US3902899A - Austenitic castable high temperature alloy - Google Patents

Abstract

An austenitic castable high temperature alloy having improved high temperature strength coupled with good corrosion resistance and ductility in the as-cast condition and comprising about 16 to about 22% chromium; about 6 to about 18% nickel; about 6 to about 10% molybdenum, up to about 3% tungsten; about 0.5 to about 2.5% boron; about 0.01 to about 0.4% carbon; about 0.15 to about 7% manganese; up to about 3% silicon; from zero to about 1% niobium; and the balance substantially all iron together with normal residuals and incidental impurities present in usual amounts.

Description

United States Patent Sponseller 1 Sept. 2, 1975 [5 1 AUSTENITIC CASTABLE HIGH 3.352.666 11/1967 Foster Ct al. 75/1221 F x TEMPERATURE ALLOY I Primurv ExaminerL. Dewayne Rutledge 7 I D d L. S A A b 5] nvemor 23 ponsener nn r or Assistant bxu'mznerArthur J. Stemer Attorney. Agent, or FirmHarness, Dickey & Pierce [73] Assignee: AMAX Inc., New York, N.Y.

[22] Filed: May 13, 1974 [57] ABSTRACT Appl. No.: 469,347

US. Cl. 75/128 A; 75/128 C; 75/128 F;

75/128 G; 75/128 W Int. Cl. C22c 39/26; C220 39/50 Field of Search 75/128 F. 128 W, 128 A,

An austenitic castable high temperature alloy having improved high temperature strength coupled with good corrosion resistance and ductility in the as-cast condition and comprising about 16 to about 22% chromium; about 6 to about 18% nickel; about 6 to about 10% molybdenum, up to about 3% tungsten; about 0.5 to-about 2.5% boron; about 0.01 to about 0.4% carbon; about 0.15 to about 7% manganese; up to about 3% silicon; from zero to about 1% niobium; and the balance substantially all iron together with normal residuals and incidental impurities present in usual amounts.-

3 Claims, N0 Drawings AUSTENITIC CASTABLF. HIGH TEMPERATURE ALLOY BACKGROUND OF THE INVENTION There has been an increasing need for a comparatively low-cost alloy possessed of good mechanical properties and corrosion resistance at elevated temperatures to enable its use for the fabrication of cast components such blades. vanes. and integral wheels for gas turbine engines. and exhaust valves for internal combustion engines. This need has been accentuated by the potential widespread use of moderate-size gas turbine engines in a variety of consumer products including trucks and automobiles. garden tractors. as well as snowmobiles. boats and miscellaneous recre ational vehicles. While a variety of so-called superalloys of the nickel. cobalt or iron-base are suitable for use due to their excellent high temperature properties. the relatively high cost of such materials and the difficulty of fabricating them into components has detracted from their widespread use in the manufacture of competitively-priced consumer products.

In order to overcome the cost disadvantages associated with superalloy materials. various iron-base alloys. particularly austenitic-type alloys containing chromium. have heretofore been proposed for use in the fabrication of components which are to be exposed to high temperature and high stress environments during service. While some austenitic high temperature alloys have provided a satisfactory low-cost substitute in cer tain situations. the inferior mechanical properties at high temperatures. and/or a general lack of ductility of such alloys has detracted from a more widespread adoption thereof. In most instances. such prior high temperature alloys have required relatively complex melting and casting procedures and costly post-heat treatments to attain adequate properties which also have detracted from a greater acceptance of such materials.

The present invention overcomes many of the problems and disadvantages associated with castable high temperature alloys of similar type heretofore known by providing an iron-base austenitic alloy which can readily be air-melted and. because of very high fluidity. can be cast in accordance with standard foundry practices and wherein the resultant castings are possessed of excellent properties in the as-cast condition. avoiding further costly and time-comsuming heat treating practices of the types heretofore employed. In addition to the excellent high temperature stress rupture and corrosion resistant properties, the comparatively low cost of the present alloy renders it eminently suitable for the manufacture of blades. vanes. and integral wheels for gas turbine engines. and poppet-type cxhaust valves for internal combustion engines, which are exposed to corrosive environments at temperatures as high as about 1500F during service. Significant cost savings without any sacrifice in performance are obtainable by substituting the present alloy for nickelbase or cobalt-base superalloys for the fabrication of components which are intended for use under service conditions of moderate severity in which the use of such high cost superalloys ordinarily cannot be fully justified.

SUMMARY OF THE INVENTION The benefits and adyantages of the present invention are achieved by an iron-base austenitic alloy which is comprised of a carefully selected group of alloying constituents employed in controlled amounts to provide a material having unique and balanced properties. cnabling the alloy to be air-melted and cast in accordance with standard foundry practices and wherein the resultant east components are possessed of excellent room temperature and high temperature mechanical properties and corrosion resistance in an as-cast condition. dispensing with the need of costly and time-consuming post-heat treatments. The alloy is further characterized as one having a stable microstructure which retains the improved properties of the alloy even after prolonged exposure to elevated temperatures during service.

The castable austenitic alloy of the present invention contains about 16% to about 22% chromium. about 6% to about 18%- nickel. and 6% to about 1071 molybdenum. up to about 3% tungsten. 0.5% to about 2.571 boron. about 0.01.71 to about 0.471 carbon. about 0.15% to about 7% manganese. up to about 3% silicon. from zero to about 1% niobium. and the balance substantially all iron together with normal residuals and incidental impurities present in conventional amounts. The alloy is further characterized as one having a substantially austenitie microstructure in which borides and carbides are present in the interdendritic and intergra n ular boundary phase networks. as well as finely dispersed borides and carbides within the grains themselves. The foregoing microstructure. together with the unique combination of the alloying elements employed, provide for the substantially improved high tempera ture stress rupture properties and mechanical strength of the alloy. as well as its substantially increased ductility. rendering it eminently suitable for the simple and economical fabrication of a variety of components and parts to be used in elevated temperature service conditions.

Additional benefits and advantages of the present invention will become apparent upon the reading of the description of the preferred embodiments and the specific examples provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS TABLE 1 Alloy Composition Percent by Weight I Nominal Constituent Permissible Preferred Chromium l6 2: l7 20 ll Nickel 6- l8 8 l4 l2 Molybdenum 6 10 7 9 8 Tungsten 3' U h 3,

Boron 0.5 2.5 (Lo 1.3 1.0

TABLE l-Continued Alloy Composition Percent by Weight It will be understood that in addition to the specific alloying elements enumerated in Table 1, the alloy of the present invention can also contain conventional residuals and normal impurities present in the amounts normally encountered in commercial steel-making practices. Such residuals and impurities, when present in normal quantities, do not adversely affect the properties of the alloy which provides for increased flexibility and efficiency in the utilization of scrap iron in accordance with commercial foundry practices.

The chromium constituent as set forth in the table can be employed in amounts ranging from 16 up to about 22%. while quantities in the range of about 17 to 20% are usually preferred. The chromium constituent contributes oxidation or corrosion resistance to the alloy and also comprises a boride former to produce precipitated chromium boride in the interdendritic and intergranular boundary phase network within the austenitic microstrueture of the alloy. Quantities of chromium less than about 16% result in inadequate corrosion resistance of the alloy, while quantitiesin excess of about 22% result in instability of the microstructure and the formation of undesirable phases during elevated temperature service of components'east of the alloy.

Nickel can be employed in amounts broadly ranging from about 6 up to about 18% and contributes to the formation and stability of the austenitic structure of the alloy. Quantities of nickel less than about 6% result in the formation of an undesirable proportion of ferrite, while quantities in excess of about 18% provide'no appreciable benefits over that obtained by the use of lesser amounts and. therefore, the use of amounts greater than about 18% is undesirable from an economic standpoint.

Molybdenum is employed in an amount ranging from about 6 to about 10%, and preferably from about 7 to about 9%. The molybdenum constituent contributes strength to the austenitic alloy. both through solidsolution strengthening and by the formation of the intergranular and interdendritic boride and carbide boundary phases. Quantities of molybdenum less than about 6% result in alloys which are generally of inadequate high temperature strength; while quantities in excess of about 10%. depending upon the quantities of other alloying agents present. tend to impart instability to the microstructure of the alloy and the formation of certain undesirable phases during service The use of tungsten in the alloy is optional and quantities up to about 3% can be employed for enhancing the strength of the alloy by both solid-solution strengthening and the formation of carbide and boride phases in the boundary network. Quantities of tungsten greater than about 3% are undesirable due to the in creased instability of the microstructure and the formation of undesirable phases during service of cast componentsat elevated temperatures. As will be noted in Table l a particularly satisfactory alloy of the enumerated nominal composition need not contain any tungsten in order to achieve the excellent high temperature stress rupture properties coupled with comparatively superior roomternperature ductility.

The boron alloying constituent contributes to the interdendritic and intergranular boride phase strengthening mechanism by the formation of borides containing chromium and molybdenum, as well as tungsten when present. A fine dispersion of borides within the austenite grains also contributes toward the improved strength properties of the alloy at elevated temperatures. Generally. quantities of boron less than about 0.5% result in an alloy of inadequate strength due to the insufficient formation of the interdendritic and intergranular boride phase network, whereas quantities above about 2.5% generally result in an excessive embrittlement and an undesirable loss of ductility of the as-cast material. It is for this reason that the element boron is controlled within the specific ranges as set forth in Table l.

The presence of carbon contributes to the elevatedtemperature strength of the alloy by the formation of intergranular phases consisting of precipitated carbides of niobium, molybdenum and tungsten, if present. The carbon also contributes to improved mechanical properties by the formation of finely-dispersed carbide phases, principally niobium carbide, in the austenite. As set forth in Table l, the carbon content can range from as low as about 0.01 to as high as 0.4%, while quantities of from about 0.05 to about 0.25% are preferred. The carbon constituent is controlled within the aforementioncd range since quantities in excess of about 0.4% result in excessive brittleness and a loss in the ductility of the alloy.

The use of manganese may broadly range from as low as about 0.15 to as high as about 7%, although quantities of about 0.5 to about 5% are preferred. As in the case of nickel. the managanese contributes toward the stability of the austenitic structure of the alloy and the specific concentration employed in the alloy can be varied in consideration of the quantity of nickel employed. Normally. quantities of manganese in excess of about 7% are objectionable because of the high degree of reactivity of the molten alloy with the melting vessel, as well as the material of which the casting mold is comprised. When the alloy is cast into molds that have been preheated to high temperatures, such as approximately 18()0F, it is desirable to employ a manganese content of 3% or less.

Silicon comprises an optional constituent in the alloy and generally can be tolerated in amounts up to about 3%, while concentrations of about 0.15%. which corresponds to a normal residual level. up to about 1% are preferred. When silicon is present in amounts generally greater than about 3%. the presence of such excessive quantities promotes the formation of ferrite and, accordingly. concentrations of this magnitude and above are objectionable] Niobium also comprises an optional ingredient. although its presence in concentrations of about 0.2 to about 0.7% is preferred. The inclusion of niobium in the alloy results in an enhancement of the high temperature strength properties of the alloy as a result of the formation offinely-dispersed niobium carbide phase in the austenite. as well as in the interdendritic and intergranular phase network. The use of niobium in amounts generally greater than about it: is undesirable due to the resultant reduction in ductility of the alloy.

The balance of the alloy consists essentially-of iron while the nickel yvas added as electrolytic nickel and from that present in ferroalloys. was added as eommercially pure iron. The melts were deoxidi7ed with alumi along with conventional residuals and incidental impu- 5 num and \\'er. e poured into test bar specimens using rities present in the usual quantities. A further benefitpreheated lost wax-type molds. The specific composiof the alloy of the present invention is in its apparent tion of the experimental alloys is set forth in Table 2. toleration of trace quantities of such elements as lead. tin. arsenic. antimony, copper. sulfur. phosphorus. etc.. i with any significant detrimental effects on its properl H TABLE 2 ties, 1

Commercial-sized heats of the alloy can be prepared Composition of Experimental Alloys utilizing standard Mr-melting practices and the boron v v Pcrwnhy weight constituent is preferably added ust pr or to pouring to 51cm! n ,7 All") 3 AH). C avoid appreciable oxidation thereof. The alloy is castad. l 1 v ble in accordance with standard casting practlce. and mm. the cast components. after suitable finishing and ma- Molybdenum I 0 7.5 v n chining as may be required. can be employed directly 3 3 without need for further'heat treatment. It will be apparent from the foregoing thatthe comparatively low 2 2%:- t 1 0.0. cost of the alloycoupled with the ease 1n casting comjf jj (H5 (H5 I 3 ponents therefrom which do not require further heat Silicon 0.25 0.25 0.25 treatment provides for significant cost savings over cas- Mime table austenitic steel alloys of the types heretofore 7 known. T Y 1 M In order to further illustrate the austenitic alloy of the The test bar specimens obtained of each experimenpresent invention,the following examples are provided. tal alloy were subjected to physical and mechanical It will be understo d that th s exa p r ppl testing, including the determination of their respective merely for illustrative purposes and are not intended to w room t m erature-- density, their room temperature be limiting of the scope of the invention as herein de 7 Y 01% offs t i ld Strength (Y5), d l i il scribed and as set forth in the subjoined claims. strength (U l S). their. elevated-temperature ultimate EXAMPLE 1 tensile strength (UTS), as wellas their percent elonga-1 tion at room temperature and elevated temperatures as Three experimental alloys. designated A. B d 15 an indication of the ductility of the sample alloys. The respectively. were prepared by air-melting 55-p nd alloys were also subjected to high temp'erature'stressheats in an induction furnace. Th Chr m um. molybrupture tests and the IOU-hour rupture'strength was dedenum. tungsten, niobium, boron, manganese and silitermined. The data obtained are summarized in Table con alloying elements were introduced as ferroalloys; 3.

TABLE 3 Properties of Experimental Alloys 100 Hr. Rupture Temp. Density Tensile Properties Strength Alloy F (lb/in) YS (ksi) UTS (ksi) Elong.('71 (ksi) 800 94.2 3.0 1000 89.0 3.0 1200 86.9 2.5 as 1300 83.3 3.5 52 1350 44 1400 77.1 3.5 37 1500 (16.3 6.0 I600 49.3 7.0 17

1200 91.2 4.5 an 1300 241.9 4.0 1350 40 1400 m0 5.0 as 1500 6L0 0,0 25 I600 4&4 1x0 17 Elongation at 75W determined at the moment of fracture h resistance strain gauges.

It will be apparent from a review of the' data as set forth in Table 3. that thecreep-rupture properties of ity of each of the experimental alloys as indicated by the percent elongation thereof is unexpectedly high in comparison to the ductility of steel alloys of similar type heretofore known. This is a consequence of the relatively high boron and relatively low carbon contents of these alloys. The experimental alloys are also observed to possess good high temperature corrosion resistance. With respect to the density of the alloys. it will be noted that the values obtained for the alloys of the present invention are significantly lower than densi ties of cobalt-base superalloys and some of the more recently developed nickel-base superalloys. which are suitable for use under similar service conditions, thereby providing for a significant reduction in the stress of rotating gas turbine components.

EXAMPLE 2 An experimental alloy. designated as D, is prepared by air-melting a heat containing the alloying elements asset forth in Table 1 'under the heading Nominal Composition." From the results obtained on the experimentalvalloys of Example 1. the anticipated properties of alloy- D are a density of 0.287 pounds per cubic inch;

a 0.271 offset yield strength of .65 ksi, an ultimate tensile strength of 105 ksi. an elongation of -l.257 all at room temperature. and a rupture strength at l.350F of 48 ksi at 100 hours. The average cocfficient of thermal expansion of the alloy is anticipated to be 9.2 X l0 inch per inch per F. over the range of 70F to 1,350F.

'While it will be apparent that the invention herein disclosed iswell calculated to achieve the benefits and advantages herein set forth, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the'spirit thereof. What is claimed is: v l. Anau'stenitic castable high temperature alloy consisting essentially of about 16 to about 22% chromium, about 6 to about l 87( nickel, about'6 to about 10% molybde num. up to about 371 tungsten, and about 0.5 to about 2.571 boron, about 0.01 to about 0.47rcarbon, about 0. l 5 to about 7% manganese, up to about 3% silicon. from zero to about 1% niobium, and the balance substantially all iron together with normal residuals and incidental impurities'present in usual amounts, said alloy further characterizedas having a IOO-hour rupture strength at l,500F of at least about 25 ksi;

2. The austenitic castable high temperature alloy as defined in claim 1, in whichjch'romium'is present in an amount of about 17 to about 20%, nickel'is present in an amount of about 8 to about 14 /1. molybdenum is present in an amount of about 7 to abou t97z boron is present in an amount of about 0.6 to about l .370, carbon is present in an amount of about 0.05 to about 0.25%. manganese-is present in an amount of about 0.5 to about 5%. silicon is present in anamo unt' of about 0.15 to about l /r. and niobium is present in an amount of about 0.2 to about 0.7%. i i

3. The austenitic castable high temp er' ture alloy as defined in claim 1, in which chtorniu'rr'i is'p'resent in an amountof about 18%, nickel is present in an amo'unt

Claims (3)

1. AN AUSTENITIC CASTABLE HIGH TEMPERATURE ALLOY CONSISTING ESSENTIALLY OF ABOUT 16 TO ABOUT 22% CHROIUM, ABOUT 6 TO ABOUT 18% NICKEL, ABOUT 6 TO ABOUT 10% MOLYBDENUM, UP TO ABOUT 3% TUNGSTEN, AND ABOUT 0.5 TO ABOUT 2.5% BORON, ABOUT 0.01 TO ABOUT 0.4% CARBON, ABOUT 0.15 TO ABOUT 7% MAGANESE, UP TO ABOUT 3% SILICON, FROM ZERO TO ABOUT 1% NIOBIUM AND THE BALANCE SUBSTANTIALLY ALL IRON TOGEHTER WITH NORMAL RESIDUALS AND INCIDENTAL IMPURITIES PRESENT IN USUAL AMOUNTS, SAID ALLOY FURTHER CHARACTERIZED AS HAVING A 100-HOUR RUPTURE STRENGTH AT 1,500*F OF AT LEAST 25 KSI.

2. The austenitic castable high temperature alloy as defined in claim 1, in which chromium is present in an amount of about 17 to about 20%, nickel is present in an amount of about 8 to about 14%, molybdenum is present in an amount of about 7 to about 9%, boron is present in an amount of about 0.6 to about 1.3%, carbon is present in an amount of about 0.05 to about 0.25%, manganese is present in an amount of about 0.5 to about 5%, silicon is present in an amount of about 0.15 to about 1%, and niobium is present in an amount of about 0.2 to about 0.7%.

3. The austenitic castable high temperature alloy as defined in claim 1, in which chromium is present in an amount of about 18%, nickel is present in an amount of about 12%, molybdenum is present in an amount of about 8%, boron is present in an amount of about 1%, carbon is present in an amount of about 0.2%, manganese is present in an amount of about 1%, silicon is present in an amount of about 0.5% and niobium is present in an amount of about 0.5%.

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