Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-03-17
2001-10-16
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C138S143000, C148S325000, C148S519000, C148S592000, C420S057000, C420S067000, C428S682000, C428S683000, C428S684000, C428S685000, C428S686000, C428S924000, C428S925000
Reexamination Certificate
active
06303237
ABSTRACT:
The present invention relates to the use of a ferritic iron chromium alloyed construction material for the production of multi layered compound tubes, which must meet the demands for good resistance against oxidation, carburisation and so called “metal dusting” in applications such as bayonet tubes, superheater and reformer tubes in steam reforming plants. The invention also relates to the compound tubes per se. Such an outer or inner material component is especially advantageous in a co-extruded tube, where the inner, alternatively the outer, material component consists of a conventional steel or a nickel base alloy with good strength.
With compound tube is intended a tube consisting of two layers with so called metallurgical bonding between the components. Metallurgical bonding is necessary in order to maintain a good thermal conductivity. The compound tube is made by so called co-extrusion.
Steam reforming means the process steps for the production of so called synthesis gas, for generation of for instance ammonia, methanol and hydrogen gas, where water vapour is mixed with hydrocarbons in order to form hydrogen gas and carbon oxide. With reformer tubes are meant the catalyst filled tubes in which steam and hydrocarbons are converted wholly or partially to hydrogen gas and carbon oxide at high temperatures. Bayonet tubes are in this context a type of tubes placed inside the reformer tubes and function as a heat exchanger as the process gas which flow through them emits its heat to the gas which flows on the outside. Superheater tubes are placed, in the form of coils or cores, after the reformer and are used in order to cool down the process gas by super heating of steam.
The solutions, which are used today for bayonet tubes, superheater and reformer tubes, where metal dusting constitutes a problem, are generally nickel base alloys or stainless steels. However, these materials have limited resistance against metal dusting, which gives a shortened length of life or results in that non optimum process parameters for the exchange must be used in the steam reforming. The nickel base alloys are further very expensive due to high amounts of alloying elements and demanding manufacturing processes.
A first aim with the present invention is consequently to develop a more resistant product to a lower cost than the present solutions. This aim has been achieved by using alloys having a composition according to the present invention in the production of compound tubes.
The production of the compound tube is done in a way that the two different components are made to bars in a conventional manner. The bars are drilled and turned with close tolerance demands and are put together to a co-extrusion blank. The corrosion protective ferritic iron chromium alloyed material usually constitutes between 20-50% of the total wall thickness.
The blank is heated to a temperature between 900 and 1200° C. and is co-extruded into a tube. The co-extruded tubes cools in air in order to minimise bent tubes due to thermal tensions created during the cooling. Cold working operations (cold rolling) to finished dimension follows if necessary.
During the co-extrusion process the metallurgical bonding is created. This, like the layer thickness of both the components; is attested by means of a control of the finished product ready for delivery.
The present invention is based on the discovery that compound tubes in specific alloying combinations can fulfil all the demands set on construction materials intended to be used as bayonet tubes and superheater and reformer tubes in steam reforming plants. The demands that must be satisfied are good resistance against metal dusting, oxidation resistance, sufficient mechanical properties (as strength) and structure stability.
Testing in laboratory scale and in production plants has shown that the ferritic iron chromium alloy is superior to the materials normally used today in steam reforming plants. Previously known materials are described in for instance Stahl and Thomsen: Survey of Worldwide Experience with Metal Dusting, presented at the AIChE symposium on ammonia safety, Tucson, Ariz., Sept. 18-20, 1995; Grabke, Krajak and Muiller-Lorenz: Werkstoffe und Korrosion 44:89-97 (1993), and Richardson: Nitrogen No. 205, September-October 1993.
The invention includes the use of an iron chromium alloy with ferritic structure and containing, in weight-%:
LEVEL
1
1
2
3
carbon
<0.05
<0.10
<0.3
chromium
20-30
15-40
15-60
nickel
<2
<10
molybdenum
<2
<5
silicon
<2
<5
nitrogen
<0.05
<0.10
<0.3
manganese
<2
<5
iron
rest (except usual
impurities)
1
Level 3: suitable content
Level 2: preferred content
Level 1: specially preferred content
The alloy above will constitute the, for corrosion by metal dusting and carburisation, exposed component in a compound tube made by co-extrusion, where the other, load carrying component consists of a lower alloyed carbon steel, a so called 9-12% chromium steel, a conventional stainless steel or a nickel base alloy. Which of the components is the outer or inner component depends on if the process gas flows on the in- or outside of the tube.
The environments where metal dusting and carburisation arise are characterised by a high carbon activity and a relatively low oxygen partial pressure in the process gas, and a normal temperature of 450-900° C. In order to be resistant against this type of corrosion a metallic material is required to have a good ability to form a protective oxide on the surface. Decisive for this ability is mainly the content of the oxide forming element in the material and the micro structure of the material. Due to the relatively low oxygen content in the gas, only three types of protective oxides can practically be formed in the actual environment: aluminium oxide, chromium oxide and silica. Steel alloys or nickel base alloys with aluminium or silicon in order to promote formation of these types of oxides result in deteriorated ductility of the alloy, which makes the making very difficult. The diffusion of the oxide forming element to the surface is critical, why it is a prerequisite in the actual temperature region that the alloy has a micro structure with a ferritic matrix.
The ferritic iron chromium alloyed material of the invention has on the other hand very low strength at high temperatures and can also be embrittled during operation by formation of so called sigma phase. It is therefore not suitable for use in applications that work under mechanical stress. The low strength makes further that it is easily deformed by creep, which is negative for instance for protection against metal dusting, since the protective oxide is easily broken up. That means that the ferritic iron chromium alloyed material as such can not be used as bayonet tubes, superheater and reformer tubes in steam reforming plants.
The joining of corrosion resistant ferritic iron chromium alloyed material, which usually constitutes 20-50% of the total wall thickness, with an alloy with high strength in the form of a compound tube, so that that the iron chromium alloyed material is exposed to the corrosive process gas, a product that manages both the demands on resistance to metal dusting and mechanical hot strength is obtained. The tubes may have an outer diameter of 15-200 mm and a total wall thickness of 2-20 mm.
The choice of load carrying component, i e the component on which the ferritic iron chromium alloyed corrosion protection steel shall be applied, depends on the working temperature and the mechanical stress of the component. Besides demands on strength there are demands on resistance against oxidation in combustion gases or water vapour for the load carrying component. It can generally be said that the oxidation properties become more decisive the higher the working temperature of the component. Oxidation resistance is generally achieved by alloying with chromium. Suitable alloys for the load carrying component are therefore at higher(T≧550° C.) temperatures, austenitic stainless steels or Ni—Cr-alloys. At
Forsberg Urban
Linden Johan
Burns Doane Swecker & Mathis L.L.P.
Koehler Robert R.
Sandvik AB
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