U.S. patent application number 10/560072 was filed with the patent office on 2006-06-15 for method of continuous vacuum carburization of metal wire, metal band or metal pipe and apparatus therefor.
This patent application is currently assigned to Nachi-Fujikoshi Corp. Invention is credited to Hirokuni Amano, Yasushi Hara, Kiyohito Ishida, Tetsushi Machi.
Application Number | 20060124203 10/560072 |
Document ID | / |
Family ID | 33562638 |
Filed Date | 2006-06-15 |
United States Patent
Application |
20060124203 |
Kind Code |
A1 |
Ishida; Kiyohito ; et
al. |
June 15, 2006 |
Method of continuous vacuum carburization of metal wire, metal band
or metal pipe and apparatus therefor
Abstract
A continuous vacuum carburizing process carburizes material such
as a steel wire 7 with a carbon content equal to or less than a
desired carbon content by passing it under reduced pressure
continuously through a carburizing atmosphere 5 with constant
pressure and constant gas composition. A continuous vacuum
carburizing apparatus for carrying out this process has at least
one furnace core tube 1, 11, or 12 and a feeding/taking-up
mechanism 13, 14 for the steel wire 7 in a vacuum container 1.
Carburizing medium gas is supplied to the furnace core tube 1 via
pipes 2, 4 to form the carburizing atmosphere 5. A heater 10 heats
the furnace core tube 1, thereby activating carbon of the
carburizing medium gas. In this way, material having a small
thickness of, for example, 0.02 mm to 3 mm is carburized with less
variation in the amount of carburization and no surface oxidation
or sooting.
Inventors: |
Ishida; Kiyohito; (Miyagi,
JP) ; Amano; Hirokuni; (Toyama, JP) ; Hara;
Yasushi; (Toyama, JP) ; Machi; Tetsushi;
(Toyama, JP) |
Correspondence
Address: |
VENABLE LLP
P.O. BOX 34385
WASHINGTON
DC
20045-9998
US
|
Assignee: |
Nachi-Fujikoshi Corp
1-1-1, Fujikoshi-Honmachi, Toyama-shi
Toyama-ken
JP
Kiyohito Ishida
5-20, Kamisugi 3-chome, Aoba-ku Sendai-shi
Miyagi
JP
|
Family ID: |
33562638 |
Appl. No.: |
10/560072 |
Filed: |
June 3, 2004 |
PCT Filed: |
June 3, 2004 |
PCT NO: |
PCT/JP04/09181 |
371 Date: |
December 9, 2005 |
Current U.S.
Class: |
148/222 ;
148/223; 148/235 |
Current CPC
Class: |
C23C 8/20 20130101; C23C
8/36 20130101 |
Class at
Publication: |
148/222 ;
148/223; 148/235 |
International
Class: |
C23C 8/22 20060101
C23C008/22; C23C 8/20 20060101 C23C008/20; C23C 8/36 20060101
C23C008/36 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 4, 2003 |
JP |
2003-271038 |
Claims
1. A continuous vacuum carburizing process comprising: under a
reduced pressure of 5 kPa or less, forming at least one carburizing
atmosphere in which pressure and gas composition are constant with
one of chain saturated hydrocarbon, chain unsaturated hydrocarbon
gas and cyclic hydrocarbon used as a carburizing medium; activating
carbon in the carburizing atmosphere; and passing one material of a
metal wire, a metal strip and a metal pipe, which has a carbon
content equal to or less than a desired carbon content,
continuously through the carburizing atmosphere and thereby
carburizing the material.
2. The continuous vacuum carburizing process according to claim 1,
further comprising heating a fixed area, through which the material
passes following the carburizing atmosphere and in which the
carburizing medium does not exist, and causing the carbon
carburized in the material to be diffused into inner sections of
the material.
3. The continuous vacuum carburizing process according to claim 1,
wherein said activating carbon comprises heating the carburizing
atmosphere to 850.degree. C. to 1050.degree. C.
4. The continuous vacuum carburizing process according to claim 1,
wherein said activating carbon comprises bringing the carbon into a
plasma state and heating the carburizing atmosphere to 400.degree.
C. to 1050.degree. C.
5. The continuous vacuum carburizing process according to claim 1,
further comprising lowering pressure in a surrounding area of the
carburizing atmosphere than the pressure of the carburizing
atmosphere.
6. The continuous vacuum carburizing process according to claim 2,
further comprising supplying and discharging carrier gas to/from
the fixed area and forming carrier gas atmosphere in the fixed
area.
7. The continuous vacuum carburizing process according to claim 2,
said passing the material through the carburizing atmosphere and
then through the fixed area is repeated multiple times.
8. The continuous vacuum carburizing process according to claim 1,
wherein carburizing is performed until the material reaches or
exceeds the desired carbon content.
9. The continuous vacuum carburizing process according to claim 1,
wherein the material has a diameter of 0.02 mm to 3 mm in case of
the metal wire, a thickness or width of 0.02 mm to 3 mm in case of
the metal strip and a wall thickness of 0.02 mm to 3 mm in case of
the metal pipe, and the material is carburized to a center of its
cross section.
10. The continuous vacuum carburizing process according to claim 1,
wherein the material is carburized only in a surface layer
thereof.
11. The continuous vacuum carburizing process according to claim 1,
wherein the material comprises one of carbon steel for machine
construction, alloy steel for machine construction, tool steel,
spring steel and stainless steel.
12. The continuous vacuum carburizing process according to claim 1,
wherein the material comprises one of a nickel alloy and a cobalt
alloy containing one or more of carbide-forming elements of boron,
titanium, vanadium, chromium, zirconium, niobium, molybdenum,
hafnium, tantalum and tungsten.
13. The continuous vacuum carburizing process according to claim 1,
wherein the material comprises one of a metal and an alloy which
has as a main component one of carbide-forming elements of boron,
titanium, vanadium, chromium, zirconium, niobium, molybdenum,
hafnium, tantalum and tungsten.
14. A continuous vacuum carburizing apparatus comprising: a furnace
core portion formed to enclose a fixed space through which one
material of a metal wire, a metal strip and a metal pipe is passed
continuously; means for supplying as a carburizing medium one of
chain saturated hydrocarbon, chain unsaturated hydrocarbon gas and
cyclic hydrocarbon to the furnace core portion under a reduced
pressure of 5 kPa or less and discharging the carburizing medium to
form at least one carburizing atmosphere in which pressure and gas
composition are constant; and means for activating carbon of the
carburizing medium within the furnace core portion.
15. The continuous vacuum carburizing apparatus according to claim
14, wherein said means for activating carbon comprises an electric
heater for heating the furnace core portion to 850.degree. C. to
1050.degree. C.
16. The continuous vacuum carburizing apparatus according to claim
14, wherein said means for activating carbon comprises a discharger
for causing glow discharge in the furnace core portion and an
electric heater for heating the furnace core portion to 400.degree.
C. to 1050.degree. C.
17. The continuous vacuum carburizing apparatus according to claim
14, further comprising a feeding/taking-up mechanism for passing
the material through the furnace core portion, and a vacuum
container for receiving the furnace core portion, the
supply/discharge means and the heating means, said vacuum container
being kept in its inside at a lower pressure than pressure in the
furnace core portion.
18. The continuous vacuum carburizing apparatus according to claim
14, further comprising means for supplying and discharging a
carrier gas to/from the furnace core portion to form, on a
downstream side of the carburizing atmosphere with respect to a
travel direction of the material, at least carrier gas atmosphere
without the carburizing medium.
19. The continuous vacuum carburizing apparatus according to claim
14, wherein said furnace core portion and said supply/discharge
means are adapted to form a plurality of carburizing atmospheres in
the furnace core portion.
Description
TECHNICAL FIELD
[0001] The present invention generally relates to a process and
apparatus for manufacturing metallic material which has superior
toughness and wear resistance. More particularly, the invention
relates to a continuous vacuum carburizing process for metal wires,
metal strips, or metal pipes and an apparatus for carrying out the
process.
BACKGROUND ART
[0002] Most of steels used as wear resistant material have a high
carbon content and low cold-workability. Thus, in a cold drawing
process of a steel wire, stress relief annealing has to be
performed frequently to reduce hardness of the work-hardened wire.
Such frequent stress relief annealing increases process lead
time.
[0003] Besides, in the case of ingot material, during its
solidification process, large primary carbides are produced in the
material. The large carbides are not destroyed completely and
remain even after subsequent hot working or cold working.
Consequently, when this material is used as wires, the large
carbides serves as stress concentration sources, causing chips or
breakage.
[0004] To deal with the above problems, Japanese Patent No. 3053605
discloses a technique for working low-carbon steel stock with a
limited ingredient balance into sheet or thin wire shapes and
subsequently carburizing them to the center. This technique
produces, with high manufacturing efficiency, metallic material in
which hard carbides are distributed finely and uniformly to provide
superior toughness and wear resistance. However, the patent makes
no mention of problems involved in carburizing wires or flat
strips.
[0005] JP-A-6-192814 and JP-A-7-126829 disclose methods for
carburizing metal strips continuously. However, they neither
disclose nor suggest anything about carburizing material uniformly
to its center as discussed by the patent mentioned above.
[0006] As concerns carburizing depth when carburizing steels, known
techniques for carburizing the objects with substantially
semi-infinite carburizing depth include gas carburizing which
involves carburizing the objects by adjusting the carbon potential
of carburizing gas and vacuum carburizing which involves
carburizing the objects under reduced pressure.
[0007] In the case of small diameter material such as wire rods,
since the carburizing depth coincides with the radius of the
material, if the material has application of a carburizing process
(hereinafter referred to as batch processing), such as described in
Japanese Patent No. 3053605, which involves adding a carburizing
medium after putting a workpiece in a furnace, variations in
carburizing conditions are reflected directly in the amount of
carbon in the inner section of the material.
[0008] In addition, gas carburizing, in particular, tends to cause
problems that carbon penetration increases due to adhesion of soot
to surface portions of the material, resulting in coarse carbides,
that which surface defects occur due to surface oxidation, and that
carbon becomes insufficient and predetermined heat-treated hardness
is not obtained.
[0009] In batch processing of material having a small wire
diameter, in particular, predetermined carbon penetration is
reached at an initial stage of introducing carburizing atmosphere
gas and its control is very difficult. Furthermore, affection of
surface defects cannot be ignored because of a large specific
surface area.
DISCLOSURE OF THE INVENTION PROBLEM TO BE SOLVED BY THE
INVENTION
[0010] The present invention, in view of the problems described
above, has an object to provide a carburizing process for metal
wires, metal strips or metal pipes which has far less variation in
the amount of carburization in the material and is free of surface
oxidation or sooting.
[0011] Another object of the invention is to provide a carburizing
apparatus for effectively carrying out the above process.
MEANS FOR SOLVING THE PROBLEM
[0012] To achieve the above first object, according to the
invention, there is provided a continuous vacuum carburizing
process comprising, under a reduced pressure of 5 kPa or less,
forming at least one carburizing atmosphere in which pressure and
gas composition are constant with one of chain saturated
hydrocarbon, chain unsaturated hydrocarbon gas and cyclic
hydrocarbon used as a carburizing medium, activating carbon in the
carburizing atmosphere, and passing one material of a metal wire, a
metal strip and a metal pipe, which has a carbon content equal to
or less than a desired carbon content, continuously through the
carburizing atmosphere and thereby carburizing the material.
[0013] To effectively carry out the above process, a continuous
vacuum carburizing apparatus according to another aspect of the
invention comprises a furnace core portion formed to enclose a
fixed space through which one material of a metal wire, a metal
strip and a metal pipe is passed continuously, means for supplying
as a carburizing medium one of chain saturated hydrocarbon, chain
unsaturated hydrocarbon gas and cyclic hydrocarbon to the furnace
core portion under a reduced pressure of 5 kPa or less and
discharging the carburizing medium to form at least one carburizing
atmosphere in which pressure and gas composition are constant, and
means for activating carbon of the carburizing medium within the
furnace core portion.
[0014] With the above construction, the carburizing atmosphere is
in the reduced pressure condition in which no oxide layer is formed
on a material surface and the carburizing medium causes no soot on
the material surfaces. Also, the carburizing atmosphere has
constant pressure and constant gas composition in a condition that
the carbon is activated. The material to be carburized is moved
through this atmosphere, and it is possible to perform appropriate
carburization with less variation in the amount of carburization.
The material can be carburized continuously, making it possible to
process a large amount of material efficiently.
[0015] Preferably, the above continuous vacuum carburizing process
further comprises heating a fixed area, through which the material
passes following the carburizing atmosphere and in which the
carburizing medium does not exist, and causing the carbon
carburized in the material to be diffused into inner sections of
the material.
[0016] In this fixed area, a carrier gas atmosphere may be formed
by supplying and discharging carrier gas.
[0017] For this end, the continuous vacuum carburizing apparatus
preferably further comprises means for supplying and discharging a
carrier gas to/from the furnace core portion to form, on a
downstream side of the carburizing atmosphere with respect to a
travel direction of the material, at least carrier gas atmosphere
without the carburizing medium.
[0018] By providing such an area, it is possible to diffuse a
desired amount of carbon reliably into the material.
[0019] Preferably, the activation of carbon is performed by heating
the carburizing atmosphere to 850.degree. C. to 1050.degree. C. The
carburizing medium gas is decomposed by the heating and produces
active carbon. This temperature range facilitates reaction of the
carburizing medium gas and diffusion of the carbon which penetrates
into the material while inhibiting grain growth in the
material.
[0020] The activation of carbon may be performed by bringing the
carbon into a plasma state in addition to the heating of the
carburizing atmosphere. For this end, the continuous vacuum
carburizing apparatus preferably comprises an electric heater for
heating the furnace core portion to 400.degree. C. to 1050.degree.
C. and a discharger for causing glow discharge. By accelerating
carbon ions, carburization of the material can be performed more
efficiently.
[0021] The continuous vacuum carburizing apparatus may further
comprise lowering pressure in a surrounding area of the carburizing
atmosphere than the pressure of the carburizing atmosphere.
[0022] The above apparatus preferably further comprises a
feeding/taking-up mechanism for passing the material through the
furnace core portion, and a vacuum container for receiving the
furnace core portion, the supply/discharge means and the heating
means, which is kept in its inside at a lower pressure than
pressure in the furnace core portion.
[0023] By keeping the surrounding area at a lower pressure, the gas
degraded as a result of reaction is discharged out of the
carburizing atmosphere quickly, and contaminated gas is prevented
from flowing into the carburizing atmosphere from the outside. This
makes it possible to stably keep the gas composition in the
carburizing atmosphere in desirable condition.
[0024] In the continuous vacuum carburizing process, the passing of
the material through the carburizing atmosphere and then through
the fixed area having no carburizing medium may be repeated
multiple times.
[0025] To carry out the vacuum carburizing process, in the
continuous vacuum carburizing apparatus, the furnace core portion
and the supply/discharge means are preferably adapted to form a
plurality of carburizing atmospheres in the furnace core
portion.
[0026] In some ferrous material, there is a fear that a coarse
network of carbide crystals will be deposited on a surface if the
entire amount of carbon required to carburize the material to its
center is introduced into the material at once. Pulse carburizing
which repeats a short cycle of carburizing and diffusion multiple
times is effective in this situation. By forming the plurality of
carburizing atmospheres in the furnace core portion, it is possible
to carry out such pulse carburizing.
[0027] Preferably carburizing is performed until the material
reaches or exceeds the desired carbon content.
[0028] This process makes it possible to use material with a low
carbon content, cold-work it into a desired shape and then
carburize the material appropriately, thereby facilitating the
working of the material and giving desired strength to the
material.
[0029] The material to be carburized may have a diameter of 0.02 mm
to 3 mm in case of the metal wire, a thickness or width of 0.02 mm
to 3 mm in case of the metal strip and a wall thickness of 0.02 mm
to 3 mm in case of the metal pipe.
[0030] The continuous vacuum carburizing process according to the
invention, by continuously feeding the material into the fixed
carburizing atmosphere, causes far less variation in carburizing
even in the case of a thin material which has a thickness almost
equal to a carburizing depth.
[0031] The carburization may be made to the center of the cross
section of the material or only to its surface layer.
[0032] Material to be carburized may be carbon steel for machine
construction, alloy steel for machine construction, tool steel,
spring steel, or stainless steel.
[0033] Alternatively, the material to be carburized may be a nickel
alloy or cobalt alloy containing one or more carbide-forming
elements out of boron, titanium, vanadium, chromium, zirconium,
niobium, molybdenum, hafnium, tantalum, and tungsten.
[0034] Alternatively, the material to be carburized may be a metal
or alloy having as main constituent one carbide-forming element of
boron, titanium, vanadium, chromium, zirconium, niobium,
molybdenum, hafnium, tantalum, and tungsten.
ADVANTAGES OF THE INVENTION
[0035] As described above, according to the continuous vacuum
carburizing method and apparatus of the invention, by moving
material through the fixed carburizing atmosphere, it is possible
to perform appropriate carburizing with far less variation in the
amount of carburization. In particular, when material has a small
diameter or thickness, it has conventionally encountered problems
that desired hardness is not obtained after heat treatment and that
coarse carbide is generated. The process and apparatus of the
invention can eliminate these problems.
BEST MODE FOR CARRYING OUT THE INVENTION
[0036] A continuous vacuum carburizing process and apparatus of the
invention will be described in detail based on the embodiments
shown in the accompanying drawings.
[0037] FIG. 1 schematically shows a manufacturing process of a tool
steel wire with the use of the continuous vacuum carburizing
apparatus according to the embodiment of the invention. The process
uses wire rods of low-carbon alloy steel as material and includes a
continuous wire drawing step, a continuous stress relief annealing
step, a carbide dispersion carburizing step, and a
quenching/tempering step.
[0038] In the continuous wire drawing step, the wire rod is fed
continuously from a supply side to a take-up side and drawn
efficiently through a plurality of dies. The wire rod having a
diameter of 5 to 8 mm is moved through the dies 5 to 20 times and
its cross-sectional area is reduced to 1/5 or less.
[0039] The wire rod hardened in this step is then transferred to
the continuous stress relief annealing step, and the wire rod is
heated to a predetermined temperature in a continuous stress relief
furnace and reduced in hardness. Subsequently, the wire rod is
returned to the continuous wire drawing step and it is drawn again
until its cross-sectional area is reduced to 1/5 or less. The wire
drawing and continuous stress relief are repeated until the wire
rod reaches a predetermined wire diameter.
[0040] The wire rod, when the wire drawing has been completed to
the predetermined wire diameter, is transferred to the carbide
dispersion carburizing step. In this step, the continuous vacuum
carburizing apparatus of the invention provides carburization
treatment on the wire rod into its inner section.
[0041] The wire rod completed of carburization is transferred to
the quenching/tempering step. In this step, the wire rod is
quenched and tempered continuously in a continuous
quenching/tempering furnace and consequently attains a
predetermined hardness.
[0042] FIG. 2 shows the continuous vacuum carburizing apparatus or
furnace according to this embodiment in detail.
[0043] The continuous vacuum carburizing furnace has an elongate
vacuum container 9, a plurality of furnace core tubes 1, 11, and 12
(three in the illustrated example) placed in the vacuum container
along a longitudinal direction thereof, and a feeding/taking-up
mechanism for passing the steel wire 7, which has been drawn to the
predetermined diameter, through a furnace core portion formed of
the furnace core tubes.
[0044] Each core tube 1, 11, or 12 has an elongate shape with both
ends open and is equipped with a carburizing gas inlet pipe 2, a
carrier gas inlet pipe 3, and a pair of exhaust pipes 4.
Furthermore, each furnace core tube is equipped with an electric
heater 10 along its longitudinal direction.
[0045] The inlet pipes 2, 3 and the exhaust pipes 4, 4 extend
through the vacuum container 9 and are connected to the furnace
core tube to introduce carburizing gas and carrier gas into the
furnace core tube from outside the vacuum container and discharge
them outside the vacuum container.
[0046] The exhaust pipes 4, 4 are arranged on both sides of the
carburizing gas inlet pipe 2 with regard to the longitudinal
direction of the furnace core tube. The inside of the furnace core
tube between the exhaust pipes constitutes a carburizing portion 5
occupied by the carburizing gas. The carrier gas inlet pipe 3 is
arranged on the downstream side of the carburizing gas inlet pipe 2
and the exhaust pipes 4, 4 with regard to the travel direction of
the steel wire 7. That inside of the furnace core tube on this
downstream side constitutes a diffusing portion 6 filled with the
carrier gas.
[0047] By the way, although in FIG. 2 only the furnace core tube 1
is given reference numerals 2 to 6 and 10, the core tubes 11, 12
have similar construction.
[0048] The vacuum container 9 has an exhaust pipe 8 equipped with
an evacuation valve (not shown) and can evacuate the inside of the
container.
[0049] The feeding/taking-up mechanism includes a feed bobbin 13
and a take-up bobbin 14, which are installed on both sides of the
furnace core tubes 1, 11, and 12 in the vacuum container. The
bobbins 13, 14 are rotatively driven, reel out the steel wire 7
wound around the bobbin 13, pass it through the furnace core tubes
1, 11, and 12 and take up it on the take-up bobbin 14.
[0050] Incidentally, the feeding/taking-up mechanism may be
installed outside the vacuum container. In that case, it is
preferable that a differential exhaust mechanism is provided to
prevent air from entering the vacuum container along with the
travel of the steel wire 7.
[0051] The continuous vacuum carburizing furnace operates as
follows according to an embodiment of the process of the
invention.
[0052] First, the steel wire 7 is led through the furnace core
tubes 1, 11, and 12 from the feed bobbin 13 and connected to the
take-up bobbin 14. Then, the entire vacuum container 9 is evacuated
sufficiently through the exhaust pipe 8. When the inside of the
vacuum container reaches a predetermined degree of vacuum below 10
Pa, electric current is delivered to the electric heater 10 and the
furnace core tubes 1, 11, and 12 are heated to a predetermined
temperature of between 850.degree. C. and 1050.degree. C.
[0053] Then, the carburizing gas such as ethylene and the carrier
gas such as nitrogen or argon are introduced into the furnace core
tubes 1, 11, and 12 through the carburizing gas inlet pipes 2 and
the carrier gas inlet pipes 3. At the same time, the vacuum in the
vacuum container 9 is controlled through adjustment of the
evacuation valve of the exhaust pipe 8 and the pressure in the
furnace core tubes 1, 11, and 12 is restored to 5 kPa or lower,
preferably to 1 to 3 kPa.
[0054] After the adjustment of the atmosphere, the steel wire 7 is
passed through the furnace core tubes 1, 11, and 12 and taken up on
the take-up bobbin 14 by operation of the feeding/taking-up
mechanism. When a required amount of steel wire is obtained, the
furnace is cooled, the vacuum of the vacuum container is removed,
and the steel wire 7 is taken out of the furnace together with the
bobbin. Thus, the steel wire processed to the predetermined
diameter and carburized is obtained.
[0055] The carburizing medium gas is introduced and discharged
continuously into/from each furnace core tube heated to 850.degree.
C. to 1050.degree. C. via the carburizing gas inlet pipe 2 and the
exhaust pipes 4, 4, and it functions as the carburizing atmosphere
which has a constant pressure and constant constituent gases and is
capable of vacuum carburizing. This atmosphere carburizes the steel
wire 7 passing through it. Then, the carburized steel wire 7 passes
through the heated diffusing portion 6 of each core tube. The
diffusing portion has no gas serving as a carburizing medium, and
the carbon carburized from the surfaces of the steel wire 7
diffuses into the inner section of the alloy.
[0056] The carburized portion may be limited to regions near the
surface, or the entire material may be carburized to its
center.
[0057] The continuous vacuum carburizing process of the invention
is performed under a reduced pressure of 5 kPa or less and uses
chain saturated hydrocarbon, chain unsaturated hydrocarbon gas, or
cyclic hydrocarbon as the carburizing medium. This is because at a
pressure higher than 5 kPa, soot would be produced on the surface
of the treated material, disabling proper carburizing. Also, the
reason for the carburizing atmosphere with the reduced pressure is
that carburizing performed under normal pressure would produce an
oxide layer of 5 to 10 .mu.m on the surface of the treated
material. This defect will have large influence especially on
small-diameter wire rods with large specific surface areas.
[0058] The above-mentioned heating temperature condition of the
carburizing atmosphere is because at 850.degree. C. or below, the
gas serving as the carburizing medium (except special gases such as
acetylene) does not start reaction for forming cementite on the
material surface, and consequently the material is not carburized.
Also, the diffusion rate of carbon in steel is low at 850.degree.
C. or below, making carburizing/diffusion operations inefficient.
On the other hand, the upper limit of 1050.degree. C. is because at
temperatures above 1050.degree. C., steel wire undergoes marked
grain growth, degrading its mechanical properties.
[0059] The material to be treated by the continuous vacuum
carburizing process of the invention is preferably 0.02 mm to 3 mm
in diameter in the case of wire rods, for example. Material below
0.02 mm is difficult to control carburizing depth. With diameters
above 3 mm, as it takes a long time to carburize the material to
its center and variation in the time required to introduce gas have
less influence, there is no necessity of specifically using the
process of to the invention.
[0060] Nevertheless, the process of the invention is of course
effective for the case of carburizing only surface layers of
material at a predetermined density regardless of the size of the
material.
[0061] Although in the above embodiment, the carbon in the
carburizing medium gas is activated by heating the furnace core
tubes 1, 11, and 12, plasma may be used in addition to this
activation.
[0062] FIG. 3 shows the essential part of the continuous vacuum
carburizing apparatus according to another embodiment which
performs such vacuum plasma carburizing. The apparatus may have the
same or similar components as the embodiment of FIG. 2 except a
portion for performing the generation of plasma. The same or
similar components are denoted with the same reference numerals,
and description thereof will be omitted.
[0063] The continuous vacuum carburizing apparatus of this
embodiment has a discharger 15 in addition to the apparatus
construction of FIG. 2. The discharger 15 is electrically connected
with the steel wire 7 via the furnace core tube 1 and the bobbin
13. During operation of the apparatus, the discharger 15 applies
voltage between the furnace core tube 1 as an anode and the steel
wire 7 as a cathode. This produces glow discharge in the furnace
core tube 1 and makes the introduced carburizing medium gas into
plasma. In addition, the electric heater 10 heats the furnace core
tube 1 to 400.degree. C. to 1050.degree. C.
[0064] The carbon in the carburizing medium gas is ionized and
carbon ions adhere to the surfaces of the steel wire 7 effectively.
In this way, the apparatus of the embodiment facilitates the
carburizing of the steel wire 7 by converting the carburizing
medium gas into plasma.
[0065] FIG. 4A shows the gas inlet pipes 2, 3 and the exhaust pipes
4 of the continuous vacuum carburizing apparatus of FIG. 2 in an
enlarged scale.
[0066] The pipe layout in FIG. 4A is intended to form the
carburizing gas atmosphere or carburizing portion 5 only in the
shaded portion of each furnace core tube and to form an adjacent
area on the right side in the figure as the diffusing portion 6 in
which no carburizing medium gas exists. Specifically, the
carburizing medium gas and the carrier gas, which are introduced
into the furnace core tube simultaneously, tend to mix with each
other in the furnace core tube. By disposing the exhaust pipe 4
between the carburizing gas inlet pipe 2 and the carrier gas inlet
pipe 3, and by evacuating the furnace core tube from between the
two gases independently, it is possible to prevent the carburizing
gas from entering the right side of the furnace core tube.
[0067] The introduction and discharge of the carburizing medium
into/from the furnace core tube is intended to keep the carburizing
medium in the furnace core tube at appropriate pressure and in
appropriate atmosphere. The gas serving as the carburizing medium
exists between the introduction site and the discharge site of the
carburizing gas. To prevent the gas from leaking into the diffusing
portion, a carrier gas inlet pipe for blocking may be installed
near a border with the diffusing portion.
[0068] Alternatively, the furnace core tube may be divided between
the carburizing portion and diffusing portion to let the
carburizing gas introduced into the carburizing portion escape into
the vacuum container, thereby preventing the carburizing gas from
leaking into the diffusing portion. In any case, it is important to
maintain the carburizing gas atmosphere in the carburizing portion,
and the atmosphere free of a carburizing medium in the diffusing
portion.
[0069] FIGS. 4B and 4C show modifications of the layout in FIG. 4A,
intended to prevent leakage of the carburizing gas to the diffusing
portion.
[0070] In the example in FIG. 4B, carrier gas inlet pipes 31 to 33
and exhaust pipes 41, 42 are installed additionally to more
reliably prevent the carburizing gas from entering an area on the
right side of the furnace core tube.
[0071] In the example in FIG. 4C, a furnace core tube for
carburizing area and a furnace core tube for diffusing area are
provided completely separately. In this case, the carburizing gas
and the carrier gas escape into the vacuum container from their
respective furnace core tubes, and thus there is no need to
discharge them from the furnace core tubes directly.
[0072] Wire rods were produced by way of trial from carbon steel
for machine construction, alloy steel for machine construction,
tool steel, spring steel, and stainless steel by the manufacturing
process described above. Results of the trial are shown in Table 1.
The trial products were by wire-drawing rolled coils of the various
steel materials and then by carburizing them with the process of
the invention or with a conventional batch process. The amounts of
carbon were measured at six spots after carburizing to evaluate
variations in the carbon amount, and Table 1 shows the
variations.
[0073] In table 1, trial product Nos. 1 to 9 are tool steel wire
rods, trial product Nos. 10 to 15 are stainless steel wire rods,
trial product Nos. 16 to 17 are carbon steel wire rods, trial
product Nos. 18 to 19 are alloy steel wire rods, trial product Nos.
20 to 21 are spring steel wire rods. TABLE-US-00001 TABLE 1
Chemical composition of rolled coil (wt. %) Manufacturing method
Adjustment of carbon No C Cr W Mo V Co Fe content Carburizing
process 1 0.20 3.9 6.1 4.9 1.9 The rest Carburizing after Process
of Invention 2 drawing Conventional process 3 0.90 3.9 6.0 4.8 2.0
The rest No adjustment of Ingot 4 0.20 3.9 6.1 4.9 1.9 The rest
Carburizing after Process of Invention 5 drawing Conventional
process 6 0.89 4.0 6.1 4.9 1.9 The rest No adjustment of Ingot 7
0.35 4.0 6.3 5.1 2.8 8.0 The rest Carburizing after Process of
Invention 8 drawing Conventional process 9 1.25 4.0 6.2 5.2 2.7 7.9
The rest No adjustment of Ingot 10 0.25 12.5 1.0 0.3 The rest
Carburizing after Process of Invention 11 drawing Conventional
process 12 0.90 16.3 0.5 The rest Carburizing after Process of
Invention 13 drawing Conventional process 14 0.30 13.5 The rest
Carburizing after Process of Invention 15 drawing Conventional
process 16 0.05 The rest Carburizing after Process of Invention 17
drawing Conventional process 18 0.04 1.0 The rest Carburizing after
Process of Invention 19 drawing Conventional process 20 0.05 0.9
0.2 The rest Carburizing after Process of Invention 21 drawing
Conventional process Wire Carburizing Carburizing Diffusion
Diffusion diameter temperature time temperature time C% after
carburizing No mm .degree. C. (min.) .degree. C. (min.) 1 2 3 4 5 6
Results 1 0.1 950 0.27 1000 1.5 0.92 0.92 0.93 0.92 0.93 0.93 2
0.82 0.25 1.32 2.20 1.01 0.90 3 4 3.0 1030 10.00 1040 90.0 0.95
0.95 0.95 0.95 0.96 0.95 5 0.95 0.93 0.96 0.95 0.93 0.93 6 7 0.2
950 0.58 1020 3.0 1.24 1.26 1.26 1.25 1.26 1.26 8 1.33 1.58 0.10
0.25 1.45 0.75 9 10 0.8 1000 3.00 900 12.0 1.55 1.54 1.53 1.53 1.54
1.54 11 1.00 1.20 0.50 0.87 0.76 1.35 12 0.2 900 0.85 850 3.0 1.45
1.45 1.47 1.45 1.45 1.45 13 1.24 1.36 1.04 1.25 1.17 1.45 14 0.2
880 0.85 850 3.0 0.55 0.56 0.54 0.56 0.54 0.54 15 0.74 1.03 0.88
0.99 0.55 0.88 16 0.5 900 2.00 850 4.0 1.25 1.26 1.24 1.24 1.24
1.26 17 1.00 2.65 1.25 1.75 0.35 0.55 18 0.4 900 2.00 850 4.0 1.24
1.24 1.26 1.24 1.26 1.25 19 2.35 0.95 0.05 1.05 1.98 1.22 20 1.0
1000 1.20 1.19 1.21 1.20 1.19 1.20 21 1.20 2.55 2.32 1.05 0.08
1.96
[0074] As shown in Table 1, when the diameter is 0.1 mm, the
variation in carbon content according to the conventional
carburizing process is approximately 2.0%, and the variation in
carbon content according to the process of the invention, is 0.01%.
When the diameter is 0.2 mm, the variation in carbon content
according to the conventional carburizing process is approximately
1.5%, and the variation in carbon content according to the process
of the invention is 0.02%. Thus, the continuous vacuum carburizing
process of the invention gives good results.
[0075] Among the tool steel wire rods, probe pins were produced
from the 0.1 mm diameter steel wire made of trial product Nos. 1 to
3 equivalent to SKH51, drills were produced from the 3 mm diameter
steel wire made of trial product Nos. 4 to 6 equivalent to SKH51,
and dot pins were produced from the 0.2 mm diameter high-speed
cobalt tool steel wire made of trial product Nos. 7 to 9. Results
of their performance evaluation are shown in FIGS. 5, 6, and 7.
[0076] The graph of FIG. 5 comparatively shows flexural strength of
the probe pins produced on the experimental basis.
[0077] Trial product No. 1 was produced by drawing a rolled coil of
5.5 mm diameter with a lower carbon content than a desired value to
a wire of 0.1 mm diameter and then by increasing the carbon content
of the wire to the desired value using the continuous vacuum
carburizing process of the invention. Trial product No. 2 was
produced by drawing the same rolled coil as trial product No. 1 in
the same manner and then by carburizing it with the conventional
process so as to obtain the desired carbon content. With trial
product No. 2, the results of carburizing exhibited a wide
variation and thus only the samples whose carbon content was within
a predetermined range were used for the probe pins. Trial product
No. 3 was produced by drawing a rolled coil of 5.5 mm diameter
already containing the desired carbon content to a wire of 0.1 mm
diameter.
[0078] The graph of FIG. 6 comparatively shows the life of the
experimentally produced drills when they were used for machining
under the conditions specified in the figure.
[0079] Trial product No. 4 was produced by drawing the rolled coil
of 5.5 mm diameter with the lower carbon content than the desired
value to a wire of 3 mm diameter and then by carburizing the steel
wire with the continuous vacuum carburizing process of the
invention to the desired carbon content. Trial product No. 5 was
produced by drawing the same rolled coil as trial product No. 4 in
the same manner and then by carburizing it with the conventional
process to the desired carbon content. Trial product No. 6 was
produced by drawing the rolled coil of 5.5 mm diameter already
containing the desired carbon content to a wire of 3 mm
diameter.
[0080] The graph of FIG. 7 comparatively shows the flexural
strength of the experimentally produced dot pins.
[0081] Trial product No. 7 was produced by drawing the rolled coil
of 5.5 mm diameter with the lower carbon content than the desired
value to a wire of 0.2 mm diameter and then by carburizing it with
the continuous vacuum carburizing process of the invention to the
desired carbon content. Trial product No. 8 was produced by drawing
the same rolled coil as trial product No. 7 in the same manner and
then by carburizing it with the conventional process to obtain the
desired carbon content. With trial product No. 8, the results of
carburizing exhibited a wide variation and thus only samples whose
carbon content was within a predetermined range were used for the
dot pins. Trial product No. 9 was produced by drawing the rolled
coil of 5.5 mm diameter already containing the desired carbon
content to a wire of 0.2 mm diameter.
[0082] Among the tool steel wire rods shown in the graphs of FIGS.
5 to 7, trial product Nos. 1, 2, 4, 5, 7, and 8 were produced by
highly efficiently drawing the steel wire of the lower carbon
content than the desired carbon content and then by carburizing it,
thereby obtaining the desired carbon content.
[0083] In terms of the flexural strength of the probe pins and the
dot pins as well as the cutting life of the drills shown in FIGS. 5
to 7, the trial products carburized after drawing by either the
process of the invention or the conventional process are superior
to the trial products produced from the rolled coils containing the
desired carbon content as given as the comparison examples. Between
the two processes, the process of the invention provides better
results. This is because that the process of the invention provides
excellent carburizing with less liability to generate coarse
carbides.
[0084] The continuous vacuum carburizing process of the invention
was applied to nitinol wire rods composed of nickel and titanium.
Carbon contents were measured at six spots after carburizing either
with the process of the invention or the conventional process to
evaluate variations in the carbon content. The result is shown in
Table 2.
[0085] It will be seen from Table 2 that the carburizing process of
the invention causes far less variation in the amounts of carbon
than do the conventional process as is the result of the steel
stocks in Table 1. TABLE-US-00002 TABLE 2 Chemical Carburiz- Dif-
composition of Manufacturing method Wire ing Carburiz- fusion Dif-
supplied wire Adjustment dia- tempera- ing tempera- fusion (wt. %)
of carbon Carburizing meter ture time ture time C% after
carburizing No C Ti Ni content process mm .degree. C. (min.)
.degree. C. (min.) 1 2 3 4 5 6 1 0.01 45.0 55.0 Supplied Process of
0.5 950 1.00 950 3.0 2.20 2.21 2.20 2.20 2.20 2.20 wire are
Invention 2 carburized Conventional 0.50 0.25 0.99 3.20 2.45 1.91
process
[0086] The present invention has been described with reference to
the embodiments. The invention, however, is not limited solely to
these specific forms, and various modifications may be made to the
described, specific forms within the scope of the appended claims,
or the invention may take other forms as well.
[0087] For example, although the above embodiments have been
described as carburizing the steel wires steel wires, the invention
is effective not only for wires with a circular cross-section but
also for materials of other shapes such as a pipe shape and a
strips shape as long as they have small cross sections.
[0088] Besides, it is also possible to introduce, for instance,
nitrogen gas instead of the carburizing medium in the latter half
part of the first furnace core tube or the second or third furnace
core tube, form a nitride layer on the surface of the carburized
metal wire during it passes through the furnace core tube, and
thereby produce functionally gradient material.
INDUSTRIAL APPLICABILITY
[0089] By using tool steel of a lower carbon content than a desired
amount and by carburizing it according to the invention, it is
possible to manufacture thin tool steel wire with high
manufacturing efficiency, greatly reducing lead time on the tool
steel wire used for dot printer pins, probe pins, drills, etc.
[0090] Also, by applying the carburizing according to the invention
to stainless steel, it is possible to form uniform carburized
layers near the surface with a depth accuracy which cannot be
achieved by conventional carburizing processes. This results in an
ultra-thin stainless steel wire whose inner section has flexibility
and which has been carburized to a certain depth from the surface
and has moderate rigidity. Thus, the applicability of the stainless
steel wire is extended to machine parts which require corrosion
resistance and wear resistance.
[0091] Alternatively, when the nitinol, an alloy of nickel, is
carburized, fine carbides are separated out on the surface or in
its inner sections. The alloy is applicable to curtain guide wires
and the guide wires thus obtained have flexibility, moderate
rigidity, and excellent operability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0092] FIG. 1 is a schematic diagram showing the manufacturing
process of a tool steel wire in which the continuous vacuum
carburizing apparatus according to an embodiment of the invention
is applied;
[0093] FIG. 2 is a longitudinal sectional view of the continuous
vacuum carburizing apparatus of FIG. 1;
[0094] FIG. 3 is a sectional view showing the essential part of the
continuous vacuum carburizing apparatus according to another
embodiment of the invention;
[0095] FIG. 4A is a view showing the layout of gas inlet pipes and
exhaust pipes in the continuous vacuum carburizing apparatus of
FIG. 2;
[0096] FIG. 4B is a view showing a modification of the pipe layout
of FIG. 4A;
[0097] FIG. 4C is a view showing another modification of the pipe
layout of FIG. 4A;
[0098] FIG. 5 is a graph comparatively showing flexural strength of
probe pins produced experimentally by the process of the invention
and by a conventional process;
[0099] FIG. 6 is a graph comparatively showing the machining
performance of drill materials produced experimentally by the
process of the invention and by a conventional process; and
[0100] FIG. 7 is graph comparatively showing the flexural strength
of dot pins produced experimentally by the process of the invention
and by a conventional process.
* * * * *