Precipitation Strengthened Alloys

Abstract

A precipitation strengthened alloy is provided having the composition: wherein the ratio of tantalum to tungsten is between about 11/2 to 21/2 and the molybdenum is less than 1 percent.

Description

This invention relates to precipitation strengthened alloys and particularly to precipitation strengthened cobalt-base alloys.

While good cobalt-base alloys exist for high temperature service (for example, HAYNES alloy No. 25 and HAYNES alloy No. 188), these alloys are characterized by "solid solution strengthening" as contrasted to strengthening by the formation of precipitates (for example, .gamma.' precipitation in nickel-base alloys). Those cobalt-base alloys that have been strengthened by precipitation reactions have generally employed aluminum and/or titanium as reacting elements and these alloys are characterized by improved strength near 1,200.degree.F, but ineffective strengthening at 1,600.degree.F or above.

Others have employed beryllium, columbium (niobium) or tantalum. These elements will cause precipitation reactions in cobalt alloys, but heretofore the strengthening mechanism(s) have not been effective at 1,600.degree.F and above.

We have found that cobalt-base alloys having improved strength at 1,600.degree.F and higher can be achieved by what appears to be a precipitation reaction caused by the presence of tantalum to tungsten in the ratio of about 11/2 to 21/2 and preferably in the ratio of 2 to 1 by weight percent and the molybdenum is limited to impurity levels and the carbon is maintained below about 0.3 percent.

It is an object of this invention to provide cobalt-base alloys with improved strength at 1,600.degree.F and higher.

Another object is to provide a high strength wrought cobalt-base alloy.

Yet another object is to provide a high strength cobalt-base alloy which is oxidation resistant.

Other objectives are to provide cobalt-base alloys that are formable and which retain good engineering strengths at temperatures as high as 2,000.degree.F.

Still other objects will be apparent from the following description and claims.

A cobalt-base alloy in accordance with the present invention is broadly an alloy consisting essentially of about:

Weight % ______________________________________ Tantalum 5-20 Tungsten 2-15 Chromium up to 30 Iron 0-10 Carbon 0-0.3 Nickel 0-30 Silicon 0-1 Yttrium 0-0.2 Lanthanum 0-0.2 Manganese 0-2 Cobalt + incidental impurities balance ______________________________________

Wherein the ratio of tantalum to tungsten in weight percent is between about 11/2 to 21/2 and the molybdenum is less than 1 percent.

A preferred range of the alloy providing useful strengths is an alloy consisting essentially of about:

Weight % ______________________________________ Tantalum 5-20 Tungsten 2-15 Chromium 15-30 Iron 0-10 Carbon 0-0.2 Nickel 8-30 Silicon 0-1 Yttrium 0-0.2 Lanthanum 0-0.2 Manganese 0-2 Cobalt + incidental impurities balance ______________________________________

wherein the ratio of the tantalum to tungsten in weight percent is between about 11/2 to 21/2 and the molybdenum is less than one percent. A more preferred range of the alloy is:

Weight % ______________________________________ Tantalum 5-18 Tungsten 2-12 Chromium 15-30 Iron 0-10 Carbon 0-0.2 Nickel 8-30 Silicon 0-1 Lanthanum 0-0.2 Yttrium 0-0.2 Manganese 0-2 Cobalt + incidental impurities balance ______________________________________

Wherein the ratio of the tantalum content to the tungsten content in weight percent is between about 11/2 to 21/2 and the molybdenum content is less than one percent.

In addition to the specifically mentioned constituents, other alloying elements may be added without departing from the spirit of the invention and without negating the criticality of the tantalum to tungsten ratio and the minimization of the molybdenum and carbon contents. Such elements would include, but would not necessarily be limited to, Hf to about 5 w/o, Ti and/or Zr to about 2 w/o, Cb (Nb) and Re to about 4 w/o, aluminum to about 1 w/o, and magnesium and/or boron to about 0.04 w/o.

It has been discovered, as part of the present invention, that alloys as described above develop unusual strengths for a cobalt-base alloy through at least 1,700.degree.F, when the tantalum content of the alloy is approximately twice that of the tungsten content and when the carbon is maintained at a relatively low level. When the critical Ta/W ratio is not adhered to, effective strengthening is not obtained.

The strengths developed apparently are caused by a precipitation reaction of some sort because, as will be shown, in the annealed condition (heat treated about 2,200.degree.F and rapid cooled) the materials are relatively soft, ductile, and comparatively weak. However, during exposure at elevated temperatures between about 1,200.degree.-1,800.degree.F, the alloy becomes much stronger and, as would be expected from a precipitation reaction, harder and somewhat less ductile.

Further, it has been discovered that molybdenum is detrimental. With molybdenum present, the strengths achieved are significantly less. Therefore, molybdenum is considered as a detrimental impurity in the alloy and is only tolerated to 1 w/o for economic reasons.

Apparently, carbon also inhibits effective strengthening by the mechanism and therefore is limited to a maximum of 0.3 weight percent. Carbon in excess of this amount drastically reduces the strength achieved.

Since the alloy is designed, in general, for high temperature use, balanced quantities of chromium, silicon, manganese, lanthanum and/or yttrium are added to the alloy to promote oxidation resistance. Typically, for oxidation resistance, these elements are present as follows in weight percent: 15-30Cr, 0.1-0.6Si, 0.5-1.5Mn, small but effective amounts to 0.15 of Y and/or La.

Nickel and iron are used as needed to stabilize the face center cubic crystal structure of the base alloy and it is anticipated but not proven that iron and nickel would function as a partial participant in the strengthening mechanism.

Table I hereinbelow lists the compositions of alloy used to demonstrate the invention.

TABLE I - CHEMICAL ANALYSIS, w/o __________________________________________________________________________ Alloy Al C Co* Cr Fe La Mn Mo Ni Si Ta W __________________________________________________________________________ 5 0.33 0.13 Bal 21.56 1.88 0.05 0.66 0.37 23.60 0.35 7.81 9.31 7 0.38 0.12 Bal 21.07 1.86 0.04 0.67 0.33 23.60 0.40 16.74 ** 9 0.37 0.12 Bal 20.81 1.36 0.05 0.53 0.42 23.50 0.27 10.53 4.50 10 0.45 0.12 Bal 20.07 1.38 0.04 0.54 4.04 22.40 0.25 10.14 4.39 24 0.19 0.55 Bal 20.88 1.18 0.07 0.48 -- 21.63 0.22 10.08 4.95 __________________________________________________________________________ * Cobalt plus incidental impurities ** No W added to melt

In general, the alloys of Table I were vacuum induction melted (although other methods might have been used), cast into nominally 20-pound round tapered ingots, forged from a furnace operating about 2,150.degree.F and then rolled into sheet, annealed between 2,150.degree. and 2,200.degree.F, and rapid cooled.

Alloy 24 was vacuum induction melted and then cast into test specimen mold prepared per the loss wax process.

Alloy 9 is considered an example of the invention, while alloy 10 demonstrates the detrimental effects of molybdenum, alloy 7 demonstrates that tantalum without tungsten is not effective, alloy 5 demonstrates that near equal amounts of tantalum and tungsten are not effective, and alloy 24 illustrates that excessive amounts of carbon are to be avoided.

Table II hereinbelow lists comparative tensile strengths of the alloys. The FIGURE compares a typical alloy of the invention, alloy 9, with other experimental alloys and the commercial HAYNES alloy No. 188 by means of a conventional Larson-Miller plot.

TABLE II __________________________________________________________________________ TENSILE DATA* __________________________________________________________________________ Test Temp. YS UTS Elongation Alloy .degree.F ksi ksi % __________________________________________________________________________ 5 Room 75.7 147.7 49 Room 77.4 146.5 49 1600 48.8 63.7 65 1600 49.3 63.3 60 2000 9.6 17.8 49 2000 10.3 17.9 50 7 Room 84.7 152.9 32 Room 80.5 154.2 32 1600 73.4 103.2 8 1600 42.2 99.2 13 2000 4.3 15.0 94 2000 4.3 16.4 71 9 Room 55.0 133.6 63 Room 54.6 133.2 60 1600 63.1 81.8 10 1600 70.9 86.9 12 2000 11.6 18.4 40 2000 7.2 18.7 40 Annealed Room 132.7 183.0 20 + 16 hrs. 1500.degree.F 1200 108.3 151.0 20 " 10 Room 58.7 136.5 62 58.0 136.7 61 1600 71.5 87.2 11 1600 66.0 88.6 11 2000 11.6 17.5 65 2000 10.3 17.7 61 24 Room 57.3 97.2 5 As cast Room 53.0 103.9 8 As cast + 24 hrs. at 1800.degree.F Room 47.7 106.5 7.9 As cast + 16 hrs. at 1500.degree.F 1800 20.4 27.7 38 As cast 1800 23.2 27.1 36 As cast + 24 hrs. at - 1800.degree.F __________________________________________________________________________ * Annealed unless otherwise noted.

The data of TABLE II show that alloys 7, 9, and 10 all have improved intermediate temperature strength as compared to commercial alloy 188, which for comparison has tensile properties at 1,600.degree.F of about 38 ksi 0.2 percent offset yield strength, 61 ksi ultimate strength and 69 percent elongation. The tensile data for alloy 5 illustrate that when the critical Ta/W ratio of about 2 is not adhered to, the strengthening of the alloy is minimum. Alloy 5 has 7.81 w/o Ta and 9.31 w/o W.

It is worthy of note that alloys 9 and 10 had higher yield strength at 1,600.degree.F than at room temperature. This is thought to be the result of a precipitation reaction occurring during the 30 to 60 minute stabilization period at temperature prior to tensile testing. Further proof that a precipitation reaction occurs in the alloy of the invention is that alloy 9 aged 16 hours at 1,500.degree.F had more than double the yield strength at room temperature than did the alloy without an aging treatment.

Alloys similar to the alloy of the invention (alloy 24), but with carbon levels exceeding about 0.3 weight percent apparently are not strengthened in the same manner as the alloys of the invention.

For example, alloy 24 was aged 24 hours at 1,800.degree.F then tested at room temperature with no significant increase in strength being observed over a similar test bar tested in the as-cast condition.

Another sample of alloy 24 was aged 16 hours at 1,500.degree.F and then tested at room temperature also with no significant increase in strength. See Table II.

While all of the alloys 5, 7, 9 and 10 show superior intermediate temperature tensile properties only alloys 9 and 10 have exceptionally good longer term strength as depicted by stress-rupture life. The data of Table III hereinbelow show the stress rupture properties of the alloys. These data and the graph of the FIGURE show clearly the stress-rupture life properties.

TABLE III __________________________________________________________________________ STRESS RUPTURE DATA __________________________________________________________________________ Test Temp. Stress Life Elongation Alloy .degree.F ksi hrs. % __________________________________________________________________________ 5 1500 25 81.8 10 1500 25 91.7 8 1700 13 12.6 27 1700 13 13.4 26 1900 4.5 27.9 17 1900 4.5 26.4 18 7 1500 25 27.6 12 1500 25 28.5 29 1500 40 4.1 9 Annealed + 16 hrs. at 1500.degree.F 1700 13 9 32 1700 13 15.7 14 1900 4.5 10.5 27 1900 4.5 11.3 23 9 1500 25 654.0 11 1500 25 626.2 8 1500 40 47.6 28 Annealed + 16 hrs. at 1500.degree.F 1700 13 74.8 4 1700 13 51.1 6 1900 4.5 25.3 12 1900 4.5 17.9 9 10 1500 25 229.3 8 1500 25 231.8 6 1700 13 26.7 6 1700 13 23.6 9 1900 4.5 12.8 29 1900 4.5 15.4 24 __________________________________________________________________________ * Samples annealed 0.05" thick sheet unless noted otherwise.

A comparison of the data from alloys 9 and 10 illustrates a good example of the detrimental effects of molybdenum on the strength of the alloys of the invention. Alloys 9 and 10 were produced from the same melt of material, forged at the same time, rolled at the same time and otherwise processed identically. The only significant difference being that, after the ingots for alloy 9 were cast from the melt, a late addition of molybdenum was made to modify the composition. A lanthanum addition of 18 gms and 30 gms of a 20 Mg 80 Ni alloy were also added to replace the losses of lanthanum and magnesium which occurred during the melt-in of the molybdenum late addition.

The data from alloys 9 and 10 show that alloy 10 containing 4.04 w/o molybdenum has approximately one half the rupture life of alloy 9 containing only an impurity level of 0.42 w/o molybdenum.

The foregoing data clearly show that a superior cobalt-base alloy is provided by this invention having a critical relationship between the tantalum content and the tungsten content and wherein the molybdenum and carbon contents must be critically controlled.

It will be apparent to men skilled in this art that, while we have illustrated and described certain preferred embodiments of this invention in the foregoing specification, the invention may be otherwise embodied within the scope of the following claims.

Claims

1. A precipitation hardened alloy consisting essentially in weight percent of about: Tantalum 5-20 Tungsten 2-15 Chromium up to 30 Iron 0-10 Carbon 0-0.3 Nickel 0-30 Silicon 0-1 Yttrium 0-0.2 Lanthanum 0-0.2 Manganese 0-2 Cobalt + incidental impurities balance

wherein the ratio of tantalum to tungsten is between about 11/2 to 21/2 and

2. An alloy as claimed in claim 1 consisting essentially in weight percent of about:

wherein the ratio of the tantalum content to the tungsten content is about

3. An alloy as claimed in claim 1 consisting essentially in weight percent of about:

4. An alloy as claimed in claim 1 consisting essentially in weight percent of about:

wherein the ratio of the tantalum to tungsten content is about 11/2 to 21/2

5. An alloy as claimed in claim 1 consisting essentially in weight percent of about:

wherein the tantalum to tungsten ratio is about 11/2 to 21/2 and the

6. An alloy as claimed in claim 1 wherein the ratio of tantalum to tungsten is about 2 to 1.