U.S. patent application number 14/558099 was filed with the patent office on 2015-04-02 for plasma process and reactor for treating metallic pieces.
The applicant listed for this patent is UNIVERSIDADE FEDERAL DE SANTA CATARINA (UFSC), Whirlpool SA. Invention is credited to Cristiano Binder, Roberto Binder, Gisele Hammes, Aloisio Nelmo Klein.
Application Number | 20150090403 14/558099 |
Document ID | / |
Family ID | 41119449 |
Filed Date | 2015-04-02 |
United States Patent
Application |
20150090403 |
Kind Code |
A1 |
Binder; Roberto ; et
al. |
April 2, 2015 |
Plasma Process and Reactor for Treating Metallic Pieces
Abstract
The plasma reactor defines a reaction chamber provided with a
support for the metallic pieces and an anode-cathode system, and a
heating means is mounted externally to said plasma reactor. The
plasma process, for a cleaning operation, includes the steps of
connecting the support to the grounded anode and the cathode to a
negative potential of a power source; feeding an ionizable gaseous
charge into the reaction chamber and heating the latter at
vaporization temperatures of piece contaminants; applying an
electrical discharge to the cathode; and providing the exhaustion
of the gaseous charge and contaminants. A subsequent heat treatment
includes the steps of: inverting the energization polarity of the
anode-cathode system; feeding a new gaseous charge to the reaction
chamber and maintaining it heated; applying an electrical discharge
to the cathode; and exhausting the gaseous charge from the reaction
chamber.
Inventors: |
Binder; Roberto; (Joinville,
BR) ; Klein; Aloisio Nelmo; (Joinville, BR) ;
Binder; Cristiano; (Florianopolis, BR) ; Hammes;
Gisele; (Florianopolis, BR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Whirlpool SA
UNIVERSIDADE FEDERAL DE SANTA CATARINA (UFSC) |
Sao Paulo
Florianopolis |
|
BR
BR |
|
|
Family ID: |
41119449 |
Appl. No.: |
14/558099 |
Filed: |
December 2, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12737125 |
Jan 18, 2011 |
8926757 |
|
|
PCT/BR2009/000165 |
Jun 9, 2009 |
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14558099 |
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Current U.S.
Class: |
156/345.37 |
Current CPC
Class: |
H01J 2237/335 20130101;
H01J 37/32522 20130101; B22F 2998/00 20130101; H01J 37/32568
20130101; B22F 2998/00 20130101; H01J 37/32834 20130101; H01J
37/32623 20130101; H01J 37/3244 20130101; B22F 3/11 20130101 |
Class at
Publication: |
156/345.37 |
International
Class: |
H01J 37/32 20060101
H01J037/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 11, 2008 |
BR |
PI0803774-4 |
Claims
1. Plasma reactor for treating metallic pieces and comprising a
metallic casing defining, internally, a reaction chamber provided
with: a support; an anode-cathode system associated with an
electric power source; an ionizable gaseous charge inlet; and a
gaseous charge exhaustion outlet connected to a vacuum system,
characterized in that it comprises a heating means mounted
externally to the metallic casing in order to heat the latter and
the interior of the reaction chamber.
2. Reactor, according to claim 1, characterized in that the heating
means transfers heat to the interior of the reaction chamber by
radiation from the metallic casing.
3. Reactor, according to claim 2, characterized in that the heating
means is formed by at least one resistor in thermal contact with
the metallic casing.
4. Reactor, according to claim 3, characterized in that the support
comprises multiple parallel and spaced apart ordering structures
electrically coupled to the same electrode of the anode-cathode
system and which are intercalated by conducting elements coupled to
the other electrode of the anode-cathode system, each of said
ordering structures carrying at least one metallic piece to be
treated.
5. Reactor, according to claim 4, characterized in that the
direction of the heat radiation produced from the metallic casing
to the interior of the reaction chamber occurs according to a
direction parallel to that of the ordering structures of the
support.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a divisional of and claims priority to
U.S. patent application Ser. No. 12/737,125, filed on Jan. 18,
2011, the contents of which are incorporated herein by reference in
their entirety.
FIELD OF THE INVENTION
[0002] The present invention refers to a plasma process and reactor
for the treatment of metallic pieces, particularly porous metallic
pieces obtained by powder metallurgy, said treatment comprising a
cleaning operation with dissociation and removal of oil and other
organic and inorganic contaminants existing on the surface or in
the pores of metallic pieces, and generally also an operation of
thermochemically treating the surface of said metallic pieces,
which operations are effected in a plasma reactive environment and,
preferably, in the interior of the same reactor.
BACKGROUND OF THE INVENTION
[0003] In most of the cases, the pieces produced by powder
metallurgy need to be calibrated after the sintering step due to
the dimensional variations that occur during sintering. Lubricant
oil is used in the calibration to reduce friction and wear of the
machine tools, as well as to facilitate extraction of the pieces
from the calibration matrix. Oil is likewise used for storing
sintered pieces and pieces produced by other manufacturing
technique. For example, refrigerant oil is used for machining high
precision pieces.
[0004] Aiming at improving the properties of the finished pieces,
such as wear resistance, corrosion resistance and fatigue
resistance, there are often used surface thermochemical treatments,
such as nitration, cementation, carbonitration, etc. In order to
effect these thermochemical treatments, the presence of oil on the
surface and in the pores of the pieces is prejudicial, especially
when the thermochemical processing is effected via plasma. For
example, during plasma nitration, the oil retained in the pores and
on the surface of the pieces produces instabilities in the
electrical discharge, contamination of the reactor, inadequate
formation of the superficial layers formed (for example, nitrates)
and contamination with carbon of the material submitted to
treatment by means of an inefficient cleaning. Thus, the oil must
be completely removed before the thermochemical treatments of
surface hardening.
[0005] Conventionally, a chemical cleaning is carried out in
ultrasound with organic solvents (for example hexane, petroleum
ether or alcohol) further followed by a heat treatment in
atmosphere containing hydrogen or oxygen in an industrial electric
oven, aiming at eliminating completely all the organic residues
from the pieces. When communicating residual pores are present,
which generally occurs in sintered steels, the cleaning is
especially difficult, besides being pollutant due to the pollutant
products used.
[0006] In some known treating methods, the operations of cleaning
and thermochemically treating the surfaces are carried out in two
separate steps in distinct equipment, which requires a very long
processing time, typically 20 hours, leading to low productivity
and high cost.
[0007] With the purpose of obtaining a complete removal of the oil
and other organic and inorganic contaminants from the surface or
pores of the metallic pieces, and also simplifying and abbreviating
a subsequent surface thermochemical treatment operation of said
pieces in the same thermal cycle, there has been proposed the
process of cleaning and surface treatment object of Brazilian
patent application PI-0105593-3, of the same applicant, according
to which the pieces to be cleaned are positioned on a support
provided inside the plasma reactor and connected to an anode of the
latter, the cathode of said reactor being connected to a negative
potential. The assembly defined by the support and pieces is
surrounded by an ionized gas at low pressure and containing ions,
neutral atoms and electrons, known as plasma and which is generated
by an abnormal electrical discharge. The electrons provoke an
electronic bombardment on the assembly defined by the support and
pieces and connected to the reactor anode.
[0008] The generation of gaseous plasma in the interior of the
reactor allows the plasma reactive environment formed around the
pieces to be used to catalyze the reaction of dissociating the
molecules of the oil and of other possible contaminants existing in
the pieces, allowing the vaporization of said contaminants and the
complete elimination thereof through exhaustion, under vacuum, from
the inside of the reactor. The heat generated by the plasma, by the
collision of fast ions and neutral atoms against the cathode, is
usually sufficient to provide vaporization of the molecularly
dissociated oil, without requiring relevant changes in the plasma
parameters more adequate to catalyze the reactions of interest in
each cleaning operation.
[0009] However, in plasma reactors of certain dimensions (and in
certain chemical reactions of molecular dissociation of the
contaminants), it may occur that different inner regions of the
reactor remain at a temperature which is sufficiently low to allow
condensation of the contaminant vapors prior to the dissociation
and progressive deposition thereof in these relatively cold inner
regions of the plasma reactor, contaminating the system with
carbon-based compounds that are harmful to the subsequent surface
treatments.
[0010] Moreover, in many surface thermochemical treatment
operations occurring subsequently to the cleaning operation and
carried out inside the same reactor, the heat generated by the
plasma is not enough to maintain the heating rate and the process
temperature required to obtain the desired surface treatment.
[0011] Thus, even if the surface treatment operation has been
executed, as disclosed in said prior patent application, by
reverting the electrical circuit between the anode and the cathode,
and also by surrounding the pieces with gaseous plasma of ions with
high kinetic energy, and by applying an electrical discharge to the
cathode so as to provoke an ionic bombardment on the pieces, there
is a need to adjust the plasma parameters, not as a function of the
reactions of interest, but aiming to obtain temperature levels
inside the plasma reactor that are sufficient and necessary for the
desired surface treatment. In this case, the temperature variations
required inside the reactor are obtained as a function of the
electrical discharge parameters, but some situations may exist in
which the intensity of the electrical discharge required for the
production of determined temperatures leads to the formation of
electrical arcs in the reaction environment, causing superficial
damages (marks on the pieces) and contamination by carbon deposits
on the surfaces of the pieces, impairing the subsequent
thermochemical treatments, besides the fact that the thermal
gradient negatively influences the formation and homogeneity of the
formed layer.
[0012] The provision of a resistive heating in plasma reactors is
known in the art.
[0013] In one of these known reactors, there is provided an
external resistive heating for removing the binders and possible
contaminants from the pieces obtained by sintering. However, in
this known construction, the pieces to be submitted to a treatment
for removing binders and contaminants are applied to the reactor
cathode, leading to the formation of electric arcs and consequent
contamination of the pieces with carbon, which is harmful to the
subsequent surface treatments.
[0014] In another known type of reactor disclosed in patent U.S.
Pat. No. 6,579,493, there is provided an inner resistive heating to
obtain high temperatures sufficient to remove the binders and
certain contaminants from the metallic pieces obtained by powder
metallurgy and also to provide sintering of the pieces.
Nevertheless, the provision of resistive heating in the interior of
this type of reactor requires the use of high cost materials, such
as molybdenum and the provision of heat radiation reflecting
elements between the inner resistance and the reactor wall, and
also the provision of cooling in said outer wall. This solution is
inadequate for cleaning the organic contaminants from the pieces
under treatment, since it allows the volatile vapors of oils and
other contaminants to be condensed and deposited on the cold
regions of the heat radiation reflecting elements and on the
reactor wall, before they are exhausted from the reaction
environment, contaminating the latter and the pieces contained
therein with carbon-based compounds that impair the subsequent
surface thermochemical treatments of the pieces.
[0015] From the above, there is a need for the provision of a
solution which allows obtaining, in the interior of the reactor,
homogeneous and even high temperatures as a function of the desired
surface treatment, in a way that is independent from the electrical
discharge parameters that are more adequate for catalyzing the
reactions of interest in each case.
SUMMARY OF THE INVENTION
[0016] Thus, it is an object of the present invention to provide a
plasma process and reactor for treating metallic pieces by means of
gaseous plasma and at temperatures that are generated and
controlled in a way totally independent from the plasma generation
parameters.
[0017] It is another object of the present invention to provide a
plasma process and reactor, as mentioned above and which allow the
plasma generation parameters to be maintained in levels that are
sufficient and adequate for catalyzing the reactions of interest,
without leading to the formation of electrical arcs in the reaction
environment.
[0018] It is a further object of the present invention to provide a
plasma process and reactor, as mentioned above and which allow a
cleaning operation to be carried out through molecular dissociation
through gaseous plasma and through vaporization and exhaustion of
the dissociated contaminants, by maintaining the interior of the
plasma reactor at temperatures higher than those of condensation of
said contaminants.
[0019] It is also another object of the present invention to
provide a plasma process and reactor, as mentioned above and which
allow the operation of thermochemically treating the surface of the
metallic pieces to be carried out, also by gaseous plasma, in the
same reactor in which the cleaning operation is effected and at
temperatures that are obtained and controlled by preferably
resistive heating.
[0020] These and other objects are attained through a plasma
process for treating metallic pieces in a plasma reactor defining a
reaction chamber provided with: a support; an anode-cathode system
associated with an electrical power source; an ionizable gaseous
charge inlet; and a gaseous charge exhaustion outlet connected to a
vacuum system.
[0021] The plasma process for treating metallic pieces of the
present invention comprises the following cleaning steps: a)
connecting the support to the grounded anode and the cathode to a
negative potential of the electric power source; b) positioning the
metallic pieces on the support in the interior of the reaction
chamber; c) surrounding the support and the metallic pieces with an
ionizable gaseous charge fed into the reaction chamber; d) heating
the interior of the reaction chamber, from the outer side of the
plasma reactor, at vaporization temperatures of contaminants to be
dissociated from the metallic pieces being treated in the interior
of the reaction chamber; e) applying an electrical discharge to the
cathode, in order to provoke the formation of a gaseous plasma of
ions with high kinetic energy surrounding the metallic pieces and
the support, and a bombardment of electrons in the metallic pieces
for molecular dissociation of the contaminants; f) providing the
exhaustion of the gaseous charge and of the contaminants maintained
in gaseous state, from the interior of the reaction chamber. In the
cases in which the metallic pieces are submitted, after cleaning,
to a thermochemical treatment, the plasma process for treating
metallic pieces of the present invention comprises, after step "f"
of the cleaning operation, the further steps of thermochemically
treating the surface of the metallic pieces, in the same reactor,
said steps comprising: g) inverting the energization polarity of
the anode-cathode system, so that the support, with the metallic
pieces, defines the cathode; h--surrounding the support and the
metallic pieces with a new ionizable charge fed into the reaction
chamber; i) maintaining the interior of the reaction chamber
heated, from the outer side of the plasma reactor, and conducting
the temperature therein to the levels required in the desired
surface thermochemical treatment; j) applying an electrical
discharge to the cathode, so as to provoke the formation of a
gaseous plasma of ions surrounding the metallic pieces and the
support, as well as an ionic bombardment in the metallic pieces;
and k) providing the exhaustion of the gaseous charge from the
interior of the reaction chamber.
[0022] The present invention also presents a plasma reactor for
treating metallic pieces and in which the process steps described
above are carried out, said reactor presenting a metallic casing
defining, internally, a reaction chamber, as already described, and
a heating means mounted externally to the metallic casing, in order
to heat the latter and the interior of the reaction chamber.
According to one aspect of the present invention, the heating means
is formed by at least one resistor in thermal contact with the
metallic casing.
[0023] According to a particular aspect of the present invention,
the support comprises multiple parallel and spaced apart ordering
structures which are electrically coupled to the same electrode of
the anode-cathode system and intercalated by conducting elements
coupled to the other electrode of the anode-cathode system, each of
said ordering structures carrying at least one metallic piece to be
treated. In a preferred construction, the metallic casing portions,
producing heat radiation to the inside of the reaction chamber, are
disposed according to a direction orthogonal to the mounting
direction of the ordering structures.
[0024] Even in the cases in which the surface treating operations
to be effected inside the reactor require temperatures that are not
so high as those required in sintering operations and which reach
values of about 1100 degrees C., the positioning of the heating
means externally to the metallic casing of the reactor allows the
latter to present a simpler and cleaner inner construction,
avoiding the formation of intermediary regions, between the heating
means and said casing, which are susceptible to the capture and
subsequent deposition of contaminants.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The invention will be described below, with reference to the
attached drawings, given by way of example of embodiments of the
invention and in which:
[0026] FIG. 1 represents, schematically, a plasma reactor
constructed according to the present invention, illustrating some
metallic pieces provided on a support mounted in the interior of
said plasma reactor; and
[0027] FIG. 2 represents a simplified and rather schematic vertical
section view of a plasma reactor constructed according to the
present invention and housing, inside the reaction chamber, a piece
support comprising a plurality of horizontal ordering
structures.
DESCRIPTION OF THE INVENTION
[0028] As mentioned above and as illustrated in the appended
drawings, the invention relates to a plasma process and reactor for
treating metallic pieces 1, said process being carried out in a
plasma reactor 10 comprising a metallic casing 20, having an
ionizable gaseous charge inlet 21e and a gaseous charge exhaustion
outlet 22, said metallic casing 20 defining, internally, a reaction
chamber 23 inside which is usually positioned a support and an
anode-cathode system 40, associated with an electric power source
50 external to the metallic casing 20. A reaction chamber 23 is
coupled to a vacuum system 60 connected to the outlet 22 of the
metallic casing 20. The reaction chamber 23 is maintained hermetic
for plasma generation therewithin, the inlet 21 is hermetically
coupled to an ionizable gas supply source (not illustrated) and the
outlet 22 is hermetically coupled to the vacuum system 60.
[0029] The metallic casing 20 is preferably formed in refractory
steel (as, for example stainless steel AISI 310 or 309) and the
support 30 in refractory steel (as, for example stainless steel
AISI 310 or 309), but other type of material can be used, depending
on the adequate process temperatures.
[0030] The metallic casing 20 presents a prismatic shape, for
example, a cylinder, having wall extensions 20a which, in the
cylindrical shape, comprises a surrounding side wall and an upper
end wall 20b.
[0031] The metallic casing 20 is inferiorly open so as to be
removably and hermetically seated and locked on a base structure B
to which are adequately mounted component parts operatively
associated with the reactor and which will be described ahead.
[0032] The plasma reactor 10 of the present invention further
comprises a heating means 70 mounted externally to the plasma
reactor 10, that is, to its metallic casing 20, in order to heat
the latter and the interior of the reaction chamber 23, for
example, producing heat radiation from the metallic casing 20 to
the interior of the reaction chamber 23.
[0033] The plasma reactor 10 is also externally provided with an
outer cover 11, generally made of carbon steel coated with an
adequate heat insulating means (aluminade and silicade fibers, for
example) presenting an adequate shape so as to surround laterally
and superiorly the assembly defined by the metallic casing 20 and
by the heating means 70, defining a heating chamber 13 around the
metallic casing 20 and inside which is positioned the heating means
70.
[0034] The heating means 70 is generally formed by at least one
resistor 71 mounted in thermal contact with the metallic casing 20,
inside the heating chamber 13 defined between the metallic casing
20 and the outer cover 11. According to a way of carrying out the
present invention, it is further provided a ventilation system 80
comprising at least one air circulating means 81 generally
positioned external to the outer cover 11 and provided with at
least one suction nozzle 81a and at least one discharge nozzle 81b
that are opened to the interior of the heating chamber 13, said air
circulating means 81 being able to produce a circulating airflow in
at least part of the interior of the heating chamber 13 and through
the suction and discharge nozzles 81a, 81b. In case the air
circulating means 81 is mounted externally to the heating chamber
13, the suction and discharge nozzles 81a, 81b take the form of
tubular extensions opened to the interior of the heating chamber
13.
[0035] The ventilation system 80 can further comprise at least one
air exchanging means 82, generally with a construction similar to
that of the air circulating means 81 and also positioned externally
to the outer cover 11. The air exchanging means 82 is provided with
at least one suction nozzle 82a and at least one discharge nozzle
82b opened to the interior of the heating chamber 13, said air
exchanging means 82 being also connected to an air admission duct
83, generally opened to the atmosphere, and to an air exhaustion
duct 84, generally opened to the atmosphere. The air exchanging
means 82 can be constructed in any adequate manner known in the art
so as to provide a controlled supply of atmospheric air to the
interior of at least one respective region of the heating chamber
13, while it extracts and expels to the atmosphere, through the air
exhaustion duct 84, a corresponding amount of heated air removed
from at least one respective inner region of the heating chamber
13, allowing effecting a certain heating degree of the inner
regions of the heating chamber 13.
[0036] The intensity of the air circulation or air exchange within
the heating chamber 13 can be achieved by different ways, such as,
for example, by varying the operational speed of a ventilating
means, not illustrated, or by varying the positioning of the inner
deflecting means, also not illustrated.
[0037] In the illustrated embodiment, there are provided multiple
air circulating means 81 which also perform the function of air
exchanging means 82, said means being constructed with ventilating
and deflecting means adequately controlled to allow performing any
one of the functions of air circulation and air exchange mentioned
above.
[0038] In the construction illustrated in the attached drawings,
the reaction chamber 23 is provided, superiorly, with an inlet 21
positioned in the vertical axis of the metallic casing 20 of the
plasma reactor 10, in order to homogenously distribute the
ionizable gaseous charge from said inlet 21. In this construction,
the support 30 is formed by a plurality of ordering structures 31
that are horizontally or substantially horizontally disposed, thus
defining piece support or mounting planes that are orthogonal to
the direction in which the gaseous charge is fed through the inlet
21, said ordering structures 31 having through openings to allow
the gaseous charge to reach the pieces mounted in the ordering
structures 31 that are more distant from the inlet 21.
[0039] The ionizable gaseous charge is admitted to and exhausted
from the reaction chamber 23 by command of control valves, not
illustrated, which are automatically driven, for example, commanded
by a control unit or other specific control means (not
illustrated), but said control valves can be manually driven. While
not illustrated either, the switching of the anode-cathode system
can be carried out automatically or not, it being observed that
these means for controlling the valves and the anode-cathode system
are particular aspects that do not restrict the concept presented
herein.
[0040] In the embodiment illustrated and as mentioned above, the
support 30 comprises multiple parallel and spaced apart ordering
structures 31 electrically coupled to the same electrode 41 of the
anode-cathode system 40 and which are interposed by conducting
elements 42 coupled to the other electrode 41 of the anode-cathode
system 40, each of said ordering structures 31 carrying at least
one metallic piece 1 to be treated.
[0041] The conducting elements 42 coupled to the other electrode
are positioned in the interior of the reaction chamber 23, between
the ordering structures 31, by using any adequate support structure
that can be defined by structural columnar elements 32 of the
support 30 itself that carries the ordering structures 31, it being
only necessary to mount said conducting elements 42 electrically
insulated in relation to the structure of the support 30 provided
with the respective ordering structures 31.
[0042] According to the present invention and as exemplarily
illustrated, the heating means 70 is arranged so as to heat
adjacent wall extensions of said metallic casing 20 and extending
according to a direction, usually that coinciding with the
direction of the height of the reaction chamber 23 and which is
orthogonal to the mounting planes of the ordering structures 31.
With this arrangement, the heat radiated from said wall extensions
of the metallic casing 20 to the interior of the reaction chamber
23 follows a direction parallel to the mounting direction of the
ordering structures 31, making more efficient the distribution,
among the ordering structures 31, of the heat radiated from said
wall extensions of the metallic casing 20.
[0043] The anode-cathode system 40 has its electrodes 41 defined by
the anode and cathode of said energizing system. During the
cleaning operation of the plasma process of the invention, the
electrode 41, which defines the anode of the anode-cathode system
40, is coupled to the ordering structures 31 of the support 30, in
which the metallic pieces 1 are positioned, said electrode 41 being
grounded, whereas the other electrode 41 which defines the cathode
of the anode-cathode system is electrically coupled to the electric
power source 50.
[0044] In the plasma processes in which after the cleaning
operation there is effected, in the same reactor, processes of
thermochemical treatment, the electrode 41, which defined the anode
of the anode-cathode system, is coupled to the power source 50,
whereas the other electrode 41 is grounded.
[0045] As described below, the present invention allows performing
the cleaning and the thermochemical treatment (for example,
nitration, carbonitration, cementation, oxynitration,
oxycarbonitration and others), in which both steps are aided by
plasma, in the same equipment and in the same thermal cycle and in
which the anode-cathode system is confined in the interior of the
reaction chamber 23 in order to allow plasma generation and
consequently utilize plasma reactive environment to catalyze the
reaction of dissociating the contaminant molecules found in the
metallic pieces 1, such as oil organic molecules in the cleaning
operation.
[0046] During the cleaning operation, the metallic casing 20 is
heated jointly with the gaseous charge, which is also submitted to
a certain heating degree inside the reaction chamber 23 upon plasma
generation. The formation of the gaseous plasma of ions contributes
to the heating of the interior of the reaction chamber 23 and to
the vaporization of the contaminants being dissociated, both in the
cleaning operation and in the thermochemical treating
operation.
[0047] Besides contributing to the vaporization of contaminants in
the interior of the reaction chamber 23, the external heating of
the latter allows the gaseous plasma formed therewithin to be
obtained with less energy consumption. The resistive heating,
provided externally to the reaction chamber 23, avoids the
existence of cold walls in the interior of the latter, that is, in
the environment in which the metallic pieces 1 are subject to the
plasma treating process. It is necessary to avoid the existence of
cold walls in the interior of the reaction chamber 23, for example,
in the initial phase of heating the metallic pieces 1, since the
oil evaporated from the pieces being treated tends to deposit on
the not sufficiently heated inner regions of the reaction chamber
23.
[0048] Thus, the additional and generally resistive external
heating avoids the existence of walls or regions of the reaction
chamber 23 presenting temperatures lower than those of vaporization
of the contaminants, that is, of the vaporized oil, impeding the
condensation and deposition of contaminants in these cooler regions
of the reaction chamber 23, before said contaminants are exhausted,
by suction, through the vacuum system 60, through the outlet 22 of
the metallic casing 20.
[0049] For the cleaning operation, the metallic pieces 1 to be
processed are positioned on the ordering structures 31 of the
support 30 mounted inside the reaction chamber 23, electrically
insulated from its metallic casing 20. During the cleaning
operation, the support 30 defines the anode of the anode-cathode
system 40, which anode is grounded, whereas the conducting elements
42 are connected to an outlet of the electric power source 50, in
negative potential, acting as the cathode of the electrical
discharge. The interior of the reaction chamber 23 is maintained at
a sub-atmospheric pressure and with desired values for the
formation of plasma in the cleaning operation, by using the vacuum
system 60. A charge of ionizable gases is fed into the reaction
chamber 23, through inlet 21 of the metallic casing 20, before
providing the electrical discharge in the cathode of the
anode-cathode system 40.
[0050] In a way of carrying out the present invention, the
ionizable gaseous charge, in the cleaning operation, comprises
hydrogen, and it can also comprise a gaseous mixture containing
hydrogen and at least one of the gases consisting of argon,
nitrogen, or a mixture comprising oxygen and other gases, as for
example, nitrogen. The selection of the process gases will depend
on the nature of the substance to be eliminated from the metallic
piece (for example, oil).
[0051] For example, the gaseous charge will comprise:
[0052] hydrogen, when the contaminants to be removed from the
metallic pieces present reactivity with hydrogen, or are based on
hydrocarbon chains that are dissociated in gases based on carbon
and hydrogen (methane (CH.sub.4), for example);
[0053] oxygen, when the contaminants to be removed from the
metallic pieces present reactivity with oxygen, or are based on
hydrocarbon chains that are dissociated in gases based on carbon
and oxygen (carbon dioxide (CO.sub.2), for example);
[0054] mixtures of argon and hydrogen or oxygen, when a higher
electron density is desired for dissociating the contaminants to be
removed from the metallic pieces; and
[0055] mixtures of nitrogen and argon or hydrogen or oxygen, when
the contaminants to be removed from the metallic pieces have their
dissociation facilitated by the mixture of said gases.
[0056] The dissociation of other contaminant bases is also possible
with the principle of the present invention.
[0057] The main principle of the cleaning operation consists in
dissociating the oil molecules by electron bombardment, resulting
in lighter molecules or gaseous radicals which are eliminated from
the reaction chamber 23 by exhausting the gaseous charge and
contaminants from the inside thereof. In a way of carrying out the
present invention, the exhaustion occurs under vacuum, via
bombardment through the vacuum system 60, producing an efficient
cleaning of the pieces, as well as maintaining the interior of the
reaction chamber 23 deprived of oil deposits and other contaminant
products, mainly the organic ones, the cleaning operation being
effected at low temperatures, in the range of from about 30 degrees
C. to 500 degrees C., depending on the nature of the contaminants
to be eliminated.
[0058] In such embodiment, the support 30 and the metallic pieces 1
to be treated are surrounded by the plasma generated with the
electrical discharge and bombarded mainly by electrons generated in
the plasma. The second electrode 41, which in the cleaning step
receives the electrical discharge and actuates as the cathode, is
bombarded mainly by ions and consequently heated. As the heat
produced in the cathode warms the metallic pieces 1, for the
cleaning operation, the heating means 70, external to the reaction
chamber 23, supplies the additional amount of heat necessary to
obtain the heating rate and temperature required to avoid
condensation of the contaminants on the inner walls of the reaction
chamber 23, said heating rate and temperature being programmed
independently of the plasma parameters. These plasma parameters are
adjusted or programmed so as to catalyze the reaction of
dissociating the molecules of the contaminant, such as for example,
oil. As already mentioned, the formation of gaseous plasma can also
contribute with part of the heating of the interior of the reaction
chamber 23 required to avoid condensation of contaminants on the
inner walls of the reaction chamber 23.
[0059] The use of a heating means 70, external to the reaction
chamber 23, presents the advantage of allowing a homogeneous
temperature to be obtained in the interior of the reaction chamber
23, as well as avoiding the deposition of vapors and soot resulting
from the plasma reaction in the metallic pieces 1 inside the plasma
reactor 10. Another advantage, resulting from the geometry used in
the confined anode-cathode system 40 is that the species generated
in the plasma surround, completely, the metallic pieces 1, leading
to an efficient removal of the contaminants, such as oil, from the
metallic pieces 1.
[0060] The dissociation of the oil molecules produces lighter
radicals and molecules, which maintain the gaseous physical state
at the working temperature and are pumped outwardly from the plasma
reactor 10 through the vacuum system 60.
[0061] The contaminant vapor is discharged from the reaction
chamber 23 jointly with the other gases produced in the plasma
operation, upon completion of the cleaning operation of the
metallic pieces 1. Since there are no residues inside the reaction
chamber 23, because the oil and other contaminants are completely
eliminated by the molecular dissociation activated by the active
species generated in the plasma, the load of metallic pieces 1 can
be treated inside the same plasma reactor 10, upon completion of
the cleaning operation of said metallic pieces 1, by raising the
temperature in the interior of the reaction chamber 23 to values
compatible with those required in a determined thermochemical
treatment.
[0062] In the solution of the present invention, the plasma reactor
20 further comprises a switching system 90, which allows inverting
the polarity between the anode and the cathode of the anode-cathode
system 40, so that the metallic pieces 1, which during the cleaning
operation with dissociation of oil and contaminants are necessarily
connected to the anode, are connected to the cathode of the
anode-cathode system 40 for the thermochemical treatment by
plasma.
[0063] With the present invention, the cleaning and thermochemical
treatment operations carried out by plasma occur in the same plasma
reactor, with no need of interrupting the heating.
[0064] After the cleaning operation, with the removal of
contaminants, particularly oil, the thermochemical treatment
operation is started in the same plasma reactor 10, by introducing,
through the inlet 21 of the metallic casing 20, a charge of
ionizable gases into the interior of the reaction chamber 23, which
can be similar to the one used in the cleaning operation or contain
determined specific gases for the desired thermochemical treatment,
said new ionizable gaseous charge being fed to the interior of the
reaction chamber 23, so as to surround the support 30 and the
metallic pieces 1.
[0065] The ionizable gases of the thermochemical treatment
operation are fed into the interior of the reaction chamber 23,
after exhausting the gases and vapors of the cleaning operation
therefrom.
[0066] The polarity between the cathode and the anode is then
inverted, so that the metallic pieces 1 are connected to the
cathode and, upon generation of electrical discharge in a gas
mixture specifically defined for a determined thermochemical
treatment of the already cleaned metallic pieces, carrying out this
treatment in the same plasma reactor 10 and in the same thermal
cycle.
[0067] It should be noted that the present thermochemical treatment
process can present alteration in this sequence of steps of feeding
a charge of ionizable gases and of inverting the polarity, without
changing the result obtained.
[0068] After the admission of a new gaseous charge and the
inversion of polarity of the anode-cathode system 40, the
thermochemical treatment process further comprises the steps of:
maintaining the interior of the reaction chamber 23 heated from the
outer side of the plasma reactor 10 and conducting the temperature
therewithin to the levels required in the desired surface
thermochemical treatment; applying an electrical discharge to the
cathode, in order to provoke the formation of a gaseous plasma of
ions surrounding the metallic pieces 1 and the support 30, and an
ionic bombardment on the metallic pieces 1; and providing the
exhaustion of the gaseous charge from the interior of the reaction
chamber 23.
[0069] In the thermochemical treatment operation, the gaseous
charge supplied to the reaction chamber 23 comprises, for example:
a gaseous mixture of hydrogen and nitrogen, when the thermochemical
treatment is nitration; a gaseous mixture containing hydrogen,
nitrogen and carbon, when the surface thermochemical treatment is
nitrocarburization or carbonitration; a mixture containing
hydrogen, argon and carbon, when the surface treatment is
cementation; and a gaseous mixture containing oxygen, hydrogen,
nitrogen, argon and carbon, when the surface thermochemical
treatment is oxynitration, oxynitrocarburization or
oxycarbonitration. Other gases can be used, depending on the
desired thermochemical process.
[0070] For the thermochemical treatment operation, the support 30
is connected to the negative potential of the electric power source
50, through the electrode 41 which actuates as the cathode of the
electrical discharge, whereas the electrode 41 which had the
cathode function before is grounded, actuating as the anode of the
electrical discharge. After this inversion of polarity, that is,
with the metallic pieces 1 on the support 30 connected to the
cathode of the electrical discharge, the desired step of
thermochemical treatment by plasma is carried out in the same
plasma reactor 10 and in the same thermal cycle. The gaseous charge
to be ionized in the reaction chamber 23 is submitted and
maintained, in each of the cleaning and thermochemical treatment
operations, at a sub-atmospheric pressure of the order of
1.33.times.10.sup.1 Pascal (0.1 Torr) a 1.33.times.10.sup.4 Pascal
(100 Torr), which pressures are obtained by action of the vacuum
system 60 comprising, for example, a vacuum pump. The cleaning and
heat treatment operations utilize DC electrical discharge, which
can be delivered in an atmosphere under low pressure containing an
ionizable gas charge, as defined above, so as to produce electrons
and reactive atomic hydrogen or other species, depending on the
gases utilized for plasma generation.
[0071] The process of the present invention, including the cleaning
and thermochemical treatment operations by plasma, can be used for
metallic pieces 1 produced by powder metallurgy or by other
manufacturing processes (for example, machining, stamping, cold
extrusion, and others). The process of the present invention
promotes a cleaning of the metallic pieces 1 in the plasma reactor
10 of the present invention in a period of time of about 3 hours,
with the total time, including the cleaning operation and the
thermochemical treatment (for example, nitration) being of about 6
hours. This processing time can be changed, for longer or shorter
periods of time, as a function of the nature of the contaminant and
of the thermochemical process.
[0072] The heating means 70, external to the reaction chamber 23,
warms the inner walls of the metallic casing 20, avoiding
deposition of contaminants, such as oil drops. It is possible to
maintain the plasma stable and to dissociate the whole charge of
contaminants from the metallic pieces by the plasma generation, as
well as maintain the reaction chamber 23 deprived of oils and soot,
allowing to give continuity to the heating and to subsequently
carry out the process of thermochemical treatment with the help of
plasma in the same plasma reactor. By using the auxiliary resistive
heating system and keeping the reaction chamber 23 at about 500
degrees C., there is no occurrence of any type of deposition and
soot formation, and the electrical discharge is kept totally
stable, allowing the complete removal of contaminants, such as oil,
in approximately 3 hours, via molecular dissociation. Both the
final temperature and the cleaning time can be altered as a
function of the chemical nature of the contaminant.
[0073] After this cleaning step, in which the metallic pieces I
were connected to the anode, the polarity is inverted and the
support 30 of the metallic pieces 1 is connected to the
cathode.
[0074] After exhaustion of the gases used in the cleaning process,
the temperature in the interior of the reaction chamber is raised,
for carrying out the process of thermochemical treatment, to values
between about 350 degrees C. and about 900 degrees C.
[0075] Upon changing the charge of the gaseous mixture to, for
example, hydrogen and nitrogen and heating the reaction chamber at
temperatures between 480 degrees C. and 590 degrees C., there is
effected the plasma nitration, or other thermochemical treatment,
in the same plasma reactor 1 and in the same thermal cycle,
resulting in the reduction of the total processing time and energy
consumption, thus reducing the production cost. Other temperature
ranges are possible (temperatures higher or lower than the range
cited as an example) and that are defined as a function of the type
of thermochemical treatment to be performed.
[0076] The metallic pieces 1 were treated in the plasma reactor in
an industrial scale, in which the operations of plasma cleaning and
thermochemical treatment, such as plasma nitration, were carried
out in the same thermal cycle. Said metallic pieces 1 were analyzed
by optical and electronic microscopy, as well as by X-ray
diffraction analysis. Results show that the nitrated layer obtained
is similar to that obtained by a conventional process, that is, not
effected in a single thermal cycle and carrying out the cleaning
operation by traditional processes with organic solvents and
removing heat using other equipment. However, it is important to
point out that the total treatment time, when the treatment is made
by this novel process using plasma to remove oil, is significantly
shorter, resulting in higher productivity. Moreover, the process of
removing oil by plasma does not use pollutant reactants such as
hexane and others, which are traditionally used in the chemical
cleaning process. A further advantage of the present process is
related to the use of the confined cathode-anode system 40 which
allows smaller distances to be used among the pieces in the
support, thereby allowing the provision of a greater amount of
pieces in the same volume of the reaction chamber 23 and/or the
utilization of equipment with reduced dimensions for the same
productivity, as compared to the other known prior art systems.
Finally, the use of the same plasma reactor 10 to perform the steps
of cleaning (for example, oil removal) and thermochemical treatment
(such as nitration), by using the confined cathode-anode system
with an external resistive heating, leads to a substantial
investment reduction.
[0077] While only one way of carrying out the present invention has
been illustrated herein, it should be understood that alterations
can be made in the form and arrangement of the constitutive
elements, without departing from the constructive concept defined
in the claims that accompany the present specification.
* * * * *