U.S. patent application number 09/998684 was filed with the patent office on 2002-08-08 for method for nitriding suspension components.
Invention is credited to King, Tom, Medeiros, Leo.
Application Number | 20020104587 09/998684 |
Document ID | / |
Family ID | 23014203 |
Filed Date | 2002-08-08 |
United States Patent
Application |
20020104587 |
Kind Code |
A1 |
Medeiros, Leo ; et
al. |
August 8, 2002 |
Method for nitriding suspension components
Abstract
A controlled nitrogen diffusion process is employed on a steel
coil spring of a suspension system. One known process is that known
as the Nitreg.RTM. process. Ammonia is introduced into a furnace
atmosphere and nitrogen diffuses into the coil spring, creating a
hardened diffusion zone which resists fatigue and fractures. The
Nitreg.RTM. nitrogen diffusion process is a computer controlled
menu driven process which continually monitors the process
parameters. By regulating the nitriding potential, the monitored
process parameters are automatically adjusted to achieve optimal
results. The nitriding potential is the tendency of nitrogen to be
absorbed by steel and is expressed as the ratio of the partial
pressure of ammonia to the partial pressure of hydrogen.
Additionally, the depth of the white compound layer can be
regulated or eliminated, reducing distortions.
Inventors: |
Medeiros, Leo; (Toronto,
CA) ; King, Tom; (Milton, CA) |
Correspondence
Address: |
CARLSON, GASKEY & OLDS, P.C.
400 WEST MAPLE ROAD
SUITE 350
BIRMINGHAM
MI
48009
US
|
Family ID: |
23014203 |
Appl. No.: |
09/998684 |
Filed: |
November 30, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60266350 |
Feb 2, 2001 |
|
|
|
Current U.S.
Class: |
148/215 ;
148/318 |
Current CPC
Class: |
C23C 8/80 20130101; C21D
9/02 20130101; F16F 2226/02 20130101; C23C 8/26 20130101 |
Class at
Publication: |
148/215 ;
148/318 |
International
Class: |
C23C 008/26 |
Claims
What is claimed is:
1. A method for surface hardening a steel coil spring of a
suspension system comprising the steps of: nitriding a surface of
said coil spring; and regulating a nitriding potential in a
nitriding atmosphere to control the step of nitriding said coil
spring, said nitriding potential being the tendency of nitrogen to
be absorbed by said steel coil spring.
2. The method as recited in claim 1 wherein the step of regulating
said nitriding potential further includes monitoring at least one
process parameter.
3. The method as recited in claim 1 wherein the step of nitriding
said coil spring further includes introducing ammonia into said
nitriding atmosphere.
4. The method as recited in claim 1 further comprising the steps
of: cleaning said surface of said coil spring; heating said coil
spring; and cooling said coil spring.
5. The method as recited in claim 4 wherein the step of heating
said coil spring includes heating said nitriding atmosphere to a
temperature between 380.degree. C. and 480.degree. C.
6. The method as recited in claim 1 wherein the step of nitriding
said coil spring produces a diffusion zone having a depth between
30.mu.m and 100.mu.m.
7. The method as recited in claim 1 wherein the step of nitriding
said coil spring further includes forming a compound layer on said
surface of said coil spring having a depth between 0 and
2.mu.m.
8. The method as recited in claim 1 further comprising the step of
shot peening said surface of said coil spring.
9. The method as recited in claim 9 wherein the surface of said
coil spring is shot peened with a 0.8 mm diameter shot and a 0.3 mm
diameter shot.
10. A method for surface hardening a steel coil spring of a
suspension system comprising the steps of: cleaning said surface of
said coil spring; heating said coil spring; nitriding a surface of
said coil spring; regulating a nitriding potential in a nitriding
atmosphere to control the step of nitriding said coil spring, said
nitriding potential being the tendency of nitrogen to be absorbed
by said steel coil spring; cooling said coil spring; and shot
peening said surface of said coil spring.
11. A steel coil spring of a suspension system comprising: a steel
body portion having a surface; and a diffusion zone produced by
nitriding said surface of said coil spring by regulation of a
nitriding potential.
12. The coil spring as recited in claim 11 wherein said surface of
said coil spring is nitrided by introducing ammonia into a
nitriding atmosphere.
13. The coil spring as recited in claim 11 wherein a nitriding
atmosphere is heated to a temperature between 380.degree. C. and
480.degree. C.
14. The coil spring as recited in claim 11 wherein said diffusion
zone has a depth between 30.mu.m and 100.mu.m.
15. The coil spring as recited in claim 11 wherein said coil spring
further includes a compound layer having a depth between 0 and
2.mu.m.
Description
[0001] This application claims priority from provisional
application serial No. 60/266,350 filed Feb. 2, 2001.
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to a method for
nitriding suspension components.
[0003] Vehicles are commonly equipped with suspension systems for
absorbing road shock and other vibrations, while providing for a
smooth and comfortable ride. Steel coil springs are utilized as a
part of the vehicle suspension system. The coil springs must be
able to resist stresses which cause fractures and decrease the
fatigue life of the coil spring.
[0004] A nitriding process has been utilized to improve the fatigue
life of a coil spring utilized in a vehicle suspension system.
Nitriding produces compressive residual stresses on the surface of
the coil spring which counteract the tensile stresses produced by
everyday use which cause fractures.
[0005] Additionally, the nitriding process forms a white layer on
the exterior surface of the steel. For most applications, this
white layer has no useable properties. The layer is very hard, but
brittle, and may spall during use. As it has no useable properties,
it is often removed by treatment grinding or finishing.
SUMMARY OF THE INVENTION
[0006] A controlled nitrogen diffusion process is employed on the
exterior surface of a steel coil spring of a suspension system to
create a hardened layer which reduces fractures and improves
fatigue properties. One known process is the Nitreg.RTM. process.
The Nitreg.RTM. process is a computer controlled menu driven
process which regulates the nitriding potential of the furnace
atmosphere. The nitriding potential is the tendency of nitrogen to
be absorbed by steel and is expressed as the ratio of the partial
pressure of ammonia to the partial pressure of hydrogen. Electric
sensors and furnace components balance the nitriding atmosphere
required to maintain the desired nitriding potential.
[0007] The steel coil spring is first cleaned to remove scale from
the exterior surface. In an atmospheric furnace, the coil spring is
heated and ammonia is released. Nitrogen from the ammonia diffuses
into the exterior surface of the steel coil spring, creating a
hardened diffusion zone on the coil spring. After cooling the coil
spring, the coil spring is subject to shot peening to instill high
compressive residual stresses on the surface of the coil.
[0008] The nitriding potential of the furnace atmosphere is
regulated depending on the type of steel utilized and the
application requirements. By regulating the nitriding potential,
the depth of the diffusion zone can be controlled. Additionally,
the depth of the white compound layer can be regulated or
eliminated to reduce distortions.
[0009] These and other features of the present invention will be
best understood from the following specification and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The various features and advantages of the invention will
become apparent to those skilled in the art from the following
detailed description of the currently preferred embodiment. The
drawings that accompany the detailed description can be briefly
described as follows:
[0011] FIG. 1 illustrates a schematic side cross sectional view of
a nitrided steel coil spring;
[0012] FIG. 2 illustrates a graph relating the hardness of various
steels to the depth below the surface for a prior art nitriding
process; and
[0013] FIG. 3 illustrates a flowchart of the controlled nitrogen
diffusion process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0014] This invention relates to a method for nitriding a coil
spring of a suspension system utilizing a controlled nitrogen
diffusion process. One known process is the Nitreg.RTM. process.
The Nitreg.RTM. nitrogen diffusion process is a surface hardening
heat treatment wherein the nitriding potential of the furnace
atmosphere is regulated depending on the type of steel utilized and
the application requirements.
[0015] FIG. 1 illustrates a side cross sectional view of a nitrided
steel coil 8. Nitrogen is introduced into the surface of the steel
at a specific temperature range while in the ferritic condition to
harden the surface of the steel. Nitrogen is partially soluble in
iron. At nitrogen contents up to approximately 6%, nitrogen forms a
solid solution with ferrite. When the nitrogen content reaches
about 6%, a white layer having a composition of Fe.sub.4N is
formed. As shown in FIG. 1, this layer is the compound layer A. The
compound layer A is very hard, but is brittle and may spall in
use.
[0016] When the nitrogen content reaches 8%, the equilibrium
reaction product Fe.sub.3N is formed, illustrated as diffusion zone
B in FIG. 1. The diffusion zone B is hardened by the formation of
the Fe.sub.3N compound. The diffusion zone B is the layer which
provides surface hardening. Below the diffusion zone B is steel
zone C. Steel zone C is the inner steel portion of the coil spring
in which there is no nitrogen diffusion. The hardness of various
steels at the different zones are illustrated in FIG. 2.
[0017] Nitrided steels generally contain strong nitride-forming
elements such as aluminum, vanadium, molybdenum, titanium and
chromium. When these steels are nitrated, the nitride-forming
elements form particles with the nitrogen which create
strengthening dislocations by straining the ferrite lattice.
[0018] The Nitreg.RTM. nitrogen diffusion process of the present
invention is employed on a steel coil spring 8 to create a hardened
layer and improve fatigue properties. The Nitreg.RTM. nitrogen
diffusion process is a computer controlled menu driven process
which hardens the surface of the coil spring 8 by regulation of the
nitriding potential of the furnace atmosphere. The nitriding
potential is the tendency of nitrogen to be absorbed by steel and
is expressed by the ratio of the partial pressure of ammonia to the
partial pressure of hydrogen. Electronic sensors and furnace
components help balance the nitriding atmosphere to maintain the
specific nitriding potential. The sensors continually monitor and
adjust the process parameters to regulate the nitriding potential.
The nitriding potential is programmed depending on the type of
steel utilized and the application requirements.
[0019] The nitriding process 10 is illustrated schematically in
FIG. 3. The coil spring 8 is first cleaned 12 to remove scale from
the exterior surface. The scale is removed either by shot peening
or shot blasting. The exterior scale can also be removed by
utilizing hydrochloric acid on the surface of the coil spring 8.
The coil spring 8 is positioned in an atmospheric furnace. In the
furnace, the coil spring 8 is heated 14 to a temperature between
380.degree. C. and 480.degree. C. After heating 14, ammonia is
released 16 into the furnace for approximately 3 to 8 hours.
[0020] The amount of ammonia released 16 depends on the desired
nitriding potential.
[0021] Nitrogen from the ammonia diffuses 20 into the exterior
surface of the steel coil spring 8, creating a hardened diffusion
zone B. By regulating the nitriding potential 18 by continually
monitoring the process parameters with sensors, the depth of the
diffusion zone B can be controlled. The diffusion zone B is
preferably between 30 .mu.m and 100 .mu.m deep. Once the nitrogen
from the ammonia has diffused into the steel coil spring 8, the
coil spring 8 is then cooled 22. The entire cycle lasts
approximately 12 to 20 hours.
[0022] By regulating the nitriding potential 18 depending on the
type of steel utilized and the application requirements, the white
compound layer A can be controlled or eliminated. In traditional
nitriding, the process produces uncontrollable white layer growth
which is removed by post treatment grinding or finishing. The
Nitreg.RTM. process produces a hard and non-spalling white compound
layer A on the exterior surface of the coil spring 8. The growth of
the compound layer A can be controlled by regulating the nitriding
potential. In the preferred embodiment, the compound zone A is 0 to
2 .mu.m thick.
[0023] After the nitriding process, the coil spring 8 is subject to
a shot peening process 24 to instill high compressive residual
stresses on the surface of the coil spring 8.
[0024] Preferably, a two-step process is employed. In the first
step, the first peening is done with a 0.8.mu.mm diameter shot, and
in the second step, the second peening is done with a 0.3 mm
diameter shot. By employing the additional shot peening step, there
is at least a six fold increase in the fatigue life of the coil
spring 8 having a 0.5 inch diameter wire subjected to a stress of
564 +/-476 Mpa.
[0025] The Nitreg.RTM. nitrogen diffusion process provides surface
hardening. The fatigue properties of the coil spring 8 can be
improved, allowing for higher stress design and/or lighter weight
springs. The process strengthens the exterior surface of the coil
spring 8, decreasing fractures and increasing the fatigue life.
Additionally, as the process is computerized, the results are
repeatable.
[0026] The foregoing description is only exemplary of the
principles of the invention. Many modifications and variations of
the present invention are possible in light of the above teachings.
The preferred embodiments of this invention have been disclosed,
however, so that one of ordinary skill in the art would recognize
that certain modifications would come within the scope of this
invention. It is, therefore, to be understood that within the scope
of the appended claims, the invention may be practiced otherwise
than as specially described. For that reason the following claims
should be studied to determine the true scope and content of this
invention.
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