DE3039473A1 - CARBON AND CORROSION PROTECTED NICKEL BASE ALLOY - Google Patents

Description

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Society for 24.525.9

High temperature reactor technology mbH D-5060 Bergisch Gladbach 1
5

Carburizing and corrosion-protected nickel-based alloy

The present invention relates to a nickel-based alloy according to the preamble of the first claim. Nickel-based alloys are preferably used where high strength values must be guaranteed at elevated temperatures. Such a use case is z. B. the helium-operated cooling circuit of a high-temperature nuclear reactor; the use of such reactors is contemplated, inter alia, for coal refining, e.g. B. the production of synthetic natural gas. This means that the presence of small impurities from methane and hydrogen must be expected in the cooling circuit. It has been found that the alloys mentioned at the high operating temperatures of 800 - 1000 ⁰ C tend to carburization and to intergranular corrosion. 25th

The property of aluminum is known to be dense in an oxidizing atmosphere, and further after its formation Forming oxidation inhibiting oxidation layer. A simple addition of aluminum to nickel-based alloys However, it is not sufficient to work on the surface of workpieces made from such alloys Lead formation of a protective oxide layer. Rather, the aluminum oxide forms more irregularly Particles below the surface, where not only does it not have a protective effect, but even corrosion favored.

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This behavior is due to the fact that the oxygen diffuses faster into the alloy, than the aluminum to the surface. In tests with one, preferably for the manufacture of gas turbine blades used, known alloy of the following composition: (here and in the following all information in

Weight%)

Si Mn Cr Nl Fe Mo Co Tl Al W 0.1 <0.1 9.24 remainder 0.2 3.0 14.4 4.63 5.48 0.1

It has been shown that when titanium is added in a certain ratio to aluminum, the formation of a A continuous oxide layer on the workpiece is favored and the corrosion resistance is increased as a result.

Based on this knowledge, the object of the invention is to specify an alloy composition, those in an environment with low oxygen partial pressure to form a continuous, dense oxide skin capable of resisting carburization and corrosion. The alloy should also (in contrast to the last-mentioned) have mechanical properties that allow components such. B.

Manufacture tubes for heat exchangers in nuclear reactor plants of the type described. The protective layer-forming properties of the alloy should be retained in the entire temperature range of its use, ie from room temperature up to about 1000 ^{° C.} The solution to this problem consists of an alloy which satisfies the features set out in the characterizing part of the first claim. The features of special configurations of this general inventive concept are mentioned in claim 2.

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By adding titanium in the specified ratio to aluminum, it is achieved that the interstitial diffusion of oxygen into the workpiece is prevented (presumably by the formation of titanium-oxygen compounds near the workpiece surface), so that the diffusion of aluminum to the surface to form a stable aluminum oxide layer on the workpiece surface. In order not to hinder this oxide formation / the content of other oxygen-affine elements in the alloy is kept as low as possible, in particular the specified content of chromium should not be exceeded. It has been shown that alloys which can form chromium oxide at intermediate temperatures up to 800 ⁰ C, do not do this in the intended cases of application at temperatures around 1000 ⁰ C more. However, it is not advisable to completely dispense with the addition of chromium, since then the protection against corrosion during processing and heat treatment of the workpieces is less. The other alloy components mentioned are known to serve to ensure sufficient mechanical strength with good deformability.

For experimental purposes, an alloy of the following composition was produced that met the features of the invention:

C Si Mn Cr Ni Pe Mo Co Ti Al W S Cu

0.058 0/06 (0.01 4.9 60.5 14.9 <0.05 7.8 1.12 0.34 10.3 0.006 <0.01 30th

Test pieces made of this alloy were exposed to a helium atmosphere of 1.8 bar at temperatures of 850, 900 and 950 ^° C. for periods of up to 2000 hours each.

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This atmosphere contained the following impurities:

500 ^ bar H ₃ , 50 / t-bar CH ₄ , 40 ^ bar CO u.!, S ^ bar H ₃ O.

At the same time, comparison pieces were made from a commercially available alloy with the composition

C Si Mn Cr Nl Fe Mo Cp Tl Al S
0.07 0.08 0.02 22.3 54.3 0.15 9.09 12.5 0.35 1.06 0.007

exposed to these conditions. As the subsequent metallographic examination showed, a thin, continuous and uniform oxide layer of about 1 / m had formed on the pieces of the alloy according to the invention, which effectively prevented deeper corrosion of the material. The comparison pieces
In contrast, showed irregular oxide particles reaching up to 50 / un deep. This does not provide any protection against corrosion. The same applies to protection against carburization. After 1855 hours at 900 ^° C., the alloy according to the invention showed a carbon uptake of between 0.003 and 0.006 percent by weight, the comparison alloy that of 0.042 percent by weight.

Comparative measurements on unused test pieces and on those which were exposed to the conditions described above show that the strength properties of the alloy according to the invention are not significantly changed by use. With test pieces 44 mm long and an average diameter of 4.7 mm, the following values were achieved in the unused state (the values for a further test piece which was exposed to ^{a temperature of 900 ° C. for 870 hours):}

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0.2% elongation at 236 MPa (239 MPa), elongation at break of 57.3% (56.4%) at 615 MPa (619 MPa).

The resistance of the specimens identified above was achieved without having to undergo any special surface treatment. It was enough rather, a usually bare, z. B. ground or stained surface.

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Claims (2)

-1- 24.525.9 Protection claims 1. Highly heat-resistant nickel-based alloy for use in an environment with low oxygen partial pressure with aluminum components, which lead to the formation of a corrosion-inhibiting protective layer of aluminum oxide on the surface of workpieces made from such alloys, characterized by the following features:
10 a) the alloy contains a further component, the one that diffuses into the workpiece from the environment Oxygen binds to the layers of the workpiece close to the surface. b) Other alloy components with an affinity for oxygen are included, as far as the strength properties of the alloy are concerned allow through non-oxygen-affine constituents replaced or their salary is as low as possible. 2. Alloy according to claim 1 characterized by the following features (all data in% by weight): a) An aluminum content of 0.3 to 0.8% and a titanium content of 0.3 to 1.5%, the ratio being from titanium to aluminum is greater than 1. b) A chromium content of 3 to 10% and additions of iron, cobalt, molybdenum and / or tungsten. c) A manganese content below 0.1% and a silicon content below 0.1%. We / 10/17/80
tvs217011 INSPECTED