Welded structure made of low thermal expansion coefficient...

Details Welded structure made of low thermal expansion coefficient... Welded structure made of low thermal expansion coefficient...

2001-03-30

2003-03-04

Yee, Deborah (Department: 1742)

Alloys or metallic compositions

Ferrous

Over 10 percent nickel containing

C420S095000, C148S336000, C428S681000, C428S682000, C428S683000

Reexamination Certificate

active

06528012

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an article serving as storage tanks, pipelines, and various equipments associated with them for cryogenic substances, such as liquefied natural gas (LNG), in which all or part of members of the article are formed of a Fe—Ni low thermal expansion coefficient alloy and are assembled by welding. (Such an article will be collectively referred to as “welded structure” herein.) The present invention also relates to a welded pipe used in the above pipelines. Furthermore, the present invention relates to a welding material (wire) suitable for use in manufacturing the welded structure and the welded pipe as described above.
BACKGROUND OF THE INVENTION
Heretofore, austenitic stainless steels, such as JIS-SUS 304, have been used as a material of storage tanks and pipelines for cryogenic substances, such as the liquefied natural gas (LNG). However, the austenitic stainless steel has a high thermal expansion coefficient. Thus, for example in the pipeline, it is required to measure for absorbing the deformation due to expansion and contraction by interposing a loop pipe for each given length of the pipeline. If the pipeline can be formed of a material having a remarkably low thermal expansion coefficient which allows the loop pipe to be eliminated, an elbow pipe involved in the loop pipe will become unnecessary and thereby the diameter of a tunnel for letting the pipeline pass through will be able to be reduced. This will allow maintenance operations for thermal insulations of the pipeline or the like to be minimized, and will open the way to significantly economize construction costs and operation-maintenance expenses.
Among materials available for the transport or storage equipment of cryogenic substances, such as LNG, in view of mechanical and chemical properties thereof, it is known that a Fe—Ni alloy of a particular component ratio has an extremely low linear expansion coefficient. Typical examples of such an alloy are Fe—36% Ni alloy and Fe—42% Ni alloy, which are collectively referred to as Invar alloy (“%” concerning each content of components means “weight %” herein). These alloys are used as a material for equipment in which the expansion, and/or contraction due to temperature variation, is undesirable.
When a structure formed of the above Fe-Ni low thermal expansion coefficient alloy is assembled by welding, it is desireable to apply a welding material having a linear expansion coefficient similar to that of the base material. Thus, there are proposed some welding materials similar to the base metal, which are disclosed in Japan patent Laid-Open Publication No. 4-231194 and Japanese Patent Laid-Open Publication No. 8-267272 for example.
The welding material disclosed in the above Japanese Patent Laid-Open Publication No. 4-231194 includes C: 0.05 to 0.5% and Nb: 0.5 to 5% as well as Ni (Co) and Fe, and, as needed, selectively includes Mn, Ti, Al, Ca, Mg, Si, Cu, Ag, B, Sn, and Zn, whereby cracking in welding operations can be prevented.
The welding material disclosed in the Japanese Patent Laid-Open Publication No. 8-267272 includes Ni: 30 45%, C:0.03 to 0.3%, Nb: 0.1 to 3%, P:0.015% or less, S:0.005% or less, Si: 0.05 to 0.6%, Mn: 0.05 to 4%, Al: 0.05% or less, and O (oxygen): 0.015% or less, wherein the relationship between Nb and C is defined by (%Nb) × (%C) ≧ 0.01, whereby the reheat cracking in the multi-layer welding operation is prevented and the toughness of the welded zone is also improved.
However, when the above proposed welding materials are applied to assemble a large size structure, particularly to the structure of pipelines, storage tanks or the like for the liquefied natural gas, the following problems arise; various alloying elements, such as Nb, Ti, Ce, Mg, B, Ca et al., included in these welding materials, deteriorate workability of the alloy, resulting in a complicated manufacturing process of a welding material (wire). In particular, Nb contributes to the creation of a large size of oxide, resulting in not only deteriorated hot-workability, but also deteriorated cold-workability. Even if a welding material is successfully produced, the following problems remain.
When welded structures, such as pipelines or storage tanks for liquefied natural gas, are assembled, in view of the efficiency of operations and the quality of welded zones, it is desired that it is capable of automatic welding based on the TIG or plasma welding process and of welding in various positions for on-site operations. That is, the welding material is required to have an excellent weldability in fabrication. Specifically, in automatic welding, it is required not to allow weld defects to arise, such as incomplete formation of root pass bead or lack of fusion cased by insufficient weld penetration or missing of a weld line straightness, and not to allow burn though or lack of penetration in root pass to arise even during the welding in an overhead position.
The proposed welding materials described above were developed under consideration for improving welded joint properties, such as cracking resistance and toughness. However, the aforementioned workability and weldability in fabrication of the welding material were left out of view.
The welding materials disclosed in the Japanese Patent Laid-Open Publication No. 4-231194 were invented under consideration of the solidification cracking in welding operations. However, it does not discuss any measure for the reheat cracking arising in a multi-layer welding operation for a thick member, such as a large size structure. Further, despite of the necessity that the region, which contacts to liquefied natural gas, should have a sufficient toughness under very low temperature, i.e., −196° C., this point is also left out of view.
The term “solidification cracking” means a cracking which arises in a weld metal (bead) during solidification. The term “reheat cracking” means a cracking which arises in an initially formed weld metal (bead), which originally had no cracking, by a thermal affection at the time when the initially formed weld metal is reheated by an additional weld metal superposed thereon.
While a measure against the reheat cracking was also considered in the Japanese Patent Laid-Open Publication No. 8-267272, an effect of preventing the reheat cracking is not always sufficiently provided only by arranging the chemical composition of the welding material because a certain dilution of the alloy components due to the welding methods or groove shapes used in welding operations is involved. Thus, it is difficult to actually apply this welding material for structures handling cryogenic substances, such as the liquefied natural gas.
In these structures, such as storage equipment and pipelines for LNG, intended for handling the cryogenic substances, the weld metal is required to have an adequate stress corrosion cracking resistance, as well as toughness under a low temperature, as described above. This is required because the above structures are often coated with a an insulating material (thermal insulator), such as urethane resin, which includes a small amount of Cl
—
, and these structures are often situated close to an ocean and exposed to the atmosphere containing Cl
—
due to sea water.
The above invention disclosed in the Japanese Patent Laid-Open Publication No. 8-267272 has an objective to improve the cold toughness of welded zone. However, it does not discuss any measure for the stress corrosion cracking resistance.
In the meantime, the welding material used for fabricating the above welded structure is required to be readily produced, or to be readily converted into a wire (i.e. required to have good workability), and is also required to facilitate welding operations when such a material is applied (i.e. required to have good welding weldability in fabrication).
For manufacturing of welding materials, i.e., welding wires, a hot working of raw materials and a cold working for wire drawing are essential. As described above, the welding materials proposed until the present

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