Patent Application
20110240715
The present invention generally relates to welding processes and materials. More particularly, this invention relates to a corrosion-resistant filler metal and welding process particularly well suited for fabricating and repairing components of nuclear power plants (NPP).
In boiling water reactors (BWR) and pressurized water reactors (PWR) of nuclear power plants, coolant water is heated by heat generated by a fission reaction. In boiling water reactors, the temperature of the coolant water can reach or exceed about 550 F (about 290 C) at elevated pressures. As such, reactor components subjected to hot coolant waters are exposed to a highly corrosive environment.
Various alloys have been employed in the fabrication of reactor components, including low alloy steels, stainless steels, and nickel-based alloys. Notable examples of the latter include NiCrFe alloys such as the well-known Alloy 600 (76Ni-15Cr-8Fe) and Alloy 690 (62Ni-29Cr-9Fe). Methods of fabricating and repairing reactor components have included the use of various welding processes, for example, weld joining, weld overlay and weld buildup, and various welding techniques, for example, gas-tungsten arc welding (GTAW), metal gas arc welding (GMAW), shielded metal-arc welding (SMAW), laser welding, electron beam welding, and ultrasonic welding. Nickel-based filler metals are extensively used to weld components formed of low alloy steels, stainless steels, and nickel-based alloys.
Notable examples of nickel-based filler metals used to fabricate and repair structures formed of Alloys 600 and 690 include commercially available alloys known as Alloy 82, Alloy 182, Alloy 52, Alloy 152, and Alloy 132. Alloy 82 is generally reported to have a composition containing, by weight, 18.0-22.0% chromium, 2.5-3.5% manganese, 2.0-3.0% niobium+tantalum, not more than 3.0% iron, not more than 0.75% titanium, not more than 0.50% silicon, not more than 0.50% copper, not more than 0.75% cobalt, not more than 0.10% carbon, not more than 0.030% phosphorus, not more than 0.015% sulfur, and a minimum nickel content of 67.0%. Alloy 182 is generally reported to contain, by weight, 13.0-17.0% chromium, 5.0-9.5% manganese, 1.0-2.5% niobium+tantalum, not more than 10.0% iron, not more than 1.0% titanium, not more than 1.0% silicon, not more than 0.50% copper, not more than 0.12% cobalt, not more than 0.1% carbon, not more than 0.030% phosphorus, not more than 0.015% sulfur, and a minimum nickel content of 59.0%. Alloy 52 is a more recent alloy having a higher chromium content than Alloys 82 and 182. The composition of Alloy 52 is generally reported to be, by weight, 28.0-31.5% chromium, 7.0-11.0% iron, not more than 1.0% manganese, not more than 0.10% niobium+tantalum, not more than 1.0% titanium, not more than 1.10% aluminum, not more than 1.5% titanium+aluminum, not more than 0.50% silicon, not more than 0.30% copper, not more than 0.040% carbon, not more than 0.05% molybdenum, not more than 0.020% phosphorus, not more than 0.015% sulfur, with the balance nickel. Alloy 152 is generally reported to have a composition containing, by weight, 28.0-31.5% chromium, not more than 5.0% manganese, 1.0-2.5% niobium+tantalum, 7.0-12.0% iron, not more than 0.50% titanium, not more than 0.75% silicon, not more than 0.50% copper, not more than 0.05% carbon, not more than 0.030% phosphorus, not more than 0.015% sulfur, not more than 0.50% aluminum, not more than 0.50% molybdenum, with the balance nickel. Alloy 132 is generally reported to have a composition containing, by weight, 13.0-17.0% chromium, not more than 3.5% manganese, 1.5-4.0% niobium+tantalum, not more than 11.5% iron, not more than 0.75% silicon, not more than 0.50% copper, not more than 0.12% cobalt, not more than 0.080% carbon, not more than 0.030% phosphorus, not more than 0.020% sulfur, and a minimum nickel content of 62.0%. In addition to the above, several types of Alloy 52 have been proposed, including alloys introduced by Special Metals Corp. and a composition reported by Knolls Atomic Power Laboratory as Alloy 52i.
Alloy 82 and particularly Alloy 182 have been identified as susceptible to stress corrosion cracking (SCC) under certain nuclear power plants operating conditions, particularly when exposed to hot coolant water environments, including but not limited to coolant water at temperatures of about 290° C. or higher. Though exhibiting improved corrosion resistance compared to Alloys 82 and 182, Alloy 52 has proved to be more difficult to weld due to ductility dip cracking (DDC) and hot tearing. As such, it would be desirable if a nickel-based weld filler metal were available that was capable of exhibiting improved stress corrosion cracking resistance as well as improved weldability in order to overcome limitations of the prior art.
The present invention provides a nickel-based alloy and welding processes that use the alloy as a weld filler metal to fabricate and repair components, including components of nuclear power plant reactors that contact the hot coolant water of the reactor.
According to a first aspect of the invention, the nickel-based alloy consists of, by weight, 26 to about 30% chromium, 2 to about 4% iron, 2 to about 4% manganese, 2 to about 3% niobium, 1 to about 3% molybdenum, not more than 0.6% titanium, not more than 0.03% carbon, not more than 0.05% nitrogen, not more than 0.6% aluminum, not more than 0.5% silicon, not more than 0.01% copper, not more than 0.02% phosphorus, not more than 0.01% sulfur, with the balance nickel and incidental impurities.
Other aspects of the invention include welding consumables formed of the alloy described above, and processes of welding components formed of nickel-based alloys using the alloy described above. A particular but nonlimiting example is the welding of components formed of NiCrFe alloys to produce components and structural elements of light water reactors for nuclear power plants.
A technical effect of this invention is that the nickel-based alloy is capable of exhibiting improved stress corrosion cracking resistance as well as improved weldability in comparison to conventional alloys such as Alloys 52, 82 132, 152 and 182.
Other aspects and advantages of this invention will be better appreciated from the following detailed description.
According to a preferred aspect of the invention, the weldments 16 and 22 are formed from a welding consumable formed of a nickel-based weld filler alloy, and the chemistry of the alloy is tailored to exhibit enhanced weldability and promote the corrosion and cracking resistance of the weldments 16 and 22. As used herein, welding consumable refers to electrodes, wires, powders and various other forms in which the alloy can be used during a welding process to produce the weldments 16 and 22. Particularly notable welding techniques include gas-tungsten arc welding (GTAW), metal gas arc welding (GMAW), shielded metal-arc welding (SMAW), laser welding, electron beam welding, and ultrasonic welding. Though the weldments 16 and 22 are represented as a butt joint and weld overlay, respectively, it should be understood that the alloy and welding technique can be used to form various other types of weldments, and can also be used to form weldments that fill cavities, form cladding or buildup material, and repair various other types of defects that might be present in the surface of a component.
Generally, the nickel-based weld filler alloy used to form the weldments 16 and 22 is formulated to have similar physical and mechanical properties to the alloys of the components 12, 14 and 20 to be welded. To this extent, the weld filler alloy of this invention contains nickel, chromium and iron, as do prior art weld filler alloys, such as Alloys 52, 82, 132, 152 and 182 that have been previously used to weld NiCrFe alloys such as Alloys 600 and 690, stainless steels, and low alloy steels. However, the weld filler alloy of this invention differs significantly in terms of its chromium and iron contents, as well as intentional additions of significant amounts of molybdenum and nitrogen. Broad, preferred and nominal chemistries for the alloy are summarized in Table I below. In addition to the listed constituents, the alloy may further contain incidental impurities, though such impurities preferably do not account for more than about 0.5 weight percent of the alloy.
The above alloying levels for chromium in the weld filler alloy were selected with the desire to promote resistance to stress corrosion cracking in hot water. The amount of chromium in the weld filler alloy is significantly higher than in Alloys 182 and 82 for the purpose of providing improved resistance to stress corrosion cracking (SCC).
In addition, the weld filler alloy contains an intentional addition of molybdenum well in excess of its levels permitted in Alloys 52, 82, 132, 152 and 182, and also contains an intentional addition of nitrogen. The molybdenum addition was chosen for the purpose of improving the weldability properties of the alloy. Molybdenum promotes the formation of molybdenum carbides by reacting with the carbon in the alloy, reducing the amount of carbon available to react with chromium and produce chromium carbides, which is a primary cause for ductility dip cracking (DDC). As known in the art, ductility dip cracking is a measure of the weldability of the alloy. The addition of molybdenum also improves the corrosion resistance of the alloy in high temperature water. The maximum amount of molybdenum is kept at 3 to avoid solidification cracking.
Finally, the levels of the remaining constituents of the alloy, and particularly the levels of iron, niobium, carbon, manganese, silicon, nitrogen, phosphorus and sulfur, were chosen for various reasons. Maintaining the iron content relatively low (less than in Alloy 52) is for the purpose of improving weldability by reducing the likelihood of forming Laves phases and M23O6 carbides. Laves phases are brittle and are one of the main causes of solidification cracking. The weld filler alloy has a similar niobium content to Alloy 82, which has good weldability properties. Alloy 52 does not contain niobium and experiences poor weldability. Therefore, the addition of niobium was for the purpose of improving weldability by favoring the formation of niobium carbides instead of chromium carbides. The amount of carbon (not more than 0.03%) is lower than Alloys 182. Lower amounts of carbon are believed necessary to avoid sensitization, for example, during welding. The addition of manganese is believed beneficial since it improves weldability by enhancing the fluidity of the weld pool. The low level of silicon contained in the weld filler alloy inhibits the formation of Laves phases and the incidence of solidification cracking. Though the solubility of nitrogen in nickel alloys is limited, nitrogen in the specified limited amounts should promote weldability. Phosphorous and sulfur levels should be kept low to reduce hot cracking during welding.
In view of the above, weldments formed with the weld filler alloy of this invention are expected to exhibit excellent weldability during fabrication, repair and overlay processes using the alloy as a welding consumable, and the resulting weldments are expected to be resistant to cracking when the welded components are subjected to corrosion and elevated temperatures, as in the case of light water reactor components that contact coolant water at high temperatures, for example, about 290° C. or more. Consequently, a technical advantage of the present invention is the ability to achieve improvements in the overall performance of welded structures formed of low alloys steels, stainless steels and nickel-based alloys, and particularly NiCrFe alloys such as Alloys 600 and 690 used in nuclear power plants. Commercial advantages include potential cost savings and the minimization of repairs and replacements during shutdowns of nuclear power plants.
While the invention has been described in terms of a preferred embodiment, it is apparent that other forms could be adopted by one skilled in the art. Therefore, the scope of the invention is to be limited only by the following claims.
