IL45227A - Iron base alloy suitable for use at elevated temperatures - Google Patents

Description

κηοηι 7T*a ?y SOD -3 an ηΛΐολβ n in ra A ni-»ie*iaa»3 «HDWV Iron base alloy .saits&le for use at ele ted ©ffipet¾fc ujpee Cs- 3289 FIELD OF THE INVENTION: , This invention relates to at least partially austenitic iron-base alloy compositions consisting predominantly of iron, chromium, nickel, carbon, and tungsten. The iron-base alloys of the present invention combine elevated temperature strength and high temperature corrosion-resistance with low cost. The alloys are particularly useful for making vehicular and industrial gas turbine components, such as cast turbine blades, turbine vanes, shroud housings, and combustion section components, where elevated temperature capabilities at low cost are emphasized. In addition, furnace furniture and high temperature machinery used in the chemical and petro- chemical industries may be fabricated from the alloys of the present invention. The alloys are well suited to such applications because of the unexpectedly high elevated temperature strength obtained in a material within the iron-base alloy price range.

BACKGROUND OF THE INVENTION: Industries involved in the development and manufacture of high temperature machinery or process equipment normally have had to rely on expensive nickel and cobalt-base alloys for applications requiring load- carrying or structural capability at high temperature. The advent of nickel and cobalt -base superalloys and the subsequent growth and success of that technology is evidence of the need for alloys having high strength at elevated temperatures.

While nicker and cobalt-base superalloys have proven suitable for applications requiring high strength at elevated temperatures, such alloys are very expensive. Nickel or cobalt typically cost between 10 a 30 times as much as iron. Accordingly, numerous attempts have been made in the past to develop high temperature, high strength, iron-base alloys as a method for reducing the overall costs of high temperature components.

Iron-base castings intended for elevated temperature service a in widespread use. However, due to the lack of strength of such alloys, their applications have been confined to conditions of very low stress.

Heat resistant iron-base cast alloys may be divided into two broad classes: ferritic and austenitic. Of the two, it is well known in the art that austenitic alloys, characterized by a face - centered- cubic (FCC) crystal lattice structure possess the superior load-carrying capability at elevated temperatures. Although the austenitic alloys are the stronger of the two alloy classes, the level of strength exhibited by such alloys is far below that considered the minimum requirement for most hot section gas turbine components operating in the 1400° F. -2000° F. temperature range.

In general, iron -base austenitic alloys contain chromium, nickel, and carbon. The chromium is present in sufficient quantity to enhance resistance to high temperature corrosion and oxidation and th nickel is present in sufficient quantity to stabilize an FCC crystal structure at room temperature. The carbon, which is an additional austenitic stabilizer, serves primarily as a chromium carbide forming constituent to enhance elevated temperature strength and to reduce the susceptibility of the iron-chromium-nickel alloys to precipitation of undesirable embrittling phases.

In accordance with the present invention, a range of at least partially austenitic iron-base alloys have been discovered which possess in the cast form, high temperature strength (e. g. , about 1000 ° F. to about 210 ° F. ) and corrosion resistance superior to many commerciall available, and currently employed, nickel and cobalt-base superalloys. The alloys of the present invention have a critical composition balance, particularly in the tungsten and nickel content. Generally, this composition balance produces an age- hardenable alloy. In addition, the alloys of the present invention preferably have a boron addition which combines with the composition balance to produce an alloy with elevated temperature creep-rupture strength superior to such commercial cobalt base alloys as HS- 31. Such cobalt-base alloys are extensively used where high temperature strength is required.

Refractory metal additions, including molybdenum, tungsten, columbium, and tantalum, have been made to austenitic- iron-base alloys in the past. Such additions have been in small quantities for the purposes of solid solution s trengthening, carbide stabilization, or forming strengthening refractory metal carbides. Strength improvements have been reali zed with many of these additions, particularly at temperatures up to 1600 °F. However, because the austenitic matrix has been lean in refractory metal content, diffusion rates are very high at temperatures near about *200Q°F. The resultant effect is a sharp deterioration in creep-rupture strength at temperatures above 1800° F. due to rapid coalescence or dissolution of the strengthening phases.

To circumvent these difficulties, the alloys of the present invention include unusually large amounts of a refractory metal, in particular tungsten. These unusually large concentrations of refractory metal retard diffusion rates and thereby enhance elevated temperature strength. A simple large refractory metal addition to a balanced austenitic iron-base composition will not, by itself, achieve an alloy having the desired properties. Large refractory metal additions can result in the formation of deleterious, embrittling phases and deterioration or elimination of the desirable FCC lattice structure.

In accordance with the present invention, it has been discovered that the desired refractory metal additions may be made without deleterious effects, if the nickel content is increased to accommodate the tungsten while simultaneously providing a matrix crystal structure which is at least partially austenitic. Carbon can also be used as an effective austenite stabilizer, but is usually retained at conventional levels.

It has been further discovered in accordance with the present invention, that boron, in amounts unusually large for iron-base alloys, will in concert with, or in partial replacement of, carbon, cause an unusual and unexpected increase in elevated temperature strength.

Boron is known to increase alloy fluidity in the liquid state, thereb enhancing castability. Thus, the addition of boron is particularly attractive where complex configurations are to be cast.

Small percentage additions of boron to iron-base alloys have been made in the past. However, these additions are generally in the range of 0. 001 to 0. 01 weight percent and only rarely over 0. 03 wt. %. U. S. Patent No. 3, 250, 612 notes that boron additions to the particular iron-base alloys therein disclosed in amounts above 0. 025 wt. % resulted in properties that were inferior to alloys containing less than that amount of boron, whether the alloys were in the as -cast or heat treated condition In U. S. Patent No. 3, 165, 400, additions of boron up to 0. 2 wt. % to particular iron-base alloys were noted as giving a slight improvement in strength. However, no further improvement was evident above that level nor is there any indication in the patent that boron would substantially improve strength.

The boron additions of the present invention may be employed in concentrations substantially above the amount conventionally employed in the prior art. The maximum observed improvement in 100 hour creep-rupture strength (61% improvement in 1400° F. testing) was effected by a boron addition of 0. 4 wt. % to one of two identical alloy compositions.

At least partially austenitic, low cost, iron-base cast alloys with elevated temperature strength superior to many nickel and cobalt-base superalloys are provided by the present invention. The iron-base cast alloys of the present invention resist structural deteriora- tion, phase coalescence, agglomeration, and instability, and retain to a substantial degree the original alloy properties during long-time exposure at temperatures up to about 2100°F. These alloys also provide resistance to hot corrosion and oxidation, permitting service in severe environmental conditions at elevated temperatures.

SUMMARY OF THE INVENTION; In general terms, the present invention pertains to at least partially austenitic iron-base alloy compositions adapted to be employed in cast shapes under conditions requiring relatively high strength at high temperatures. The invention also concerns cast components made from such alloys for use in gas turbine engines and other environments requiring strength and corrosion and oxidation-resistance at elevated temperatures.

The alloys of the present invention are predominantly iron^ i. e. , at least about 35% iron, and contain in varying amounts, chromiu nickel, tungsten, and carbon. One or more of the elements boron, titanium, manganese, and silicon may also be included in the alloys. In addition, the alloys of the present invention may contain minor amounts of other elements ordinarily included in iron-base alloys by those skilled in the art, which elements will not substantially deleterious ly affect the important characteristics of the alloy, or which are inadver tently included in such alloys by virtue of impurity levels in commercial grades of alloying ingredients.

Relatively large 'amounts of tungsten are included in the alloys of the present invention to enhance elevated temperature strength. In addition, the alloys contain a larger amount of nickel than is present in conventional iron-base alloys to insure a matrix crystal structure that is at least partially austenitic. Preferably, the alloys also contain a boron addition which further increases the elevated temperature stren Table I sets forth a broad range and two different narrower ranges, in terms of percent by weight of elements employed in the alloy of the present invention. It should be understood that the tabulation of Table I relates to each element individually and is not intended to solely define composites of broad and narrow ranges. Nevertheless, composit of the narrower ranges specified in Table I represent preferred embodiments. It should be further understood that the alloys of Table I may additionally include manganese and silicon in amounts normally employe in iron-base alloys to achieve castability and deoxidation (generally less than 2% each). It also should be understood that the tungsten content of the alloys of Table I may be partially replaced by molybdenum in amount up to 50 atomic percent of the tungsten.

TABLE I One group of preferred alloys within the scope of the present invention contains about 10 to about 14%, (and more preferably from about 11% to about 13%) by weight tungsten, coupled with either substantially no boron or a boron content between about 0. 3 and about 0. 5% by weight. A particularly preferred alloy composition, in per- ■ ' ■ centages by weight, consists essentially of about 22% to about 24% chromium, about 15% to about 17% nickel, about 11% to about 13% tungsten, about 0. 4% to about 0. 8% (and more preferably about 0. 5% to about 0. 6%) carbon, about 0. 25% to about 0. 65% (and more preferably about 0. 3% to about 0. 5%) boron, a combined carbon and boron content of up to about 1. 05%, less than about 1% (and more preferably about 0. 3% to about 0. 8% each) of manganese and silicon, and the balance essentially iron and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy.

Another particularly preferred alloy composition, in percentage by weight, consists essentially of about 22% to about 24% chromium, about 15% to about 17% nickel, about 4% to about 10% (and more preferably 4. 5% to 6. 5%) tungsten, about 0. 35% to about 0. 5% carbon, about 0. 1% to about 0. 2% boron, less than 1% (and more preferably about 0. 3% to about 0. 8%) each of manganese and silicon, and the balance essentiall iron and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy.

Not only does the present invention provide high tungsten, partially austenitic alloys, but many of the compositions within the scope of the invention respond to an age-hardening heat treatment with a noticeable resultant improvement in creep-rupture strength. Man of the alloys of the present invention may be improved in high temperature strength capability by thermal exposure at about 1500°F. to 1700°F. for at least two hours, followed by air cooling.

BRIEF DESCRIPTION OF THE DRAWING; The figure represents a Larson-Miller creep-rupture paramete plot for several alloys. One of the alloys of the plot is within the ambit of the present invention while the others represent commercially available alloys outside the ambit of the present invention.

DESCRIPTION OF EXAMPLES AND PREFERRED EMBODIMENTS: The critical composition balance between nickel and tungsten to obtain structural stability and strength in the alloys of the present invention, and the unusually potent strengthening effect of relatively high boron additions to these alloys, was determined using materials melted in air and cast in standard test bar molds. Thirty to fifty pound heats were melted and cast-to-size test bars, were produced for mechanical property evaluation. Exposure at temperatures between 1400°F. and 1800°F. was conducted on all allo compositions to determine the optimum age, heat-treating sequence. A heat treatment of 1600°F. in air for 18 hours followed by cooling in still air was found - - to be optimum for most alloys responding to aging heat treatment and was thus adopted as the standard heat treatment prior to testing.

Creep-rupture testing was conducted at temperatures between 1400°F. and 2000°F. under loads which would enable comparison of the resultant properties with commercially available alloys.

Analysis of 15 example alloys is presented in Table II in terms of weight percent of the alloying constituents. The results of creep-rupture testing with respect to most of the example alloys are given in Table III. The data in Table III includes time to rupture in hours under various conditions of temperature and stress, the tolerated final total elongation or linear creep strain, the reduction in area of the specimen diameter in the area of fracture, and a calculated equivalent stress to produce rupture in 100 hours at 1600°F. for each alloy. The calculated stress is used to demonstrate the effect of various alloy modifications and the behavior of the example alloys in comparison with commercially available iron, nickel and cobalt-base alloys.

TABLE II Example Alloy Fe Cr Ni W C B Ti Mn Si 1 (1) 23 5 20 0.01 0.50 0.5 2 (1) 23 5 20 0.10 -- 0.50 C.5 3 (1) 28 5 20 0.10 — 0.50 0.5 4 . (1) 28 5 20 0.60 0.50 0.5 5 (1) 23 16 12 0.60 -- -- ■ 0.50 0.5 6 (1) 23 16 12 0.60 0.40 0.50 0.5 7 (1) 28 16 12 0.60 0.40 0.50 0.5 8 (1) 23 16 12 0.80 0.20 0.50 0.5 9 (1) 23 16 12 0.20 0.80 0.50 0.5 10 (1) 23 16 12 0.20 0.80 0.20 0.50 0.5 11 (1) 23 16 6 0.60 0.40 0.50 0.5 12 (1) 23 20 12 0.60 0.40 0.50 0.5 13 (1) 23 20 12 0.20 0.80 0.20 0.50 0.5 14 (i) 28 20 12 0.60 0.40 -- 0.50 0.5 15 (1) 28 20 12 0.20 0.80 0.20 0.50 0.5 (1) Balance TA T E TIT Creep Rupture Properties Example Test Conditions 1600F-100 hrs.

Alloy Temp, °F Stress, psi Life, hrs. %E1. %RA Rupture Stress 2 1400 15, 000 172.4 1.8 2.5 3 1400 15,000 180.5 5.6 4.6 ' 5 1400 30, 000 23.8 10.5 14.9 1400 30, 000 13.6 12.2 20.1 1800 10, 000 26.8 6.5 11.7 1800 10, 000 26.3 8.2 12.9 -- -- 15, 000 psi 6 1400 45, 000 53.0 3.5 9.9 1400 45, 000 49.5 3.0 5.5 1600 25, 000 35.9 4.5 7.2 1600 25, 000 42.7 .3.7 9.9 1800 12, 000 69.6 3.5 6.3 1800 12, 000 47.5 4.6 10.7 2000 4» 000 124.3 3.9 11.2 2000 4, 000 72.1 3.6 10.0 --. -- 22, 200 psi 7 1400 45, 000 68.5 3.1 3.8 1400 45, 000 66.6 3.2 3.9 1800 10, 000 38.5 16, 1 30.0 1800 10.000 43.4 7.2 14.8 -- . -- -- 19, 000 psi 8 1400 45, 000 16.3 6.5 5.9 1400 45, 000 9.0 4.1 5.3 1800 12, 000 17.7 34.3 37.1 ■ ' 1800 12, 000 18.4 35.6 50.0 . -- 18, 000 psi 9 1400 45, 000 34.5 7.5 9.1 1400 45,000 28.4 7.1 8.3 1800 12, 000 36.7 18.5 47.7 1800 12, 000 37.5 23.9 38.2 21, 000 psi 10 1400 45, 000 9.2 7.6 8.3 1400 45, 000 10.8 7.8 9.2 1800 12.000 17.3 22.5 46.0 1800 12,000 13.3 30.0 51.0 -- ' — -- -- 17, 000 psi TABLE III 'cont'd)' Creep Rupture Properties Example Test Conditions 1600- 100 hrs. Alloy Temp, °F Stress, psi Life, %E1 RA Rupture Stress 11 1400 45, 000 19. 3 7. 5 10. 1 1400 45, 000 18. 2 7. 3 9. 0 1800 12, 000 31. 8 15. 6 36. 3 1800 12, 000 25. 6 8„ 7 31. 1 19, 500 psi 12 1400 45, 000 41. 7 4. 5 4. 8 1400 45, 000 24. 3 4. 8 5. 0 1800 12, 000 37. 6 15. 5 16. 8 1800 12, 000 31. 3 11. 8 20. 9 20, 500 psi 13 1400 45, 000 8. 4 8. 3 8. 9 1400 45, 000 11. 2 6. 9 7. 8 1800 12, 000 25. 1 23. 3 48. 5 1800 12, 000 20. 3 19. 6 36. 2 19, 000 psi 14 1400 45, 000 17. 3 3. 8 4. 1 1400 45, 000 20. 6 4. 5 4. 6 1800 12, 000 19. 7 13. 9 15. 3 1800 12, 000 18. 2 11. 4 22. 6 18, 500 psi 15 1400 45, 000 9. 2 6. 8 7. 1 1400 45, 000 8. 7 6. 8 7. 3 1800 12, 000 8. 7 24. 5 36. 6 1800 12, 000 7. 9 30. 1 41. 2 14, 000 psi Example alloys 1 through 4 demonstrate creep- rupture behavior typical of heat-resistant iron-base alloys. However, these alloys are not particularly austenitic, as the nickel and carbon contents are too low to accommodate the large 20% by weight tungsten addition. Example alloys 1 - 4 are hard, but low in ductility. Rockwell hardness numbers, C-scale, (R-c) of 50 were recorded in both the as -cast and heat treated conditions.

A composition lower in tungsten and higher in nickel is demonstrated by example alloy 5. The 1600°F. age heat treatment raised the alloy hardness from Rc 22 as-cast to R 32. Creep-rupture tests at temperatures of 1400°F. and 1800°F. show a strength improvement of more than double the tungsten-free composition. A strength improvement of this magnitude is totally unexpected. The toughness, castability, and structural stability were found to be excellent and the micros tructure of the alloy was identified as partially austenitic.

The strength capability of example alloy 5 is superior to the vast majority of alloys classified as iron-base super alloys. A few iron-base alloys possess comparable or superior strength. However, such alloys generally contain aluminum and/ or titanium, and thus rely on a conventional age-hardening reaction between nickel, aluminum and titanium. Alloys containing these reactive constituents must be melted and cast in a vacuum and are therefore necessarily more expensive to produce.

In contrast, the age-hardenable alloys of the present invention rely on a previously unrecognized age-hardening mechanism between nickel and tungsten, and thus may be free from aluminum and titanium. Such alloys exhibit excellent castability in air, thereby minimizing processing costs. The alloys of the present invention require no special processing skills other than conventional foundry practice, and may be readily cast in sand, shell, or investment molds. Furthermore, the alloys achieve a degree of strength improvement unrecognized in prior art tungsten additions to iron- chromium-nickel alloys .

The effect on a boron addition to the alloys of the present invention is also demonstrated in Tables II and III. Example alloy 6 represents an addition of 0. 4 wt. % boron to example alloy 5. As shown in Table III, the boron addition effected an increase from 1 ¾000 to 22,200 in the stress necessary to produce rupture in 100 hours at 1600°F. In testing not reflected by Table III, the boron addition effected an increase from 26, 000 psi to 41, 700 psi in the stress necessary to produce rupture in 100 hours at 1400°F. This represents an increase in load-carrying ability of 61%. At 1800°F. the stress to produce rupture in 100 hours increased from 9, 000 psi to 11, 000 psi for an improvement of 22%.

Creep-rupture testing at 2, 000°F. with examp le alloy 6 shows a stress of 4, 000 psi necessary to produce rupture in 100 hours. This level of strength is superior to several nickel and cobalt-base superalloys.

The superior level of- strength of example alloy 6 compared to several commercial iron, nickel or cobalt-base superalloys is demonstrated in the Larson-Miller parameter plot of the figure. . The data represented by the plot was obtained from example alloy 6 and five commercial alloys, designated A, B, C, D, and E, employing testing procedures in accordance with ASTM standard EI 39-66T. Commercial alloy A is an iron-base alloy, commercial alloy B is a cobalt-base alloy, and commercial alloys C, D, and E are nickel-base alloys. The analyses of commercial alloys A, B, C, D, and E, are shown in Table IV.

TABLE IV Commercial Alloy Fe Ni Co Cr Mo W Zr A (1) 20 -- 25 -- -- -- I B 2.0 10.5' (1) 26 -- 7.5 C 2.5 (1) -- 22 9.0 -- -- D 18.5 (1) 1.5 22 9.0 0.6 -- E (max) 2.5 (1) -- 12.5 4.2 -- 0.1 (1) Balance The figure shows that only commercial alloy E possesses creep- rupture properties superior to those of examp le alloy 6. Howeve commercial alloy E is a nickel-base alloy containing significant amount of the reactive metals aluminum and titanium. . Since commercial alloy E is a nickel-base alloy, it is inherently more expensive than the iron-base alloys of the present invention. In addition, the presence of the age-hardening reactive metals in commercial alloy Έ necessitates special processing.

An important characteristic of example alloy 6 shown in the plot of the figure is that creep-rupture strength for the example alloy is not deteriorating as rapidly at the high temperature (high parameter number) end of the curves as is the creep-rupture strength of the superalloys shown in comparison. This reflects the long time, high temperature stability properties of the alloys of the present invention resulting from a high refractory metal content. Thus, the alloys of the present invention provide superior service in retaining original alloy properties as required for low stress, long time, high temperatur applications.

The magnitude of the strength improvement effected by the high boron addition is unexpected. The combined effects of high tungsten and boron produces elevated temperature strength capability in the alloys of the present invention superior to known commercial cast iron-base alloys. This superiority at temperatures up to about 2100°F. is believed to be the result of the combined effects of a dispersion of tungs en- chromium borides and the nickel- tungsten balance which will generall produce an age-hardening mechanism.

Compositional refinements of the major constituents in the < example alloys reveal that increasing the chromium content up to about 28 Wt. % or more for enhanced resistance to sulfur bearing gases at -high temperature, will result in some deleterious phase precipitation. However, simultaneously increasing the nickel content to about 20 wt. % or greater, will restore structural stability. Higher chromium content alloys tend to be somewhat weaker at elevated temperatures. Nickel contents at about 20 wt. % or more improve toughness and insure stability of the austenite (FCC)phase, but produce little or no effect on creep-rupture strength. To insure an at least partially austenitic alloy, the nickel content is preferably raised as the tungsten content is raised. Tungsten content may be lowered somewhat in high boron content alloys with only a minor loss in strength.

The optimum boron-carbon ratio for creep-rupture strength is close to 1. 0. Strength decreases above and below that value. The decrease is less rapid with high boron, low carbon alloys. The boron plus carbon content should be sufficiently low (1. 05% by wt. ) to maintain toughness, but sufficiently high (0. 2% by wt. ) to insure structural stability. Carbon effects structural stability by stabilizing austenite while boron effects stability by reacting with the sigma phase forming elements chromium and tungsten to form borides. Carbon and boron are strengthening elements, but they also are embrittling. Accordingly, if the combined carbon plus boron content exceeds 1. 05% by weight, the ductility of the alloys falls to undesirably low levels.

Titanium may be used as a deoxidizer in the alloy melt.

Residual titanium of about 0. 2% by weight normally would be included only in high nickel or low chromium alloys. Manganese and silicon may be used as normal foundry additions in conventional amounts to control melt deoxidation and fluidity with no detrimental effect on alloy properties.

Table V sets forth the stress to produce rupture in 100 hours at 1600°F. for the heat resistant cast commercial superalloys A through D of Table IV. A comparison of this data with the creep-rupture data for example alloys 5 through 15, set forth in Table III, demonstrates the alloys of the present invention possess superior properties to commercial iron-base alloy A and commercial nickel-base alloys C & D; and comparable properties to cobalt-base commercial alloy B.

TABLE V Commercial 1600F/ 100 hour Alloy Rupture Stress, psi A 7, 800 B 19, 500 C 11, 000 D 10, 000 Example alloy 6, heat treated at 1600°F. for 20 hours and air cooled, has a density of 0. 301 lb/ cu. in. (8. 33 g/ cm^), an insipient melting temperature of 2290-2320°F. , and a shrinkage factor of 1. 6%. This alloy had, as cast, an R^ hardness value of 30-32, while after heat treating at 1600 °F. for 20 hours and air cooling the Rc value was 35. Impact properties on example alloy 6, measured at room temperature, ranges from 2 to 5 foot-pounds. This range is typical of cast cobalt-base superalloys.

Static oxidation tests were conducted at 800°F. for 500 hours with cylindrical test specimens. Test specimens of examp le alloy 6 which were cooled to room temperature every 24 hours, exhibited a total weight loss of 0. 24 mg. /cm^ and a surface recession of 0. 0016 i Crucible sulfidation testing was conducted at 1700°F. in a mixture of 10% NaCl/ 90% Na2SC>4 for 300 hours on alloys within the scope of the present invention. The results of this testing along with comparative results for commercial alloys 713 C, U-500, and IN- 738 taken from previously published data employing identical test conditions is presented in Table VI. Example alloy 6 possesses sulfidation resistances far superior to either commercial alloys U-500 or 713 C and only is slightly inferior to expensive nickel-base alloy IN- 738 TABLE VI Alloy Time, hours Remarks Alloy 713 C 2 - 4 Destroyed U-500 (cast) 40 - 100 Gross attack Example Alloy 6 300 25-30% reduction in specimen diameter IN- 738 250 - 300 Slight attack Tensile properties on heat treated specimens of example alloy 6 were determined in accordance with ASTM standards E8-66 and E21 -66T. The results of this testing is presented in Table VII. As is apparent from the table, the alloys of the present invention possess good high temperature yield strength and ultimate tensile strength.

TABLE VII Temperature, F. 0. 2%Y. S. U. T. S. %B1 %RA 1000 56, 500 85, 400 1. 8 4. 6 1000 53, 900 85, 600 1. 3 5. 3 1600 38, 500 61, 200 2. 3 5. 1 1600 35, 100 60, 300 3. 1 3. 2 The alloys of the present invention may be heliarc welded with a tensile joint efficiency of up to 90% or even greater at 1600°F. when using matching filler. Welding may be readily employed to produce structural joints or for repair purposes in sections under 0. 5 inches. For thicker section sizes or restraining joints, a more ductile filler metal may be required.

The alloys of the present invention have thermal coefficients of expansion similar to most iron-base alloys. Table VIII sets forth the thermal coefficient of expansion for example alloy 6 over a broad temperature range.

TABLE VIII Thermal Expansion Mean Coefficient of Expansion Temperature from 70 F s:o Indicated Temperature •(F) · (per F) 400 8. 10 x 10" 6 800 8. 25 1200 8. 80 1600 9. 00 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. Such modifications are considered to be within the purview and scope of the invention and appended claims.

Claims (21)

45227/2 WHAT IS CLAIMED IS:

1. An at least partially austenitic iron-base alloy consisting essentially of the following elements in the weight percent ranges set forth: Chromium 20% to 32% Nickel 14# to 20f° Tungsten 4 ° to 14?^ Carbon 0. 01% to 0. 8% Boron 0. 00% to 0. 85% Titanium 0. 00% to 0. 2% the balance of the alloy being essentially iron and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the all oy, said iron being present in an amount of at leas t about 35% by weight and the combined amount of carbon plus boron being within the range of about 0. 2% to 1. 05% by wt.

2. An iron-base alloy in accordance with claim 1 in which the tungsten content is partially replaced with molybdenum in an amount of up to 50 atomic percent of the tungsten.

3. An iron-base cast alloy in accordance with claim 1 containing about 11% to about 13% tungsten and substantially no boron.

4. An iron-base cast alloy as defined in claim 1 containing about 11% to about 13% tungsten and about 0. 3% to about 0. 5% boron.

5. A cast component for use in the turbine section of a gas turbine engine formed of the alloy of claim 1.

6. A process of heat treating an object formed of the alloy of claim 1 to improve high temperature strength capability comprising exposing said object to a temperature of about 1500°F. to about 1700°F. for at least two hours, followed by air cooling.

7. An at least partially austenitic iron-base alloy consisting essentially of the following elements in the weight percent ranges set forth: Chromium 21% to 28% Nickel 14% to 20% Tungsten 4% to 14% Carbon 0. 2% to 0. 65% Boron 0. 1% to 0. 5% Titanium 0. 00% to 0. 2% the balance of the alloy bein essentially iro and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy, said iron being present in an amount of at least about 40% by weight, and the combined amount of carbon plus boron not exceeding about 1. 05% by weight.

8. A cast component for use in the turbine section of a gas turbine engine formed of the alloy of claim 7.

9. The alloy of claim 7 in which the tungsten content is within the range of from about 4% to about 10% by weight. s

10. The alloy of claim 7 in which the tungsten content is partially replaced with molybdenum in an amount up to 50 atomic percent of the tungsten.

11. An iron-base alloy consisting essentially of the following elements in the weight percent ranges set forth: Chromium 21% to 25% Nickel 14% to 18% Tungsten 10% to 14% Carbon 0. 1% to 0. 8% Boron 0. 25% to 0. 8% Titanium 0 to 0. 2% the balance of the alloy being essentially iron, and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy, the combined amount of carbon plus boron not exceeding 1. 05% by weight.

12. The alloy of claim 11 in which the tungsten content is partially replaced with molybdenum in an amount up to 50 atomic percent of the tungsten.

13. A cast componerit for. use in the turbine section of a gas turbine engine formed of the alloy of claim 11.

14. An at least partially austenitic iron-base alloy consisting essentially of the following elements iii the weight percentage ranges set forth: Chromium 22% to 24% Nickel 15% to 17% Tungsten 11% to 13% Carbon 0. 4% to 0. 8% ' Boron 0. 25% to 0. 65% Manganese less than 1% Silicon less than 1% the balance of the alloy being essentially iron, and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteristics of the alloy, the combined amount of carbon plus boron being not more than about 1. 05% by weight.

15. A cast component for use in a turbine section of a gas turbine engine formed of the alloy of claim 14.

16. The alloy of claim 14 in which the carbon content is within the range of from about 0. 5% to about 0. 6% and the boron content is within the range of from about 0. 3% to 0. 5%.

17. The alloy of claim 16 in which the manganese and silicon conte are each within the range of from about 0. 3% to about 0. 8%.

18. An at least partially austenitic iron-base alloy consisting essentially of the following elements iii the weight percent ranges set forth: Chromium 22% to 24% . Nickel 15% to 17% Tungsten 4% to 10% Carbon 0. 35% to 0. 5% Boron 0. 1% to 0. 2% Manganese less than 1% Silicon less than 1% the balance of the alloy being essentially iron, and minor amounts of impurities and incidental elements which do not detrimentally affect the basic characteris tics of the alloy.

19. A cast component for use in the turbine section of a gas turbine engine formed of the alloy of claim 18.

20. The alloys of claim 18 in which the tungsten content is within the range of from about 4. 5% to about 6. 5% by weight.

21. The alloy of claim 20 in which the manganese and silicon are each present in amounts within the range of from about 0. 3% to about 0.