Nickel based alloy for high temperature technology

The invention relates to a creep-resistant, corrosion-resistant nickel-based alloy for applications in high-temperature technology.

Metallic materials, in particular for hot work tools, for components of gas turbines and engines, for elements in furnace construction and in the chemical industry, are increasingly subject to increased mechanical and corrosion-chemical requirements at operating temperatures of over 900 ° C. Because of the corrosion stresses, but also with regard to the strength and creep properties of the material, nickel-based alloys containing chromium are well suited for use at the highest temperatures.

A nickel-based alloy with the abbreviation NiCr 7030 according to DIN material number 2.4658 is considered heat-resistant and is used for heating conductors, furnace components and the like. Although such a material has good oxidation resistance, depending on the silicon and aluminum content, it has low strength and low durability properties as well as high creep values ​​at operating temperatures of around 1000 ° C.

A highly heat-resistant nickel-based alloy has become known from DE 4411228 C2. This highly heat-resistant, oxidation-resistant, solidly embroidered, hot and cold deformable nickel-based alloy consists essentially of (in% by mass) 0.001 to 0.15 carbon, 0.10 to 3.0 nitrogen, 25.0 to 30.0 chromium, more than 0.3 to 1.2 nitrogen, 0.001 to 0.01 boron, 0.01 to 0.5 yttrium, cerium, lanthanum, hafnium and tantalum, individually or in combination, the remainder being nickel with a proportion of at least 64.0%. Although solid solution strengthening can be achieved through the carbon content, the mainly effective elements of the above alloy with regard to the high-temperature properties are chromium and nitrogen. Chromium and nitrogen form chromium nitrides, which improve the creep strength, with nitrogen also providing solid solution strengthening. With the alloy according to DE 4411228 C2, significantly improved creep strength and high temperature strength values ​​appear to be achievable.

In the case of the known nickel-based materials, which are increasingly exposed to higher stresses, so-called creep of the material under load and a deterioration in corrosion resistance, especially with cyclical loading, occur in the temperature range between 900 ° C and 1200 ° C. The object of the present invention is to eliminate this deficiency and to create an improved nickel-based alloy for high-temperature applications.

This task is solved by a creep-resistant, corrosion-resistant nickel-based alloy consisting of in% by weight

0.0015 to 0.60 carbon (C)

0.20 to 0.90 nitrogen (N)

22.0 to 32.0 chromium (Cr)

5.0 to 20.0 elements of groups 4.5 and 6 of the periodic table excluding Cr

0.03 to 3.0 aluminum (AI)

0.4 to 3.0 silicon (Si) to 0.15 elements of group 3 of the periodic table excluding actinides up to 0.60 manganese (Mn) up to 14.8 iron (Fe) up to 0.01 boron (B) max 0.014 phosphorus (P) max 0.004 Sulfur (S) min 51 Nickel (Ni) and / or cobalt (Co) and impurities from the melting process.

The advantages achieved with the invention are essentially based on the fact that grain boundary sliding in the material at temperatures up to 1200 ° C. is largely prevented by stable precipitates in the grain boundary regions and increased solid solution strengthening is achieved. Furthermore, the adhesive strength of the chromium spinels or the like layers on the surface is increased, which results in an improved high-temperature corrosion resistance of the parts.

The following is the effect and interaction of the elements of the invention

AT 408 665 B

Nickel-based alloy are described in more detail.

The elements of group 4, 5 and 6 (except chromium) are essentially titanium (Ti), zirconium (Zr), hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), molybdenum ( Mo) and tungsten (W) have solid solution strengthening and have different activities with regard to the non-metallic elements carbon and nitrogen. Ta and Nb, for example, form thermally highly stable nitrides, whereas the nitrogen affinity of the strong carbide formers Mo and W is low. It has been shown that the elements of groups 4, 5 and 6 (with the exception of Cr) are embedded in the atomic lattice of the matrix with a concentration in the material of at least 5, but not more than 20 wt Carbonitride precipitates form, which increase the grain boundary strength and thus make it difficult to slide the grain boundary at temperatures above 1000 ° C. Furthermore, the precipitates effectively prevent grain growth under these conditions.

The extent of solid solution strengthening can be adjusted by the carbon content and the content of strong carbide formers. If, for example, the carbon content of the alloy is low, the strongly carbide-forming elements are increasingly built into the crystal lattice of the mixed crystals and brace it.

Another decisive advantage of the alloy according to the invention is that the elements listed above, in particular the elements Mo and W, shift the peritectic transformation of the II phase to higher temperatures by substituting Cr atoms and thereby stabilize the II precipitates under application conditions is effected. A transformation γ + π -> γ + ε corresponding to γ ​​+ Cr which takes place with increasing temperature at around 1000 ° C₁₃Ni7N₄ -> γ + Cr₂N in Ni-Cr-N alloys, which with a volume change of about 1 x 10 '³ % is connected, for example, as can be seen from Table 1, by a Mo concentration of 4% by weight to a temperature of over 1210 ° C. In the case of cyclical exposure to temperature and material stress, the addition of Mo, for example, does not result in volume changes even at high operating temperatures, which improves the high-temperature corrosion resistance because there is no initiation of parts of the chromium spinel surface layer to flake off.

Carbon with a content of greater than 0.0015% by weight promotes the formation of nitride and carbonitride, but with a content of greater than 0.6% by weight of the alloy removes excessively large amounts of carbide-forming elements, which counteracts solidification of the matrix. Carbon contents of 0.16 to 0.5% by weight are preferred.

If the ratio of nitrogen to carbon content in the alloy is advantageously in the range from 0.5 to 5.5, preferably 1.0 to 4.0, optionally 1.0 to 3.0, particularly effective and stable carbonitride precipitates are obtained and an efficient solid solution strengthening is achieved.

In order to achieve the most stable π-phase possible at high usage temperatures of the material, but also at the same time an effective solid solution hardening, it is advantageous if the nickel-based alloy has a total concentration of molybdenum and tungsten in% by weight according to the context

W.

Mo + - = 3.0 to 10, preferably 4.0 to 8.0.

Chromium contents in% by weight of 25 to 30. To minimize high-temperature corrosion, it is important that the material contains at least 0.03% by weight of Al and at least 0.4% by weight of Si. Contents higher than 3.0% by weight Al lead to disadvantageous precipitation behavior, stress cracks and coarse grain formation, and contents higher than 3.0% Si impair the hot deformability of the alloy.

The corrosion resistance at high temperatures can be increased if the material is alloyed with elements from group 3 of the periodic table, i.e. scandium (Sc), yttrium (Y), lanthanum (La) and lantanide up to a concentration of 0.15% by weight is. Contents between 0.01 and 0.12% by weight are preferred.

The invention is to be further explained below:

Nickel-based alloys with a composition according to the invention can with the help of

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Pressure metallurgy, in which the liquid melt is kept under high pressure until it solidifies, (e.g. DESU process) or powder metallurgy. When using PM technology, a metal powder with the desired content of metallic elements is first produced, then this powder is embroidered over the gas phase at an elevated temperature and hot isostatically pressed.

A deformation of the cast or sintered block usually takes place after the material has been homogenized at 1250 ° C and reshaped at 1200 ° C. Grain sizes of 35 to 80 μm and nitride precipitates with a diameter of 1 to 5 μm are created in the material.

As mentioned earlier, the transition temperature of the II phase is increased by the presence of Group 4,5 and 6 elements (other than Cr). Table 1 shows the determined dissolution and formation temperatures, the composition of the II phase and that of the mixed crystal for a Mo-free Ni-Cr-N alloy and for those with a Mo content of 4 and 8% by weight and one with 4 wt .-% W indicated. At concentrations of 8% by weight Mo and 0.7% by weight N, for example, both temperature values ​​for a π θ ε conversion are above 1300 ° C.

The II phase has a reduced chromium content of 45% by weight with a molybdenum concentration of 11% by weight. With a reduced nickel concentration, the γ mixed crystal has increased chromium values ​​of 29% by weight and a molybdenum content of 6.5% by weight.

Tab. 1 .: Influence of the molybdenum and tungsten content on the γ + Cr interval₂N transformation temperature ΔΤ (dilatometer - investigations)

<td>Chemical composition [wt. %]</td><td></td><td>Ni 30Cr 0.9N</td><td>Ni 30Cr 4W 0.7N</td><td>Ni 30Cr 4Mo 0.7N</td><td>Ni 30Cr 8Mo 0.7N</td>

<td></td><td></td><td></td><td></td><td></td><td></td>

<td>ΔΤ (heating)</td><td></td><td>1120-1185 ° C</td><td>1160-1180 ° C</td><td>1210-1280 ° C</td><td>&gt; 1300 ° C</td>

<td>ΔΤ (cooling)</td><td></td><td>1180-1195 ° C</td><td>1180-1240 ° C</td><td>1260-1280 ° C</td><td>&gt; 1300 ° C</td>

<td>π phase Composition [wt. %]</td><td></td><td>42Ni 58Cr</td><td>41 Ni 53Cr 4.5W</td><td>41 Ni 51 Cr 8 Mon.</td><td>43Ni 45Cr 11Mo</td>

<td>γ matrix Composition [wt. %]</td><td></td><td>77Ni 23Cr</td><td>68Ni 26Cr 5W</td><td>69Ni 28Cr 3.5Mo</td><td>65Ni 29Cr 6.5Mo</td>

Table 2 shows the chemical composition of alloys according to the invention (alloys 1 to 5) and comparison alloys (alloys 6 to 9).

In Table 3 are the mechanical properties of the alloys at 800 ° C. at 1000 ° C and at 1100 ° C.

In comparison, it can be established that the alloying measures according to the invention significantly increase the 0.2% elongation limits (Rp0.2) of the material and the elongation at break (A) in each case has lower values. Compared to the prior art, in particular the creep resistance at 1% elongation of the nickel-based alloys according to the invention is significantly improved.

The resistance to high-temperature corrosion of the alloys according to the invention was improved by about 16% (alloy 3 by more than 22%) compared to those of the prior art.

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<td>alloy</td><td></td><td>Leg. 1</td><td>I leg. 2 |</td><td>1 leg. 3 |</td><td>Leg. 4th</td><td>Leg. 5</td><td></td><td>Leg. 6th</td><td>I leg. 7 I.</td><td>I leg. 8th ]</td><td>I leg. 9 |</td>