Austenitic stainless steel alloy

Abstract: The alloy comprises in weight percent: Iron plus impurities Balance Carbon (C) 0.02 to 0.04 max. Chromium 18 to 20 Nickel 9 to 11 Manganese 1.5 to 2 Niobium plus Tantalum 14 x wt%C with maximum of 0.25wt% Niobium. The alloy may comprise small amounts of other elements.

Description: This invention relates to austenitic stainless steel compositions. An illustrative embodiment of the invention is concerned with an austenitic stainless steel alloy composition having both a high resistance to irradiation promoted corrosion and reduced long term irradiation induced radioactivity and reference is made herein to such an alloy by way of example. Stainless steel alloys, especially those of high chromium-nickel type, are commonly used for components employed in nuclear fusion reactors due to their well known good resistance to corrosive and other aggressive conditions. For instance, nuclear fuel, neutron absorbing control units, and neutron source holders are frequently clad or contained within a sheath or housing of stainless steel of Type 304 or similar alloy compositions. Many such components, including those mentioned, are located in and about the core of fissionable fuel of the nuclear reactor where the aggressive conditions such as high radiation and temperature are the most rigorous and debilitating. Solution or mill annealed stainless steels are generally considered to be essentially immune to intergranular stress corrosion cracking, among other sources of deterioration and in turn, failure. However, stainless steels have been found to degrade and fail due to intergranular stress corrosion cracking following exposure to high irradiation such as typically encountered in service within and about the core of fissionable fuel of water cooled nuclear fission reactors. Such irradiation related intergranular stress corrosion cracking failures have occurred notwithstanding the stainless steel metal having been in the so-called solution or mill annealed condition, namely having been treated by heating up to within a range of typically about 1,010° to about 1,120°C, then rapidly cooled as a means of solutionizing carbides and inhibiting their nucleation and precipitation out into grain boundaries. Accordingly, it is theorized that high levels of irradiation resulting from a concentrated field or extensive exposure, or both, are a significantly contributing cause of such degradation of stainless steel, due among other possible factors to the irradiation promoting segregation of the impurities therein. Efforts have been made to mitigate intergranular stress corrosion cracking of stainless steels which have not been desensitized by solution or mill annealing, or irradiated, including the development of "stabilized" alloys. For example, alloys have been developed containing a variety of alloying elements which are intended to form stable carbides. Such stabilizing carbides should resist solutionizing at annealing temperatures of at least 1038°C whereby the carbon is held so that the subsequent formation of chromium carbide upon exposure to high temperatures is prevented. Included among the alloying elements proposed are titanium, niobium and tantalum. An example of one type of such a stainless steel alloy is marketed under the designation of Type 348. The Metals Handbook , Ninth Ed., Vol. 3, page 5, American Society for Metals, 1980 gives the alloy composition for Type 348 in weight percent as follows: C Mn Si Cr Ni P S Cu Nb + Ta 0.08 max. 2.00 max. 1.00 max. 17.0-19.0 9.0-13.0 0.045 max. 0.03 max. 0.2 max. 10 x %C min. Aspects of the invention are set out in the claims to which attention is invited. Embodiments of this invention comprise stainless steel alloy compositions having specific ratios of alloying elements for service where exposed to irradiation. The austenitic stainless steel alloy composition of such embodiments provides resistance to the degrading effects of the irradiation, and/or is of reduced long term irradiation induced radioactivity. An embodiment of this invention is particularly directed to a potential deficiency of susceptibility to irradiation degradation which may be encountered with chromium-nickel austenitic stainless steels comprising Type 304 and related high chromium-nickel alloys such as listed in Tables 5-4 on pages 5-12 and 5-13 of the 1958 edition of the Engineering Materials Handbook , edited by C.L. Mantell. These alloys comprise austenitic stainless steels of about 18 to 20 percent weight of chromium and about 9 to 11 percent weight of nickel, with up to a maximum of about 2 percent weight of manganese, and the balance iron with incidental impurities. US-A-4162930 discloses a chromium-nickel austenitic stainless steel having improved resistance to intergranular stress corrosion cracking. The steel has low carbon and phosphorus content or carbon and phosphorus in solid solution fixed by niobium addition. Further resistance to transgranular stress corrosion cracking is realized with a low molybdenum content. The steel is particularly useful in applications involving exposure to high-temperature and high-pressure water and attack by chlorides. This embodiment comprises a modified Type 304 austenitic stainless steel and a specific alloy composition including precise ratios of added alloying ingredients, as well as given limits on certain components of the standard austenitic stainless steel alloy. The present invention accordingly provides a stainless steel alloy for service exposed to irradiation, having resistance to irradiation promoted stress corrosion cracking and reduced long term irradiation induced radioactivity consisting of up to 0.04% carbon 1.5 to 2% manganese 18 to 20% chromium 9 to 11% nickel a minimum of a combination of both niobium plus tantalum of about 14 x wt% carbon content up to a maximum of niobium plus tantalum of 0.65 wt%, and up to a maximum of 0.25 wt% niobium, optionally up to 0.005 wt% phosphorus up to 0.004 wt% sulphur up to 0.03 wt% silicon up to 0.03 wt% nitrogen up to 0.03 w% aluminium up to 0.01 wt% calcium up to 0.003 wt% boron up to 0.05 wt% cobalt the balance being iron with incidental impurities. A preferred stainless steel alloy according to the present invention consists of up to 0.04% carbon 1.5 to 2% manganese 18 to 20% chromium 9 to 11% nickel a minimum of niobium plus tantalum of 14 x wt% carbon content up to a maximum of 0.65 wt%, and up to a maximum of 0.25 wt% niobium, the balance being iron with incidental impurities. Preferably, the tantalum can range up to about 0.4wt percent of the overall alloy, the minimum content of niobium plus tantalum is 0.28 wt percent of the overall alloy and the carbon content is in the range of 0.02 to 0.04 wt percent. The foregoing preferred specific austenitic stainless steel alloys composition, among other attributes, provides a high degree of resistance to stress corrosion cracking regardless of exposure to irradiation of high levels and/or over prolonged period, without incurring long term induced radioactivity. As such, the alloy composition of this invention is well suited for use in the manufacture of various components for service within and about nuclear fission reactors whereby it will retain its integrity and effectively perform over long periods of service regardless of the irradiation conditions. Moreover, the alloy composition of this invention additionally minimizes irradiation induced long term radioactivity whereby the safety and cost requirements for its disposal following termination of service are reduced, and of greatly shortened period. The following comprises an example of a preferred austenitic stainless steel alloy composition of this invention. Alloy Ingredient Percent Weight Carbon 0.033 Chromium 19.49 Nickel 9.34 Tantalum 0.40 Niobium 0.02 Sulfur 0.003 Phosphorus 0.001 Nitrogen 0.003 Silicon 0.03 Iron Balance Physical Properties Yield, MPa 275 - 323 Elongation, % 48 - 52 Grain Size (ASTM) 9.5 Hardness. R B Embodiments of the austenitic stainless steel alloy may provide: an austenitic stainless steel alloy composition having effective resistance to the deleterious effects attributable to prolonged exposure to high levels of radiation; an austenitic stainless steel alloy composition which essentially maintains its physical and chemical integrity when subjected to high levels of irradiation over long periods; an austenitic stainless steel alloy composition which provides effective resistance to irradiation promoted intergranular stress corrosion cracking; an austenitic stainless steel alloy composition which minimized the long term imposed radioactivity resulting from exposure to extensive high levels of irradiation in service; and/or an austenitic stainless steel alloy composition which exhibits low radiation emissions following its irradiation whereby it can be disposed of at low cost.

Claims: A stainless steel alloy for service exposed to irradiation, having resistance to irradiation promoted stress corrosion cracking and reduced long term irradiation induced radioactivity consisting of up to 0.04% carbon 1.5 to 2% manganese 18 to 20% chromium 9 to 11% nickel a minimum of a combination of both niobium plus tantalum of about 14 x wt% carbon content up to a maximum of niobium plus tantalum of 0.65 wt%, and up to a maximum of 0.25 wt% niobium optionally up to 0.005 wt% phosphorus up to 0.004 wt% sulphur up to 0.03 wt% silicon up to 0.03 wt% nitrogen up to 0.03 w% aluminium up to 0.01 wt% calcium up to 0.003 wt% boron up to 0.05 wt% cobalt the balance being iron with incidental impurities. A steel according to Claim 1, consisting of up to 0.04% carbon 1.5 to 2% manganese 18 to 20% chromium 9 to 11% nickel a minimum of niobium plus tantalum of 14 x wt% carbon content up to a maximum of 0.65 wt%, and up to a maximum of 0.25 wt% niobium, the balance being iron with incidental impurities. A steel according to Claim 1 or 2 having a carbon content in the range of 0.02 to 0.04 wt%. A steel according to any of Claims 1 to 3, wherein the combination of Nb plus Ta is at least 0.28 wt%. A steel according to any one of Claims 1 to 4 having up to 0.4 wt% tantalum.

Inventors: COATES DAVID JOHN, GORDON GERALD MYRON, JACOBS ALVIN JOSEPH, SANDUSKY DAVID WESLEY, COATES, DAVID JOHN, GORDON, GERALD MYRON, JACOBS, ALVIN JOSEPH, SANDUSKY, DAVID WESLEY
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1989
Main classification: C22C38
IPC codes: C22C38