DE1921359B2 - Process for increasing the ductility at high temperatures of cast nickel-based alloys - Google Patents

Description

The invention relates to a method for increasing the ductility in the range from 704 to 871 ° C of cast nickel-based alloys, which are melted from a batch with no more than 5% return scrap, in which the / phase is the basic phase in the crystalline matrix available and which are suitable for use under stress at temperatures up to 1038 ⁰ C.

A number of nickel-based alloys of sufficient strength for use at temperatures in the range of 926 ° C to 1067 ° C are known. These alloys have been proposed for use in hot stage gas turbines and have been adapted for this application. In the past few years, nickel-based alloys containing chromium, cobalt, molybdenum, tungsten, niobium, tantalum, aluminum, titanium, carbon, zirconium and boron have been the focus of alloy development in this field. In general, the morphology of such alloys comprises a grid structure with certain phases dispersed in the bulk of each crystal and at the crystal boundaries. In the recently developed nickel-based alloys for use at extremely high temperatures, the basic phase of the crystalline matrix consists of a / phase which is likely to consist essentially of NijAl, with nickel partly replaced by cobalt, chromium, iron etc. and aluminum can be partially replaced by titanium, niobium and other elements. Mainly among the intra-granular and cone-boundary phases, there are various formulas of carbide phases, generally defined by the formulas MC, M ₂ SC ₆ and M ₆ C. As a general rule, it can be said that the carbide phase contributes to the reinforcement of the alloy without affecting one Loss of ductility is significantly involved when the carbon phases form relatively small particles and are generally distributed throughout the basic structure of the alloy crystals. It is also a general rule that the carbides cause significant adverse effects such as B. Loss of ductility, when the carbide phases are preferentially deposited in the areas of the grain boundaries. This feature is not limited to carbide phases are deposited in areas of the grain boundaries, contribute to a loss of ductility.

In general, as the strength of the alloys increases (usually by increasing the additive elements), the ductility decreases. The metallurgists have learned to accept a compromise between strength and ductility. The alloys specified and specifically described below represent commercially acceptable compromises between strength and ductility. However, the alloys described here are not simple compromises in the sense that a relatively uniform change in ductility is expected for a given strength at a given temperature got to. It has long been known that most nickel-based alloys have strength properties sufficient that the alloys can be used at temperatures of about 926 ° C. and higher, but that, as is commonly expressed, they have a ductility trough at slightly lower Temperature, usually around 760 ° C. If one plots the tensile elongation against the temperature, the usual curve shows only a small change between room temperature and about 704 ° C. Between 704 ⁰ C and about 871 ° C

the ductility curve shows a minimum, and this minimum usually has a value below that for the percent elongation measured at room temperature a technically acceptable elongation is achieved

Recently there has been a very surprising phenomenon with a certain high temperature alloy, especially found in terms of ductility at 760 ° C. In the usual way this became special Alloy based on test results obtained using normal size cast Test specimens were obtained, selected and developed After the examination with the normal, technical As usual test procedures, these cast-to-size specimens showed all signs of a technical acceptable ductility in the range of about 760 ° C. However, if turbine metal parts, such as e.g. B. turbine blades, molded in a conventional manner showed the obtained castings often a ductility at around 760 ° C that was worse than desired.

Further investigations of the samples separated from metal parts showed that a mass effect occurs which has an adverse effect on the ductility in the critical area at about 760 ° C. It has been found that samples which have been separated from the actual part cast to size very often Much lower values of the percentage elongation and the percentage reduction in area in the tensile and creep test at 760 ° C gave than the values obtained in tests on similarly sized elongation and creep specimens. From a practical point of view, it is the job of a metallurgist to make a metal part that has good technical properties. The fact that the properties measured on normal size cast samples are not representative of the properties of the actual metal parts is an unpleasant and difficult problem, to which is added the problem posed by the presence of the known ductility trough about 760 ⁰ C is caused.

In FR PS 12 27 686 it is described that when certain nickel alloys are cast in such a way that the molds are poorly fed, which is especially true when casting high-temperature alloys of the FaI! may be, the castings become porous and the strength of the alloy is reduced. This problem was only perceived with poorly fed castings. According to the French patent, the addition of zirconium in amounts of 0.2 to 1% and in particular 0.5% to the alloys is intended to neutralize the loss of strength and to eliminate the porosity. These are nickel-chromium-cobalt alloys with a content of 11 to 15% chromium, 5 to 15% cobalt, 5 to 10% tungsten, 3 to 6% aluminum, 1.5 to 5% titanium, 0, 05 to 0.20% carbon, 0.003 to 0.08% boron and 0.2 to 1.0%, preferably 0.5%, zirconium. The zirconium can be completely or partially replaced by hafnium according to the equation 1.0%> Zr + Hf / 2> 0.2%. In tests of the French Pateniischrift was determined according to that ^e: n addition of 5% zirconium, the ductility lowers even in well-fed castings made of the alloy, so that such alloys for the production of highly heat-resistant castings that have adequate creep under load is unsuitable .

British patent specification 9 43141 relates to a Process for the heat treatment of wrought nickel alloys, including for the manufacture of gas turbine machines should be used. The heat treatment is to be applied to wrought nickel alloys which can have the following composition:

aluminum 0.1 to 9.0% titanium 0.1 to 64% cobalt 0 to 30% chrome 5 to 30% molybdenum 1.0 to 15% tungsten 0 to 15% niobium 0 to 7% hafnium 0 to 8% Tantalum 0 to 5% Vanadium 0 to 6% boron 0 to 03% zirconium Obis 1.2% carbon 0.01 to 0.3 « manganese Obis 1.0% Silicon Obis 1.5% iron 0 to 5.0% beryllium 0 to 0.5% nitrogen Obis 0.15% copper 0 to 0.9% Rare earth 0 to 0.2% sulfur 0.01 max. phosphorus 0.02 max. Calcium 0.08 max. magnesium 0.15 max. Remainder nickel (at least 35%)

The British patent does not provide the solution to the problem of improving the ductility of nickel alloys. The heat treatment of the known wrought alloys is intended to stabilize them by reducing the migration of the intermetallic compounds at the crystal boundaries of the alloy during aging. The creep correspond to the creation of cast alloys for the production of highly heat-resistant castings, the special requirements with respect to at a temperature of 760 ⁰ C under a load, the British Patent can not be removed.

The object of the invention is to create a method for increasing the ductility at high Temperatures for nickel-based alloys that contain no more than 5% return scrap and Can be processed directly into high-temperature castings and in particular for the production of high-temperature gas turbine parts are suitable.

This object is achieved according to the invention by a method for increasing the ductility at temperatures in the range from 704 to 871 ⁰ C of cast alloys based on nickel, which are melted from a batch with no more than 5% return scrap, in which the y'- phase is present as a base phase in the crystalline matrix, and which are suitable g.-for use under stress at temperatures up to 1038 ⁰ C, consisting of 0.02 to 0.20% carbon, 7 to 13% chromium, up to 8% molybdenum, up to 6% tantalum, up to 14% tungsten, up to 35% cobalt, 4 to 7% aluminum, 0.5 to 6% titanium, up to 3% niobium, up to 1.5% vanadium, 0.002 to 0.02% boron, 0 .01 to 0.20%

Zirconium, the remainder at least 36% nickel and up to 2% of the usual manufacturing-related impurities, which is characterized in that the melted charge 0.5 to 4% hafnium after deoxidation be admitted.

In the present context, all Percentages based on weight.

From US patent specification 30 05 705 are nickel-chromium-iron alloys and cobalt-nickel-chromium alloys are known. These are wrought alloys, which are used in the manufacture of forged goods.

In US patent specification 30 05 705 these alloys are referred to as high temperature alloys, which can be used at temperatures of about 650 to 870.degree. From the US patent “” ΙΐΛΐ-ιιηι · eAnfX / -ti nc · η I omarimnon Ar-ktoKlioKn Μαπιτοπ

of iron or chromium, which is considerably higher than that of the alloys obtained according to the invention allowable quantities are. If aluminum is present in the known alloys, it is in significantly lower amounts than in the case of the cast alloys according to the invention.

Because of these differences, the alloys described in US Pat unsuitable for use at much higher temperatures, i.e. H. Temperatures up to 1038 ° C, for which the alloys obtained according to the invention, however, are very suitable. Even at low Temperatures, such as 815 ° C, are significantly below the strength values for the known alloys those for the cast alloys according to the invention. The limits of the ranges of the compositions as well as the requirements for the nickel-based cast alloys of the invention differ basically from those for the known wrought alloys. These differences lead to several Properties and uses of the known alloys compared to the according to the invention. Because of these fundamental differences in composition and properties was - even with knowledge of US patent specification 30 05 705 - the effect of the addition of Hafnium to the alloys according to the invention cannot be foreseen.

From US patent specification 30 05 705 information about the effect of the addition of hafnium is only for one Alloy can be seen which has large amounts of cobalt, chromium and iron and only 30% nickel, however does not contain aluminum and has no / phase.

It is because of this single example that the US patent claims, but neither shows nor shows demonstrated that the addition of hafnium also reduces the dictility of other alloys, such as. B. from the alloy C disclosed in US Pat. No. 30 05 705, but in no way soft Requirements relating to the composition of the alloys according to the invention meet, improve would.

However, it was found that the addition of 1% hafnium in an alloy, soft, the composition of the alloy C according to the US patent, had a loss of fracture life and a reduction caused eggs ductility at 750 ⁰ C and a strength of 45.7 kg / mm ² .

In the method according to the invention, the composition of the cast alloys becomes particularly so controlled that in addition to the usual / phase also eutectic or primary / phase is formed. These eutectic precipitates are interlinked and thus prevent the formation of cracks on the Grain boundaries. At the same time, in the case of the casting alloys to be used according to the invention, the training of Chinese characters resembling carbide precipitations of the constitution MeC prevented and this rather it was eliminated in a more spherical constitution.

The drawings serve to explain the invention, namely shows

F i g. 1 is a graph of the tensile strength, yield strength, and percent elongation of two Alloys plotted versus temperature, one within the scope of the invention and the other is outside

C rr T ntrtn D Anrn ^

linn ninnr · KA t \ f rAnkrt tnnri r \ Y \ ii

of an alloy that is outside the invention,

F i g. 2A is a reproduction of a photomicrograph of an alloy according to the invention (comparable to the in F i g. 2 alloy shown),

Figure 3 is a reproduction of a photomicrograph of another alloy outside the scope of the invention,

Fig. 3A is a representation of a photomicrograph of a ^r Rrfindungsgemäßen alloy (similar to the F g in i. Shown alloy 3).

The cast alloys based on nickel, their ductility on at temperatures in the range "is increased from 704 to 871 ⁰ C by the inventive method are suitable for use under stress at temperatures up to 1038 ° C in the investment casting of eg. As turbine blades, vanes, integrally In the process according to the invention, the alloys are modified in such a way that so much hafnium is added to them that they contain 0.5 to 4% hafnium in the solid state an equal weight percent nickel, tantalum and optionally other refractory elements can be used in the alloy Preferably, hafnium is used in amounts from about 0.7% to about 1.2% for the same weight percent tantalum when tantalum is in the In general it can be said that it is possible that the strength properties of the base alloy are present alloy cannot be changed significantly if hafnium is used for elements such as B. tantalum, is set in an alloy e ^: but at the same time the ductility of the alloy is significantly increased. If hafnium is used instead of alloy additives, such as e.g. Nickel, is used, the ductility of the alloy is significantly improved while at the same time it is possible for the strength properties of the base alloy to be improved is provided with hardening elements and is particularly intended for the highest strength values at the highest temperatures, without paying attention to the ductility, does not necessarily lead to an increase in the ductility to a generally technically acceptable value at temperatures of the "trough"

The chemical composition in percent by weight of some well-known nickel-based alloys, which can be modified according to the invention is given in Table I below.

Table I.

Cr

Co

Mon

Ta

Ti

ΛΙ

Zr

Cb

Ni

0.15

9.0
10.0
10.0

2.5

1.5

1.5

5.5

0.015

0.05

rest

0.10 0.15 - 0.12 0.18 3.0 1.0 8.0 9.0 rest 12.5 10.0 - rest 10.0 10.0 - 15.0 4.7 - 12.5 - - 5.5 6.0 - 4.2 0.014 4.25 - - 0.06 1.0 2.0 0.8 - 6.0 5.0 6.1 0.015 0.015 0.012 0.10 0.05 0.10 - 1.0 2.0 - - rest rest

The alloys can also contain up to a total of about 2% of random elements such as e.g. B. Manganese, silicon, Iron, etc., included. Non-metals, such as B. sulfur, oxygen and nitrogen, and harmful metals such as z. * \ Lead, bismuth, arsenic, etc., are made in accordance kept to as low a value as possible with technical practice. Preferably all will in Table 1 mentioned alloys and the alloys to be used according to the invention by vacuum melting produced and still fiirz under vacuum in a precision casting mold. B. Cast gas turbine metal parts.

Table II

0.02 to 0.20%
7 to 13%
up to 35%
up to 14%
up to 8%
up to 6%

Ti

Al

Zr

Nb

0.5 to 6%
4 to 7%
0.002 to 0.02% 0.01 to 0.2%
up to 3%
up to 1.5%

According to the invention, alloy compositions such as those shown below are used in particular Table 111 are given.

Table III

Ni - balance (not less than about 36%) essentially

The addition of hafnium or, preferably, the replacement of hafnium in amounts from about 0.5% to about 1.5% or even 4% in balanced alloy compositions within the range given in Table II increases the ductility of the alloy at temperatures in the range of 704 to 704 87 Γ C, especially in the area of the »trough« at around 760 ^° C, without any adverse effects on other important technical properties of the alloys. The addition of hafnium to the alloy occurs without difficulty after deoxidation and the modified alloy is then treated in the same way as if no modification had taken place. Alloys within the range given in Table II can, particularly with respect to the elements tantalum and tungsten (if tungsten is present at all), by maintaining the tungsten content above about 8% by weight and the total content of tantalum plus tungsten below about 13 or even 10 wt .-% can be compensated.

Alloy I.

Alloy 2

C. 0.10-0.18% 0.03-0.13% Cr 7-11% 7-10% Co 6-13% 6-13% W. 8-12% up to 2% Mon 2-3% 4-8% Ta up to 3% 2.5-4.5% Ti 1-2% 0.5-1.5% Al 5-6% 5.5-6.5% B. 0.004-0.02% 0.004-0.02% Zr 0.02-0.2% 0.02-0.2% Hf 0.5-4% 0.5-4%

Ni

essentially the rest

Alloys 1 and 2 were melted and made into sample bars, parts and specimens under vacuum. !: en cast for chemical analysis. From the alloys A and B specified above, comparison samples were produced which, with the exception of hafnium, den Alloys 1 and 2, respectively, were essentially similar. In the following tables, examples 1-1,1-2 etc. different samples of alloy 1 and examples 2-1, 2-2, etc. represent different samples of alloy 2 (see Table VIII).

The information in Table IV shows the properties of Alloy 2 versus Alloy B at Size cast specimens with respect to tensile strength values at room temperature.

In order to adapt to technical practice, all of them have been described in Tables IV to VI Tests carried out with vacuum-melted and cast samples, which were subjected to heat treatment for 4 Hours at 1080 ° C with the following 10 hours of continuous heating to 900 "C were subjected.

Table IV

example
No.

7. Hf

"W

at RT

"02

at RT

(N / mm ² ) (N / mm ² )

Alloy B 0 940 707 8.0 2-1 0.5 913 760 8.5 2-2 1 932 795 6.0 2-3 1.5 1000 798 14.0

Table V contains the creep values which were obtained at 760 ^° C. and a load of 66.1 kg / mm- '.

Table V

Example % Uf

No.

Time to break

(Hours)

Creep amount *)

Alloy B

2-1

2-2

2-3

0.5

I.
1.5

48
58.3
86.0
130.6

1.9 2.9 4.27 6.39

*) Kricchver! / Eigl before crawling within 2 hours Fracture.

The data in Tables IV and V show that the mechanical properties of alloys B and 2 are at least the same, except that at 760 ° C the hafnium-containing alloys are a lot exhibit greater ductility under conditions that cause creep, with the rupture time being substantial is increased.

Fi g. 1 shows the tensile strength in a graph

10

Properties of alloys B and 2 (Examples 2-6) at temperature ranges from room temperature to about 982 ° C. Curves 11, 13 and 14 show the ultimate tensile strength, the yield point and the percentage elongation of the alloy from Example 2-6. Curves 12 and 15 show the ultimate tensile strength and the percentage elongation of alloy B. From FIG. 1 shows the improvement in ductility from the incorporation of 1.5% hafnium into alloy 2-6, which is evident from the improvement in the percentage elongation. The additional tensile strength values at 760 ^° C. for both examples 2-1 and 2-3 show 9% elongation, which proves excellent ductility at the temperature of the "depression".

Figures 2, 2A, 3 and 3A are reproductions of photomicrographs of alloy samples within and outside the scope of the invention. Each

VJl CSC Γ LfIIUW

500x magnification, which after polishing with an etching liquid, which can be obtained by adding 20 ml Hydrogen fluoride and nitric acid produced to 60 ml of glycerine, was etched. F i g. 2 shows alloy B. and FIG. 2A shows an alloy produced according to the invention, which consists essentially of the composition of alloy B plus 1.5% hafnium instead of the same amount of nickel. It can be seen that the writing-like carbide phases as shown in FIG. 2 do not appear in Fig. 2A. Farther appears to be much more primary / phase in FIG. 2A than in FIG. To be 2. The same situation is with regard to that primary / phase when comparing FIG. 3 and 3A can be seen. These figures represent microstructures of the Alloy A and an alloy produced according to the invention (Examples 1 to 3) of the alloy A is identical with the exception that 3.3% hafnium is for all of the tantalum and some of the nickel (total 3.3%) of alloy A are set.

The values in Table VI show the results when part of the tantalum was replaced by hafnium in Alloy B. The values were obtained with test bars cast to size.

Table VI example

No.

7th, St.

"H
at RT

(N / mm ² )

"(12

at RT (N / mm ² )

*) Creep amount shows the creep within 2 hours before breakage.

Creep behavior Creep amount *) 760 C ", 598 N / mm ²

(Hours.)

Alloy B 0 940 771 8.0 - 2-4 0.5 933 763 8.0 65.3 2-5 I. 967 778 11.0 90.5 2-6 1.5 1078 785 14.5 101.2

2.56
3.67
3.64

From Table V (I are similar values for the alloy A and similar alloys made according to the invention can be seen. These values were based on size obtained cast test bars, which were subjected to a heat treatment at 843 ° C. for 50 hours became. In the alloys produced according to the invention, hafnium was used instead of all of the tantalum and a portion of the nickel up to a total weight percentage of nickel plus tantalum equal to the Percentage by weight of the hafnium was used.

Table VII % No at RT <7 (I2 (%) Time land behavior δ Creep behavior example (N / mm ² ) at RT 4.3 760 C, 740 N / mm ² 982 Γ, ni N / mm ² No. 1008 (N / mm ² ) 5.0 (Hours.) (%) (SId.) O 1110 901 7.0 48 3.75 35 Alloy A 1.5 1241 959 4.0 113.9 9 25.9 1-1 2.2 1145 1005 69.9 9.5 18.4 1-2 3.3 1018 123.1 9 11.9 1-3

Table VII shows the applicability of the invention to alloy A and particularly proves that the desired improvement in ductility when used.

The analyzes of some alloys used to determine the stated test results

Λ
VJUIIg VVJI ti rv 1.HIgI-I ου l-l - 1-2 "·" '"' IUlIlIl. 2-1 • .vurucn, are in the table λ / ΙΜ _a u.ge.ü..rt. 2-6 Tabel VIII 5.48 5.70 5.83 6.15 (Weight percent) 0.017 0.017 0.015 0.016 0.12 0.14 1-3 0.09 2-2 2 -.1 2-4 2-5 0.07 Al 8.78 8.60 5.60 7.78 6.03 6.03 6.13 6.06 7.98 B. 10.1 10.0 0.017 9.88 0.018 0.017 0.018 0.015 9.72 C. 2.52 2.47 0.14 6.00 0.10 0.10 0.05 0.07 6.12 Cr rest rest 8.55 rest 7.82 7.73 8.05 7.95 rest Co 1.50 1.54 9.85 1.08 9.80 9.78 9.79 9.79 1.08 Mon 9.90 9.80 2.43 <0, l 5.77 5.75 6.05 6.10 - Ni 0.12 0.11 rest 0.09 rest rest rest rest 0.16 Ti 1.50 2.20 1.59 0.49 1.06 1.05 1.03 1.08 1.55 W. - 9.55 4.40 <0.l <0.l - - 2.73 Zr. 0.14 0.07 0.13 0.13 0.15 Hf 3.30 1.10 1.40 0.53 1.03 Ta - 4.27 4.32 3.88 3.25

In the method according to the invention for producing hafnium-containing alloys and especially in the treatment of the batches containing return scrap, hafnium was essentially used as an element or in the form of a compound which, under the alloying conditions, becomes metallic Hafnium decomposed, added. Thus, hafnium can be metallic hafnium, hafnium master alloy or possibly are added as intermetallic hafnium compounds. The hafnium should be the melted one Alloy can be added after the melt in vacuo by adding carbon has been completely deoxidized (carbon boiling) to prevent the formation of excessive amounts of the very resistant Avoid hafnium oxide. As those skilled in the art know, it is very advantageous to melt and pour Alloys of the type described here carry out under high vacuum in order to avoid harmful amounts of Avoid oxygen, nitrogen, etc., in the alloy. Under certain conditions you can however, the alloys described herein are melted under inert gas and cast in air.

The nickel-based casting alloys obtained by the method according to the invention with a increased ductility compared to other cast alloys contribute substantially to the extent that the Manufacture of excellent high-temperature gas turbine components that are extremely heavy-duty at high loads Exposure to temperatures is possible.

For this purpose 3 sheets of drawings

Claims (6)

Patent claims: 1. Method of increasing ductility at temperatures in the range of 704 to 87 Γ C by Nickel-based cast alloys obtained from a batch with no more than 5% return scrap are melted, in which the / phase is present as the basic phase in the crystalline matrix and which are suitable for use under stress at temperatures up to 1038 ° C from 0.02 to 0.20% carbon, 7 to 13% chromium, up to 8% molybdenum, up to 6% tantalum, up to 14% Tungsten, up to 35% cobalt, 4 to 7% aluminum, 0.5 to 6% titanium, up to 3% niobium, up to 13% vanadium, 0.002 up to 0.02% boron, 0.01 to 0.20% zirconium, the remainder at least 36% nickel and up to 2% production-related usual impurities, characterized in that the melted Charge 03 to 4% hafnium to be added after deoxidation. 2. The method according to claim 1, wherein the cast alloy based on nickel from 0.10 to 0.18% Carbon, 7 to 11% chromium, 6 to 13% cobalt, 8 to 12% tungsten, 2 to 3% molybdenum, up to 3% tantalum, 1 to 2% titanium, 5 to 6% aluminum, 0.004 to 0.02% boron, 0.02 to 0.2% zirconium, the remainder nickel characterized in that so much hafnium is added to the alloy that it is in the solid state Contains 0.5 to 4% hafnium. 3. The method according to claim I, in which the cast alloy on Nickelba * iS from 0.03 to 0.13% Carbon, 7 to 10% chromium, 6 to 13% cobalt, up to 2% tungsten, 4 to 8% molybdenum, 23 to 4.5% tantalum, 03 to 13% titanium, 5.5 to 6.5% aluminum, π 0.004 to 0.02% boron, 0.02 to 0.2% zirconium, the remainder nickel, characterized in that the Alloy so much hafnium is added that it contains 0.5 to 4% hafnium in the solid state. 4. The method according to any one of claims I to 3, characterized in that the hafnium as a replacement for the same amount of one or more refractory elements is added to the batch. 5. The method according to any one of claims 1 to 4, characterized in that the alloy in an -n is melted in a known manner under high vacuum or in an inert atmosphere and that before the addition of or replacement by hafnium the melt z. B. by adding carbon is completely deoxidized in vacuo. -> o 6. The use of any one of claims 1 to 5 produced cast alloy nickel-based gas turbine components which are suitable for temperatures up to 1038 ⁰ C and at temperatures 704-871 ^C C increased ductility, namely r a creep, elongation of at least 2 , 56% at a load of 59.8 kp / mm ² at 760 ^° C.