U.S. patent application number 10/109338 was filed with the patent office on 2003-03-06 for method of strengthening ti alloy.
This patent application is currently assigned to Fuji Oozx Inc.. Invention is credited to Asanuma, Hiroaki, Hirose, Masahito.
Application Number | 20030041922 10/109338 |
Document ID | / |
Family ID | 19091920 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030041922 |
Kind Code |
A1 |
Hirose, Masahito ; et
al. |
March 6, 2003 |
Method of strengthening Ti alloy
Abstract
Ti alloy is heated in an atmosphere of CO.sub.2 in a heating
furnace. O and C atoms are introduced into the Ti alloy to harden
it without forming Ti oxide, thereby increasing hardness by Ti--O
and Ti--C solid solutions thus formed.
Inventors: |
Hirose, Masahito;
(Fujisawa-shi, JP) ; Asanuma, Hiroaki;
(Fujisawa-shi, JP) |
Correspondence
Address: |
Zarley Law Firm, P.L.C.
Suite 200
Capital Square
400 Locust Street
Des Moines
IA
50309-2350
US
|
Assignee: |
Fuji Oozx Inc.
Kanagawa-ken
JP
|
Family ID: |
19091920 |
Appl. No.: |
10/109338 |
Filed: |
March 28, 2002 |
Current U.S.
Class: |
148/217 ;
148/237 |
Current CPC
Class: |
C23C 8/28 20130101 |
Class at
Publication: |
148/217 ;
148/237 |
International
Class: |
C23C 008/28 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 3, 2001 |
JP |
2001-265462 |
Claims
What is claimed is:
1. A method of strengthening Ti alloy, comprising the step of:
heating the Ti alloy in an atmosphere of CO.sub.2 at 600 to
900.degree. C. in a heating furnace to diffuse C and O atoms into
the Ti alloy.
2. A method as claimed in claim 1 wherein the method is carried out
for 0.5 to 50 hours.
3. A method as claimed in claim 1 wherein the method is carried out
at 800 to 850.degree. C.
4. A method as claimed in claim 3 wherein the method is carried out
for 1 to 3 hours.
5. A method as claimed in claim 1 wherein CO.sub.2 is always
introduced into the heating furnace.
6. A method as claimed in claim 1 wherein the method is carried out
at about 800.degree. C. for 1 to 2 hours.
7. A method as claimed in claim 6 wherein the Ti alloy is used to
make a poppet valve in an internal combustion engine.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a method of strengthening Ti alloy
to improve wear resistance.
[0002] In automobile industries, poppet valves and other
valve-operating parts are made of Ti alloy that provides high
strength and low specific gravity. Poppet valves require wear
resistance and scuff resistance at portion which is engaged with
different valve-operating parts.
[0003] In order to strengthen Ti alloy material to provide wear
resistance and scuff resistance, various methods have been
developed. For example, oxides are formed on the surface of Ti
alloy in Japanese Patent Pub. No. 62-256956. Nitrides are formed on
the surface in Japanese Patent Pub. No. 61-81505. Carburizing is
carried out to diffuse carbon atoms into Ti alloy in Japanese
Patent No. 2,909,361.
[0004] Wear resistance and scuff resistance in the foregoing
methods are improved in Ti alloy material, but the surface is so
hard that different parts to be engaged are likely to be
attacked.
[0005] Japanese Patent Application No. 2001-25415 discloses a Ti
alloy poppet valve in which Ti--O and Ti--C solid solutions are
formed, and a method of manufacturing a Ti alloy poppet valve,
comprising the steps of heating the Ti alloy valve at temperature
lower than beta transformation point in a plasma vacuum finance
which contains oxygen less than stoichiometric amount for forming
Ti oxides to diffuse O and C atoms to form O and C diffusion layer
which comprises Ti--O and Ti--C solid solutions to strengthen a
valve body.
[0006] To diffuse O and C atoms, in the presence of O.sub.2 less
than stoichiometric amount for forming titanium oxides, heat
treatment is carried out at about 800.degree. C. Glow discharge is
made in the presence of a gas for ionized carburizing, or plasma
carburizing is carried out while oxygen less than stoichiometric
amount for forming titanium oxide is supplied. Oxygen/carbon
diffusion layer thus obtained not only improves wear and scuff
resistance, but also decreases attacking property to other
members.
[0007] However, as mentioned above, heat treatment is carried out
in the presence of oxygen in a plasma vacuum finance and ionizing
carburizing is carried out by glow discharge, which is complicate.
Furthermore, it is necessary to employ a vacuum discharge device
and plasma power source in a plasma vacuum finance to increase
cost.
SUMMARY OF THE INVENTION
[0008] In view of the disadvantages in the prior art, it is an
object of the present invention to provide a method of
strengthening Ti alloy to diffuse oxygen and carbon atoms without
forming titanium oxide.
[0009] According to the present invention, there is provided a
method of strengthening Ti alloy, comprising the step of heating
the Ti alloy in an atmosphere of CO.sub.2 at 600 to 900.degree. C.
in a heating furnace to diffuse C and O atoms into the Ti
alloy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] The features and advantages of the present invention will
become more apparent from the following description with respect to
appended drawings wherein:
[0011] FIG. 1 is a micrograph of Ti alloy treated by Example 1 of
the present invention;
[0012] FIG. 2 is a graph that shows oxygen and carbon atom
concentration of the Ti alloy material in FIG. 1;
[0013] FIG. 3 is a graph of hardness to depth of Ti alloy material
in FIG. 1;
[0014] FIG. 4 is a micrograph of Ti alloy treated in Example 3 of
the present invention;
[0015] FIG. 5 is a micrograph of Ti alloy treated in Comparative
Example 2;
[0016] FIG. 6 is a graph of the results of wear test to Ti alloy
materials; and
[0017] FIG. 7 is a schematic view of a device for the wear
test.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0018] Heat treatment of the present invention will be described as
below.
[0019] Ti alloys include alpha alloys such as Ti--5Al--2.5Sn;
near-alpha alloys such as Ti--6Al--2Sn--4Zr--2Mo(hereinafter refer
to "Ti6242") and Ti--8Al--Mo--V; alpha-beta alloys such as
Ti--6Al--4V, Ti--6Al--6V--2Sn and Ti--6Al--2Sn--4Zr--6Mo; and beta
alloys such as Ti--13V--11Cr--3Al and Ti--15Mo--5Zr--3Al. Ti6242
may be preferably used.
[0020] In heat treatment, Ti alloy is put in a heating furnace, and
air in the furnace is purged by CO.sub.2. It is heated in an
atmosphere of CO.sub.2 at 600 to 900.degree. C., preferably 800 to
850.degree. C.
[0021] Below 600.degree. C., diffusion speed of carbon atoms is too
slow, which is disadvantageous in cost. Above 900.degree. C., oxide
layer is formed and the temperature exceeds beta transformation
point of Ti to change its configuration, which is not
preferable.
[0022] In heat treatment, to supplement CO.sub.2 consumed by
introduction of C and O into Ti alloy and to maintain CO.sub.2
atmosphere in the furnace, CO.sub.2 may be always fed into the
heating furnace. Feeding rate may be 0.5 to 3.0 l/min, preferably
1.0 to 2.5 l/min.
[0023] Time for treatment in CO.sub.2 affects wear resistance or
hardness, and may be preferably 1 to 3 hours. By the heat
treatment, O and C atoms are diffused at depth of 25 to 50 .mu.m
from the surface, and surface hardness is HV 550 to 1000.
[0024] When a poppet valve in an internal combustion engine of an
automobile is made of Ti alloy, suitable Vickers hardness is HV 700
to 850. The valve treated by the method of the present invention
not only provides wear and scuff resistance, but also improves
attacking property to the other member.
EXAMPLE 1
[0025] In a muffle furnace which has volume of 24 l, a poppet valve
made of Ti6242 was put as sample and CO.sub.2 was introduced to
purge air. CO.sub.2 was fed into the furnace at the flow rate of 1
l/min and the sample was heated till 800.degree. C. and maintained
at the temperature for two hours. Then, the valve was cooled to
room temperature without contacting air. After cooling, the sample
was taken out of the furnace and various tests were carried
out.
[0026] FIG. 1 illustrates a micrograph of a section of the sample.
As illustrated in the micrograph, O and C atoms were introduced at
the depth.
[0027] FIG. 2 is a graph which shows averages of concentrations of
O and C atoms measured at each depth by an
electric-field-radiation-type Auger electronic spectrometer. In the
graph, an axis of abscissa denotes depth (.mu.m) from the surface
of the sample, and an axis of ordinate denotes concentration
(atomic %) of O and C atoms. The unit of concentration "atomic %"
means rate of O and C atoms with respect to analyzed total atoms.
The graph shows oxygen and carbon atoms in the diffusion layer of
the sample.
[0028] X-ray diffraction in X-ray microdiffraction device
identifies TiC, but does not find titanium oxide. From the result,
oxygen atoms do not combine with titanium, but remain as atoms.
Carbon atoms partially combine with titanium to form TiC, but the
remaining is diffused as carbon atoms.
[0029] Section hardness of the sample thus obtained was measured by
a Micro-Vickers hardness tester of Shimazu Corp. FIG. 3 shows
distribution of hardness. An axis of abscissa means depth (.mu.m)
from the surface, and an axis of ordinate means hardness (HV) under
100 gf. It shows improvement in hardness up to depth of 50 .mu.m
according to the method of the present invention.
[0030] FIGS. 2 and 3 prove that existence of oxygen and carbon
atoms contributes improvement in hardness of Ti alloy.
[0031] As shown in FIG. 3, surface hardness was HV 830.
EXAMPLES 2 TO 9 AND COMPARATIVE EXAMPLES 1 to 3
[0032] Surface treatment was carried out under different
temperatures and time with respect to Ti6242, and the following
Table shows the results.
1 TABLE Surface Temperature Time Hardness Oxide (.degree. C.) (h)
(HV) Layer Example 2 750 3 570 none 3 800 0.5 630 none 4 710 50 680
none 5 800 1 710 none 6 800 1.5 790 none 7 800 3 870 none 8 850 1
930 none 9 850 2 960 none Comparative 850 55 1030 formed Example 1
2 900 1 980 formed 3 1000 0.5 1030 formed
[0033] In an atmosphere of CO.sub.2, Ti6242 was heated at 710 to
850.degree. C. for 0.5 to 50 hours, so that O and C atoms were
introduced into Ti alloy without forming oxide.
[0034] FIG. 4 illustrates a microgragh of a Ti alloy poppet valve
treated In the Example 3, and O and C diffusion layer was
formed.
[0035] A poppet valve is used in an internal combustion engine of
an automobile and is subjected to severe condition such as high
temperature. Such a valve requires hardness of HV 700 to 850. In
Examples 1, 5 and 6, a sample requires to be subject to the
conditions of time for 1 to 2 hours at 800.degree. C.
[0036] As clarified in Comparative Example 1, the temperature
850.degree. C. was the same as those in Examples 8 and 9, but it
took 55 hours to attain HV 1030. But it was so long that an oxide
layer was formed on the surface. Deformation is large and it is not
suitable.
[0037] In Comparative Examples 2 and 3, when the temperature was
over 900.degree. C., surface hardness was sufficient, but a thick
oxide layer was formed to cause large deformation, which was not
suitable for actual use.
[0038] FIG. 5 shows a micrograph of a poppet valve in Comparative
Example 2, in which an oxide layer was formed on an O and C
diffusion layer.
[0039] FIG. 6 illustrates results of wear tests of Ti6242 in
Examples 1 and 3, Comparative Example 2, untreated Ti alloy and
tuftriding-treated heat-resistant steel.
[0040] To carry out the test, as shown in FIG. 7, a test piece 2 is
engaged in a valve guide 1 made of Fe-sintered material. Vertical
weight "W" for 6 kgf was loaded and the test piece 2 was
reciprocally slid for 50 hours while lubricating oil was supplied
between them.
[0041] The test piece made of untreated Ti6242 was the maximum in
wear, and wear becomes smaller in order of Example 3, Example 1,
heat-resistant steel and Comparative Example 2. Example 1 is
equivalent to the heat-resistant steel in wear. Owing to difference
in surface hardness, Example 3 is larger than Example 1 in wear.
The minimum wear in Comparative Example 2 seems to be due to an
oxide layer on the surface. Comparative Example 2 was too rigid, so
that wear of the valve guide 1 engaged therewith was the
maximum.
* * * * *