Patent Application
20060186093
The invention relates to a method for manufacturing metal composite block material (billet, slab) from separately manufactured composite block elements.
In the industrial field of thermal-chemical process technology pipes are used that are comprised of at least two different metallic materials for fulfilling often complex process requirements in corrosive thermal environments. For example, the outer part of such pipes, referred to also as composite pipes, can be comprised preferably of an austenitic corrosion-resistant material in order to provide corrosion protection, while the inner pressure-conducting pipe part is made of heat-resistant carbon steel. The reverse arrangement of the pipe material components is also known for certain applications.
Composite pipes are used primarily in the boiler area of waste incinerators of waste-fueled power plants. They are moreover used in connection with lye recovery devices in the pulp industry—in the black liquor recovery boiler of the Kraft process—and moreover as heat exchanger pipes in thermal catalytic cleavage processes.
There are essentially two methods for producing such composite pipes. The first approach via composite block material—the forming-technological prestage of pipe manufacture—is characterized in that the aforementioned exemplary material combination of austenitic/carbon steel is already arranged in a defined state in the blank. During the subsequent hot forming process of the composite block material, a composite pipe of at least two different steel materials is produced that are bonded to one another. In this connection, the wall thickness proportion of each steel material component in regard to the total wall thickness of the finished composite pipe must be taken into consideration already during composite material construction by taking into account the material-specific flow behavior during hot forming.
The second approach for manufacturing composite pipes is the so-called cladding of pipes that is based on welding technology. The pipes are produced initially by conventional methods. During cladding, the additional material component is applied onto the pipe surface by welding by means of a fusion welding process until the desired layer thickness of generally a few millimeters has been reached. In the prior art, cladding is realized either automatically by an automated process or by hand.
In all types of composite pipe manufacture it is important that between the individual material components of the pipe a metallic bonding can be produced in a reproducible way. This metallic bond, that should not be destroyed during cold forming processes to which the pipes are subjected during the course of further processing, ensures that in operational use thermal conduction through the entire pipe wall is optimal and that no efficiency loss will occur when the pipes are used in boilers.
In a method for composite block material manufacture in accordance with U.S. Pat. No. 6,242,112, a second liquid material component is applied by means of gas atomizing to deposit finely distributed metal droplets onto a round rod or pipe made of carbon steel. This spraying method that is also referred to as Osprey technology is disclosed in detail in U.S. Pat. No. 3,826,301 and G.B. 1,472,939. The spray-formed (Osprey) composite block material produced in this way is subsequently formed by extrusion to a composite pipe.
U.S. Pat. No. 6,296,953 discloses a method for composite material manufacture in which the two material components of the future composite pipe is first conventionally rolled by hot rolling to round rods of different diameters. After hot rolling, the round rods are cut to length and provided with concentric longitudinal bores in such a way that the round rod with the greater diameter has a bore into which the round rod of the smaller diameter can be inserted. After positioning of the smaller round rod in the bore of the greater rod, this combination is formed by hot extrusion to a composite pipe.
U.S. Pat. No. 4,630,351 discloses a method for manufacturing seamless externally clad steel pipes by extrusion wherein a composite block material is produced in such a way that a high-alloy cladding material is welded in a pre-determined thickness onto an appropriate round core material. After prior drilling of the core material, forming of the composite material to a composite pipe by extrusion and/or cold pilger rolling is carried out.
U.S. Pat. No. 4,795,078 discloses a method for producing composite pipes in which two or three individual pipes are first heated to different temperatures and are then inserted into one another for producing a shrink fit. Subsequently, by means of an electron beam welding device on both end faces of the pipe a seal-tight welding seam is applied on the end face edges of the individual pipes for connecting the individual pipes to one another. Subsequently, the pipe is inserted into a high-temperature isostatic press. In the argon atmosphere of the press, a metallurgical bonding to the pipe boundaries of the composite pipe takes place; subsequently, the composite pipe is subjected to a stretching process.
In JP 06170534 A an automatic cladding method for welding high-alloy steel material onto the outer pipe surface is disclosed; JP 09314384 discloses a device for cladding pipes on the interior. The disadvantage of cladding is that, as a result of the process, the fusion welding process always causes intermixing to a greater or lesser degree of the base material of the pipe and the applied high-alloy material. This leads to changes of the chemical and mechanical material properties. Aside from the welding parameters, the mixing effect depends on the root penetration, the average layer thickness, and the number of layers. Moreover, the cladding process produces more or less rough pipe surfaces on which combustion and reaction products adhere and which can be of the point of origin of cracks as a result of stress concentration.
When employing the composite block material technology, a smooth pipe surface can be generated already by extrusion. Also, excessively strong mixing is prevented in general.
The disadvantage of composite block material technology based on spray-forming is that a special manufacturing method must be employed that is complex and not always available. Also, the nature of the composite block manufacture based on spray-forming requires a certain excess of material and a minimum number of manufactured pieces that may not be too small.
When the composite block technology is based on joining conventionally produced composite block elements, three different technical criteria must be fulfilled:
1. It should be possible to manufacture the composite block element that forms the outer or inner area of the composite block (that can be viewed as a tick-wall pipe) with minimal material waste.
2. The manufacturing method must ensure an always optimal metal bonding without any disruptive oxide layer between the material components.
3. The individual material components of the composite block material must be position-stable relative to one another so that they survive the hot forming process without any damage.
It is an object of the present invention to propose a method with which an inexpensive and at the same time qualitatively superior manufacture of composite pipes can be ensured. In this connection, preferably easily available possibilities of semi-finished product manufacture are to be used and material waste should be minimized.
In accordance with the present invention, this is achieved in that the composite block elements are machine-cut at their joining surfaces to a metallically blank or bright state and subsequently are joined by electron beam welding to a composite block material and the composite block material is subsequently formed by steel extrusion to a composite pipe.
In this connection, the manufacture of metal composite block material from steel is realized by joining initially separately manufactured composite block elements whose boundary surfaces are machine-cut to a metallically blank or bright state before joining. This is realized by machine-cutting the surfaces preferably at such dimensional tolerances that the composite block elements can be easily inserted into one another. In a subsequent step the elements are then fixedly connected to one another to form a composite block material (billet, slab) by partial electron beam welding (EB welding). The composite block material is then formed subsequently on a steel extrusion device to a composite pipe.
Advantageous embodiments of the method according to the invention are disclosed in the dependent claims.
When producing the composite block elements 1, 2 illustrated in
A diameter difference of DI−da of approximately 0.2 mm to 0.4 mm as a result of the machine-cutting process is desirable in view of a favorable geometric configuration of the joining gap for the later electron beam welding process (EB welding).
For such a tolerance range of the joining surfaces, the elements 1, 2 can be inserted into one another without problem. Special auxiliary steps such as, for example, prior heating of the outer element 1, are not required.
After completion of workpiece preparation and insertion, partial electron beam welding is carried out in accordance with the schematic illustrated in
Partial EB welding as an essential manufacturing step of the method according to the invention has the following advantages:
1. The electron beam welding process is the only fusion welding process that is able to penetrate without welding filler material very deep into the material to be welded (in the case of steel to a weld depth of approximately 150 mm) and that generates in this connection a welding seam of only a few tenths of a millimeter. Because of this characteristic property of electron beam welding, two important requirements for the method according to the invention are realized:
a) a satisfactory weld depth LEB=100 mm is generated that provides sufficient mechanical strength so that the strong shearing forces that occur during extrusion between the composite block elements 1, 2 can be taken up safely without the elements 1, 2 being torn apart;
b) sufficiently narrow welding seams 3 are produced so that mixing of the materials at the joining area can be significantly reduced and the original workpiece properties are hardly negatively affected.
2. Electron beam welding, aside from the special case of atmospheric electron beam welding, is carried out in industrial welding devices at fine vacuum up to high vacuum. The system pressures are accordingly in the range of 10−2 mbar to approximately 5×10−4 mbar. In this pressure range the partial pressure of oxygen in the gas chamber is so minimal that noticeable metal oxidation in the joining area LEB joined by electron beam welding is precluded.
3. The composite block material area of the length LPS according to
After completion of the EB welding process the steel extrusion of the composite block material 4 is carried out; the steel extrusion step is important for the present invention. For this purpose, the composite block material is first heated to a temperature between 1,100 degrees Celsius and 1,300 degrees Celsius and preferably between 1,150 degrees Celsius and 1,250 degrees Celsius; the heated material is inserted into the extruder 20 illustrated schematically in
The entire pressing process of the composite block material 4 takes only a few seconds. Because of the speed of the processes that are being performed, an excellent metallurgical joint having high thermal and mechanical load resistance is provided at the boundary surface between the two individual elements 1, 2 of the composite block material as a result of the pressure-enhanced diffusion processes. In the die 22 an approximately 20-fold hot forming of the composite block material 4 having an initial length of approximately 0.7 to 1.5 m can be achieved.
The special advantage of the extrusion process resides therefore in the diffusion-caused metallurgical processes that are performed in fractions of a second. As a result of the extremely fast plastifying deformation the boundary surfaces of the individual elements 1, 2 are extremely enlarged. In this way, fresh boundary surfaces are produced whose outer atoms are neither impaired by neighboring metal atoms nor by atoms of third contaminating substances in their reactivity. The high pressure (corresponding to the smallest atom distances of the reaction partners relative to one another) and the high reactivity of the atoms in the newly formed boundary surfaces lead to the diffusion-based bonding process achieved by the inventive method to be derived from the aforementioned short processing time.
In addition to its main task of the forming the composite block material 4 to the composite pipe, the extrusion process which is part of the process chain according to the invention can provide the additional effect that the compound block elements 1, 2 can be produced with reduced waste of expensive material. For example, it is possible that the outer composite block element 1 can be produced by an axial punching process in accordance with the method of indirect pressing carried out on the extruder instead of by drilling or internal turning of the inner diameter DI. While in the case of a cutting process by drilling, when supposing that conventional blank dimensions are DA=170 mm and DI=140 mm, a material waste of approximately 60 percent to 70 percent occurs, this can be reduced by applying the punching process inclusive of the subsequent external turning to 20 percent to 25 percent.
A further manufacturing alternative that saves material and thus material costs results by employing sheet metal bending techniques in combination with beam welding. According to
In a further embodiment it is proposed that a composite block element 1 corresponding to a thick-wall pipe according to the illustration of
While specific embodiments of the invention have been shown and described in detail to illustrate the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2005 008 250.5 | Feb 2005 | DE | national |
