US3459539A - Nickel-chromium-iron alloy and heat treating the alloy - Google Patents

Description

United States Patent 3,459,539 NICKEL-CHROMIUM-IRON ALLOY AND HEAT TREATING THE ALLOY Herbert L. Eiselstein and Thomas H. Bassford, Huntington, W. Va., assignors to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Feb. 15, 1966, Ser. No. 527,490 Int. Cl. C22c 39/20; C21d 1/26', 1/60 U.S. Cl. 75128 8 Claims ABSTRACT OF THE DISCLOSURE The invention is directed to an alloy containing about 29% to about 40% nickel, about 19% to about 25% chromium, about 0.2% to about 0.5% carbon, about 0.25% to about 1.25% titanium, up to about 1% aluminum, and the balance essentially iron. The alloy may be prepared by air melting and has high creep and rupture properties when heat treated at temperatures of about 2300" F. to about 2350 F. for at least about two hours.

The present invention is directed to nickel-chromiumiron alloys and, more particularly, to special nickelchromium-iron alloys having controlled composition and having high creep and rupture strength while at the same time being relatively inexpensive as compared to known alloys having a comparable high-temperature strength capability.

The production of alloys resistant to the effects of elevated temperatures, e.g., 1400 F. or 1600 F. to 2000 F., or higher, while being subjected to stress has long been a concern of the metallurgical art. In applications such as piping and other structural forms for use in power and petrochemical plants (e.g., ethylene furnaces) many different alloys are employed. These alloys usually are of the nickel-chromium and nickel-chromium-iron types, With other elements being employed for special purposes. Thus, elements such as cobalt, tungsten, molybdenum, columbinm, aluminum, titanium, etc., are employed to contribute strength, precipitation-hardening capability, oxidation resistance, etc., to the alloys. Certain of the alloying elements which are commonly employed in heat-resistant alloys are expensive in themselves and are subject, from time to time, to being available only in limited supply. Many of the commonly used alloys, including, for example, the HK and HOM stainless steels, cannot be produced in wrought form, such as tubing, and are thus only available in cast form, including centrifugal castings. Furthermore, many of the commonly used alloys, particularly the less expensive types, becomes embrittled during long-time exposure to the combined effects of stress and temperature. In addition, many of the commonly used alloys are difiicult to weld, while others must be vacuum melted thereby further raising cost.

The art has accordingly been faced with a long-standing problem; to wit, that of providing a relatively inexpensive alloy which would be structurally stable when subjected to the combined eifects of stress and temperature, which would have good resistance to creep and rupture at elevated temperatures, which could be produced in the form of large ingots by air melting and air casting techniques, and which could readily be converted from the ingot stage to usual wrought forms such as tubing, plate, sheet, rod, bar, etc., by usual mill techniques.

We have now discovered a nickel-chromium-iron alloy capable of being produced in large ingots by air melting and air casting techniques, which ingots can be commercially converted into common wrought forms which in use have outstanding resistance to grain growth and embrittlement during mill processing and long-time exposure to elevated temperatures and have good stress-rupture properties, while being relatively inexpensive.

It is an object of the present invention to provide a nickel-chromium-iron alloy having high resistance to creep and rupture.

It is a further object of the invention to provide a nickelchromium-iron alloy having high resistance to grain growth when heated to temperatures near the melting point.

It is another object of the invention to provide a nickelchromium-iron alloy having the capability of being produced in wrought form.

Another object of the invention is to provide a nickelchromium-iron alloy which is relatively immune to embrittling effects when exposed to stress at elevated temperature while at the same time having high resistance to creep and rupture.

A further object of the invention is to provide a heat treatment process which contributes high rupture strength to the alloy contemplated in accordance with the invention.

Other objects and advantages of the invention will become apparent from the following description.

Broadly stated, the present invention is directed to a creepand rupture-resistant nickel-chromium-iron alloy containing, in weight percent, about 29% to about 40% nickel, about 19% to about chromium, about 0.2%

- to about 0.5% carbon, about 0.25% to about 1.25%

titanium, up to about 1% aluminum, up to about 0.75% silicon, up to about 1.5% manganese, and the balance, including small amounts of incidental elements and impurities not exceeding about 3 being essentially iron.

Advantageously, the alloys contemplated in accordance with the invention contain about 30% to about 35% nickel, abput 10% to about 23% chromium, about 0.35% to about 0.75 titanium, about 0.2% to about 0.5% carbon, and the balance essentially iron. The advantageous alloy compositions display a rupture life of at least about hours at 1600 F. and 12,000 pounds per square inch (p.s.i.) and, in many cases, a rupture life of about 200 hours or more under these conditions. A particularly advantageous alloy contains about 20% chromium, about 30% nickel, about 0.4% carbon, about 0.5% titanium, and the balance essentially iron.

In preparing the alloy, the chromium and nickel contents are controlled in interrelated amounts in order to maintain satisfactory scaling resistance and creep-rupture resistance in the alloy. Thus, nickel is at least about 29% and chromium is at least 19% in order to maintain scaling resistance but nickel does not exceed 40% and chromium does not exceed 25 to maintain creep-rupture strength. Carbon is a highly important element in the alloy in order to obtain the desired carbide dispersion-hardening therein. Carbon is at least 0.2% to obtain the requisite strength in combination with the other ingredients in the alloy but does not exceed about 0.5 as otherwise the requisite malleability in big ingots, such as ingots having a section size of about 20 inches square weighing about 8,750 pounds and slab ingots having a section of about 17 inches by 55 inches weighing about 17,300 pounds. Titanium is another highly important alloying ingredient and it is controlled within the range of about 0.25% to about 1.25% to provide, in combination with the other alloying ingredients, the requisite dispersion strengthening of the alloy. More advantageously, titanium is controlled within the range of about 0.35% to about 0.75% or about 0.9% or about 1%. Control of titanium and of carbon in combination is particularly important in order to permit obtaining the requisite creep-rupture properties in the alloy. In air melting techniques as applied to the alloy, an aluminum addition to the molten alloy prior to the titanium addition performs the useful effect of protecting the titanium addition from untoward effects, such as oxidation and the like, which could cause unwanted and/or undesirable results. Accordingly, an amount of aluminum of up to about 1% resulting from the aforementioned aluminum addition can be present in the alloy with useful results. Silicon may be present in the alloy in amounts up to about 0.75% without encountering harmful effects on the malleability or weldability of the alloy. Those skilled in the art will appreciate that silicon frequently forms a constituent of nickel alloy scrap of the kind which can be employed usefully in melting the alloy. Manganese similarly is found in scrap materials which may usefully be employed in melting the alloy and may be present therein in amounts up to as much as about 1.5% without harmful effect. Columbium, molybdenum and tungsten may also be found in scrap materials, such as mill revert scrap, employed to prepare the alloy. These elements are unnecessary for the production of the special properties developed in the alloy but may be present in amounts up to about 1% each. The impurities sulfur and phosphorus should be present only in limited amounts, e.g., in amounts not exceeding 0.015% each and, preferably, in amounts not exceeding about 0.007% each.

countered. To the extent that incipient melting is avoided, the annealing temperature may exceed 2350 F. We have observed incipient melting in the alloy after heating to 2400 F. for two hours. The data obtained in creeprupture testing of the alloy indicate that the anneal should be for a period of about two hours as the maximum improvement in creep-rupture properties is then obtained, with little or no improvement resulting upon heating for longer times. It is found that, despite the high annealing temperature employed as aforedescribed, the alloy resists grain growth. Advantageously, the metal is rapidly cooled after the anneal, e.g., by water quenching or cooling in air.

In order to give those skilled in the art a better understanding of the advantages of the invention, the following illustrative examples and data are set forth.

A number of commercial scale melts were prepared in an arc furnace using conventional air melting practice to provide alloys having the compositions set forth in Table I hereinafter. In each instance, melts having the specified contents of nickel, chromium, iron, carbon and incidental elements was prepared. Shortly before casting the molten bath, an amount of aluminum less than about 1% by weight of the bath was introduced therein, whereupon the requisite titanium addition was made and the molten metal thus treated was cast into ingot molds.

TAB LE I Percent Alloy No. 0 Mn Fe S Si Cu Ni Cr Al Ti 0.40 0.83 48.00 0. 007 0.45 0.22 30.11 19. 96 O. 63 0. 51 0.44 0. 85 44. 63 0. 007 0.47 0.27 32. 64 20. 67 0.38 0. 50 0. 47 0. 75 46. (i2 0. 007 0. 30 0. 24 32. 27 19. 23 0. 42 0. 52 0. 40 0.87 44. 94 0. 007 o. 56 o. 213 31.10 21. 87 0. 43 0. 58 0. 40 0. 70 45. 73 0. 007 0. 42 0. 24 31. 47 20. 02 0. 43 0. U0 0. 41 r). so 46. 0. 007 0. 3!) 0. 23 31. 50 20. 40 (1. 53 0. 64 0. 3f) 0. 73 45. E17 0. 007 0. 38 0. 30 31. 78 20. 42 0. 38 0. 55 0. 41 0. 60 44. 43 0. 0 0. 34 0. 24 33. TU 20. 16 0. 0. 52 0.34 0. 80 45. 3!) 0. 007 0. 34 0. 41 33. 21 10. 48 0. 57 0. 53 0. 39 O. 83 44. 80 0. 007 0. 38 0. 32 87 1!). 38 0 0. 52 0. 41 0. 70 45. 55 0. 007 0. 3S 0. 41 32. 52 10.111 0. 52 0. 55 0. 41 0. 84 45. 25 0. 007 0.33 O. 00 31. 00 21. JO 0. 57 0. 55 0. 0. 79 44. 11 0. 007 0. 37 0. 36 33. 04 10. 33 0. 50 0. 53 0.42 0. 81 44. 29 0.007 0.41 0. 38 32.54 19. 80 0. 5S 0 52 NorE.The alloys were malleable over temperature ranges or about 1,700 F. to about 2,300 F. as determined by usual production control tests. The alloys contained molybdenum in amounts up to a nut 0.26% and not more than 0.015% phosphorus.

Big ingots produced from the alloy may be converted to common mill forms by conventional operations, including hot rolling, forging, extrusion, cold rolling, etc., with usual mill process anneals at temperatures of the order of 1900 F. to about 2100 F. as required consistent with good mill practice.

In order to contribute high creep-rupture resistance to the alloy, we find that a heating in the temperature A portion of metal from Alloy No. 1 was converted into extruded tube having an outside diameter of 6 inches and a Wall thickness of one-half inch. Portions of the tube were subjected to annealing treatments at various temperatures from 2150 F. to 235 0 F. and water quenched. Test specimens of the thus-treated metal were subjected to stress-rupture testing at 1600 F. and 12,000 p.s.i. stress with the results set forth in the following Table II.

TABLE II Min. creep Annealing rate, Rupture life, Elongation, Average grain temp, F. Time, hrs. percent/hr. hrs percent RA. percent size, in.

range of about 2300 F. to about 2350 F. is necessary. The results set forth in the foregoing Table II demonstrate We find that the annealing temperature should be at least about 2300 F. or the high level of creep-rupture properties is not obtained but that the annealing temperature should not exceed about 2350 F. as otherwise the possibility exists that incipient melting may be en- The alloy contemplated in accordance with the invention becomes harder and stronger when aged in the temperature range of about 1200 F. to about 1600 F. It is found, however, that prolonged heating of the alloy in the temperature range in which aging takes place does not result in any embrittlement as revealed by short-time tensile tests and by the Charpy V-Not-ch impact test. Thus, portions of 6-inch diameter extruded tube having a one-half inch wall produced from Alloy No. 1 were annealed for two hours at 2325 F. and water quenched. Test speciments of the material were subjected to short-time tensile tests at various temperatures with the results set forth in the following Table III.

TABLE III Yield strength Tensile strength, Elongation, Tcn1p., F. (0.2% ofiset), p.s.i. p.s.i. percent Material of similar origin to that reported in Table III was subjected to heating at 1400 F. for 1000 hours. Test Specimens of this material were subjected to short-time tensile testing with the results set forth in the following Table IV.

It is to be noted that the 1400 F. ductility trough found for the as-annealed material was removed by the long-time exposure to 1400 F. The Charpy V-Notch impact strength of the material in the as-annealed condition was 30 foot-pounds. It was found that the long-time exposure at 1400 F. for 1000 hours had little effect upon the impact toughness, since the impact value after the exposure was 26 foot-pounds.

Hot rolled rod material from Alloy No. 1 was annealed at 2300 F. for one hour and water quenched. Rotating beam fatigue data were obtained upon this material with the results set forth in the following Table V.

TABLE V Fatigue strength, p.s.i.

10 cycles cycles 10' cycles 10 cycles TABLE VI Parameter (P), 1% plastic strain Parameter (P) rupture Stress p.s.i.

For the parameter plot, the temperature-parameter abscissa relationship is set forth in the following Table VII.

TABLE VII Parameter Parameter Parameter Parameter for for 1,000 for 10,000 for 100,000 Temp, F. hour life hour life hour lite lieu; life It was found that an aging treatment for 8 hours at 1600 F. increased the 1% strain parameter at 1800" F. and 4,000 p.s.i. from 38.7 to 39.5.

While the mechanism involved in providing the high creep-rupture strength found in the alloy of the invention as a result of annealing at temperatures circa 325 F. is not fully understood, X-ray diffraction studies of residues extracted from a slab forging of Alloy No. 3 indicated that the basic carbide type of the as-forged material was M C and no titanium carbide was detected. However, after the material had been annealed at 2300 F. for 10 hours, the basic carbide type was Mqc3 and there was definite evidence of titanium carbide. X-ray diffraction analysis of residues from annealed material which had been exposed to temperatures on the order of 1400 F. indicated that a precipitation of M 0 type carbides in a fine dispersion in the matrix had occurred. Regardless of the actual mechanism involved in achieving the high creep-rupture strength through the high temperature annealing technique in alloys of the invention, it is still found that surprisingly high creep-rupture strength is developed in the alloys, although hardening by gamma prime precipitation apparently does not take place. It appears that the strengthening mechanism involving carbides which operates to provide the high creep-rupture properties developed in the special nickel-chromium-iron alloys provided in accordance with the invention is unique thereto and is not obtained in other matrix compositions. Furthermore, with the ability to work the alloys in conventional equipment as is the case, and with the fact that none of the expensive, more exotic alloying ingredients is required, the basic cost of the alloy in wrought forms capable of industrial application is low in comparison to other metallic materials which have a similar strength capability. The alloys are weldable by the inert-gas shielded process using either tungsten-arc or metal-arc procedures. Filler wire of matching composition is employed. Best results are obtained in welding annealed material.

The alloy resists scaling upon exposure to heat under oxidizing conditions, resists sulfidation and other corrosive conditions and resists carburization at elevated temperatures. These properties, together with the high stressrupture properties of the alloy, make it advantageous in many applications, including furnace equipment, baskets, trays, muffies, radiant tubes, etc, in the petrochemical field for reformer and cracker tubes, hot die platens and many others.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A nickel-chromium-iron alloy consisting essentially of about 29% to about 40% nickel, about 19% to about 25% chromium, about 0.2% to about 0.5% carbon, about 7 0.25% to about 1.25% titanium, up to about 1% aluminum, up to about 0.75% silicon, up to about 1.5% manganese, and the balance, including small amounts of incidental elements and impurities, being essentially iron.

2. An alloy according to claim 1 wherein the nickel content is about 30% to about 35%, the chromium content is about 19% to about 23%, and the titanium content is about 0.35% to about 1%.

3. An all-y according to claim 1 in the condition resulting from a heating in the temperature range of at least about 2300 F. to about 2350 F. for at least about two hours, whereby the resistance of the alloy to creep and rupture is greatly increased.

4. An alloy according to claim 1 having a microstructure characterized by the presence of titanium carhide and of carbides having the types M7C3 and M C 5. The method for producing improved creep-rupture strength in alloy consisting essentially of 29% to 40% nickel, about 19% to chromium, 0.2% to 0.5% carbon, 0.25 to 1.25% titanium, and the balance essentially iron, which comprises annealing a wrought article made of said alloy at a temperature of 2300 F. to 2350 F. for at least two hours.

6. The method according to claim 5 wherein the alloy contains 30% to nickel, 19% to 23% chromium and 0.35% to 1% titanium.

7. An alloy consisting essentially of about 0.34% to about 0.47% carbon, about 30.11% to about 33.87% nickel, about 19.23% to about 21.90% chromium, about References Cited UNITED STATES PATENTS 2,597,173 5/1952 Patterson -128.8 X 2,606,113 8/1952 Payson 148136 X 2,661,284 12/1953 Nisbet 148l36 X 2,686,116 8/1954 Schempp 148-136 X 2,813,788 11/1957 Skinner 148128.8 2,879,194 3/1959 Eichelberger 75-128.8 3,184,577 5/1965 Witherell 75--128.8

OTHER REFERENCES Delta Ferrite Formation and Its Influence on the Formation of Sigma in a Wrought Heat Resisting Steel, pp. 11-12, preprint 1948 by Gilman et 211., published by American Society for Metals.

HYLAND BIZOT, Primary Examiner US. Cl. X.R. 148-136 mg UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,459 539 Datedw Inventofls) Herbert L. Eisels a I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 36, for "10%" read --l9%--.

Column 5, Table III, second column, for "26,000" read R ---26,500-. Same Table, same column, for "24,000" read Column 5, Table IV, last column, last number, for "6.0" read Column 7, Claim 5, line 2, before "alloy" insert --an--.

Same Claim, line 3, delete "about" Column 8, Claim 8, line 1, before "chromium" insert the-.

Signed and sealed this 11 th dag, of May 1971 (SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SGHUYLER, JR. Attesting Officer Commissioner of Patents