USE OF A NICKEL-CHROMI-IRON ALLOY

Description

[0001] The invention relates to the use of a nitrogen-alloyed nickel-chromium-iron alloy for a new application in the field of thermal recycling.

[0002] EP 2 632 628 A1 discloses a malleable homogeneous austenitic nickel alloy with high corrosion resistance to aggressive liquid media, both under oxidizing and reducing conditions, and excellent resistance to local corrosion in acidic, chloride-containing media. The alloy consists of (by mass) chromium 26.0–28.0%, molybdenum 6.0–7.0%, iron max. 33.5%, manganese 1.0–4.0%, silicon max. 0.1%, boron 0.001–0.004%, copper 0.5–1.5%, aluminum 0.01–0.3%, magnesium 0.001–0.15%, carbon max. 0.01%, nitrogen 0.1–0.25%, nickel 33.5–35%, rare earth elements > 0 to 1.0%, and other impurities resulting from the melting process. The alloy is suitable as a material for components that must be resistant to chemical attack.

[0003] Currently, nickel alloys such as FM 625 (UNS N06625), FM 622 (UNS N06022) and FM 686 (UNS N06686) are mostly used as cladding materials for overlay welding or flame spraying in applications for thermal recycling such as waste incineration plants, substitute material combustion plants or biomass plants.

[0004] Corrosion stresses in components and flue gas-contacted surfaces of thermal waste-to-energy plants are diverse and complex. Various diffusion-driven high-temperature corrosion types occur, such as corrosion by halogens with chlorine and increasingly bromine, sulfidation, carburization, molten salts, or corrosion by low-melting-point liquid metals. Furthermore, the materials used can be subjected to additional stress from wet corrosion mechanisms during downtime and maintenance periods when temperatures drop below the dew point or during cleaning operations. Further material stress arises from the thermal cycling during plant start-up and shutdown, or from local and temporary flame bursts in the combustion chamber.

[0005] Despite the corrosion protection of heat exchanger tubes, heating surfaces, flue gas contact surfaces and other components by cladding with these known materials, depending on the material used and operating conditions, wear and tear occurs on the superheater tubes and other thermally stressed components, forcing the operator to shut down, carry out costly maintenance work and possibly to replace parts.

[0006] The material described in EP 2 632 628 A1 has so far been used exclusively in wet corrosion applications, where electrochemical reactions in conjunction with electrolytes cause corrosion. Known areas of application include: chemical processes with phosphoric acid, sulfuric acid, applications in seawater and brackish water, and pickling plants with nitric-hydrofluoric acid.

[0007] German patent DE 10 2007 062 810 A1 discloses a plant for generating energy from biomass. Parts of this plant can be made of heat-resistant and corrosion-resistant materials, preferably stainless steel. Stainless steels with higher chromium and molybdenum contents are specified. However, the materials specified therein are not suitable for weld overlay, as these relatively low-alloy materials, particularly in combination with the iron dilution in the weld metal that occurs during weld overlay, form increased residual deltaferrite in the microstructure, which generally severely limits their use under both wet and high-temperature corrosion conditions.

[0008] WO 2012 / 059080 A2 describes a nickel-iron-chromium-molybdenum alloy with the following composition (in wt%): Ni 33–35%, Cr 26–28%, Mo 6–7%, Cu 0.5–1.5%, Mn 1.0–4%, Si max. 0.1%, Al 0.01–0.3%, C max. 0.01%, N 0.1–0.25%, B 0.001–0.004%, SE > 0–1%, Fe balance including unavoidable impurities. This material is considered a wet-corrosive material.

[0009] EP 3 499 172 A1 describes a superheater comprising a pipe arrangement exposed to combustion gases, wherein the combustion gases are selected, in particular, from the combustion fuels consisting of municipal solid waste, waste, biomass, and sewage sludge. The superheater is to be made of a nickel-based alloy or a special stainless steel with the following composition (in wt.%): at least 18%, preferably 20–32% Cr, 6–20% Mo, 0.5–40% Fe, balance Ni, wherein it may contain components totaling less than 10% of Al, Nb, W, Mn, Si, Ti, Co, Cu, C, N, and trace elements.

[0010] The WO 2008 / 081407 A2 permits the use of a material and / or fuel produced from biomass. This material and / or fuel may be produced using a process in which the biomass is treated at a temperature above 100°C and a pressure above 5 bar for a treatment period of at least one hour.

[0011] The aim of the invention is to make the alloy, which according to the prior art is only approved for low temperatures up to max. 450 °C, available for a new field of application.

[0012] This goal is achieved by using an alloy of the composition (in mass-%) Ni 33,5 - 35,0% Cr 26,0 - 28,0% Mon 6,0 - 7,0% Fe < 33,5% Mn 1,0 - 4,0% Si ≤ 0,1% Cu 0,5 - 1,5% Al 0,01% - 0,3% C ≤ 0,01% P ≤ 0,015% S ≤ 0,01% N 0,1 - 0,25% B 0,001 - 0,004% SE > 0 - 1,0% as needed W ≤0,2% Co ≤0,5% Note ≤ 0,2% Ti ≤ 0,1%, as well as impurities resulting from smelting, as a welding cladding material in the area of ​​heat exchanger tubes of thermal recovery plants, in particular waste, biomass, sewage sludge and alternative fuel plants, wherein the welding cladding material, after the overlay welding, selectively forms sigma phase and other hard particles in the weld metal structure in a fully austenitic microstructure matrix under operational stress.

[0013] Sigma phase formation causes a dispersion of hard particles within the weld metal structure, leading to an increase in the weld metal's hardness and resulting in unexpectedly high resistance to erosion-induced removal of protective coatings. Thus, under operational stress, the formation of the sigma phase leads to a disproportionate increase in the resistance of such weld overlays in thermal recovery plants. The formation of chromium carbides at the application temperature further contributes to protection against erosion and erosion-induced corrosion. Therefore, it is only under operational stress that the weld metal acquires its exceptionally high resistance to mechanical friction and thus also to dust and particle erosion through the precipitation of intermetallic phases such as the sigma phase.

[0014] Even with very long operating times of over 10,000 hours, it can be expected that under the changing conditions of a thermal recycling plant, where not only purely diffusion-controlled / electrochemical corrosion plays a role, but especially also the combination with the resistance of a material to mechanical stress, e.g. by scattering and smoke particles (erosion or erosion corrosion), this material has a novel property profile.

[0015] Furthermore, the formation of iron(II) chloride or iron(III) chloride, which normally occurs in iron-containing materials, is strongly suppressed, especially at low oxygen partial pressures, with associated material dissolution.

[0016] Various laboratory tests and welding trials under production conditions have demonstrated that this material exhibits excellent weldability – high crack resistance and good wettability – for both tungsten inert gas (TIG) and metal active gas (MIG) welding processes. The application of the weld cladding layers can be achieved not only by welding but also, for example, by flame or plasma spraying using powder or wire. Advantageously, this alloy is used as a cladding material in thermal treatment plants for materials such as waste, biomass, sewage sludge, and refuse-derived fuel incineration plants, when applied using welding, flame, or plasma spraying processes.

[0017] In the ASTM G 48C wet corrosion test, the critical pitting temperature for the base material in the as-delivered condition is typically greater than or equal to 85°C. Resistance to pitting corrosion is reduced by the formation of the sigma phase; however, the alloy is so highly alloyed that the chromium content present in the austenitic matrix ensures passivity.

[0018] Advantageous further developments of the invention can be found in the dependent claims.

[0019] The alloy is particularly suitable for coating steels via the liquid phase, such as welding or flame spraying, and exhibits high corrosion resistance to aggressive media that can arise during thermal recycling.

[0020] Preferred chemical compositions (in mass %) are listed below: Ni 33,5 - 35,0% Cr 26,0 - 28,0% Mon 6,0 - 7,0% Fe < 33,5% Mn 1,8 - 3,0% Si ≤ 0,1% Cu 1,0 - 1,5% Al 0,05% - 0,3% C ≤ 0,01% P ≤ 0,015% S ≤ 0,01% N 0,2 - 0,25% B 0,001 - 0,004% SE 0,020 - 0,060% as needed W ≤0,2% Co ≤0,5% Note ≤ 0,1% Ti ≤ 0,5%, as well as impurities resulting from the melting process.

[0021] During investigations of the aforementioned material in the form of weld overlays on 16Mo3 pipes, it was surprisingly and unexpectedly found that it can also be used advantageously in the temperature range and under the specific conditions of thermal recycling.

[0022] The invention is explained in more detail below using an example: Figure 1This shows a cross-section of a real heat exchanger tube, typically used as a steam generator tube in a waste incineration plant. The inner tube is made of 16Mo3 carbon steel and has a material thickness of 5 mm and a diameter of 38 mm. Using the metal active gas arc welding (MIG) process, the FM 31plus weld overlay material was applied in a single layer with a thickness of 2.0–2.4 mm, creating an outer layer of weld overlay and a metallurgical bond between the carbon steel tube and the weld metal. The following welding parameters were used to produce the weld overlay: welding current (pulsed) with = 108 A, welding voltage U = 26 V, overlap = 50%. A four-component shielding gas consisting of argon, helium, hydrogen, and carbon dioxide was used. The wire diameter of the FM 31plus was 1.0 mm from batch 118903. Figure 2 and Figure 3 metallographic cross-sections of this weld overlay show Figure 2 represents the transition from carbon steel to FM 31plus weld metal and Figure 3 The pure, fine-dendritic solidified fully austenitic weld metal from FM 31plus is shown. Figure 4 This figure shows a comparison of the measured wear after an aging test of weld-clad heat exchanger tubes, welded with FM 625 and FM 31plus, after 1000 hours under realistic boiler room conditions of a waste incineration plant, maintaining a defined temperature gradient between 360 °C and 540 °C steam temperature on the tube inner wall throughout the entire aging period. The temperature load relevant for the cladding on the tube outer surface is significantly higher and is essentially above 450 °C. The investigations unexpectedly revealed that the FM 31plus weld overlay is fundamentally equivalent to the FM 625 weld overlay with regard to the observed wear and is even significantly superior over a wide temperature range, despite the fact that the iron content of FM 31plus, which is otherwise particularly detrimental under chlorinating conditions, is at least 28.5% higher by mass than that of FM 625.

[0023] Table 1 lists the compositions of the welding material according to the invention as well as of alternative materials used to date. Table 1 material FM 31plus FM 625 FM 622 Batch No. 118903*) 115949 122001 C 0,003 0,015 0,005 S 0,002 0,002 0,004 N 0,22 0,018 0,016 Cr 26,6 22,3 21,4 Ni 34,0 64.3 (remainder) 59.2 (remainder) Mn 1,94 0,01 0,16 Cu 1,24 0,01 0,01 Si 0,02 0,07 0,03 Mon 6,47 9,21 13,7 Fe 29,13 0,20 2,2 Al 0,07 0,06 0,11 B 0,0024 <0,001 0,001 V 0,03 <0,01 0,17 W 0,10 0,02 2,87 sE 0,04 * Melting-related impurities: Co, P, Nb, Ti

[0024] The material FM 31plus, used as a weldable cladding material for components in thermal waste-to-energy plants, distinguishes itself from comparable materials through the autogenous formation of property-enhancing microstructure phases within the operating temperature range. Calculations were performed using the Calphad software J-MatPro. Figure 5 and Figure 6 They describe how this effect is caused, among other things, by the formation of intermetallic phases, such as the sigma phase. This can also be confirmed by metallographic investigations.

Claims

1. Use of an alloy having the following composition in % by mass: Ni33.5-35.0 %Cr26.0-28.0 %Mo6.0-7.0 %Fe< 33.5 %Mn1.0-4.0 %Si≤ 0.1 %Cu0.5-1.5%Al0.01 %-0.3 %C≤ 0.01 %P≤ 0.015 %S≤ 0.01 %N0.1-0.25 %B0.001-0.004 %REE> 0-1.0 %as neededW≤ 0.2 %Co≤ 0.5 %Nb≤ 0.2 %Ti≤ 0.1 %; and melting-related impurities as a clad-welding material in the field of heat exchanger pipes of thermal recycling plants, in particular waste, biomass, wastewater sludge and substitute fuel facilities, wherein after buildup welding the clad-welding material forms sigma phase and other hard particles in targeted manner in the structure of the weld material in a full austenitic structural matrix in the operationally stressed state.

2. Use according to Claim 1 having the following composition in % by mass: Ni33.5-35.0 %Cr26.0-28.0 %Mo6.0-7.0 %Fe< 33.5 %Mn1.8-3.0%Si≤ 0.1 %Cu1.0-1.5 %Al0.05 %-0.3 %C≤ 0.01 %P≤ 0.015 %S≤ 0.01 %N0.2-0.25 %B0.001-0.004 %REE0-020-0.060 %as neededW≤ 0.2 %Co≤ 0.5 %Nb≤ 0.1 %Ti≤ 0.5 % and melting-related impurities.

3. Use according to Claim 1 or 2, characterized in that the clad-welding material remains fully austenitic and does not form delta ferrite even during welding-induced blending with iron due to the nickel content of at least 33.5 % in the welding material.

4. Use according to any one of Claims 1 to 3, wherein the clad-welding material is used for repairs.

5. Use according to any one of Claims 1 to 4, characterized in that the clad-welding material is in the form of a wire.

6. Use according to any one of Claims 1 to 4, characterized in that the clad-welding material is in the form of a welding strip for submerged arc welding or electroslag welding.

7. Use according to any one of Claims 1 to 4, characterized in that the clad-welding material is in powder form.