Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
1999-06-10
2001-06-05
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C148S516000, C148S527000, C148S428000, C219S076100, C219S121110, C219S1370WM, C219S145220, C219S145230, C219S146230, C420S446000, C420S447000, C420S448000, C420S449000, C420S450000, C420S452000, C420S454000, C420S459000, C420S460000, C428S386000, C428S544000, C428S546000, C428S577000
Reexamination Certificate
active
06242113
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a nickel, chromium, iron welding alloy, articles made therefrom for use in producing weldments, and weldments and methods for producing these weldments.
2. Brief Description of the Prior Art
In various welding applications, including equipment used in nuclear power generation, weldments are required that provide resistance to various cracking phenomenon. This includes not only stress corrosion cracking but hot cracking, cold cracking, and root cracking as well.
Commercial and military nuclear power generation have only existed within the second half of the 20
th
century. During this time, the industry has replaced the first generation of NiCrFe alloys having 14 to 15 percent chromium with alloys having higher chromium contents on the order of 30 percent. This change was predicated on the discovery that stress corrosion cracking in nuclear pure water could be avoided with alloys of this type that contained chromium in this amount. These alloys have been in use for about 20 to 25 years.
The specific application for nuclear power generation equipment that requires the majority of welding and welded products within the nuclear power plant is the fabrication of the nuclear steam generator. This equipment is essentially a large tube and shell heat exchanger that generates steam from secondary water from primary nuclear reactor coolant. The key component of this steam generator is the tubesheet. It is sometimes 15 to 20 feet in diameter and well over a foot thick and is usually forged from a high strength low alloy steel that must be weld overlaid with a NiCrFe alloy that has good fabric ability and is resistant to stress corrosion cracking in nuclear pure water. Due to the size of the tubesheet, the weld deposit sustains substantial residual stress during overlay. Furthermore, the weld metal overlay must be capable of being rewelded after being drilled to provide openings therein to receive thousands of small steam generator tubes. These tubes must be seal-welded to the overlay weld deposit to make helium-leak-tight welds. These welds must be of extraordinary high quality and must provide 30 to 50 year life with high predictability. In addition, in both the overlay weld deposit and the welded steam generator tubes, excellent crack resistance must be provided. This requirement, with respect to resistance to hot cracking, also termed “solidification cracking,” and stress corrosion cracking has been met by most of the existing 30% chromium weldments.
In addition to hot cracking resistance and stress corrosion cracking resistance, the tube-to-tubesheet welds require root cracking resistance. The tube-to-tubesheet weld is made by melting the tube end together with a ring of the weld overlay material surrounding the tube (with or without the use of additional filler metal) to thereby seal the space between the tube wall and the opening in the tubesheet. There is a tendency for these welds to crack at the intersection of the weld at the joiner of the tube to the tubesheet. This type of cracking is referred to as “root cracking” because it occurs at the root of the weld. The existing 30% chromium welding alloys are not resistant to root cracking.
A third type of cracking that may be encountered is cold cracking, also known as “ductility dip cracking.” This cracking only occurs in the solidified state after weld solidification has been completed. After solidification occurs, shrinkage stresses begin to develop as a result of the reduction in volume of the welding alloy at lower temperature. At the same time, once solidification is complete, ductility recovery occurs rapidly for a few hundred degrees, followed by a sharp temporary loss in ductility, and again followed by a more gradual continuous recovery of ductility until ambient temperature is reached. If the residual stress of cool-down is sufficiently large when the alloy exhibits this sharp ductility loss, solid state cracking may occur. This results from portions of the microstructure not having sufficient strength or ductility to resist the stress at the prevailing temperature. The commercially available 30% chromium welding alloys presently available are not sufficiently resistant to cold cracking.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a nickel, chromium, iron welding alloy and weldments made therefrom that provides the desired strength and corrosion resistance in addition to resistance to hot cracking, cold cracking, root cracking, as well as stress corrosion cracking.
A further object of the invention is to provide a welding alloy of the nickel, chromium, iron type that is particularly adapted for uses in fabricating equipment used in nuclear power generation.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a nickel, chromium, iron alloy for use in producing weld deposits. The alloy comprises, in weight percent, about 27 to 31.5 chromium; about 7 to 11 iron; about 0.005 to 0.05 carbon; less than about 1.0 manganese, preferably 0.30 to 0.95 manganese; about 0.60 to 0.95 niobium; less than 0.50 silicon, preferably 0.10 to 0.30 silicon; 0.01 to 0.35 titanium; 0.01 to 0.25 aluminum; less than 0.20 copper; less than 1.0 tungsten; less than 1.0 molybdenum; less than 0.12 cobalt; less than 0.10 tantalum; less than about 0.10 zirconium, preferably 0.002 to 0.10 zirconium; less than about 0.01 sulfur; less than about 0.01 boron, preferably 0.001 to 0.01 boron; less than about 0.02 phosphorous; and balance nickel and incidental impurities.
The alloy will exhibit adequate stress corrosion cracking resistance in view of the chromium content. The alloy may be in the form of a weld deposit, a welding electrode, a welding electrode in the form of a wire with a flux cover, a welding electrode in the form of a sheath with a flux core, a weld deposit overlay or a weldment comprising an alloy substrate, such as steel with an overlay of the invention alloy. It may be used in a method for producing a weld deposit or weldment in the form of a flux-covered electrode used for producing a weld deposit that includes welding performed by submerged arc welding or electroslag welding. The weldment may be in the form of a tubesheet of a nuclear reactor. It may be further used as an article for producing a weldment, with the article being in the form of wire, strip, sheet, rod, electrode, prealloyed powder, or elemental powder. The method for producing the weld deposit may include producing a flux-covered electrode of a nickel, chromium wire, or a nickel, chromium, iron wire and melting the electrode to produce a weld deposit.
REFERENCES:
patent: 4010309 (1977-03-01), Petersen
patent: 5077006 (1991-12-01), Culling
patent: 5543109 (1996-08-01), Senba et al.
patent: 1086288 (1967-10-01), None
patent: 1481831 (1977-08-01), None
PCT Search Report, PCT/US00/11855; Sep. 11, 2000.
“The Microstructure and Corrosion Resistance of Nickel-Based Filler Metals with High Chromium Contents”,Welding Journal, Feb., 1990, No. 2, Miami FL, pp. 60-67, by C. L. Briant and E. L. Hall.
Dropkin, Esq. Robert F.
INCO Alloys International, Inc.
Jones Deborah
Koehler Robert R.
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