US 3740274 A
A method of treating certain stainless steels to improve the ductility characteristics after irradiation at high temperatures involving working at room temperatures or lower to a 90 percent reduction of area followed by low temperature recrystallization annealing.
Description (OCR text may contain errors)
United States Patent 1191 Chow June 19, 1973 HIGH POST-IRRADIATION nucrrurv 3,573,109 3/1911 Levy .Q ..14s/12.3
OC 3,623,920 11/1971 Kondo et a]. 148/123 Inventor: Joe G. Y. Chow, Northport, N.Y.
Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.
 Filed: Apr. 20, 1972  Appl. No.: 246,046
 0.8. CI. 148/12, 148/123  Int. Cl. C2ld 1/00, C2ld 7/02  Field of Search..... 148/123, 12
 References Cited UNITED STATES PATENTS 3,347,715 10/1967 Pfeil 148/12 Kondo et al. 148/123 Primary ExaminerW. W. Stallard AttorneyRoland A. Anderson  ABSTRACT A method of treating certain stainless steels to improve the ductility characteristics after irradiation at high temperatures involving working at room temperatures or lower to a 90 percent reduction of area followed by low temperature recrystallization annealing.
7 Claims, 2 Drawing Figures HIGH POST-IRRADIATION DUCTILITY PROCESS BACKGROUND OF THE INVENTION The development of breeder nuclear reactors which will operate at high temperatures for long periods of time between shutdowns requires the use of materials which will not become embrittled and crack under the adverse conditions of temperature and intense radiation. For example, it is known that the effect of irradiation on stainless steel at temperatures of about 1000 F. is normally to reduce substantially the ducility of the metal. The problem of cracking in service is one of the more severe difficulties faced by breeder reactor designers.
In US. Pat. No. 3,620,252 there is described a process for treating cobalt alloys to improve their ducility under the described conditions. However, as cobalt during reactor service produces radioactive cobalt 60 which has a very long half life as a gamma source and some of it tends to circulate with the coolant, the resulting maintenance problems make it preferable to avoid the cobalt alloys.
SUMMARY OF PRESENT INVENTION In accordance with this invention, it has been found that chromium nickel stainless steel alloys, which are otherwise suitable for reactor use and breeder reactors in particular, can be improved in their ability to remain ductile after being subject to irradiation at high temperatures for long periods of time. By chromium-nickel stainless steel alloys is meant for the purposes of this invention an austenitic alloy of iron having as its major alloying ingredients chromium and nickel, with chromium as the major additive and nickel as the minor additive, and have no more than 2 percent manganese. More detailed information on such stainless steel alloys including AISI designated types are given in the Metals Handbook, 8th E., Vol. 1, pub. by American Society of Metals, pp. 408- 409. I
In accordance with this invention, a stainless steel semifinished sample or article is cold worked until the material is too hard to work, usually when the cross section area is reduced at least 50 percent, is then given an intermediate low temperature recrystallization, is cold worked again until the maximum dimension of substantially all grains in the sample do not exceed 3 microns, the cross section area being reduced at least 90 percent, and is then given a final annealing at recrystallization temperatures to produce long term stability.
It is thus a principal object of this invention to provide an improved process for treating stainless steel a1- loys to retain ductility during and after irradiation.
Other objects of this invention will hereinafter become obvious from the following description of preferred embodiments of this invention.
BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a photo-micrograph (1000 X) of a section of normal type 316 stainless steel after beingsubject to irradiation.
FIG. 2 is a photo-micrograph (1000 X) of similar material which had been first treated in accordance with a preferred embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with a preferred embodiment of this invention a cylindrical rod of a chromium-nickel stainless steel alloy at a temperature up to 300 F, but preferably at or below room temperature is cold worked, such as by either swaging or rolling, reducing the cross section by at least 50 percent or until it becomes too hard to work, is given an intermediate low temperature anneal by heating it to at least recrystallization temperature, in the range of 1200-1500 F. until it is soft enough to resume working, after which working is resumed until the maximum dimension of substantially (that is, at least percent) all grains in the alloy is not in excess of 3 microns, and then low temperature annealing is repeated for 10-16 hours or a sufficient length of time to produce long term stability of the grain structure. The final annealing is for asubstantially longer time than the intermediate annealing.
More than one intermediate annealing step'may be employed, or in some cases, it may bepossible to produce the 90 percent reduction in area without an intermediate annealing. Multiple annealings as described insure that an extremely fine-grain structure results, in
which the maximum dimension of substantially all grains does not exceed 3 microns. The following examples illustrate this invention;
Example 1 A 1 inch cylindrical sample ofType 316 stainless steel was cold swaged at room temperature to a diameter reduced by 45 percent and recrystallized at 870 C.
for 16 hours.
The worked sample was then irradiated to 1.3 X 10 nvt (E 0.82 MeV) and specimens were prepared and tested for ductility at several temperatures with the results shown in Table Az TABLE A Test Tamp., C Tensile Strength, psi. Total E1ong.,% 25 148,000 10 650 42,800 25 750 29,500 7 800 17,800 6.5
FIG. 1 is a photomicrograph of the microstructure prior to irradiation.
Example 2 TABLE B Yield Tensile Total Test Temp. Strength Strength Elong. C. psi psi 25 100,000 121 .000 23 650 26,000 35,100 32 750 12,800 15,200 35 800 10,600 12,000 30 FIG. 2 is a photomicrograph of the microstructure of the specimen prior to irradiation showing the small grain size.
A comparison of the results shown in'Tables A and B shows a remarkable increase in ductility of the stainless steel when the present inventive process is applied.
Examples 3 and 4 of substantially all grains do not exceed 3 microns;
d. annealing said sample at a temperature at or above recrystallization to stabilize the crystal structure.
2. The process according to claim 1 in which working takes place up to a temperature of 300F.
3. The process according to claim 2 in which the intermediate annealing temperature is in the range of l200-l500 F.
4. The process according to claim 3 in which final annealing takes place at a temperature in the range of l200l500F. until recrystallization takes place.
5. The process according to claim 4 in which the working temperature is room temperature.
6. The process according to claim 4 in which more than one intermediate annealing is employed.
7. The process according to claim 3 in which the final annealing is for a substantially longer period than the intermediate anneal to increase long term stabilization characteristics of the grain structure.