U.S. patent application number 14/441971 was filed with the patent office on 2015-10-15 for method for treating a cast iron workpiece and workpiece formed thereby.
The applicant listed for this patent is GM Global Technology Operations LLC. Invention is credited to Jeff Wang, Xiaochuan Xiong.
Application Number | 20150292053 14/441971 |
Document ID | / |
Family ID | 50772004 |
Filed Date | 2015-10-15 |
United States Patent
Application |
20150292053 |
Kind Code |
A1 |
Xiong; Xiaochuan ; et
al. |
October 15, 2015 |
METHOD FOR TREATING A CAST IRON WORKPIECE AND WORKPIECE FORMED
THEREBY
Abstract
A method for treating a cast iron workpiece to increase a useful
life thereof includes machining the workpiece to provide a finish
surface thereon and deforming the finish surface of the workpiece
by rubbing the finish surface against a blunt tool (80,80'),
thereby forming a nanocrystallized surface layer (70). The
workpiece is nitrocarburized, the nanocrystallized surface layer
accelerating diffusion of nitrogen atoms and carbon atoms
therethrough. The nitrocarburizing taking place: i) if the
workpiece is stress relived prior to machining, for about 1 hour to
about 2 hours at a temperature ranging from about 550.degree. C. to
about 570.degree. C., or ii) if the workpiece is not stress
relieved prior to machining, for about 5 hours to about 10 hours at
a temperature ranging from about 370.degree. C. to about
450.degree. C. The nitrocarburizing renders the nanocrystallized
surface layer into i) a friction surface, or ii) a
corrosion-resistant surface.
Inventors: |
Xiong; Xiaochuan; (Shanghai,
CN) ; Wang; Jeff; (Nanjing, CN) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM Global Technology Operations LLC |
Detroit |
MI |
US |
|
|
Family ID: |
50772004 |
Appl. No.: |
14/441971 |
Filed: |
November 29, 2012 |
PCT Filed: |
November 29, 2012 |
PCT NO: |
PCT/CN2012/085510 |
371 Date: |
May 11, 2015 |
Current U.S.
Class: |
148/219 ;
148/318 |
Current CPC
Class: |
C21D 5/00 20130101; C21D
9/0068 20130101; C22C 37/00 20130101; C21D 8/005 20130101; C23C
8/56 20130101; B24B 39/045 20130101; C21D 7/08 20130101; C23C 8/04
20130101; Y10T 29/47 20150115; C23F 17/00 20130101; C23C 8/32
20130101; C21D 7/00 20130101; G01B 21/30 20130101 |
International
Class: |
C21D 5/00 20060101
C21D005/00; C23C 8/56 20060101 C23C008/56; C23C 8/04 20060101
C23C008/04; C22C 37/00 20060101 C22C037/00; C21D 9/00 20060101
C21D009/00; C21D 7/00 20060101 C21D007/00; C21D 8/00 20060101
C21D008/00; C23C 8/32 20060101 C23C008/32; C23F 17/00 20060101
C23F017/00 |
Claims
1. A method for treating a cast iron workpiece to increase a useful
life thereof, the method comprising: either i) stress relieving the
workpiece, or ii) refraining from stress relieving the workpiece;
machining the workpiece to provide a finish surface thereon;
deforming the finish surface of the workpiece by rubbing the finish
surface against a blunt tool, thereby forming a nanocrystallized
surface layer at the finish surface; and nitrocarburizing the
workpiece, the nanocrystallized surface layer accelerating
diffusion of nitrogen atoms and carbon atoms therethrough, the
nitrocarburizing taking place: 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.degree. C. to about
570.degree. 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.degree. C. to about 450.degree.
C., thereby rendering the nanocrystallized surface layer into i) a
friction surface, or ii) a corrosion-resistant surface by the
nitrocarburizing.
2. The method as defined in claim 1 wherein the workpiece is a
rotational member of a vehicle brake.
3. The method as defined in claim 1 wherein the workpiece is a
shaft or an engine block cylinder liner.
4. The method as defined in claim 1 wherein machining is
accomplished by a process selected from turning, milling, sand
blasting, grit blasting, grinding, and combinations thereof.
5. The method as defined in claim 1 wherein nitrocarburizing
includes a gas nitrocarburizing process, a plasma nitrocarburizing
process, or a salt bath nitrocarburizing process.
6. The method as defined in claim 1 wherein the nitrocarburizing
comprises: immersing at least the nanocrystallized friction surface
of the workpiece into a nitrocarburizing salt bath; and then
immersing the at least the nanocrystallized friction surface into
an oxidizing salt bath.
7. The method as defined in claim 1 wherein rubbing the finish
surface against the blunt tool is accomplished by rotating the
finish surface against the blunt tool.
8. The method as defined in claim 7 wherein four passes are made
over the finish surface with the blunt tool.
9. The method as defined in claim 7 wherein deforming further
comprises advancing the blunt tool into the rotating finish surface
of the workpiece by about 0.03 mm beyond first contact between the
rotating workpiece and the blunt tool.
10. The method as defined in claim 1 wherein the blunt tool
includes a blunt pellet operatively associated therewith, the
pellet to rubbingly contact the finish surface.
11. The method as defined in claim 10 wherein the pellet is formed
from a material chosen from iron-tungsten alloys, silicon carbide,
boron nitride, titanium nitride, diamond, and hardened tool
steel.
12. The method as defined in claim 10 wherein the pellet has a
shape chosen from a sphere shape, a spherical cap shape, a roller
shape, and a parabolic shape.
13. The method as defined in claim 1 wherein a thickness of the
nanocrystallized surface layer ranges from about 3 .mu.m to about
15 .mu.m.
14. A rotational member formed by the method of claim 1 wherein the
rotational member comprises a brake rotor, a brake drum, or a
combination thereof.
15. The rotational member as defined in claim 14 wherein the
rendered surface is a friction surface, and wherein the friction
surface exhibits hardness of between about 56 HRC and about 64 HRC.
Description
TECHNICAL FIELD
[0001] The present disclosure relates generally to methods for
treating cast iron workpieces and workpieces formed thereby.
BACKGROUND
[0002] 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
[0003] A method for treating a cast iron workpiece to increase a
useful life thereof includes machining the workpiece to provide a
finish surface thereon and deforming the finish surface of the
workpiece by rubbing the finish surface against a blunt tool,
thereby forming a nanocrystallized surface layer. The workpiece is
nitrocarburized, the nanocrystallized surface layer accelerating
diffusion of nitrogen atoms and carbon atoms therethrough. The
nitrocarburizing taking place: i) if the workpiece is stress
relieved prior to machining, for a period of time ranging from
about 1 hour to about 2 hours at a temperature ranging from about
550.degree. C. to about 570.degree. C., or ii) if the workpiece is
not stress relieved prior to machining, for a period of time
ranging from about 5 hours to about 10 hours at a temperature
ranging from about 370.degree. C. to about 450.degree. C. The
nitrocarburizing renders the nanocrystallized surface layer into i)
a friction surface, or ii) a corrosion-resistant surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] 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.
[0005] FIG. 1 is a perspective view of a disc brake assembly in an
example of the present disclosure;
[0006] FIG. 2 is a side view of a drum brake assembly in an example
of the present disclosure;
[0007] FIG. 3 is a schematic perspective view showing an example of
a workpiece and a tool operating thereon;
[0008] 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;
[0009] FIG. 4 is an enlarged cross-sectional schematic view showing
an example of the workpiece in a nitrocarburizing environment;
[0010] FIG. 4A is a schematic depiction of a section view showing
an example of a compound layer after ferritic nitrocarburization at
a microscopic enlargement;
[0011] FIG. 5 is a perspective view of a brake disc in an example
of the present disclosure;
[0012] FIG. 6 is a scanning electron microscope (SEM) image,
similar to the view of FIG. 4, but showing an example of an actual
workpiece depicting the microstructure of the substrate and the
nanocrystallized surface layer;
[0013] FIG. 7 is a perspective view of a brake drum in an example
of the present disclosure;
[0014] FIG. 8 is a perspective view showing the inside of the brake
drum depicted in FIG. 7;
[0015] FIG. 9 is a perspective view of a drum-in-hat rotational
member;
[0016] FIG. 10 is a cross sectional view of the drum-in-hat
rotational member depicted in FIG. 9;
[0017] FIG. 11A is a flow diagram depicting an example of a method
according to the present disclosure;
[0018] FIG. 11B is a flow diagram depicting another example of a
method according to the present disclosure;
[0019] FIG. 12 is a perspective view of a shaft in an example of
the present disclosure;
[0020] FIG. 13 is a perspective view of an engine block cylinder
liner in an example of the present disclosure; and
[0021] 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.
DETAILED DESCRIPTION
[0022] Examples of the present disclosure advantageously provide a
surface nanocrystallization process for faster or more energy
efficient ferritic nitrocarburizing (FNC) treatments of cast
iron.
[0023] Generally, methods according to examples of the present
disclosure include surface nanocrystallization by, e.g., deforming
against a blunt tool, and accelerated diffusion of nitrogen and
carbon atoms through the nanocrystallized surface layer, to form a
substantially rust-free and high wear/fatigue resistant case on
cast iron components/workpieces.
[0024] It is to be understood that in examples of the present
disclosure, the deformation against the blunt tool 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.
[0025] Conventional Ferritic NitroCarburizing (FNC) usually takes
about 5 to 6 hours at about 570.degree. 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.
[0026] Referring first to FIG. 11A, an example 100 of the method of
the present disclosure 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; machining the workpiece to provide
a finish surface thereon, as shown at reference numeral 106;
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 surface layer
at the finish surface, as shown at reference numeral 108; 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.degree. C. to about 570.degree. C., as shown at reference
numeral 110.
[0027] The nanocrystallized surface layer accelerates/facilitates
diffusion of nitrogen atoms and carbon atoms therethrough. It is to
be understood that the nanocrystallized surface layer (described
further below at reference numeral 70) has any suitable thickness.
However, in an example of the present disclosure, the thickness of
nanocrystallized surface layer 70 ranges from about 3 .mu.m to
about 15 .mu.m. In a further example, the thickness of
nanocrystallized surface layer 70 is about 8 .mu.m.
[0028] Referring now to FIG. 11B, another example 100' of the
method of the present disclosure includes casting a cast iron
workpiece, as shown at reference numeral 102; machining the
workpiece to provide a finish surface thereon, as shown at
reference numeral 106; 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 surface layer at the finish surface, as shown at
reference numeral 108; 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.degree. C. to about 450.degree.
C., as shown at reference numeral 110'. The nanocrystallized
surface layer accelerates/facilitates diffusion of nitrogen atoms
and carbon atoms therethrough.
[0029] In each of the examples above of the present method, the FNC
renders the nanocrystallized surface layer 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. 4A 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.
[0030] This formation of the nanocrystallized surface layer 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.
[0031] 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 in some instances; and 3. the surface
nanocrystallized microstructure may itself contribute to better
wear and fatigue performance of the workpiece.
[0032] 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 finish
machining of the cast iron workpiece). 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) 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), 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.
[0033] The tool 80, 80' applies a deforming force to the 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. 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 part because of
elastic deformation of the workpiece, the pellet 82, and the
holding fixture of the blunt tool 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.
[0034] In an example, the tool 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).
[0035] FIG. 3A is an enlarged schematic view showing the pellet 82
of the tool 80 forming a nanocrystallized surface layer 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, 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).
[0036] In examples of the present disclosure, a coolant may be
applied to the tool and/or the workpiece. It is to be understood
that the heat transfer properties of the coolant may improve tool
life and nanocrystallization characteristics, however, lubrication
may have deleterious effects on the method disclosed herein in some
instances. Examples of suitable coolants are water, air, carbon
dioxide gas, and nitrogen gas which generally do not have high
lubricity but have good heat transfer characteristics.
[0037] FIG. 4 is an enlarged cross-sectional schematic view showing
an example of the workpiece in a nitrocarburizing environment. The
nanocrystallized surface layer 70, due, e.g., to a large number of
grain boundaries, facilitates diffusion of the nitrogen and carbon
therethrough, toward the base material substrate 84 during the FNC
process(es) 110, 110'.
[0038] 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.
[0039] An example of a cast iron workpiece is a rotational member
of a vehicle brake. A brake 10 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.
[0040] 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.
[0041] 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.
7 and 8). 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.
[0042] FIG. 7 shows a perspective view of a brake drum 56 in an
example of a rotational member 12' . FIG. 8 is a rotated
perspective view of the brake drum 56 shown in FIG. 7, showing an
inside view of the brake drum 56. The friction surface 46' is
visible in FIG. 8. 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. 7
and 8, 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 surface layer 70 (not shown in
FIG. 14) and then into friction surface 46' after ferritic
nitrocarburization (FNC). As shown in both FIGS. 7 and 8, examples
of a brake drum 56 may include fins 68.
[0043] It is to be understood that a disc brake 20 may be combined
with a drum brake 50. As shown in FIGS. 9 and 10, 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.
[0044] The rotational member 12, 12', 12'' includes a 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.
[0045] 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.
[0046] 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 surface layer 70 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', corrosion-resistant
surface 86, 86'. The compound layer 70' may have an exposed surface
in contact with an atmosphere, for example, air.
[0047] As depicted in FIG. 4A, compound layer 70' further may
include an oxide layer 72 having Fe.sub.3O.sub.4 disposed at the
exposed surface (friction surface 46, corrosion-resistant surface
86). An iron nitride layer 74 including epsilon Fe.sub.3N iron
nitride and gamma prime Fe.sub.4N iron nitride may be generally
subjacent the oxide layer 72 and containing a majority of epsilon
Fe.sub.3N 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 iron nitride layer 74. As shown in FIG. 4A, 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) that is beyond the reach of ferritic
nitrocarburization (not shown).
[0048] 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.degree. C. An
experiment using the test procedure in Surface Vehicle Recommended
Practice J2707, Issued Feb. 2005 by SAE International may be
conducted. An Akebono NS265 Non Asbestos Organic (NAO) friction
material may be used in the experiment.
[0049] Referring now to FIG. 5, a perspective view of a brake disc
39 in an example is shown. Rotational member 12 is a brake disc 39
with vent slots 38.
[0050] FIG. 6 is a scanning electron microscope (SEM) image showing
an example of an actual workpiece depicting the microstructure of
the workpiece substrate and the nanocrystallized surface layer (the
thickness of the nanocrystallized surface layer 70 in this example
is about 8 microns). A scale indicator is provided in FIG. 6 to
facilitate estimation of relative sizes.
[0051] 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.
[0052] Machining 106 may be accomplished by, for example, turning,
milling, sand blasting, grit blasting, grinding, and combinations
thereof.
[0053] It is to be understood that nitrocarburizing 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.
[0054] 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.
[0055] Further, examples of the present methods 100, 100' may
improve corrosion resistance similarly to FNC methods performed
without first forming nanocrystallized surface layer 70.
[0056] 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.degree. C.). Alternately, examples may enable low temperature
FNC (reduced from 570.degree. C. to about 400.degree.
C.-450.degree. 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.
[0057] 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.
[0058] In describing and claiming the examples disclosed herein,
the singular forms "a", "an", and "the" include plural referents
unless the context clearly dictates otherwise.
[0059] 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.
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