U.S. patent application number 10/586626 was filed with the patent office on 2007-09-06 for method for activating surface of metal member.
This patent application is currently assigned to PARKER NETSUSHORI KOGYO K.K.. Invention is credited to Hiroshi Eiraku, Kaoru Hoshino, Takashi Kawamura, Takumi Kurosawa, Makoto Miyashita, Toshiko Totsuka, Kuniji Yashiro.
Application Number | 20070204934 10/586626 |
Document ID | / |
Family ID | 34792370 |
Filed Date | 2007-09-06 |
United States Patent
Application |
20070204934 |
Kind Code |
A1 |
Hoshino; Kaoru ; et
al. |
September 6, 2007 |
Method for Activating Surface of Metal Member
Abstract
A passivated film on a surface of a high-alloy steel member
makes it difficult to apply diffusion treatment, such as gas
nitriding or gas carburizing, that forms a nitrided layer,
carburized layer or carbonitrided layer on the surface of the steel
member. An activating treatment method is provided for the surface
of the metal member. This method is not accompanied by problems of
conventional activation treatment with a halide, such as furnace
deposits, furnace wall erosion and effluent gas detoxification
treatment, and is useful as pretreatment for diffusion treatment.
According to this method, the passivated surface of the high-alloy
steel member can be activated by using a gas commonly employed in
gas heat treatment, and forming HCN gas in a heating furnace while
making use of catalytic action of the steel member or a surface of
the furnace.
Inventors: |
Hoshino; Kaoru; (Tokyo,
JP) ; Miyashita; Makoto; (Tokyo, JP) ;
Kawamura; Takashi; (Tokyo, JP) ; Totsuka;
Toshiko; (Tokyo, JP) ; Eiraku; Hiroshi;
(Tokyo, JP) ; Yashiro; Kuniji; (Tokyo, JP)
; Kurosawa; Takumi; (Tokyo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
PARKER NETSUSHORI KOGYO
K.K.
16-8, Nihonbashi 2-chome, Chuo-ku,
Tokyo
JP
103-0027
|
Family ID: |
34792370 |
Appl. No.: |
10/586626 |
Filed: |
January 19, 2005 |
PCT Filed: |
January 19, 2005 |
PCT NO: |
PCT/JP05/00607 |
371 Date: |
July 19, 2006 |
Current U.S.
Class: |
148/225 |
Current CPC
Class: |
C23C 8/02 20130101 |
Class at
Publication: |
148/225 |
International
Class: |
C23C 22/00 20060101
C23C022/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 20, 2004 |
JP |
2004-012328 |
Claims
1. A method for activating a surface of a metal member, which
comprises heating a mixed gas of a carbon donor compound, which is
gaseous at normal temperature and pressure, and ammonia as
essential components to at least 300.degree. C. in a heating
furnace to form HCN under catalytic action of said metal member, a
metal-made inner wall of said furnace or a metal-made jig in the
thus-heated mixed gas, and causing the thus-formed HCN to act on
said surface of said metal member.
2. A method according to claim 1, wherein said carbon donor
compound is at least one compound selected from acetylene,
ethylene, propane, butane and carbon monoxide.
3. A method according to claim 1, wherein said metal-made inner
wall of said heating furnace or said metal-made jig contains at
least one metal selected from Fe, Ni, Co, Cu, Cr, Mo, Nb, V, Ti and
Zr.
4. A method according to claim 1, wherein HCN is formed to at least
100 mg/m.sup.3 in said heating furnace and a furnace atmosphere gas
has a dew point not higher than 5.degree. C.
Description
TECHNICAL FIELD
[0001] The invention of the present application relates to a method
for the pretreatment of a metal member to activate a surface of the
metal member before applying diffusion treatment such as nitriding
or carburizing to the metal member.
BACKGROUND ART
[0002] To improve mechanical properties such as abrasion resistance
and fatigue strength, gas nitriding or gas carburizing that forms a
nitrided layer or carburized layer in a surface of a metal member
is widely applied primarily to members made of iron-based
material.
[0003] Upon applying such treatment to a surface of a member made
of alloy steel, especially high-alloy steel, the penetration and
diffusion of nitrogen or carbon into the surface of the metal
member is prevented by a passivated film (an oxide or the like)
which exists on the surface of the member, thereby possibly
resulting in the occurrence of poor treatment or uneven treatment
of the member as a problem. Before such diffusion treatment,
activation treatment is hence applied to the surfaces of metal
members. It is methods making use of chloride compounds, led by
Malcomizing, that are most widely adopted as such surface
activation treatment. As the chloride compounds, vinyl chloride
resin, ammonium chloride, methylene chloride and the like are
used.
[0004] Such a chloride is placed together with a metal member in a
treatment furnace and is heated there. By this heating, the
chloride is decomposed to form HCl, and the thus-formed HCl
decomposes a passivated film on a surface of the metal member to
activate the surface so that diffusion treatment such as nitriding
or carburizing as a next step is assured.
[0005] However, the surface activation of a metal member by such a
chloride results in the erosion of a furnace wall made of bricks or
a metal by HCl formed through decomposition, and in gas nitriding
or gas softnitriding, HCl so formed reacts with ammonia as
atmosphere gas to form ammonium chloride, which not only deposits
in the furnace or an exhaust system to cause troubles but also
remains on the surface of the metal member (work) to induce
reductions in the corrosion resistance and fatigue strength of the
member.
[0006] As a substitute method for the above-described methods
making use of chlorides, an activation method of the surface of a
metal member with a compound of fluorine which belongs to the same
halogen group, NF.sub.3, has been put into practical use in recent
years (for example, Patent Document 1). Upon heating, NF.sub.3 is
decomposed to form fluorine, and the thus-formed fluorine converts
a passivated film on the surface of the metal member into a
fluoride film to activate the surface of the metal member. The
activation method of the surface of the metal member with the
fluorine compound (NF.sub.3), however, requires sophisticated
treatment for the detoxification of NF.sub.3 and HF contained in
effluent gas, which prevents the wide-spread adoption of the
method.
[0007] The above-described activation methods for the surfaces of
metal members, which make use of halides, respectively, involves
problems such as troublesome furnace deposits, furnace wall erosion
and the need for detoxification treatment facilities for effluent
gas. From the foregoing background, developments of activation
methods for the surfaces of metal members, said methods making use
of no halide, are under way.
[0008] The ammonia gas nitriding method disclosed in Patent
Document 2 reductively activates a passivated film on a surface of
a high-chromium alloy steel member by forming reducing radicals and
CO at the surface of the alloy steel member through the pyrolysis
of acetone. According to this method, acetone is pyrolyzed on the
heated surface of the high-chromium alloy steel member in
accordance with the below-described formula (1) so that reducing
radicals and CO are formed at the surface of the high-chromium
alloy steel member. 2(CH.sub.3)CO.fwdarw.2CH.sub.3.+CO (1)
[0009] An oxide film (MO) on the surface of the metal member is
reduced in accordance with the following formula (2):
5MO+2CH.sub.3..fwdarw.5M+2CO+3H.sub.2O (2)
[0010] As the principal component of the surface oxide film of the
high-chromium alloy steel member is Cr.sub.2O.sub.3,
5Cr.sub.2O.sub.3+6CH.sub.3..fwdarw.10Cr+6CO+9H.sub.2O (3)
[0011] The CO formed in accordance with the formulas (1) to (3)
reacts with ammonia as atmosphere gas, and forms HCN in accordance
with the following formula (4): CO+NH.sub.3.fwdarw.HCN+H.sub.2O
(4)
[0012] The HCN formed in accordance with the formula (4) reduces
the passivated film on the surface of the high-chromium alloy steel
member in accordance with the following formula:
Cr.sub.2O.sub.3+6HCN.fwdarw.2Cr(CN).sub.3+3H.sub.2O (5)
[0013] The Cs and Ns in the resulting Cr(CN).sub.3 diffuse into the
surface of the high-chromium alloy steel member, and contribute to
carburizing and nitriding so that no residue is formed on the
surface of the member.
[0014] The above-described chloride-dependent activation method of
a surface of a high-chromium alloy steel member, on the other hand,
can be expressed by the following formula (6):
Cr.sub.2O.sub.3+6HCl.fwdarw.2CrCl.sub.3+3H.sub.2O (6)
[0015] The chromium chloride remains on the surface of the member,
and acts as a causative substance for the corrosion of the member.
[0016] Patent Document 1: JP-A-3-44457 [0017] Patent Document 2:
Japanese Patent Application No. 9-38341
PROBLEMS TO BE SOLVED BY THE INVENTION
[0018] As has been described above, the method disclosed in Patent
Document 2 is good in that it has theoretically solved the problems
of the chloride-dependent activation method for a surface of a
metal member as disclosed in Patent Document 1. Nonetheless, the
method disclosed in Patent Document 2 is accompanied by a drawback
that the use of acetone, which is liquid at normal temperature and
pressure, requires facilities for the introduction of acetone vapor
and the difficult flow rate control of acetone makes it hard to
obtain a metal member having an evenly-activated surface.
[0019] With a view to solving the above-described problems, the
present inventors have struggled to develop a method that makes use
of a compound, which is gaseous at normal temperature and pressure,
in place of acetone involving the problems in handling, leading to
the completion of the present invention.
[0020] Described specifically, the present invention provides:
[0021] 1. A method for activating a surface of a metal member,
which comprises heating a mixed gas of a carbon donor compound,
which is gaseous at normal temperature and pressure, and ammonia as
essential components to at least 300.degree. C. in a metal-made
heating furnace to form HCN under catalyticaction of the metal
member, a metal-made inner wall of the furnace or a metal-made jig
in the thus-heated mixed gas, and causing the thus-formed HCN to
act on the surface of the metal member.
[0022] 2. A method as described above under 1., wherein the carbon
donor compound is at least one compound selected from acetylene,
ethylene, propane, butane and carbon monoxide.
[0023] 3. A method as described above under 1., wherein the
metal-made inner wall of the heating furnace or the metal-made jig
contains at least one metal selected from Fe, Ni, Co, Cu, Cr, Mo,
Nb, V, Ti and Zr.
[0024] 4. A method as described above under 1., wherein HCN is
formed to at least 100 mg/m.sup.3 in the furnace and a furnace
atmosphere gas has a dew point not higher than 5.degree. C.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0025] A passivated film on a surface of a high-alloy steel member
makes it difficult to apply diffusion treatment, such as gas
nitriding or gas carburizing, that forms a nitrided layer,
carburized layer or carbonitrided layer on the surface of the steel
member. According to the present invention, an activating treatment
method is provided for the surface of the metal member. This method
is not accompanied by problems of conventional activation treatment
with a halide, such as furnace deposits, furnace wall erosion and
effluent gas detoxification treatment, and is useful as
pretreatment for diffusion treatment. According to this method, the
passivated surface of the high-alloy steel member can be activated
by using a gas commonly employed in gas heat treatment, and forming
HCN gas in a heating furnace while making use of catalytic action
of the steel member or a surface of the furnace.
BEST MODES FOR CARRYING OUT THE INVENTION
[0026] The present invention will next be described in more detail
based on best modes for carrying out the invention.
[0027] According to Patent Document 2 referred to in the above,
CH.sub.3. (methyl radicals) formed by the pyrolysis of acetone in
the formula (1) reduce an oxide film on a surface of a metal
member. The CO formed in the above-described formula (1) and (2)
reacts with ammonia as atmosphere gas on the metal surface to form
HCN. HCN acts on the metal oxide film in accordance with the
above-described formula (5).
[0028] From a comparison between the formula (2) and the formula
(5), the CH.sub.3. formed by the pyrolysis of acetone and HCN (the
reaction product of CO, the other pyrolyzate, with ammonia as
atmosphere gas) are similar to each other in their action on the
passivated film. The present inventors, therefore, presumed that
the existence of both CH.sub.3. and HCN would be a sufficient
condition for the activation of the surface of a high-chromium
alloy steel member but would not absolutely be a necessary
condition. Paying attention to HCN, the present inventors,
therefore, endeavored to develop a method for the formation of HCN
on a metal surface and also to ascertain effects of HCN for the
activation of the surface of a metal member.
[0029] An investigation was conducted on the formation of HCN by
introducing a nitriding atmosphere gas (NH.sub.3:N.sub.2=1:1 by
molar ratio) together with gases selected from various
carbon-containing compounds, which are gaseous at normal
temperature and pressure, respectively into a Muffle furnace made
of SUS310S and heating them to 550.degree. C. As a result, it has
been clearly ascertained that carbon monoxide, carbon dioxide,
acetylene, ethylene, propane and butane each forms HCN when
combined with ammonia.
[0030] An experiment was then conducted in a similar manner as
described above except that the inner wall of the Muffle furnace
was replaced by bricks, and an analysis was performed for the
amount of HCN formed. In each case, HCN was not detected. From
those results, it has become evident that the catalytic action of a
metal surface is an essential condition for the HCN-forming
reactions between ammonia and these gases.
[0031] The HCN-forming reactions between ammonia and the
above-mentioned carbon-containing compounds can be expressed by the
following formulas, respectively: NH.sub.3+CO.fwdarw.HCN+H.sub.2O
(7) 2NH.sub.3+2CO.sub.2.fwdarw.2HCN+H.sub.2O+O.sub.2 (8)
2NH.sub.3+C.sub.2H.sub.2.fwdarw.2HCN+3H.sub.2 (9)
2NH.sub.3+C.sub.2H.sub.4.fwdarw.2HCN+4H.sub.2 (10)
3NH.sub.3+C.sub.3H.sub.8.fwdarw.3HCN+7H.sub.2 (11)
4NH.sub.3+C.sub.4H.sub.10.fwdarw.4HCN+9H.sub.2 (12)
[0032] To compare the amounts of HCN to be formed by the reactions
between the nitriding atmosphere gas (NH.sub.3:N.sub.2=1:1 by molar
ratio) and the gases selected from the various carbon-containing
compounds, the reactions of the above-described formulas (7) to
(12) were each conducted by incorporating the corresponding
carbon-containing compound at 1% in terms of equivalent ratio in
the nitriding atmosphere gas (NH.sub.3:N.sub.2=1:1 by molar ratio),
introducing the resultant mixed gas into a Muffle furnace the inner
wall of which was made of SUS310S, and then heating the mixed gas
at 550.degree. C. for 30 minutes. As a result, the amounts of HCN
formed from the respective carbon-containing compounds decreased in
the following order:
C.sub.2H.sub.2>CO>C.sub.2H.sub.4>C.sub.4H.sub.10>C.sub.3H.sub-
.8>CO.sub.2
[0033] With respect to these carbon-containing compounds
ascertained to form HCN through their reactions with the nitriding
atmosphere gas, these compounds were each introduced into a heating
furnace at an initial stage of nitriding treatment and then
assessed with an SUS304 plate to determine whether or not they have
activating effect. As a result, compared with control nitriding
treatment without introduction of any carbon-containing compound,
C.sub.2H.sub.2, CO, C.sub.2H.sub.4, C.sub.4H.sub.10 and
C.sub.3H.sub.8 have been found to be equipped with profound effects
on such SUS304 plates in both the evenness of nitriding and the
weight increase by the penetration of nitrogen. When CO.sub.2 was
used, on the other hand, no difference was observed from the
control nitriding treatment in both the evenness of nitriding
treatment and the weight increase of the specimen. Concerning
CO.sub.2, no activating effect was, therefore, recognized for the
surface of the SUS304 plate.
[0034] The availability of no activating effect for the surface of
the SUS304 plate despite the formation of HCN in the furnace by the
introduction of CO.sub.2 is presumably attributed to the
re-oxidation of the surface of the SUS304 plate under the oxidation
action of O.sub.2 and H.sub.2O, the byproducts of the HCN-forming
reaction in the formula (8). Concerning CO, HCN is formed as
mentioned above. This is inconsistent with the phenomenon that
stainless steel is not evenly nitrided in a gas softnitriding
atmosphere in which ammonia and CO-containing RX gas exist. This
inconsistency may be explained by reasons to be described below. It
is to be noted that the term "RX gas" means a gas, which is formed
by mixing substantially equal chemical equivalents of a hydrocarbon
gas (for example, propane gas, butane gas, or natural gas) and air
and causing them to decompose in a catalyst layer maintained at
1,000.degree. C., contains CO and H.sub.2(N.sub.2) as a primary
component and small amounts of CO.sub.2 and H.sub.2O, and is widely
used as a nitriding gas.
[0035] The CO contained in NH.sub.3:RX gas=1:1 by molar ratio, a
typical composition for gas softnitriding, amounts to about 10% in
terms of volume percentage. HCN, which is required for the
activation of a surface of a metal member, is therefore presumed to
exist sufficiently in a gas softnitriding furnace.
[0036] In an RX gas the dewpoint of which is not controlled,
however, there are a significant amount of H.sub.2O (around 2 vol.
%) and about 0.5 vol. % of CO.sub.2. It is, therefore, judged that
by their oxidizing action, the activated surface of the SUS304
plate is re-oxidized to prevent the penetration of nitrogen into
the surface of the plate.
[0037] When CO gas is selected as a carbon donor compound for the
activation of a surface of a metal member, it is thus desired to
use CO gas singly instead of RX gas. Because the amount of CO gas
required to be injected in the present invention is as little as
1/10 (by volume) or so of a gas softnitriding atmosphere, the
effects of H.sub.2O and CO.sub.2 in RX gas are reduced so that RX
gas may be used as a CO source in some instances.
[0038] Judging from the formulas on the right-hand sides in the
reaction formulas (7) to (12), the byproducts in the case of
CO.sub.2 have the highest oxidizing action among these compounds
having cyan-forming effect, followed by CO, and the hydrocarbon
compounds all form reducing hydrogen. To avoid re-oxidation, it is,
therefore, desired to choose a hydrocarbon compound as a carbon
donor compound.
[0039] The activating effect for the surface of the alloy steel
member in the present invention is attributed to HCN. The
above-described activating effect is dependent on the concentration
of HCN in the furnace atmosphere. To obtain satisfactory activating
effect, the concentration of HCN can appropriately be in a range of
from 100 to 30,000 mg/m.sup.3. At an HCN concentration lower than
100 mg/M.sup.3, the above-described activating effect cannot be
expected. At an HCN concentration higher than 30,000 mg/m.sup.3, on
the other hand, the above-described activating effect is saturated,
resulting not only in an economical disadvantage but also in the
occurrence of sooting (the formation of carbon in the furnace) by
pyrolysis of the carbon donor compound. Therefore, HCN
concentrations outside the above-described range are not
preferred.
[0040] Further, the dew point of the furnace atmosphere gas may
preferably be 5.degree. C. or lower. If the dew point is higher
than 5.degree. C., the metal surface activated by HCN gas is
re-oxidized with H.sub.2O in the atmosphere and accordingly, is
passivated back again.
[0041] The method according to the present invention is also
advantageous from the environmental standpoint in that as explained
in the reaction formula (5), the HCN attributed to the activation
of the surface of the metal member is absorbed into the member and
attributes to the nitriding and carburizing of the member to leave
no residue on the surface of the member and the HCN discharged as
effluent gas without any contribution to the reaction can be
readily burned and detoxified in an ammonia combustion facility
arranged as an attachment for the nitriding facility to obviate any
new additional facility.
[0042] A further advantage of the present invention is that the
time of nitriding treatment can be shortened owing to the smooth
progress of the steps in the nitriding treatment process. Gas
nitriding of a metal member is generally conducted in such a
schedule as will be described below.
[0043] The metal member is set in a furnace, and subsequent to
vacuum purging or nitrogen gas replacement of the air in the
furnace, the temperature is raised to a nitriding temperature of
the metal member and is then maintained constantly at the
temperature, both while introducing the nitriding atmosphere gas
(NH.sub.3+N.sub.2) at a rate as much as 1 to 10 times the internal
volume of the furnace per hour. During the treatment, the internal
pressure of the furnace is maintained at atmospheric pressure+0.5
kPa or so by a pressure control valve, and the force-out effluent
gas is caused to burn and decompose in an effluent gas combustion
facility.
[0044] According to the method disclosed in Patent Document 1 and
making use of the fluorine-based gas, it is necessary, subsequent
to the introduction of the fluorine-based gas and the activation
treatment of the member, to exhaust the fluorine-based gas and then
to introduce the nitriding atmosphere gas into the furnace as
disclosed in the examples of the specification of Japanese Patent
No. 2,501,925.
[0045] In the present invention, on the other hand, the carbon
donor compound is introduced into the nitriding atmosphere gas
during the step in which the metal member is heated to the
nitriding treatment temperature. As a consequence, HCN is formed to
activate the surface of the metal member, and the subsequent
termination of the introduction of the carbon donor compound makes
it possible to advance directly to the nitriding step. As a result,
the treatment time of the nitriding step is substantially
shortened, thereby making it possible to fundamentally eliminate
the re-oxidation phenomenon of the surface of the metal member
which has until now remained as a problem in the conventional
treatment upon advancing from the activation step to the nitriding
step.
[0046] The present invention has such technical features and
advantageous effects as described above. A description will
hereinafter be made about certain preferred embodiments of the
present invention. In the treatment furnace for use in the present
invention, the inner wall can preferably be made of metal. Even if
the inner wall is not made of metal, the present invention can
still be practiced provided that the metal member to be treated
acts as a catalyst for the formation of HCN or a jig adapted to
hold the metal member within the furnace is made of metal. The
metal that makes up the metal-made inner wall, metal member or jig
may preferably contain, for example, one or more metals selected
from Fe, Ni, Co, Cu, Cr, Mo, Nb, V, Ti and Zr.
[0047] Examples of metal members which can be subjected to surface
activation treatment by the method of the present invention include
members of cold-working die steel, hot-working die steel, plastic
die steel, high-speed tool steel, powder metal high-speed tool
steel, chrome-molybdenum steel, maraging steel, austenitic
stainless steel, ferritic stainless steel, martensitic stainless
steel, martensitic heat-resisting steel, austenitic heat-resisting
steel or nickel-based superalloys. In the above-described treatment
furnace, these metal members are held by suitable jigs and are
subjected to surface activation treatment in a manner known per se
in the art.
[0048] The surface treatment gases to be fed into the furnace are
the carbon donor compound, which is gaseous at normal temperature
and pressure, and ammonia, which are fed from their own gas
cylinders into the furnace. After the metal member is set in the
furnace and the internal air of the furnace is purged under vacuum
or is replaced with nitrogen gas, the nitriding atmosphere gas
(ammonia alone, ammonia+nitrogen gas, or ammonia+nitrogen
gas+hydrogen gas) is introduced into the furnace to establish a
reducing atmosphere. Subsequently, heating is initiated, followed
by the introduction of the carbon donor compound useful in the
present invention. The ammonia gas and carbon donor compound form
HCN under the catalytic action of the metal surface when they are
heated to 300.degree. C. or higher in the furnace. The ratio of the
flow rate of ammonia as a nitriding atmosphere gas to that of the
introduced carbon donor compound should be controlled within a
range of from 1:0.0001 to 1:0.1. If the flow rate of the carbon
donor compound is so low that the flow rate ratio becomes smaller
than 1:0.0001, HCN is formed too little to bring about its
activating effect. If the flow rate of the carbon donor compound is
so high that the flow rate ratio becomes greater than 1:0.1, on the
other hand, the activating effect is saturated to result in an
economical disadvantage.
[0049] The carbon donor compound is composed of one or more gaseous
compounds selected from acetylene, ethylene, propane, butane and
carbon monoxide as described above, and can be fed into the
treatment furnace concurrently with the ammonia-containing gas as
mentioned above. It is preferred for the efficient utilization of
the carbon donor compound to initiate the introduction of the
carbon donor compound at the time point that the temperature of the
ammonia-containing gas within the furnace has reached about
300.degree. C. To raise the concentration of the carbon donor
compound in the furnace atmosphere at such an early stage as
permitting shortening the treatment time, however, it is desired to
introduce the carbon donor compound at the same time as the
initiation of heating and to assure the formation of HCN from the
initial stage.
EXAMPLES
[0050] Based on examples and a comparative example, the present
invention will hereinafter be described more specifically. It is to
be noted that the following examples and comparative example were
conducted using a treatment furnace of the construction illustrated
in FIG. 1. FIG. 1 shows a Muffle furnace 1, an outer shell 2 of the
Muffle furnace, a heater 3, an internal container (retort) 4, a gas
inlet pipe 5, an exhaust pipe 6, a motor 7, a fan 8, a metal-made
jig 9, a gas guide cylinder 10, an inverted funnel 11, a vacuum
pump 12, an effluent gas combustion facility 13, a carbon donor
compound gas cylinder 14, an ammonia gas cylinder 15, a nitrogen
gas cylinder 16, a hydrogen gas cylinder 17, a flow rate meter 18,
and a gas control valve 19.
Example 1
[0051] Using the SUS310S Muffle furnace of 100-L internal capacity
shown in FIG. 1, SUS304 plates were set in the furnace, NH.sub.3
gas and N.sub.2 gas were fed at flow rates of 200 L/H,
respectively, and the furnace atmosphere was heated from room
temperature to 550.degree. C. in 75 minutes. At the time point that
the atmosphere temperature had reached 100.degree. C. in the course
of the heating (at the 18.sup.th minute after the initiation of the
heating), an injection of acetylene gas was initiated at 2 L/hr.
After heated to 550.degree. C., the atmosphere temperature was
maintained for 2 hours. At that time point, the injection of
acetylene gas was terminated and instead, NH.sub.3 gas and N.sub.2
gas were then fed at 550.degree. C. for 4 hours to allow nitriding
to proceed. Subsequently, the heating was stopped and N.sub.2 gas
alone was continuously fed to cool down the furnace. When the
atmosphere temperature had dropped to 100.degree. C. or lower, the
specimens were taken out of the furnace.
[0052] Effluent gas from the furnace was branched off to have a
portion of the effluent gas absorbed in a 2 wt. % aqueous solution
of caustic soda, and an analysis was performed for HCN. From the
analysis results of the HCN-absorbed solution, the average HCN
concentration in the furnace atmosphere during the acetylene gas
injection period was 8,000 mg/m.sup.3. Some of the SUS304 specimens
were weighed to determine a weight increase after the nitriding
treatment. As a result, the weight increase was determined to be 20
g/m.sup.2. Some of the SUS304 specimens were cut, and their cut
surfaces were polished, etched with Marble's solution, and then
observed under an optical microscope. Nitrided layers of 50-.mu.m
uniform thickness were found to be formed (a 500.times. micrograph
is shown in FIG. 2). Some of the remaining specimens were measured
for surface hardness at 5 points by a Vickers hardness tester. All
the values (Hv) distributed between 1,200 and 1,250.
Example 2
[0053] SUS304 plates were set in the Muffle furnace employed in
Example 1, NH.sub.3 gas and N.sub.2 gas were fed at flow rates of
200 L/H, respectively, and the furnace atmosphere was heated from
room temperature to 550.degree. C. in 75 minutes. At the time point
that the atmosphere temperature had reached 100.degree. C. in the
course of the heating (at the 18.sup.th minute after the initiation
of the heating), an injection of propane gas was tinitiated at 5
L/hr. After heated to 550.degree. C., the atmosphere temperature
was maintained for 2 hours. At that time point, the injection of
propane gas was terminated and instead, NH.sub.3 gas and N.sub.2
gas were then fed at 550.degree. C. for 4 hours to allow nitriding
to proceed. Subsequently, the heating was stopped and N.sub.2 gas
alone was continuously fed to cool down the furnace. When the
atmosphere temperature had dropped to 100.degree. C, or lower, the
specimens were taken out of the furnace.
[0054] Effluent gas from the furnace was branched off to have a
portion of the effluent gas absorbed in a 2 wt. % aqueous solution
of caustic soda, and an analysis was performed for HCN. From the
analysis results of the HCN-absorbed solution, the average HCN
concentration in the furnace atmosphere during the propane gas
injection period was 400 mg/m.sup.3. Some of the SUS304 specimens
were weighed to determine a weight increase after the nitriding
treatment. As a result, the weight increase was determined to be 18
g/m.sup.2. Some of the SUS304 specimens were cut, and their cut
surfaces were polished, etched with Marble's solution, and then
observed under an optical microscope. Nitrided layers of 45-.mu.m
uniform thickness were found to be formed. Some of the remaining
specimens were measured for surface hardness at 5 points by a
Vickers hardness tester. All the values (Hv) distributed between
1,200 and 1,250.
Example 3
[0055] SUS304 plates were set in the Muffle furnace employed in
Example 1, NH.sub.3 gas and N.sub.2 gas were fed at flow rates of
200 L/H, respectively, and the furnace atmosphere was heated from
room temperature to 550.degree. C. in 75 minutes. At the time point
that the atmosphere temperature had reached 100.degree. C. in the
course of the heating (at the 18.sup.th minute after the initiation
of the heating), an injection of CO gas was initiated at 5 L/hr.
After heated to 550.degree. C., the atmosphere temperature was
maintained for 2 hours. At that time point, the injection of CO gas
was terminated and instead, NH.sub.3 gas and N.sub.2 gas were then
fed for 4 hours to allow nitriding to proceed. Subsequently, the
heating was stopped and N.sub.2 gas alone was continuously fed at
550.degree. C. to cool down the furnace. When the atmosphere
temperature had dropped to 100.degree. C. or lower, the specimens
were taken out of the furnace.
[0056] Effluent gas from the furnace was branched off to have a
portion of the effluent gas absorbed in a 2 wt. % aqueous solution
of caustic soda, and an analysis was performed for HCN. From the
analysis results of the HCN-absorbed solution, the average HCN
concentration in the furnace atmosphere during the CO gas injection
period was 1,000 mg/m.sup.3. Some of the SUS304 specimens were
weighed to determine a weight increase after the nitriding
treatment. As a result, the weight increase was determined to be 18
g/m.sup.2. Some of the SUS304 specimens were cut, and their cut
surfaces were polished, etched with Marble's solution, and then
observed under an optical microscope. Nitrided layers of 45-.mu.m
uniform thickness were found to be formed. Some of the remaining
specimens were measured for surface hardness at 5 points by a
Vickers hardness tester. All the values (Hv) distributed between
1,200 and 1,250.
Example 4
[0057] SUS304 plates were set in the Muffle furnace employed in
Example 1, NH.sub.3 gas and N.sub.2 gas were fed at flow rates of
200 L/H, respectively, and the furnace atmosphere was heated from
room temperature to 550.degree. C. in 75 minutes. At the time point
that the atmosphere temperature had reached 100.degree. C. in the
course of the heating (at the 18.sup.th minute after the initiation
of the heating), an injection of C.sub.2H.sub.4 gas was initiated
at 5 L/hr. After heated to 550.degree. C., the atmosphere
temperature was maintained for 2 hours. At that time point, the
injection of C.sub.2H.sub.4 gas was terminated and instead,
NH.sub.3 gas and N.sub.2 gas were then fed at 550.degree. C. for 4
hours to allow nitriding to proceed. Subsequently, the heating was
stopped and N.sub.2 gas alone was continuously fed to cool down the
furnace. When the atmosphere temperature had dropped to 100.degree.
C. or lower, the specimens were taken out of the furnace.
[0058] Effluent gas from the furnace was branched off to have a
portion of the effluent gas absorbed in a 2 wt. % aqueous solution
of caustic soda, and an analysis was performed for HCN. From the
analysis results of the HCN-absorbed solution, the average HCN
concentration in the furnace atmosphere during the C.sub.2H.sub.4
gas injection period was 1,200 mg/m.sup.3. Some of the SUS304
specimens were weighed to determine a weight increase after the
nitriding treatment. As a result, the weight increase was
determined to be 18 g/m.sup.2. Some of the SUS304 specimens were
cut, and their cut surfaces were polished, etched with Marble's
solution, and then observed under an optical microscope. Nitrided
layers of 45-.mu.m uniform thickness were found to be formed. Some
of the remaining specimens were measured for surface hardness at 5
points by a Vickers hardness tester. All the values (Hv)
distributed between 1,200 and 1,250.
Example 5
[0059] SUS304 plates were set in the Muffle furnace employed in
Example 1, NH.sub.3 gas and N.sub.2 gas were fed at flow rates of
200 L/H, respectively, and the furnace atmosphere was heated from
room temperature to 550.degree. C. in 75 minutes. At the time point
that the atmosphere temperature had reached 100.degree. C. in the
course of the heating (at the 18.sup.th minute after the initiation
of the heating), an injection of C.sub.4H.sub.10 gas was initiated
at 5 L/hr. After heated to 550.degree. C., the atmosphere
temperature was maintained for 2 hours. At that time point, the
injection of C.sub.4H.sub.10 gas was terminated and instead,
NH.sub.3 gas and N.sub.2 gas were then fed at 550.degree. C. for 4
hours to allow nitriding to proceed. Subsequently, the heating was
stopped and N.sub.2 gas alone was continuously fed to cool down the
furnace. When the atmosphere temperature had dropped to 100.degree.
C. or lower, the specimens were taken out of the furnace.
[0060] Effluent gas from the furnace was branched off to have a
portion of the effluent gas absorbed in a 2 wt. % aqueous solution
of caustic soda, and an analysis was performed for HCN. From the
analysis results of the HCN-absorbed solution, the average HCN
concentration in the furnace atmosphere during the C.sub.4H.sub.10
gas injection period was 600 mg/m.sup.3. Some of the SUS304
specimens were weighed to determine a weight increase after the
nitriding treatment. As a result, the weight increase was
determined to be 18 g/m.sup.2. Some of the SUS304 specimens were
cut, and their cut surfaces were polished, etched with Marble's
solution, and then observed under an optical microscope. Nitrided
layers of 45-.mu.m uniform thickness were found to be formed. Some
of the remaining specimens were measured for surface hardness at 5
points by a Vickers hardness tester. All the values (Hv)
distributed between 1,200 and 1,250.
Comparative Example 1
[0061] SUS304 plates were set in the Muffle furnace employed in
Example 1, NH.sub.3 gas and N.sub.2 gas were fed at flow rates of
200 L/H, respectively, and the furnace atmosphere was heated from
room temperature to 550.degree. C. in 75 minutes. After heated to
550.degree. C., the atmosphere temperature was maintained for 6
hours. NH.sub.3 gas and N.sub.2 gas were continuously fed to allow
nitriding to proceed. Subsequently, the heating was stopped and
N.sub.2 gas alone was continuously fed to cool down the furnace.
When the atmosphere temperature had dropped to 100.degree. C. or
lower, the specimens were taken out of the furnace.
[0062] Effluent gas from the furnace was branched off to have a
portion of the effluent gas absorbed in a 2 wt. % aqueous solution
of caustic soda, and an analysis was performed for HCN. From the
analysis results of the HCN-absorbed solution, HCN was not detected
at all, thereby ascertaining that HCN did not exist at all in the
furnace atmosphere. Some of the SUS304 specimens were weighed to
determine a weight increase after the nitriding treatment. As a
result, the weight increase was determined to be 10 g/M.sup.2. Some
of the SUS304 specimens were cut, and their cut surfaces were
polished, etched with Marble's solution, and then observed under an
optical microscope. Nitrided layers of uneven thicknesses of from 8
to 18 .mu.m were found to be formed (a 500.times. micrograph is
shown in FIG. 3). Some of the remaining specimens were measured for
surface hardness at 5 points by a Vickers hardness tester. The
values (Hv) considerably varied from 500 to 1,100, and their
absolute values were found to be lower compared with the
corresponding values of the examples.
INDUSTRIAL APPLICABILITY
[0063] A passivated film on a surface of a high-alloy steel member
makes it difficult to apply diffusion treatment, such as gas
nitriding or gas carburizing, that forms a nitrided layer,
carburized layer or carbonitrided layer on the surface of the steel
member. According to the present invention, an activating treatment
method is provided for the surface of the metal member. This method
is not accompanied by problems of conventional activation treatment
with a halide, such as furnace deposits, furnace wall erosion and
effluent gas detoxification treatment, and is useful as
pretreatment for diffusion treatment. According to this method, the
passivated surface of the high-alloy steel member can be activated
by using a gas commonly employed in gas heat treatment, and forming
HCN gas in a heating furnace while making use of catalytic action
of the steel member or a surface of the furnace.
BRIEF DESCRIPTION OF THE DRAWINGS
[0064] [FIG. 1] A diagram illustrating the construction of a
treatment furnace useful in the present invention.
[0065] [FIG. 2] A micrograph of a cut surface of a specimen of
Example 1.
[0066] [FIG. 3] A micrograph of a cut surface of a specimen of
Comparative Example 1.
[0067] 1: Muffle furnace [0068] 2: Outer [0069] 3: Heater [0070] 4:
Internal container (retort) [0071] 5: Gas inlet pipe [0072] 6:
Exhaust pipe [0073] 7: Motor [0074] 8: Fan [0075] 9: Metal-made jig
[0076] 10: Gas guide cylinder [0077] 11: Inverted funnel [0078] 12:
Vacuum pump [0079] 13: Effluent gas combustion facility [0080] 14:
Carbon donor compound gas cylinder [0081] 15: Ammonia gas cylinder
[0082] 16: Nitrogen gas cylinder [0083] 17: Hydrogen gas cylinder
[0084] 18: Flow rate meter [0085] 19: Gas control valve
* * * * *