U.S. patent application number 10/576479 was filed with the patent office on 2007-03-29 for wear-resistant elements and method of making same.
Invention is credited to Hiroyuki Fukuhara, Kensuke Hirata, Toshihiko Homma, Kenji Sasaki.
Application Number | 20070071630 10/576479 |
Document ID | / |
Family ID | 34463425 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070071630 |
Kind Code |
A1 |
Fukuhara; Hiroyuki ; et
al. |
March 29, 2007 |
Wear-resistant elements and method of making same
Abstract
A material is shaped and sintered into a compact using
iron-based alloy powder containing Cr; and a nitriding treatment
having no carburizing action is conducted to the compact so that a
surface of the compact may have a mixed structure 3 of an Fe--Cr--N
compound layer 2, an Fe--Cr--N diffused layer, and a matrix.
Inventors: |
Fukuhara; Hiroyuki; (Shiga,
JP) ; Sasaki; Kenji; (Shiga, JP) ; Hirata;
Kensuke; (Kanagawa, JP) ; Homma; Toshihiko;
(Kanagawa, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK L.L.P.
2033 K. STREET, NW
SUITE 800
WASHINGTON
DC
20006
US
|
Family ID: |
34463425 |
Appl. No.: |
10/576479 |
Filed: |
October 19, 2004 |
PCT Filed: |
October 19, 2004 |
PCT NO: |
PCT/JP04/15429 |
371 Date: |
April 20, 2006 |
Current U.S.
Class: |
419/13 ; 148/230;
419/29 |
Current CPC
Class: |
C23C 8/26 20130101; C23C
8/02 20130101; B22F 3/24 20130101; B22F 2003/241 20130101 |
Class at
Publication: |
419/013 ;
148/230; 419/029 |
International
Class: |
C23C 8/26 20060101
C23C008/26; B22F 3/24 20060101 B22F003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 21, 2003 |
JP |
2003-361003 |
Claims
1. A method of making a wear-resistant element, comprising: shaping
and sintering a material into a compact using iron-based alloy
powder containing Cr; and conducting a nitriding treatment having
no carburizing action to the compact, thereby causing a surface of
the compact to have a mixed structure of an Fe--Cr--N compound
layer, an Fe--Cr--N diffused layer, and a matrix.
2. A method of making a wear-resistant element, comprising: shaping
and sintering a material into a compact using alloy powder in which
at least one metallic element selected from Mn, Ti and V is
contained in iron-based alloy powder containing Cr; and conducting
a nitriding treatment having no carburizing action to the compact,
thereby causing a surface of the compact to have a mixed structure
of an Fe--Cr--N compound layer, an Fe--Cr--N diffused layer, and a
matrix.
3. The method according to claim 1 or 2, wherein the compact has
pores formed in the surface thereof, the Fe--Cr--N compound layer
being formed at locations adjacent the pores, the mixed structure
of the Fe--Cr--N diffused layer and the matrix being formed at
locations remote from the pores.
4. A method of making a wear-resistant element, comprising: shaping
and sintering a material into a compact using iron-based alloy
powder containing Cr; and conducting a nitriding treatment having
no carburizing action to the compact, thereby causing a surface of
the compact to have a mixed structure of an Fe--Cr--N compound
layer, an Fe--Cr--N diffused layer, and a matrix of a sorbite
structure.
5. The method according to claim 4, wherein the compact has pores
formed in the surface thereof, the Fe--Cr--N compound layer being
formed at locations adjacent the pores, the mixed structure of the
Fe--Cr--N diffused layer and the matrix of the sorbite structure
being formed at locations remote from the pores.
6. A method of making a wear-resistant element, comprising: shaping
and sintering a material into a compact using iron-based alloy
powder containing Cr; quenching and tempering the compact;
conducting a nitriding treatment having no carburizing action to
the compact; and partially removing a surface of the compact,
thereby causing the surface of the compact to have a mixed
structure containing at least an Fe--Cr--N compound layer.
7. The method according to claim 1, further comprising conducting
an atmospheric treatment to the compact before the nitriding
treatment.
8. The method according to claim 7, wherein the atmospheric
treatment is conducted at a temperature of 380.degree. C. or
more.
9. A wear-resistant element comprising: a sintered and nitrided
material having a surface; and a mixed structure of an Fe--Cr--N
compound layer, an Fe--Cr--N diffused layer, and a matrix formed in
the surface of the sintered and nitrided material, wherein the
surface of the sintered and nitrided material is entirely covered
with grains or protrusions of 0.1.about.0.5 .mu.m.
10. The method according to claim 2, further comprising conducting
an atmospheric treatment to the compact before the nitriding
treatment.
11. The method according to claim 3, further comprising conducting
an atmospheric treatment to the compact before the nitriding
treatment.
12. The method according to claim 4, further comprising conducting
an atmospheric treatment to the compact before the nitriding
treatment.
13. The method according to claim 5, further comprising conducting
an atmospheric treatment to the compact before the nitriding
treatment.
14. The method according to claim 6, further comprising conducting
an atmospheric treatment to the compact before the nitriding
treatment.
15. The method according to claim 10, wherein the atmospheric
treatment is conducted at a temperature of 380.degree. C. or
more.
16. The method according to claim 11, wherein the atmospheric
treatment is conducted at a temperature of 380.degree. C. or
more.
17. The method according to claim 12, wherein the atmospheric
treatment is conducted at a temperature of 380.degree. C. or
more.
18. The method according to claim 13, wherein the atmospheric
treatment is conducted at a temperature of 380.degree. C. or
more.
19. The method according to claim 14, wherein the atmospheric
treatment is conducted at a temperature of 380.degree. C. or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to wear-resistant elements
having an increased hardness by nitriding and also to a method of
making the same.
BACKGROUND ART
[0002] A vane mounted in, for example, rotary compressors is
slidably received within a vane groove defined in a cylinder.
Because the vane is held in sliding contact at its side surfaces
with side walls of the vane groove and at its end portion with a
roller, the vane must have wear resistance. For this purpose, a
material having a base material for which steel, a sintered metal
or cast iron containing chromium is used and which is soft-nitrided
has been proposed. This material has a first compound layer or
surface layer of Fe--Cr--N and a second compound layer of the same
composition formed below the first compound layer (see Document
1).
[0003] Another material has been proposed having a nitrided layer
that is formed by nitriding the surface of a base material of
stainless steel (see Document 2).
[0004] A further material has been proposed wherein an iron-based
powdery material is used. This material is obtained by quenching
and tempering sintered iron having a porosity or void volume below
10% or 15% to cause the matrix to have a martensitic structure, and
subsequently nitriding or soft-nitriding the surface thereof to
form an Fe--N compound layer therein and a nitrogen-diffused layer
below it (see Document 3 or 4).
[0005] Document 1: Japanese Laid-Open Patent Publication No.
60-26195
[0006] Document 2: Japanese Laid-Open Patent Publication No.
11-101189
[0007] Document 3: Japanese Laid-Open Patent Publication No.
2001-140782
[0008] Document 4: Japanese Laid-Open Patent Publication No.
2001-342981
[0009] In the above-described materials, the surface is formed with
an Fe--Cr--N or Fe--N compound layer or an Fe--Cr--N diffused layer
and, hence, has a single composition and a uniform hardness.
Accordingly, if a wear-resistant element such as a vane for use in
a compressor is made of one of such materials, the vane tends to
wear uniformly during operation of the compressor. As a result, the
surface is hard to maintain desired oil retaining properties, and
there is a possibility of seizing.
[0010] The present invention has been developed to overcome the
above-described disadvantages.
[0011] It is accordingly an objective of the present invention to
provide a highly reliable wear-resistant element having a surface
of different hardness in which minute oil holes are formed to
enhance the oil retaining properties during operation and to
eliminate the deficiencies such as seizing.
DISCLOSURE OF THE INVENTION
[0012] In accomplishing the above objective, a method of making a
wear-resistant element according to the present invention is
characterized by shaping and sintering a material into a compact
using iron-based alloy powder containing Cr, and conducting a
nitriding treatment having no carburizing action to the compact,
thereby causing a surface of the compact to have a mixed structure
of an Fe--Cr--N compound layer, an Fe--Cr--N diffused layer, and a
matrix.
[0013] In another aspect of the present invention, a method of
making a wear-resistant element is characterized by shaping and
sintering a material into a compact using alloy powder in which at
least one metallic element selected from Mn, Ti and V is contained
in iron-based alloy powder containing Cr, and conducting a
nitriding treatment having no carburizing action to the compact,
thereby causing a surface of the compact to have a mixed structure
of an Fe--Cr--N compound layer, an Fe--Cr--N diffused layer, and a
matrix.
[0014] It is preferred that the compact has pores formed in the
surface thereof, the Fe--Cr--N compound layer being formed at
locations adjacent the pores, the mixed structure of the Fe--Cr--N
diffused layer and the matrix being formed at locations remote from
the pores.
[0015] In a further aspect of the present invention, a method of
making a wear-resistant element is characterized by shaping and
sintering a material into a compact using iron-based alloy powder
containing Cr, and conducting a nitriding treatment having no
carburizing action to the compact, thereby causing a surface of the
compact to have a mixed structure of an Fe--Cr--N compound layer,
an Fe--Cr--N diffused layer, and a matrix of a sorbite
structure.
[0016] In this case, the compact preferably has pores formed in the
surface thereof, the Fe--Cr--N compound layer being formed at
locations adjacent the pores, the mixed structure of the Fe--Cr--N
diffused layer and the matrix of the sorbite structure being formed
at locations remote from the pores.
[0017] In a still further aspect of the present invention, a method
of making a wear-resistant element is characterized by shaping and
sintering a material into a compact using iron-based alloy powder
containing Cr, quenching and tempering the compact, conducting a
nitriding treatment having no carburizing action to the compact,
and partially removing a surface of the compact, thereby causing
the surface of the compact to have a mixed structure containing at
least an Fe--Cr--N compound layer.
[0018] Before the nitriding treatment, an atmospheric treatment,
i.e., a slight oxidation treatment may be conducted to the compact,
and the preferred temperature of the atmospheric treatment is
380.degree. C. or more.
[0019] A wear-resistant element includes a sintered and nitrided
material having a mixed structure of an Fe--Cr--N compound layer,
an Fe--Cr--N diffused layer, and a matrix formed in the surface
thereof. It is preferred that the surface of the sintered and
nitrided material be entirely covered with grains or protrusions of
0.1.about.0.5 .mu.m.
EFFECTS OF THE INVENTION
[0020] The present invention has the above-described features and
offers the following effects.
[0021] A material is first shaped and sintered into a compact using
iron-based alloy powder containing Cr or using alloy powder in
which at least one metallic element selected from Mn, Ti and V is
contained in iron-based alloy powder containing Cr, and a nitriding
treatment having no carburizing action is subsequently conducted to
the compact, thereby causing a surface of the compact to have a
mixed structure of a compound layer, a diffused layer, and a
matrix. Accordingly, the amount of processing of the soft matrix
portion increases during the finishing of the wear-resistant
element to thereby form minute hollows, i.e., minute oil holes.
When the wear-resistant element is in operation, slight wear occurs
in the soft matrix portion to thereby form oil holes, making it
possible to realize a highly reliable wear-resistant element free
from seizing.
[0022] Furthermore, when the alloy powder in which at least one
metallic element selected from Mn, Ti and V is contained in
iron-based alloy powder containing Cr is used, the compound layer
and the diffused layer come to contain at least one of Cr, Mn, Ti
and V. Fe or Cr acts to ensure a desired hardness, Mn acts to
further enhance the hardness, Ti acts to promote the nitriding
treatment, and V acts to make the nitriding depth deep, making it
possible to further enhance the reliability of the wear-resistant
element.
[0023] Also, iron-based alloy powder containing Cr is first shaped
and sintered into a compact, which is in turn quenched and
tempered, and a nitriding treatment having no carburizing action is
subsequently conducted to the compact, thereby causing a surface of
the compact to have a mixed structure of a compound layer, a
diffused layer, and a matrix of a sorbite structure. Accordingly,
the amount of processing of the soft matrix portion increases
during the finishing of the wear-resistant element to thereby form
minute hollows, i.e., minute oil holes. When the wear-resistant
element is in operation (relative frictional motion), slight wear
occurs in the matrix portion that is softer than the compound layer
and the diffused layer to thereby form oil holes. In addition,
because the quenching and tempering increase the hardness of the
matrix structure, a subsequent nitriding treatment further enhances
the hardness of the compound layer and the diffused layer, making
it possible to realize a seizing-free and highly reliable
wear-resistant element having an increased wear resistance.
[0024] Alternatively, a material is first shaped and sintered into
a compact, which is in turn quenched and tempered, a nitriding
treatment having no carburizing action is subsequently conducted to
the compact, and a surface of the compact is then partially
removed, thereby causing the surface of the compact to have an
Fe--Cr--N compound layer and also have variations in hardness.
Accordingly, the amount of processing of the soft matrix portion
increases during the finishing of the wear-resistant element to
thereby form minute hollows, i.e., minute oil holes. When the
wear-resistant element is in operation (relative frictional
motion), slight wear occurs in the soft portions to thereby form
oil holes. The oil holes act to enhance the lubricating properties,
while the portion other than the oil holes, i.e., the portion of
the compound layer acts to maintain the wear resistance, making it
possible to enhance the reliability of the wear-resistant
element.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a photograph depicting a section of a
wear-resistant element according to the present invention upon
etching.
[0026] FIG. 2 is a photograph depicting a surface of the
wear-resistant element of FIG. 1 upon grinding and etching.
[0027] FIG. 3 is a photograph depicting impressions formed on
another surface of the wear-resistant element of FIG. 1 when Micro
Vickers hardness measurements have been conducted.
[0028] FIG. 4 is a graph depicting hardness distribution curves of
the wear-resistant element of FIG. 1.
[0029] FIG. 5 is a diagram depicting heat treatment patterns
conducted to materials of samples.
[0030] FIG. 6 is a photograph depicting a surface condition of
sample X at .times.40 magnification after nitriding.
[0031] FIG. 7 is a photograph depicting a surface condition of
sample Y at .times.40 magnification after nitriding.
[0032] FIG. 8 is a photograph depicting a surface condition of
sample Z at .times.40 magnification after nitriding.
[0033] FIG. 9 is a photograph depicting a surface condition of
sample X at .times.200 magnification after nitriding.
[0034] FIG. 10 is a photograph depicting a surface condition of
sample X at .times.1000 magnification after nitriding.
[0035] FIG. 11 is a photograph depicting a surface condition of
sample X at .times.5000 magnification after nitriding.
[0036] FIG. 12 is a photograph depicting a surface condition of
sample X at .times.20000 magnification after nitriding.
[0037] FIG. 13 is a photograph depicting a surface condition of
sample Y at .times.200 magnification after nitriding.
[0038] FIG. 14 is a photograph depicting a surface condition of
sample Y at .times.1000 magnification after nitriding.
[0039] FIG. 15 is a photograph depicting a surface condition of
sample Y at .times.5000 magnification after nitriding.
[0040] FIG. 16 is a photograph depicting a surface condition of
sample Y at .times.20000 magnification after nitriding.
[0041] FIG. 17 is a photograph depicting a surface condition of
sample Z at .times.200 magnification after nitriding.
[0042] FIG. 18 is a photograph depicting a surface condition of
sample Z at .times.1000 magnification after nitriding.
[0043] FIG. 19 is a photograph depicting a surface condition of
sample Z at .times.5000 magnification after nitriding.
[0044] FIG. 20 is a photograph depicting a surface condition of
sample Z at .times.20000 magnification after nitriding.
[0045] FIG. 21 is a photograph depicting a surface condition of
another portion of sample Z at .times.5000 magnification after
nitriding.
[0046] FIG. 22 is a photograph depicting a surface condition of
another portion of sample Z at .times.20000 magnification after
nitriding.
[0047] FIG. 23 is a graph depicting maximum contents of alloying
elements at a location adjacent a surface layer of each sample.
[0048] FIG. 24 is a graph depicting oxygen contents at the maximum
content portions of the alloying elements at the location adjacent
the surface layer of each sample.
EXPLANATION OF REFERENCE NUMERALS
[0049] 1 pore [0050] 2 compound layer [0051] 3 mixed structure
[0052] 8 Micro Vickers impression between pores
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0053] An embodiment of the present invention is explained
hereinafter with reference to the drawings.
[0054] A wear-resistant element according to the present invention
is used for a vane or the like that is mounted in, for example, a
rolling piston. The wear-resistant element is obtained by vacuum
sintering iron-based alloy powder containing Cr, such as powdery
HSS (powdery high-speed steel), at a temperature of about
1200.degree. C. to mold it into a desired shape, conducting a
quenching treatment to turn it into a martensitic structure,
conducting a tempering treatment at a temperature of 480.degree.
C..about.580.degree. C. to turn it into a sorbite structure, and
conducting a gas nitriding treatment having no carburizing action
for about six hours at a temperature of 400.degree. C. below the
tempering temperature.
[0055] FIG. 1 depicts a section structure of the wear-resistant
element according to the present invention obtained in the
above-described manner. The wear-resistant element shown therein
has been etched after the gas nitriding treatment, making it
possible to easily observe a compound layer.
[0056] Because the material has been made by sintering and molding
a green compact, the density thereof increases up to only about
80.about.90%, and many pores or voids 1 exist therein. Furthermore,
a gas used for the nitriding treatment has passed through the pores
1 to thereby nitride inner parts of the material, and a white
compound layer 2 has been formed around the pores 1. As the
distance from the pores 1 increases, the number of black portions 3
increases. The black portions 3 have a mixed structure of a
diffused layer and a matrix.
[0057] FIG. 2 depicts a surface of the wear-resistant element at
.times.450 magnification when it has been cut in a direction
perpendicular to the surface of FIG. 1 (that is, it has been cut at
a predetermined depth from the surface), and the cut surface has
been ground.
[0058] As shown in FIG. 2, the pores 1 peculiar to the sintered and
molded green compacts exist in the machined surface, and because
the nitriding gas entering the pores 1 has progressed the nitriding
around the pores 1, an Fe--Cr--N compound layer 2 around the pores
1 has become white upon etching. The white portion decreases at
locations remote from the surfaces of the pores 1, and a mixed
structure 3 of an Fe--Cr--N diffused layer and the matrix exist
there. That is, the surface of the wear-resistant element having
the pores 1 therein has the mixed structure 3 of the compound layer
2, the diffused layer, and the matrix structure.
[0059] FIG. 3 is a photograph of impressions formed on a section of
the wear-resistant element when Micro Vickers hardness measurements
have been conducted. The smaller the impressions are, the harder
the Micro Vickers hardness is. As is clear from the size of the
Micro Vickers impressions, the portions around the pores 1 have
small impressions, while the size of the Micro Vickers impressions
between the pore 1 and the pore 1 is large compared with the
portions around the pores 1, indicating a reduction in hardness. It
is conceivable that because the compound layer 2 has been created
around the pores 1 by the nitriding gas entering such portions, and
the portions 8 between the pore 1 and the pore 1 have turned into
the mixed structure 3 of the diffused layer and the matrix, the
portions 8 have a hardness lower than that of the portions around
the pores 1.
[0060] Because the surface hardness varies moderately, the amount
of processing of the soft matrix portion increases during the
finishing of the wear-resistant element, thus forming minute
hollows, i.e., minute oil holes. When the wear-resistant element is
in operation, the portions having the soft matrix structure are
slightly worn away and serve as the oil holes. Accordingly, in
addition to the pores in the sintered and molded green compact, the
oil holes having a high wedge effect are created over the entire
surface of a movable element or an element confronting it. As a
result, the surface comes to have increased oil retaining
properties and improved lubricating properties as a whole, while
the compound layer and the diffused layer around the pores can
ensure the wear resistance, making it possible to ensure a desired
reliability, compared with the wear-resistant elements that are
hard throughout the entire surface thereof.
[0061] It is to be noted here that although the above-described
embodiment has been explained taking the case of a quenched and
tempered article of powdery HSS, the material may be made of
generally available alloy powder. Similar effects can be obtained
using alloy powder in which at least one metallic element selected
from Mn, Ti and V is contained in iron-based alloy powder
containing Cr.
[0062] FIG. 4 depicts hardness distribution curves of the
wear-resistant element of FIG. 1 after the nitriding treatment. As
shown therein, the hardness at position A of a depth of more than
0.4 mm from the surface is still almost the same as the hardness at
surface B. The surface structure is, when the surface of this
wear-resistant element has been machined or ground by about 0.1 mm
and subsequently etched at position C of a depth of 0.1 mm from the
surface, shown in FIG. 2.
[0063] In the case of the sintered green compact of powdery HSS,
even a short-time nitriding treatment can allow the nitriding gas
to easily enter the inner parts of the material because of the
presence of the pores therein, resulting in deep nitriding. Whereas
the normal nitriding treatment requires a nitriding treatment after
a rough machining of the material and a subsequent finish
machining, the sintered green compact of powdery HSS can easily
obtain a desired hardness by a nitriding treatment of the material
and a subsequent finish machining. Even if the machining allowance
becomes ununiform due to deformation caused by the quenching and
tempering of the material, the deep nitriding can minimize the
variations in the surface hardness of the compound layer that is
the highest one in a finished product. Although the surface removal
exposes the mixed structure of the Fe--Cr--N diffused layer and the
matrix as well as the Fe--Cr--N compound layer, this can be readily
understood from the fact that low hardness portions exist at
locations close to the surface. Because the wear-resistant element
according to the present invention can dispense with the rough
machining, it can be manufactured at a low cost while maintaining a
superior wear resistance.
EXAMPLE 1
[0064] Three kinds of iron-based alloy powder were formed into a
predetermined shape, and each compact so shaped was vacuum-sintered
at a predetermined temperature (for example, 1180.degree. C.) to
thereby form a sintered compact, which was in turn heat-treated in
a predetermined fashion, and thereafter, the surface of each
compact was inspected. The materials of the samples (sintered
compacts) correspond to SKH51, and the samples are hereinafter
referred to as samples X, Y and Z.
[0065] Table 1 indicates results of a component analysis of samples
X, Y and Z after the heat treatment. TABLE-US-00001 TABLE 1 unit:
wt % Others Material W Mo Cr V Si C (Fe etc.) Sample X 5.5.about.
4.0.about. 3.5.about. 1.4.about. 0.4.about. 1.2.about. .ltoreq.1.0
6.7 6.0 5.0 2.4 0.9 1.8 Sample Y 5.5.about. 4.0.about. 3.5.about.
1.4.about. 0.4.about. 0.9.about. .ltoreq.1.0 6.7 6.0 5.0 2.4 0.9
1.4 Sample Z 5.5.about. 4.0.about. 3.5.about. 1.4.about. 0.4.about.
0.9.about. .ltoreq.1.0 6.7 6.0 5.0 2.4 0.9 1.4
[0066] FIG. 5 depicts heat treatment patterns conducted to the
materials of the samples. Table 2 indicates the material
characteristics of the samples. TABLE-US-00002 TABLE 2 Density
Hardness Transverse Rupture Material (g/cm.sup.3) (HRA) Strength
(MPa) Sample X 6.65.about.6.75 63.0.about.66.0 544.9.about.765.9
(Average 64.3) (Average 617.6) Sample Y 6.71.about.6.75
63.6.about.68.1 743.6.about.902.7 (Average 65.0) (Average 774.4)
Sample Z 6.71.about.6.75 63.6.about.68.1 743.6.about.902.7 (Average
65.0) (Average 774.4)
[0067] A six-hour nitriding treatment was conducted at a
temperature of 400.degree. C. with respect to samples X and Y,
while a three-hour atmospheric treatment (slight oxidation
treatment) was first conducted at a temperature of 480.degree. C.
and a six-hour nitriding treatment was subsequently conducted at a
temperature of 400.degree. C. with respect to sample Z.
[0068] The surface configuration and surface nature of each sample
were then inspected and evaluated using a scanning electron
microscope having magnifying powers of 40 to 20000.
[0069] FIGS. 6 to 8 depict surface conditions of samples X, Y and Z
at .times.40 magnification after the nitriding treatment. These
figures reveal that samples Y and Z present similar surface
conditions, while sample X presents minute granular configurations
indicating active surface conditions, compared with samples Y and
Z.
[0070] FIGS. 9 to 12 depict surface conditions of sample X at
.times.200, .times.1000, .times.5000 and .times.20000
magnifications, respectively, while FIGS. 13 to 16 depict the
surface conditions of sample Y at .times.200, .times.1000,
.times.5000 and .times.20000 magnifications, respectively. FIGS. 17
to 20 depict the surface conditions of sample Z at .times.200,
.times.1000, .times.5000 and .times.20000 magnifications,
respectively, while FIGS. 21 and 22 depict surface conditions of
another portion of sample Z at .times.5000 and .times.20000
magnifications, respectively.
[0071] FIGS. 9 to 12 reveal that there are small grains or
particles bonded and sintered under pressure on the surface of
sample X, innumerable minute protruding deposits exist on surfaces
between the sintered grains, and deposition of nitride grains
occurred around such minute deposits. That is, it can be understood
that sample X was nitrided to the inner parts by the six-hour
nitriding treatment at a temperature of 400.degree. C.
[0072] FIGS. 13 to 16 reveal that sample Y has sintered grains
larger than those of sample X. As can be seen by a comparison
between FIGS. 9 and 10 and FIGS. 13 and 14, sample Y presents a
relatively planar surface condition. The inspections at more than
.times.5000 magnifications also reveal that sample Y has a lesser
number of minute protruding deposits than sample X, indicating
unstable (inactive) surface conditions.
[0073] FIG. 17 (.times.200 magnification) reveals that sintered
grains on the surface of sample Z are analogous to those on the
surface of sample Y and are larger than those on the surface of
sample X. However, the inspections at more than .times.1000
magnifications as shown in FIGS. 18 to 22 reveal that sample Z has
a more number of minute deposits than sample X on the surface
thereof and between the sintered grains and, hence, sample Z
microscopically presents surface conditions analogous to those of
sample X.
[0074] A difference between sample Y and sample Z resides in
whether or not the material underwent the atmospheric treatment
after the sintering. The former was in an untreated condition,
while the latter underwent the atmospheric treatment. The untreated
material presents the planar and stable surface conditions as
explained hereinabove, while sample Z treated in the atmosphere has
innumerable protruding deposits on the surface thereof, which is
hence active, like sample X.
[0075] On the other hand, the hardness of each sample after the
nitriding is such as shown in Table 3. TABLE-US-00003 TABLE 3
Distance from Surface (mm) Sample X Sample Y Sample Z 0.01 1163 Hv
837 Hv 1240 Hv 0.50 1115 Hv 767 Hv 1175 Hv 1.00 1111 Hv 792 Hv 1087
Hv 1.50 1112 Hv 744 Hv 1080 Hv
[0076] As can be seen from Table 3, the hardness of sample Z at
depths of 0.01 mm and 0.05 mm from the surface is the highest with
that of sample X and that of sample Y following in this order.
[0077] Comparing the hardness shown in Table 3 and the
aforementioned surface configurations, the deposited minute grains
on the surface of sample Z after the nitriding have the highest
density, followed by that of sample Y and then followed by that of
sample X, as shown in FIGS. 9 to 22. Because sample Z has minute
oxide grains that were formed on the surface thereof by treating it
in the atmosphere before the nitriding treatment, it is conceivable
that a nitriding reaction in sample Z was facilitated by activating
the surface thereof with the minute oxide grains deposited
thereon.
[0078] In the case of sample Y, the hardness at a depth of 0.5 mm
from the surface sometimes increased up to more than 900 Hv by
increasing the nitriding temperature (for example, about
430.degree. C.) or by lengthening the nitriding period of time (for
example, about 10 hours) even when the nitriding temperature was
400.degree. C., but the nitriding was unstable and cracks sometimes
occurred.
[0079] Sample Z was treated in the atmosphere at varying
temperatures of 280.degree. C., 380.degree. C., 480.degree. C. and
580.degree. C. while maintaining the treating period of time
constant (three hours). The treatment at a temperature of
280.degree. C. decreased the hardness below 900 Hv at a depth of
1.5 mm from the surface, but the treatments at temperatures of more
than 380.degree. C. increased the hardness over 900 Hv throughout a
range from the surface to a depth of 1.5 mm.
[0080] That is, samples Y and Z are poor in nitriding capability,
but a three-hour atmospheric treatment at a temperature of
380.degree. C. and a subsequent six-hour nitriding treatment at a
temperature of 400.degree. C. enhanced the nitriding capability up
to the level of sample X.
[0081] It is recognized that there are differences in the alloying
elements (Cr, W, Mo, V) in proximity to the surface layer and in
the oxygen content among the samples, and the difference in the
element content distribution has influence on the nitriding
reaction.
[0082] More specifically, upon inspection of the surface layer
composition of the samples down to a depth of about 50 .mu.m, each
material indicated the same value in the content at a depth more
than 3 .mu.m and, hence, the content distribution of the elements
constituting the surface layer was analyzed based on data down to a
depth of 3 .mu.m.
[0083] As a result, it was recognized that sample X had a
concentrated region in which Cr, W, Mo and V were concentrated
about two or three times those of the matrix composition at a depth
of about 0.2 .mu.m from the surface layer, and the oxygen content
close to about 30% was detected in the outermost layer.
Furthermore, in the case of sample Y, W, Mo and V were concentrated
about 1.5 times those of the matrix composition at a location close
to the surface layer, but an element removal phenomenon was
recognized with Cr. Also, the oxygen content in the outermost layer
was about 6%, which is low relative to sample X.
[0084] Like sample X, it was recognized in sample Z that W and Mo
were concentrated about 1.5 to 2 times those of the matrix
composition at a depth of about 0.2 .mu.m from the surface layer,
and an about 6% oxygen content was detected in the outermost layer.
On the other hand, the state of Cr and V was analogous to that of
sample Y, and an element removal phenomenon was recognized with the
former, while the latter was concentrated in the outermost
layer.
[0085] From the above, it is conceivable that sample Z has an
intermediate aspect between sample X and sample Y in the
concentration of the elements constituting the surface layer.
[0086] FIG. 23 depicts maximum contents of the alloying elements at
a location adjacent the surface layer of each sample. As shown
therein, sample X has the highest Cr, W, Mo and V contents in the
surface layer. Sample Y and sample Z have substantially the same Cr
content, but sample Z has higher contents of the other elements
than sample Y. From these facts, it appears that the nitriding
tends to increase the hardness of the samples in the order of
samples X, Z and Y.
[0087] Each sample tends to be easily nitrided by the atmospheric
treatment, and FIG. 24 depicts oxygen contents at the maximum
content portions of the alloying elements at the location adjacent
the surface layer. This graph reveals that sample X has high oxygen
contents at locations other than a location having a maximum V
content, compared with other samples. On the other hand, sample Y
that is hard to be nitrided has lower oxygen contents than other
samples. From these facts, it is assumed that the degree of
difficulty in the nitriding depends upon the oxygen contents at the
maximum content portions of Cr, W and Mo created at the location
adjacent the surface layer.
[0088] Sample Z has the highest oxygen content at a location having
a maximum V content, and the reason for this is that sample Z has
the highest V content in the outermost layer, while other samples
have the highest V content at a location inside the outermost
layer. This is caused by a difference in oxygen absorption and is
not deeply related to the V content.
[0089] According to the example referred to above, as is the case
with sample X, sample Z has a surface covered with minute nitride
grains deposited thereon, and the density thereof seemingly exceeds
that of sample X. This determines the order of hardness in the
outermost layer, and it is assumed that the minute grain deposits
caused by a change in surface configuration that is in turn caused
by the atmospheric treatment contributed to nitrogen absorption. It
was observed that the surfaces of the sintered materials after
nitriding samples X and Z were almost entirely covered with grains
or protrusions of about 0.1 to 0.5 .mu.m.
[0090] From the above, samples X and Z can be preferably used for
the wear-resistant element according to the present invention,
while the use of sample Y is not preferred.
[0091] Also, the above results in the following.
[0092] (1) The surface of the material after the nitriding
treatment maintains a surface state of a raw material. A large
number of minute deposits are recognized on the surfaces of sample
X and sample Z, while sample Y has a lesser number of deposits and
is hence planar.
[0093] (2) After the nitriding, sample Z has the highest hardness
at depths of 0.01 mm and 0.05 mm with sample X and sample Y
following in this order. The hardness of the surface layer is
closely related to the density of the deposits on the surface.
[0094] (3) The degree of difficulty in the nitriding is dominated
by the content distribution of the alloying elements in the surface
layer and the surface activation phenomenon of the minute oxide
grains and the like.
INDUSTRIAL APPLICABILITY
[0095] The wear-resistant element according to the present
invention is effectively used for sliding components or the like in
an engine or a compressor because the amount of processing of the
soft matrix portion increases during the finishing of the
wear-resistant element to thereby form minute hollows, i.e., minute
oil holes and because slight wear occurs in the soft matrix portion
to form the oil holes to thereby enhance the wear resistance while
avoiding seizing.
* * * * *