U.S. patent application number 13/639254 was filed with the patent office on 2013-05-16 for method for shot peening a gas carburised steel.
The applicant listed for this patent is Yuji Kobayashi, Toshiya Tsuji. Invention is credited to Yuji Kobayashi, Toshiya Tsuji.
Application Number | 20130118220 13/639254 |
Document ID | / |
Family ID | 44675768 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130118220 |
Kind Code |
A1 |
Kobayashi; Yuji ; et
al. |
May 16, 2013 |
METHOD FOR SHOT PEENING A GAS CARBURISED STEEL
Abstract
The present invention is to provide a method for shot peening
for producing a high compressive residual stress in a gas
carburized steel that has a soft layer. In this method, a depth
where the maximum compressive residual stress is generated is
estimated and the hardness on the surface or near the surface is
not used. The depth where the maximum compressive residual stress
is generated is estimated by multiplying the depth where the
maximum stress is generated under contact stresses caused by the
collision of shot media by the constant K. A processed steel that
comprises a gas carburized steel and that has a hardness at that
depth that exceeds 750 HV is used. Shot media that have a hardness
that is greater than that of the processed steels at that depth by
50 HV or more are shot onto the processed steels to produce a high
compressive residual stress in the processed steels.
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: |
44675768 |
Appl. No.: |
13/639254 |
Filed: |
August 4, 2011 |
PCT Filed: |
August 4, 2011 |
PCT NO: |
PCT/JP2011/004416 |
371 Date: |
October 4, 2012 |
Current U.S.
Class: |
72/53 |
Current CPC
Class: |
B24C 1/10 20130101; C21D
11/00 20130101; C23C 8/80 20130101; C21D 7/06 20130101 |
Class at
Publication: |
72/53 |
International
Class: |
B24C 1/10 20060101
B24C001/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2010 |
JP |
2010-176681 |
Claims
1. A method for shot peening, wherein a processed steel comprises a
gas carburized steel that has hardness at 750 HV or more at a depth
z, where a maximum compressive residual stress is generated, the
depth z being estimated by using Equations (1) to (4), and wherein
shot media that have a hardness that is greater than the hardness
of the processed steel by 50 HV or more are shot onto the processed
steel to produce a compressive residual stress in the processed
steel: .alpha. = 1 8 E * 3 5 ( 5 .pi. 4 ) s 5 .rho. 3 5 D 3 V 6 5 (
1 ) 1 E * = 1 - v 1 2 E 1 + 1 - v 2 2 E 2 ( 2 ) z = 0.48 K .alpha.
( 3 ) K = 1.25 ( 4 ) ##EQU00003## where .alpha.: contact radius by
shot media (m), p: specific gravity of shot media (kg/m.sup.3), D:
diameter of shot media (m), V: shot speed(m/s), E*: equivalent
elastic modulus E.sub.1: Young's modulus of the processed
steel(Pa), v.sub.1: Poisson's ratio of the processed steel,
E.sub.2: Young's modulus of shot media (Pa), v.sub.2: Poisson's
ratio of shot media, K: constant, and z: depth where the maximum
compressive stress is generated (m).
2. The method for shot peening of claim 1, wherein diameters of the
shot media are in a range from 0.2 mm to 1.0 mm.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for shot peening.
Specifically, it relates to a method for shot-peening a gas
carburized steel.
BACKGROUND ART
[0002] Conventionally, shot peening has been known to produce
compressive residual stresses and to increase hardness to improve
the fatigue strength of gears (see authored by the Society of Shot
Peening Technology of Japan; Fatigue of Metals and Shot Peening;
published by Gendai Kogaku-sha; 2004). Further, to achieve high
fatigue strength aimed at reducing the weight of parts, a method
for increasing compressive residual stresses produced by shot
peening has been known (see Kazuyoshi Ogawa and Takashi Asano;
Theoretical Prediction of Residual Stress Produced by Shot Peening
for Hardened Steel; Transactions of JSSR, No. 48 (2003) pp.
31-38).
[0003] However, in these studies only the mean values of the
hardness on the surface of, or near the surface of, the material
are focused on. Thus the methods by these studies are difficult to
be applied to a gas carburized steel that has a soft layer on the
surface, since residual stresses are affected by plastic
deformations caused by the collision of shot media and the
mechanical properties of the processed steels that relate to the
suppression of the plastic deformation.
DISCLOSURE OF THE INVENTION
[0004] The object of the present invention is to provide a method
for shot peening for producing high compressive residual stresses
in a gas carburized steel that has a soft layer on the surface. In
this method, a depth where the maximum compressive residual stress
is generated is estimated, and no data on the hardness on the
surface or near the surface is used.
[0005] The method for shot peening of the first aspect of the
present invention is to produce a compressive residual stress in a
processed steel by peening shot media onto the processed steel. The
processed steel comprises a gas carburized steel having a hardness
of 750 HV or higher at the depth z, where the maximum compressive
residual stress is generated. The depth z is estimated by using
Equations (1) to (4) below. The shot media have a hardness that is
above that of the processed steel by 50 HV or more.
.alpha. = 1 8 E * 3 5 ( 5 .pi. 4 ) s 5 .rho. 3 5 D 3 V 6 5 ( 1 ) 1
E * = 1 - v 1 2 E 1 + 1 - v 2 2 E 2 ( 2 ) z = 0.48 K .alpha. ( 3 )
K = 1.25 ( 4 ) ##EQU00001##
[0006] where
[0007] .alpha.: contact radius by shot media (m),
[0008] p: specific gravity of shot media (kg/m.sup.3),
[0009] D: diameter of shot media (m),
[0010] V: shot speed(m/s),
[0011] E*: equivalent elastic modulus
[0012] E.sub.1: Young's modulus of the processed steel(Pa),
[0013] v.sub.1: Poisson's ratio of the processed steel,
[0014] E.sub.2: Young's modulus of shot media (Pa),
[0015] v.sub.2: Poisson's ratio of shot media,
[0016] K: constant, and
[0017] z: depth where the maximum compressive stress is generated
(m).
[0018] The method for the shot peening of the second aspect of the
present invention is characterized in that the diameters of shot
media are in the range from 0.2 mm to 1.0 mm in the method for the
shot peening of the first aspect.
[0019] By the method for the shot peening of the first aspect of
the present invention, the position where the maximum compressive
residual stress is generated is estimated by multiplying the depth
by a constant. The depth is determined as the depth where the
maximum stress is generated by the collision of shot media. By
shot-peening a gas carburized steel a maximum compressive residual
stress that is equal to, or more than, 1,600 MPa, is produced. At
that depth the gas carburized steel has a hardness that is equal
to, or more than, 750 HV. The shot media have a hardness that is
above that of the processed steel by 50 HV or more. Thus the
processed steel is processed to have a high fatigue strength. That
is, by estimating the depth z, where the maximum compressive stress
is generated, from Equations (1) to (4), it is possible to produce
a high compressive residual stress in a gas carburized steel that
has a soft layer.
[0020] By the method for the shot peening of the second aspect of
the present invention, since the diameters of shot media are in the
range from 0.2 to 1.0 mm, the maximum compressive residual stress
can be securely produced in the processed steel.
[0021] The basic Japanese patent application, No. 2010-176681,
filed Aug. 5, 2010, is hereby incorporated by reference in its
entirety in the present application.
[0022] 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.
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. 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
[0023] FIG. 1 is a graph showing the distribution of the hardness
of the processed steels that were used in the embodiments of the
present invention.
[0024] FIG. 2 is a table showing the conditions and the results of
the shot peening that were used in the embodiments of the present
invention.
[0025] FIG. 3 is a graph showing the relationship between the
estimated values and measured values where the maximum compressive
residual stress is generated.
[0026] FIG. 4 is a graph showing the relationship between the
differences between the hardness of shot media from the hardness at
the depth where the maximum residual stresses is generated and the
maximum compressive residual stresses.
BEST MODE FOR CARRYING OUT THE INVENTION
[0027] Below, the embodiments of the present invention are
described with reference to the drawings.
[0028] FIG. 1 is a graph showing the distribution of the hardness
of the processed steels that were used in the embodiments. The
abscissa denotes depths (micrometer) from the surface of the steel,
and the ordinate denotes the Vickers hardness. A gas carburized
steel is used for the processed steels. In the drawing, "TP.A" and
"TP.B" denote the steels which have been tempered at 180 degree C.
and 180 degree C., respectively.
[0029] FIG. 2 is a table showing the conditions and the results of
shot peening that were used in the embodiments. A compressive-air
shot peening system was used. Shot media that have a hardness of
700 HV to 1,000 HV and diameters (the mean diameters) of 0.2 to 1.0
mm were used.
[0030] The
"Maximum .sigma..sub.R"
in the table denotes the maximum compressive residual stresses in
the processed steels. The compressive residual stresses were
measured by using a micro-stress analyzer that is available from
Rigaku Corporation (X-ray tube: Cr-K.alpha.; diffractive surface:
(220); stress constant: -318 MPa/deg; Bragg angle of the
strain-free 2.theta.: 156.4.degree.).
[0031] The "Depth of Peak" in the table denotes the depth from the
surface where the maximum compressive residual stress is generated.
The "Hardness at Peak" denotes the hardness at the "Depth of Peak,"
i.e., the Vickers hardness of the processed steels at the depth
where the maximum compressive residual stress is generated. The
"Relative Hardness" denotes the differences between the hardness of
shot media and that of the processed steels, specifically the value
that is calculated by subtracting the hardness at peak of the
processed steels from the hardness of shot media. As shown in the
table, if the relative hardness is 50 HV or more, the maximum
compressive residual stress of -1,600 MPa or more can be obtained.
The maximum compressive residual stress of -1,600 MPa is a typical
value that is required for gear materials.
[0032] FIG. 3 is a graph showing the estimated values and measured
values of the depths where the maximum compressive residual stress
is generated. The estimated values are calculated by using the
following Equations (1) to (4).
.alpha. = 1 8 E * 3 5 ( 5 .pi. 4 ) s 5 .rho. 3 5 D 3 V 6 5 ( 1 ) 1
E * = 1 - v 1 2 E 1 + 1 - v 2 2 E 2 ( 2 ) z = 0.48 K .alpha. ( 3 )
K = 1.25 ( 4 ) ##EQU00002##
[0033] where
[0034] .alpha.: contact radius by shot media (m),
[0035] p: specific gravity of shot media (kg/m.sup.3),
[0036] D: diameter of shot media (m),
[0037] V: shot speed(m/s),
[0038] E*: equivalent elastic modulus
[0039] E.sub.1: Young's modulus of the processed steel(Pa),
[0040] v.sub.1: Poisson's ratio of the processed steel,
[0041] E.sub.2: Young's modulus of shot media (Pa),
[0042] v.sub.2: Poisson's ratio of shot media,
[0043] K: constant, and
[0044] z: depth where the maximum compressive stress is generated
(m).
[0045] As shown in FIG. 3, the estimated values where the maximum
compressive residual stress is generated are generally coincident
with the measured values. That means that the depths where the
maximum compressive residual stress is generated can be estimated
by multiplying the depth where the maximum stress is generated
under contact stresses caused by the collision of shot media, by
the constant K.
[0046] FIG. 4 is a graph showing the relationship between the
differences between the hardness of shot media and the hardness at
the depth where the maximum compressive residual stress is
generated. Specifically, it shows the values (the relative
hardness) that are calculated by subtracting the hardness of the
processed steels at the peak depth from the hardness of shot media
on the abscissa and the maximum compressive residual stresses (MPa)
of the processed steels on the ordinate.
[0047] As shown in FIG. 4, if the value that is calculated by
subtracting the hardness of the processed steels at the peak depth
from the hardness of shot media is less than 50 HV, the maximum
compressive residual stress does not reach -1,600 MPa. This is
because shot media are subject to plastic deformation when they are
shot onto the processed steel. Thus energy is insufficiently
transmitted from shot media to the processed steel.
[0048] On the contrary, if the value that is calculated by
subtracting the hardness of the processed steels at the peak depth
from the hardness of shot media is 50 HV or more, the maximum
compressive residual stress exceeds -1,600 MPa. Since the maximum
compressive residual stress is generally expressed as a minus
value, that means that the absolute value exceeds 1,600 MPa. This
is because shot media are seldom subject to plastic deformation
when they are shot onto the processed steel. Thus sufficient energy
is transmitted from shot media to the processed steel.
[0049] Further, as shown in FIG. 4, if the hardness of the
processed steel at the depth where the maximum compressive residual
stress is generated is less than 750 HV, the maximum compressive
residual stress does not reach -1,600 MPa even when the value that
is calculated by subtracting the hardness of the processed steels
at the peak depth from the hardness of shot media is 50 HV or more.
The maximum compressive residual stress to be produced in a steel
is known as being limited by the yield strength of the steel. The
yield strength is proportional to the hardness. Thus, unless the
hardness of the processed steel at the depth where the maximum
compressive residual stress is generated is 750 HV or more, the
yield strength that is required to produce the maximum compressive
residual strength of -1,600 MPa cannot be ensured.
[0050] The threshold for the difference in the hardness of shot
media and the processed steels, i.e., 50 HV, is determined as
follows. As shown in FIG. 4, the maximum compressive residual
stresses are shown in relation to the values that are calculated by
subtracting the hardness of the processed steels at the "Depth of
Peak" from the hardness of shot media. An estimated curve is drawn
by the least square method. Based on the curve the threshold is
determined. The threshold for the hardness at the depth where the
maximum compressive residual stress is generated, i.e., 750 HV, is
determined as follows. The maximum compressive residual stresses
are shown in relation to the hardness of the processed steels. An
estimated curve is drawn by the least square method. Based on the
curve the threshold is determined.
[0051] As discussed above, in the embodiments of the present
invention processed steels are used that have a hardness that is
greater than 750 HV at the depth z, where the maximum compressive
residual stress is generated. The depth z is estimated from
Equations (1) to (4). The shot media that have the hardness that is
greater than that of the processed steels at the depth z by 50 HV
or more are shot onto the processed steels. In these ways a
compressive residual stress is produced in the processed steels. In
other words, the depth where the maximum compressive residual
stress is generated is estimated by multiplying the depth where the
maximum stress is generated under contact stresses caused by the
collision of shot media by the constant K. The processed steels
that have a hardness at that depth that exceeds 750 HV are used.
The shot media that have a hardness that is greater than that of
the processed steels at the estimated depth by 50 HV or more are
shot onto the processed steels. As a result, a maximum compressive
residual stress that is 1,600 MPa or more can be produced in the
processed steels. Thus the processed steels can be improved in
fatigue strength. That is, by estimating the depth z, where the
maximum compressive residual stress is generated, from Equations
(1) to (4), a high compressive residual stress can be produced in a
gas carburized steel that has a soft layer.
[0052] Further, by having the diameters of shot media being in the
range from 0.2 mm to 1.0 mm, a maximum compressive residual stress
that is 1,600 MPa or more can be securely produced in the processed
steels.
[0053] In this invention, any shot media can be used. However, shot
media made of steels, etc., are preferable.
* * * * *