GB2047742A - Iron-based nickel alloy - Google Patents

Abstract

An iron-based alloy which is austenitic and stable with respect to stress-induced martensitic transformation at room temperature including at least 50 weight % iron, 34 to 45 weight % nickel, and minor amounts of titanium, and tantalum, (alone or with niobium) and preferably including aluminium and scavenger metals such as molybdenum, vanadium, boron, and/or zirconium. The material may be hardened by heat treatment to high strength while retaining good toughness. One preferable heat treatment sequence is (a) ageing at a temperature of 700 to 800 DEG C and then (b) ageing to a temperature of 550 to 750 DEG C. The range of compositions is <IMAGE>

Description

SPECIFICATION Iron-based nickel alloy The present invention was made at the Berkeley campus of the University of California under Contract No.

RP636-2 between Electric Power Research Institute, Inc. and the University of California.

The invention relates to an iron-nickel alloy which is austenitic, stable with respect to stress-induced martensitictransformation at room temperature and hardenable by heat treatment.

Modern industrial applications of structural alloys often require that they be non-magnetic, corrosion resistant, or have particular thermal expansion properties. Such requirements often rule out the use of common "ferritic" structural steels, having a body centered cubic crystal structure, and lead to the specification of austenitic steels (having a face-centered cubic crystal structure) or non-ferrous alloys.

However, the non-ferrous alloys are, in general, either relatively low in structural strength (for example, aluminium) or are costly and difficult to process and handle (for example, titanium and nickel-based alloys).

Typical austenitic steels are also relatively low in strength unless they are mechanically worked.

One particular application for alloys of the foregoing type is for use in retaining rings in large electrical generators. The operating conditions of the generator require that the alloy be non-ferromagnetic; hence, high strength ferritic structural steels cannot be used.

The alloys currently in use in the manufacture of generator retaining rings are soft austenitic alloys (e.g., 18 Mn-5Cr-0.5C austenitic steel) in their original condition which are hardened by die expansion or explosive forming of the ring to a maximum yield strength on the order of 170 ksi. Such hardening is a complex and delicate step in retaining ring manufacture. Also, higher yield strengths are required for larger generators now under consideration.

There are some commercially available heat treatable precipitation hardened austenitic iron-based alloys stabilized by the addition or nickel (or manganese) and chromium, and including either interstitial atomic species such as carbon and nitrogen, which harden the alloy on heat treatment by forming carbides or nitrides with iron or chromium, or substitutional species such as titanium or chromium which harden the alloy on heat treatment forming the intermetallic compound Ni2(Ti, Al) in the intermediate cubic y' structure.

A variety of austenitic steels of the two classes described in the previous paragraph have been developed and are in current use. However, such alloys are unsuited for certain applications, for example, retaining rings for larger electrical generators, because their maximum strength (consistent with good retention of toughness) is too low. For example, the maximum yield strength of known commercially available tough, heat treatable austenitic (i.e., the AISI 600 series alloys) is less than 140,000 Ibs/in2. Somewhat higher yield strengths, 170 - 190,000 Ibs/in2, are attained in heat-treated nickel-based alloys, for example, INCO 718 and Pyromet CTX-1. However, these alloys are nickel-based, rich in alloy addition such as chromium, and are consequently very expensive. The nickel alloy base also introduces melting and handling difficulties which may be avoided in an iron-based system.

Avariety of commercially available alloys are set out in Table 5 herein in which each alloy is identified by trade name and composition, and the yield strengths are shown.

Summary of the invention and objects It is a general object of the invention to provide an economical iron-based steel containing nickel and other alloying elements which is austenite, stable with respect to stress-induced martensitic transformation at room temperature and capable of heat treatment to high strength without excessive loss of toughness.

Further objects include providing such an alloy which is non-magnetic, rendering it particularly effective for forming retaining rings for electrical generators. Further objects and features of the invention will be apparent from the following description.

In accordance with the above objects, the present invention relates to an alloy predominantly composed of at least 50 weight % iron and about 33 to 45 weight % nickel. The alloy includes additional metals which render the austenite stable at room temperature and capable of heat treatment to impart high strength. Such other alloy elements include titanium (1 to 4 weight %), and tantalum (1 - 5 weight %). The nickel precipitates which titanium and aluminium in an intermetallic compound of the general formula Ni3X, wherein X is titanium or auminum. The tantalum is also included in the precipitate and provides higher strength. In addition, the alloy preferably includes scavengering metals such as molybdenum, vanadium, boron, zirconium, or mixtures of them which improve ductility and toughness by preventing intergranular brittleness and impeding intergranular precipitation.The precipitation of the Ni3X compound is accomplished by thermal aging. Exceptional properties are produced by a two-stage aging treatment in which the alloy is aged at 700"C to 800"C in the first stage and then 550"C to 750"C in the second stage wherein the second stage is at a temperature of at least 20"C less than the first stage. Such processing renders the product particularly adapted for use as a retaining ring in large electrical generators as it is non-magnetic and heat treatable to high strength and toughness.

Detailed description of the preferred embodiments The elements of the iron-based alloy of the present invention are selected within the indicated ranges to be austenite stable with respect to stress-induced transformation at room temperature. In addition, it is characterized by the ability to be heat treated to high strength compared to other iron-based austenitic steels while retaining acceptable ductility.

The following Table 1 iilustrates an alloy within the scope of the present invention while Table 2 illustrates a preferred alloy.

TABLE 1 (Basic composition) Element Weight% Fe over 50 (balance) Ni 33-45 Ti 1-5 Ta 1-5 Nb 0-4 TotalofTa+Nb 1-6 Al 0-2 Moand/orV 0-3 B and/or Zr 0-0.1 TABLE 2 (Preferred alloy) Element Weight% Fe over 50 (balance) Ni 33-42 Ti 2-4 Ta 2-4 Al 0.2-1 Mo and/or V 0.5 - 2 B and/or Zr 0.001 - 0.1 Atheoretical explanation for the exceptional properties of a composition of the foregoing type is set out below. In that regard, the following particularly effective alloy, designated herein "Alloy A" has been extensively studied. A composition of that alloy is set forth in Table 3 below. A variance of + 20% or more with respect to the first four elements on the table will produce similar exceptional properties.The last three named scavengering elements may be varied to an even more significant extent, or, in certain instances, eliminated.

TABLE 3 (Alloy A) Element Weight% Fe 56 Ni 36 Ti 3 Ta 3 Al 0.5 Mo 1.3 V 0.3 B 0.01 There is a significant interplay of the alloying elements in the foregoing compositions. Some of the more significant effects will be set out below on an element by element basis.

Iron provides the alloy matrix which interplays with the other alloy elements. It provides good melting and forming properties. Also, it is relatively inexpensive and so a high iron content is used consistent with retention of the desired properties. It has been found that an excess of 50 weight % to as high as 56 weight % or more of iron may be employed. It constitutes the balance of the alloy over the other elements.

Nickel provides a number of essential properties to the present product. In general, a minimum nickel content to provide the desired strength is on the order of 34 weight %. Higher nickel contents may provide higher strengths but may also lead to significantly lower ductilities. Balancing these physical properties, the optimum maximum limitation in nickel is on the order of 42 - 45 weight %.

The nickel content stabilizes the austenite structure of the alloy and provides the major component of intermetallic precipitates which form on heat treatmentqo provide the high strengths. During heat treatment, the nickel may form y' intermediate precipitate structure of the general formula Ni3X. The symbol X may be titanium, aluminium, tantalum, niobium and mixtures of these elements. By proper selection of the type and quantity of X, the precipitate is made to provide substantial strength to the alloy without causing serious loss of ductility. As set out hereinafter, it is important to suppress integranular precipitation so that high hardness can be obtained without an unacceptable loss of toughness.

Another function of the nickel content is that its presence determines whether or not the alloy is magnetic.

The determinant factor is the amount of nickel retained in the iron matrix after the precipitation of Ni3X. For an alloy to be used as a retaining ring for an electrical generator, it is essential that it not be ferromagnetic. It is suitably paramagnetic with a residual permeability of no more than about 1 gauss/orsted. To satisfy this magnetic requirement, the residual nickel content of the matrix after precipitation should be less than about 30 weight %. While this total amount of nickel may vary to achieve this level, depending upon the remaining alloy elements, it has been found that a maximum nickel content to avoid ferromagnetism is about 37 weight %. Thus, the preferred range for this application is 33 weight % to 37 weight %.

Titanium participates in the precipitation reaction which strenghens the alloys. It forms a strong precipitate having they' (No2X) structure. Its composition is chosen to impart high strength while avoiding grain boundary precipitation. For this purpose, suitable titanium contents are from 1 to 4% while preferred contents are from 2 to 4% and optimal contents are in the order of 3 weight %. Aluminium is interchangeable with a portion of the titanium as it performs a similar function. It does not impart as high a strength in the Ni3X precipitate as does the titanium. However, it does decrease the lattice mismatch to inhibit grain boundary precipitation. Thus, it may be present in an amount of about 0 to 2 weight % and, preferably, on the order of 0.2 to 1 weight %.

Tantalum is an essential element to the high strength attained in the present alloy. It also participates in the Ni3X precipitate to improve the ability of that precipitate to harden the alloys. It is believed that this is associated with the increased lattice mis-match between the precipitate and the lattice due to the relatively large size of the tantalum atom. Niobium performs a similar function to the tantalum and may be substituted for tantalum in the intermetallic precipitation. Niobium is not as effective as tantalum as an alloy addition.

However, since it is less expensive then tantalum it may be used as a partial substitute.

The total amount of tantalum and niobium is suitably from about 1 to 6 weight %, preferably from 2 to 4 weight % and optimally on the order of 3 weight %. As set out above, tantalum is preferred over niobium and at least 1 weight % tantalum is used. Where not prohibitive from a cost standpoint, it is preferably present at about 2 to 4 weight %.

The intermediate precipitate which would develop during heat treatment of the alloy, including titanium and aluminium but not tantalum or niobium, is the cubic y' (Ni3X). However, in the preferred mode of the present invention, the tantalum alone or in combination with niobium, content is relatively large with respect to the titanium and aluminium content. If so, the tetragonal y" Ni3X may be formed as the intermediate precipitate structure. This is due to the presence of the relatively large atoms of tantalum and niobium in the precipitate. It is believed that this structure provides significantly increased strength.

Scavengering elements may be added to improve alloy ductility and toughness by preventing intergranular brittleness and impeding intergrannular precipitation. Suitable scavengering elements include molybdenum, vanadium, boron, and zirconium, while the first three named elements are preferred. The total amount of molybdenum and vanadium may range up to say 3 weight % while a preferred range is on the order of 0.5 to 2 weight %. In a suitable mixture, molybdenum is in a range of about 1 to 1.5 weight %; while vanadium is in a range of about 0.1 to 0.5 weight %. Boron and/or zirconium serve as surfactants in the grain boundaries and should be used in only small amounts, e.g., 0.001 weight % to 0.1 weight %.

In general, the larger the quantity of alloying elements in addition to iron and nickel, the higher the strength of the resulting product. However, the higher strength is normally accompanied by a certain amount of brittleness. These factors should be balanced. For that purpose, it is preferable that the total amount of such alloying elements in addition to iron and nickel be between 4 and 10 atomic percent and preferably from 7 to 8 atomic percent.

A major advantage of alloys of the foregoing type is their ability to be formed into final configuration at elevated temperatures in a soft condition and then to be thermally processed to design strength. This ability is particularly important for use in the production of large scale parts such as electrical generator retaining rings. By controlling the heat treatment times and temperatures in a sequence adapted for the particular alloy composition, yield strengths on the order of 150,000 to 225,000 psi may be obtained with good residual ductility.

The specific processing conditions to be employed depend upon the alloy of choice. The following sequence is adaptable to many of the alloys within the scope of the invention. In a first step, the alloy is normally homogenized under conventional conditions such as at a temperature of 1200"C for about 24 hours. Then, the alloy is preferably warm worked or forged at a temperature close to the recrystallization temperature, e.g., 1 100"C, preferably not less than 1000"C. Such warm working breaks the as-cast character of the alloy and causes some dislocations to occur. Thereafter, the alloy is quenched to room temperature, preferably by conventional water quenching. If desired, prior to quenching, the alloy may be annealed at a temperature of 950 - 1 200"C. Annealing temperatures should be low enough so that substantial grain growth is prevented and high enough so that severe intergrannular precipitation does not occur.

A significant increase in alloy strength has been found to occur by a two-stage aging process of the following type. In the first stage, the alloy is aged at 700 - 800 C for at least 2 hours and less than the time when overaging commences (100 hours). Aging for four hours has been found to be sufficientforthis stage.

Good results are obtained when the first step is performed in the temperature range of 7250C to 775 C.

Optimum results are obtained at a temperature on the order of 750"C for about fou r hours. Subsequent to the first stage, the alloy may be water quenched prior to the second stage. Alternatively, it may be furnace cooled from the first stage to a lower temperature second stage followed by water quenching.

The second aging treatment is performed at a temperature in the range of 550"C to 750"C to further improve the properties of the alloy. The same time constraints as the first stage apply here. It is preferable that the second stage be performed at a temperature at least 20"C cooler than the first stage. Good results are obtained at a temperature of from about 650"C to 700"C while optimum results are obtained at 670"C. After the second stage, the alloy should be cooled rapidly to room temperature as by water quenching.

Transmission electron microscopy was performed on alloy A heat treated according to the above preferred conditions. The alloy was forged at 1 1 OO"C, and retained a significant amount of the "warm" work.

It is characterized by a banded structure with a relatively high density of dislocations. After the two-stage aging treatment the product had a fine distribution of y' precipitates embedded in the austenite matrix. Alloy A had a yield strength of 205 ksi and a Kic of 112.5 ksi in2. With no annealing prior to aging, the Rc hardness was about 45 to 46. With annealing prior to aging the Rc hardness varied from about 37 to about 41 for only the first aging step and from about 41 to about 44 for the two step aging.

Examples of various compositions in accordance with the present invention, their heat treatment and resulting properties of the alloy are set forth in the following Table 4. Abbreviations of the properties in the table are as follows: Y.S. - maximum yield strength; T.S. - tensile strength; el. - elongation; and R.A. - reduction in area TABLE 4 Examples of Fe-based austenitic allys containing Ta and their tensile properties

COMPOSITION Y.S. T.S. el. R.A. HEATTREATMENT Fe Ni Ti Ta Tb Al Mo V B (ksi) (ksi) (%) (%) 1. bal. 31.5 - 4.5 .015 50 80 38 71 AN + 750 C/4 hours 2. bal. 33 3 3 - - - - - 105* AN + 720 C/12 hours 3. bal. 36 3 3 - - - - - 193 Af + 720 C/12 hours + 680 C/12 hours 4. bal. 36 3 3 - .5 - - - 155* AN + 725 C/8 hours + 625 C/8 hours 5. bal. 36.5 3 3 - .3 - - .01 181 211 20 52 AN + 720 C/8 hours-F.C.# 620 c/8 hours 6. bal. 36 3 3 - .5 1.3 .3 0.1 205 232 19 67 Af + 750 C/4 hours + 670 C/4 hours 7. bal. 36 3 3 2.5 - 1 .3 .01 205 222 15 27 AN +750 C/2 hours-F.C.# 220 230 18 47 Af 625 C/6 hours 8. bal. 42 3.5 3.5 - .6 1.6 .4 .0.12 220 238 818 23 Af + 750 C/4 hours 9. bal. 42 3.5 3 2 .6 1.6 .4 .0.12 222 248 13.4 37 + 670 C/4 hours 10. bal. 40 3 3 3 1 1 .3 .01 214 259 10 16 11. bal. 40 3 3 - 1 1 .3 .01 179 225 23 54 12. bal. 40 3 2.5 2.5 1 1 .3 .01 200 246 18 28 13. bal. 40 3 2 3 1 1 .3 .01 185 237 26 33 AN + 720 C/8 hours-F.C.3 14. bal. 40 3 3 2 1 1 .3 .01 193 245 22 26 620 C/8 hours 15. bal. 40 3 1.5 2.5 1 1 .3 .01 183 236 25 44 16. bal. 40 3 2.5 1.5 1 1 .3 .01 188 236 25 50 17. bal. 38 3 4.5 - 3 - - .01 181 220 15 28 * - Proportional limit of bending test.

Af-Forged at 1100 C and water quenched.

AN-Annealed at 1000 C for 1 hour and water quenched.

F.C.-Furnace cooled.

Referring to the above Table 4, the best combinations of physical properties are found in samples 3 and 5 18. It is believed that the reason why the yield strengthS were low for samples 1,2, and 4 is that the austenite in the alloy was not stable and transformed to martensite before the plastic yield strength was reached. This highlighted the important interplay of the elements to produce an alloy which is stable with respect to stress-induced martensitic transformation at room temperature. It is noted that sample 6 is paramagnetic after heat processing.

Table 5 lists various examples of commercial high strength, non-metallic alloys. The iron-based alloys (1 7) are substantially lower in strength than the alloys of the present invention, while the nickel-based superalloys(8- 21) are vastly more expensive and, in certain instances, are inferior in strength.

TABLE 5 Commercial high-strength, non-metallic alloys COMPOSITION, wt % YIELD TENSILE STRENGTH STRENGTH DESIGNATION Fe Ni Cr Mn Co Mo Ti Nb Al W C B Others (ksi) (ksi) 1. A-286 bal. 26 15 1.4 - 1.25 2.15 - 0.2 - 0.05 0.003 +0.3V 100 146 2. V-57 bal. 26 15 0.25 - 1.25 3 - 0.25 - 0.06 0.008 +0.5V 119 172 3. W 545 bal. 26 13.5 1.65 - 1.75 3 - 0.15 - 0.02 0.05 133 181 4. Unitemp 212 bal. 25 16 0.05 - - 4 0.5 0.15 - 0.08 0.06 +0.05Zr 134 187 5. Armco 22-4-9 bal. 4 22 9 - - - - - - 0.55 - +0.4N 102 162 6. 16-25-6 bal. 25 16 2 - 6 - - - - 0.1 - +0.15N 112 142 7. Uniloy 888 bal. 7.5 7.5 9 - - - - - - 0.5 - +1.5V 115 160 8. Inconel 700 0.7 bal. 15 0.1 28.5 3.75 2.2 - 3 - 0.12 - +0.05Cu 104 171 9. Inconel 713 2.5 bal. 13 0.25 - 9.5 0.75 2.3 6 - 0.14 - 107 121 10. Udimet 500 4 bal. 19 - 18 4 3 - 4 - 0.15 0.008 110 197 11. IN-100 - bal. 10 - 15 3 5 - 5.5 - 0.18 0.015 +1V+0.05Zr 118 135 12. Naspaloy 2.0 bal. 19.5 0.5 13.5 4.3 3 - 1.3 - 0.1 0.005 +0.085Zr 120 188 13. Nicrotung - bal. 12 - 10 - 4 - 4 - 0.1 0.05 +0.06Zr 120 130 14. M 252 2.1 bal. 19 0.02 10 10 2.6 - 1.0 - 0.15 +0.06Zr 122 180 15. Unitemp 1753 9.5 bal. 16 0.05 7.2 1.6 3.2 - 1.9 8.4 0.24 0.08 +0.06Zr 130 194 16. Incoloy 901 34 bal. 13 0.15 - 6 3 - 0.2 - 0.05 0.015 130 175 17. Udimet 700 1.0 bal. 15 - 18.5 5 3.5 - 4.4 - 0.15 0.025 140 205 18. D 979 28 bal. 15 0.75 - 4 3 - 1 4 0.05 0.01 146 104 19. Rene 41 5 bal. 19 0.1 11 10 3.1 - 1.5 - 0.09 154 193 20. Incoloy 706 40 bal. 16 0.2 - - 1.75 2.9 0.2 - 0.03 - 161 193 21. Inconel 718 18 bal. 19 0.2 - 3 0.8 5.2 0.6 - 0.04 - +0.2Cu 175 212

Claims (23)

1. An iron-nickel alloy which is austenite stable to stress induced transformation at room temperature, which alloy includes at least 50 weight % iron, about 33 - 45 weight % nickel, about 1 - 4 weight % titanium, and about 1 - 5 weight % tantalum.

2. An alloy as claimed in claim 1 which additionally includes 0.2 - 2 weight % aluminium.

3. An alloy as claimed in claim 1 or claim 2 which additionally includes about 0.5 - 3.0weight % of a scavenger which is molybdenum, vanadium, or a mixture thereof.

4. An alloy as claimed in any one of the preceding claims which additionaly includes about 0.001 - 0.1 weight of boron.

5. An alloy as claimed in any one of the preceding claims which is non-ferromagnetic and includes less than 30 weight % unprecipitated residual nickel content.

6. An alloy as claimed in any one of the preceding claims in which the nickel content is from about 33 to 37% weight %.

7. An alloy as claimed in any one of the preceding claims in which the titanium content is from about 2 to 4weight %.

8. An alloy as claimed in any one of the preceding claims in which the tantalum content is from about 2 to 4 weight %.

9. An alloy as claimed in any one of the preceding claims in which the total amount of other alloying agents in addition to iron and nickel is about 4 to 10 atomic %.

10. An alloy as claimed in claim 9 in which the total amount of the other alloying agents is about 7 to 8 atomic %.

11. An alloy as claimed in any one of the preceding claims which is the form of a retaining ring for an electrical generator.

12. An alloy as claimed in any one of the preceding claims which has been strengthened by thermal ageing to precipitate an intermetallic compound of the general formula Ni3Xwherein Xis titanium, aluminium, tantalum, niobium, or a mixture thereof.

13. An iron-based alloy which is austenite stable at room temperature comprising at least 50 weight % iron, about 34 - 45 weight % nickel, about 2 - 4 weight % titanium, about 2 - 4 weight % tantalum, and about 0.2 - 2 weight % aluminium.

14. An alloy as claimed in claim 13 which additionally includes at least 0.5 to 3 weight % of a scavenger which is molybdenum, vanadium, boron, zirconium, or a mixture thereof.

15. An alloy as claimed in claim 1 substantially as hereinbefore described.

14. An alloy as claimed in claim 1 as hereinbefore identified in Table 4 or Table 5.

17. A method for heat treating an iron-nickel alloy including at least 50 weight % iron, about 33 to 45 weight % nickel, about 1 - 4 weight % titanium, and about 1 - 5 weight % tantalum, which method comprises subjecting the alloy to a first ageing treatment at a temperature ranging from about 700"C to 8000C for from 2 to 100 hours, and then subjecting it to a second ageing treatment at a lower temperature of from about 550"C to 7500C for at least 2 hours, the second ageing treatment being performed at a temperature of at least 20"C less than the temperature of said first ageing treatment, to form an alloy which is austenite stable to strain induced transformation at room temperature.

18. A method as claimed in claim 17 in which prior to the first ageing treatment, the steel is first homogenized and then forged at a temperature of at least 1000"C.

19. A method as claimed in claim 17 or claim 18 in which the first ageing treatment is performed at a temperature from about 725"C to 7750C and the second ageing treatment is performed at a temperature from about 650"C to 7000C.

20. A method for heat treating an iron-nickel alloy including at least 50 weight % iron, about 34 - 45 weight % nickel, about 1 - 4 weight % titanium, about 1 - 8 weight % of a nickel precipitating agent which is tantalum, niobium, or a mixture thereof, and about 0.2 - 2 weight % aluminium, which method comprises thermally ageing the alloy to precipitate an intermetallic compound of the general formula Ni3X, wherein X is titanium, aluminium, tantalum, niobium or a mixture thereof.

21. A method for heat treating an iron-nickel alloy as claimed in claim 17 substantially as hereinbefore described.

22. A method for heat treating an iron-nickel alloy as claimed in claim 20 substantially as hreinbefore described.

23. An iron-nickel alloy which has been heat treated by a method as claimed in any one of claims 17 to 22.