Field of the invention

The invention relates to a method for coating a steel plate with another steel plate, a steel plate coated with another steel plate, and a vessel formed of a steel plate coated with another steel plate.

Background of the invention

In pulp mills, for example, the walls of pulp towers or cooking vessels are normally made of structural steel plates whose inside is coated with a stainless steel plate; that is, steel plates whose one surface is provided with another steel plate (so-called coating plate). The function of the coating plate of stainless steel is to prevent corrosion of the walls of the vessel caused by aqueous pulp in the vessel. Similar designs may also be used in other vessels made of steel and used for storage or processing of gases, liquids or pulp-like substances which are under a relatively high pressure and which are corrosive to ferrite steels. In principle, the walls of such a large vessel could be made entirely of stainless steel, but due to the considerably higher price of stainless steel compared with ferrite steel, this would not be cost-effective.

Figures 1 and 2 illustrate a prior art principle of manufacturing a steel plate coated with another steel plate 10'. In the method of prior art for coating walls of a large vessel of the above-described kind, made of steel plate, with another steel plate, one surface of the steel plate 1 to be coated (i.e. the surface forming the inner face of the vessel) is provided with stainless steel plate strips of a given width (below, "plate strips") by welding them next to each other on top of the steel plate 1 to be coated. Figure 1 shows a part of a wall of a vessel made by the method of prior art, seen from the direction of a coating plate 12'. Figure 2, in turn, shows a cross-section A-A of the wall shown in Fig. 1 and formed of steel plates coated with other steel plates 10', shown in Fig. 1. As shown in Figs. 1 and 2, when forming a steel plate coated with another steel plate 10' of prior art, a first plate strip S1 is welded at its first edge R11 and its second edge R12 to a steel plate 11' to be coated. Next, a second plate strip S2 is welded at its second edge R22 to the steel plate 11' to be coated, adjacent to the first plate strip S1 , so that the first edge R21 of the second plate strip S2 is placed, in a suitable (relatively short) section, on top of (to overlap) the second edge R12 of the first plate strip S1 , as shown in Fig. 2. After this, the first edge R21 of the second plate strip S2, placed on top of the second edge R12 of the first plate strip S1 , is welded to the first plate strip S1 . Next, a third plate strip S3 is welded, in turn, first at its second edge R32 to the steel plate 11' to be coated, so that its first edge R31 is placed, in a suitable (relatively short) section, on top of (to overlap) the second edge R22 of the second plate strip S2; and further, the first edge R31 of the third plate strip S3, placed on top of the second edge R22 of the second plate strip S2, is welded to the second plate strip S2. The procedure is continued by fixing further plate strips until the steel plate 1 to be coated has been fully coated with plate strips S1 , S2, S3, etc. forming the coating steel plate 12' but leaving installation strip zones 2T with a width of about 200 mm uncoated at the edges of the steel plate to be coated, as shown in Figs. 1 and 2.

Normally, one large vessel is composed of a plurality of wall elements formed by adjacent steel plates coated with another steel plate and fastened to each other, which elements are welded alongside one another by butt welds between the steel plates 11 to be coated. On top of these uncoated installation strip zones 2T at the joints, installation strips are welded to cover these zones uncoated with another steel plate, to complete the coating with a coating steel plate 12' forming the inner face of the vessel.

The method of prior art has the drawback that the plate strips S1 , S2, S3, etc. are fastened onto the steel plate to be coated in an overlapping way so that triangular empty spaces T are formed between the second edge of each plate strip forming the coating steel plate, and the plate to be coated, as shown in Fig. 2 from the direction of the cross-section. [ At these empty spaces T, the second edge of the preceding plate strip is against the lower surface of the edge zone on the side of the first edge of the next plate strip. As a result, stress concentrations are formed in the plate strips forming the coating steel plate, subjecting the plates to a shearing load. For example in the case of various vessels, when the structure is thus subjected to long-term local loads, this has been found to cause tearing off and leaking of the coating steel plate 12' if the structure is also subjected to some dynamic loading ( e.g . pressure variation, pressure shocks, vibration, etc.) which is in many cases very common under industrial conditions. In the case of a vessel, if the coating steel plate 12' leaks, the coated steel plate 11' will be subjected to corrosion, which, in turn, e.g. shortens the safe service life of the vessel. Furthermore, the method of prior art has the drawback that the coating steel plate 12' has to be formed of narrow plate strips, whose usable width is partly lost because the strips are fastened to each other by placing them in a partly overlapping way, as described above.

Brief summary of the invention

It is an aim of the invention to present a novel method for coating a steel plate with another steel plate so that the steel plate coated with another steel plate can be made stronger and more durable than before, without increasing the thickness of the steel plates used in the structure or without changing the material properties (strength) of the steel plates; and that the single pieces of steel plate used for coating the steel plate to be coated can be larger than before. Furthermore, it is an aim of the invention to present a steel plate coated with another steel plate, formed by the method according to the invention, as well as a vessel made of such steel plates.

In the method according to the invention, the steel plate to be coated is coated with another, coating steel plate which is formed of one or more pieces of steel plate to be welded next to each other and/or one after another by means of butt welds, and which pieces of steel plate are welded to the steel plate to be coated by welds at welding points inside the edges of the coating steel plate or the pieces of steel plate forming the same, so that the welds fasten the coating steel plate, or the pieces of steel plate forming the same, to the steel plate to be coated, at least at the welding points. To put it more precisely, the method according to the invention is characterized in what will be presented in the independent claim 1 ; the steel plate according to the invention, coated with another steel plate, is characterized in what will be presented in the independent claim 7; and the vessel according to the invention, made of steel plates coated with other steel plates, is characterized in what will be presented in the independent claim 13. Dependent claims 2 to 6 present advantageous embodiments of the method according to the invention; dependent claims 8 to 12 present advantageous embodiments of the steel plate according to the invention, coated with another steel plate; and dependent claims 14 to 17 present advantageous embodiments of the vessel according to the invention, made of steel plates coated with other steel plates.

The method according to the invention has the advantage that the steel plates coated with other steel plates and used in large vessels, such as pulp towers and cooking boilers of the pulp industry, can be made significantly stronger and more durable than before. In pressure resistance tests carried out by the applicant on such steel plates coated with other steel plates, a pressure resistance was achieved that was on average 2.5 fold in a test where liquid (water) to be pressurized was introduced between the coated steel plate and the coating steel plate, the pressure being increased until the structure was broken and/or the pressurized liquid started to leak from between the steel plates. The method also has the advantage of reducing the quantity of the coating steel plate needed for each steel plate to be coated. For example, in the case of pulp towers or cooking vessels used in the pulp industry, the number of separate pieces of steel plates is reduced to four, but in a smaller application, the coating steel plate could be, in principle, only one piece of steel plate, having the same size as the steel plate to be coated, because the coating plate is no longer fastened merely at its edges to the steel plate to be coated. Furthermore, in a steel plate made by the method according to the invention and coated with another steel plate, stress concentrations of the above-described type, caused by the butt weld of adjacent pieces of steel plate, are no longer formed in the coating steel plate; consequently, by using a steel plate whose material properties correspond to those of prior art and which is coated with another steel plate, it is possible to achieve a significantly more durable structure, for example, for various vessels whose inner faces are coated with other steel plates. Description of the drawings

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a steel plate coated with another steel plate by a method of prior art, seen from the direction of the coating steel plate;

Fig. 2 shows the steel plate of Fig. 1 coated with another steel plate by a method of prior art, in a cross-section along a line A-A shown in Fig. 1 ;

Fig. 3 shows a steel plate coated with another steel plate by a method according to the invention, seen from the direction of the coating steel plate; Fig. 4 shows the steel plate of Fig. 3 coated with another steel plate, in a cross- section along a line B-B shown in Fig. 3;

Fig. 5 shows part of a wall of a vessel made of steel plates according to Figs. 3 and 4, coated with other steel plates, seen from the direction of the coating steel plate.

Detailed description of preferred embodiments of the invention

Figures 3 and 4 show a steel plate coated with another steel plate 10 according to an embodiment of the method of the invention. Here, a coating steel plate 12 has been fastened to one surface of a steel plate to be coated 11 by welding it onto said planar surface. Typically, the steel plate to be coated 11 consists of a steel grade different from the coating steel plate 12. The material of the steel plate to be coated 11 may be any weldable steel, such as unalloyed carbon steel, austenitic or ferritic stainless or acid-proof steel, or austenitic or ferritic duplex, lean duplex or superduplex steel. The material of the coating steel plate 12 may be any material corresponding to the material of the steel plate to be coated 11 . In practice, however, the material of the steel plate to be coated 11 is often a steel having a lower price per kilogram, for example ferritic structural steel, whereas the coating steel plate 12 is such a steel grade that has a higher price per kilogram and has a special feature with respect to the steel plate to be coated 11 . For example, in the case of vessels to be built for liquids that are corrosive to ferritic structural steels, this special feature is better resistance to corrosion than with the steel plate to be coated 11 , whereby the coating steel plate 12 is normally made of stainless steel, and the coated steel plate 11 is made of a structural steel suitable for the purpose. In the case of the embodiment shown in Figs. 3 and 4, the coating steel plate 12 has been fastened to the steel plate to be coated 11 by providing the coating steel plate 12 with welding holes 22 extending through the coating steel plate 12. After this, the coating steel plate 12 has been placed onto the steel plate to be coated 11 in such a way that the welding holes 22 being at the position of the welding points 13 at the coating steel plate is positioned within the steel plate to be coated 11, after which the coating steel plate 12 has been fastened to the steel plate to be coated 11 with welds 14 through the welding holes 22 in the coating steel plate 12. In the embodiment of Figs. 3 and 4, the welding is performed in such a way that the weld 14 fills and plugs the welding holes 22 in the coating plate 12 in a liquid and/or gas tight manner; in other words, in such a way that gas and/or liquid existing or introduced on the side of the coating steel plate 12 cannot leak via the welding holes 22 through the coating steel plate 12 to the side of the steel plate to be coated 11.

In another embodiment corresponding to the embodiment of Figs. 3 and 4, it is possible to apply a welding method where no welding holes 22 are needed, but the coating steel plate 22 can be fastened at the welding points 13 by welds which are otherwise e.g. similar to the welds 14 shown in Figs. 3 or 4 but have been formed without making welding holes, by melting the material of the coating steel plate 12 in a welding process to fasten the coating steel plate 12 to the steel plate to be coated 11, for example by welds similar to the welds 14, at least at the welding points 13. In such an embodiment, additives required for the welding may have been used for forming the welds, for example to provide welds meeting the strength requirements according to standards.

In the case of the embodiment with welding holes as shown in Figs. 3 and 4, the welding holes 22 are elongated slots or cuts. In this case, they are parallel to the longer edge of the rectangular coating steel plate 12 and the likewise rectangular steel plate to be coated 11 , and several holes are provided at regular intervals, one after another and next to each other, as shown in Fig. 3. The welding holes 22 are made e.g. by laser or flame cutting in the coating steel plate 12 before it is fastened to the steel plate to be coated 11. For making the welding holes 22, in some cases, depending on the thickness of the coating steel plate 12, it might also be possible to apply a mechanical cutting or working method, such as punching. In other embodiments of the invention, similar to those shown in Figs. 3 and 4, the welding holes 22 provided at the welding points 13 (or the mere welding points 13) in the coating steel plate 22, and thereby the welds 14 joining the steel plate to be coated 11 and the coating steel plate 12, may have any shape. Moreover, the mere welding points 13 and/or the welding points and the welding holes 22 may be arranged at any location in the coating steel plate 12 and the steel plate to be coated 11 , and in any orientation with respect to these and each other. Furthermore, there may be any number of welding points 13 and/or welding points and welding holes 22 in the coating steel plate 12.

In the manufacture of the steel plate coated with another steel plate 10, shown in Figs. 3 and 4, the coating steel plate 12 is welded to the steel plate to be coated 11 by an arc welding method. However, the welding might be performed by a number of other welding methods as well, including various arc welding, beam welding, friction welding, or gas welding methods, or by any combination (hybrid welding) or variation of the above mentioned welding methods. Depending on the welding method, the coating steel plate 12 may thus be provided with welding holes 22 at the welding points 13 when this is required for the welding method to be applied. The welding may be performed manually or mechanically, and/or totally or partially under automatic control. In an embodiment of the method for manufacturing of a steel plate similar to that shown in Figs. 3 and 4 and mechanically coated with another steel plate, when the coating steel plate 12 is welded onto the steel plate to be coated 11 , the welding device and/or the welding head therein is guided to move in the direction of the welding points 13 and/or the welding holes 22 by a control device, to which the welding device and/or the welding head is fastened.

In the embodiment of Figs. 3 and 4, the coating steel plate 12 covering one steel plate to be coated 11 is formed of several pieces 15 of steel plate to be fastened to each other, next to each other and/or one after another. The pieces 15 of steel plate forming the coating steel plate 12 are fastened to each other by welding to each other by butt welds 16. Before the pieces 15 of steel plate are fastened to each other by the butt welds 16, they can be first attached to the steel plate to be coated 11 at welding points 13 suitable with respect to each other, in the above described ways via or without the welding holes 22. In this respect, the order may also be reversed, for example, so that the pieces 15 of steel plate are fastened to the steel plate to be coated 11 by a temporary fastening method for the time of welding the butt welds 16. Irrespective of the welding order, the pieces 15 of steel plate are placed in such a way with respect to each other that suitable gaps, required for the butt welds 16, are left between the ends of the pieces 15 of steel plate. The edges of the pieces 15 of steel plate, to be fastened to other, adjacent pieces 15 of steel plate, may also be shaped (e.g. outwards bevelled) in a way suitable for making the butt welds 16, whereby the welding of the pieces 15 of steel plate to each other can be performed as smoothly as possible. Fastening of the pieces 15 of steel plate forming the coating steel plate 12 to the steel plate to be coated 11 is followed by welding the butt welds 16. This, too, can be carried out manually, mechanically, fully or partly automatically, and by any of the above-mentioned welding methods.

In the embodiment of the steel plate coated with another steel plate 10, as shown in Figs. 3 and 4, the ends and the side edges of the coating steel plate 12 are fastened by edge welds 17 to the edge zones of the steel plate to be coated, from which an installation strip 21 begins, forming a joint zone between adjacent steel plates coated with another steel plate 10. In this way, the pieces 15 of steel plate are fastened to the steel plate to be coated 11 , not only at the welding points 13 but also at their end and side edges.

Uses of the steel plate coated with another steel plate 10, as shown in Figs. 3 and 4, typically include any such steel plate structures in which steel surfaces are needed to meet different requirements on different sides of the steel plate. For example, in various vessels built for containing a liquid, a gas, or a solid bulk substance, or a mass-like substance, it may be necessary to ensure the surface quality requirements, the cleanliness of the surface, or the corrosion resistance of the steel on the inner face of the vessel. Flowever, conventional structural steel with a low price per weight is quite suitable as the material for the outer surface of such a vessel. Thus, one wall, part or all of the walls may be made of a steel plate coated with another steel plate 10, as shown e.g. in Figs. 3 and 4, the coating steel plate 12 meeting the quality requirements for the inner face of the vessel. A steel plate according to the invention, coated with another steel plate, e.g. a steel plate coated with another steel plate 10 as shown in Figs. 3 and 4, may be planar or bent or formed to a desired shape, according to the use. For example, for a wall of a vessel, a steel plate coated with another steel plate is bent to a curved shape corresponding to the shape of the wall of the vessel. The steel plate coated with another steel plate may also have a shape different from the rectangular shape. For example, for making the walls of a conical vessel, it may have the shape of a sector of a circle, or for the bottom or the lid of a cylindrical vessel, it may have a circular shape.

Particularly in the case of a vessel, the steel plate for coating the steel plate to be coated is on the inner face of the vessel. Flowever, the structure may also be utilized in vessels in the reversed way so that the coating steel plate is on the outer face of the container. However, in many cases the coating steel plate is thinner than the steel plate to be coated, because the coating steel plate is usually the one having a required special property and having a higher price per weight than the steel plate to be coated. By using a steel plate according to the invention, coated with another steel plate, it is thus possible to save significant sums in the manufacturing costs of the vessel.

In an embodiment of a vessel according to the invention, part of the wall 18 being shown in Fig. 5, the vessel contains two or more steel plates coated with another steel plate 10, as shown in Figs. 3 and 4, and are fastened to each other. In the wall 18 of the vessel shown in Fig. 5, a steel plate coated with another steel plate 10 according to Figs. 3 and 4 is used, the steel plate 12 coating the steel plate to be coated 11 being provided on the inner face of the vessel. For forming the wall 18, the steel plates 11 to be coated and forming the steel plates coated with another steel plate 10 are welded to each other by butt welds 19, and the above-described installation strips 20 are attached onto the installation strip zones 21 . These joints are also liquid and gas tight in the case of the vessel shown in Fig. 5. The installation strips 20 may be, for example, steel plate strips fastened by welding onto the joints formed by the butt welds 19. In other respects, the vessel consisting of walls 18 according to Fig. 5 may have a structure similar to such vessels, and contain all the parts corresponding to such vessels, in which a steel plate coated with another steel plate 10' has been used by a method of prior art, as shown in Figs. 1 and 2. The method, the steel plate coated with another steel plate, and the vessel according to the invention may differ from the above presented example embodiments in a number of ways. For example, in such an embodiment of the method where the coating steel plate consists of a single piece of steel plate in the size of the piece of steel plate to be coated, the edges of the coating steel plate does not necessarily have to be welded to the steel plate to be coated, but this can be done first when a steel structure (e.g. a vessel) is formed of such a steel plate, coated with another steel plate, by joining steel plates of this kind to each other. Furthermore, in a case of a steel plate coated with another steel plate, made by a method according to the invention, both sides of the steel plate to be coated may be provided with a coating steel plate (in other words, the method may be used to implement a three-ply steel plate). In this case, the steel plate fastened to the first side of the steel plate to be coated could be a steel plate of the same steel grade or a different steel grade with respect to the steel grade of the second steel plate to be fastened to the second side. Also, the coating steel plates may have equal or different thickness, as needed. Normally, the thickness of the coating steel plate/s is smaller than the thickness of the steel plate to be coated, but in some cases it could also be equal, or the coating steel plate/s could be thicker than the steel plate to be coated. Steel plates coated on both sides with another steel plate could be used, for example, in such vessels or structures where a special property is required of both surfaces that is not achieved with conventional structural steel but where a great material thickness has to be used to achieve sufficient strength and rigidity. In this case, the strength and rigidity requirements of such a steel plate structure having different special properties on its different sides can be met at minimum material costs by utilizing a steel plate according to the invention, coated with another steel plate. The method, the steel plate coated with another steel plate, and a vessel made of a steel plate coated with another steel plate according to the invention are not limited by the above presented example embodiments but they may vary within the scope of the appended claims.