U.S. patent application number 13/695541 was filed with the patent office on 2013-06-27 for method for shot peening.
The applicant listed for this patent is Yuji Kobayashi, Toshiya Tsuji. Invention is credited to Yuji Kobayashi, Toshiya Tsuji.
Application Number | 20130160510 13/695541 |
Document ID | / |
Family ID | 44675767 |
Filed Date | 2013-06-27 |
United States Patent
Application |
20130160510 |
Kind Code |
A1 |
Kobayashi; Yuji ; et
al. |
June 27, 2013 |
METHOD FOR SHOT PEENING
Abstract
The present invention is to provide a method for shot peening
for producing a compressive residual stress that exceeds 60% of the
yield strength at 0.2% offset without using stress shot peening.
Shot media are peened onto a processed steel that has an amount of
retained austenite in a range between 5 to 30%, and any change in
the amount of retained austenite is controlled to be in a range of
2 to 30% before and after shot peening to produce the compressive
residual stress in the processed steel.
Inventors: |
Kobayashi; Yuji;
(Toyokawa-shi, JP) ; Tsuji; Toshiya;
(Toyokawa-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kobayashi; Yuji
Tsuji; Toshiya |
Toyokawa-shi
Toyokawa-shi |
|
JP
JP |
|
|
Family ID: |
44675767 |
Appl. No.: |
13/695541 |
Filed: |
August 4, 2011 |
PCT Filed: |
August 4, 2011 |
PCT NO: |
PCT/JP2011/004414 |
371 Date: |
October 31, 2012 |
Current U.S.
Class: |
72/53 |
Current CPC
Class: |
C21D 7/06 20130101; C21D
2211/001 20130101 |
Class at
Publication: |
72/53 |
International
Class: |
C21D 7/06 20060101
C21D007/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2010 |
JP |
2010-176682 |
Claims
1. A method for shot peening, wherein shot media are peened onto a
processed steel that has an amount of retained austenite in a range
between 5 to 30%, and wherein a change in the amount of retained
austenite is controlled to be in a range of 2 to 30% before and
after shot peening to produce a compressive residual stress in the
processed steel.
2. The method for shot peening of claim 1, wherein a change in the
amount of retained austenite at the depth where the maximum
compressive residual stress is generated is controlled to be in a
range of 2 to 30% before and after shot peening to produce a
compressive residual stress in the processed steel.
3. The method for shot peening of claim 1 or 2, wherein the
processed steel is a gas carburized steel.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for shot peening.
Specifically, it relates to a method for shot-peening a steel.
BACKGROUND ART
[0002] Conventionally, shot peening has been known to produce
compressive residual stresses to improve the fatigue strength of
parts made of a steel (see authored by the Society of Shot Peening
Technology of Japan; Fatigue of Metals and Shot Peening; published
by Gendai Kogaku-sha; 2004). Further, it has been known that
increasing the maximum value of compressive residual stresses is
very effective in improving the fatigue strength of the parts (see
Masahiko Mitsubayashi, Takashi Miyata, and Hideo Aihara; Prediction
of Improvement in Fatigue Strength by Shot Peening and Selection of
Most Effective Peening Conditions; Transactions of JSME, Vol. 61,
No. 586 (June, 1995) pp. 28-34).
[0003] However, it is also known that the maximum value of
compressive residual stresses produced by shot peening is
approximately 60% of the yield strength at 0.2% offset (Hideki
Okada, Akira Tange, and Kotoji Ando; Relationship among Specimen's
Hardness, Residual Stress Distribution and Yield Stress on the
Difference of Shot Peening Methods; Journal of High Pressure
Institute of Japan, Vol. 41, No. 5 (2003) pp. 233-242). Thus by
applying stress shot peening, i.e., shot-peening a part that is
under a pre-stressed condition, a maximum compressive residual
stress that exceeds 60% of the yield strength at 0.2% offset can be
obtained (see the above reference).
[0004] Though the stress shot peening can be used for a part, like
a spring that can be stressed while shot-peening it, there have
been problems in that stress shot peening cannot be used for a part
like a gear that cannot be stressed while shot-peening it.
DISCLOSURE OF THE INVENTION
[0005] The object of the present invention is to provide a method
for shot peening for producing maximum compressive residual
stresses that exceed 60% of the yield strength at 0.2% offset by
controlling the properties of the material or the conditions for
the heat treatment of the processed steel and the conditions for
shot peening, without using the stress shot peening.
[0006] The method for shot peening of the first aspect of the
present invention is to produce a compressive residual stress in a
processed steel that has an amount of retained austenite in a range
of 5 to 30%, by peening shot media onto the processed steel. The
amount of retained austenite is controlled to keep the change in
the amount within a range of 2 to 30% before and after the shot
peening.
[0007] In the method for shot peening of the second aspect of the
present invention, the shot peening is controlled to keep the
change in the amount of retained austenite at the depth where the
maximum compressive residual stress is generated at a range of 2 to
30% before and after the shot peening.
[0008] In the method for shot peening of the third aspect of the
present invention the processed steel is a gas carburized
steel.
[0009] By the method for shot peening of the first aspect, a
maximum compressive residual stress can be obtained that exceeds
60% of the yield strength at 0.2% offset. Thus no jig for stressing
the processed steel for the shot peening is required. Further,
efficient shot peening can be used for a part such as a gear that
has a complicated shape.
[0010] By the method for shot peening of the second aspect, the
method for shot peening of the first aspect can always be
performed.
[0011] By the method for shot peening of the third aspect, a
processed steel that has a desired amount of retained austenite can
be easily obtained by changing carburizing.
[0012] The basic Japanese patent application, No. 2010-176682,
filed Aug. 5, 2010, is hereby incorporated by reference in its
entirety in the present application.
[0013] The present invention will become more fully understood from
the detailed description given below. However, the detailed
description and the specific embodiment are illustrations of
desired embodiments of the present invention, and are described
only for an explanation. Various possible changes and modifications
will be apparent to those of ordinary skill in the art on the basis
of the detailed description.
[0014] The applicant has no intention to dedicate to the public any
disclosed embodiment. Among the disclosed changes and
modifications, those which may not literally fall within the scope
of the present claims constitute, therefore, a part of the present
invention in the sense of the doctrine of equivalents.
[0015] The use of the articles "a," "an," and "the" and similar
referents in the specification and claims are to be construed to
cover both the singular and the plural, unless otherwise indicated
herein or clearly contradicted by the context. The use of any and
all examples, or exemplary language (e.g., "such as") provided
herein, is intended merely to better illuminate the invention, and
so does not limit the scope of the invention, unless otherwise
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 is a table showing the properties of the processed
steels that were used in the embodiments of the present invention.
FIG. 2 is a table showing the conditions of the shot peening that
were used in the embodiments of the present invention.
[0017] FIG. 3 is a table showing the properties of the processed
steels after the shot peening.
[0018] FIG. 4 is a supplemental table giving data that are similar
to those in Table 3.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] Below, the embodiments of the present invention are
described with reference to the drawings.
[0020] FIG. 1 is a table showing the properties of the processed
steels that were used in the embodiments of the present invention.
Steel-A to Steel-G are prepared as the processed steels. The carbon
contents (wt %), the conditions for heat treatment, and the yield
strengths at 0.2% offset (MPa), as properties of the materials, and
the tensile strengths (MPa), the hardness at the surfaces (HV0.3),
and the amount of retained austenite .gamma. (Gamma).sub.R (%),
are all shown in the table. The processed steels are prepared from
the steels that are based on a chromium steel or a
chromium-molybdenum steel and that have different carbon contents,
i.e., between 0.2 and 0.8 wt %, and the steels that are based on a
chromium-molybdenum steel that have a carbon content of 0.8 wt %,
and that are tempered in different conditions. These processed
steels are gas carburized steels.
[0021] FIG. 2 is a table showing the conditions of the shot peening
that were used in the embodiments of the present invention. Two
types of conditions for shot peening (the conditions for peening
shot media onto the processed steels) were used. A compressive-air
shot peening system was used in both types. The hardness (HV), the
diameters (mm), and the air pressure for peening shot media are all
shown in the table. The coverage, which represents the amount of
shot media being peened, was 300% in all cases.
[0022] FIG. 3 is a table showing the properties of the processed
steels after the shot peening. The table also shows the properties
before the shot peening. It shows the properties of Steel-A to
Steel-G in the upper and lower sides for two respective types of
conditions for shot peening.
[0023] That table shows the maximum compressive residual stress
.sigma. (Sigma).sub.R (MPa),
[0024] Gamma.sub.R at the peak depth (%), Gamma.sub.R
(max)/Gamma.sub.0.2, and the rate of change in Gamma.sub.R at the
peak depth (%), as the properties of the processed steels after
shot peening.
[0025] The maximum compressive residual stress Gamma.sub.R (MPa)
means the maximum value of the compressive residual stresses that
are measured at various depths from the surface (since a
compressive residual stress is generally expressed as a negative
value, it is the maximum value in absolute values). The compressive
residual stresses were measured by using a micro-stress analyzer
that is available from Rigaku Corporation (X-ray tube:
Cr-K.alpha.(.sub.Alpha); diffractive surface: (220); stress
constant: -3] MPa/deg; Bragg angle of the strain-free 2.theta.:
156.4.degree.).
[0026] The Gamma.sub.R at the peak depth (%) denotes the amount of
retained austenite at the depth where the maximum compressive
residual stress is generated. The amounts of retained austenite
were also measured by using a micro-stress analyzer that is
available from Rigaku Corporation (X-ray tube: Cr--K.sub.Alpha;
diffractive surface: (220); Gamma-diffraction plane: (311); time
for measuring on Alpha-plane: 60 sec; range of diffraction on
Alpha-plane: 156.4 degree C.).
[0027] The Gamma.sub.R (max)/Gamma.sub.0.2 denotes the maximum
compressive residual stress compared to the yield strength at 0.2%
offset. The rate of change in Gamma.sub.R at the peak depth (%)
denotes a rate of change in the amount of retained austenite before
and after the shot peening at the depth where the maximum
compressive residual stress is generated.
[0028] As seen in FIG. 3, the Gamma.sub.R (max)/Gamma.sub.0.2
exceeds 60%, which is the target value, for Steel-B, -C, -D, -E,
and -G. FIG. 4 shows supplemental data for FIG. 3.
[0029] From these data, it was found that the processed steels that
have the maximum compressive residual stress that exceeds 60% of
yield strength at 0.2% offset can be obtained by the following
process, i.e., peening shot media onto a processed steel that has
the amount of retained austenite in a range between 5 to 30%. The
rate of change (reduction) in the amount of retained austenite at
the depth where the maximum compressive residual stress is
generated is controlled to be in a range between 2 to 30%.
[0030] The threshold value of the amount of retained austenite,
i.e., 5 to 30%, is determined based on the maximum value in the
range that is representative for industrial materials. The upper
limit for the rate of change in the amount of retained austenite,
i.e., 30%, is specified based on the maximum value of the amount of
retained austenite. The lower limit for the rate of change in the
amount of retained austenite, i.e., 2%, is determined by plotting
the Gamma.sub.R (max)/Gamma.sub.0.2 in relation to the rate of
change in Gamma.sub.R at the peak depth (%) and drawing an
approximate curve by the least square method.
[0031] If the rate of change (reduction) in the amount of retained
austenite of the processed steel at the depth where the maximum
compressive residual stress is generated is controlled to be in a
range between 2 to 30%, the maximum compressive residual stress
becomes over 60% of the yield strength at 0.2% offset. This is
because the retained austenite expands by the deformation-induced
martensitic transformation and thus the mechanical properties
improve by the expansion of the retained austenite.
[0032] As discussed above, in the embodiments of the present
invention processed steels that have the amount of retained
austenite in a range between 5 to 30% are subject to shot peening.
The change in the amount of retained austenite before and after
shot peening is controlled to be in a range of 2 to 30%, so as to
produce the compressive residual stress in the processed steel.
Thus, a maximum compressive residual stress that exceeds 60% of the
yield strength at 0.2% offset can be produced. Therefore, no jig
for stressing the processed steel for the stress shot peening is
required. Further, a part such as a gear, which has a complicated
shape, can be efficiently shot-peened.
[0033] Further, by changing the amount of retained austenite at the
depth where the maximum compressive residual stress is in the range
between 2 to 30% before and after shot peening, a maximum
compressive residual stress that exceeds 60% of the yield strength
at 0.2% offset can always be produced.
[0034] Further, since the processed material is a gas carburized
steel, a processed steel that has a desired amount of retained
austenite can be easily obtained by adjusting the conditions for
carburizing.
[0035] Any steels can be used for the processed steels, but a gas
carburized steel that has a large amount of retained austenite is
preferable.
* * * * *