EP2801443B1

EP2801443B1 - Processing medium for processing stainless steel or other metallic surfaces, method for processing stainless steel or other metallic surfaces using such a processing medium and nozzle arranged to be fitted on a process gun

Info

Publication number: EP2801443B1
Application number: EP13166877.4A
Authority: EP
Inventors: Philippe Remi Aloys Bourdeaud'hui
Original assignee: Phibo Industries BVBA
Current assignee: Phibo Industries BVBA
Priority date: 2013-05-07
Filing date: 2013-05-07
Publication date: 2015-11-04
Anticipated expiration: 2033-05-07
Also published as: EP2801443A1; BE1021089B1; DK2801443T3

Description

Field of the Invention

The present invention generally relates to a processing medium for processing stainless steel or other metallic surfaces such as aluminium, copper, bronze, metal alloys, etc., wherein said processing medium is adapted to be ejected out of a nozzle of a process gun by compressed air (see for example DE 10 2010 043 285 A1 ).
The present invention also relates to a method for processing stainless steel or other metallic surfaces by means of a processing medium which is ejected out of a nozzle of a process gun by compressed air. With processing is meant that the surface is at least cleaned and conditioned, the latter meaning that the topography of the surface is changed to a certain required state.
For the identification and appellation of the different types of stainless steel (also called inox steel or inox), in the industry usually the American normalisation (American Iron and Steel Institute) is used. Dependent of the application, different types of stainless steel can be used. The American 3-A Sanctuary Standards dictates AISI 316 for all stainless steel surfaces that come into contact with food. Certain parts of an installation, the utilities such as piping, are an exception to that and therefore, also AISI 304 can be used. AISI 304 comprises 18 % chromium and 8 % nickel. This alloy is in annealed state non-magnetic and not curable and in cold formed state weak magnetic. This stainless steel is less sensitive for excretion of chromium during welding. A more corrosion resistant but more expensive type of stainless steel is AIS1316 with 16 % chromium and 10 % nickel and 2% molybdenum. AISI 316 is more resistant against salt corrosion and is frequently applied in the chemical industry. AISI 316L has a low carbon content in order to obtain easily weldable stainless steel and to reduce the corrosivity after welding. Another way to improve the weldability of steel is to add titanium to the alloy, arriving to the AISI 316Ti-type. This solution is technical almost equal. Only when one considers architectural applications, account has to be taken with a typical grinding pattern of stainless steel alloys with titanium.
The hardness of the stainless steel typically lays within the range of 150 to 400 HB (Brinell hardness) or 1 to 43 HRc (Rockwell C hardness).
The magnetic properties of stainless steel are determined by the crystal structure, thus by the composition of the type stainless steel. Types of stainless steel having between 6 and 26 % by mass nickel (the 300-series out of the AISI) are austenitic and therefore non-magnetic. These are excellent formable (folding, deep drawing, straightening) and also shock proof throughout the temperature range from very cold to very hot temperatures. Nickel takes care that steel remains in its austenitic state during cooling. The remaining elements increase the corrosion resistance and the processability of the steel. When the stainless steel is however strongly cold deformed, the crystal structure changes through which magnetic properties occur with austenitic stainless steel. Martensitic, ferritic and duplex stainless steel types to the contrary are magnetic. Austenitic stainless steel such as AISI 304 and AISI 304H and AISI 316 is the most commonly used type of stainless steel. In Europe, austenitic stainless steels of grade AISI 304 are broadly used in food industries because of their good corrosion resistance. Austenitic stainless steels containing other elements such as molybdenum (AISI 316) enhancing their anticorrosive properties are frequently used in dairy industries).
Ferritic stainless steel is used in an environment that is little aggressive and when the look is less important. Ferritic stainless steel comprises no nickel and consequently is cheaper. Duplex stainless steel is a seaworthy steel with a mixed austenitic and ferritic structure and a high yield strength. These alloys mostly are cold formed. The high strength characteristics allow a thinner, more elegant and at the same time stronger design. Martensitic stainless steel is characterised by its high hardness, through which these kind of materials are extremely suitable for the production of knifes. These steel types contain a little or no nickel, implicating that the material can be used as surgical steel.
Stainless steel has the advantage that it has a high corrosion resistance. This corrosion resistance of stainless steel is due to a 'passive' chromium-rich oxide layer (dichromium trioxide Cr₂O₃) that is formed in a natural way at the surface of the stainless steel at normal environmental temperatures. This is the natural appearance and is described as the passive state. Stainless steel will also automatically passivate when a pure surface is exposed to an environment that can provide sufficient oxygen in order to form the chrome-rich surface layer. This automatically and progressively occurs as long as sufficient oxygen is present at the surface of the steel. This passivation layer still increases some more in thickness after its initial formation. Natural phenomena, such as contact with air and aerated water, form and perpetuate the corrosion resistant passive state. Consequently, stainless steel can maintain its corrosion resistance even when mechanical damage such as scratches or chipping occurs. Consequently, an inherent self-healing mechanism is present.
This self-healing mechanism of stainless steel is primordially determined by the amount of chrome present in the stainless steel. Contrary to carbon or low alloy steel, stainless steel is a steel alloy containing a minimum of 10,5 % chromium content by mass. The corrosion resistance of these steels can be enlarged by addition of other alloy elements such as nickel, molybdenum, nitrogen and titanium (or niobium). This provides in a list of steel types with corrosion resistant properties over a large scale of user conditions as well as enlarged properties in the field of formability, strength and heat (fire) resistance. Stainless steel cannot be considered as corrosion resistant under all conditions since in function of the type of steel, conditions can occur in which the passive state is broken en cannot be restored. This makes the surface 'active' such that corrosion can occur. The surface of stainless steel can become active in restricted, deoxygenated zones such as mechanical connections, sharp corners or poorly groomed welds.
In comparison with other materials, the use of stainless steel as a construction material has a number of important advantages: a high mechanical strength, a relative good workability, a good cleanability, a high corrosion resistance and an inert surface. The good cleanability of stainless steel is closely related with the condition of the surface, which is in its turn influenced by the surface treatment.
The invention furthermore relates to a nozzle arranged to be fitted onto a process gun, said nozzle being adapted for ejecting a processing medium by compressed air on a stainless steel or other metallic surface. Said nozzle furthermore comprises a distal end section having a nozzle-exit and a distal end, wherein said nozzle-exit is located at said distal end.

Background of the Invention

The fabrication and maintenance of components and constructions in stainless steel, but also in other metals such as aluminium, copper, bronze, metal alloys, etc. implies cleaning and finishing of the (final) fabrication.
For food industries, hygiene is a permanent concern since they must commercialize high quality products in order to comply with the legislation (EN 1672-2 and EN ISO 14159 hygienic design requirements) and the expectations of the consumers. Therefore, the hygienic state of the surface is a critical parameter with respect to the performances of the production process and to the final quality of the product. For this reason, cleaning and disinfection are essential to ensure the microbiological safety of the product and to avoid higher production costs. However, the efficiency of the cleaning process will not only depend on the optimization of the process by itself and on the equipment design, but also on the characteristics of the soiled surface, i.e. mainly its roughness, surface chemical composition and surface energy. Since interactions between the soil and the surface of the stainless steel fabrication are responsible for the deposition and the adhesion of the soil on the surface, modifying the surface of the stainless steel fabrications will affect the degree of adhesion or the detachment. In the case of stainless steel, a ubiquitous material in food and pharmaceutical industries, its surface roughness and its surface topography are believed to play an important role in the hygienic behaviour, thus influencing its cleanability.
Among the different parameters characterizing a surface, another surface property dependent on surface roughness that should be considered besides cleanability is corrosion resistance. Rougher stainless steel or other metallic surfaces are more prone to corrosion which can lead to the apparition of pitting and crevices, increasing the adhesion possibilities of the micro-organisms and decreasing the cleanability of the surface. Therefore, corrosion resistance is another good reason to justify the need of equipment with smooth surfaces as they will also contribute to reduce the additional costs owing to possible damages of the installation.
For optimum corrosion resistance, stainless steel and other metallic surfaces must also be pure and free of organic contamination (grease, oil, paint). Stainless steel surfaces also have to be free of metallic contamination such as iron contamination.
Known treatment techniques for stainless steel surfaces are:

high pressure cleaning;
grinding;
pickling and passivation; and
(electrolytic) polishing.

The disadvantages thereof however are:

high pressure cleaning: only has a temporary effect;
grinding: the surface becomes very rough and the risk on adhesion of soil, tension corrosion and fracture enlarges;
pickling and passivation: has many practical objections, can only be used to treat a limited number of types of stainless steel and is furthermore environmental technical not always responsible; and
(electrolytic) polishing: is a very complex and costly process.

Shot peening is a more recent technique to induce residual surface compressive stresses in metal parts to increase fatigue strength and resistance to stress-corrosion cracking. Shot peening is a cold working process in which small spherical media called shot bombard the surface of a fabrication. During the shot peening process, each piece of shot that strikes the material acts as a tiny peening hammer, imparting to the surface a small indentation or dimple. To create the dimple, the surface of the material must yield in tension. Below the surface, the material tries to restore its original shape, thereby producing below the dimple, a hemisphere of cold-worked material highly stressed in compression. According to the Aerospace Material Specifications AMS-13165 (nowadays updated to AMS 2340), automatic shot peening requires specific peening intensities and coverage with a saturation point as can be seen in the table 1 below.

Table 1

Material	Steel under 200.000 psi	Steel over 200.000 psi Titanium and Titanium alloys	Aluminum alloys (metallic shots / ceramic shot)	Aluminum alloy (non-metallic shot)
Under 0,090 inch thickness	-	-	-	0,004 to 0,008 N
0,090 to 0,375 inch thickness	0,008 to 0,012 A	0,006 to 0,010 A	0,006 to 0,010 A	0,008 to 0,012 N
Over 0,375 inch thickness	0,012 to 0,016 A	0,006 to 0,010 A	0,010 to 0,014 A	0,012 to 0,016 N

In EP 0 638 416 for instance, a shot peening process is disclosed. In the shot peening process as described in this European patent application, a mixture of at least two sizes of shot is used to obtain a textured finish press plate with desirable gloss features. The press plate is impacted with a mixture of shot having at least two different sizes to simultaneously obtain a texture and gloss control.
In EP 1 184 135 , a single step shot peening-process is disclosed wherein a product is projected by shot in which two or three kinds particles, each having an average particle diameter within the range of the predetermined average particle diameters, the ranges differing from one another, and having a predetermined average particle diameter ratio to one another, are combined in a predetermined weight ratio. In a first embodiment of this method, the shot consists of large-diameter particles having an average particle diameter of 300 - 1.000 µm and small-diameter particles having an average particle diameter of 20 - 300 µm, the ratio of the average particle diameter of said small-diameter particles to that of said large-diameter particles being 1/3 - 1/15, are combined in a weight ratio such that the coverage of each of the particles is 100 % or more in the same projection time. In a second embodiment of this method, the shot consists of large-diameter particles having an average particle diameter of 500 - 1.000 µm, medium-diameter particles having an average particle diameter of 100 - 500 µm and small-diameter particles having an average particle diameter of 20 - 100 µm, the ratio of the average particle diameter of said medium-diameter particles to that of said large-diameter particles and the ratio of the average particle diameter of said small-diameter particles to that of said medium-diameter particles each being 1/2 - 1/15, are combined in a ratio such that the coverage of each of the particles is 100 % or more in the same projection time.
In EP 0 962 539 , a one step-method is disclosed wherein mixture shots including at least two types of shots comprised of different or same materials consisting of high hardness metal or metallic component and having different shot diameters between 0,6 and 0,03 mm are injected onto the surface of a metallic product at an injection pressure of not less than 0,29 MPa or not less than 50m / sec, the residual compressive stress of the surface of the metallic product and that of a lower surface layer are made at least - 1200 MPa and that of a portion having a depth of about 50 mµ below the surface of the metallic product is made - 1300 MPa or higher.
In EP 2 353 782 , a process for treating a surface of a component to improve its surface finish and induce residual compresses stresses in a near-surface region of the component. The process entails performing a first peening operation to form residual compressive stress layers in the near-surface region of the component, and then performing at least a second peening operation to cause surface smoothing of the surface of the component while retaining compressive stresses in the near-surface region of the component. The first peening operation comprises wet glass bead peening at a first intensity with a first glass bead media, and the second peening operation comprises wet glass bead peening at a second intensity with a second glass bead media, wherein the second intensity is lower than the first intensity.
The shot peening process has the following big disadvantages:

because of the tension that arises, there is a great risk on deformation of the treated fabrication;
the process parameters are very critical through which they have to be monitored permanently during the peening process which is very labour intensive, through which only highly qualified personnel can perform such process and through which it is only applicable to solid and sturdy fabrications which furthermore are generally limited in size;
the shot peening process generally is a dry process, through which no polishing effect of the treated surface can be obtained and through which there is an annoying development of dust;
because the effect of shot peening is determined by the kinetic energy E_c = M x V²/2, wherein M is the mass of the shot and V is the speed of the projection (wherein with air blasting, V² is a function of the air pressure), only macro sized shot can be used having a minimum average particle size of between 100 and 3000 µm, through which it is not possible to obtain the appropriate R_a of smaller than 0,6 µm (see more detailed information about R_a below); and
static electricity is excited.

In EP 2 353 782 furthermore, a more complicated process is disclosed comprising two peening steps.
There consequently exists the need to provide a simple method for processing stainless steel and other metallic surfaces of fabrications of any possible size and any possible (complex) shape, as well as a processing medium for use in such a method.
It is furthermore a need to provide such a method and a processing medium, wherein a specific surface topography is obtained meaning:

a clean surface, i.e. with as less as possible contaminations;
a smooth, polished surface with as less as possible irregularities;
an isotropic or uniform surface exhibiting the same surface topography and the same roughness values when measured along axes in all directions;
a surface with easy-to-clean properties.

For the food industry and pharmaceutical industry, it is an object to obtain such a method and a processing medium leading to a hygienic surface which is less sensitive to bacterial and soil adhesion.
Furthermore, it is a requirement to obtain such a method and a processing medium

restricting as much as possible deformation of the fabrication,
excluding the need for chemical pre- or after-treatment, and
obtaining substantially no dust formation.

The conventional nozzles which are used on process guns and which are arranged for ejecting a processing medium generally have a straight outer surface.
The disadvantage of these conventional nozzles is that the particle distribution into the stream (flow) ejected out of the nozzle is not optimal and not consistent, thus reducing the efficiency and productivity of the surface treatment. Furthermore, conventional nozzles produce noise at the outlet thereof because turbulence of air is created at the nozzle outlet.
There consequently exists a need to provide a nozzle which is arranged to be fitted on a process gun and which is arranged to eject a processing medium on a stainless steel or other metallic surface, said nozzle being adapted to obtain a better distribution of the particles of the processing medium within the stream leading to a better efficiency and a more optimal use of the energy of the stream ejected out of the nozzle. It is a further object of the invention to provide such a nozzle reducing the noise at the nozzle outlet.

Summary of the Invention

According to a first aspect of the invention, a processing medium for processing stainless steel or other metallic surfaces, wherein said processing medium is adapted to be ejected out of a nozzle of a process gun by compressed air, wherein said processing medium consists out of a suspension comprising a liquid and a mixture of at least two different types of products consisting of chemically inert abrasive particles
said particles at least comprise particles having an irregular shape, said particles being dispersable in said liquid,
said irregular shaped particles consists of fused alumina particles,
said fused alumina particles are substantially iron-free.
In the physical chemistry, a suspension is a mixture of two substances of which one substance in very small parts is mixed with another substance, which mixture is not separating quickly. In general, it relates to a solid substance which is suspended in a liquid.
Chemically inert particles are particles which will chemically not interact with other products and which will not resolve in a liquid.
With an irregular shaped particle is meant any form of particle which is not spherical, said particle more specifically having round or sharp angles.
More advantageously, said fused alumina particles have an Al₂O₃-content of 95 % - 99,80 % by weight. For processing stainless steel, it is necessary to use very pure and iron-free Al₂O₃-particles.
In a more preferred embodiment of a processing medium according to the invention, said particles also comprise particles having a spherical shape, said particles being dispersable in said liquid.
In an advantageous embodiment of a processing medium according to the invention, said particles have an average particle size of between 0,9 µm and 110 µm.
In a favourable embodiment of a processing medium according to the invention, said suspension is a balanced suspension. A balanced suspension is a suspension having particles with a similar average particle size. This is important for the filtration step of the suspension performed with the processing apparatus and for the separation of the particles from the liquid. When processing stainless steel, the use of a balanced suspension is important for obtaining the desirable surface topography.
In a favourable embodiment of a processing medium according to the invention, the mixture weight ratio of said spherical shaped particles with respect to said irregular shaped particles is preferably 60 % - 96 % / 4 % - 40 %, more preferably 70 % - 96 % / 4 % - 30%, and most preferably 80% / 20%.
By changing the ratio of the types of abrasive particles in respect with the required total concentration of the suspension, the visually aesthetic finish can be modified.
In an advantageous embodiment of a processing medium according to the invention, said spherical shaped particles consists of glass beads.
More advantageously, said glass beads have a SiO₂-content of 50 % - 80 % by weight.In a favourable embodiment of a processing medium according to the invention, said glass beads have a relative hardness of 4 Mohs - 6 Mohs, preferably 5 Mohs and said fused alumina particles have a relative hardness of 8 Mohs - 10 Mohs, preferably 9 Mohs. The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material.
Because of the relative hardness of said fused alumina particles, thermic oxidation such as the oxidation of a weld can be removed in an efficient manner.
In a preferred embodiment of a processing medium according to the invention, said suspension has a concentration in use of 10 % to 70 % particles in liquid, more preferably 10 % to 40 %, and most preferably 15 % to 25 %.
In an advantageous embodiment of a processing medium according to the invention, said mixture of particles has a balanced bulk density of from 1 kg/dm³ - 2 kg/dm³, and more preferably of approximately 1,7 kg/dm³.
In order to further extend the capability of the method and the processing medium according to the invention, to improve degreasing or to enable the surface to be disinfected during the same operation, or to inhibit oxidation, the processing medium according to the invention can comprise a soluble chemical additive. Preferably, said soluble chemical additive has a concentration of approximately 1 vol-% to 15 vol.-% on the total liquid of the suspension. Use of a soluble chemical additive is for instance advantageous when reconditioning old surfaces.
According to a second aspect of the invention, a method for processing stainless steel or other metallic surfaces by means of a processing medium which is ejected out of a nozzle of a processing gun by compressed air, wherein said method is a single step-method wherein said surfaces are processed with a processing medium according to the invention as described above.
This method according to the invention is first of all applicable on any size of fabrication and on any type of surface, even on complex and irregular shaped surfaces, cavities and hollow tubular structures. No deformation of the fabrication or damage of the surface occurs and size limits of the processed fabrication are maintained. No special skills are required for the operator. Furthermore, build-up of static electricity is eliminated.
Because the method is a wet method using a suspension, a polishing effect of the treated surface is obtained. The liquid in the suspension furthermore forms a liquid buffer that takes care that there is no direct impact of the particles of the suspension into the treated surface, through which the risk of damaging the surface and impregnation of particles in the surface is seriously reduced. Also, no dust formation is obtained. Furthermore, very small particles can be used.
Furthermore, the method results in a hydrophobic surface partially due to the polishing effect. This hydrophobic surface also is obtained because of the suspension used.
The method furthermore exhibits the same surface topography and the same roughness values when measured along axes in all directions, with specific (visual) cosmetic properties such as a smooth satin finish. It allows achievement of a very fine surface finish with a roughness average R_a < 0,6. The method thus provides in a repeatable, non-directional, uniform finish from part to part.
The method according to the invention provides in a thorough cleaning method and allows to remove simultaneously oil, grease, carbon, discolouration, rust, paint and general dirt from the surface and surface oxidation that arises during the welding of stainless steel (typical discoloration by oxidation of the metal surface). The method is very well suited for cleaning and degreasing during reconditioning, as for overhauling and manufacturing. After cleaning, the surface is ready for close inspection and crack detection if required.
The method according to the invention also creates compressive residual stress in the surface improving the durability and the corrosion resistance of the manufactured stainless steel structures and components. With this method, specific peening intensities and coverage with a saturation point of 0,008"N, and more preferably not more than 0,005"N, as described and determined for shot peening according to AMS 13165 (nowadays superseded by AMS 2340) are obtained, this without the need of special skills of the process operator. Strict peening intensity criteria however is not critical to the method as most of the shot peening applications are intended to improve the fatigue strength. The consistency in obtaining the desired topography remains the most important advantages of the method.
Other advantages of the method according to the invention are that

an iron free surface is obtained when processing stainless steel and other not sensitive corrosion metallic surfaces such as aluminium, etc;
an isotropic surface topography with a specific 2D profile and 3D topography through which a better cleanability of the surface is obtained;
the surface topography created by the process is less sensitive for bacterial and soil adhesion;
it allows the restoration of the self-healing mechanism of passivation (Cr₂O₃ auto passivation) after the removal of the chromium-poor layers that for instance occur as a result of welding;
it avoids the distortion of the stainless steel surfaces;
it avoids the use of dangerous or toxic chemicals;
it is a very cost effective process;
it is flexible and versatile; and
there are no health and environmental issues and the process is substantially free of dust.

A very suitable application area of the method according to the invention is stainless steel food-contact surfaces from small components to extensive manufactured stainless steel structures. Other application areas like aerospace, automotive, etc. are also possible.
In an advantageous embodiment of a method according to the invention, said suspension is ejected out of said nozzle at a low nozzle output pressure from 0,5 to 5 bar (0,05 MPa to 0,5 MPa), and more preferably from 1,5 to 4 bar (0,15 MPa to 0,4 MPa).
By adjustment of the nozzle output pressure, it is possible to remove scratches and discolorations, to reduce blemishes and to provide the surface with a specific texture, resulting in an easy to clean surface with a burnished, satin look.
In a preferred embodiment of a method according to the invention, said suspension is ejected out of said nozzle at a balanced and controllable volume output of from 20 I/minute up to 130 I/minute, and more preferably from 30 I/minute up to 100 I/minute.
According to a third aspect of the invention, use of a nozzle arranged to be fitted on a process gun during a method for processing stainless steel or other metallic surfaces according to the second aspect of the invention by ejecting a processing medium according to the first aspect of the invention out of the nozzle by compressed air on a stainless steel or other metallic surface, said nozzle comprising a distal end section having a nozzle comprising a distal end section having a nozzle-exit and a distal end, said nozzle-exit being located at said distal end, wherein said distal end section comprises an outer profile arranged to induce an external suction air flow around said nozzle-exit, said outer profile is tapered towards said distal end and comprises a plurality of longitudinally extending notches arranged around the circumference of said distal end section.
Such a nozzle has the advantage that the performance of the stream of the processing medium is enhanced, and a better spread and consistency of the processing medium is obtained.
This special shape of the nozzle reduces the processing noise resulting from the compressed air used for the acceleration of the processing medium.

Brief Description of the Drawings

Fig. 1 illustrates a 3D surface topography of sample 1 of a stainless steel substrate that is treated with a method according to the invention using a first set of parameters;
Fig. 2 illustrates a 2D roughness profile descriptor of sample 1 of which the 3D surface topography is shown in figure 1;
Fig. 3 illustrates a 3D surface topography of sample 4 of a stainless steel substrate that is treated with a method according to the invention using a fourth set of parameters;
Fig. 4 illustrates a 2D roughness profile descriptor of sample 4 of which the 3D surface topography is shown in figure 3;
Fig. 5 illustrates an example of a saturation curve plotting Almen strips arc heights versus exposure time to a peening process on a graph;
Fig. 6 illustrates an example of an intensity determination curve of a peening process;
Fig. 7 illustrates the obtained saturation curve of two Almen strips as prescribed according to the AMS-S-13165 which are treated with the method according to the invention as applied on samples 2 and 4;
Fig. 8a - 8d illustrate the results of the Salt Spray Tests carried out following ASTM A967 "Standard Specification for Chemical Passivation Treatments for Stainless Steel" after treating welded stainless steel parts with the method according to the invention;
Fig. 9a illustrates the surface wettability by the behaviour of water on an untreated stainless steel surface;
Fig. 9b illustrates the surface wettability by the behaviour of water on a stainless steel surface which is treated with the method according to the invention;
Fig. 10a illustrates a top view of a first type of processing platform for processing a stainless steel fabrication by the method according to the invention;
Fig. 10b illustrates a front view of the processing platform as shown in figure 10a;
Fig. 11 a illustrates a front view of a second type of processing platform for processing a stainless steel fabrication by the method according to the invention;
Fig. 11 b illustrates a top view of the processing platform as shown in figure 11a;
Fig. 11 c illustrates a detailed view of the floor sweepers of the processing platform as shown in Fig. 11 a and 11 b in free moving mode of the sweepers;
Fig. 11d illustrates a detailed view of the floor sweepers of the processing platform as shown in Fig. 11a and 11b in sweep or recovery mode of the sweepers;
Fig. 12a illustrates a front view of a nozzle of a process gun for streamlining, accelerating and ejecting processing medium;
Fig. 12b illustrates a cross section of the nozzle as shown in Fig. 12a.

Detailed Description of Embodiment(s)

The processing medium according to the invention for processing stainless steel or other metallic surfaces, wherein this processing medium is adapted to be ejected out of a nozzle of a process gun by compressed air, consists out of a suspension, also called "slurry", comprising a liquid, preferably water (resulting in an aqueous suspension), and a mixture of at least two different types of products consisting of chemically inert abrasive particles (or in other words two products consisting of chemically inert abrasive particles that are not the same). The products. These abrasive particles have an average particle size of between 0,9 and 110 µm. These particles at least comprise irregular shaped abrasive particles. The particles furthermore preferably comprise spherical shaped abrasive particles. These particles preferably are substantially massive (solid) in order to prevent that these particles would float. In order to preferably obtain a balanced suspension, the particles of the two different products preferably have a similar particle size. The mixture weight ratio of the spherical shaped particles with relation to the irregular shaped particles is 60 % - 96 %, more preferably 70 % - 96 % / 4 % - 30 %, and most preferably 80% / 20%. The mixture has a balanced bulk density of from 1 to 2 kg/dm³ and preferably of approximately 1,7 kg/dm³.
The particles preferably are of the mineral type. The spherical shaped particles preferably consist of glass beads having a SiO₂-content of 50% to 80% by weight. The irregular shaped particles preferably consist of fused alumina particles having an Al₂O₃-content of 95 % - 99,8 % by weight. More preferably, white fused alumina is used. When stainless steel is treated, very pure and iron-free fused alumina particles have to be used since, when there is a risk of iron-inclusion in the stainless steel surface, then there consists a risk that oxidation or corrosion of the surface occurs. The glass beads have a relative hardness of 4 - 6 Mohs, preferably 5 Mohs and the fused alumina particles have a relative hardness of 8 - 10 Mohs, preferably 9 Mohs. The suspension has a concentration in use of 10 % to 60 %, more preferably 10 % to 40 % and most preferably 15 % to 25% of particles in liquid, preferably water.
The suspension furthermore can comprise a soluble chemical additive in a concentration of preferably approximately 1 - 15 vol.-% on the total liquid (water + soluble chemical additive) of the suspension. For instance, following soluble chemical additives can be added to the suspension:

a biocide agent in order to disinfect the processed surface. This is especially important in the food, diary or pharmaceutical industry.
a degreasing agent. This is especially interesting to be used in the reconditioning of old surfaces.
a corrosion inhibitor. This is especially recommended to be used when corrosion sensitive materials such as steel and cast iron are wet processed in order to protect the treated surfaces against rust attack or rust formation.
a passivation agent can be used for accelerating the auto passivation of stainless steel.

The method according to the invention is a single step-method wherein a stainless steel surface or another metallic surface such as aluminium, copper, bronze, metal alloys, etc. are processed with the suspension according to the invention as disclosed above. This suspension is ejected out of a nozzle of a process gun by compressed air onto the surface to be treated. The suspension furthermore is preferably streamlined and accelerated in this nozzle. The suspension is thus ejected onto the target surface in a highly energized but controllable stream. The nozzle air output pressure is a low pressure from 0,5 to 5 bar and preferably from 1 to 4 bar. This nozzle output pressure optionally is measured using a pressure transducer and optionally using an electro pneumatic pressure regulator. This allows a reliable closed loop control of this parameter. The suspension pressure being fed to the blast gun will settle itself when the blasting pressure is adjusted. The suspension is ejected out of the nozzle at a balanced and controlled volume output of 20 I/minute up to 130 I/minute, and preferably from 30 I/minute up to 100 I/minute (per process gun).
The ratio between the liquid pressure and the air pressure in the nozzle is variable to allow the liquid buffer as already disclosed above to be maintained between the suspension and the treated surface. Under normal working conditions, i.e. at recommended processing pressure applied in the invention, this liquid buffer effect is maintained.
In the following paragraphs, 4 identical samples of inox AISI 304 were processed with a method according to the invention having the following different process parameters:

Sample 1: 8 % - 12 % fused alumina particles and 88 % - 92 % glass beads at a pressure of 2,5 - 3,5 bar and at a concentration of particles in water of 15 % - 25 %;
Sample 2: 8 % - 12 % fused alumina particles and 88 % - 92 % glass beads at a pressure of 3,5 - 4,5 bar at a concentration of particles in water of 15 % - 25 %;
Sample 3: 13 % - 17 % fused alumina particles and 83 % - 87 % glass beads at a pressure of 2,5 - 3,5 bar and at a concentration of particles in water of 15 % - 25 %;
Sample 4: 13 % - 17 % fused alumina particles and 83 % - 87 % glass beads at a pressure of 3,5 - 4,5 bar and at a concentration of particles in water of 15 % - 25 %.

In order to quantify the surface topography of these processed samples, different roughness parameters were measured.
A first important parameter is the roughness average R_a. The surface roughness is a measure of the texture of a surface. It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small the surface is smooth. R_a is by far the most common used roughness parameter in use. R_a is the arithmetic average of the absolute values and is defined as $\frac{1}{n} \sum_{i = 1}^{n} |y_{i}| .$
R_a is expressed as units of height.
The EHEDG (European Hygienic Engineering and Design Group) recommends surface roughness of less than 0,8 µm and all 3-A sanitary criteria (USA standards) also include surface finish requirements which demand R_a values smaller than 0,8 µm R_a. The American Meat Institute Equipment Design Task Force also recommends that surfaces should not have R_a values exceeding 0,8 µm.
The method according to the invention results in an R_a of lower than 0,6 µm and more preferably between 0,3 and 0,6 µm.
However, next to the R_a-parameter, other roughness parameters are important to investigate the relation between the surface topography and its cleanability, such as

the waviness height W_t (total height of W-profile) being the sum of the largest profile peak height and the largest profile valley depth of the W-profile within the evaluation length In (reference length);
the maximum roughness depth R_max being the largest single roughness depth within the evaluation length;
the peak count RP_c being the number of roughness profile elements per cm which consecutively intersect the specified upper profile section level c₁ and the lower profile section level c₂;
the percentage of open space in the valleys; and
the reduced valley depth R_vk being the mean depth of the valleys protruding from the roughness core profile.

In table 2, the roughness average R_a, the waviness height W_t, the maximum roughness depth R_max and the peak count RP_c, the percentage of open space in the valleys and the reduced valley depth R_vk of the samples 1 to 4 as mentioned above are set out:

Table 2

Sample no.	R_a	W_t	R_max	RP_c	Percentage of open space in the valleys	R	_vk
1	0,340	0,460	3,080	7,5	11,35%	0,495
2	0,505	0,725	4,520	26,5	10,60%	0,980
3	0,320	0,325	2,935	9,0	11,55%	0,615
4	0,535	0,745	4,125	30,5	10,25%	0,625

Out of table 2, it can be concluded that when the output nozzle pressure increases at a constant concentration of the fused alumina particles and the glass beads, all roughness parameters increase. Furthermore, out of this table 2, it can be derived that changing the concentration of the fused alumina particles and the glass beads while remaining the concentration constantly has no significant influence on the roughness parameters. Other tests however have demonstrated that the latter then has an effect on the peening intensity and the removal of thermic oxides.
In figure 1, the 3D surface topography and in figure 2, the 2D roughness descriptor of the surface (cross section over the total sample length) of sample 1 are shown. In figure 3, the 3D surface topography and in figure 4, the 2D roughness descriptor of the surface of sample 4 are shown. The definitions and surface texture parameters are in conformity with EN ISO 4287 - 4288 & ASME B46.1. It can be concluded from these figures that the method according to the invention creates a uniform finish with low roughness values exhibiting properties with the same values when measured along axes in all directions (= isotropic surface). Furthermore, the low and well controlled roughness leads to surfaces with minimal retention of soils and with high cleanability. Such a surface topography shows an ideal engineered surface for food-contact surfaces.
When there is a suspicion of surface contamination, multiple tests are conducted to substantiate this. The American standards ASTM A380 & A967 are usually used for testing and measuring procedures for the quality of stainless steel.

In the table 4 below, results of the Salt Spray Tests carried out following the ASTM A967-norm "Standard Specification for Chemical Passivation Treatments for Stainless Steel" after the treatment of the 4 different samples as mentioned above which were welded and thereafter were treated with the method according to the invention are shown.

Table 4

Sample no.	Surface condition after the salt spray test	Weld condition after the salt spray test	General condition after the salt spray test
1	unchanged	unchanged	unchanged
2	unchanged	unchanged	unchanged
3	unchanged	unchanged	unchanged
4	unchanged	unchanged	unchanged

Out of table 4, it can be concluded that the surface after being processed with the method according to the invention is free of organic and metallic contamination, or in other words a pure surface is obtained. Furthermore, it is possible that a new oxide skin arises by the self-healing passivation mechanism restoring the optimal corrosion resistance of the stainless steel surface.
Also Oxilyser tests with the Oxilyser 3 were performed on the samples 1 to 4 showing that the passivation took properly place. This Oxilyser test is very suitable to control the corrosion resistance of stainless steel and more specifically the quality of the passivation on large fabrications. By spot-checking on several critical places such as edges, stains, welds, etc, large surfaces can be efficiently checked within a short time.
As already stated above, the specific process parameters of the method according to the invention induce, during the surface processing of the stainless steel substrates, compressive residual stress in the surface improving the durability and the corrosion resistance of the manufactured stainless steel structures and components, as is the case with shot peening.
In the table 5 below, the results of the shot peening tests carried out following the process parameters of the method according to the invention are shown. As already mentioned above, the adopted procedure to measure the intensity of the shot peening is based on the specification of the US Mil. Spec. adopted as an SAE standard, namely the SAE-AMS-S-13165, now superseded by AMS 2430, who covers procedure requirements for shot peening of metal parts, to induce residual compressive stresses in specified surfaces, for the purpose of improving resistance to fatigue, stress corrosion cracking and galling. The used Almen strips to measure the intensity of the peening are type N-1S (± 0,0005" flatness tolerance). The Almen test strip specimens conforming in dimensions and mechanical properties to the required test strip specifications of AMS-S-13165 (and SAE AMS 2430S) are exposed to the suspension stream in a manner which simulates the operator working conditions used in the process according to the invention.

Test procedure

After exposure, the test strips are removed from the holder and the amount of deflection is measured with a micrometre gage that meets the technical requirements of AMS-S-13165. A saturation curve is produced by exposing individual test strips to the suspension stream for increasing time periods and plotting the results (exposure time vs. arc height). A minimum of four points other than zero shall be used to define the curve; one of the four points used to indicate saturation shall be at least double the time of the saturation point. When the arc height is plotted against time, the intensity of the suspension stream can be determined by evaluating the first point or intensity on the best fit line where, when the exposure time is doubled, there is a 10% increase in arc height. Then saturation is achieved (see figure 5). The intensity of the peening can be determined using an intensity determination curve (see figure 6).

In the table 5 below, the Almen plate deflection, expressed in µm and inches, of the

samples

2 and 4 as mentioned above during an exposure time from 0 to 180 seconds is set out. In figure 7, the graphic corresponding with the data in table 5 is shown.

Table 5

	Sample 2	Sample 4
Exposure time (s) / (min)	Almen plate deflection (µm) / (inches)
0	0	0
15 / 0,25	142 / 0,006	137 / 0,005
30 / 0,5	177 / 0,007	164 / 0,006
60 / 1	209 / 0,008	190 / 0,007
80 / 1,2	221 / 0,009	199 / 0,008
140 / 2,2	238 / 0,009	213 / 0,008
180 / 3	242 / 0,010	216 / 0,009

As can be seen in figure 7, the saturation intensity (square on the respective curve) of sample 2 is 218,58 µm or 0,0086 inch during an exposure time of 1,25 minutes. The saturation intensity (the square on the respective curve) of sample 4 is 193,06 µm or 0,0076 inch during an exposure time of 1,11 minutes.
Out of table 5, it can be concluded that the method according to the invention creates a gentle peening effect easy to reproduce, inducing during the surface conditioning of the stainless steel substrates compressive residual stress in the surface exhibiting peening intensities which are benefit for the durability and the corrosion resistance of the manufactured stainless steel structures and components. Next to the engineered smooth stainless steel surface, the process according to the invention allows to obtain specific peening intensities and coverage with a saturation point of 0,008"N in order to avoid or to reduce the risk of distortion and high residual tensile stresses that may result from the process according to the invention in the core material of very thin stainless steel substrates. Generally, it may be accepted that a peening intensity of approximately 0,005"N in accordance with SAE-AMS-S-13165 will be achieved after ± 15 sec. processing time.
In figures 9a and 9b, the behaviour of water on a stainless steel surface treated with the process according to the invention (figure 9b) versus a stainless steel surface treated with a conventional dry bead blasting method (figure 9a) is shown. As can be seen in figure 9b, this surface shows a hydrophobic surface (a surface having little or no tendency to absorb water) with a surface energy repelling the water (few water droplets are left on the slightly inclined surface), while the surface as shown in figure 9a shows a water film on the surface, typical state of a hydrophilic surface (a surface exhibiting an affinity for water). Hydrophilic materials possess a high surface energy and furthermore have the ability to develop hydrogen bond between surface and water molecule. Under such conditions, water will spread spontaneously on the clean high energy surface. A hydrophobic surface possesses the characteristic of opposite response to water interaction compared to hydrophilic surfaces. Mostly, it possesses low surface energy value and lack of active groups in its surface chemistry for formation of hydrogen bond with water. Hydrophobic surfaces have a low wettability and high value of contact angles such as organic polymers and wax.
It can thus be concluded that out of figure 9b that the method according to the invention creates a hydrophobic surface easy to reproduce, improving the surface properties of the stainless steel substrates and exhibiting a surface topography which is benefit for the cleanability of stainless steel substrates.
The method according to the invention can be operated using a wide range of standard machinery (generally in the form of closed-loop processing cabinets) of varying sizes or existing custom designs that are adapted in order to be compatible with the specific technical requirements of the method according to the invention. However, for processing extensive and complex shaped constructions of stainless steel or other metals as the ones disclosed above, specially designed equipment for manual operation are recommended. Manual operation includes larger booths in which the operator, wearing protective clothing, enters the blast cubicle using a trigger operated processing gun. Booths are offered as fully equipped and installed rooms or offered in the form of processing platform (generator) with the vital technical components in the form of the required pumps, processing gun(s), filtration devices and control units for installation.
The most important parameters of the process according to the invention as described above having an impact on the processing machinery are the following:

the type of suspension according the invention;
the pressure of the compressed air,
the consistency of the suspension in use,
the total particle concentration (in the invention being the mixture of the particles of the two different products as described above) within the suspension,
the pressure and the flow of the suspension.

In the figures 10a and 10b, a first type of processing platform (1) is shown comprising a sump unit having a funnel (2) which is placed below one or more grids (not shown on the figures) which are adapted that an operator can stand on it and which are adapted to allow the passage and the recuperation of the suspension into the funnel (2). The funnel (2) preferably is made in the form of a tub. In order to obtain a homogeneous suspension, one or more pumps (3a), preferably in the form of a Vortex pump, each driven by an electromotor (3b), are provided. These pumps (3a) are arranged to recycle the suspension to the process gun. The processing funnel (2) is engineered by means of a turbulent flow in order to keep the suspension in motion (see the arrows), and this especially during the processing method, in order to prevent that the particles would precipitate and stick together and to guarantee the consistency of the suspension to the process gun. Furthermore, a large range of effluent handling and filtration systems (working on the sedimentation principle, cyclonic separation and/or using cartridge filters, etc) are available to allow open, rec-circulatory or fully closed loop processing machines.
In the figures 11 a and 11 b, a second type of processing platform (4) is shown comprising a flat collection and recovery tub having, preferably elastomer, floor sweepers (5) (also called wipers) that are driven by an electromotor (6) in order to reciprocate the sweepers (this means back and forth movement of the sweepers) (see the arrows). These floor sweepers (5) are placed below grids (not shown on the figures) which are adapted that an operator can stand on it and which are adapted to allow the passage and the recuperation of the suspension into a receptacle (7). As can be seen on figure 11b, inside the space bounded by this receptacle (7), a plurality of these floor sweepers (5) are arranged at the underside of the receptacle (7). These floor sweepers (5) preferably are mounted on a frame (10) in order to be connected to one another. When the floor sweepers (5) are in a recovery or in sweep mode (see figure 11d), the suspension (meaning the abrasive particles (8) and the liquid (9), preferably water), are brought together and finally (see figure 11d), the floor sweepers (5) bring the suspension (8 + 9) towards a funnel (11) whereafter the suspension (8 + 9) is recycled by means of a pump driven by an electromotor (not shown on the figures) similar as the one described in the first type of processing platform (1).
In both processing platforms (1, 4), the concentration of the suspension can be measured using a graduated abrasive concentration-level device which is fitted into the circuit of the suspension.
The method according to the invention can be operated using a process gun in the form of an injector gun, also called venturi gun. Any type of conventional nozzle can be fitted onto such a process gun for ejecting the processing medium out of the nozzle onto the surface to be treated.
In figures 12a and 12b however, a specially engineered nozzle (20) is shown that is adapted for ejecting a processing medium by compressed air and which is adapted to be fitted by means of its proximal end (30) (also called nozzle inlet end) onto a process gun. This nozzle is furthermore preferably adapted to streamline and accelerate the processing medium therein. The processing medium can be in the form of the suspension according to the invention as described above, but can also be any other suitable processing medium for processing a stainless steel or other metal surface such as a dry blasting medium (i.e. a medium that is ejected out of a nozzle with compressed air but without the use of a liquid).
This nozzle (20) comprises a distal end section (21) (also called nozzle outlet section) (see figures 12a and 12b) having a nozzle-exit (22) (see figure 12b) through which the processing medium passes and is ejected in the processing mode of the process gun. This nozzle (20) furthermore comprises a distal end (23) (also called nozzle outlet end) at which the nozzle-exit (22) is located.
The distal end section (21) furthermore comprises an outer profile (24) that is arranged to induce an external suction air flow around the nozzle-exit (22). As can be seen on figure 12a, this outer profile (24) more specifically is enlarged with respect to the remaining part of the nozzle (20). This outer profile (24) furthermore is tapered towards the distal end (23) and comprises a plurality of longitudinally extending notches (25) that are arranged around the circumference of the distal end section (21). These notches (25) are arranged to draw air between each two notches (25) from the back (25b) to the front (25a) of these notches (25).
As can be seen on figure 12b, the nozzle (20) is built up out of an outer jacket (26), preferably made out of aluminium, and an inner section (27), preferably made out of boron carbide. Boron carbide (B4C) is an extremely hard boron-carbon ceramic material having a Mohs hardness of approximately 9,5.
Although the present invention has been illustrated by reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various changes and modifications without departing from the scope thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being defined by the appended claims.

Claims

A processing medium for processing stainless steel or other metallic surfaces, wherein said processing medium is adapted to be ejected out of a nozzle of a process gun by compressed air,
said processing medium consists out of a suspension comprising a liquid and a mixture of at least two different types of products consisting of chemically inert abrasive particles, CHARACTERISED IN THAT
said particles at least comprise particles having an irregular shape, said particles being dispersable in said liquid,
said irregular shaped particles consist of fused alumina particles,
said fused alumina particles are substantially iron-free.
A processing medium according to claim 1, CHARACTERIZED IN THAT said fused alumina particles have an Al₂O₃-content of 95 % - 99,80 % by weight.
A processing medium according to claim 1 or 2, CHARACTERIZED IN THAT said particles also comprise particles having a spherical shape, said particles being dispersable in said liquid.
A processing according to any one of claims 1 to 3, CHARACTERIZED IN THAT said particles have an average particle size of between 0,9 µm and 110 µm.
A processing medium according to claim 4, CHARACTERIZED IN THAT said suspension is a balanced suspension.
A processing medium according to claim 4 or 5, CHARACTERIZED IN THAT the mixture weight ratio of said spherical shaped particles with respect to said irregular shaped particles is 60 % - 96 % / 4 % - 40 %.
A processing medium according to any one of claims 3 to 6, CHARACTERIZED IN THAT said spherical shaped particles consists of glass beads.
A processing medium according to claim 7, CHARACTERIZED IN THAT said glass beads have a SiO₂-content of 50 % - 80 % by weight.
A processing medium according to any one of claims 7 to 8, CHARACTERIZED IN THAT said glass beads have a relative hardness of 4 Mohs - 6 Mohs and said white fused alumina particles have a relative hardness of 8 Mohs - 10 Mohs.
A processing medium according to any one of claims 1 to 9, CHARACTERIZED IN THAT said suspension has a concentration in use of 10 % to 70 % particles in liquid.
A processing medium according to any one of claims 1 to 10, CHARACTERIZED IN THAT said mixture of particles has a balanced bulk density of from 1 kg/dm³ - 2 kg/dm³.
A processing medium according to any one of claims 1 to 11, CHARACTERIZED IN THAT said suspension comprises a soluble chemical additive.
A processing medium according to claim 12, CHARACTERIZED IN THAT said soluble chemical additive has a concentration of approximately 1 vol-% - 15 vol.-% on the total liquid of said suspension.
A method for processing stainless steel or other metallic surfaces by means of a processing medium which is ejected out of a nozzle of a process gun by compressed air,
CHARACTERIZED IN THAT said method is a single step-method wherein said surfaces are processed with a processing medium according to any one of claims 1 to 13.
A method according to claim 14, CHARACTERIZED IN THAT said suspension is ejected out of said nozzle at a low nozzle output pressure from 0,5 to 5 bar.
A method according to claim 14 or 15, CHARACTERIZED IN THAT said suspension is ejected out of said nozzle at a balanced and controlled volume output of 20 I/minute up to 130 I/minute.
Use of a nozzle (20) arranged to be fitted on a process gun during a method for processing stainless steel or other metallic surfaces according to any of the claims 14 to 16 by ejecting a processing medium according to any of the claims 1 to 13 out of the nozzle (20) by compressed air on a stainless steel or other metallic surface, said nozzle (20) comprising a distal end section (21) having a nozzle-exit (22) and a distal end (23), said nozzle-exit (22) being located at said distal end (23), wherein said distal end section (11) comprises an outer profile (24) arranged to induce an external suction air flow around said nozzle-exit (22), said outer profile (24) being tapered towards said distal end (23) and comprises a plurality of longitudinally extending notches (25) arranged around the circumference of said distal end section (21).