EP2350329B1 - Nickel-chromium alloy - Google Patents

Description

For high-temperature processes, petrochemicals require materials that are resistant to both temperature and corrosion and, in particular, have grown on the one hand from the hot product and, on the other hand, from the hot combustion gases of, for example, steam crackers. Their coils are subject to external oxidizing aufstickenden combustion gases with temperatures up to 1100 ° C and more and in the interior at temperatures up to about 900 ° C and optionally also high pressure of a carburizing and oxidizing atmosphere.

In contact with the hot combustion gases, it is therefore, starting from the outer pipe surface to a nitriding of the pipe material and the formation of a scale layer.

The carburizing hydrocarbon atmosphere inside the pipes is associated with the risk that diffuses from there the carbon in the pipe material, the carbides in the material increase and from the existing carbide M ₂₃ C ₉ with increasing carburizing the carbon-rich carbide M ₇ C ₆ forms. The consequence of this is internal stresses due to the increase in carbide volume associated with carbide formation or conversion as well as a reduction in the strength and toughness of the tubing material. Furthermore, it comes on the inner surface to the emergence of a firmly adhering, up to several millimeters thick coke layer. Cyclic temperature loads, as they occur as a result of shutdown of the system, lead the further that the tubes shrink to the coke layer due to the different coefficients of thermal expansion of the metallic tube and the coke layer. This leads to high stresses in the pipe, which lead to the formation of cracks in the inner pipe surface. Through such cracks, hydrocarbon gas can then increasingly enter the pipe material.

From the U.S. Patent 5,306,358 is a weldable by the TIG process nickel-chromium-iron alloy with up to 0.5% carbon, 8 to 22% chromium, up to 36% iron, up to 8% manganese, silicon and niobium, up to 6% aluminum, to 1 % Titanium, to 0.3% zirconium, to 40% cobalt, to 20% molybdenum and tungsten and to 0.1% yttrium, balance nickel known.

Furthermore, the German patent specification describes 103 02 989 a nickel-chromium casting alloy also suitable as a material for coils of cracking and reforming furnaces with up to 0.8% carbon, 15 to 40% chromium, 0.5 to 13% iron, 1.5 to 7% aluminum, to 0, 2% silicon, to 0.2% manganese, 0.1 to 2.5% niobium, to 11% tungsten and molybdenum, to 1.5% titanium, 0.1 to 0.4% zirconium and 0.01 to 0 , 1% yttrium, balance nickel. This alloy has proven itself in particular when used as a pipe material, although the practice continues to call for pipe materials with extended life.

Finally, Japanese Laid-Open Publication describes 2004 052 036 a chromium-nickel-iron alloy suitable as a material for high-temperature furnaces with 0.1 to 0.6% carbon, 20 to 40% chromium, 1.5 to 4% aluminum, up to 3% silicon, up to 3% manganese, 0, 5 to 2% niobium, 0.5 to 5% tungsten, 0.01 to 0.5% titanium, 0.01 to 0.5% zirconium, 0.5 to 5% molybdenum and 20 to 65% nickel; Rest iron.

The invention is therefore directed to a nickel-chromium alloy having improved durability under conditions such as cracking and reforming of hydrocarbons.

This object is achieved by a nickel-chromium alloy according to claim 1, a method according to claim 2 or the uses of the nickel-chromium alloy according to claim 1 according to claims 11 to 14.

The alloy according to the invention is characterized in particular by its comparatively high contents of chromium and nickel and by a compelling carbon content within a comparatively narrow range.

Of the alloying ingredients, the silicon improves the oxidation and carburization resistance. The manganese also has a positive effect on the oxidation resistance and additionally favorable on the weldability, deoxidizes the melt and binds the sulfur stable.

Niobium improves creep strength, forms stable carbides and carbonitrides; It also serves as a mixed crystal hardener. Titanium and Tantalum improve creep strength. Even at very low levels, very finely divided carbides and carbonitrides form. At higher levels, titanium and tantalum act as mixed crystal hardeners.

Tungsten improves the creep rupture strength. Particularly at high temperatures, tungsten improves the strength by means of solid solution hardening, since the carbides partly dissolve at higher temperatures.

Cobalt also improves creep strength by means of solid solution hardening, zirconium through the formation of carbides, especially in conjunction with titanium and tantalum.

Yttrium and cerium obviously not only improve the oxidation resistance and especially the adhesion and growth of the Al 2 O 3 cover layer. In addition, yttrium and cerium improve the creep resistance even at very low levels, since they stably bind the remaining free sulfur. Low levels of boron also improve creep strength, prevent sulfur segregation, and retard aging by coarsening the M23C6 carbides.

Molybdenum also improves the creep rupture strength, especially at high temperatures by means of solid solution hardening. Especially because at high temperatures, the carbides partially go into solution. The nitrogen improves the creep rupture strength by means of carbonitride formation, while hafnium, even at low levels, improves the oxidation resistance by means of better adhesion of the cover layer and has a positive effect on the creep rupture strength.

Phosphorous, sulfur, zinc, lead, arsenic, bismuth, tin and tellurium are among the impurities, their contents should therefore be as low as possible.

Under these conditions, the alloy is particularly suitable as a casting material for components of petrochemical plants, for example for the production of pipe coils for cracking and reforming furnaces, reformer tubes, but also as a material for iron ore direct reduction plants and similarly loaded components. These include furnace parts, radiant tubes for heating ovens, rolls for annealing furnaces, parts of Strip and strip casting plants, hoods and sleeves for annealing furnaces, parts of large diesel engines and shaped bodies for catalyst fillings.

Overall, the alloy is characterized by a high oxidation and carburization resistance as well as good creep strength and creep resistance. In addition, the inner surface of cracking or reformer tubes is characterized by a catalytically inert, aluminum-containing oxide layer, thus preventing the formation of catalytic coke strands, known as carbon nanotubes. The properties that characterize the material also remain with multiple burn-out of the coke which inevitably deposits on the inner wall of the pipes during cracking.

Particularly advantageous is a use of the alloy for producing centrifugally cast tubes, if they are drilled with a contact pressure of 10 to 40 MPa, for example 10 to 25 MPa. In such a boring occurs due to the contact pressure to a cold deformation or work hardening of the pipe material in a near-surface zone with depths of, for example, 0.1 to 0.5 mm. When the tube is heated, the cold-worked zone recrystallizes, resulting in a very fine-grained microstructure. The recrystallization structure enhances the diffusion of the oxide-forming elements aluminum and chromium, which promotes the formation of a closed layer of high density and stability consisting primarily of alumina.

The resulting adherent aluminum-containing oxide forms a closed protective layer of the tube inner wall, which is largely free of catalytically active centers such as nickel or iron and even after a prolonged cyclic heat stress is still stable. This aluminum-containing oxide layer prevents, in contrast to other pipe materials without such a cover layer, the penetration of oxygen into the base material and thus an internal oxidation of the pipe material. Furthermore, the cover layer suppresses not only the carburizing of the pipe material, but also corrosion by impurities in the process gas. The top layer consists mainly of Al ₂ O ₃ and the mixed oxide (Al, Cr) ₂ O ₃ and is largely inert to a catalytic coke formation. It is poor in elements that catalyze coke formation, such as iron and nickel.

Of particular advantage for the formation of a durable oxide protective layer is the heat treatment, which can take place in a very economical manner in situ; it serves to condition, for example, the inner surface of steam cracker pipes after their installation when the relevant furnace is heated to its operating temperature.

This conditioning can be carried out as heating with interposed isothermal heat treatments in a furnace atmosphere, which is set during the heating according to the invention, for example in a very weakly oxidizing water vapor-containing atmosphere with an oxygen partial pressure of at most 10 ^-20 , preferably at most 10 ^-30 bar.

Particularly suitable is a protective gas atmosphere of 0.1 to 10 mol% of water vapor, 7 to 99.9 mol% of hydrogen and hydrocarbon individually or side by side and 0 to 88 mol% noble gases.

The atmosphere during the conditioning preferably consists of an extremely weakly oxidizing mixture of water vapor, hydrogen, hydrocarbons and noble gases in an amount such that the oxygen partial pressure of the mixture at a temperature of 600 ° C is less than 10 ^-20 bar, preferably less than 10 ^-30 bar is.

The initial heating of the tube interior after a previous mechanical removal of a surface layer, d. H. the separate heating of the resulting cold-formed surface zone is preferably carried out under very weak oxidizing inert gas in several phases each at a rate of 10 to 100 ° C / h initially to 400 to 750 ° C, preferably about 550 ° C at the inner surface of the tube. This heating phase is followed by a one to fifty-hour hold within the temperature range mentioned. The heating takes place in the presence of a water vapor atmosphere as soon as the temperature has reached a value which precludes the formation of condensed water. Following this holding the tube is then brought to the operating temperature, for example to 800 to 900 ° C and is ready for operation.

However, the tube temperature gradually increases in the cracking operation as a result of the deposition of pyrolytic coke and finally reaches about 1000 ° C or even 1050 ° C on the inner surface. At this temperature, the inner layer consisting essentially of Al ₂ O ₃ and to a small extent of (Al, Cr) ₂ O ₃ converts from a transition oxide such as γ, δ or θ-Al ₂ O ₃ into stable α-aluminum oxide.

Thus, the tube has reached its operating state with its mechanically removed inner layer in a multi-stage, but preferably einzügigen method.

However, the process does not necessarily have to run in one stage, but can also start with a separate preliminary stage. This precursor includes initial heating after abrading the inner surface to holding at 400 to 750 ° C.

The pipe thus pretreated can then be further processed in situ, for example in another manufacturing facility, starting from its cold state in the manner described above, that is to say in another factory. H. be brought to the operating temperature in the installed state.

The mentioned separate pretreatment is not limited to tubes, but is also suitable for a partial or complete conditioning of surface zones of other workpieces, which are then treated according to their nature and use as in the invention or by other methods, but with a defined initial state.

By way of example, five nickel alloys in comparison with ten other nickel alloys, whose composition is shown in Table I and which are particularly suitable for their contents of carbon (alloys 5 and 6), chromium (alloys 4, 13 and 14), aluminum (alloys 12 , 13), cobalt (alloys 1, 2) and iron (alloys 3, 12, 14, 15), differ from the first five nickel-chromium-iron alloy.

As can be seen from the diagram in Fig. 1, alloy 9 experiences no internal oxidation even after more than 200 cycles of annealing at 1150 ° C for 45 minutes, whereas the two comparative alloys 12 and 13 show increasing weight loss after only a few cycles as a result of catastrophic oxidation.

Furthermore, the alloy 9 is also characterized by a high carburization resistance; because, according to the diagram of FIG. 2, it has the lowest weight gain after all three carburizing treatments, compared with the conventional alloys 12 and 13, due to the low weight gain.

Furthermore, the diagrams of FIGS. 3a and 3b show that the creep strength of the nickel alloy 11 is even better in a substantial range than in the case of the two comparative alloys 12 and 13. The exception here is the alloy 15, which is not covered by the invention because of its low iron content. however, with their much lower oxidation, carburization and coking resistance.

Finally, based on the diagram according to FIG. 4, the creep strength of the alloy 11 is far better than that of the comparative alloy 12.

Furthermore, in the series of cracking simulations, several nickel alloy tube sections were used in a laboratory plant to carry out heating experiments with different gas atmospheres and heating conditions followed by a thirty minute cracking phase at a temperature of 900 ° C to initiate the initial phase of the catalytic coke formation. and to investigate and evaluate the propensity for catalytic coke formation.

The data and the results of these experiments with samples of alloy 11 from Table I are summarized in Table II. They show that the respective gas atmosphere in conjunction with a temperature control according to the invention is associated with a considerable reduction in the already low catalytic coke formation.

Examples of the surface condition of the tube interior of furnace tubes with the composition of the alloy 8 are shown in FIGS. 5 and 6. The FIGS Figure 6 (Experiment 7 according to Table II) shows the superiority of a surface after a conditioning according to the invention in comparison to the Figure 5 , which relates to a not according to the invention conditioned surface (Table II, Experiment 2).

Shown in Figures 7 (Alloy 14) and 8 are shallow areas in cross section. The samples were heated to 950 ° C and then subjected to 10 crack cycles of 10 hours each in an atmosphere of water vapor, hydrogen and hydrocarbons. After each cycle, the sample tubes were burned out for one hour to remove the coke deposits. For this purpose, the micrograph of the image 7 in the form of the dark areas shows the large-area and thus bulky result of internal oxidation on the inside of a tube in a conventional nickel-chromium casting alloy compared to the micrograph of the image 8 of the alloy 9, which is virtually none Internal oxidation, although both samples were similarly subjected to multiple cyclic treatment from cracking on the one hand and removal of the carbon deposits on the other.

The experiments show that in the samples from the conventional alloys, starting from surface defects, there is a strong internal oxidation on the inside of the tube. As a result, small metallic centers with a high proportion of nickel are formed on the inner surface of the tube, which forms a considerable amount of carbon in the form of carbon nanotubes (Figure 11).

On the other hand, sample 9 does not have a carbon nanotube after the same tenfold cyclic cracking and subsequent aging in a coking atmosphere which is due to a substantially continuous, catalytically inert, aluminum-containing oxide layer. In contrast, Figure 11 relates to an SEM top view of the conventional sample shown in Figure 7 in section; Due to the missing cover layer, it shows a catastrophic oxidation and a corresponding catastrophic formation of catalytic coke in the form of carbon nanotubes.

The stability of the oxide layer on an alloy is particularly evident in the course of the aluminum concentration over the depth of the edge zone after ten cracking phases with respective removal of the coke deposits by burnout in an intermediate phase compared to the diagrams in Figs 9 near the near surface due to the local failure of the protective overcoat and then onset of strong internal aluminum oxidation of the material is depleted of aluminum, the aluminum concentration in the diagram of the image 10 moves approximately at the starting level of the casting material. This clearly shows the importance of a continuous, dense and in particular firmly adhering inner aluminum-containing oxide layer in the tubes according to the invention.

The stability of the aluminum-containing oxide layer was also investigated by long-term tests in a laboratory plant under process-related conditions. The samples of alloys 9 and 11 were heated to 950 ° C. under steam and then subjected to cracking at this temperature three times each for 72 hours; they were then subjected to burnout at 900 ° C for four hours each. Image 12 shows the closed aluminum-containing oxide layer after the three crack cycles and beyond how the aluminum-containing oxide layer covers the material itself over chromium carbides in the surface. It can be seen that chromium carbides present on the surface are completely covered by the aluminum-containing oxide layer.

Even in disturbed surface areas, in which primary carbides of the base material are heaped up and therefore particularly susceptible to internal oxidation, the material is protected by a uniform aluminum-containing oxide layer, as the micrograph of the image 13 makes clear. It can be seen how oxidized former MC carbide overgrown aluminum-containing oxide and thus is encapsulated.

The micrographs of the near-surface zone according to Figures 14 and 15 show that even after the long-term cyclic tests, no internal oxidation has occurred, which is due to the stable and continuous aluminum-containing oxide layer. In these experiments, samples of alloys 8 to 11 were used.

Overall, the inventive nickel-chromium-iron alloy is characterized, for example, as a pipe material after removal of the inner surface under mechanical pressure and a subsequent multi-stage in situ heat treatment for conditioning the inner surface by a high oxidation, corrosion and especially high Creep rupture and creep resistance.

Particularly noteworthy, however, is especially the extraordinary carburization resistance of the material, which is due to a rapid construction of a substantially closed and stable oxide or Al ₂ O ₃ layer. Above all, in the case of steam cracker and reformer tubes, this layer largely suppresses the formation of catalytically active centers with the risk of catalytic coke formation. These material properties are also not lost after a plurality of respectively significantly extended cracking cycles, each associated with a burnout of the deposited coke. Table I alloy C Si Mn P S Cr Not a word Ni Fe W Co Nb al Ti Hf Zr Y Ta Ce 1 0.44 0.30 0.02 0,002 0,003 29,50 0.20 46,90 18.20 0.07 0.40 0.68 3.05 0.15 0.15 0.06 - - - 2 0.44 0.30 0.02 0,002 0,003 29,60 0.15 46,75 17.90 0.07 0.30 0.67 3.18 0.16 0.60 0.06 - - - 3 0.49 0.02 0.01 0,010 0,004 30,80 0.01 51,60 12.50 0.08 0.01 0.64 3.58 0.10 - 0.06 0,004 0.01 0.005 4 0.42 0.03 0.03 0,007 0.005 26.70 0.02 46.10 rest 0.07 0.01 0.69 2.24 0.08 - 0.05 0,004 0.01 - 5 0.20 0.01 0.01 0,010 0,003 30,40 0.01 52,30 rest 0.07 0.01 0.52 3.17 0.12 - 0.06 0,004 - - 6 0.38 0.11 0.01 0,006 0,003 29,75 0.05 44,50 19,70 0.03 0.05 0.68 4.25 0.19 0.20 0.06 - - - 7 0.48 0.11 0.01 0,007 0,003 30.35 0.05 44,00 19.40 0.38 0.05 0.69 4.05 0.13 - 0.04 - - - 8th 0.47 0.59 0.13 0,006 0,002 29,50 0.07 42,70 20.72 0.09 0.06 0.80 4.54 0.18 - 0.06 0.24 - - 9 0.44 0.16 0.09 0,006 0,002 30.35 0.07 42,20 rest 0.03 0.01 0.78 3.17 0.1 - 0.07 0,013 - - 10 0.50 1.43 0.17 0,006 0,002 30,10 0.01 rest 19.20 0.05 0.05 0.78 4.00 0.15 - 0.07 0.18 - - 11 0.42 0.07 0.09 0,007 0,003 30.30 0.02 rest 21.20 0.04 0.01 0.77 3.28 0.23 - 0.11 0.15 - - 12 0.45 1.85 1.26 0,007 0,003 35.02 0.01 45,70 14.85 0.01 0.05 0.81 0.10 0.20 - 0.05 - - 0.01 13 0.44 1.72 1.23 0,010 0.005 25.02 0.01 34.40 rest 0.04 0.01 0.84 0.13 0.10 - 0.02 - - - 14 0.45 0.14 0.06 0.01 0,003 25.7 0.02 57.50 11.40 0.04 0.01 0.53 3.90 0.15 - 0.05 0.04 - - 15 0.44 0.05 0.19 0.01 0,002 30.4 0.07 55.27 10.71 0.05 0.09 0.10 2.40 0.14 - 0.05 0.024 - - attempt Gas composition during the heating phase: Temperature profile during the heating phase: Relative surface coverage of catalytic coke *: 1 100% air From 150 ° C to 875 ° C, 50 ° C / h; 40 h Hold at 875 ° C 1.4% 2 100% water vapor 1.1% 3 70% water vapor 1.2% 30% methane 4 3% water vapor 0.37% 97% methane 5 3% water vapor From 150 ° C to 600 ° C, 50 ° C / h; 40 hours hold at 600 ° C; 0.26% 97% methane (+ H ₂ S-shock **) from 600 ° C to 875 ° C, 50 ° C / h 6 3% water vapor 0.08% 97% ethane (+ H ₂ S-shock **) 7 3% water vapor 97% ethane 0.05% *: This value was determined by counting the coke threads on a defined pipe surface.
**: After reaching operating temperature for 1 h treatment with 250 ppm sulfur (H ₂ S) in water vapor.

Claims (14)

1. Nickel-chromium alloy with high resistance to oxidation and carburisation, creep rupture strength and creep resistance, from,

   0.4 to 0.6% carbon

   28 to 33 % chromium

   17 to 22% iron

   3 to 4.5% aluminium

   0.01 to 1% silicon

   0.01 to 0.5% manganese

   0.01 to 1.0% niobium

   0.01 to 0.5% tantalum

   0.01 to 0.6% tungsten

   0.001 to 0.5% titanium

   0.001 to 0.3% zirconium

   0.001 to 0.3% yttrium

   0.001 to 0.3% cerium

   0.01 to 0.5% molybdenum

   0.001 to 0.1% nitrogen

   The remainder is nickel including impurities from the smelting process.
2. Method with at least partial conditioning of objects made from a nickel-chromium alloy with high resistance to oxidation and carburisation, creep rupture strength and creep resistance, from

   0.4 to 0.6% carbon

   28 to 33% chromium

   15 to 25% iron

   2 to 6% aluminium

   up to 2% silicon

   up to 2% manganese

   up to 1.5% niobium

   up to 1.5% tantalum

   up to 1.0% tungsten

   up to 1.0% titanium

   up to 1.0% zirconium

   up to 0.5% yttrium

   up to 0.5% cerium

   up to 0.5% molybdenum

   up to 0.1 % nitrogen

   The remainder is nickel including impurities from the smelting process in a surface zone by mechanical removal with a contact pressure of 10 to 40 MPa, following by heating up at a heating rate of 10 to 100 °C/hr to a temperature on the surface of 400 to 740 °C under weak oxidising conditions and avoiding a build-up of condensation.
3. Method according to claim 2, characterised in that the contact pressure is 15 to 30 MPa.
4. Method according to claim 2 or 3, characterised in that the heating takes place in an inert gas.
5. Method according to claim 2 to 4, characterised in that, during removal, a surface zone is cold-formed to a depth of 0.1 to 0.5 mm.
6. Method according to any one of claims 2 to 5, characterised by a final annealing, held at 400 to 750 °C for one to fifty hours, then finally heated to the operating temperature at a heating rate of 10 to 100 °C/hr.
7. Method according to claim 6, characterised in that the holding temperature is 550 to 650 °C.
8. Method according to any one of claims 6 to 7, characterised in that the annealing atmosphere consists of a weak oxidising mixture of water vapour, hydrogen, hydrocarbons and inert gases with an oxygen partial pressure below 10^-20 bar at 600 °C.
9. Method according to claim 8, characterised by an oxygen partial pressure below 10^-30 bar.
10. Method according to any one of claims 2 to 9, characterised in that the annealing atmosphere consists of 0.1 to 10 mol % of water vapour, 7 to 99.9 mol % of water vapour and hydrocarbons individually or together and 0 to 88 mol % of inert gases individually or together.
11. Application of an alloy conforming to claim 1 as a material to produce castings.
12. Application of an alloy conforming to claim 1 as a material to produce petrochemical equipment.
13. Application of an alloy conforming to claim 1 as a material to produce pipe coils for cracking and reforming furnaces, preheaters, reforming pipes and iron direct reduction plants.
14. Application of an alloy conforming to claim 1 as a material to produce furnace components, radiant tubes for the heating of furnaces, rollers for annealing furnaces, components for continuous casting plants and strip casting plants, hoods and sleeves for annealing furnaces, components for large diesel engines and moulding bodies for catalyst fillings.

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