WO1994019511A1 - Method of manufacture of a metal part coated with mineral materials, part obtained and use thereof - Google Patents

Abstract

Method of manufacture of a metal part (1) coated with several layers (4, 5, 6) of mineral materials, said method allowing some oxidation of the metal part's surface. The invention also concerns a hollow revolving part manufactured according to the above-mentioned method. The manufacture of the hollow revolving part comprises the following stages wherein a liquid casting is subjected to: a treatment using magnesium, an inoculation process and a centrifugal operation during which the first layer (4) of the coating (2) is applied to the metal part. The metal part of the invention is for use in the transport of aggressive fluids.

Description

 Method of manufacturing a metal part coated with mineral materials, oiècp nht.pnue et. its use.

The present invention relates to the manufacture of a metal part coated with mineral materials. In particular, the invention relates to the manufacture of a metal part coated with a mineral coating based on silica and sodium oxide.

The manufacture of glass-coated cast iron objects is known (see for example document FR-495,190).

The mineral coating constitutes an effective protection of the metal part and has good chemical inertness with respect to a fluid in contact with it.

The object of the invention is to obtain a metal part whose coating which provides a protective role and chemical inertness, is adhered to the substrate, and has few open porosities so as to form a passive barrier between the fluid and the substrate.

To reduce the porosity and improve the adhesion of the mineral coating to the metal substrate and to ensure very good closure of the coating so as to protect the substrate from corrosion, several layers of said mineral coating are successively applied to at least part of the surface of the metal part. The elements of mineral material are present in variations of different proportion according to the layers. The first coating layer must be sufficiently covering to form an antioxidant film and said layer must be thin enough to allow degassing and decantation of the gas bubbles resulting from the reaction between the first layer and the metal.

A first coating layer with a thickness of less than 200 μm is produced.

In order to carry out the coating at least in part while the metal is at an elevated temperature above 700 ° C., the production of the first coating layer is carried out by application of the mineral material in powder form, in particular a dry powder. . During the production of the first coating layer, the fluidity of the mineral material must be adapted to ensure a fusion of the powder, necessary for coating the substrate and for adhesion to the substrate, and for limiting the chemical reactions between the substrate and the first coating layer.

A reducing substrate, such as cast iron, and a first layer of oxidizing coating present a risk of redox reaction with degassing and appearance of blisters.

A composition that is too fluid can cause a flow of the coating, with the defects that would follow, loss of the covering appearance and oxidation of the substrate. An insufficiently fluid composition will not allow good melting of the powder, and therefore a lack of adhesion and poor coverage, hence a risk of oxidation of the substrate.

Oxidation of the substrate is particularly marked during heat treatments at high temperature.

The fluidity of the composition largely depends on the silica content and the content of fluxes.

In order for the material to have the required fluidity, the mineral material applied to form the first coating layer is an enamel having the following composition by weight:

Si0 ₂ 50 to 70%

CaO + MgO 5 to 20% B2O3 less than 10% CoO less than 2%

Fe ₂ C> 3 less than 5% AI2O3 less than 10% Na ₂ 0 + K ₂ 0 15 to 25% F ₂ less than 2% NiO less than 2% In order to limit the redox reactions between the coating and the substrate, the mineral material is reduced during its preparation.

In order to allow the powder to melt, for a good coating and good adhesion, the first coating layer with a thickness of 100 μm is produced.

To deposit the mineral material on a hot part, the second coating layer is produced by applying the mineral material in the form of dry powder.

To deposit the mineral material on a part at room temperature, the second coating layer is produced by applying a slip of the mineral material. The slip is a very fluid mixture of finely ground mineral material and water with various adjuvants such as antioxidants and suspending agents.

In order to absorb the first coating layer, to dissolve the skin of iron oxides formed on the surface of the metal substrate, when the latter consists of a ferrous metal, and to cause a surface attack on the substrate, the second coating layer of mineral material by application of said mineral material which is an enamel of the following composition by weight:

Si0 ₂ 40 to 60%;

CaO + MgO less than 5%

Li ₂ 0 less than 5%

Ti0 ₂ 5 to 15% N NiiOO less than 2%

ZnO less than 2%

A1 ₂ 0 ₃ less than 10%

Na 0 + K ₂ 0 15 to 20%

^B 2 ° 3 5 to 15% C CooOO less than 2% sb ₂ o ₃ less than 2% In order to obtain good adhesion of the coating, by absorption, dissolution and surface attack as defined above, the second coating layer is produced at a thickness greater than 100 μm. To prevent the second coating layer from being too porous, it is produced at a thickness of 200 μm.

In order to ensure the closure of the coating and to obtain a good regularity in thickness of the coating, which limits the open porosities and gives the coating a smooth surface, a coating is produced on the surface of the part of revolution by application. three layers of mineral materials, the material forming the third layer being applied in the form of powder or slip. The third layer is produced by applying an enamel having the following composition by weight:

Si0 ₂ 40 to 60%;

CaO + MgO less than 5%

Li ₂ 0 less than 5% T TiiO0 _2, 5 to 15%

NiO less than 2%

ZnO less than 2%

A1 ₂ 0 ₃ less than 10%

Na 0 + K ₂ 0 15 to 20%

CoO less than 2% sb ₂ o ₃ less than 2% This composition which may be the same as that of the second layer or which may vary with respect to the latter, makes it possible to coat the previous layers by forming a covering film.

To ensure the closing and covering functions, the third layer is produced at a thickness greater than 100 μm. In order to obtain a good fusion on deposit, the third layer is produced at a thickness of 200 μm. The manufacture of tubular cast iron parts coated with glass is known (see for example document FR-2 297 817 in the name of the Applicant).

The manufacturing according to the invention of a metallic piece of revolution is done according to a process comprising at least the following successive steps:

- a magnesium treatment,

- an inoculation

- A centrifugation operation of said cast iron, operation during which the first coating layer as defined above is produced.

This process makes it possible to deposit the coating during the manufacture of the metallic piece of revolution and to take advantage of the thermal energy of the metal to achieve the melting of the first layer.

The integration of the steps of depositing the coating with the steps of manufacturing the metallic piece of revolution makes it possible to reduce the number of manufacturing operations and therefore to make the manufacturing process less expensive.

The subject of the invention is also a hollow metal part of revolution, coated on the internal part of its surface with mineral materials, this part being manufactured in accordance with the method described above.

This part makes it possible to transport aggressive fluids without the characteristics of these fluids being altered or the hollow part of revolution being deteriorated. The invention also relates to the use of the part obtained according to the process defined above for the transport of aggressive fluids.

The present invention is represented by an exemplary implementation applied to an element of water supply or sanitation piping in ductile iron, illustrated by appended figures, in which: - Fig. 1 shows in perspective a pipe element obtained -i according to the method of one invention;

- Fig. 2 is a micrograph at 100 magnification of the enamel coating with the joint area of the cast iron substrate of the piping element shown in FIG. 1.

In this example of manufacturing a piping element 1 having an internal coating 2, the liquid iron undergoes a magnesium treatment then an inoculation. The inoculation of the cast iron is carried out using an inoculant such as ferro-silicon introduced into the liquid cast iron in the form of powder or wire, in known manner, for example the document FR-A-2 546 783 in the name of the Applicant.

The method includes an operation of centrifuging the cast iron. During the centrifugation operation, the first layer 4 of coating 2 is produced at a thickness of 100 μm, by application of a reduced enamel of the following composition:

Si0 ₂ CaO + MgO balance 10% B ₂ 0 ₃ 5%

CoO 1% ^Fe 2 ° 3 ^2%

A1 ₂ 0 ₃ 5%

Na ₂ 0 + K ₂ 0 22% F ₂ 1%

NiO 1% This enamel is applied as a dry powder.

After the centrifugation operation, the pipe element 1 is graphitized. Then, the second coating layer 5 2 is produced at a thickness of 200 μm, by applying an enamel in powder form, said enamel being of the following composition by weight: Si0 ₂ balance CaO + MgO 0.5% Li ₂ 0 4%

Ti0 ₂ 6.5%

NiO 0.5%

ZnO 0.5%

A1 ₂ 0 ₃ 6%

Na ₂ 0 + κ ₂ o 19%

^B 2 ° 3 11%

CoO 1.5%

Sb 0 ₃ 1% The second coating layer 5 is applied to the piping element 1 at a temperature between 800 ° and 700 ° C. Next, the piping element 1 is ferritized in a temperature range of less than 800 ° C., which ensures the baking of the second coating layer 2. Then the third coating layer 6 is produced with a thickness of 200 μm by application of enamel in the form of powder of composition identical to the composition of the enamel of the second layer 5 of coating 2 presented above. This powder is deposited on the pipe element 1 at a temperature below 800 ° C.

After the deposition of the third layer 6, the third layer is glazed, that is to say a firing of a duration of less than five minutes at a temperature between 750 ° C and 700 ° C.

In a first variant, the cast iron obtained after centrifugation is a gray perlitic cast iron with spheroidal graphite. The process then differs from that described above in that there is no graphitization of the piping element 1.

Under these conditions, it is possible to produce the second coating layer 5 during or after the centrifugation operation, by depositing the enamel in the form of a dry powder. The pipe element is subsequently subjected to a ferritization treatment in a temperature range of less than 800 ° C., and the baking of the second layer 5 coating 2 takes place simultaneously with the ferritization of the piping element 1.

In a second variant, the cast iron obtained after centrifugation is a gray ferritic cast iron with spheroidal graphite. The method then differs from the method of the first variant in that there is no ferritization of the piping element 1 and that the second coating layer 5 of coating 2 is baked at a temperature included between 800 ° and 700 ° C of said element 1 after the application of the second coating layer 5 2.

In a third variant, the method differs from that initially described in that the enamel is deposited in the form of a slip to produce the second layer 5 of coating 2, after graphitization and while the pipe element 1 is at room temperature.

In a fourth variant, the method differs from the first variant in that the enamel is deposited in the form of a slip after centrifugation and while the piping element is at ambient temperature.

In a fifth variant, the third enamel layer is deposited in the form of a slip while the pipe element 1 is at ambient temperature.

The pipe element 1 obtainable by the process according to the invention, including the five variants described above, is a ductile iron pipe 3 having on the internal part of its surface a coating 2 of enamel consisting of three layers 4, 5, 6. This pipe has at one end a plain end 8 and at its other end a socket 7 adapted to receive the plain end of another identical pipe.

The micrograph of the enamel coating 2 on a joint area of the cast iron substrate (Fig. 2), represents the substrate, the interface between the substrate 3 and the coating 2, the second layer 5 of coating 2 and the third layer 6 coating 2. The substrate 3 is a ferritic cast iron with spheroidal graphite. At the level of the interface between the substrate and the coating, the oxide skin was dissolved. The first coating layer 4 2 has been absorbed by the second coating layer 5 2. The interface between the coating and the cast iron substrate 3 is consistent, and has few porosities. The localized surface attack of the substrate leads to the formation of anchoring sites for the coating 2.

The second layer 5 has a limited porosity. The third coating layer 6 2 does not show any decohesion with the second coating layer 5 2. It has a very low porosity, forms a smooth surface and achieves very good closure of the coating. The method according to the invention makes it possible to coat even oxidized metal parts which have an adherent oxide skin with a thickness of less than 20 μm.

Thanks to the integration of the steps for manufacturing the enamel coating 2 with the operations for manufacturing the pipe 1, the invention allows a gain in productivity and in thermal energy. The invention makes it possible to eliminate one or more specific stages of firing the enamel.

Variants of the process in which the application of the enamel is in the form of a slip make it possible to obtain great flexibility in production.

The pipes obtained make it possible to carry out transport lines for aggressive fluids, such as acid solutions, solutions with a high content of dissolved C0 ₂ , abrasive fluids, industrial discharges, waste water or sewage sludge.

Claims

1.- Method of manufacturing a metal part coated with mineral materials based on silica and sodium oxide, characterized in that several layers (4, 5, 6) of mineral coating (2) are applied successively on at least part of the surface of said part, the elements of mineral material being present in variations of different proportion depending on the layers, the first layer (4) of coating (2) being produced at a thickness of less than 200 μm and l 'the second layer (5) of coating (2) with a thickness greater than 100 μm is produced. 2. - Method according to claim 1, characterized in that the production of the first layer (4) of coating (2) is carried out by application of the mineral material in powder form, in particular a dry powder. 3.- Method according to one of claims 1 or 2, characterized in that the mineral material applied to form the first layer (4) of coating (2) is an enamel having the following composition by weight: Si0 ₂ 50 to 70 % CaO + MgO 5 to 20% B ₂ 0 ₃ less than 10% CoO less than 2% ^Fe 2 ° 3 less than 5% Al ₂ 0 ₃ less than 10% Na ₂ 0 + K ₂ 0 15 to 25% F ₂ less than 2% NiO less than 2% 4.- Method according to claim 2 or 3, characterized in that the mineral material is reduced during its preparation. 5.- Method according to claim 4, characterized in that the first layer (4) of coating (2) is produced at a thickness of 100 μm. 6.- Method according to one of claims 1 to 5, characterized in that the production of the second layer (5) of coating (2) is carried out by the application of the mineral material in powder form, in particular of dry powder. 7.- Method according to one of claims 1 to 5, characterized in that the production of the second layer (5) of coating (2) is carried out by application in the form of a slip of the mineral material. 8.- Method according to any one of claims 1 to 7, characterized in that the mineral material applied to form the second layer (5) of coating (2) is an enamel of the following composition by weight: S SiiO0 ₂ , 40 to 60%; CaO + MgO less than 5% Li ₂ 0 less than 5% Ti0 ₂ 5 to 15% NiO less than 2% Z ZnnOO less than 2% A1 ₂ 0 ₃ less than 10% Na ₂ 0 + K ₂ 0 15 to 20% ^B 2 ° 3 5 to 15% CoO less than 2% ^s s ^b b2 ₂ ° o3 ₃ less than 2% 9.- Method according to one of the preceding claims, characterized in that the second layer (5) of coating (2) is produced at a thickness of 200 μm. 10.- Method according to any one of the preceding claims, characterized in that a coating (2) is produced on the surface of the part by application of three layers (4, 5, 6) of mineral materials, the material forming the third layer being applied in the form of powder or slip. 11.- Method according to claim 10, characterized in that one realizes the third layer (6) of coating (2) by applying an enamel having the following composition by weight: SiO- _> 40 to 60%; CaO + MgO less than 5% Li ₂ 0 less than 5% Ti0 ₂ 5 to 15% NiO less than 2% ZnO less than 2% A1 ₂ 0 ₃ less than 10% NNaa ₂ -, 00 ++ κK ₂ 00 15 to 20% ^B 2 ° 3 5 to 15% CoO less than 2% Sb ₂ 0 ₃ less than 2% 12.- Method according to one of claims 10 or 11, characterized in that the third layer (6) of coating (2) is produced at a thickness greater than 100 μm. 13.- Method according to the preceding claim, characterized in that the third layer (6) of coating (2) is produced at a thickness of 200 μm. 14.- Method according to claim 1, characterized in that the manufacture of the metal part from a liquid iron comprises at least the following successive steps: - a magnesium treatment, - an inoculation, - a centrifugation operation of said cast iron, operation during which the first layer (4) of coating (2) is produced in accordance with one of claims 1 to 5. 15.- Method according to claim 14, characterized in that the cast iron part is at a temperature between 950 ° C and 700 ° C when the second layer (5) of coating (2) is applied in accordance with One of claims 6, 8, 9, or 10. 16.- Method according to claim 14, characterized in that the cast iron part is at room temperature when the second layer (5) of coating (2) is applied in accordance with claim 7. 17.- Method according to one of claims 15 or 16, characterized in that after having produced the first layer (4) of coating (2), the second layer (5) of coating (2) is applied to the part after the centrifugation operation. 18.- Method according to claim 15, characterized in that after having produced the first layer (4) of coating (2), the second layer (5) of coating (2) is produced during the centrifugation operation. 19.- Method according to one of claims 15 to 18, characterized in that after the production of the second layer (5) of coating (2), a ferritization heat treatment of the part is carried out in a temperature range below 800 ° C, said ferritization heat treatment being followed by the production of the third coating layer (6) (2) in accordance with one of claims 10 to 13. 20.- Method according to one of claims 15 to 17, characterized in that after the centrifugation operation of the part and before the preparation of the second layer (5) of coating (2), a treatment is carried out thermal graphitization of cast iron. 21.- Method according to one of claims 15 to 18, characterized in that after having produced the second layer (5) of coating (2) a cooking operation of the part is carried out at a temperature between 800 ° and 700 ° C. 22.- Method according to the preceding claim, characterized in that after cooking a third layer (6) of coating (2) is applied in accordance with one of claims 10 to 13. 23.- Method according to claim 19 or 22, characterized in that after the deposition of the third layer (6) of coating (2) in the form of slip, the coating is baked at a temperature between 750 ° and 700 ° C. 24. Hollow metallic piece of revolution (1), coated on the internal part of its surface with at least one mineral material, characterized in that it is manufactured in accordance with any one of claims 1 to 23. 25.- Use of the hollow metallic part of revolution (1) according to claim 24 for the transport of aggressive fluids, in particular acid solutions, solutions with a high content of dissolved C0 ₂ , abrasive fluids, industrial waste, waste water or sludge of purification.