IL34792A - Heat treatable alloy - Google Patents

Description

HEAT TREATABLE ALLOT Martin Marietta Corporation 34792/2 The present invention is concerned with an alloy and, more particularly, with castings made from said alloy, said castings being particularly adapted for use as blades in gas turbine engines.

British patent 909,356 relates to the production of the turbine rotor and stator blades of a gas turbine engine.

Although this patent mentions that the object is to prepare gas turbine hardware, which must be capable of withstanding high temperatures and stresses, for long periods of operation, the patent is totally silent with respect to the final properties of the blades produced according to the process therein described. Specifically, the process consists of a combination of extrusion, heat treatment and quenching operation which are alleged to be suitable for making hardware from certain wrought alloys.

The process of this patent is designed to manufacture blades from alloys which ordinarily contain a high content of strengthening constituents which would make the manufacture of the blades difficult.

The crux of the process of this reference resides in the fact that the blades are formed by casting an ingot, employing a major portion of the whole of the ingot in the production of not more than four turbine rotor or stator blades and using a step of extrusion at a temperature in the range of 1020°C-1140°C. The British Patent states that the reason for using small ingots is that it is necessary to prevent the formation of large crystals, because when large ingots are used, the central portion is ver slow to cool and the slow cooling permits the growth of large crystals. In accordance with the process of this patent, the coolin of the small ingots is rapid, so that they have fine grains* The material is preferably subjected to surface cold work by shot peening, reeling or cold swaging, and is finally subjected to a recrystallization treatment and water quenching.

British Patent 909 » 356 contains at page 1 , column 2 a very broad disclosure of the alloy composition to which that particular heat treating method is applicable, and in the case of most of the listed elements, the perialssa!ile range of percentages is very broad, for example, aluminum is permitted in amounts of 2-17 and molybdenum in amounts of 5-30 tungsten in amounts of 0-20$, and chromium in amounts of 5-30$· T e only alloy actually disclosed by the British Patent in the example contains 10.2$ cobalt, a.08$ molybdenum, . 2$ titanium and 4.60$ of aluminum.

The sum of the aluminum plus titanium is 9.02$.

Although hafnium in an amouftt between 0 and 10$ is broadly disclosed in British patent 909, 356 , the only specific alloy described in detail contains no hafnium and apparenitsly the patentee did not appreciate the advantages derived from the use of an alloy containing hafnium.

Moreover, the use of too much molybdenum as in the example of British 909,356 will result in the formation of the undesirable sigma phase, resulting in unsuitable high temperature stress for cast hardware. This is the obvious explanation for the fact that the British Patent suggests extrusion complicated heat treatment and quenching operations in making hardware. Also the use of too much aluminum plus titanium in high molybdenum content and high chromium content alloys is known to result in brittle castings.

It has been previously known that hafnium may be added to certain wrought alloys for various purposes. However, the effect of the addition of hafnium to wrought alloys or to casta'ble allo s is unpredictable and the effect of the hafnium addition depends to a very large eittent on the nature and amounts of the other constituents of the alloy. The use of hafnium to improve ductility in a certain class of high temperature nickel-base castable alloys is the subject of our application S.N. 32,110 in Israel. In accordance with that invention, it was found important to limit the chromium content of the alloys to a maximum of about 13$.

The present invention relates to an improvement in an alloy which has been previously used commercially under the designation "Udimet 700", This alloy is designated as "Alloy Zw herein. That alloy is known to be used primarily as a wrought alloy and from the standpoint of being a wrought alloy, the alloy has unusually high stress in the form of wrought or extruded hardware. However, as Is disclosed in the present specification, when that particular alloy designated as "Alloy Z" is used for making cast hardware, which after casting must be subjected to solution heat treatment, serious problems have been encountered.

Thus the present invention is designed to overcome deficiencies and defects in castings of a particular alloy which wae initially developed as a heat treatable bar alloy, that is, an alloy which la designed to be wrought after being initially caot into a billet or other rudimentary form and thereaf er heat treated to provide £$ood high temperature characteristics. The nominal coQoei &cn in percent by weight of tills alloy is as follows: Carbon 0.08 Titanium 3.5 ChroKiun 15.0 Aluninura A.yfi Cobalt 18.55* Boron 0.0¾5 MolybdenuQ -2$ Nickel Balance This particular alloy, identified hereinafter as "Alloy 2 is currently being ueed oo caatinge for various high temperature usee. en cast Alloy ¾ io to be used ao certain oecond stage blades in the turbine section of a gas turbine engine, the engine manufacturer requires that specimens machined from blades (m.f.b. specimens) and heat treated by solution treatment at 11β3° C. for two hours, furnace cooled at about 55.6° in Centigrade units per hour to 1080° C, cooled to room temperature and then held at 760° C. for 1β hours exhibit a life-to-rupture of at least 23 hours with at least k It has now been discovered that by inclusion of about 0.8$ to about 2$ by weight of hafnium in the composition, non-qualifying heats of Alloy Z can be substantially eliminated provided due care, as is customary -in the art of high temperature alloy manufacture, be used in the formulating and treating of the alloy. a new, highly advantageous alloy.

Another object of the present invention is to provide novel castings made from said new alloy.

Other objects and advantages will become apparent from the following description; Generally speaking, the present invention contemplates alloys and castings made therefrom having a chemical composition within the range (in percent by weight) as set forth in Table I > TABLE I Carbon 0.05 - 0.13$ Aluminum 3.5 - 4. 5$ Chromium $ t 'Zr H% Boron 0. 005 - 0.05 Cobalt 2 % Zirconium up to 0.3 Molybdenum jt If Hafnium 0.8 - 2% Titanium 3 - % Nickel Balance Essentia The alloy. as set forth in Table I can also contain small amounts of impurities and incidental elements normally associated with the ingredients thereof and such impurities and incidental elements are intended to be included within the term "essentially" when such term modifies the phrase "balance nickel". The alloy of Table I is made by melting the alloying ingredients including, if desired, graded and selected scrap and revert in a vacuum induction furnace and castings are made therefrom by pouring the molten alloy while under vacuum into suitably shaped precision investment molds. The metal is allowed to freeze in said molds and, when cool, is removed from the molds. After gates, risers and the like are cut from the cast objects, the objects are heat treated by solution treatment at 1149° C to 1177° C. for about 2 hours, furnace cooled at a rate of about 55.6° C. per hour to about 1θ8θ° C. and then cooled freely to room temperature. Subsequently the objects are reheated to about 7βθ° C. and held, for example for 16 hours to effect precipitation of gamma prime phase in the alloy structure.

The advantages of the present invention are best exemplified by a case history of a particular heat of Alloy Z. This heat had the following composition in percent by weight : Carbon 0.06 Titanium 3.26 Iron 0.13 Chromium i4.4 Aluminum 4. 0 Manganese Cobalt 15.0 Boron .018 Silicon Molybdenum 4.29 Zirconium <.03 Sulfur 0.004 Copper <.l Nickel Balance Repeatedly metal of this heat was cast into turbine blade clusters, samples were taken and heat treated and subjected to qualification tests at 7600 C. under a load of 5980 Kg/cm2. The results are set forth in Table II.

TABLE II Average Average Cluster Life (Hours) Prior Creep ( ) 1 15.7 2.30 2 6.7 1.46 3 1 .9 I.9 4 37.6 3.08 The data in Table II show that this heat of Alloy Z is clearly not acceptable to the gas turbine engine manufacturer. Statistical analysis of the data in Table II shows that if a very large number of m.f.b. samples were made from this heat of Alloy Z3 greater than 99.9$ of these samples would fail to meet the criterion of 4^ prior creep. Furthermore, only about 33$ of the samples v/ould be expected to exceed the requirement One and one-half percent of hafnium was added to the particular heat of Alloy Z which exhibited the results set forth in Table II. The modified heat had the following composition (in percent by weight) : Carbon Ο.θβ Aluminum 4.43 Iron 0.13 Chrominum 14.2 Boron .018 Manganese Cobalt 14.8 Zirconium r °3 Silicon Molydbenu 4.23 Copper < .l Sulfur 0.004 Titanium 3.21 Hafnium 1.45 Nickel Balance The modified alloy was poured into a turbine blade cluster identical to the clusters formed with the unmodified alloy. Samples were taken and treated in exactly the same manner as had been done previously. Results of creep tests at ?6θ° C. and 5980 Kg/cm2 with this cluster are set forth in Table III: TABLE III Life-to-Rupture Blade No. (Hours ) Prior Crepe { ) 1 53.3 .30 • 2 76.0 6.49 3 8I.0 6.64 4 94.4 8.64 88.8 6.35 Statistical analysis of the data in Table III indicates that if a very large number of m.f .b. samples were made from the modified alloy, greater than 95 of such samples would exhibit greater than 4 prior creep and greater than 99 of such samples would exhibit a life-to-rupture in excess of 23 hours. The data in Table III clearly show that by inclusion of about 1.5 hafnium in the unacceptable heat of Alloy Z, the clearly acceptable under the criterion laid down by the gas turbine engine manufacturer. ~ The advantages of the present invention are obtained when the alloys of the invention contain from about O.Q to 2 or even more of hafnium. The inclusion of hafnium in said alloys is not known to significantly degrade any useful characteristic of Alloy Z and is highly advantageous in that it eliminates } for all practical purposes , heats of Alloy Z which, for no apparent reason, fail to qualify for cast turbine blade usage. The novel modified Alloy Z of the present invention can be advantageously used in the wrought form as well as in the cast form and is useful in the manufacture of various items subjected in use to stress at high temperature, e.g. gas turbine blades and other items of gas turbine hardware. ·' While the present invention has been described in conjunction with advantageous embodiments, those skilled in the art will recognize that modifications and variations may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention.

Claims (4)

34792/2 Claims

1. A heat treatable alloy adapted for forming castings for service under stress at high temperature, comprisin in percent by weight essentially about 0.05$ to about 0.13$ carbon, about14.2$ to about 17$ chromium, about 14.8$ to about 20 cobalt, about .23$ to about 6 molybdenum, about 3$ to a out 4$ titanium, about 3» 5$ to about 4, aluminum, about 0.005SI to about 0.05$ boron, up to about 0.3$ zirconium, about 0.8$ to about 2$ hafnium with the balance being essentially nickel.

2. An alloy according to claim 1, comprising about 0.88$ carbon, about 15$ chromium, about 18.5$ cobalt, about 5*2$ molybdenum, about 3·5$ titanium, about 4.3$ aluminum, about 0.03$ boron, up to about 0.3$ zirconium, about 0.8$ to about 2$ hafnium, with the balance" being essentially nickel. 3» An alloy according to claim 1 comprising 0.06$ of carbon, 14.2$ of chromium, 14.8$ of cobalt, 4.23$ of molybdenum,

3.21$ of titanium, 4.43$ of aluminum, 0.018$ of boron, up to00.03$ of zirconium, up to 0.1$ of copper, 0.13$ o iron, up to 0.1$ of manganese, up to 0.1$ of silicon, 0.004$ of sulfur, 1.45$ of hafnium, the balance being essentially nickel.

4. The method of forming a casting from the alloy of claim 1 which comprises melting said alloying ingredients in a vacuum furnace, pouring the molten alloy while under vacuum, into a precision investment mold, allowing the resulting object to cool, removing it from the mold, subjecting said object to heat treatment for about 2 34792/2 hours at 1149° - H77°C.»cooling it at ttie rate of 55.6° - 1080°C. per hour, then cooling it to room temperature, reheating to about 760 C. and holding the object to effect precipitation of the gamma prime phase. The alloy according to claim 1, wherein the alloying ingredients include scraps and reverts. A heat-treatable alloy substantially as hereinbefore described. The method of forming a casting substantially as hereinbefore described.. For pplicants Br. Yitzh k