NL8002490A - METHOD FOR HEAT TREATMENT OF AN IRON-NICKEL CHROME ALLOY. - Google Patents

Description

.Α, -Λ - 1 -

Method of heat treating an iron-nickel-chromium alloy.

The invention relates to a method for heat treating iron-nickel-chromium alloys.

The invention is particularly intended for use in nickel-chromium-iron alloys such as those described in copending U.S. Serial No. 917,832, which have strong mechanical properties and at the same time a swelling resistance under the influence of radiation, and a low neutron absorption. As such, the alloy is particularly suitable for use in pipes and coatings in fast breeder reactors.

A material of this kind is a γ'-amplifier-superalloy; and its properties can be drastically changed by varying the thermomechanical treatment to which it is subjected. For core-15 reactor applications, it is, of course, desirable to subject the alloy to a thermomechanical treatment which provides the greatest resistance to radiation-induced swelling and / or the highest strength and most importantly the highest ductility after irradiation.

According to the invention, a method of heat treating an iron-nickel-chromium alloy consisting essentially of from 25 to 45% nickel, from 10 to 16% chromium, from 1.5 to 3% molybdenum or niobium, from 1 to 3% titanium, from 0.5 to 3.0% aluminum, the remainder being essentially iron, the steps of heating the alloy to a temperature in the range of 1000 to 1100 ° C for 30 sec. to 1 hour, followed by oven cooling, 10 to 80% cold working of the alloy, and heating the alloy to a temperature of 750 to 850 ° C for 4-15 hours, followed by air cooling, and then heating the alloy to a temperature in the range of 650 to 700 ° C for 2-20 hours, followed by air cooling.

Preferably, the initial heat treatment 35 is carried out at 1025 ° C to 1075 ° C for 2-5 minutes to minimize the residence time in the oven. This initial heat treatment is followed by furnace cooling and cold working, preferably by 20 to 50% cold rolling. Thereafter, the alloy is heated to a preferred temperature of 775 ° C for 8 hours, followed by air cooling prior to the last heating and air cooling step.

The invention will now be further elucidated by means of the following examples.

EXAMPLE I

An alloy of the composition according to Table 10 below was subjected to various thermo-mechanical treatments, which are described below:

TABLE A

nickel - 45% chromium - 12% 15 molybdenum - 3% silicon - 0.5% zirconium - 0.05% titanium - 2.5% aluminum - 2.5% 20 carbon - 0.03% boron - 0.005% iron - rest

The said alloy was a γ'-reinforced superalloy. The following Table B lists the various thermomechanical treatments to which the alloy as compiled according to Table A has been subjected; while Table C gives the resulting microstructural and mechanical properties of the alloy after the heat treatment:

TABLE B

30 Designation Thermomechanical treatment

AR 1038 ° C / 1 hour / FC + 60% CW

IN-1 1982 ° C / 1 hour / AC + 788 ° C / 1 hour / AC + 720 ° C / 24 hours / AC

IN-2 1890 ° C / 1 hour / AC + 800 ° C / 11 hours / AC + 700 ° C / 2 hours / AC

EC 1927 ° C / 1 hour / AC + 800 ° C / 11 hours / AC + 700 ° C / 2 hours / AC

35 EE 1800 ° C / 1 hour / AC + 700 ° C / 2 hours / AC

800 2 4 90 after 1038 ° C / 1 hour / FC + 60% CW.

- 3 -

TABLE C

Designation Special features 650 ° C

• _ ___ UTS (ksi) 80 ksi SR (hour) AR No γ ', high dislocation - 67.9 5 bond density IN-1 Bimodal γ *, re-151.5 0.8 crystallized above γ' solvus IN-2 Trimodal γ * (dis- 141.3 81.9 10 cations) EC Trimodal γ * re- - 64.7 crystallized under.

γ 'solvus EE Bimodal γ *, cells - 235 15 with equal axes

As can be seen from table C above, the EC treatment gives stronger fracture load properties than treatment IN-1. The EC treatment resulted in a trimodal distribution of γ ', since the recrystallization temper was below the γ' solvus, and resulted in the precipitation of a small volume of size (about 600 nm) γ 'precipitates.

Of the treatments listed in Tables B and C, three treatments gave dislocation structures.

These were AR, IN-2 and EE treatments. The fracture load data of Table B showed that the heat treatment EE produced a significantly stronger material. This structure consisted of an interwoven dislocated field structure pinned by a bimodal γ 'distribution. This condition had the greatest strength of any material tested and was very stable due to the pinned nature of the dislocation cells.

The graph, shown in the drawing, illustrates the alloy swelling behavior reported in Table A 35 in three thermomechanical conditions, ST, EC and EE. The swelling temperature curves are for radial doses of 30 dpa, which is equivalent to 203 MSfd / MT (i.e.

e greater than the target fluency of 120 MWd / MT). The data shows that the ST and EE treatments produced the lowest swelling in the alloy listed in Table B. The EC treatment gave an acceptable level of swelling at target fluctuations, but the treatment was far from optimal for reactor applications.

EXAMPLE II

An alloy having the composition of Table D below was subjected to thermomechanical treatment 5 according to that of Example I:

TABLE D

nickel - 60 chromium - 15 molybdenum - 5.0 10 niobium - 1.5 silicon - 0.5 zirconium - 0.03 titanium - 1.5 aluminum · - 1.5 15 carbon - .03 boron - 0.01 iron - rest

The thermomechanical treatments given to the alloy of Table D, and the microstructures 20 and mechanical properties of the resulting alloy are listed in Tables Ξ and F below.

TABLE E

&

Designation Thermomechanical treatment

BP 1038 ° C / 1 hour / AC + 800 ° C / 11 hours / AC + 700 ° C / 2 hours / AC

25 BR 927 ° C / 1 hour / AC + 800 ° C / 11 hours / AC + 700 ° C / 2 hours / AC

BT 1038 ° C / 0.25 hours / AC + 899 ° C / 1 hour / AC +

749 ° C / 8 hours / AC

CT 30% WW at 1038 ° C + 800 ° C / 11 hours / AC +

700 ° C / 2 hours / AC

30 CU 890 ° C / 1 hour / AC + 800 ° C / 11 hours / AC + 700 ° C / 2 hours / AC +30

BU 800 ° C / 1 hour / AC + 700 ° C / 2 hours / AC

* after 1038 ° C / 1 hour / FC + 60% CW

- table F - 8002490

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- 5 -

TABLE F

Designation Special features 65 ° C

_ _ UTS (ksi) 80 ksi SR (hour) BP Small γ ', no dislocks 136.7 5 cations BR Bimodal γ1, γ cells 152.5 73 BT Bimodal γ', no 135.3 dislocations CT Bimodal γ ' , not 154.6 10 uniform structure (long-band cells, some sub-granules) CU Bimodal γ ', long-stretched 147.0 cells 15 Bü Bimodal γ', equiv 156.4 74 axial cells

The γ 'solvus and the 1 hour recrystallization temperature for the alloy of Table D were 915 ° C + 10 ° C and 1000 ° C + 20 ° C, respectively. Therefore, unlike the alloy specified in Table A, there was no temperature range in which recrystallization could be completed under aging. Consistent with this fact, the treatments BP and BT, calcining at 1038 ° C followed by double aging, both gave a dislocation-free austenite matrix and a bimodal γ 'distribution. Structures obtained by the treatments CU and BU, which did not induce recrystallization, all had a highly dislocated cell structure, containing different distributions of γ1.

Table F is an overview of the observed structures and corresponding physical properties.

It should be noted that the mechanical property values are grouped into two classes. These are dislocation-free γ1-containing structures with ultimate tensile strengths between 135 and 137 ksi at 650 ° C, and the dislocated γ'-structures, which are much stronger, with ultimate tensile strengths between 147-157 ksi. Due to their superior strength and because of the advantage of an increased swelling incubation time, the dislocated structures are preferred.

40 The treatment CU, given in tables E and F, started with a dislocated cell structure with an o Λ Λ O λ η η - 6 - trimodal γ 'distribution, which was then: 30% cold worked. The final cold working operation actually reduced the strength, as indicated by the 650 ° C ultimate tensile strength data given in Table F, apparently by destroying the cohesion of the dislocation cell walls.

The treatments BR and BU of the alloys listed in Table D both give a highly dislocated, partially recrystallized or regained cell structure with bimodal γ 'distribution. The BU treatment was preferred, since it gave slightly higher fracture load properties than the BR treatment.

The dislocation and γ 'structures for the BU treatment' gave a cell structure, which was more dispersed and interwoven, than that generated by BE treatment of the alloy given in Table A. The minimum cell thickness of the BU treatment was approximately the same as the γ * particle distance.

To further demonstrate the improvement obtained by the thermomechanical treatment of the invention, reference is made to the following Tables G and H, which show that this treatment is very effective in promoting high post-irradiation ductility. In this regard, it should be noted that there is a valley in which the ductility of these materials decreases materially when tested at a temperature 110 ° C above the irradiated temperature. Thus, the worst ductility is found at a temperature of 805 ° C when the material is irradiated at 695 ° C. This 110 ° C is responsible for all operating transition conditions of, for example, a fast breeder reactor. Therefore, the selection of the material and the heat treatment or the thermomechanical heat treatment of the material, when irradiated at 695 ° C, should be tested at 805 ° C, where the lowest post irradiation ductility occurs. For example, it is readily apparent from the following Tables G and H that the solution-treated condition of alloy D66 when irradiated at 695 ° C and tested at 805 ° C exhibits zero ductility.

40 In contrast, material subjected to the treatment according to the invention of the same alloy, irradiated at 695 ° C, and when tested at 805 ° C, shows a 1.1% uniform elongation is obtained. It is critically important to maintain a ductility of greater than 0.3% under these conditions, as this is necessary to maintain fuel pin integrity during the reactor transition conditions, and the tables show how these goals are achieved. Tables G and H further show that the higher ductility of this treatment is also accompanied by higher strength, which is highly unexpected with respect to these irradiated materials. These higher strengths confirm the fact of the excellent swell resistance exhibited by the alloys subjected to the process of the invention.

- tables G and H - 800 24 90 -8-.

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Claims (7)

A method of heat treating an iron-nickel-chromium alloy consisting essentially of from 25% to 45% nickel, 10% to 16% chromium, 1.5% to 3% molybdenum or niobium, from 1% to 3 % titanium, from 0.5 S to 3.0% aluminum, and the remainder essentially iron, characterized by the steps of heating the alloy to a temperature in the range of 1000 ° C to 1100 ° C for 30 sec . up to 1 hour, followed by oven cooling, 10% to 80% cold working of the 10 alloy, heating the alloy to a temperature of 750 ° C to 825 ° C for 4-15 hours, followed by air cooling, and then heating the alloy to a temperature in the range of 650 ° C to 700 ° C for 2-20 hours, followed by air cooling. 2. Process according to claim 1, characterized in that the alloy is initially heated to a temperature in the range from 1025 ° C to 1075 ° C for 2-5 min. 3. Method according to claim 1 or 2, characterized in that the alloy is cold worked 20% to 50% by cold rolling. 4. Process according to claim 3, characterized in that the alloy is cold rolled from 30% to 50%. 5. A method according to claim 1, characterized in that the alloy is in the form of a tube, and is cold worked by drawing the tube to produce a 15% to 35% reduction. 6. A method according to claim 5, characterized in that the reduction is in the range from 20 to 30%. 7. Process according to claim 1, characterized in that the alloy is heated to a temperature of about 775 ° C for 8 hours, after air-cooling, after cold working. 800 24 90