U.S. patent application number 10/540534 was filed with the patent office on 2006-12-28 for method of refining metal surface and metal product by method.
Invention is credited to Hideaki Kaga, Junkou Kurosaki, Yoshikazu Todaka, Kouichi Tsuchiya, Mitsugi Uemura, Minoru Umemoto.
Application Number | 20060289090 10/540534 |
Document ID | / |
Family ID | 32684234 |
Filed Date | 2006-12-28 |
United States Patent
Application |
20060289090 |
Kind Code |
A1 |
Umemoto; Minoru ; et
al. |
December 28, 2006 |
Method of refining metal surface and metal product by method
Abstract
This invention relates to a method for fining a surface of a
metal product to form crystal grains having sizes less than 1 .mu.
m at their surfaces and the metal product produced thereby. The
method is comprised of a process for forming crystal grains having
sizes less than 1 .mu. m at the surface of the metal product by
projecting or peening shots or projectiles while a power per unit
of area of the surface, which power is caused by projecting or
peening shots or projectiles, is controlled at a predetermined
value.
Inventors: |
Umemoto; Minoru; (Aichi-ken,
JP) ; Tsuchiya; Kouichi; (Aichi-ken, JP) ;
Todaka; Yoshikazu; (Aichi-ken, JP) ; Uemura;
Mitsugi; (Aichi-ken, JP) ; Kaga; Hideaki;
(Aichi-ken, JP) ; Kurosaki; Junkou; (Aichi-ken,
JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
32684234 |
Appl. No.: |
10/540534 |
Filed: |
December 25, 2003 |
PCT Filed: |
December 25, 2003 |
PCT NO: |
PCT/JP03/16669 |
371 Date: |
June 24, 2005 |
Current U.S.
Class: |
148/421 |
Current CPC
Class: |
C21D 7/06 20130101 |
Class at
Publication: |
148/421 |
International
Class: |
C22C 14/00 20060101
C22C014/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 25, 2002 |
JP |
2002-374610 |
Dec 18, 2003 |
JP |
2003-42143 |
Claims
1. A method for fining a metal surface, comprising a process for
forming crystal grains having sizes less than 1 .mu.m at the
surface of a metal product by means of projecting or peening shots
or projectiles to the surface while a power per unit of area of the
surface is controlled at a predetermined value.
2. A method for fining a metal surface according to claim 1,
wherein the shots or projectiles are made from high-carbon steel,
ferrous metallic glass, or stainless steel, and the diameters of
the shots or projectiles are 30 .mu.m to 2000 .mu.m.
3. A method for fining a metal surface according to either of
claims 1 and 2, wherein the power per unit of area is greater than
11 KJ/sec*mm.sup.2.
4. A method for fining a metal surface according to any of claims
1, 2, and 3, wherein the process for projecting or peening shots or
projectiles to the surface is carrired out while the temperature of
the metal surface is controlled to be between room temperature and
-150.degree. C.
5. A method for fining a metal surface according to any of claims
1-4, wherein the unit area is calculated by multiplying a contact
surface of a projectile or a shot by a number of the shots or
projectiles.
6. A method for fining a metal surface according to claim 5,
wherein the unit area is calculated by subtracting the overlapped
areas that is calculated based on the number of shots or
projectiles that have their contact surfaces overlap from the sum
of the contact surfaces.
7. A metal product having surfaces hardened by the method for
fining a metal surface according to any of claims 1-6.
Description
TECHNICAL FIELD
[0001] This invention relates to a method for fining a surface of a
metal product to form crystal grains having sizes less than 1 .mu.m
at its surface and for producing a metal product thereby.
BACKGROUND OF THE INVENTION
[0002] It is well known that when a surface of a metal product has
been projected upon by shot peening, the micro-structure of the
surface can be fined. (See Literature 1.) Literature 1 discloses
that a micro-structure having fine grain sizes in a surface layer
with a high dislocation density that is formed by shot peening is
useful to improve the fatigue characteristics of a metal
product.
[0003] Literature 1: A. Niku-Lari, First International Conference
on Shot Peening, United Kingdom, Pergamon Press, 1981, p. 192.
DISCLOSURES OF INVENTION
[0004] However, Literature 1 does not disclose forming crystal
grains having sizes of less than 1 .mu.m. Namely, it does not
disclose mechanisms and conditions to form crystal grains having
sizes of less than 1 .mu.m.
[0005] This invention is directed to solve this problem. Namely,
the purpose of this invention is to provide a method for fining a
surface of a metal product to form crystal grains having sizes of
less than 1 .mu.m at its surface.
[0006] Further, the purpose of this invention is to provide a metal
product that is treated by the method.
[0007] To achieve the purpose, the method according to this
invention is comprised of a process for forming crystal grains
having sizes of less than 1 .mu.m at the surface of the metal
product by projecting or peening shots or projectiles while the
power per unit of area of the surface is controlled at a
predetermined value.
[0008] According to this invention, it is possible to improve
fatigue strength, hardness, and a corrosion resistance of a metal
product by forming crystal grains having sizes of less than 1 .mu.m
at the surface of the metal product.
[0009] As explained above, this invention is comprised of a method
for forming crystal grains having sizes of less than 1 .mu.m at the
surface of the metal product by projecting or peening shots or
projectiles while the power per unit of area of the surface, which
power is caused by projecting or peening shots or projectiles, is
controlled at a predetermined value. The metal product is treated
by this method. Thus, according to this invention, the fatigue
strength, the hardness, and the corrosion resistance of the metal
product can be improved by forming crystal grains having sizes of
less than 1 .mu.m at the surface of the metal product.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 shows a perspective view of an apparatus for
projecting projectiles of a first embodiment.
[0011] FIG. 2 shows a perspective view of an apparatus for dropping
a weight to treat the surface of a metal product of the second
embodiment.
[0012] FIG. 3 shows a cross-sectional view of the apparatus for
shot-peening of the third and fourth embodiment.
[0013] FIG. 4 shows a photomicrograph of the surface of the metal
product that is treated by the method of this invention.
[0014] FIG. 5 shows a photomicrograph of the crystal grain that is
fined by the method of this invention.
PREFERRED EMBODIMENTS OF THE INVENTION
[0015] Below, the preferred embodiments of this invention are
explained. This invention relates to a method comprising a process
for forming crystal grains having sizes of less than 1 .mu.m at the
surface of the metal product by projecting or peening shots or
projectiles while the power per unit of area of the surface, which
power is caused by projecting or peening shots or projectiles, is
controlled at a predetermined value.
[0016] A steel or non-ferrous metal can be used as the material for
the metal product of this invention. The surface of the metal
product is defined as a portion near the surface that can be
affected by projecting the shots or projectiles. The depth of the
portion that is affected by projecting the shots or projectiles
depends on the velocity and mass of the shots or projectiles and
the period for projecting them when the surface of the metal
product is projected upon by them.
[0017] It is preferable that the hardness of the shots or
projectiles be equal to or higher than that of the metal product.
It is also acceptable that the hardness of the shots or projectiles
be lower than that of the metal product, if its surface can be
hardened.
[0018] The reason the power per unit of area of the surface, which
power is caused by projecting or peening shots or projectiles, is
controlled at a predetermined value, is as follows:
[0019] Literature 1 says that when the surface of the metal product
is projected upon by shot peening, a degree of fining the
micro-structure of the surface of the metal product depends on the
dislocation density and the arrangement of the dislocations, grain
sizes, and phase changes of the micro-structure. However, its
mechanisms are not explained in Literature 1.
[0020] In this invention, it is found that the power per unit of
area of the surface, which power is caused by projecting the shots
or projectiles, affects the fining of the micro-structure of the
surface of the metal product.
[0021] Namely, it is possible to produce nanocrystals without
repeatedly projecting the shots or projectiles, such as by the
shot-peening.
[0022] "A unit of area" of the surface is defined as the sum of the
contact surfaces that are projected upon by the shots or
projectiles. Namely, "a unit of area" is calculated by multiplying
the contact surface from a projectile or a shot by the number of
the shots or projectiles, based on the assumption that the marks
(surfaces contacted by the shots or projectiles) on the surface by
projecting the shots or projectiles do not overlap. Thus, when the
marks on the surface do overlap, "a unit of area" is calculated by
subtracting the overlapped areas calculated based on the number of
shots or projectiles that have their contact surfaces overlap from
the sum of the contact surfaces. Thus, basically "the unit of area"
does not correspond to the surface that is treated by the
shot-peening.
[0023] However, it is possible to use a surface area of the metal
product treated by the shot-peening as "the unit of area," under a
certain assumption.
[0024] Below, the first embodiment is explained.
[0025] FIG. 1 shows an apparatus of the first embodiment of this
invention. This apparatus 10 for projecting the projectiles can
project a metal ball 11 having a diameter of 4 mm on the surface of
the metal product 12 through the nozzle 13 with compressed gas at a
high speed. Table 1 shows the working conditions for projecting the
projectiles and the results of the treatment. This treatment
increases the strength of the portion of the surface since the
process is instantaneously completed. Namely, nanocrystals can be
formed because the small area of the surface of the metal product
12, which is projected upon by the ball 11, is quickly
processed.
[0026] In comparison with the area hardened by ordinary working or
made from base materials, the growth of the grain size in the area
having nanocrystals is very slow. Thus, the area having
nanocrystals is clearly distinguished from the other areas hardened
by ordinary working or made from base materials, based on the
change of the state of the micro-structure of the surface or its
hardness by heating them. Since the grain size in the area hardened
by working become very coarse, and the hardness of the area is
reduced by heating it (the hardness of the surface is reduced from
450 Hv to 310 Hv in Vickers hardness), it is recognized that the
growth of the grain sizes in the area having nanocrystals is very
slow, and the decrease of the hardness of the area is little (the
hardness of the area is reduced from 700 Hv to 650 Hv) by heating
it.
[0027] It can be seen that an area having, nanocrystals is formed,
by investigating the behavior of a recrystallization.
[0028] Now, the second embodiment is explained.
[0029] FIG. 2 shows an apparatus of the second embodiment of this
invention. This apparatus 20 for dropping a weight can freely drop
a weight 21 made of metal on the surface of the metal product 22
and cause a collision between the surface of the metal product 22
and the weight 21, to treat its surface. Table 1 shows the working
conditions for dropping the weight and the results of the
treatment. In this apparatus 20, the metal product 22 to be treated
to form nanocrystals on the surface 22A is located in the bottom of
the cylinder (not shown in the Figs.).
[0030] In this embodiment, the metal product 22 is already machined
so that it has a final configuration and cannot move in the
cylinder since the configuration of the outside of the metal
product closely corresponds to that of the inside of the cylinder
(not shown). The metal weight 21 is placed at the upper part in the
cylinder. As explained below, a protrusion 21A is disposed at the
surface of the metal weight 21, and the protrusion has a
predetermined height (3 mm) from the surface of the weight 21. The
protrusion 21A is disposed at the location on the weight that is
opposite the point where nanocrystals should be formed on the
surface 22A of the metal product 22.
[0031] The metal weight, which is placed at the upper part in the
cylinder, drops freely. Consequently, the protrusion 21A of the
metal weight collides with the predetermined portion of the surface
22A of the metal product 22. If the mass of the metal weight is
defined as M (Kg), and the velocity of the metal weight 21 when it
collides with the surface of the metal product 22 is defined as V
(m/sec), then V is given as follows: V= {square root over (2 gH)}
[0032] (g: acceleration of gravity; H: distance that the weight
falls)
[0033] Namely, the protrusion 21A of the metal weight 21 collides
with the surface 22A of the metal product 22 at the following
kinetic momentum: M* {square root over (2 gH)} (Kg*m/sec)
[0034] Consequently, the force, which is defined as a temporal
response to the kinetic momentum, acts on the portion of the metal
product 22 with which the protrusion 21A collides. Since the
collision is completed in a short time, the strength of the portion
of the metal product with which the protrusion collides increases
significantly.
[0035] Namely, since the small area of the metal product is
intensively worked in a very short time by the collision of the
protrusion 21A of the metal weight 21, it is easy to form
nanocrystals.
[0036] According to the result of the examination of this
embodiment, the power per unit of area, that is, the power per
depressed area produced by the protrusion or the contact area of
it, should be at least 11 KJ/sec*mm.sup.2.
[0037] Namely, the accumlated kinetic momentum of the metal weight
is not important, but the power per depressed area is
important.
[0038] If the power per depressed area is less than 11
KJ/sec*mm.sup.2, no nanocrystal is formed at the surface 22A of the
metal product 22. Namely, when the protrusion 21A collides with the
surface 22A of the metal product 22 with a power per depressed area
of more than 11 KJ/sec*mm.sup.2, a nanocrystal is formed at the
portion with which the protrusion 21A collides.
[0039] It is preferable that the protrusion be a hemispherical
protrusion that projects at a height (h) of 1-10 mm from the
surface of the metal weight 21. The protrusion may have a shape of
an ellipse. If a plurality of the portions on the surface of the
metal product to be treated to form nanocrystals are required, the
plurality of the protrusions 21A opposite the portions may be
disposed at the surface of the metal weight.
[0040] The kinetic momentum explained above is defined as a
function of the mass (M) of the metal weight 21 and the speed (V)
of it at the moment that the protrusion collides. When the metal
weight 21 having the protrusion 21A, which has a hemispherical
configuration and a height of 1-10 mm, is used in the experiments
of this invention, the mass (M) of the metal weight 21 is set
between 0.1-10 Kg, and the speed of the weight at the moment that
the protrusion collides is set at more than 1 m/sec, the power per
depressed area of more than 11 KJ/sec*mm.sup.2 is achieved, and the
nanocrystals can be formed at the surface of the metal product.
[0041] When the metal weight 21 having the plurality of the
protrusions 21A is used, it is necessary to set the weight
colliding with the surface 22A of the metal product 22 to the value
that equals the product of the number of protrusions times the mass
of the weight having a protrusion (between 0.1-10 Kg). Then, the
process is completed by dropping the weight at a speed of more than
1 m/sec. Since the value of the kinetic momentum divided by the
total depressed area caused by all the protrusions 21A and the
deformation period satisfies the power per depressed area of more
than 11 KJ/sec*mm.sup.2, the nanocrystals can be formed at the
portions of the surface of the metal product.
[0042] Next, the third embodiment is explained.
[0043] FIG. 3 shows an apparatus of the third embodiment of this
invention. This shot-peening apparatus 30 can project shots 31
having a diameter of 50 .mu.m, which shots 31 are made from steel,
on the surface of the metal product 32 with compressed air through
a nozzle 33. Table 1 shows the working conditions of the shot
peening and the results of the treatment. From Table 1, it is found
that the power per unit of area of the third embodiment to produce
nanocrystals is larger than that of the first and the second
embodiments.
[0044] In this embodiment, the pressure of the compressed air is
controlled so that the projecting speed of the shots 31 at the
metal product becomes 150-200 m/sec. If it is required that the
entire area of the surface of the metal product be treated by shot
peening, it can be treated by moving the metal product 32 so that
the shots are projected over the entire area. A layer constituted
of a fine crystal having grain sizes of less than 100 nm is formed
at the surface of the metal product 32 by means of this shot
peening process. It is found that the hardness of the layer having
the fine crystal is significantly increased. FIG. 4 shows a
photomicrograph of the surface of the metal product that is treated
by the method of the third embodiment of this invention. FIG. 5
also shows a photomicrograph of the crystal grain that is fined by
the method of the third embodiment of this invention.
[0045] As explained above, since the layer constituted of a fine
crystal can be formed at the surface of the metal product 32 by
using the method of the third embodiment of this invention, its
hardness significantly increases. Thus, the strength of the metal
product 32 also increases, and the fatigue strength and corrosion
resistance of the metal product can be improved.
[0046] Now, the fourth embodiment is explained.
[0047] FIG. 3 shows an apparatus of the fourth embodiment of this
invention, which figure is the same figure as that shown for the
third embodiment. This shot-peening apparatus 30 can project shots
31 having a diameter of 50-300 m and made from stainless steel on
the surface of the metal product 32 with compressed air through a
nozzle 33. Table 1 shows the working conditions of the shot peening
and the results of the treatment. From Table 1, it is found that
the power per unit of area of this embodiment to produce
nanocrystals is larger than that of the first and the second
embodiments.
[0048] In this embodiment, the pressure of the compressed air is
controlled so that the projecting speed of the shots 31 on the
metal product becomes 80 m/sec. If it is required that the entire
area of the surface of the metal product be treated by the shot
peening, it can be treated by moving the metal product 32 so that
the shots are projected at the entire area. As well as in the third
embodiment, a layer constituted of a fine crystal having grain
sizes of less than 100 nm is formed at the surface of the metal
product 32 by means of this shot peening process. It is found that
the hardness of the layer having the fine crystal is significantly
increased. Shots made not only from stainless steel, but also from
high-carbon steel or ferrous metallic glass, can be used. Further,
shots having a range of diameters of 30 .mu.m to 2000 .mu.m can be
used.
[0049] As explained above, since a layer constituted of a fine
crystal can be formed at the surface of the metal product 32 by
using the method of the fourth embodiment of this invention, its
hardness significantly increases. Thus, as well as in the third
embodiment, the strength of the metal product 32 also increases,
and the fatigue strength and corrosion resistance of the metal
product can be improved.
[0050] Generally, when a surface of a metal product is treated by
shot peening, a work hardening is caused at the surface. It is well
known that the degree of the work hardening of the metal is
proportional to the square root of its dislocation density. When
the process for working the metal product is continued, since the
speed of the disappearance of its dislocations caused by a merger
between the dislocations of grains increases, the rate of the work
hardening decreases gradually as the degree of working increases.
However, when the metal product is worked hard at a high strain
rate, since no disappearance of the dislocations of grains is
caused, the dislocation density of the grains increases. Then, when
the dislocation density reaches a critical value, a
dislocation-cell structure changes to a grain-boundary
structure.
[0051] Further, the fining of the micro-structure of the surface of
the metal product is improved by projecting the projectiles or the
shots to the surface while the temperature of the metal surface is
controlled to be between room temperature and -150.degree. C. It is
difficult for the number of the dislocations of the grains to reach
the critical dislocation density. This allows the grains to be
recrystallized, since the recovery rate of the dislocations of the
grains increases with the increased temperature of the surface due
to the continuius projection of the projectiles or the shots.
However, in the condition at low temperature, since the recovery
rate of the grain structure which is fined by projecting the
projectiles or the shots decreases, it becomes easy for the number
of the dislocations of the grain to increase. Namely, it become
easy for the dislocation density to reach the critical value, which
value allows the grain to be fined.
[0052] In this embodiment, liquid nitrogen (temperature:
-196.degree. C.) and liquid carbon dioxide (temperature:
-79.degree. C.) can be used to cool the metal product. It is
preferable to control the temperature of the metal product to be
between room temperature and about -150.degree. C. based on the
material of the product. It is possible to form finer grains by
using this method compared to the method for projecting the
projectiles or the shots at room temperature.
INDUSTRIAL APPLICABILITY
[0053] This invention relates to a method for fining a surface of a
metal product to form crystal grains having sizes less than 1 .mu.m
at its surface and to produce the metal product by the method. By
this method, since the fatigue strength, the hardness, and the
corrosion resistance of the metal product can be improved, the
method possesses a possibility of industrial applicability.
TABLE-US-00001 TABLE 1 Working Conditions for Each Processing
Method of Method of Mehod of Projecting Projectiles Dropping Weight
Shot Peening (Embodiment 1) (Embodiment 2) (Embodiment 3 and 4)
Diameter of Ball or Projectile 4 6 0.05 .PHI. (mm) Velocity of Ball
or Projectile 120 4.4 190 (m/sec) Working Energy per One Shot 1.9
49 9.2 .times. 10.sup.-6 (J) Depth of Deformation (.mu.m) 500 1000
5 Contact Area (mm.sup.2) 6.3 19 7.9 .times. 10.sup.-4 Deformation
Time (s) 4.2 .times. 10.sup.-6 2.3 .times. 10.sup.-5 2.6 .times.
10.sup.-8 Strain Rate (l/s) 2.4 .times. 10.sup.5 4.4 .times.
10.sup.4 3.8 .times. 10.sup.7 Power/Contact Area 72 11 450 (kJ/s *
mm.sup.2)
* * * * *