CA1133363A

CA1133363A - Method for heat treating iron-nickel-chromium alloy

Info

Publication number: CA1133363A
Application number: CA352,685A
Authority: CA
Inventors: Howard F. Merrick; Michael K. Korenko
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1979-07-27
Filing date: 1980-05-26
Publication date: 1982-10-12
Also published as: IT1136403B; IT8041570A0; NL8002490A; GB2058834A; FR2462478A1; JPH0130891B2; SE452992B; ES491749A0; ES8105787A1; DE3019931C2; SE8003879L; FR2462478B1; DE3019931A1; GB2058834B; BE883413A; JPS5620123A

Abstract

13 47,107I
ABSTRACT OF THE DISCLOSURE
A method for heat treating an age-hardenable iron-nickel-chromium alloy to obtain a bimodal distri-bution of gamma prime phase within a network of dis-locations, the alloy consisting essentially of about 25% to 45% nickel, 10% to 16% chromium, 1.5% to 3% of an element selected from the group consisting of molybdenum and niobium, about 2% titanium, about 3% aluminum and the remainder substantially all iron. To obtain optimum re-sults, the alloy is heated to a temperature of 1025°C to 1075°C for 2-5 minutes, cold-worked about 20% to 60%, aged at a temperature of about 775°C for 8 hours followed by an air-cool, and then heated to a temperature in the range of 650°C to 700°C for 2 hours followed by an air-cool.

Description

1~L33363 1 47,1071 METHOD FOR HEAT TREATING IRON-NICKEL-CHROMIUM ALLOY
BACKGROUND OF THE INVENTION
The present invention is particularly adapted ~or use with a nickel-chromium-iron alloy such as that described in U.S. Patent No. 4,236,943 issued on Decem-5 ber 2, 1980, which has strong mechanical properties and,at the same time, has swelling resistance under the in-fluence of irradiation and low neutron absorbence~ As such, it is particularly adapted ~or use as a duct~ng and cladding alloy ~or fast breeder reactors~
A material of this type is a gamma-prime strengthened superalloy; and its properties can be altered drastically by varying the thermomechanical treatment to which it is sub~ected. For nuclear reactor applications it is, of course, desirable to subject the alloy to a thermomechanical treatment which will produce the greatest irradiation-induced swelling resistance and/or the highest strength and most importantly the highest post irradiation ductility.

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2 47,107I
SUMMARY OF THE INVENTION
In accordance with the present invention, an alloy having a composition of about 25% to 45% nickel, 10% to 16~ chromium, 1.5% to 3% of an element selected from the group cons$sting of molybdenum and niobium, about 1% to 3% titanium, about 0~5% to 3.0% aluminum and the remainder substantially all iron, initially heated to a temperature in the range of about 1000C to 1100C for a period o~ 30 seconds to one hour; although the preferred heat treatment is to h~at in the range of 1025C to 1075C
for 2-5 minutes to minimize the time in the furnace. mis initial heat treatment is followed by a furnace-cool and cold-working in the range o~ about 20Yo to 60% although cold working within the range between 10% ~nd 80% is useful.
Thereafter, the alloy i~ heated to a temperature in the range of 750C to 825C for 4-15 hours and preferably 775C
for 8 hours, followed by an air-cool. mereafter, the alloy is again heated to a temperature in the range of about 6500C
to 700C for 2-20 hours ~ollowed by an air-cool.
The abo~e and oth~r objects and features of the invention will become apparent ~rom the following detailed description taken in connection with the accompanying drawing which is a plot of percent swelling versus an-nealing temperature for an alloy within the scope of the in~ention.
In order to establish the desirable re~ult~ of the invention, an alloy having the following composition was subject to uarious thermomechanical treatment~ herein-after described:
TABLE I
Nickel - 45%
Chromium - 12%
Molybdenum _ 30/O
Silicon - 0.5%
Zirconium - 0.05%
Titanium - 2.5~
Aluminum - 2.5%
Carbon - 0.~3%

~33363 2a 47 ,1 07I
Boron - 0 . 005%
Iron - BalO

3 47,107I
'Ihe foregoing alloy is a gamma-prime strength-ene(l superalloy. Its properties can be altered drasti-c~lly b~ varying its thermomechanical treatment prior to testing. The following Table II sets forth the various 'j thermomechanical treatments to which the alloy set forth in Table I was subjected; while Table III lists the re-sultin~ microstructural and mechanical properties of the alloy after heat treatment:
T~ LE II
Vesi.gna-tion_ Thermomechanical Treatment AR 103~C~l hr/FC + 60% CW
IN-I ~982C/1 hr/AC ~ 788C/1 hr/AC + 720C/24 hr/AC
IN-2 ~890C/1 hr/AC + 800C/11 hr/AC + 700C/2 hr/AC
1'j EC -~927C/1 hr/AC + 800C/ll hr/AC + 700C/2 hr/AC
EE *800C/ll hr/AC + 700C/2 hr/AC
ter l~g~C/I hr/FC + 60% CW.

TABLE_LII
Designa- 650C
20 tion Comments UTS (ksi) 80 ksi SR (hr) AR No gamma-prime, high - 67.9 dislocation density IN-l Bimodal gamma-prime, 151.5 0.,3 recrystallized above gamma-prime solvus lN-2 Trimodal gamma-prime 141.3 81.9 (~islocated) EC Trimodal gamma-prime - 64.7 recrystallized below gamma-prime solvus EE Bimodal gamma-prime, - 235 equiaxed cells As can be seen from Table III above, the EC
treatment produces higher stress rupture properties than treatment IN-l. The EC treatment results in a trimodal distribution of gamma-prime since the recrystallization 1.~33363 ~ 4 47,107I
anneal is below the gamma-prime solvus and results in the prccipitation of a small volume of large (approximately 600 nm) gamma-prime precipitates.
Of the treatments set forth in Tables II and III, three treatments produced dislocated structures.
These are treatments AR, IN-2 and EE. The stress rupture data of Table II reveals that heat treatment EE produces a significantly stronger material. This structure consists of an interwoven dislocated cell structure which is pinned by a bimodal gamma-prime distribution. This condition has the highest strength of any tested and is very stable because of the pinned nature of the dislocation cells.
The graph shown in the attached figure illus-trates the swelling behavior of the alloy set forth in Table I in three thermomechanical conditions, ST, EC and EE. The swelling versus temperature curves are for radia-tion doses of 30 dpae, which is equivalent to 203x103 ~-(i.e., greater than the goal fluence of 120x103 ~-). The data reveals that the ST and EE treatments produce the lowest swelling in the alloy set forth in Table II above.
The EC treatment produces an acceptable level of swelling at goal fluences but the treatment is far from optimum for in-reactor applications.
In similar tests, an alloy having the following composition was tested:
TABLE IV
Nickel 60 Chromium 15 Molybdenum 5.0 Niobium 1.5 Silicon 0.5 Zirconium ~ .03 Titanium 1.5 Aluminum 1.5 Carbon 0,03 Boron 0.01 Iron Bal.

47,~071 Ihe thermomechanica:l treatalents given to t:he aforesaid alloy o~` T~ble IV and the microstructures and mechanical properties of the resulting alloy are given in the follow-ing Tables V and VI:
TABLE V
Designa-tion Thermomechanical Treatment*
BP 1038C/1 hr/AC + 800 C/ll hr/AC + 700C/2 hr/AC
BR 927C/1 hr/AC + 800C/ll hr/AC ~ 700C/2 hr/AC
BT 1038C/0.25 hr/AC + 899C/l hr/AC +
749C/8 hr/AC
CT 30%CW at 1038C + 800C/ll hr/AC +
700C/2 hr/AC
CU 890C/1 hr/AC + 800C/ll hr/AC +
700C/2 hr/AC + 30 BU 800C/ll hr/AC + 700C/2 hr/AC
*A~ter TO38~7r-hr/Fc ~ 60% CW.

TABLE VI
Designa- 650C
20 tion Com,ents ~ r) BP Small gamma-prime, no 136.7 dislocations BP Bimodal gamma-prime, 152.5 73 gamma cells 25 BT Bimodal gamma-prime 135.3 no dislocations CT Bimodal gamma-prime, 154.6 non-uniform structure (long banded cells, some subgrains) CU Bimodal gamma-prime, 147.0 elongated cells BIJ Bimodal gamma-prime, 156.4 74 equiaxed cells ~5 The gamma-prime solvus and the one hour recrystalliza~ion temperature for the alloy set forth in Table IV are 915C

3~3 6 47,1071 I()"(` al-l(l lO()()QC + 20(`, respectively. Therefore, unlike thc alloy given in 'I'ab]e I, there is no temperature range in which recrystallization can be accomplished while aging. Consistent with this fact, treatments BP and BT
which involve annealing at 1038C and subsequent double-aging, both produce a dislocation-free austenite matrix and a bimodal gamma-prime distribution. Structures pro-duced by treatments CU and BU, which do not induce recrys-tallization, all contain a highly dislocated cell struc-l~ ture containing various distributions of gamma-prime.
Table VI is a summary of the observed structures and corresponding physical properties. Note that the mechanical property values are grouped into two classes.
These are non-dislocation density, gamma-prime containing structures having 650C, ultimate ten~ile strengths be-tween 135 and 137 ksi, and the dislocated gamma-prime structures, which are much stronger, with ultimate tensile strengths between 147 and 157 ksi. Because of their superior strength and because of the benefit of an in-creased incubation time for swelling, dislocated struc-tures are preferred.
Treatment CU set forth in Tables V and VI above, starts with a dislocated cell structure with a trimodal gamma-prime distribution which is subsequently cold-worked 2~j 30%. The final cold-working operation actually decreases the strength as indicated by the 650C ultimate tensile strength data set form in Table VI, apparently by des-troying the integrity of the dislocation cell walls.
Treatments BR and BU of the alloy set forth in Table IV both produce a highly dislocated, partially recrystallized or recovered cell structure with bimodal gamma-prime size distribution. The BU treatment is pre-ferre~ since it yields slightly higher stress rupture ~ than the BR treatment. The dislocation and 3~, gamma-prime structures for the BU treatment produce a cell structure which is much more dispersed and interwoven than that produced by the EE treatment of the alloy set forth in Table I. The minimum cell thickness of the BU treat-:1~ 3~363 7 47,107I
mellt is appro.~imately the satne as the gamma-prime particle C i ~l g .
Ln order to further demonstrate the improvement that is obtained by means of the thermomechanical treat-ment of the present invention, reference may be had to thefollowing Tables VII and VIII which shows that this treat-ment is very effective in promoting high post radiation ductility. In this regard it should be pointed out that there exists a trough in which the ductility of these materials is materially decreased when tested at a temper-ature which is 110C above the temperature at which the material has been irradiated. Thus, the poorest ductility would be found at a temperature of 805C where the mater-ial has been irradiated at 695C. This 110 should ac-1'> count for all transient conditions of operation of forexample a fast breeder reactor. Thus the selection of the material and the heat treatment or the thermomechanical heat treatment of the material which when irradiated at 695centigrade should be tested at 805C where the lowest post irradiation ductility has occurred. Reference to the following Tables VII and VIII make it abundantly clear for example that the solution treated condition of alloy D66 when irradiated at 695C and tested at 805C exhibits zero ductility. In contrast thereto, material which has been subjected to the treatment set forth in the claims append-ed hereto of the same alloy irradiated at 695C and tested at 805C shows that a 1.1% uniform elongation is avail-able. It is critically important to maintain a greater than 0.3% ductility under these conditions since this is necessary to maintain fuel pin integrity during reactor transient conditions and the tables demonstrate the at-tainment of those goals. Tables VII and VIII also illus-trate that the higher ductility of this treatment is also accompanied by higher strength which is a highly unex-pected as respects these irradiated materials. Thesehigher strengths also attest to the fact of the excellent swelling resistance demonstrated by the alloys ~hich are subjected to the method of this treatment.

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Although the invention has been shown in connec-tion with certain specific embodiments, it will be readily apparent to ~hose skilled in the art that various changes in method steps and compositional limits can be made to suit requirements without departing from the spirit and scope of the invention.

Claims

11 47,107I
What is claimed is:

1. A method for heat treating an iron-nickel-chromium alloy consisting essentially of about 25% to 45%
nickel, 10% to 16% chromium, 1.5% to 3% of an element selected from the group consisting of molybdenum and niobium, about 1% to 3% titanium, about 0.5% to 3.0%
aluminum and the remainder substantially all iron; which method comprises the steps of heating the alloy to a temperature in the range of 1000°C to 1100°C for 30 se-conds to 1 hour followed by a furnace-cool, cold-working the alloy 10% to 80%, heating the alloy to a temperature of about 750°C to 800°C for 4-15 hours followed by an air-cool, and then heating the alloy to a temperature in the range of 650°C to 700°C for 2-20 hours followed by an air-cool.

2. The method of claim 1 wherein the alloy is initially heated to a temperature in the range of 1025°C
to 1075°C for 2-5 minutes.

3. The method of claim l wherein said alloy is qo~/o f o ~o~c cold-worked by cold rolling 20% to 60%.

4. The method of claim 3 wherein said alloy is cold-rolled 30% to 50%.

5. The method of claim 1 wherein said alloy is in the form of a tube and is cold-worked by drawing the tube to produce a reduction of 15% to 35%.

6. The method of claim 5 wherein said reduction is within the range of 20% to 30%.

7. The method of claim l wherein, after cold-working, said alloy is heated to a temperature of about 12 47,107I
775°C for 8 hours followed by an air-cool.

8. The method of claim 1 wherein the method steps comprise heating to a temperature of 1025°C to 1075°C for 2-5 minutes followed by a furnace-cool, cold-working the alloy 20% to 60% ? heating the alloy to a temperature of about 775°C for 8 hours followed by an air-cool, and then heating the alloy to a temperature of 700°C for 2 hours followed by an air-cool.

9. The method according to claim 1 wherein said element is molybdenum.

10. me method according to claim 1 or 9 further comprising the forming of a microstructure in said alloy having dislocations and a bimodal distribution of gamma prime precipitates.

11. me method according to claim 10 wherein said dislocations comprise interwoven dislocated cell structures which are pined by said bimodal gamma prime precipitates.