EP0690140A1 - High temperature wrought alloy - Google Patents

Abstract

High temp. malleable alloy having a fine grained duplex structure comprises in wt.%; less than 0.05 C; less than 0.5 Si, less than 0.5 Mn; 8.5-11 Al; less than 0.02 P; less than 0.01 S; 4-10 Cr; 23-28 Fe; 0.025-0.2 Hf and/or rare earths, and/or Zr; less than 0.5 Ti, less than 0.005 B; balance Ni plus melting impurities.

Description

The invention relates to a high-temperature wrought alloy based on nickel, which contains aluminum, chromium, iron and hafnium. Such a formable, heat-resistant nickel-based alloy is by CA 1.166.484 with the composition 8 to 25% Cr, 2.5 to 8% Al and a small but effective Y content, as well as with a total of up to 15% Hf and other elements and up to 30% Fe known. Furthermore, up to 20% Co and up to 5% Ti are permitted. Before its intended use in kilns, in particular as supports for the ceramic products to be fired, where temperatures up to 1220 ° C can occur, this alloy is subjected to a suitable heat treatment to produce an aluminum oxide film. Overall, this known alloy is based on the fact that it does not affect the ceramic articles at the high firing temperatures. For a wide long-term use in plant construction, this alloy, which has been optimized for the special use mentioned, is less suitable due to the special framework conditions.

With the upper limit of 25% chromium, the above alloy is still close to the high-chromium alloys, for which the protective effect of chromium oxides is important. Alloys of the type X1NiCrMoCuN 31 27 4 (German material number 1.4563) and X1NiCrMoCu 32 28 7 (1.4562) are currently being tested for heat exchanger tubes in coal gasification plants. However, if one relies on the protective effect of the chromium oxides, sufficient oxygen must be available in the process medium for the oxide formation. However, this is not the case especially in plants in the petrochemical industry and energy technology, so that the permissible metal temperature of heat exchanger tubes is currently and walls must be limited to about 450 ° C in order to prevent an inadmissible sulfurization of the material.

If process temperatures are to be increased, materials are required which can form a protective oxide layer even in low-oxygen atmospheres. Of particular interest here are high aluminum alloys which can form dense, stable Al₂O₃ layers even under extreme conditions. New nickel-based alloys with high aluminum contents, for example with 9 to 12% Al, 8 to 15% Cr, 9 to 16% Fe, 0.2 to 1.5% Zr, 0.2 to 1.5% Hf and 0.05 to 0.2% B (DE 3634635) are intended for use in power engineering for turbine guide vanes. However, as typical casting alloys, they are brittle and cannot be provided as semi-finished products in the form of sheet metal, pipe or wire.

The object of the invention is to further develop the known nickel-based alloys with regard to their resistance to carburization and sulfurization in the temperature range from 400 to 1100 ° C., thereby maintaining their oxidation resistance and also the hot and cold formability.

According to the invention, therefore, a high-temperature wrought alloy with a fine-grained duplex structure is proposed, which (in mass%)
<0.05 C.
<0.5 Si
<0.5 Mn
8.5 to 11 Al
<0.02 P.
<0.01 s
4 to 10 cr
23 to 28 feet
0.025 to 0.2 Hf and / or rare earths and / or Zr
<0.5 Ti
<0.005 B
Remainder nickel and melting-related admixtures exist.

Reference is made to the advantageous narrowing of the analysis according to the subclaim, namely to a composition with
<0.05 C.
<0.5 Si
<0.5 Mn
9 to 11 Al
<0.02 P.
<0.01 s
8 to 10 cr
25 to 28 feet
0.05 to 0.15 Hf and / or rare earths and / or Zr
<0.5 Ti
<0.005 B
Rest of nickel and additions due to melting.

The alloy according to the invention is characterized by a fine-grained duplex structure. One of the phases is a disordered face-centered cubic Ni, Fe, Al, Cr mixed crystal, the second phase is a body-centered cubic B2-ordered substoichiometric intermetallic phase.

The alloy according to the invention can be forged, rolled and welded and can be used in process gases containing carbon and sulfur, even at temperatures above 750 ° C.

Table 1 shows an example of some analyzes of the alloy according to the invention (analyzes A to F) and alloys (G, H, I) of batches used for comparison purposes and lying outside the composition according to the invention. The right column shows the high resistance of the alloys A to F according to the invention in a corrosive atmosphere at 1100 ° C.

Fig. 1 shows an isothermal section through the three-substance system Fe-Ni-Al at 850 ° C regarding the influence of the alloying elements nickel, iron and aluminum. Conventional high temperature wrought alloys of the type 1.4958 (X5NiCrAlTi3120) in the single phase area (Ni). Two-phase (Ni) + α 'alloys with aluminum contents above 5% are typical for turbine blade investment casting alloys; however, these two-phase alloys are brittle and neither malleable nor rollable. Single phase alloys are brittle and susceptible to sulphidation.

As can be seen in Figure 1, the alloy according to the invention with 10% aluminum and about 55 to 60% nickel is close to the boundary between the two-phase region (Ni) + β₂ and the three-phase region (Ni) + β₂ + α '. At the β₂. Phase is a cubic, body-centered, B2-ordered intermetallic Ni (Fe) Al compound; phase (Ni) is a disordered, face-centered, cubic mixed crystal. In certain temperature ranges, the intermetallic, L12-ordered α'-phase can be present as a third structural component.

Alloys in these phase areas are usually brittle and can only be produced as cast alloys or powder metallurgy. An example with 2-20% aluminum, higher chromium-iron and tungsten contents and an extremely high content of the impermissible, embrittling oxygen of 0.1-3% is described in DE 1812144. Surprisingly, it has now been found that (Ni) + β₂ alloys or (Ni) + β₂ + α'alloys can be both cold and hot formed if the alloy composition is adjusted so that the proportion of (Ni) Phase and the β₂ phase are each about 50%. This is achieved with an aluminum content of 10% + 1%. To ensure good formability, the iron / nickel ratio must be set precisely. As can be seen in Figure 2, the best forging and hot rolling ability is given when the iron content is about 26%. When the sample alloys were entered in the diagram, it was assumed that chromium was about half the Lattice sites of iron and half of the lattice sites of nickel occupied. If the iron content is below 20%, a clear decrease in the forming capacity can be observed; iron contents that are too high reduce oxidation resistance and formability.

Figure 3 shows the carburization resistance of the alloy according to the invention in comparison with that of material 1.4958 and material 1.4877. The good carburization resistance of the alloy according to the invention results from the high aluminum content. In order to maintain the protective aluminum oxide layers even over long periods of use, an aluminum content of around 10% is favorable. As Table 2 shows on the basis of the results of tests in an H₂S-containing, sulfurizing coal gasification atmosphere at 750 ° C, the corrosion attack of the alloy according to the invention by sulphidation is marginal.

The excellent sulphidation resistance in oxygen-containing and low-oxygen media is achieved through the combination of high chromium and high aluminum contents. As shown in Fig. 4, a minimum chromium content is required for a high resistance to sulphurization in gases containing H₂S. However, if the chromium content is increased by more than 10%, a significant reduction in the formability can be determined. For these reasons, the chromium content is limited to 10%.

Since components in process technology are often exposed to atmospheric oxygen at high temperatures on the side facing away from the process medium, the materials used in process technology are usually also required to be highly resistant to oxidation. This means that the material must be stable against internal oxidation and against flaking of poorly adhering oxide layers. Good adhesion of the protective oxide layers is achieved by adding 0.1% hafnium to the alloy according to the invention. The good oxidation resistance of this alloy and the favorable influence of hafnium can be seen in Fig. 5. This diagram shows the change in mass at 1100 ° C in air, measured in a cyclic oxidation test with a 24-hour cycle, as a function of the aging time. An increase in mass means an oxygen uptake, a decrease in mass indicates that poorly adhering oxide layers flake off. While the two alloys 1.4958 and 1.4877 as well as the alloy according to the invention have a significant decrease in mass due to flaking at 1100 ° C. in air without the addition of hafnium, the hafnium alloy according to the invention remains stable. However, the hafnium content must not exceed 0.2%, since there is then a risk of the formation of inner hafnium oxides, which would lead to embrittlement of the material.

The high oxidation resistance of this alloy also makes it very suitable for use as a heating conductor material in industrial furnace construction and in other applications, for example as an alternative to the difficult to process high-alloy ferritic iron-chromium-aluminum materials.

For the same reason, the silicon and titanium contents are limited to 0.5%. Both elements can be embrittling in higher concentrations due to the formation of intermetallic phases. Manganese has an unfavorable influence on the oxidation resistance and is therefore also limited to a maximum value of 0.5%.

The phosphorus and sulfur contents should be kept as low as possible, since both elements reduce the high-temperature corrosion resistance and can promote intergranular brittle fracture by reducing the grain boundary cohesion.

Oxygen is embrittling and should therefore be kept to a minimum. Carbon is also embrittling and is therefore limited to 0.05%.

The alloy according to the invention can be produced either by block casting or by continuous casting after melting in a vacuum induction furnace or open melting. The hot shaping is carried out by hot rolling or forging, the cold shaping by rolling. The microstructure is adjusted by recrystallization annealing at a temperature above 1000 ° C; complete recrystallization of the structure is not guaranteed at lower annealing temperatures. After annealing, there is a very fine-grained, uniform duplex structure, as shown in Fig. 6. The mechanical properties determined on this structure are shown as examples in Figure 7. The tensile strength and the Rp0.2 proof stress are clearly above the values measured on material 1.4958 in the entire temperature range. The elongation at break at room temperature reaches the values of high-temperature ferritic steels; it increases with increasing temperature. At temperatures above 1150 ° C, the material can be hot formed very well. Depending on the cooling conditions, there may be a third phase in the micro-structure. The mechanical properties can be varied within a wide range by appropriate selection of the heat treatment temperature and cooling rate.

All or part of the hafnium can be replaced by rare earths such as cerium, lanthanum, mixed metal or even yttrium. In addition, it is also possible to use zirconium instead of these elements.

The alloy according to the invention is excellently suitable for the production of objects which must be resistant to sulphidation, carburization and oxidation at temperatures between 400 and 1100 ° C., namely for use in power plants and in the chemical and petrochemical industry.

The parts of the system used with the alloy according to the invention in the high-temperature part of such energy technology or chemical plants, which can also be welded, are notable for their high resistance to carburization and sulfurization. Since these parts of the plant, such as pipes and boiler walls, are often exposed to atmospheric oxygen on the side facing away from the process gas, their good oxidation resistance also comes to carry. The heat resistance required at regular temperatures between 400 and 1000 ° C is also given and is still sufficient at 1100 ° C.

Claims (3)

High temperature wrought alloy with fine-grain duplex structure consisting (in mass%) of
<0.05 C.
<0.5 Si
<0.5 Mn
8.5 to 11 Al
<0.02 P.
<0.01 s
4 to 10 cr
23 to 28 feet
0.025 to 0.2 Hf and / or rare earths and / or Zr
<0.5 Ti
<0.005 B
Rest of nickel and additions due to melting. High temperature wrought alloy according to claim 1, consisting (in mass%) of
<0.05 C.
<0.5 Si
<0.5 Mn
9 to 11 Al
<0.02 P.
<0.01 s
8 to 10 cr
25 to 28 feet
0.05 to 0.15 Hf and / or rare earths and / or Zr
<0.5 Ti
<0.005 B
Rest of nickel and additions due to melting. Use of the alloy according to claim 1 or 2 for the production of articles which are resistant to sulphidation, carburization and oxidation at temperatures between 400 and 1100 ° C.

Images