[0001] The present invention relates to a method for the surface treatment of a stainless
steel workpiece. The invention further relates to the means for implementing this
method.
[0002] The principle object of the present invention is to provide a method for the surface
treatment of a stainless steel workpiece, that allows one or more slip and non-stick
surfaces to be produced on said workpiece.
[0003] In particular, an object of the aforesaid method according to the invention is to
provide an alternative to the use of materials known as "non-stick", which are applied
to the surfaces of stainless steel articles to provide them with slip and non-stick
surfaces.
[0004] Document
US 5,057,108 A discloses a method and apparatus for manufacturing a stainless steel orthopaedic
implant, comprising the steps of shot blasting with cold-worked stainless steel shots
having a nominal size af 0.1778 mm to 0.7112 mm, the shots being originally cylindrical
and becoming spherical during blasting, and electropolishing, then passivating the
substrate.
[0005] Document
US 5,531,874 A discloses an apparatus for electroetching a workpiece, the apparatus comprising two
movable cathodes arranged opposite the surfaces of the workpiece. The cathodes are
mounted on guide rails via a traverse support member, in order to move the cathodes
along the opposing surfaces of the workpiece.
[0006] For a better understanding of the present invention, it must be noted that the surface
of a stainless steel article, perfectly smooth or mirror polished, either mechanically
or electrolytically, provides a "suction" effect, i.e., an effect of high adhesion
with respect to another body in contact with the same article.
[0007] Starting from this consideration and in order to overcome this drawback, the present
invention provides a method including at least two essential steps. According to a
first essential step of surface treatment of a stainless steel workpiece, a micro-roughness
is formed on the surface of said workpiece, i.e. said surface is provided with a plurality
of micro-peaks and micro-valleys, reducing the area of contact of the worked surface
and consequently the friction with respect to another body placed in contact with
said workpiece and limiting the aforesaid "suction" effect. However, the aforesaid
surface micro-roughness of the workpiece can have consequences that at times are worse
than the aforesaid "suction" effect, as said micro-peaks, by means of their sharp
peaks, still represent an obstacle to the surface slip of the treated workpiece.
[0008] Therefore, the method according to the present invention provides a second essential
step of treatment, in which the peaks of said micro-peaks of the surface of the workpiece
are rounded, removing surface material from these peaks, improving the non-stick and
slip properties of the workpiece. It is important, in this second essential step of
the method according to the present invention, to accurately control the parameters
relating to the operating times and procedures for carrying out the same step, as
small deviations from the correct process parameters can lead to a result that is
insufficient or even contrary to expectations.
[0009] The method for the surface treatment of a stainless steel workpiece according to
the present invention is defined by appended independent claim 1. The system for implementing
the said method is defined by appended independent claim 8.
[0010] For a better understanding of the present invention, some non-limiting examples of
the operating steps, some optional, others necessary, of the aforesaid method for
the surface treatment of a stainless steel workpiece are explained below.
[0011] Firstly, preliminary analysis is conducted on the workpiece to identify the variables
and the parameters determining the choice of the processing steps of the method according
to the invention, namely: the type of steel, its surface hardness, the type of product
or food with which the surface of the worked workpiece is destined to come into contact;
after all these evaluations the operating method is decided and the production cycle
commences.
DEGREASING (optional step)
[0012] Degreasing is a step that is almost always advisable and unavoidable. This is a simple
but important step, as all the greases and various contaminating oils that normally
deposit on the workpiece during the preceding processes are eliminated. Degreasing
can be carried out in many ways and combinations: hot, cold, by immersion or spraying,
either in chemical or electrolytic form, with alkaline or acid products, or with solvents,
etc. The best solution to obtain a perfect and reliable degreasing, especially for
concave parts or parts with complicated shapes, is hot degreasing with alkaline products
using ultrasound.
[0013] In addition to the chemical action, using this treatment it is possible to obtain
an important mechanical action called cavitation, capable of penetrating all parts
of the workpiece and ensuring effective and complete cleaning. The degreasing treatment
is followed by the usual rinsing and optional drying operations.
PICKLING (optional step)
[0014] Pickling is not obligatory as it is only necessary when oxides are present on the
surface to be treated, coming from weld areas, heat treatments or from various types
of contaminants, which deposit forming a layer of surface oxide. Even if the article
is not pickled if oxides are present, the subsequent treatment, essential in the method
of the present invention, i.e., the mechanical shot peening/shot blasting treatment,
by means of the mechanical action will in any case break the layers of oxides that
have formed on the surface of the workpiece.
[0015] In some cases it is even advisable to avoid pickling, which due to its chemical action
reacts not only on the oxidized areas of the surface, but also on the whole of the
steel surface, damaging both the chrome and nickel ion, which are important elements
underlying the corrosion resistance of stainlessness steel. Therefore, this is an
operation to be carried out only where necessary or in mild form.
[0016] Pickling treatment can be carried out in many modes and combinations: hot, cold,
with alkaline or acid products, chemically and electrolytically, or mechanically as
mentioned above.
[0017] It is advisable, based on the oxide that has formed on the surface of the workpiece,
to carry out simple and quick pickling of a few minutes by immersion with acid compound
at room temperature, only to break up and dislodge the oxides; it is not necessary
to eliminate them completely as, as explained previously, in conformity with the method
according to the invention, it is obligatory to carry out the shot peening/shot blasting
step, where the residues of surface oxides will be removed mechanically.
[0018] The pickling treatment is followed by the usual washing, rinsing, and optionally
drying, operations.
VIBRATION / TUMBLING (optional step)
[0019] Vibration or tumbling is carried out with machines with vibratory, centrifugal or
centrifugal-vibratory action, with the tanks that contain abrasives of circular, rectangular,
octagonal and other shapes. The machines can operate semi-automatically, automatically,
with a continuous or discontinuous cycle. The object of the present treatment step
is to deburr, smooth, uniform and finish the surface of the workpiece in a homogeneous
manner.
[0020] The workpiece can come from raw material or from disc grinding, lapping or belt grinding.
[0021] On average, several steps are carried out with greatly variable times, using plastic
or ceramic abrasive media or other compositions, having a variety of shapes, measurements
and grit sizes, mixed with abrasive-chemical products and water to improve abrasion
and, consequently, smoothing of the workpiece.
[0022] This treatment step is also carried out, if necessary, depending on the desired finish
results to be obtained and on the condition of the part to be treated.
DEBURRING / DISC-GRINDING / BELT GRINDING / SMOOTHING (optional steps)
[0023] Also in this case, these are operations to prepare the workpiece and it is not necessary
to perform them all. In very rare cases, if the surface of the workpiece is suitable
for the subsequent step, one or more of the aforesaid steps can even be omitted.
[0024] Usually, in the manufacture of an article it is necessary to carry out various mechanical
operations which create surface imperfections. As the basis of the invention is the
formation of a perfectly uniform surface roughness on the surface of the workpiece,
it is necessary to eliminate from the same surface all the various imperfections of
bending, welding and scratches of various depths, in order to obtain a uniform and
homogeneous surface with as little as possible roughness, according to the use and
to the contact for which the workpiece was designed and to the times and steps that
will follow.
[0025] Mechanical deburring, disc-grinding, belt-grinding and smoothing operations vary
in a very wide range, differ for types of abrasive media and for number of passes
with different grit sizes, while operations can be carried out both manually or with
semi-automatic or automatic machinery.
SHOT PEENING/SHOT BLASTING (essential step)
[0026] This is the first of two essential treatments of the method according to the present
invention.
[0027] The terms shot peening, shot blasting or, in certain cases, also sanding in layman's
terms, define the type of abrasive media used; shot peening corresponds, as the name
suggests, to a round abrasive, shot blasting to a needle shaped abrasive media, while
sanding is a mixture of various products deriving from silica, which is normally used
to remove paint or clean masonry parts.
[0028] Shot abrasive usually has less cutting capacity than needle-shaped abrasive in bead
form which, at the same blasting speed, creates much deeper and more jagged roughness.
Of these two abrasives, to ensure the formation of the roughness required according
to the method of the invention, shot is the best shape (substantially round), but
for the aforesaid reasons an abrasive with an even more optimized shape, i.e. cylindrical,
has been selected. This is due to the fact that the cylindrical shape has a higher
cutting capacity and consequently also a lower processing time than round shot. Moreover,
during processing also the cylindrical shape tends to become rounded and consequently
both the rounded and cylindrical shapes can be used, as the end result is similar.
[0029] With regard to the diameter of the spherical body, respectively of the cross section
of the cylindrical body of the abrasive, this is varies greatly depending on the roughness
to be obtained, which turn varies in several process aspects. In any case, the aforesaid
diameter can vary on average from ø 0.4 mm to ø 0.8 mm.
[0030] For the chemical composition of the abrasive material there are two options: a stainless
steel abrasive, if possible an electrocast 300 series austenitic steel, or, a better
option with higher performance, once again stainless steel but from metal wire instead
of electrocast material. This latter abrasive is the most optimized, least contaminating
and has a much greater hardness and duration than electrocast abrasive.
[0031] There are many other abrasives with various shapes and compositions that could be
used, such as corundum, quartz, ceramic and glass abrasives, the last being the most
widely used. However, all these abrasives and others still are not suitable for the
method of the invention, as they are composed of various oxides, such as silicon,
sodium, zirconium oxide, etc. With the shot peening treatment, these oxides are driven
mechanically onto the treated surface, forming a layer of oxide that, as the method
according to the invention progresses, i.e., the electrolytic polishing treatment,
does not offer the possibility of removing material from the previously shot blasted
surface. The layer of oxide thus formed is a barrier layer to the passage of electrical
current, and would therefore make it necessary to re-pickle the workpiece chemically
to be able to eliminate the surface oxides and finally allow electropolishing of the
same workpiece. This process is much longer, contaminating, pointless and costly,
but above all does not guarantee the success of correct electrolytic removal and consequently
of achieving the roughness required, according to the present invention, to ensure
that the treated surface has slip and non-stick properties.
[0032] After the shape and the chemical compound of the abrasive have been established,
the machines-systems to be used must then be established: manual or automatic, vacuum
pressure, high pressure or turbine.
[0033] This relation does not consider manual high pressure systems, as they do not guarantee
continuity and, consequently, uniform roughness, and vacuum pressure systems, as they
would not be able to convey the abrasive selected according to the present invention,
especially those made of steel coming from metal wire, which would be too heavy.
[0034] Therefore, two types of system remain, both automatic: high pressure and turbine
systems, the latter being preferred. The number of turbines of the system or machine
used depends on the size of the workpiece and therefore on the size of the cabin of
the same machinery. With this machine the workpieces are hooked on special frames,
which in turn rotate first clockwise and then counter-clockwise, moving forward and
backward along the shot peening cabin, while the turbines are capable of adjusting
the angles of the abrasive jet, which is a very important aspect.
[0035] However, the machine alone is not complete for the purposes of the present invention,
as an inverter is required, naturally only for turbine machines, to be able to adjust
the revolutions of the turbines and therefore the blasting speed of the abrasive,
above all to obtain the roughness useful according to the method of the invention
and, secondly, to avoid, where possible, deforming delicate and thin workpieces. The
inverter is essential in the method according to the invention, as it makes it possible
to calibrate, with absolute certainty, the roughness required to create a non-stick
surface. In fact, in addition to the shapes and thicknesses of the workpieces to be
treated, there is another very important factor that must be kept under control: surface
hardness. At times steel laminates made of the same material, but coming from laminations
in different steel works, do not have the same hardness. Therefore, certain time or
processing parameters of the method according to the invention may require to be modified
based on the actual hardness of the material to be treated.
[0036] At the end of the shot peening/shot blasting cycle, before carrying out the subsequent
electro-polishing treatment, the parts must be blown with compressed air, either manually
or automatically, in order to eliminate all residues of abrasive and any dust.
[0037] This is followed by testing and checking with a roughness meter in several parts
of the surface of the workpiece (in at least 5 points), to check that the roughness
falls within the parameters required in conformity with the method according to the
invention.
ELECTRO-POLISHING (essential step)
[0038] The second essential step of the method according to the invention is the electro-polishing
treatment for stainless steel.
[0039] In this case, the known process is improved by means of appropriate innovations to
the plant design, equipment and operating methods, which optimally improve the efficiency
of the method according to the invention.
[0040] Electro-polishing is carried out in an electric field and the chemical compound used
is the electrical conductor. According to the present invention, to obtain the best
process result, a particular plant construction of the tank used for electro-polishing
is implemented.
[0041] There is produced an iron tank, for galvanic treatment with electrolytic cell, coated
by means of a first inner coating made of PVC, like the normal tanks currently used
for galvanic treatment, which is however modified, to make the maximum use of the
passage of current, i.e., completely lined by means of a second inner coating made
of AISI 316/L stainless steel, thereby insulating the tank with the PVC and then electrically
connecting the second inner coating made of steel to the cathode means of the electrolytic
cell, in this case to the negative polarity, distributing the electrical field on
the total surface of said second coating (bottom and four side walls).
[0042] There is thus produced an electrolytic cell for the galvanic bath, in which said
second stainless steel coating is electrically connected to the cathode means of the
electrolytic cell, and wherein the workpiece is arranged in the galvanic bath in electrical
connection with respect to the anode means of the electrolytic cell and is maintained
during the galvanic treatment at a distance variable on average from around 100 to
150 mm with respect to said cathode means.
[0043] For this reason, after having connected the whole surface of said second inner coating
to the negative current rectifier (cathode), copper guides are applied along the edges
of two heads of the tank and are also connected to said second coating, and copper
sliding blocks to support the AISI 316/L plates are engaged, in electrical and sliding
contact, at least partially immersed in the galvanic bath and which can substantially
slide along the whole of the width of the same tank, said plates thus providing respective
moving cathodes. In particular, the workpiece, connected to the anode means, is maintained
fixed in position, substantially at the centre of the tank, while said two plates,
forming moving cathodes, are arranged on opposite sides with respect to said workpiece.
[0044] Therefore, the system according to the invention operates in a different way with
respect to normal electro-polishing systems, where the cathodes are fixed to the two
sides of the tank. According to the invention, it is instead possible to move the
cathodes considerably towards or away from the workpieces, based on their shapes and
sizes, maintaining that optimal distance of around 10/15 cm, and thus obtaining a
homogeneous passage of current and optimal operating results.
[0045] To further improve the electrolytic treatment, without prejudice to the importance
of the uniform passage of electrical current, according to the invention there are
provided auxiliary cathodes for those workpieces with complicated shapes to be treated,
such as casings, box-shaped elements, tubing and the like, which must also be treated
internally. In fact, with respect to the internal parts the electrical current is
shielded, and can only manage to pass for a few centimetres, depending on the sizes
and shape of the workpiece to be treated. To overcome this problem, there are provided
auxiliary cathodes for the internal parts of the workpiece, structured as copper cathodes-frames,
with a copper diamond mesh measuring 8x8 mm, thk 1.5 mm (naturally, other sizes and
thicknesses can also be used).
[0046] The specific auxiliary cathodes-frames, made of titanium or stainless steel, currently
used, form turbulences and consequent temperature increases inside the workpiece,
which cause defects, such as gas bubbles, marks or burns on the surface of the workpiece.
To prevent this problem, according to the invention said copper auxiliary cathodes-frames,
provided with a mesh, are used, so that any gas bubbles can rise freely to the surface
and consequently the heating points are limited.
[0047] Taking account of the above, the tank with electrolytic cell is equipped with electrical
cables, the sections of which are chosen in relation to the related amperage and which
are connected to copper strips on the cathode part of the tank, two on one side of
the tank and two on the other side, and which have a length greater than half of the
same tank, in order to easily couple and clamp the brass terminals of the same cables
to the auxiliary cathodes.
[0048] Moreover, the electrolytic compound, its additives and continuous control of the
temperature and of the density of the electrolyte in the electrolytic bath, greatly
variable during the processing steps, are of primary importance in the method according
to the invention.
[0049] In fact, during the process the density of the electrolyte increases, due to the
continuous removal of material. To overcome this problem, the temperature of the electrolyte
has a fundamental role, and must remain within specific limits, established in the
method according to the invention at around 45°C to 55°C.
[0050] Moreover, the density of the electrolyte, fundamental for the correct passage of
electrical current, must be maintained at an advantageously constant value, established
in the method according to the invention as 1.750 g/mL at 20°C.
[0051] All the aforesaid innovative process and technical-plant layout characteristics are
provided, according to the invention, to maintain an optimal and uniform removal of
material from the workpiece, a fundamental operation to obtain the result of correct
surface roughness of the workpiece and consequently the success of the method according
to the invention.
[0052] The above-described electro-polishing treatment of the workpiece is followed by a
dripping step and by various washing and drying steps, after which the workpiece,
before being transferred for any packaging, is tested and checked with a roughness
meter on various parts of the surface (in at least 5 points), ensuring that the roughness
falls within the process parameters according to the present invention.
[0053] This is the end of the production cycle.
[0054] The attached drawing is provided purely by way of example, to better illustrate the
invention, and wherein:
- Fig. 1 is a flow chart indicatively illustrating the various steps of the aforesaid
production cycle;
- Fig. 2 schematically illustrates in a vertical cross-section an example of embodiment
of the electrolytic cell system for implementing the method according to the present
invention;
- Figs. 3A and 3B are diagrammatic representations of the surface roughness of a workpiece
according to the method of the invention, respectively at the end of the shot peening
step (fig. 3A) and of the electro-polishing step (Fig. 3B).
[0055] The flow chart of Fig. 1 indicatively illustrates the various steps of the aforesaid
production cycle. Said Fig. 1 indicates how the steps that precede the shot peening/shot
blasting step are not all necessary, but can be alternative to one another or at times
also repeated. The directions of the arrows that connect the various boxes indicative
of the aforesaid steps describe the possible process sequences, also alternative or
repetitive.
[0056] Downstream of the shot peening/shot blasting step, the further essential step according
to the present invention is the electro-polishing step. Further steps, which precede
or follow this step, can relate to roughness checks and washing of the workpiece.
EXAMPLES OF EMBODIMENT OF THE METHOD ACCORDING TO THE INVENTION
EXAMPLE 1 - Cycle, times and method steps on a AISI 304 stainless steel workpiece, namely "finishing shield" of a system for a mill for processing and converting coffee beans into ground coffee.
[0057] While it is being processed, ground coffee has a compact and very sticky mass. To
overcome problems of adhesion, manufacturers of the related processing plants have
used various measures, among which the insertion of vibrators or compressed air blowers
in the walls, casings and all those areas to which the product adheres and deposits,
techniques that are very costly and do not solve the problem.
1st STAGE - DEGREASING TREATMENT
[0058] The workpiece or part, namely
"finishing shield", is chemically degreased in an immersion tank, using an alkaline product at a temperature
from 50/70°C with an immersion time of 3/5'. At the end of this step, the part passes
into a dripping tank, and is subsequently rinsed twice with filtered water and finally
dried with hot air jets.
[0059] In this case, the part is not pickled as there are no evident oxides present.
2nd STAGE - MANUAL DISC-GRINDING AND BELT-GRINDING TREATMENT
[0060] The part has various defects caused by the preceding metalworking processes on its
surface. To remedy this, a first disc-grinding pass is carried out with a pneumatic
angular grinder and with a zirconia abrasive fibre disc ø 115 gr. 80, followed by
a second pass with a flap disc gr. 150.
[0061] This is followed by manual belt-grinding using a horizontal power tool with abrasive
sleeve, also made of zirconia, with two passes with differentiated grain size, a first
gr. 150 and a second gr. 220, eliminating all the surface defects to make the part
homogeneous and with low roughness (smooth), suitable for the subsequent shot peening
treatment.
3rd STAGE - SHOT PEENING TREATMENT
[0062] The part is subjected to mild degreasing with a solvent and then hooked on a specific
frame and inserted in a machine with two turbines, loaded with cylindrically shaped
AISI 304 stainless steel abrasive obtained from metal wire ø 0.4 mm. The inverter
of the turbines is adjusted, calibrating the blast speed of the abrasive at 1600 rpm
for a treatment time of 8'.
4th STAGE - INTERMEDIATE CHECKS
[0063] After the shot peening process, the part is blown with compress air and the roughness
obtained is checked using the roughness meter in at least five different points of
the treated part, resulting in an average roughness of
Ra 1.68.
5th STAGE - ELECTRO-POLISHING TREATMENT
[0064] The part is partially hooked on a copper bar and an auxiliary cathode, made of 8x8
diamond copper mesh, thk 1.5 mm is inserted into the concave part, which is immersed
in the electrolyte having the following formulation:
50% by weight (up to) of sulfuric acid 66 Be'
50% by weight (up to) of phosphoric acid 60 Be'
0.5-2% nigrosin B.
Temperature 48°C (with variations from 45°C to 55°C)
Voltage 8V (with auxiliary cathode inserted)
Time 5'.
[0065] After the treatment, the part is inserted in a drip tank, and subsequently receives
a first acid wash with consequent dripping, followed by further two washes with demineralized
water and terminating with further dripping and drying with hot air jets.
6th STAGE - FINAL CHECKS
[0066] Before packaging, the surface roughness is subjected to a final check with the roughness
meter in an average of at least five points of the treated surface; the final result
is
Ra 1.45.
OBSERVATIONS
[0067] The part treated with the method according to the invention has excellent non-stick
properties, as shown by the practical tests carried out on plate treated in conformity
with the invention, where at an angle of just 30-35° the ground coffee slips and detaches
naturally without leaving residues, while on the mirror polished plate, even at an
angle of 90°, the ground coffee still remains firmly attached and the plate must be
moved several times to dislodge it and, notwithstanding this, small/medium-sized deposits
of ground coffee still remain.
[0068] The test confirms the validity and functionality of the non-stick properties of the
surface of the part treated with the method according to the invention.
EXAMPLE 2 - Cycle, times and method steps according to the invention on a part called "sliding surface" consisting of an AISI 304 stainless steel plate measuring 200 x 30, thk 10 mm for
a boxing system for pre-wrapped chocolates.
[0069] The part treated is part of a boxing system for chocolates, formed of various lines
that converge in a single automatic sorter called
"tank", i.e., a track composed of different stainless steel plates in which pre-wrapped chocolates
are positioned by pushing in groups of 5 to be boxed. At the end of the boxing cycle,
the track rotates allowing other chocolates to slide onto other empty plates, and
this sequence continues.
1st STAGE - DEGREASING TREATMENT
[0070] The part (consisting of a plate) follows the same methods as the previous example
1.
[0071] Also in this case the pickling treatment is not essential, as there are no signs
of weld oxides or the like.
2nd STAGE - CALIBRATION / SMOOTHING / BELT GRINDING TREATMENT
[0072] The part is treated using a semi-automatic calibration / smoothing / belt grinding
machine, on which it is arranged on a conveyor belt and subjected to the correct calibration-pressure
passing under 3 contact rollers that rotate with zirconia abrasive fibre belts with
different grain sizes: first roller gr. 180, second roller gr. 220 and third roller
gr. 320; to optimize the surface to be treated two passes through the machine are
required.
[0073] After the contact surface has been calibrated and smoothed, further manual processing
is carried out using a cleaning-belt grinding machine to deburr and bevel the sharp
lateral edges; this is slight bevelling with a single pass of the abrasive belt or
with flap wheels gr. 320.
3rd STAGE -SHOT PEENING TREATMENT
[0074] The part follows the same methods as the previous example 1.
[0075] Maintaining the same machine, the same abrasive, the same diameter, changing only
the number of revolutions of the turbine and the treatment time: turbine rpm 2000,
treatment time 10'.
4th STAGE -INTERMEDIATE CHECKS
[0076] At the end of the shot peening treatment, the part is blown with compressed air and
the roughness checks are carried out by means of a roughness meter in at least five
different points of the treated part, the average of which is
Ra 2.47.
5th STAGE - ELECTROLYTIC TREATMENT
[0077] The electro-polishing treatment is carried out on the same system and using the same
electrolytic composition as in example 1.
[0078] Only the auxiliary cathodes, no longer required as the shape of the part to be treated
is simple and linear, are eliminated; only the moving cathodes, which slide by means
of copper sliding blocks, on specific copper guides, are moved to a distance of around
10 cm from the part to be treated; the treatment is carried out at 10V for 10'.
[0079] This is followed by the same dripping, washing and drying steps as example 1.
6th STAGE - FINAL CHECKS
[0080] Before packaging, the surface roughness is subjected to a final check with the roughness
meter, on an average of at least five points of the treated surface: the final result
is
Ra 1.78.
OBSERVATIONS
[0081] The part obtained with this treatment cycle and having the aforesaid final roughness
was compared to plates measuring 200x30, thk 10 mm, made of the materials and coatings
listed below:
- stainless steel plate treated with the method according to the invention;
- satin finished stainless steel plate;
- mirror-polished steel plate;
- Teflon coated steel plate;
- Teflon coated aluminum plate;
- anodized aluminum plate;
- mirror polished aluminum plate;
- chrome-plated iron plate.
[0082] Positioning all the plates on a rack with the same inclination of around 15° and
placing wrapped chocolates thereon, it was possible to note and prove that the chocolates
slid easily downwards only on three plates: the plate treated with the method according
to the invention and two others plates, one made of stainless steel and one made of
aluminum, both Teflon-coated. Continuing the tests it was however noted that the wrapped
chocolates slid more rapidly on the stainless steel plate treated with the method
according to the invention than the chocolates arranged on the Teflon-coated plates.
Therefore, it can be confirmed that the treatment carried out with the method according
to the invention ensures greater slip with respect to the parts obtained with the
normal Teflon-coating treatment.
EXAMPLE 3 - Cycle, times and method steps according to the invention relative to a "slide/hopper"
made of AISI 304 stainless steel for spun paste.
[0083] Known machinery and systems for processing spun paste cheeses (mozzarella, scamorza,
provolone, etc.) are coated in Teflon or other similar non-stick products that, being
applied materials, over time and due to continuous rubbing and pressure of the products
treated, tend to detach and, therefore, to be discharged into the product during the
various processing steps, creating serious problems of contamination of these products.
1st STAGE - DEGREASING TREATMENT
[0084] The part being processed follows the same methods as in the previous examples 1 and
2.
2nd STAGE - PICKLING TREATMENT
[0085] Unlike the previous examples, the part has various weld seams and points, with a
great many oxides on the surface, and therefore a pickling treatment is necessary.
[0086] Chemical picking with a compound with the following formulation was chosen:
- Compound:
30% nitric acid concentrate 67%,
3-5% hydrofluoric acid concentrate 38%.
- Additives:
0.5 - 1 g/L surfactants (or inhibitors) at 10% non ionic and at 5% cationic.
Temperature:
room, optimal from 40°C to 60°C,
Average immersion time around 60' (variable according to the temperature).
[0087] The treatment is followed by dripping and by a first and a second wash. At times
it is advisable to wash complex parts with jets of pressurized water.
3rd STAGE - MANUAL DISC-GRINDING AND BELT-GRINDING TREATMENT
[0088] The part is treated with the same methods as example 1.
4th STAGE - SHOT PEENING TREATMENT
[0089] In this case, as the "slide/hopper" is in contact with a food in paste and spun form,
a much greater roughness must be created. Therefore, high pressure shot peening is
carried out in a shot peening cabin with pressure of 8 atm at a constant distance
of around 50 cm with cylindrical abrasive obtained from AISI 304 metal wire ø 0.4
mm; treatment time 5'.
5th STAGE -INTERMEDIATE CHECKS
[0090] At the end of the shot peening procedure, the part is blown with compressed air and
the surface roughness is checked with the roughness meter in at least five different
points of the treated part, the average of which is
Ra 4.79.
6th STAGE - ELECTROLYTIC TREATMENT
[0091] The electro-polishing treatment is carried out in the same system and using the same
electrolytic composition described in examples 1 and 2.
[0092] Once again the auxiliary cathode made of copper mesh is inserted into the inner part
of the channel of the "slide/hopper", as the shape itself forms a shield for the electrical
current, which can prevent the electrolytic treatment from removing the material uniformly,
altering the uniformity of the roughness.
[0093] To obtain greater roughness and obtain deeper and wider valleys, it was decided to
use electro-polishing with very high times, with repetitions of 10' each, for 4 passes
with a total treatment time of 40' at 10V.
[0094] The time of 40' cannot be continuative as, not having suitable means for slight movement
of the part, preferential channels could be formed due to gas bubbles, which could
alter the removal of material and consequently the roughness.
[0095] The material must be treated for 40' for the following reasons:
- To obtain, from the preceding shot peening treatment, wider and deeper valleys, much
higher peaks were also formed, which must be removed and rounded, and therefore treatment
times must be extended.
- By treating the material for 40', a viscous and water-repellent anodic film forms,
which further improves the slip and non-stick properties of products coming into contact
with it.
- Extension of the treatment times considerably increases the resistance to corrosion
by both acid and alkaline products.
[0096] This is followed by the same dripping, washing and drying steps described in examples
1-2.
7th STAGE - FINAL CHECKS
[0097] Before packaging, the surface roughness is subjected to a final check with the roughness
meter, in an average of at least five points of the treated surface; the final result
is
Ra 2.85.
OBSERVATIONS
[0098] With reference to Figs. 3A and 3B, the following can be observed:
- Fig. 3A illustrates an example of surface roughness created on a workpiece with the
shot peening treatment according to the method of the invention, which considerably
reduces the contact surface, but forms pointed peaks that hold the materials or products
in contact with the aforesaid surface;
- Fig. 3B illustrates an example of surface roughness after the electro-polishing treatment
according to the method of the invention, in which the aforesaid peaks are rounded
by removal of material.
[0099] The result is more than satisfactory, as the spun paste detaches easily from the
"slide/hopper" in a similar manner, as a result of slipping, to Teflon coated parts.
[0100] It must nonetheless be mentioned that Teflon or other similar products are always
and in any case coating products and therefore, as a result of wear, subject to consumption
and detachment, which must not be underestimated, above all if the parts treated are
used in contact with pharmaceutical or, as in the case in hand, food products.
[0101] A strength and one of the advantages of the present invention lies in the possibility
of reducing contamination, while maintaining the effectiveness of the non-stick properties
of the treated part intact. In this case, with the same non-stick results, the present
invention in fact achieves the important and considerable advantage of preventing
the possibility of contamination of the aforesaid products.
[0102] Therefore, the method for the surface treatment of a stainless steel workpiece according
to the invention enables the following advantages and the improvements indicated below
to be achieved with respect to the current state of the art.
- Excellent slip of the treated surface, making it completely non-stick. In this regard,
see examples 1, 2 and 3 described above.
- Overcoming, due to the absence of the addition of different material than that of
the workpiece, all critical issues deriving from the detachment of material from the
treated workpiece, as instead commonly occurs in products treated with known applications
of non-stick materials, thereby avoiding serious problems of contamination. In this
regard, see examples 1, 2 and 3 described above.
- Improvement of the resistance to fatigue of the treated workpiece, increasing its
useful life.
[0103] In relation to resistance to fatigue, besides creating the particular surface roughness
useful for slip and non-stick properties, by means of the continuous surface hammering
operation, the shot peening/shot blasting treatment of the method according to the
invention compresses the fibres of the material, removing the residual tensile stresses,
generated during the previous operations, from the metal. The resulting effect is
an increase in the life of all those parts subjected to repeated stresses (fatigue
strength).
- Improvement of corrosion resistance. In this regard, the electro-polishing treatment
that according to the present invention provides for levelling of the roughness, removing
material first on the peaks, rounding them, and then on the valleys, also has the
advantage, in this removal step, of eliminating all the surface impurities, forming
a passivating gaseous anodic film, which considerably increases the corrosion resistance.
- Reduction of the release of chrome, nickel and manganese, obtaining suitability for
contact with foodstuffs in any situation and with any food, and also for products
for personal use, pursuant to Ministerial Decree 21 March 1973 and subsequent updates
in conformity with the EC Directives. Suitability for contact with foods of the treated
material represents an extremely advantageous and important point, as the method according
to the present invention allows a noteworthy decrease (compared with other known processes)
in the release of metals, above all chrome and nickel. Therefore, the treated part
falls favourable and fully within the framework of the limits and tolerances required
by the current law (Ministerial Decree 21 March 1973 and subsequent updates in conformity
with EC Directives), in relation to contact with foods and products for personal use.
It is therefore possible to certify the suitability of the method according to the
present invention.
- Excellent visual appearance of the surface finish of the workpiece treated. Although
the object of the present invention is that of providing the surface of the treated
workpiece with slip and non-stick properties, at the same time during the steps of
the treatment cycle of the method according to the invention a surface finish is formed
that makes the treated part uniform and provides it with a visually pleasing silvery,
semi-gloss appearance.
[0104] With reference to Fig. 2, there is described below an example of embodiment of the
electrolytic cell system for implementing the method according to the present invention.
[0105] Said system is indicated as a whole with 10. The system 10 comprises an iron tank
11 with a galvanic bath with electrolytic cell, which has a first inner coating 12
made of insulating plastic material, such as PVC, and a second inner coating 13 made
of AISI 316/L stainless steel, superimposed on the first coating. Said second stainless
steel coating 13 is electrically connected to cathode means (negative polarity) of
the electrolytic cell, with formation of an electrical field extended to the total
surface of said second inner coating 13 (bottom and side walls), while the workpiece
P, arranged in the galvanic bath, is electrically connected to the anode means (positive
polarity) of the electrolytic cell and is maintained, by means of fixed support means
14, ad a distance generally variable from around 100 to 150 mm with respect to the
cathode means.
[0106] It must be noted that the whole surface of said second coating 13 is electrically
connected with respect to the current rectifier of said cathode means, while along
the opposite edges of the said inner coating 13 there are arranged sliding support
means, which comprise copper guides, connected to said second coating 13 and along
which there are engaged, in electrical and sliding contact, copper sliding blocks
15, which support plates 16 opposite one another, at least partially immersed in the
galvanic bath and forming respective moving cathodes, made of AISI 316/L stainless
steel, sliding each for around half the width of said inner coating 13. The fixed
support means 14 of the workpiece P, and the workpiece P itself, are arranged between
said moving cathodes 16.
[0107] Therefore, contrary to normal electro-polishing systems, in which the cathodes are
fixed to the two sides of the tank, according to the invention it is possible to move
the cathode means towards, respectively away from, the workpiece P, taking account
of its shapes and sizes, so as to maintain a predetermined optimal distance, for example
10/15 cm, between the cathode means and the workpiece. In this way, a homogeneous
passage of current and excellent results can be achieved.
[0108] Moreover, the system according to the invention may be provided with auxiliary cathodes,
structured as copper frames with diamond mesh, to be used in the galvanic bath for
the inner parts of a workpiece of complex shape.
1. Method for the surface treatment of a stainless steel workpiece,
characterized in that it comprises at least the following steps:
- shot peening/shot blasting of at least one surface of said workpiece, wherein there
are used shots/beads made of electrocast austenitic stainless steel, respectively
stainless steel obtained from metal wire, and which are essentially spherical, respectively
cylindrical, with a diameter of the spherical body, respectively of the cross section
of the cylindrical body, substantially from around 0.4 to 0.8 mm; said method being
implemented in an automatic turbine system with inverter, which controls the continuous
variation of the rotation speed of the turbine motor and therefore the blasting speed
of the shot/beads, so that on said at least one surface of said workpiece there is
provided a surface micro-roughness, having micro-peaks and micro-valleys that reduce
the area of contact of the worked surface, and
- electro-polishing of said at least one surface of the workpiece by means of tank
with an galvanic bath with electrolytic cell, wherein said tank, made of an electroconductive
material, comprises a first inner coating made of insulating plastic material and
a second inner coating made of stainless steel, superimposed on the said first coating
and electrically connected to cathode means of the electrolytic cell, with formation
of a corresponding electrical field extended to the total surface of said second inner
coating, and in that guides made of a good electrically conductive material are applied to the two opposite
edges of said tank and are electrically connected with respect to said second coating
and to the corresponding cathode means of the electrolytic cell, while support bars
with respective sliding blocks made of a good electrically conductive material are
engaged, in electrical contact and sliding, along said guides, and are electrically
connected with at least two steel plates, at least partially immersed in the galvanic
bath, forming moving cathodes and arranged on opposite sides with respect to the workpiece,
which is supported fixed in said galvanic bath and electrically connected with respect
to the anode means of the electrolytic cell, and which is maintained during the galvanic
treatment at a distance variable from around 100 to 150 mm with respect to the said
moving cathode means, so that said surface microroughness is reduced, lowering and
rounding by removal of material of said micro-peaks of the surface of the workpiece.
2. A method according to claim 1, characterized in that said tank of the galvanic bath is provided with a first inner coating made of an
insulating plastic material and is further completely lined by means of a second inner
coating made of stainless steel, wherein said second coating made of stainless steel
is electrically connected to the cathode means of the electrolytic cell, with formation
of an electrical field extended to the total surface of the bottom and of the side
walls of said second coating, arranging said workpiece in the galvanic bath electrically
connected with respect to the anode means of the electrolytic cell and maintaining
it during the galvanic treatment at a distance variable from around 100 to 150 mm
with respect to said cathode means.
3. A method according to claims 1 and/or 2,
characterized in that it also comprises in the electro-polishing step:
- introducing into the galvanic bath auxiliary cathodes for the internal parts of
the workpiece, which are produced in the form of copper cathode frames, which are
provided with a copper mesh;
- electrical connection to said auxiliary cathodes of two pairs of electrical cables,
provided with respective electrical terminals that clamp said auxiliary cathodes.
4. A method according to claim 1, 2 and/or 3, characterized in that in said electro-polishing step density control of the electrolyte of the galvanic
bath, maintaining the density substantially constant at a value of around 1.750 g/mL
at 20°C, and temperature control of said electrolyte at a temperature range of around
45°C to 55°C are carried out.
5. A method according to claim 1, 2, 3 and/or 4, characterized in that at the end of the shot peening/shot blasting cycle, the workpiece is subjected to
a surface cleaning by means of a jet of compressed air, to eliminate all possible
processing residues present on the surface.
6. A method according to claim 1, 2, 3, 4 and/or 5, characterized in that after the electrolytic polishing treatment the workpiece is subjected to at least
one washing and subsequent dripping with subsequent drying.
7. A method according to claim 5 and/or 6, characterized in that after cleaning with compressed air, respectively after drying, the workpiece is tested
and checked with a roughness meter in several parts of the surface, ensuring that
the roughness falls within predefined process parameters.
8. A system (10) for implementing the method according to one or more of the preceding
claims,
characterized in that it comprises:
- an automatic turbine system with inverter for controlling the continuous variation
of the rotation speed of a turbine motor and therefore the blasting speed of shot/beads,
- an iron tank (11) with a galvanic bath with electrolytic cell, comprising a first
inner coating (12) made of insulating plastic material and a second inner coating
(13) made of stainless steel, superimposed on the first coating and wherein said second
stainless steel coating is electrically connected to cathode means (negative polarity)
of the electrolytic cell, with formation of an electrical field extended to the total
surface of said second inner coating (13),
and in that guides made of a good electrically conductive material are applied to the two opposite
edges of said tank and are electrically connected with respect to said second coating
and to the corresponding cathode means of the electrolytic cell, while support bars
with respective sliding blocks made of a good electrically conductive material are
engaged, in electrical contact and sliding, along said guides, and are electrically
connected with at least two steel plates, at least partially immersed in the galvanic
bath, forming moving cathodes and arranged on opposite sides with respect to the workpiece,
which is supported fixed in said galvanic bath and electrically connected with respect
to the anode means (positive polarity) of the electrolytic cell, and fixed support
means (14) maintaining the workpiece at a distance variable from around 100 to 150
mm with respect to said moving cathode means.
9. A system (10) according to claim 8, characterized in that the surface of said second coating (13) is electrically connected with respect to
the current rectifier of said cathode means, while along two opposite edges of said
tank (11) there are arranged sliding support means (15), which comprise copper guides,
connected to said second coating (13) and along which there are engaged, in electrical
and sliding contact, copper sliding blocks, which support plates (16) opposite one
another and forming moving cathodes, sliding, each for around half the width of said
tank (11), while said fixed support means (14) of the workpiece (P), and said workpiece
(P) immersed in the galvanic bath, are arranged between said moving cathodes (16),
so that it is possible to move the cathode means towards, respectively away from,
the workpiece (P), taking account of its shapes and sizes, to a predetermined optimal
distance between said cathode means (16) and the workpiece (P).
10. A system (10) according to claim 8 and/or 9, characterized in that it comprises auxiliary cathodes structured as copper frames with mesh, immersed in
the galvanic bath for the internal parts of a workpiece of complex shape.
1. Ein Verfahren zur Oberflächenbehandlung eines Werkstücks aus rostfreiem Stahl,
dadurch gekennzeichnet, dass es mindestens die folgenden Schritte umfasst:
- Kugelbestrahlung/Schleifen von mindestens einer Oberfläche des besagten Werkstücks,
bei dem elektrogegossenes Schrot/Korn aus austenitischem rostfreiem Stahl verwendet
wird, das wiederum aus rostfreiem Stahl, gewonnen aus Draht, erhalten wird und eine
im Wesentlichen kugelförmige bzw. zylindrische Form und einen Durchmesser des kugelförmigen
Körpers, bzw. des Querschnitts des zylindrischen Körpers von im Wesentlichen zwischen
etwa 0,4 und 0,8 mm aufweist; wobei das besagte Verfahren in einem automatischen Turbinensystem
mit Wechselrichter ausgeführt wird, der die kontinuierliche Veränderung der Rotationsgeschwindigkeit
des Turbinenmotors und somit die Schrot-/Korn-Abschussgeschwindigkeit steuert, so
dass die besagte mindestens eine Oberfläche des besagten Werkstücks mit einer Mikrorauigkeit
der Oberfläche versehen ist, die Mikrospitzen und Mikrosenken aufweist, die die Kontaktfläche
der bearbeiteten Oberfläche reduzieren, sowie
- Elektropolieren der besagten mindestens einen Oberfläche des Werkstücks mittels
einer galvanischen Wanne mit elektrolytischer Zelle, wobei die besagte Wanne, die
aus elektrisch leitendem Material hergestellt ist, eine erste Innenbeschichtung aus
isolierendem Kunststoffmaterial und eine zweite Innenbeschichtung aus rostfreiem Stahl
aufweist, die über der besagten ersten Innenbeschichtung liegt und elektrisch mit
den Kathodenvorrichtungen der elektrolytischen Zelle verbunden ist, wobei ein entsprechendes
elektrisches Feld gebildet wird, das sich auf die gesamte Oberfläche der besagten
zweiten Innenbeschichtung erstreckt,
und
dadurch gekennzeichnet, dass Führungen, die aus einem elektrisch gut leitenden Material hergestellt sind, in Übereinstimmung
mit zwei gegenüberliegenden Rändern der besagten Wanne angebracht sind und elektrisch
in Bezug auf die besagte zweite Beschichtung und mit den entsprechenden Kathodenvorrichtungen
der elektrolytischen Zelle verbunden sind, während Stützstäbe mit entsprechenden Kufen,
hergestellt aus einem elektrisch gut leitenden Material, in elektrischem und gleitendem
Kontakt entlang der besagten Führungen in Anspruch genommen und elektrisch mit mindestens
zwei Stahlblechen verbunden werden, die mindestens teilweise in das galvanische Bad
eingetaucht sind, und die bewegliche Kathoden bilden und an gegenüberliegenden Seiten
in Bezug auf das Werkstück angeordnet sind, das fest in dem besagten galvanischen
Bad und in elektrischer Verbindung mit den Anodenvorrichtungen der elektrolytischen
Zelle gehalten werden, und während der galvanischen Behandlung in einem Abstand zwischen
etwa 100 und 150 mm in Bezug auf die besagten beweglichen Kathodenvorrichtungen gehalten
wird, so dass die besagte Mikrorauigkeit der Oberfläche verringert wird, indem die
Spitzen der Mikrospitzen der Werkstückoberfläche abgesenkt und durch Materialabtrag
abgerundet werden.
2. Ein Verfahren gemäß Anspruch 1, dadurch gekennzeichnet, dass die besagte galvanische Wanne mit einer ersten Innenbeschichtung aus isolierendem
Kunststoffmaterial versehen ist und weiter vollständig mit einer zweiten Innenbeschichtung
aus rostfreiem Stahl ausgekleidet ist, wobei die besagte zweite Beschichtung aus rostfreiem
Stahl elektrisch mit den kathodischen Mitteln der elektrolytischen Zelle verbunden
ist, mit Bildung eines elektrischen Feldes, das sich auf die gesamte Oberfläche des
Bodens und der Seitenwände der besagten zweiten Beschichtung erstreckt, wobei das
besagte Werkstück in dem galvanischen Bad in elektrischer Verbindung mit den Anodenvorrichtungen
der elektrolytischen Zelle angeordnet und während der galvanischen Behandlung in einem
variablen Abstand zwischen etwa 100 und 150 mm in Bezug auf die besagten Kathodenvorrichtungen
gehalten wird.
3. Ein Verfahren gemäß Anspruch 1 und/oder 2,
dadurch gekennzeichnet, dass es außerdem in der Elektropolierphase auch Folgendes umfasst:
- die Einführung von Hilfskathoden in das galvanische Bad, für die inneren Teile des
Werkstücks, die in Form von Rahmenkathoden aus Kupfer hergestellt werden;
- die elektrische Verbindung zu den besagten Hilfskathoden von zwei Paaren elektrischer
Kabel, versehen mit entsprechenden elektrischen Klemmen, die die besagten selbigen
Hilfskathoden festklemmen.
4. Ein Verfahren gemäß den Ansprüchen 1, 2 und/oder 3, dadurch gekennzeichnet, dass in der besagten Elektropolierphase die Kontrolle der Dichte des Elektrolyten des
galvanischen Bades kontinuierlich durchgeführt wird, wobei die Dichte selbst im Wesentlichen
konstant auf einem Wert von etwa 1,750 g/mL bei 20°C und die Temperaturkontrolle des
besagten Elektrolyten innerhalb eines Temperaturbereiches von etwa 45°C bis 55°C gehalten
werden.
5. Ein Verfahren gemäß den Ansprüchen 1, 2, 3 und/oder 4, dadurch gekennzeichnet, dass das Werkstück am Ende des Kugelstrahl-/Schleifzyklus einer Oberflächenreinigung durch
Blasen von Druckluft unterzogen wird, um auf der Oberfläche vorhandene Bearbeitungsrückstände
zu beseitigen.
6. Ein Verfahren gemäß den Ansprüchen 1, 2, 3, 4 und/oder 5, dadurch gekennzeichnet, dass das Werkstück nach der elektrolytischen Polierbehandlung mindestens einem Waschvorgang
und einem anschließenden Abtropfen mit anschließendem Trocknen unterzogen wird.
7. Ein Verfahren gemäß Anspruch 5 und/oder 6, dadurch gekennzeichnet, dass am Ende der Reinigung mit Druckluft bzw. nach dem Trocknen das Werkstück an mehreren
Stellen der Oberfläche mit einem Rauheitsmessgerät geprüft und kontrolliert wird,
wobei überprüft wird, dass die Rauheit innerhalb vordefinierter Prozessparameter liegt.
8. Eine Anlage (10) für die Durchführung des Verfahrens gemäß einem oder mehreren der
oben genannten Ansprüche,
dadurch gekennzeichnet, dass sie Folgendes umfasst:
- ein automatisches Turbinensystem mit Wechselrichter zur Steuerung der kontinuierlichen
Veränderung der Rotationsgeschwindigkeit eines Turbinenmotors und damit der Schrot-/Korn-Abschussgeschwindigkeit,
- eine galvanische Wanne (11) aus Eisen für ein galvanisches Bad mit elektrolytischer
Zelle, umfassend eine erste Innenbeschichtung (12) aus isolierendem Kunststoff und
eine zweite Innenbeschichtung (13) aus rostfreiem Stahl, die der ersten Beschichtung
überlagert ist und mit der die besagte zweite Beschichtung aus rostfreiem Stahl elektrisch
mit Kathodenvorrichtungen (negative Polarität) der elektrolytischen Zelle verbunden
ist, mit Bildung eines elektrischen Feldes, das sich auf die gesamte Oberfläche der
besagten zweiten Innenbeschichtung (13) erstreckt, und dadurch gekennzeichnet, dass Führungen aus elektrisch gut leitfähigem Material in Übereinstimmung mit zwei gegenüberliegenden
Rändern der besagten Wanne angebracht sind und elektrisch mit der besagten zweiten
Beschichtung und mit den entsprechenden Kathodenvorrichtungen der elektrolytischen
Zelle verbunden sind, während Tragstäbe mit entsprechenden Gleitkufen aus elektrisch
gut leitendem Material in elektrischem und gleitendem Kontakt entlang der besagten
Führungen in Anspruch genommen und elektrisch mit mindestens zwei Stahlblechen verbunden
werden, die zumindest teilweise in das galvanische Bad eingetaucht sind, bewegliche
Kathoden bilden und auf gegenüberliegenden Seiten in Bezug auf das Werkstück angeordnet
sind, das in dem besagten galvanischen Bad und in elektrischer Verbindung in Bezug
auf die Anodenvorrichtungen (positive Polarität) der elektrolytischen Zelle fixiert
gehalten wird, und feste Stützvorrichtungen (14), die das Werkstück in einem variablen
Abstand zwischen etwa 100 und 150 mm in Bezug auf die besagten Kathodenvorrichtungen
halten.
9. Eine Anlage (10) gemäß Anspruch 8, dadurch gekennzeichnet, dass die Oberfläche der besagten zweiten Beschichtung (13) in Bezug auf den Stromgleichrichter
der besagten Kathodenvorrichtungen elektrisch verbunden ist, während entlang zweier
gegenüberliegender Ränder der besagten Wanne (11) Gleitstützvorrichtungen (15) angeordnet
sind, die Kupferführungen umfassen die mit der besagten zweiten Beschichtung (13)
verbunden sind und längs welchen Kupferkufen in elektrischem und gleitendem Kontakt
in Anspruch genommen werden, die einander gegenüberliegende und bewegliche, gleitende
Kathoden bildende Bleche (16) tragen, ein jedes für etwa die Hälfte der Breite der
besagten Wanne (11), während die besagten festen Stützmittel (14) des Werkstücks (P)
und das besagte Werkstück (P), das in das galvanische Bad eingetaucht ist, zwischen
den besagten beweglichen Kathoden (16) angeordnet sind, so dass es möglich ist, die
Kathodenvorrichtungen in Bezug auf das Werkstück (P) unter Berücksichtigung seiner
Formen und Abmessungen in einem vorbestimmten optimalen Abstand zwischen den besagten
Kathodenvorrichtungen (16) und dem Werkstück (P) anzunähern bzw. zu entfernen.
10. Eine Anlage (10) gemäß Anspruch 8 und/oder 9, dadurch gekennzeichnet, dass sie Hilfskathoden in Form von Kupferrahmen mit Netz aufweist, die in das galvanische
Bad für die inneren Teile eines Werkstücks mit komplexer Form eingetaucht sind.
1. Procédé pour le traitement de surface d'une pièce en traitement en acier inoxydable,
caractérisé par le fait qu'il comprend au moins les étapes suivantes :
- grenaillage de précontrainte/sablage d'au moins une surface de ladite pièce en traitement,
où sont utilisées des billes/ grenailles en acier inoxydable austénitique électrofondu,
respectivement en acier inoxydable obtenu à partir de fil métallique, et qui ont une
forme essentiellement sphérique, respectivement cylindrique, avec un diamètre du corps
sphérique, respectivement de la section transversale du corps cylindrique, substantiellement
compris entre 0,4 et 0,8 mm environ ; ladite méthode étant mise en œuvre dans un système
automatique à turbine avec convertisseur, qui contrôle la variation continue de la
vitesse de rotation du moteur de la turbine et donc la vitesse de projection des billes/grenailles,
de sorte que sur ladite au moins une surface de ladite pièce en traitement, il est
prévu une micro-rugosité de surface, ayant des micro-crêtes et des micro-vallées qui
réduisent la superficie de contact de la surface traitée, et
- électropolissage de ladite au moins une surface de la pièce en traitement au moyen
d'une cuve avec un bain galvanique à cellule électrolytique, où ladite cuve, en matériau
électroconducteur, comprend un premier revêtement interne en matériau plastique isolant
et un second revêtement interne en acier inoxydable, superposé sur ledit premier revêtement
interne et électriquement connecté aux moyens à cathode de la cellule électrolytique,
avec formation d'un champ électrique correspondant étendu à la surface totale dudit
second revêtement interne,
et
par le fait que des guides en matériau bon conducteur électrique sont appliqués au niveau de deux
bords opposés de ladite cuve et sont connectés électriquement par rapport audit second
revêtement et aux moyens à cathode correspondants de la cellule électrolytique, tandis
que des barres de support avec des patins correspondants en matériau bon conducteur
électrique sont engagées, en contact électrique et coulissant, le long desdits guides
et sont électriquement connectées avec au moins deux tôles d'acier, au moins partiellement
immergées dans le bain galvanique, formant des cathodes mobiles et disposées sur des
côtés opposés par rapport à la pièce en traitement, qui est supportée fixe dans ledit
bain galvanique et en connexion électrique par rapport aux moyens à anode de la cellule
électrolytique, et est maintenue pendant le traitement galvanique à une distance variable
entre 100 et 150 mm environ par rapport aux moyens à cathode mobiles, de sorte que
ladite micro-rugosité de surface est réduite, en abaissant et en arrondissant par
enlèvement de matière les pointes desdits micro-crêtes de la surface de la pièce en
traitement.
2. Procédé selon la revendication 1, caractérisé par le fait que ladite cuve avec bain galvanique est dotée d'un premier revêtement interne en matériau
plastique isolant et est ultérieurement revêtue complètement au moyen d'un second
revêtement interne en acier inoxydable, où ledit second revêtement en acier inoxydable
est électriquement connecté aux moyens à cathode de la cellule électrolytique, avec
formation d'un champ électrique étendu à la surface totale du fond et des parois latérales
dudit second revêtement, en disposant ladite pièce en traitement dans le bain galvanique
en connexion électrique par rapport aux moyens à anode de la cellule électrolytique
et en la maintenant durant le traitement galvanique à une distance variable entre
100 et 150 mm environ par rapport auxdits moyens à cathodes.
3. Procédé selon la revendication 1 et/ou 2,
caractérisé par le fait qu'il comprend également dans la phase d'électropolissage :
- l'introduction dans le bain galvanique de cathodes auxiliaires, pour les parties
internes de la pièce en traitement, qui sont réalisées en forme de châssis-cathodes
en cuivre, lesquelles sont dotées d'une grille en cuivre ;
- la connexion électrique auxdites cathodes auxiliaires de deux paires de câbles électriques,
dotées de borniers électriques respectifs qui serrent lesdites cathodes auxiliaires.
4. Procédé selon la revendication 1, 2 et/ou 3, caractérisé par le fait que dans ladite phase d'électropolissage le contrôle de la densité de l'électrolyte du
bain galvanique, en maintenant substantiellement constante la densité même à une valeur
d'environ 1,750 g/ml à 20 °C, et le contrôle de la température dudit électrolyte dans
une plage de température de 45 °C à 55 °C environ sont effectués de manière continue.
5. Procédé selon la revendication 1, 2, 3 et/ou 4, caractérisé par le fait qu'à la fin du cycle de grenaillage de précontrainte/sablage, la pièce en traitement
est soumise à un nettoyage de surface au moyen d'un jet d'air comprimé, pour éliminer
tous les éventuels résidus de traitement présents sur la surface.
6. Procédé selon la revendication 1, 2, 3, 4 et/ou 5, caractérisé par le fait qu'après le traitement électrolytique de polissage, la pièce en traitement est soumise
à au moins un lavage puis un égouttage avec séchage suivant.
7. Procédé selon la revendication 5 et/ou 6, caractérisé par le fait qu'à la fin du nettoyage avec air comprimé, respectivement après le séchage, la pièce
en traitement est testée et contrôlée avec un rugosimètre sur plusieurs parties de
la surface, en vérifiant que la rugosité se situe dans les paramètres de processus
procédé prédéfinis.
8. Système (10) pour la mise en œuvre du procédé selon un ou plusieurs des revendications
précédentes,
caractérisé par le fait qu'il comprend :
- un système automatique à turbine avec convertisseur pour contrôler la variation
continue de la vitesse de rotation d'un moteur de la turbine et donc la vitesse de
projection des billes/grenailles,
- une cuve (11) en fer pour bain galvanique en cellule électrolytique, comprenant
un premier revêtement interne (12) en matériau plastique isolant et un second revêtement
interne (13) en acier inoxydable, superposé au premier revêtement et où ledit second
revêtement en acier inoxydable est électriquement connecté aux moyens à cathode (polarité
négative) de la cellule électrolytique, avec formation d'un champ électrique étendu
à la surface totale dudit second revêtement interne (13), et par le fait que des guides en matériau bon conducteur électrique sont appliqués au niveau de deux
bords opposés de ladite cuve et sont connectés électriquement par rapport audit second
revêtement et aux moyens à cathode correspondants de la cellule électrolytique, tandis
que des barres de support avec des patins correspondants en matériau bon conducteur
électrique sont engagées, en contact électrique et coulissant, le long desdits guides
et sont électriquement connectées avec au moins deux tôles d'acier, au moins partiellement
immergées dans le bain galvanique, formant des cathodes mobiles et disposées sur des
côtés opposés par rapport à la pièce en traitement, qui est supportée fixe dans ledit
bain galvanique et en connexion électrique par rapport aux moyens à anode (polarité
positive) de la cellule électrolytique, et des moyens de supports fixes (14) qui maintiennent
la pièce en traitement à une distance variable entre 100 et 150 mm environ par rapport
auxdits moyens à cathode.
9. Système (10) selon la revendication 8, caractérisé par le fait que la surface dudit second revêtement (13) est électriquement connectée par rapport
au redresseur de courant desdits moyens à cathode, tandis que le long de deux bords
opposés de ladite cuve (11) sont disposés des moyens de support coulissant (15), lesquels
comprennent des guides en cuivre, connectés audit second revêtement (13) et le long
desquels sont engagés, en contact électrique et coulissant, des patins en cuivre,
qui supportent des tôles (16) opposées entre elles et formant des cathodes mobiles,
coulissantes, chacune sur la moitié de la largeur de ladite cuve (11), tandis que
lesdits moyens de support (14) fixes de la pièce en traitement (P), et ladite pièce
en traitement (P) immergée dans le bain galvanique, sont disposés entre lesdites cathodes
mobiles (16), de sorte qu'il est possible d'approcher, respectivement d'éloigner les
moyens à cathodes par rapport à la pièce (P) en traitement, compte tenu de ses formes
et dimensions, à une distance prédéterminée optimale entre lesdits moyens à cathode
(16) et la pièce en traitement (P).
10. Système (10) selon la revendication 8 et/ou 9, caractérisé par le fait qu'il comprend des cathodes auxiliaires conformées comme des châssis en cuivre avec grille,
immergées dans le bain galvanique pour les parties internes d'une pièce en traitement
de forme complexe.