US3466171A - Nickel-chromium-niobium alloy - Google Patents

Description

United States Patent US. Cl. 75-171 Claims ABSTRACT OF THE DISCLOSURE Nickel-chromium alloys containing niobium, substantial amounts of carbon and, advantageously, molybdenum, wherein the niobium, carbon, chromium and molybdenum constitutents of the alloys are carefully correlated to obtain good stress-rupture properties at 900 C. and higher.

This invention relates to nickel-base alloys capable of sustaining high stress at elevated temperatures for prolonged periods before fracture occurs and, more particularly, to nickel-chromium alloys, containing both niobium and carbon, which display these high strength properties.

. Most prior art nickel-base alloys depend for their hightemperature strength upon the formation of precipitates of phases containing nickel and also titanium or aluminum or both, commonly referred to as the gamma prime phase (gamma) represented by the formula Ni (Ti, Al). Other prior art alloys contain niobium and are strengthened, though less effectively, by a precipitated nickel niobide phase, and in yet others, phases of both kinds are present. In most of these alloys, however, the carbon content is kept low, below 0.2% or even less, in order to minimize the formation of carbides of the addition elements at the expense of the desired intermetallic phases. Such carbides are undesirable because in the temperature range in which the intermetallic phases are effective to harden and strengthen the alloys, carbides are less effective for this purpose.

As the temperature increases, the stress-rupture life of the alloys of the types above described decreases. This is due mainly to the instability of the intermetallic phase which tends to coarsen at elevated temperatures, thereby becoming less efiective to strengthen the alloys, and eventually to dissolve and, to a lesser extent, to loss of strength by the matrix. At temperatures above 900 C., the stressrupture life of even the strongest alloys hardened by inter- 1 metallic phases falls ofi rapidly.

Patented Sept. 9, 1969 It is an object of this invention to provide nickel-base alloys which maintain high strength at temperatures of 900 C. and above.

Another object of this invention is to provide nickelbase alloys which, when compared with nickel-base alloys hardened by precipitation of the gamma prime phase, do not appreciably harden on cooling from high temperatures.

Generally speaking, the present invention contemplates alloys comprising (percent by weight) about 2% to about 6% niobium, about 0.3% to about 0.5% carbon, about 19% to about 33% chromium, an amount of molybdenum such that:

(i) When the chromium content is less than 24% the molybdenum content is at least that given by the express1on percent Mo=% (24-percent Cr) (ii) When the chromium content does not exceed the molybdenum content is from O to that given by the expression percent Mo=3.5 +0.5 (SO-percent Cr) and (iii) When the chromium content is greater than 30% the molybdenum content is from 0 to about 3.5%, up to about 0.01% boron, up to about 0.1% zirconium, up to about 3% iron, up to about 2% cobalt, up to about 2% silicon, up to about 1% manganese, and the balance being essentially nickel with residual impurities and deoxidants, e.g., magnesium and/ or calcium, in ordinary amounts which do not affect the basic characteristics of the alloys.

--Afurthe1' disadvantage of alloys containing titanium-..

and aluminum is that upon cooling from hot-working temperatures the alloys harden very rapidly. Hence, when the alloys are to be used in the form of sheet they must be quenched after hot working in order that the sheet may be soft enough to be conveniently fabricated by'cold forming. It is very difficult to quench the sheet material successfully without distortion occurring, especially in large In accordance herewith we have found that niobium contributes importantly to the stress-rupture strength of the alloys. For adequate stress-rupture life at least 2.0% niobium is required, while preferably at least 2.5% is present and most advantageously at least 3.5%. Increasing the niobium content above the maximum of 6%, however, leads to a rapid drop in stress-rupture strength, and the impact strength of the alloys also decreases somewhat as the niobium content increases.

Carbon is of great importance in the alloys, since it plays a large part in controlling the hardening and strengthening mechanisms. As the carbon content is increased above 0.2%, the stress-rupture life rapidly increases, but at the same time the impact strength decreases and at carbon contents above 0.5% it becomes inadequate.

To obtain the benefit of the strengthening effects of niobium and carbon, the contents of chromium and molybdenum must be correlated. Chromium contributes to the stress-rupture strength of the alloys, and in the absence of molybdenum the alloys must contain at least 24% chromium. Chromium also contributes: to corrosion resistance, and if the highest corrosion resistance is required the chromium content should be high, i.e., at least 28%.

' On the other hand with increasing chromium contents the impact strength decreases and if the highest impact strength is important, the chromium content should not exceed 25%. Alloys with high chromium contents are also diflicult to work, and if the chromium content exceeds 33% they are unforgeable.

We have surprisingly found that although molybdenum does not improve the stress-rupture strength of alloys that are free from niobium, it is very effective in the niobium-containing alloys of the invention. When the chromium content is less than 24% it must be present, and at all chromium contents at least 1% molybdenum is preferred. The effects of chromium and molybdenum are complementary, and as the chromium content increases the amount of molybdenum needed to achieve a given stress-rupture strength decreases. Excessive amounts of molybdenum cause the stress-rupture strength to fall again, and the maximum amount that may be present decreases with increasing chromium content in accordance with the formula percent Mo=3.5+0.5 (30percent Cr) until at chromium contents of 30% and above the molyb- Despite the relative insolubility of the hardening phases at high temperatures, enough can be dissolved for substantial further precipitation to occur on subsequent aging at a lower temperature and, in order to develop the stressrupture properties to the fullest extent, the alloys require dehhlh content h not exceed The Optimum 5 heat treatment by solution-heating and aging. The higher Stress-rupture Propertles are Obtained When the Chromium the temperature of solution heating, the better the stressl molybdenum Contents are Correlated in accordance rupture properties after aging. Advantageously the solu- Wlth the formula tion heating temperature is at least 1125 C., e.g., 1150 Pement -p Cr) C. or even 1250 C., but it should not be so high that I contrast to alloys Strengthened by intermetauic incipient melting occurs. A suitable period of heating is phases containing titanium and aluminum, the stressfrom h' h to elght hours though longer Perlods may rupture properties of the alloys of the invention are not be used If deslredimproved by additions of boron or zirconium. If desired, The alloys may be coolhd from h solhhoh'heahhg however, th elements may be present in amounts up to temperature at any convenient rate either directly to the 0.01% boron 1% Zirconium or both e'g" 0 0O5% aging temperature or to alower temperature, conveniently boron and 0.03% zirconium. Nevertheless such additions room temperature, followed by reheating to the aging impair th ld bi i f the alloys, so boron and Zip temperature. The rate of cooling from the solut1on-heato i are r f bl not added in making Sheet and ing temperature has l1ttle effect upon the hardness, and other fo where good Weldability is important even after a1r-cool1ng to room temperature the hardness As to the other constituents, iron and cobalt impair the of the alloys 1n the form for ample, of mch hlameter stress-rupture lives of the alloys. For this reason the bar or Sheet is generally low enough to Permlt heavy amount thereof should not exceed 3% iron and 2% machining of h bar sgtock and cold'forming of h cobalt respectively and advantageously should not be sheet; Q F 1f the hlghest stmss'rupthre l 15 above i it 1 1 Pr f bl both are Substantially required t 13 advantageous to C001 the alloys rapldly from absent, Sili i i Weldability and if good Weldability the solution-heating temperature, e.g., by water-quenchis req ir d th ili Content Should not exceed If lng. Aging is suitably performed in the range of 750 C. weldability is not of importance, the silicon content may to 10600 C, -i C, for about 1 to 48 hourshe as hi h as 05% or even 1% or 2% Up to 1% The duration of aging depends on the temperature, and ganese may b present as an impurity. at 850 C. is conveniently 16 hours.

To facilitate hot working the alloys it is desirable that For the Purpose Of Y g those il n the art a they should nt i ll id l amounts f magnesium ter understanding of the 1nvent1on, the results of numerous or al i up to Such amounts are tests carried out on alloys of different composit1ons both monly re ent afte th use th r f as deoxidants in air within and without the invention are reproduced in several meltin nd if Vacuum-melting i employed it is also 35 tables shown below. As is reflected by these results, the advantageous t dd h b f Casting the alloy good stress-rupture propertles of the alloys of this inven- An advantageous combination of results is obtained hon depend on a delicate balance among the alloylng when the alloys contain about 24% to about 33% chroelements- The alloys used were Prepared y air-melting miurn ab t 03% to about 05% carbon about to with a conventional addition of magnesium or calcium as about 6% niobium, abo t 1% t b t 35% 1 b a deoxidant, casting to ingots, forging to /8 inch diameter denum, up to bout 0 ()05% b up to about 3% bars, solution-heating for 2 hours at 1150 C., air-cooling, Zirconium, up to about 0.3% silicon, up to about 1% and finally aging for 16 hours at Suitable test manganese, up to about 0,035% magnesium, up to about samples were thereafter machined from the aged bars. 0.035% calcium, and the balance essentially nickel. A stl'ess-l'uptum Properties Were dfilefmined at and particularly advantageous alloy has the composition 30% 'py impact Strengths Were determined at room Chromiurn, 5% niobium, 2.5% molybdenum, 0.4% carperature after soaking for 1,000 hours at 850 C. to reveal bon and the balance essentially i k l any tendency of the alloys to embrittle in service.

The hardening phases that form in th alloys f r Table I contains stress-rupture and impact data for invention remain largely undissolved at temperatures at alloys containing y g amounts of niobium and y which the gamma phase dissolves in prior art nickeldenum with each alloy nominally containing 30% chromium alloys hardened by titanium and aluminum. chromium and 0.4% carbon, the balance being nickel. Moreover, the hardening phases of our alloys are not Alloys Nos. 1 to 4 are alloys according to the invention completely dissolved even at temperatures only slightly whereas Alloy A, which contained no niobium, and alloys below the melting-point of the alloys. Thus, the alloys in B and C, which nominally contained 7% niobium, are not.

TABLE I Stress-rupture properties 3 t.s.i./900 c. 2 t.s.i./900 C.

Impact Nh Mo Life Elong. Life Elong. strength Alloy N 0. (percent) (percent) (hr.) (percent) (hr.) (percent) (it.lb.)

0 0 167 3 0 151 687 24 5 0 492 23 2,063 26 11 7 0 9s 51 2. 5 2. 5 2,145 30 20 5 2. 5 s54 27 1, 063 0. 7 (i 7 2.5 86 23 1 Test discontinued because of very low creep rate.

accordance herewith retain high strength at temperatures of 900 C. and above. In addition, the alloys do not harden as rapidly and as extensively on cooling from high temperature as do the alloys hardened by gamma phase.

Alloys A and B, in which the niobium contents were respectively lower and higher than are required by the invention, had much inferior stress-rupture lives in comparison with Alloys Nos. 1 and 2, and Alloy C was likewise much inferior to Alloys Nos. 3 and 4. Comparison of Alloy No. 2 with Alloy No. 1 and of Alloy No. 4 with Alloy No. 3 also illustrates the concurrent increase in stress-rupture life and decrease in impact strength that results from increasing the niobium content.

The role of carbon is shown by the results reported It is essential that the alloy contains both niobium and carbon in order to form strengthening phases rich in niobium.

Although no reliance is placed upon any particular theory to explain the invention, it is believed that the in Table II relating to alloys containing 30% chromium 5 good stress-rupture properties possessed. by the alloys of with varying amounts of carbon, niobium and molybour invention at temperatures of 900 C. and above are denum, the balance being nickel. Alloys Nos. 1, 2, 3 and due to the presence in dispersed form of niobium car- 4, being examples of theinvention, are reproduced for bide NbC, with or without the nickel niobide Ni Nb. The purposes of the comparison with Alloys D, E, F, G and 10 phase Cr C is frequently present and may also cooperate H which are not examples of this invention since the carwith the niobium-rich phases to strengthen the alloys, bon content is either too low or too high. though it is not very effective alone at high temperatures.

TABLE II Stress-rupture properties 3 t.s.i./900 o. 2 t.s.i./900 0.

Impact C -Nb Mo Life Elong. Life Elong. strength Alloy N (percent) (percent) (percent) (hn) (percent) (hr.) (percent) (in-lb.)

1:. f. Q AlloyNo.

The importance of the presence of niobium-rich phases is shown by the results in Table IV, in which niobium-free Alloys A, M, N and J are hardened by Cr C alone and have relative poor stress-rupture lives and are, therefore, not within the scope of this invention. In contrast, Alloy No. 2 contains 5% niobium and is strengthened by the precipitation of the niobide phases, NbC and Ni Nb, and.

is, therefore, an alloy within the teachings of this invention.

TABLE IV Stress-rupture prop erties, 2 t.s.i./900 C.

Nb Mo W Life Elong. Phases (percent) (percent) (percent) (hr.) (percent) present 5 0 0 2, 063 26 NbC, Ni Nb ClzgCn 0 0 0 167 ClzaCo 0 0 5. 0 187 38 CmCu 0 2. 5 2. 5 479 CrzaCe 0 4. 9 0 118 24 CIzaCo (neither of which is in accordance with the invention) shows the ineffectiveness of a molybdenum addition in the absence of niobium. The series of Alloys 2, 4 and K and the series Nos. L, 6, 7 and8 show the elfects of increasing the molybdenum content at chromium contents of 30% and respectively: all these alloys are examples of the invention except Alloy K, which contains too much molybdenum with chromium, and Alloy L, which contains only 20% chromium and no molybdenum. Each of these had poor stressrupture properties. The elfect of varying chromium alone is shown by comparison of wise much inferior to Alloys Nos. 3 and 4. Comparison of Alloys 2, 5 and L.

It is worthy of mention that tungsten is not equivalent to molybdenum in the alloys and in. addition is never beneficial and frequently detrimental to both the stress-rupture.

TABLE III Stress-rupture properties 3 t.s.i./900 C. 2 t.s.l./900 C.

Im act Cr Nb Mo Life Elong. Life Elong. strerigth Alloy No. (percent) (percent) (percent) (hr.) (percent) (hr.) (percent) (it.lb.)

TABLE V Stress-rupture properties, 3 t.s.l./900 c.

Nb Mo W Life Elong. strength Alloy No. (percent) (percent) (percent) (hr.) (percent) (it.lb.)

TABLE VI Stress-rupture properties, 3 t.s.i./900 C.

Nb Ta Mo Elong. Alloy No. (percent) (percent) (percent) Life (hr.) (percent) Tantalum should therefore not be added to the alloys,

though it may be tolerated as an impurity in the amounts commonly found as an impurity in commercial sources of niobium, i.e. in amounts up to about one-tenth of the weight of niobium.

The good corrosion-resistance of an alloy according to the invention is shown by the results of a test in which a specimen of Alloy No. 2 was half-immersed for 16 hours in a molten bath of a mixture of 25% sodium chloride and 75% sodium sulphate at 900 C. The loss in weight of the specimen was only 14 mg./cm.

The advantageous properties of the alloys of the invention make them suitable for use at elevated temperatures which may exceed 900 C., in the form of sheet and components fabricated therefrom. They are also useful for articles and parts that require a combination of strength and resistance to corrosion at such temperatures, for example as parts of heat-treatment furnaces, furnace belts, e.g., wire mesh belts, trays for articles to be heat-treated, and the like.

We claim:

1. An alloy, age-hardenable by precipitation of at least one niobium-rich phase, and consisting essentially of about 2% to about 6% niobium, about 0.3% to about 0.5% carbon, about 19% to about 33% chromium, and amount of molybdenum such that when the chromium content is less than 24% the molybdenum content is at least (24-percent Cr) percent; when the chromium content does not exceed 30% the molybdenum content is from 0 to 3.5+0.5 (30percent Cr) percent; and when the chromium content is greater than 30% the molybdenum content is from 0 to 3.5%, up to about 0.01% boron, up to about 0.1% zirconium, up to about 3% iron, up to about 2% cobalt, up to about 2% silicon, up to about 1% manganese, and the balance essentially nickel.

2. The alloy set forth in claim 1 and containing at least 2.5 niobium.

3. The alloy set forth in claim 2 and containing at least about 1% molybdenum.

4. The alloy set forth in claim 3 in which boron does not exceed about 0.005% and zirconium does not exceed 0.03%.

5. The alloy set forth in claim 4 in which silicon does not exceed 1%.

6. The alloy recited in claim 1 and containing about 24% to about 33% chromium, about 2% to about 6% niobium, about 1% to about 3.5% molybdenum and about 0.3% to about 0.5% carbon.

7. The alloy set forth in claim 6 in which the chromium content is at least 28%.

8. The alloy set forth in claim 7 in which silicon does not exceed 1%.

9. The alloy set forth in claim 6 in which the niobium content is at least about 3.5%.

10. The alloy set forth in claim '6 and containing about 30% chromium, about 5% niobium, about 0.4% carbon, about 2.5 molybdenum and the balance essentially nickel.

References Cited UNITED STATES PATENTS 2,587,275 2/1952 Bash -171 2,994,605 8/1961 Gill et a1 75l71 3,046,108 1/ 1962 Eiselstein 7517l 3,069,258 12/1962 Haynes 75-171 3,046,108 7/ 1962 Eiselstein 75--171 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 148162 1313;" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3iu6l7l Dated September 9: 9 9

Inventor(s) Alfred John Fletcher and Edward Gordon Richards It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 5, line 60, delete entire line reading "wise much inferior to Alloys Nos. 3 and 1 Comparison of" Column 6, line 32, for "relative", read -relatively-.

Column 6, line 57, for "contain", read --contains-.

Column 7, line 50, for "and", read --a.n-.

sibwial mu, SEALED Mil-M12.

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