EP1568795B1 - Heat resistant superalloy and its use - Google Patents

Description

Field of the invention

The present invention relates to a heat-resistant superalloy, particularly to a nickel-based one. Such alloys are used in turbines for a variety of components, but also for other purposes, such as parts of ovens and devices to be installed in ovens. However, the invention also relates to a particular use of this superalloy.

Background of the invention

As mentioned above, alloys for similar purposes are known in numerous forms, such as the U.S. Patent Nos. 3,466,171 ; 4,236,921 JP-A-11131162 or 5,439,640 evident. Also on the market is the alloy marketed under the designation MAR 247 LC, which is particularly used for achieving higher vibration resistance on turbine wheels and consists of eleven elements, including a large proportion of cobalt, but also a relatively large proportion of tantalum and Hafnium is located. This makes this alloy relatively inexpensive.

In the applications described above, a corrosion resistance to hot gas, a long service life (creep strength), but also creep resistance will generally play a role in the utility value. In the case of turbine wheels, and in particular in the case of turbocharger high-speed turbines, the vibration resistance is added because the wheels are subjected to high vibration stresses at varying temperatures.

Summary of the invention

• Kohlenstoff   0,01 - 0,2 Gew.-%
• Chrom   8 -10 Gew.-%
• Aluminium   4 - 6 Gew.-%
• Titan   2 - 4 Gew.-%
• Molybdän   1,5 - 2,8 Gew.-%
• Wolfram   10 -13,5 Gew.-%
• Niob   1,5 - 2,5 Gew.-%
• Bor   0 < B ≤ 0,04 Gew.-%
• Zirkonium   0 < Zr≤ 0,15 Gew.-%
• der Gehalt an Hafnium und Lanthan zusammen beträgt 0 < Hf + La ≤ 1,5 Gew.-%,
• gegebenenfalls Spuren von Tantal,
• wobei der Rest Nickel ist.

The invention has for its object to provide an alloy with improved vibration resistance properties and, if possible, lower costs. According to the invention, this object is achieved by the fact that the alloy satisfies the following conditions:

• Carbon 0.01-0.2% by weight
• Chromium 8-10% by weight
• Aluminum 4 - 6 wt .-%
• Titanium 2 to 4% by weight
• Molybdenum 1.5-2.8% by weight
• Tungsten 10 -13.5% by weight
• Niobium 1.5-2.5% by weight
• Boron <B <0.04 wt%
• Zirconium 0 <Zr≤ 0.15 wt%
• the content of hafnium and lanthanum together is 0 <Hf + La ≤ 1.5% by weight,
• possibly traces of tantalum,
• the remainder being nickel.

Thus, this alloy has no cobalt and only small amounts of tantalum and hafnium and is therefore less expensive than before. It allows directional solidification, is resistant to breakage of grain boundaries during casting, is also suitable for thin wall thicknesses, and exhibits an improved carbide microstructure, improved carbide stability, and, most importantly, a relatively high ductility over the prior art. In any case, the traces of tantalum should be below 2% by weight, preferably below 1.5% by weight, in particular below 1% by weight.

In addition, due to the relatively high proportion of tungsten and molybdenum, which have a strong bond to nickel, an increase in the modulus of elasticity, an increase in the γ'-solution temperature and not least also an optimization of the life with respect to the fatigue strength. These proportions, namely of tungsten and molybdenum, together are preferably> 14% by weight.

Here, the γ'-formation Ni3 is due to the proportions of aluminum and titanium, which preferably together make up a proportion of> 7 wt .-%. The proportion of aluminum serves a dual purpose, namely on the one hand to form the γ'-phase of nickel and on the other hand to obtain a long-term corrosion protection, because it forms a protective layer of Al ₂ O ₃ on the surface, which at high temperatures, in particular the Turbocharger of a turbocharger driving exhaust gas, is particularly effective. The elements Ti, Nb and Al are responsible for the precipitation hardening and the intermetallic compound, the latter being particularly dense in the alloy according to the invention. These three elements together should therefore be preferred together have a larger proportion than 9.5 wt .-%. The precipitation hardening achieves a higher level of nominal strength, so that the material matrix has to endure less plastic than elastic thermodynamic oscillation amplitudes and thus also achieves a higher resistance to vibration.

It should be emphasized that the general microstructural effect of the present invention only low Ti content reduces the size of the formation of eutectic needles (dendrites) of the γ / γ'-phases and the volume fraction in the eutectic. This in turn is very significant for the reduction of grain boundary fractures.

In addition to the protective layer of Al ₂ O ₃ contributes to the corrosion resistance and the interaction of the basic elements of the matrix with the element lanthanum. Of course, grain boundary refinement is important to the desired improved ductility. The elements B, C, Zr, Hf and La contribute to this. Especially hafnium and lanthanum (which in this case has a multiple and synergetic function) achieve micro-alloys which bring about an absolute increase in the ductility or cohesion / adhesion ratios at the grain boundaries of the matrix. However, it is preferred if the content of hafnium and lanthanum together amounts to a maximum of 0.7% by weight. Thus, then the lanthanum content will be at least 0.0035 wt .-% and will advantageously not exceed 0.015 wt .-%, preferably not more than 0.01 wt .-%. On the other hand, the hafnium content should be at least 0.3% by weight and advantageously at most 0.7% by weight, preferably at most 0.6% by weight. These levels will counteract the dislocation tendency within the material matrix, resulting in a positive time delay for low-cycle fatigue and thus a significant improvement in life.

In the case of the superalloy according to the invention, however, there are still further favorable (multiple and synergetic) functional mechanisms. For example, in the alloy, the element hafnium is incorporated into the γ'-phase of nickel, thereby increasing its strength. At the same time, the hafnium content also reduces hot cracking during the casting of the alloy, especially for materials with columnar dendrites (stem grains).

Elements B and Zr improve creep strength, creep rupture strength and ductility (to which several elements of this alloy contribute) by grain boundary cohesion. Both elements prevent the formation of carbide films on the grain boundaries. However, these elements are only to be introduced in such tracks to the grain boundaries saturate. Therefore, it is preferable that the content of boron is between 0.01 and 0.035 wt% and / or the content of zirconium is between 0.02 and 0.08 wt%.

Finally, it should also be pointed out that the element niobium substitutes the aluminum in the γ 'phase and thereby increases the γ' content in a desired manner. However, low-cycle fatigue fatigue is strongly influenced by the fineness of the γ 'phase, and it is now the niobium element which effectively counteracts γ' coarsening The matrix according to the invention also has the role of a mixed crystal former.

Overall, it has been found that the alloy in environments up to 900 ° C is free from the formation of a sigma phase. This, together with the improved low-cycle fatigue, makes the alloy of the invention particularly suitable for use with turbine wheels, in particular turbochargers.

Brief description of the drawings

• Fig. 1 ein Mikroschliffbild einer erfindungsgemäßen Legierung, von welcher
• Fig. 2 einen vergrößerten Ausschnitt zur Verdeutlichung der Korngrenzen darstellt.

Further details of the invention will be discussed below with reference to the drawings. Show it:

• Fig. 1 a microsection of an alloy according to the invention, of which
• Fig. 2 an enlarged section illustrating the grain boundaries.

Detailed description of the drawing

In Fig. 1 is a microsection of an alloy according to the later discussed Example 1 to see. The surface of the alloy with the corrosion-protective Al ₂ O ₃ layer is not visible in this figure. However, it clearly shows the γ'-phase, which in dense almost oblong hexagonal crystallites with surprisingly low mutual dislocation and directional solidification for an extremely high strength and low fatigue at low cycle fatigue ("low-cycle fatigue) ensured Breaking grain boundaries during casting and is also suitable for thin wall thicknesses, as they are particularly required for the rotor blades of turbine rotors, in particular a high-temperature turbine as in a turbocharger.Eutectic needles (dendrites) of γ / γ'-phase are in not recognizable at all in this picture.

The grain boundaries show edges, which are even clearer Fig. 2 are recognizable (10 times magnification), just as thin layer of predominantly titanium, tantalum, hafnium and lanthanum that - as can be seen - the grain surface is just covered. This has two major advantages, on the one hand, the proportion of the last-mentioned, expensive elements can be kept very low, on the other effect, as already mentioned, the elements hafnium and lanthanum an absolute increase in the ductility or cohesion / adhesion ratios at the grain boundaries effect the matrix where they possibly act together with the molybdenum content, such as a "lubricant" at the grain boundaries, which allows good ductility, but ultimately also contributes to low fatigue. Fig. 2 makes it clear why the said elements are present according to the invention in such small amounts.

The invention will now be better understood by reference to the following examples.

Example 1:

An alloy of the following composition (% by weight) was used to form the remainder of nickel: C Cr al Ti Not a word W Nb B Zr Hf La Ta 0.1 9 5 3 2.5 12.5 2 0.02 0.05 0.4 0.01 0.2

Thus, this gave a nickel content of 65.22 wt .-%. It should be noted that this alloy thus had a total content of tungsten and molybdenum of 15 wt .-% and a total content of aluminum and titanium of 8 wt .-%, wherein the sum of the contents of titanium, niobium and aluminum 10 wt. -% mattered. The content of hafnium and lanthanum accordingly made up 0.41% by weight, that is to say far below the maximum content and even below the preferred maximum value of 0.7% by weight.

Temp.:

500°C; Schwingspielzahl 10³ × 10³ : minimal 305 N/mm² Schwingamplitudenspannung;

Temp.:

750°C; Schwingspielzahl 10³ × 10³ : minimal 360 N/mm² Schwingamplitudenspannung;

Temp.:

900°C, Schwingspielzahl 10³ × 10³ : minimal 380 N/mm² Schwingamplitudenspannung.

The alloy so formed was then hot isostatically pressed at 1200 ° C and a pressure of 1400 bar for four hours. Samples were then prepared and tested according to ASTM, Standard E 139. These samples were subjected to a fatigue strength test at 500 ° C, 750 ° C and 900 ° C at a frequency of 1 • s ^-1 and 5 • s ^-1 , respectively, ie a test series of a total of 6 tests. In all tests, the expected and improved longer service lives were achieved until the samples were used, with performance in the fatigue limit defined as follows:

Temp .:

500 ° C; Number of oscillating cycles 10 ³ × 10 ³ : minimum 305 N / mm ² oscillation amplitude voltage;

Temp .:

750 ° C; Number of oscillating cycles 10 ³ × 10 ³ : minimum 360 N / mm ² oscillation amplitude voltage;

Temp .:

900 ° C, number of oscillations 10 ³ × 10 ³ : minimum 380 N / mm ² amplitude of oscillation amplitude.

The corrosion resistance was tested in a hot gas test and this showed a micrograph under the scanning electron microscope with a clear aluminum layer on the surface, which oxidized to Al ₂ O ₃ and thus provided a corrosion protection layer. This micrograph also clearly indicated the saturation of the grain boundaries by boron and zirconium. Neither dendrites nor columnar crystals were formed; rather, a fairly uniform grain was found, as one might wish (cf. Fig. 1 )

Part of the samples were then shown to have excellent ductility and elasticity, which is particularly important in turbine blades.

Example 2:

A second alloy having the following composition (in% by weight) was used to form the remainder of nickel: C Cr al Ti Not a word W Nb B Zr Hf La 0.09 9.5 5.5 2.5 2 13 1.75 0,025 0.08 0.45 0.005

So this gave a nickel content of 65.1 wt .-%. It should be noted that this alloy thus had a total content of hafnium and lanthanum of 0.455 wt .-%, a total content of tungsten and molybdenum of 15 wt .-% and a total content of aluminum and titanium of 8 wt .-%, wherein the sum of the contents of titanium, niobium and aluminum was 9.75% by weight. So no tantalum was used here.

The thus formed alloy was then subjected to the same tests as in Example 1, whereby the elasticity was slightly improved over Example 1.

Example 3:

A third alloy having the following composition (in% by weight) was used to form the remainder of nickel: C Cr al Ti Not a word W Nb B Zr Hf La Ta 0.12 8.5 4.5 3.5 2.75 11.5 2.3 0.01 0.03 0.6 0,004 0.6

Thus, this gave a nickel content of 65.586 wt .-%. It should be noted that this alloy thus had a total content of hafnium and lanthanum of 0.604 wt .-%, a total content of tungsten and molybdenum of 15 wt .-% and a total content of aluminum and titanium of 8 wt .-%, wherein the sum of the contents of titanium, niobium and aluminum was 10% by weight.

The tests carried out as in Example 1 gave a slightly increased ductility. However, when an endurance test in a corrosive atmosphere (combustion gases of a gasoline engine at about 900 ° C) was carried out, a somewhat reduced corrosion resistance was exhibited as compared with a similar test of the samples of Examples 1 and 2.

Example 4

This example served, after the previous good results with alloys of Examples 1 to 3, primarily for the purpose of being able to estimate the tendencies that would result in somewhat more extreme proportions of the elements. Therefore, a fourth alloy having the following composition (in% by weight) was used to form the remainder of nickel: C Cr al Ti Not a word W Nb B Zr Hf La 0.18 10.0 6 2 1.5 10.5 1.5 0.03 0.02 0.8 0.02

Thus, this gave a nickel content of 67.45 wt .-%. It should be noted that this alloy thus had a total content of hafnium and lanthanum of 0.82 wt .-%, a total content of tungsten and molybdenum of 12 wt .-% and a total content of aluminum and titanium of 8 wt .-% , wherein the sum of the contents of titanium, niobium and aluminum made up 9.5 wt .-%. Again, they had dispensed with the addition of tantalum.

It should be readily understood that the samples made therewith did not provide any additional improvement in properties compared to those of Examples 1-3. Despite the higher proportion of hafnium and lanthanum, ductility was rather lower, possibly due to the higher proportion of C and Cr, but possibly also to the absence of tantalum.

Other examples have been made to determine the marginal proportions of the laying elements, with the values set forth in the claims and discussed above.

From the alloys of the above examples, turbocharger turbine rotors were also prepared which were first subjected to solution annealing at 1200 ° C for 8 hours and then precipitation hardening at 860 ° C for 16 hours, each time with subsequent air cooling. All test rotors were subjected to a long-term test and proved to be above expectations.

Claims (10)

1. Heat-resistant superalloy, which satisfies the following conditions:

   - carbon 0.01 - 0.2 wt.%

   - chromium 8 - 10 wt.%

   - aluminium 4 - 6 wt.%

   - titanium 2 - 4 wt.%

   - molybdenum 1.5 - 2.8 wt.%

   - tungsten 10 - 13.5 wt.%

   - niobium 1.5 - 2.5 wt.%

   - boron 0 < B ≤ 0.04 wt.%

   - zirconium 0 < Zr ≤ 0.15 wt.%

   - the hafnium and lanthanum contents together are 0 < Hf + La ≤ 1.5 wt.%,

   - optionally traces of tantalum less than 2 wt.%,

   - the remainder being nickel.
2. Superalloy according to Claim 1, characterized in that the boron content satisfies at least one of the following conditions:

   (a) it is at least 0.01 wt.%;

   (b) it is at most 0.035 wt.%.
3. Superalloy according to Claim 1 or 2, characterized in that the zirconium content satisfies at least one of the following conditions:

   (a) it is at least 0.02 wt.%;

   (b) it is at most 0.08 wt.%.
4. Superalloy according to one of the preceding claims, characterized in that the lanthanum content satisfies at least one of the following conditions:

   (a) it is at least 0.0035 wt.%;

   (b) it is at most 0.015 wt.%, preferably at most 0.01 wt.%.
5. Superalloy according to one of the preceding claims, characterized in that the hafnium and lanthanum contents together are at most 0.7 wt.%, the following additional conditions preferably being satisfied:

   (a) the hafnium content is at least 0.3 wt.%;

   (b) the hafnium content is at most 0.7 wt.%, preferably at most 0.6 wt.%.
6. Superalloy according to one of the preceding claims, characterized in that the tungsten and molybdenum contents together are greater than or equal to 14 wt.%.
7. Superalloy according to one of the preceding claims, characterized in that the aluminium and titanium contents together are greater than or equal to 7 wt.%.
8. Superalloy according to one of the preceding claims, characterized in that the titanium, niobium and aluminium contents together are greater than or equal to 9.5 wt.%.
9. Superalloy according to one of the preceding claims, characterized in that the tantalum content is less than 2 wt.%, preferably less than 1.5 wt.%, in particular less than 1 wt.%.
10. Use of an alloy according to one of the preceding claims for casting a turbine wheel, in particular of a turbocharger.

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