Metal fusion bonding – Process – Repairing – restoring – or renewing product for reuse
Reexamination Certificate
2002-08-27
2004-09-21
Stoner, Kiley (Department: 1725)
Metal fusion bonding
Process
Repairing, restoring, or renewing product for reuse
C228S164000, C228S219000, C228S262410
Reexamination Certificate
active
06793119
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a process for welding duplex steel in the presence of a shielding gas.
Duplex steel is a stainless steel with a ferritic-austenitic structure with the two phases having different compositions. The duplex structure means that chromium and molybdenum are predominantly present in the ferrite phase and nickel and nitrogen in the austenite phase.
Duplex steel is often used in corrosive environments on account of its corrosion resistance. Duplex steel is successfully used especially in urea plants, where it comes into contact with the corrosive ammonium carbamate solutions and particularly in the high-pressure section of urea plants. Here, the most critical items such as high-pressure vessels, seals around manholes, piping, flanges and valves are manufactured from duplex steel.
The use of duplex steel in a plant requires that duplex steel components be able to be attached to each other. The welding of duplex steel is a process known to those skilled in the art for joining duplex metal parts. Duplex metal parts are welded together in the presence of a gas, a so-called shielding gas, that provides an inert atmosphere. Generally, those skilled in the art will use gases such as argon and helium as a shielding gas. These shielding gases prevent oxidation during weld fabrication.
The drawback of duplex steel welding is that duplex steel parts cannot be welded together without using a welding aid that takes care of the welded joint if the specific properties of the steel are to be retained. Specific properties of the steel include mechanical properties, such as tensile strength, ductility and corrosion resistance. Particularly with welding techniques such as IBW (Internal Bore Welding), laser beam welding or electron beam welding, duplex steel cannot be used without impairing its specific properties, because these welding techniques do not allow the use of welding aids. These welding techniques are for example needed for fabricating crevice-free tube-to-tubesheet joints. Those skilled in the art will normally use welding aids such as coated welding electrodes and solid or filled filler metal.
The aim of the present invention is to eliminate the above-mentioned drawbacks and to enable the welding of duplex steel without welding aids, without the specific properties of the duplex steel being impaired.
The applicant has found a process for welding duplex steel in the presence of a shielding gas without use being made of welding aids and with nitrogen being added to the shielding gas. In particular, a process was found for welding duplex steel in the presence of a shielding gas without using welding aids and with nitrogen being added to the shielding gas and with an austenitic-ferritic duplex steel being used with a chromium content of between 28 and 35 wt. % and a nickel content of between 3 and 10 wt. %.
Preferably an austenitic-ferritic duplex steel with the following composition is used:
C: maximum 0.05 wt. %
Si: maximum 0.8 wt. %
Mn: 0.3-4.0 wt. %
Cr: 28-35 wt. %
Ni: 3-10 wt. %
Mo: 1.0-4.0 wt. %
N: 0.2-0.6 wt. %
Cu: maximum 1.0 wt. %
W: maximum 2.0 wt. %
S: maximum 0.01 wt. %
Ce: maximum 0.2 wt. % the balance consisting of Fe and common impurities and additives and the ferrite content ranging from 30 to 70 vol. %.
More preferably the C content is maximum 0.03 wt. % and in particular maximum 0.02 wt. %, the Si content is maximum 0.5 wt. %, the Cr content 29-33 wt. %, the Ni content 3-7 wt. %, the Mo content 1-3 wt. %, in particular 1-2 wt. %, the N content 0.36-0.55 wt. % and the Mn content 0.3-1 wt. %.
The ferrite content is more preferably 30-55 vol. %. The Cr content of the austenite phase is more preferably at least 25 wt. % and in particular at least 27 wt. %.
Preferably 1-10 vol. % nitrogen is added to the shielding gas, in particular 1-5 vol. % and more in particular 1-3 vol. %.
The advantage of the process according to the present invention is that it allows duplex steel to be IBW-welded, laser beam welded and electron beam welded while retaining the specific properties.
A widely used embodiment of the welding together of ferritic-austenitic steel comprises attaching the steel components to each other via a so-called overlay weld. Here, a contact surface is fabricated by applying a layer of the ferritic-austenitic duplex steel to a base, generally a cheaper carbon steel, using a known welding process. Another object of duplex steel or with a duplex steel contact surface can be attached to the said overlay weld using the welding process according to the present invention. However, objects thus welded often break off as a result of hairline cracks developing in the duplex steel of the overlay weld.
It has now been found that this drawback can be eliminated by a process enabling two or more duplex steel parts to be joined via an overlay weld. In the process, the contact surfaces of the metal parts are partly or completely fabricated from ferritic-austenitic steel with a chromium content of between 28 and 35 wt. % and a nickel content of from 3 to 10 wt. %, after which the topmost 0.1-1.0 mm, preferably 0.2-0.8 mm and in particular 0.3-0.7 mm, of the contact surfaces is removed, after which the metal parts are welded together. The said top layer can be removed by any process known to those skilled in the art, such as grinding, filing, using abrasives, etc.
The welding and overlay welding processes according to the invention are particularly useful in the manufacture of piping components such as flanges and valves inasmuch as such piping components will be less susceptible to crevice corrosion. Such components are fabricated from a duplex ferritic-austenitic steel with a chromium content of between 28 and 35 wt. % and a nickel content of between 3 and 10 wt. % and are attached to each other by means of a welding process or overlay welding process without using welding aids and with 1-10 vol % nitrogen being added to the shielding gas.
Especially welded joints in areas where flanges and valves are present in a high-pressure section of a urea plant can be advantageously fabricated according to the present invention.
Accordingly, the invention relates in particular to application of the welding process and overlay welding process in a urea plant.
The aforementioned piping components are sources of leakage. Where materials susceptible to crevice corrosion are utilized, corrosion will in the course of time develop in the cervices, which are inherent in the said piping components, which results in leakage of process fluid. This occurs especially in for example the high-pressure section of a urea plant. The plant will then need to be shut down because of safety and environmental regulations. If such piping components are fabricated using the process of the present invention, they will be much less susceptible to crevice corrosion and leaks will be prevented.
Where the aforementioned duplex steel is utilized, the reduced susceptibility to corrosion allows more pumps to be used in place of gravitation for conveying process streams. This obviates the need to position items of equipment one above the other, such as the high-pressure carbamate condenser and the reactor in a urea plant. All equipment items can be placed on the ground so that considerable investment savings can be achieved.
It is important that the duplex steel used in the present invention have a good ferrite-austenite distribution. If the distribution is not optimal, then the properties of the steel will decrease. This may lead to fracture of piping and equipment and the steel will also be more susceptible to corrosion. It has been found that the homogeneity of the ferritic-austenitic steel can be readily tested by conducting measurements with an eddy current coil. When a piece of metal is passed through such a coil, the places where the distribution is insufficient are directly revealed. The alternative for such eddy current coil measurement is a destructive process.
Urea can be prepared by introducing (excess) ammonia and carbon dioxide into a synthesis zone at a suitable pres
Eijkenboom Joseph Maria Gerardus
Meessen Jozef Hubert
Schrijen Hendrikus Catharina Guillaume Maria
DSM IP Assets B.V.
Mayer Brown Rowe & Maw LLP
Stoner Kiley
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