二部分,以在摩擦表面的第二部分上提供第二变形层；对摩擦表面的第一部分和第二部分进行抛光,以达到预定的粗糙度；在足以使氮原子和碳原子通过变形层扩散的时间和温度下对旋转构件进行氮碳共渗,以形成厚度可变的硬化壳体。铁质旋转构件可以是具有毂表面和摩擦表面的制动转子,其中毂表面和摩擦表面具有厚度可变的硬化壳体。(A method of manufacturing a ferrous rotary member comprising the steps of: turning a first portion of the friction surface at a sufficient feed rate to provide a first deformed layer on the first portion of the friction surface; fine turning a second portion of the friction surface at a sufficient feed rate to provide a second deformed layer on the second portion of the friction surface; polishing the first and second portions of the friction surface to a predetermined roughness; nitrocarburizing the rotating member at a time and temperature sufficient to diffuse nitrogen atoms and carbon atoms through the deformation layer to form a hardened shell of variable thickness. The ferrous rotating member may be a brake rotor having a hub surface and a friction surface with a hardened shell of variable thickness.)

1. A ferrous brake rotor comprising:

a hub portion having a hub surface; and

an annular disk portion extending from the hub, wherein the disk portion includes a friction surface;

wherein the friction surface comprises a first section hardened shell thickness and the hub surface comprises a hub hardened shell thickness, an

Wherein the first section hardened shell thickness is greater than the hub hardened shell thickness.

2. The ferrous brake rotor of claim 1,

wherein the friction surface further comprises a second section hardened shell thickness, an

Wherein the first section hardened shell thickness is greater than the second section hardened shell thickness.

3. The ferrous brake rotor of claim 1, wherein the friction surface comprises a roughness of less than about 3 μ ι η.

4. The ferrous brake rotor of claim 2, wherein the first section hardened shell thickness is between a pair of second section hardened shell thicknesses.

5. The ferrous brake rotor as recited in claim 1, wherein the first segment hardened shell thickness and the hub hardened shell thickness include a hardness of 50-90 HRC.

6. The ferrous brake rotor of claim 1, wherein the first section hardened case thickness is about 1.1-2 times the hub hardened case thickness.

7. The ferrous brake rotor of claim 1, wherein the first section hardened case thickness comprises a porous layer and a non-porous layer.

8. The ferrous brake rotor of claim 7, wherein the porous layer has a thickness of about 10% of the total thickness of the non-porous layer and the porous layer.

9. A method for manufacturing a ferrous member having a friction surface, the method comprising:

turning a first portion of the friction surface at a sufficient feed rate and depth of cut to provide a first deformed layer on the first portion of the friction surface, wherein the first deformed layer comprises a nanocrystalline microstructure layer;

polishing the first portion of the friction surface to a predetermined first roughness; and

nitrocarburizing the ferrous component for a time and at a temperature sufficient to diffuse nitrogen atoms and carbon atoms through the nanocrystalline microstructure layer;

wherein turning the first portion of the friction surface comprises rotating the ferrous member about an axis of rotation at a rate of 400-1000 rev/min (RPM) and removing a layer of material from the surface of the first portion of the friction surface with a cutting tool at a feed rate of 0.25-1.00 mm/rev and a depth of cut of 0.2-0.8 mm.

10. The method of claim 9, further comprising the steps of:

fine turning a second portion of the friction surface at a sufficient feed rate to provide a second deformed layer on the second portion of the friction surface, wherein the second deformed layer comprises a nanocrystalline microstructure layer thinner than the nanocrystalline microstructure layer of the first portion of the friction surface; and

polishing the second portion of the friction surface to a predetermined second roughness;

wherein polishing the first and second portions of the friction surface comprises applying a polishing pressure of 5 to 25 MPa.

Technical Field

The present disclosure relates generally to ferrous workpieces and methods of manufacture, and more particularly, to cast iron rotary components of brake assemblies and methods of manufacturing the same.

Background

Ferrous materials are used in applications requiring resistance to surface abrasive wear. In automotive applications, cast iron is used to manufacture rotating components of brake assemblies, such as brake rotors and brake drums. Typical brake assemblies include brake pads or shoes having a friction material that engages a friction surface of a brake rotor or drum, respectively, to resist rotational movement of the rotating member and thereby slow the vehicle.

When the friction material is engaged to the friction surface of the rotating member, mechanical wear and heat cause a small amount of wear to the friction material and friction surface of the rotating member. The wear rate of the friction surface may be reduced by reducing the coefficient of friction between the friction surface and the friction material, but a lower coefficient of friction may make the brake less effective at slowing the vehicle. Furthermore, the rotating member is exposed to the harsh external operating environment of the motor vehicle. Ferrous substrates of rotating components, and more specifically frictional surfaces subject to continuous wear, form iron oxides due to exposure to water, salt, and other corrosive substances typically found in the harsh operating environment of a vehicle. Iron oxide is porous, brittle and prone to flaking, thus causing accelerated wear of the friction surfaces.

In order to increase the durability of cast iron parts with friction surfaces with long wear cycles and excellent corrosion resistance, engineers continue to develop materials and manufacturing processes to determine which microstructural features play the most important role in improving these properties and improving these material properties. U.S. patent application No. (PCT/CN 2012/085510) (PCT' 510 herein) discloses a method of polishing a friction surface by rubbing the surface with a blunt tool to form a nanocrystalline surface layer, and then diffusing nitrogen and carbon atoms through the nanocrystalline surface layer by a nitrocarburizing process to form a wear and corrosion resistant friction surface.

The method of polishing a friction surface by blunt instruments disclosed in PCT'510 provides a nanocrystalline structure for promoting diffusion of nitrogen and carbon atoms therethrough to provide a hardened shell. By applying greater force during polishing, the thickness of the nanocrystalline structure may be increased, providing a more desirable hardened shell thickness after nitrocarburizing. However, an excessive force may lead to undesirable results, such as damage to the friction surface, resulting in a surface roughness (Ra) of the friction surface of more than 3 μm, which is undesirable.

Thus, while the method of polishing and nitrocarburizing a rotating member to increase the wear resistance and corrosion resistance of a friction surface achieves its intended purpose, there is still a need for a method to further provide a thicker hardened shell to increase the wear resistance and corrosion resistance of the friction surface while providing good surface quality with a desired roughness (Ra) of 3 μm or less.

Disclosure of Invention

According to several aspects, an iron-based brake rotor is disclosed having a hub and an annular disk portion extending from the hub. The annular disk portion includes a first segment friction surface having a hardened shell with a hardened shell thickness. The hub portion includes a hub surface having a hardened shell thickness. The hardened shell thickness of the first segment friction surface is greater than the hardened shell thickness of the hub surface.

In another aspect of the present disclosure, the ferrous brake rotor further includes a second segment friction surface having a hardened shell adjacent the first segment friction surface of the disk portion. The hardened shell thickness of the first section friction surface is greater than the hardened shell thickness of the second section friction surface.

In yet another aspect of the present disclosure, the first segment friction surface and the second segment friction surface comprise a roughness of less than about 3 μm.

In yet another aspect of the present disclosure, the first segment friction surface is flush with and interspersed between the pair of second segment friction surfaces.

In yet another aspect of the present disclosure, the first segment friction surface and the hub surface comprise a hardness of 50 to 90 HRC.

In yet another aspect of the present disclosure, the hardened shell thickness of the first segment friction surface is about 1.1-2 times the hardened shell thickness of the hub surface.

In yet another aspect of the present disclosure, the hardened case thickness of the first segment friction surface includes a porous layer and a non-porous layer.

In yet another aspect of the disclosure, the porous layer has a thickness that is about 10% of the total thickness of the non-porous layer and the porous layer.

According to several aspects, a method for manufacturing a ferrous brake member having a friction surface is disclosed. The method comprises the following steps: turning a first portion of the friction surface at a sufficient feed rate and depth of cut to provide a first deformed layer having a nanocrystalline microstructure; turning a first portion of the friction surface comprises rotating a ferrous member about an axis of rotation at a rate of 400-1000 rev/min (RPM) and removing a layer of material from the surface of the first portion of the friction surface with a cutting tool at a feed rate of 0.25-1.00 mm/rev and a depth of cut of 0.2-0.8 mm; then, polishing a first portion of the friction surface to a predetermined first roughness; the ferrous component is then nitrocarburised for a time and at a temperature sufficient to allow diffusion of the nitrogen and carbon atoms through the nanocrystalline microstructure layer.

In yet another aspect of the disclosure, the method further comprises: turning a second portion of the friction surface at a sufficient feed rate to provide a second deformed layer on the second portion of the friction surface, wherein the second deformed layer comprises a nanocrystalline microstructure layer that is thinner than the nanocrystalline microstructure layer of the first portion of the friction surface; polishing a second portion of the friction surface to a predetermined second roughness; and polishing the first part and the second part of the friction surface under the condition of applying a polishing pressure of 5-25 MPa.

According to several aspects, a method for manufacturing a ferrous rotating member having a friction surface is disclosed. The method comprises the following steps: turning a first portion of the friction surface at a feed rate and depth of cut sufficient to provide a first deformed layer having a nanocrystalline microstructure on the first portion of the friction surface; after turning, polishing a first portion of the friction surface with a blunt tool to a predetermined roughness; the rotating member is then nitrocarburised at a time and temperature sufficient to diffuse the nitrogen and carbon atoms through the nanocrystalline microstructure layer to form a hardened shell (c1) having a thickness (t 1).

In another aspect of the disclosure, the step of turning the first portion of the friction surface includes rotating the friction surface about an axis of rotation at a rate of 400-1000 rev/min (RPM) and removing a layer of material from the first portion of the friction surface with a cutting tool at a rate of 0.25-1.00 mm/rev and a depth of cut of 0.2-0.8 mm.

In yet another aspect of the present disclosure, the step of turning the first portion of the friction surface includes cutting a plurality of grooves 3-8 μm deep.

In yet another aspect of the disclosure, the step of turning the first portion of the friction surface includes adjusting the RPM of the friction surface about the rotational axis such that the linear velocity of the friction surface relative to the cutting tool is 200-2000 m/min.

In yet another aspect of the present disclosure, the step of turning the first portion of the friction surface includes creating a surface roughness (Ra) of 3-8 μm.

In yet another aspect of the present disclosure, the second portion of the friction surface is polished to a roughness of less than 3 μm and a nanocrystalline microstructure layer is formed. The step of nitrocarburizing the rotating member diffuses a sufficient amount of nitrogen atoms and carbon atoms through the nanocrystalline microstructure layer of the second portion of the friction surface to form a hardened shell (c2) having a thickness (t 2).

In yet another aspect of the present disclosure, the thickness (t1) of the hardened case (c1) is greater than the thickness (t2) of the hardened case (c 2).

In yet another aspect of the present disclosure, the step of polishing the first and second portions of the friction surface includes applying a polishing pressure of 5 to 25 MPa.

In yet another aspect of the disclosure, the rotating member is a brake rotor having a hub surface, and the step of nitrocarburizing the rotating member includes diffusing a sufficient amount of nitrogen atoms and carbon atoms to form a third hardened shell on the hub surface, wherein the third hardened shell includes a third thickness that is less than a thickness of the second portion of the friction surface (t 2).

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

Drawings

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.

FIG. 1 is a flow chart depicting steps of a method according to the principles of the present disclosure;

FIG. 2 depicts processing steps of a method according to the principles of the present disclosure;

FIG. 3 depicts the pre-processing steps of a method according to the principles of the present disclosure;

FIG. 4 depicts an enlarged view of a surface finish after one of the steps of a method according to the principles of the present disclosure;

FIG. 5 depicts an enlarged view of a surface finish after another step of a method according to the principles of the present disclosure;

FIG. 6 is a perspective view of a vehicle brake assembly according to the principles of the present disclosure;

FIG. 7 is a perspective view of a brake rotor of the vehicle brake assembly according to the principles of the present disclosure;

FIG. 8 is a partial cross-sectional view of a brake rotor of the vehicle brake assembly according to the principles of the present disclosure;

FIG. 9 is a microscopic view in cross-section of a surface of a hub portion of a brake rotor of a vehicle brake assembly according to the principles of the present disclosure;

FIG. 10 is a microscopic view in cross section of a surface of a disk portion of a brake rotor of a vehicle brake assembly according to the principles of the present disclosure;

FIG. 11 is a partial cross-sectional view of a surface of a workpiece according to an exemplary embodiment;

FIG. 12 is a partial cross-sectional view of a workpiece surface according to an exemplary embodiment;

FIG. 13 is a partial cross-sectional view of a surface of a workpiece according to an exemplary embodiment; and

FIG. 14 is a time and temperature profile of a nitrocarburizing (FNC) treatment process.

Detailed Description

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. The illustrated embodiments are disclosed with reference to the accompanying drawings, wherein like reference numerals represent corresponding parts throughout the several views. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular features. Specific structural and functional details disclosed are not to be interpreted as limiting, but as a representative basis for teaching one skilled in the art how to practice the disclosed concepts.

The present disclosure provides a method for manufacturing a ferrous workpiece, such as a cast iron rotating member of a vehicle brake assembly. In the examples disclosed herein, the surface of a ferrous workpiece is first selectively machined and then pre-treated to form a nano-crystalline microstructure layer on the surface. Then, the ferrous workpiece is subjected to a Ferritic Nitrocarburizing (FNC) process in which the nanocrystalline microstructure layer accelerates and promotes diffusion of nitrogen and carbon atoms therethrough to provide a hardened layer, also referred to as a hardened case. The hardened shell may include a plurality of sections having a plurality of thicknesses. It has been surprisingly found that the combination of machining and pre-treatment of the surface can provide a thicker layer of nanocrystalline microstructure than machining or pre-treatment alone, without causing damage to the surface resulting in the desired roughness (Ra).

As used herein, the term "friction surface" refers to a functional surface of a ferrous workpiece that engages a friction material (e.g., a brake pad) in operation. As used herein, the term "finished surface" refers to a surface of a ferrous workpiece that has been exposed to a machining operation (e.g., turning). Also as used herein, the term "nanocrystalline microstructure" refers to a fine microstructure having nano-sized grains (e.g., about 5-2000 nm) at or near a finished surface.

Referring initially to fig. 1, fig. 1 is a method 100 of manufacturing a ferrous workpiece. The example shown in fig. 1 includes a first step 102 of casting a ferrous (e.g., gray iron, ductile iron, etc.) workpiece followed by an optional second step 104 of reducing stress in the ferrous workpiece. A third step 106 includes machining the workpiece to achieve a predetermined configuration and dimensions. The ferrous workpiece may be any cast iron component having a functional surface that may be symmetrically rotated about an axis during a turning operation. Turning operations or lathing is a machining process in which a cutting tool (typically a non-rotating tool) is moved more or less linearly while a workpiece is rotated symmetrically about an axis. Examples of axisymmetrically rotating workpieces include shafts having an outer cylindrical bearing surface and rotating members of a brake assembly (e.g., a brake rotor or drum) having a friction surface.

Fig. 2 shows an example of the third step 106, which shows a schematic view of machining a ferrous workpiece 118 by a turning operation using a cutting tool 121. The turning operation provides a finished surface 122 of the workpiece 118 having a heavily deformed nanolayer 124 having a depth of about 3-20 μm. The relatively deep cut is 0.2 to 0.8mm in depth and is generally indicated by reference numeral 125. The third step 106 can be accomplished using a relatively high feed rate of 0.25 to 1.00 mm/rev. The rotation speed is set to 400 to 1000rev/min, or the linear velocity can be set to 200 to 1000 m/min. After the third step, the roughness (Ra) of the semi-finished surface is about 3 to 8 μm, and the depth of the nanocrystalline microstructure on top of the surface is 3 to 20 μm.

Fig. 4 shows an image of the micro-finish remaining on the surface 122 of the workpiece 118 after the third step 106 of machining the workpiece 118. The peaks 132 and valleys 130 of the surface 122 provide the material that is plastically deformed in the next step (fourth step 108). The turning operation promotes surface nanocrystallization resulting in the formation of a nanocrystalline microstructure. The thickness of the desired nanocrystalline microstructure is from about 3 μm to about 20 μm, preferably about 8 μm. It is believed that with the FNC treatment, it produces a nanocrystalline microstructure better (e.g., compared to a surface that is not nanocrystalline).

The fourth step 108 includes plastically deforming the peaks 132 and valleys 130 of the surface 122 to form an overall thicker layer of nanocrystalline microstructure 128 at the surface. This is accomplished by using roll pressing to polish the finished surface of the workpiece 118 to plastically deform the surface of the workpiece 118. The rollers typically have a smooth surface, which is polished and has a surface roughness (Ra) of less than 1 μm. In one example, the radius of the roller is from about 1mm to about 200 mm. In another example, the radius of the roller is from about 5mm to about 10 mm. Roll polishing has a relatively short cycle time. In one example, the cycle time is from about 10 seconds to about 120 seconds per pass. Since polishing is a non-material-removing machining method, the thick and fine microstructure produced by the turning operation can be maintained or even thickened by polishing with the application of a pressure of 5 to 25 MPa. The roll polishing reduces the roughness (Ra) of the nanocrystalline microstructure from 3-6 μm to about 0.1-3.0 μm.

Fig. 3 is an illustration of a fourth step 108, showing an exemplary workpiece 118 being rotated and having a polishing tool 126 applied to the cylindrical surface 122. The polishing operation may be one cycle or several cycles. Fig. 5 is an image of the micro-finish remaining on the surface 122 of the workpiece 118 after the fourth step 108 of polishing the workpiece 118. Fig. 5 shows that peaks 132 and valleys 130 have been plastically deformed by polishing to form even thicker nanocrystalline structures 128.

The burnishing tool may also be a blunt tool of spherical, spherical crown, roller, parabolic or arbitrary shape that can be rotated relative to the workpiece to improve the surface roughness and surface quality of the semi-finished surface. Further deformation of the finished surface relative to the blunt tool (i.e., nanocrystalline) is a locally severe plastic deformation of the contact location between the blunt tool and the workpiece. The deformation takes place substantially without the formation of chips and without the removal of material during the deformation. Furthermore, the local deformation of the finished surface is different from the global deformation that may occur in wire drawing or sheet metal rolling.

Although the deformation of the present disclosure occurs near a blunt tool, by systematically applying the blunt tool to the entire surface, a very large surface of the workpiece can be nano-sized. It will be appreciated that a blunt tool may be used to make more than one pass over the finished surface, where each pass may have a different pressure, feed rate and polishing depth. It should be further appreciated that by polishing a surface that has undergone a turning operation as described above, a thicker layer of nanocrystalline microstructure 128 may be obtained at the surface than if the surface was polished without exposure to the turning operation.

The fifth step 110 of the method 100 includes nitrocarburizing at a temperature of about 550 ℃ to about 570 ℃ for a period of time (about 1 hour to about 3 hours) to diffuse nitrogen and carbon through the nanocrystalline microstructure. The combined third 106 and fourth 108 steps of the present method 100 enable the formation of thicker nanostructured layers on the finished surface without damage to the finished surface. This thicker nanocrystalline microstructure layer enables nitrogen and carbon to enter the ferrous workpiece deeper at higher diffusion rates, which results in a more efficient FNC process and a thicker hardened shell. It should be noted that the time required at the temperature of the fifth step 110 may vary based on the nitrocarburizing process, the workpiece composition and design, and the nitrogen and carbon potentials.

The method 100 of the present disclosure is relatively simple to perform and may be applied to the manufacture of various types of ferrous workpieces, such as rotating components of vehicle brake assemblies, that have axial symmetry and that can rotate during metal machining. Referring to FIG. 6, an exemplary vehicle brake assembly 210 is shown. The vehicle brake assembly 210 is an energy conversion system for decelerating, stopping, or controlling the vehicle. Although vehicles may generally include spacecraft, aircraft, and ground vehicles, in the present disclosure, the vehicle brake assembly 210 is used to slow, stop, or control a wheeled vehicle relative to the ground. More specifically, as disclosed herein, brake 210 is configured to slow, stop, or control at least one wheel of a wheeled vehicle. The vehicle brake assembly 210 may be a disc brake assembly 210 having a caliper 222 pressed against a friction surface 216 of a brake rotor 212. In another example, the vehicle brake assembly 210 is a drum brake assembly (not shown) having friction brake pads (not shown) that are pushed out over a friction surface of a brake drum. In yet another example, the vehicle brake assembly 210 may also be a combination of a disc brake assembly or a drum brake assembly.

Still referring to FIG. 6, the brake rotor 212 includes a hub portion 218 and an annular disk portion 224 extending from the hub portion 218. The brake rotor 212 may be made of a ferrous alloy (e.g., a cast iron alloy). Disk portion 224 includes a friction surface 216 that is engaged by the friction material of brake pad 220, which is held in place by caliper 222. Referring to FIGS. 7-10, the brake rotor 212 of FIG. 6 is shown independent of the remainder of the vehicle brake assembly 210. As shown in FIG. 6, the brake rotor 212 includes a disk portion 224 having a friction surface 216 and a hub portion 218. The hub portion 218 includes a plurality of holes or bores 226 through which studs 228 pass, the studs 228 being connected to a front axle or to a hub (not shown) of the vehicle. Disk portion 224 includes a friction surface 216 upon which a friction or brake pad 220 of vehicle brake assembly 210 acts to stop rotation of the wheel. Thus, the disk portion 224 and the hub portion 218 serve two different purposes; disk portion 224 converts rotational energy into thermal energy through friction, while the hub retains brake rotor 212 on the axle.

As shown in FIG. 8, the friction surface 216 of the disk portion 224 has specifications required to meet its purpose. In particular, the friction surface 216 requires a hard and high wear and corrosion resistant surface with a surface roughness of less than 3 μm. Since the friction surface 216 wears away due to the nature of the purpose, a thicker outer hardened layer (also referred to as a hardened shell) is desirable for a more durable brake rotor 212. The friction surface 216 may also include a hardened shell having multiple sections. The plurality of segments may comprise different thicknesses.

The surface 230 of the hub 218 does not require a hardened shell having the same thickness as the friction surface 216 of the disk portion 224. Thus, the thickness of the FNC housing on the friction surface 216 of the disk portion 224 is approximately 1.1-2 times the thickness of the FNC housing on the surface 230 of the hub 218. This may be accomplished by nano-crystallizing the friction surface 216 of disk portion 224 prior to exposing the entire brake rotor 212 to the FNC process. As described above, the nanocrystallization of at least a portion of the friction surface 216 is achieved by a combination of machine turning and polishing.

Fig. 9 is a microscopic image showing a cross-section of the surface 230 of the hub 218. FIG. 10 is a microscopic image showing a cross-section of the friction surface 216 of the disk portion 224 of the brake rotor. While the average thickness of the hardened shell 232 of the hub surface 230 is approximately 8 μm, the average thickness of the hardened shell 232 of the friction surface 216 of the disk portion 224 is approximately 13 μm. The friction surface 216 may exhibit a hardness of about 90HRC (rockwell hardness) and the hub may exhibit a hardness of about 50 HRC. Hardness is directly related to wear resistance, so the friction surface is more wear resistant than the hub portion.

With reference to fig. 11 and 12, and with continuing reference to fig. 7, additional examples of the present disclosure are shown and will now be described. FIG. 11 illustrates a partial cross-sectional view of the friction surface 216 of the brake rotor 212 shown in FIG. 7. The friction surface 216 has a hardened shell 234 with thicknesses t1 and t 2. The different thicknesses are achieved by selectively crystallizing the friction surface 216 by adjusting the depth of cut, high feed rate, and rotational speed during the turning operation. As a result of the selective nano-crystallization, the subsequent FNC process will produce a hardened shell deeper on the surface portion that has been nano-crystallized by turning and polishing than on the surface that has not been nano-crystallized. FIG. 12 shows a partial cross-sectional view of the friction surface 216 of the brake rotor 212, the friction surface 216 having multiple sections of the hardened shell 234 having a thickness t2, wherein an intermediate section of the hardened shell 234 has a thickness t 1. While only two examples of selective nanocrystals are shown, more patterns of variable hardened shell 134 thickness are contemplated without departing from the scope of the present disclosure.

Turning now to FIG. 13, another example of the present disclosure includes a workpiece 340, the workpiece 340 being processed to include an iron or ferrous master metal layer 342 having an unprocessed large crystal microstructure, a FNC hardened case 344, and a porous FNC hardened case 346. More specifically, as described above, the method 100 includes a FNC step wherein the FNC atmosphere contains a sufficiently high concentration of nitrogen. During the high temperature portion of the process, additional nitrogen may be dissolved in the ferrous workpiece 340. However, when the workpiece 340 cools, nitrogen no longer dissolves in the iron at the concentration at which the iron is at the higher temperature. Thus, nitrogen precipitates from the solution in the form of surface pores. In the present disclosure, the depth of the porous FNC hardened case 346 is 10% greater than the depth of the FNC hardened case 344.

The method 100 may be used to manufacture a brake rotor 212 in which a friction surface of the brake rotor is turned at a sufficient feed rate and depth of cut to provide a first deformed layer having a nanocrystalline microstructure on a first portion of the friction surface. The nanocrystalline microstructure layer may include a plurality of intermediate sections of different thicknesses formed by selectively varying the depth of cut and the feed rate. For example, the nanocrystalline microstructure layer on the surface may include a first section having a first thickness, a second section having a second thickness, a third section having a third thickness, and so on.

The step of turning the first portion of the friction surface includes rotating the friction surface about a rotation axis at a rate of 400-1000 rev/min (RPM) and removing the material layer with a cutting tool at a rate of 0.25-1.00 mm/rev and a cutting depth of 0.2-0.8 mm to form a plurality of grooves 3-8 μm deep or a surface roughness (Ra) 3-8 μm. The RPM of the friction surface about the axis of rotation may be adjusted such that the linear velocity of the friction surface relative to the cutting tool is 200-2000 m/min.

After turning, the surface has a distinct pattern of peaks and valleys suitable for polishing, as polishing promotes material flow from peaks to valleys, resulting in good surface roughness. The polishing process within the parameters disclosed herein further adds to the relatively thick fine microstructure layer formed by turning. The first part of the friction surface is then polished with a blunt tool to a roughness of less than 3 μm. The method 100 may also include polishing a second portion of the friction surface with minimal or no turning or fine turning or machining at all to achieve a roughness of less than 3 μm. The step of polishing the first and second portions of the friction surface includes applying a polishing pressure of 5 to 25 MPa.

The brake rotor is then nitrocarburised at a time and temperature sufficient to diffuse the nitrogen and carbon atoms through the nanocrystalline microstructure layer to form a first hardened shell segment (c1) having a thickness (t1) and a second hardened shell segment (c2) having a thickness (t 2). The thickness (t1) of the first hardened shell section (c1) is greater than the thickness (t2) of the second hardened shell section (c 2). Nitrocarburizing the rotating member includes diffusing a sufficient amount of nitrogen atoms and carbon atoms to form a third hardened shell on the hub surface, wherein the third hardened shell includes a third thickness that is less than a thickness of the second portion of the friction surface (t 2). It is understood that nitrocarburizing includes a gaseous 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 216 of the brake rotor 212 in a nitrocarburizing salt bath, and then immersing at least the friction surface 216 of the rotating member 212 in an oxidizing salt bath.

The combination of the turning process and the polishing process results in a thicker nanocrystalline layer resulting in a thicker hardened shell after undergoing nitrocarburizing. Nitrocarburizing may be accomplished by accelerating diffusion of nitrogen and carbon atoms through the nanocrystalline surface layer. Surface nanocrystallization and nitrocarburizing form a substantially rust-free and high wear/fatigue resistance surface on ferrous parts/workpieces.

The Ferritic Nitrocarburizing (FNC) process in gaseous form requires about 5 to 6 hours at about 560 ℃ to about 570 ℃ to obtain a hard white layer about 10 μm deep penetrating from the surface to the metal parts (e.g., brake rotor) for better wear resistance, fatigue resistance, and corrosion resistance. FIG. 14 depicts the FNC process 400 in a time versus temperature graph, wherein the y-axis 410 represents temperature and the x-axis 412 represents time. The FNC process begins with a peroxidation process 414, in which the part is heated to 450 deg.C for 90 minutes and air cooled. Next, the part is heated to a temperature of about 560 ℃ to about 570 ℃ for about 120 minutes at a temperature of 416. The part is then oil cooled 419. Examples of the method 100 of the present disclosure advantageously reduce the FNC time to achieve the same hardened layer thickness to about 1-3 hours, and thus greatly reduce processing energy costs, as compared to heating and oil cooling 420 at time and temperature 418 without using the method 100.

The digital data is presented herein in a range format. It is to be understood that such 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 of 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 individual amounts, e.g., 5.5 hours, 7 hours, 8.25 hours, etc., and sub-ranges, e.g., 8 to 9 hours, etc. Further, when "about" or "approximately" is used to describe a value, it is meant to encompass minor variations (up to +/-10%) from the stated value.

While the examples have been described in detail, those familiar with the art to which this disclosure relates will recognize various alternative designs and examples for practicing the disclosed methods within the scope of the appended claims.