CA1063836A

CA1063836A - Oxidation resistant ni-cr-al-y alloys and methods of making the same

Info

Publication number: CA1063836A
Application number: CA229,391A
Authority: CA
Inventors: Robert B.H. Herchenroeder; Howard J. Klein
Original assignee: Cabot Corp
Current assignee: Cabot Corp
Priority date: 1974-06-17
Filing date: 1975-06-16
Publication date: 1979-10-09
Also published as: FR2284683A1; GB1512984A; FR2284683B1; DE2526683A1; IT1036303B; JPS5133717A; SE7506883L

Abstract

ABSTRACT OF THE DISCLOSURE
An oxidation resistant Ni-Cr-Al-Y alloy having improved high temperature stress rupture strength and improved weldability, consisting essentially of 14-18 w/o Cr, 4.5-5.5 w/o Al, a small but effective amount to 0.04 w/o Y, the balance being predominantly nickel with usual modifiers and impurities.
The alloy may be melted by electroslag techniques.

Description

This invention relates to oxidation resistant Ni-Cr-Al-Y alloys and methods of making the same and more particularly to an alloy having improved stress rupture life, weldability, hot workability, and oxidation resistance through precise control of yttrium content and to special processing techniques for making -the same ~
The technology of today, and especially that of the .
gas turbine industry, continuously demands engineering materials -which will withstand for prolonged periods of time exposures to ~
highly oxidizing and corrosive atmosphere without serious de- .
gradation in properties. One of the newer materials is an alloy ;
consisting essentially of about 15-17 Cr, 4 5-6 AI, 0 1 to 0.3 Y, .
balance nickel. This alloy has ~xceptionally good resistance to ;~.
oxidation at temperatures as high as 2200F Unfortunately, the :
alloy has poor weldability, poor hot workability, and only ~ ~:
moderate stress rupture strength . .
We have found that by precise control of yttrium within the range of a small but effective amount but less than about 0.0~ w/o and preferably less than about 0.03 w/o a significant . :
improvement in stress rupture life can be achieved, and the alloy :
becomes more weldable, has less intergranular attack during ex-posure to high temperatures and yields more usable material after .
hot working operations ~ The achievement of consistent levels of about 0.02 w/o : yttrium in the alloy posed significant difficulties because the element yttrium is highly reactive and will reduce or interact with most refractory furnace linings, as well as the atmosphere : during melting operations, In addition, even small variations ;
in the time elapsed between adding the yttrium to the bath and 30 casting the ingot, or fluctuations in bath temperature causes inconsistencies in the yttrium retention in the final product.
Surprisingly, electroslag melting of the alloy of the invention . 1 : , ~ , ~
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has provided a very consistent retained level of about 0.02 weight percent yttrium in the alloy regardless of the starting level of yttrium providing, of course, that the starting level of yttrium in the alloy exceeded 0 02 weight percent However, the alloy of this invention can be made by other melting techni-ques if properly controlled and accordingly the alloy is not limitea to electroslag melting practices In particular, the slag used during electroslag melting of the alloy of the invention must be of a relatively stable character so t~at near chemical equilibrium between the slag and the alloy and, in particular, the yttrium in the alloy can be establ~shed within the electroslag system at some point prior to t~e completion of the reaction metal oxi~e slag + Y_~ Y203 +
metal. A particularly useful slag in this respect is a slag composed predominantly of CaF2. It has been found that when a predominantly calcium fluoride slag is used in melting the alloy of the invention that the chemical reactions are such that the yttrium retained in the alloy is consistently about 0.02 weight percent, which is about the ideal amount to provide the best combination of rupture strengths, weldability and hot worka~ility.
O~ course, given the basic concept as herein disclosed, those skilled in the science of electro-chemistry will, undoubt-edly, be able to devise many workable variations to the basic theme by altering slag composition and the electrical settings of the melting unit, all within the scope of this invention.
While the concept of the precise control of yttrium and the benefits therefrom will be demonstrated with relatively simple system,, it iq obvious that the same basic facets will apply to far more complex alloy systems.
The ba~ic Ni-Cr-Al alloy is well known a~ descri~ed herein and other elements may be added in the ranges suggested a~ follows hereafter. ;

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;~ 3~ -To illustrate, molybdenum, rhenium, hafnium, tungsten, and/or tantalum are frequently added singly or in the aggregate to about 15 wJo to ba~ic alloy~ of the type descrihed to impart improved strength. Likewise, other elements are often purposely incorporated at level~ of less than about 0.5 weight percent combined to achieve improvements in strength, phase stability, ductility, or other desirable metallurgical attributes. Such - -elements are carbon, boron, magnesium, zirconium and calcium.
Silicon is often added to about 1 weight percent and manganese to about 2 weight percent for such reasons as importing improved molten fluidity or surface stability after casting. ---When strengt~ is of such importance that some degradation in oxidation resistance can be tolerated, titanium may be incorporated within the ~ystem to about 5 percent.
Because cobalt forms a complete series of solid solutions with nickel, and is used extensively in high temperature superalloys, up to about 20 weight percent cobalt will not alter the basic concepts herein described.
When strength and stability can be compromised in ..
favour of economics, high temperature alloys are often diluted with substantial amountq of iron,`and the alloy of this invention can be similar~y diluted with iron to a content of about 30 percent~
Thus according to the invention there is provided a nickel base alloy consisting essentially by weight of about 8 to 25%, particularly 14 t~ 2i%, chromium, about 2.5 to 8~o alumi-num, an effective amount up to 0.04% of yttrium effective to im~ ~
~; prove stress rupture life and weldability and to reduce inter- ~ , granular attack, up to about 15% of one or more members of the group consisting of molybdenum, rhenium, hafnium, tungsten, and tantalum, up to about 0.5% of one or more members of the group con~isting of carbon, boron, magnesium, zirconium and calcium, ~-up to about 1%
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~ilicon, up to about ~% manganese, up to about 2~/o cobalt, up to about 3~/0 iron, up to about 5% titanium and the balance nickel with usual impurities in ordinary amounts, the nickel being at least 4~/O of the weight of the alloy.
In a particular embodiment the alloy of the invention consists essentially, by weight, of about 14 to 17%, preferably about 16% of chromium, about 4 to 6%, preferably about 5% of - aluminum; an effective amount to 0.04%, preferably about 0.02%
of yttrium and the balance nickel with usual impurities in ordinary amounts, for example, not exceeding 2% in aggregate.
In another aspect of the invention there is provided a wrought article characterized by improved stress rupture life, improved weldability, raduced intergranular corrosion at high temperature~ and by improved yield~ and formed by working an alloy of the invention.
In yet another aspect of the invention there is pro-vided a method of making a nickel base alloy characterized by improved stress rupture life, improved weldability, reduced intergranular corro~ion at high temperatures and high yields and by an effective amount up to 0.04% of yttrium effective to improve stress rupture life and weldability and to reduce intergranular attack comprising the ~teps of: (a) melting an alloy of the composition of the invention but containing at lea~t 0.02 weight percent yttrium, (b) casting the alloy into an electrode for electroslag remelting, and (c) remelting the ;
electrode by electroslag techniques.
The beneficial effects of controlling yttrium content within the critical range of a small but effective amount and 0.04 w/o and preferably between a small but effective amount and 0.03 w/o yttrium and the benefits of controlling the yttrium content by electro~lag melting will become apparent from the following examples.

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EXAMPLE I

A llO-lb. master heat of the nominal composition 17 chromium, 0.4 silicon, 0.5 manganese, 5.0 aluminum, lowest pos- -sible carbon, balance nickel, was melted in a 150-lb. vacuum --induction unit. This heat was cast into five round tapered ingots weighing approximately 17-18 pounds each. Each of the ingots was then remeltbd in a smaller 20-lb. vacuum induction furnace and late additions of 0.02, 0.05, 0.10, 0.18 and 0.25 weight percent yttrium was added to each of the melts, The melts were cast into ingot molds and chemistry sample molds.
Portions of the ingots were heated for two hours at 2050F and then forged to approximately one inch thickness.
After conditioning the billet~3 by grinding to remove minor de-fects, the billets were reheated for two hours at 2050F and were hot-rolled to nominally l/8-inch thicknesses. Several reheatings were required.
The sheet~ were all stress relieved at 2050F, fan -cooled to aTr~ient temperature, descaled, ar~d then cold rolled to 0~075-inch thickness. A final heat treatment of 10 minutes at 2050F followed by fan cooling was given the sheets prior to again descaling and testing. In all processing, all material was handled simultaneously and in the same manner.
Chemical analyses of the materials are shown in Table I as follows: ~
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Complete analyses were obtained on the master heat and the first ~
and last of the remelted heats. Yttrium was analyzed in each ; ~:
instance. Because the chromium, manganese, aluminum and silicon .
did not vary substantially in those heats checked, it can be assumed these elements did not vary in the heats 258, 259 and 260.
To demonstrate the effects of yttrium content on the stress rupture properties of the system, duplicate tests were performed at 1500F and at a stress of 15KSI.
- 10 The results of these stress rupture tests are tabulated in Table II and are shown graphically in Figure I.

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Unfortunately, the results from the second test of heat 258 (122 hours life) had ~o be discounted because of equip-ment problems, but the balance of the data show clearly the inverse relationship of stress rupture life and yttrium content.
Further, there appears to be a "~nee" in the curve of yttrium ; content versus stress rupture life at about the 0.04 weight per-cent yttrium level. The amount of degradation below 0.04 w/o yttrium per unit of yttrium present appears to be less than at higher levels of yttrium The decrease in elongation with de-creasing yttrium content shown in the table is expected because of the longer exposure at temperature these samples had.
; Using the same welding parameters of voltage, amperage, gas flow etc., attempts to fusion weld the various materials by TIG techniques were made. In each instance, those alloys of the series containing greater than 0.04 ~ developed extensive center line cracking in the weld bead to such an extent that they were considered non-weldable, whereas the material containing 0.01 Y
was welded with the minimum amount o~ cracking.
From another portion of the sheet, duplicate specimens ` 20 nominally 1/16-inch thick x 3/8-inch wide x 3-inch long were prepared from each alloy of Table I. All surfaces were uniformly ground with a 120-grit belt. These were then tested in a dynamic oxidation rig at a temperature of 2100F for 400 hours. Aviation kerosene (A640? was used for fuel, air-to-fuel ratio was set at -~
54:1; gas velocity was approximately 240 MPH, the specimens were rotated in a holder at 25 RPM, and the specimens were cycled to near room temperature each one-half hour by an automatic device which lowered the specimens from the hot zone and blasted them -- with room temperature air.
Weight changes versus exposure time curves for the alloys are plot~ed in Figure 2. (Data for heat 258 (0.04 Y) after 220 hours is not reported because the samples were mechani-cally damaged at that time).

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The data show that the alloy (257) with the lowest yttrium content has at least equivalent oxidation resistance to the higher yttrium bearing sheet. In fact, the data indicate that alloy 257 oxidizes with a parabolic rate whereas the alloys with the higher yttrium levels appear to be oxidizing at a near linear rate.
Scanning electron microscopy of the scales on the sur-faces of the specimens tested did show a distinct difference in the predominantly aluminum oxide morphology. Whereas, the scales on the alloys containing greater than 0.04 Y appeared to consist of relatively large platelets, the scale on the alloy containing only 0.01 Y seemed to have much smaller crystals of an equiaxed nature. ;
Tensile tests were performed on the alloys at room temperature and 1600F. These data are shown in TABLE III and ." .
indicate that there is no significant variation in tensile pro-perties as a result of the variation in yttrium content. ;~

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, EXAMPLE II
A 100-lb. melt was prepared by vacuum induction melting techniques and was cast into a 4-inch dia. x 25-inch long elec-trode for remelting by the electroslag procedure into a 6-inch diameter ingot. For remelting a CaF2 slag, 3000 amperes of cur- ~ ~
rent and 30 volts were used. Overall melt rate was calculated ~ -at 2.1 pounds per minute.
The chemical analyses obtained before and after remelt-ing were in weight percent:
A1 C Cr Mn ~i Si Y Heat ... . .
Before5.06 0.047 16.40 0.15 Bal. 0.13 0.21 744 After 5.30 * 16.25 * Bal. 0.15 0.02 : : -* ~ot Re-analyzed The resulting ingot was hot forged and rolled at 2050F
followed by a stress relief heat treatment at 2050F and fan cooling. This material was cold rolled about 20 percent to final thickness, re-heat treated at 2050F and fan cooled.
This electroslag remelted stock was then compared to sheet obtained by vacuum induction melting and secondary melting - 20 by vacuum arc remelting. All other processing was the same.
Chemical analysis of this heat before and after remelting was :
in weight percentO

Al C Cr Mn Ni Si Y Heat Before 5.2 0.05 16.57 0.15 Bal 0.12 0,09 746 After 5.3 * 17.07 * Bal. 0.13 0.10 -- * ~ot Re-analyzed In welding tests using the TIG method, heat 744 could be successfully joined to other nickel-base alloys using Inconel Alloy 600 filler wire, whereas, heat 746 could not be success-fully joined to other alloys because of extensive cracking along ~ v the centerline of the weld bead. In static oxidation tests at 2100F., the average metal lost per surface was 0.08 mils for _ 12 _ ~, ' ''.

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heat 744 and 0.07 mils for heat 74~. However, when the inter-granular and subsurface attack was measured by metallographic techniques, the oxide penetration on the samples of heat 744 was less than 0.1 mil whereas the penetration along the grain boun- .
daries of heat 74~ was 0.3 mil Average tensile properties at 1600F for the two alloys were:

: 0 2% Offset Yield Ultimate Strength Elongation Heat 1000 Psi Psi % . .. -744 47.8 66.2 6 3 :~
; 10 746 43 1 66 1 6.6 EXAMPLE III
Another heat was prepared by the vacuum induction electroslag technique described in E.xample II to demonstrate the reproductibility of the means of controlling yttrium to the pre- .
cise level of the preferred range of this invention.
Chemical anal~ses before and after electroslag remelt-ing are as follows in weight percent Al C Cr Mn ~i Si Y Heat :~:
Before 5 28 0.04 16.40 0.14 sal 0.11 0 21 745 . .
-~ 20 After 5 25 * 15.69 * Bal 0.10 0 02 . * ~ot Re-analyzed . It is readily observed rom these data that significant advantages can be achieved in alloys of the type described by carefully limiting the yttrium content to betwe~n a small but effective amount and less than 0.04 and preferably less than ~.
about 0.03 weight percent. Furt~er, it has been shown that the described technique of electroslag melting is an effective means of achieving a preferred level of about 0.02 weight percent .. ~ ~.
yttrium in the final product.

In the foregoing specification we have set out certain preferred practices and embodiments of our invention, however, it will be understood that this invention may be otherwise embodied within the scope of the following claims .i. .~,

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A nickel base alloy consisting essentially by weight of about 14 to 25% chromium, about 2.5 to 8% aluminum, an effective amount up to 0.04% of yttrium effective to improve stress rupture life and weldability and to reduce inter-granular attack, up to about 15% of one or more members of the group consisting of molybdenum, rhenium, hafnium, tungsten and tantalum, up to about 0.5% of one or more members of the group consisting of carbon, boron, magnesium, zirconium and calcium, up to about 1% silicon, up to about 2% manganese, up to about 20% cobalt, up to about 30% iron, up to about 5%
titanium and the balance nickel with usual impurities in ordinary amounts, the nickel being at least 40% of the weight of the alloy.

2. An alloy as claimed in claim 1, wherein the content of chromium is about 14 to 17%.

3. An alloy as claimed in claim 1 or. 2, having a yttrium content of an effective amount to 0.03%.

4. An alloy as claimed in claim 1 or 2, having a yttrium content of about 0.02%.

5. An alloy as claimed in claim 1, consisting essentially by weight of about 14 to 17% chromium, about 4 to 6% aluminum, an effective amount to 0.04% yttrium and the balance nickel with usual impurities in ordinary amounts.

6. An alloy as claimed in claim 1, consisting essentially by weight of about 16% chromium, 5% aluminum, 0.02% yttrium and the balance nickel with usual residual materials in ordinary amounts not exceeding 2% in aggregate.

7. An alloy as claimed in claim 1, 2 or 5, having been made by electroslag remelting an alloy electrode containing in excess of 0.02 weight percent yttrium.

8. An alloy as claimed in claim 6, having been made by electro-slag remelting carried out with a slag composed essentially of CaF2.

9. A method of making a nickel base alloy characterized by improved stress rupture life, improved weldability, reduced intergranular corrosion at high temperatures and high yields and by an effective amount up to 0.04% of yttrium effective to improve stress rupture life and weldability and to reduce intergranular attack, comprising the steps of:
(a) melting an alloy consisting essentially of, by weight, about 14 to 25% chromium, about 2.5 to 8%
aluminum, up to 15% of one or more members of the group consisting of molybdenum, rhenium, hafnium, tungsten and tantalum, up to about 0.5% of one or more members of the group consisting of carbon, boron, magnesium, zirconium and calcium, up to about 1% silicon, up to about 2% manganese, up to about 20% cobalt, up to about 30% iron, up to about 5% titanium and the balance nickel with usual impurities in ordinary amounts, the nickel being at least 40% of the weight of the alloy, and at least 0.02 weight percent yttrium, (b) casting said alloy into an electrode for electro-slag remelting, and (c) remelting said electrode by electroslag techniques.

10. A method as claimed in claim 9, wherein said alloy in (a) contains about 14 to 17% by weight of said chromium.

11. A method as claimed in claim 9 or 10, wherein the remelting step is carried out in the presence of a slag con-sisting essentially of CaF2.

12. A wrought article characterized by improved stress rupture life, improved weldability, reduced intergranular corrosion at high temperatures and by improved yields and formed by working an alloy consisting essentially of about 14 to 25% chromium, about 2.5 to 8% aluminum, an effective amount to 0.04% of yttrium effective to improve stress rupture life and weldability and to reduce intergranular attack, up to about 15% of one or more members of the group consisting of molybdenum, rhenium, hafnium, tungsten and tantalum, up to about 0.5% of one or more members of the group consisting of carbon, boron, magnesium, zirconium and calcium, up to about 1% silicon, up to about 2% manganese, up to about 20% cobalt, up to about 30% iron, up to about 5% titanium and the balance nickel with usual impurities in ordinary amounts, the nickel being at least 40% of the weight of the alloy.

13. A wrought article as claimed in claim 12, wherein the content of said chromium is about 14 to 17% by weight.

14. A wrought article as claimed in claim 12, formed by working an alloy consisting essentially of about 14 to 17%
chromium, and 4 to 6% aluminum, an effective amount of 0.04%
yttrium and the balance nickel with usual impurities in ordinary amounts.

15. A wrought article as claimed in claim 12, formed by working an alloy consisting essentially of about 16% chromium, 5% aluminum, 0.02% yttrium and the balance nickel with usual residual materials in ordinary amounts not exceeding 2% in aggregate.