Patent Application
20250163550
The invention relates to the use of a nickel-iron-chromium-alloy with good high-temperature corrosion resistance in highly corrosive environments and simultaneously good processability, and strength.
Austenitic nickel-iron-chromium alloys having different nickel, chromium and iron contents have long been used in furnace construction and in the chemical and petrochemical industry. For this service, a good high-temperature corrosion resistance even in highly corrosive environments such as, for example, in carburizing, sulfidizing and chlorinating environments, is required, as is a good hot strength.
In general, it must be pointed out that the high-temperature corrosion resistance of the alloys listed in Table 1 increases with increasing chromium content. All of these alloys form a chromium oxide layer (Cr2O3) with an underlying silicon oxide layer, which is more or less closed. Small additions of strongly oxygen-affine elements such as, for example, yttrium or cerium, improve the corrosion resistance. In the course of service in the area of application for establishment of the protective layer, the chromium content is slowly consumed. The useful life of the material is therefore prolonged by a higher chromium content, since a higher content of the element chromium, which forms the protective layer, delays the time at which the chromium content goes below the critical limit and oxides other than Cr2O3 are formed that are, for example, iron-containing and nickel-containing oxides. A further increase of the high-temperature corrosion resistance can be achieved by additions of silicon or aluminum. Starting from a certain minimum content, these elements form a closed layer underneath the chromium oxide layer and in this way reduce the consumption of chromium.
In carburizing environments (CO, H2, CH4, CO2, H2O mixtures), carbon may penetrate into the material and thus lead to formation of internal carbides. These cause a loss of notch impact strength. Transformation processes due to depletion of chromium in the matrix may also occur.
A high resistance to carburization is attained by materials having low solubility for carbon and low carbon diffusion rate. Nickel alloys are therefore generally more resistant to carburization than iron-base alloys, since both the carbon diffusion and the carbon solubility in nickel are lower than in iron. An increase of the chromium content brings about a higher carburization resistance by formation of a protective chromium oxide layer, unless the oxygen partial pressure in the gas is not sufficient for formation of this protective chromium oxide layer. At very low oxygen partial pressures, materials may be used that form a layer of silicon oxide or of the even more stable aluminum oxide, both of which can form protective oxide layers even at much lower oxygen contents.
In carburizing, sulfidizing environments with low oxygen partial pressure (CO, H2, H2O, CO2, H2S mixtures), sulfur may penetrate into the material and thus lead to formation of sulfides. The melting point may also sink to very low values (635° C. for the Ni—Ni3S2 eutectic, 988° C. for the Fe—FeS eutectic). In sulfidizing environments, nickel-iron-chromium with high nickel contents are often more sensitive than nickel-iron-chromium alloys with high iron content. Here also a further increase of the high-temperature corrosion resistance can be achieved by additions of silicon or aluminum.
In chlorinating environments with low oxygen partial pressure, volatile metal chlorides with high vapor pressures and/or low melting points may be formed and thus cause high corrosion rates. A high content of chromium and/or nickel improves the corrosion resistance.
In DE 41 30 139 C1, a heat-resisting, hot-formable austenitic nickel alloy is described that consists (in mass-%) of 0.05 to 0.15% carbon, 2.5 to 3.0% silicon, 0.2 to 0.5% manganese, max. 0.015% phosphorus, max. 0.005% sulfur, 25 to 30% chromium, 20 to 27% iron, 0.05 to 0.15% aluminum, 0.001 to 0.005% calcium, 0.05 to 0.15% rare earths, 0.05 to 0.20% nitrogen, with the rest nickel and the usual melting-related impurities.
The alloy described in DE 41 30 139 C1 is known by the designations “NiCr28FeSiCe”, Alloy 45™, Nicrofer 45™ or under material number 2.4889, and in the following will be designated by “45™”.
The alloy 45™ is very resistant in carburizing and sulfidizing media, which is why it is suitable for use in refuse-incineration plants or coal-gasification plants.
However, the alloy 45TM can be processed only with great difficulty. This is shown, for example, by crack formation during hot forming. 45™ likewise tends to crack formation during welding, which makes an intrinsic welding (using a weld filler in the same composition range as the material to be welded)—which would be logical for reasons of corrosion protection—impossible and makes the practical use of the material more difficult. For austenitic FeCrNi weld metals with primary austenite solidification, the formation of low-melting phases due to silicon enrichments at the austenite grain boundaries (Fe—Fe2Si eutectic: 1212° C.; NiSi—Ni3Si2 eutectic: 964° C. and Nisi eutectic: 996° C.) as well as the expanding solidification range are mentioned as the cause of the increased hot-crack formation.
In contrast, the alloy AC66 (see Table 1 for composition) has a sufficient weldability and processability, but is not very corrosion-resistant in a coal-gasification plant, as
The requirements applicable to the material become stricter when an attack by chlorine is added to the carburizing, sulfidizing conditions, just as occurs in coal-gasification plants, refuse-incineration plants, etc.
For materials used in carburizing, sulfidizing and chlorinating environments, especially atmospheres, a compromise must be reached with respect to the composition.
The hot strength is improved by a high carbon content among other possibilities. However, even high contents of solid-solution-strengthening elements such as chromium, aluminum, silicon, molybdenum and tungsten improve the hot strength.
U.S. Pat. No. 6,623,869 B1 describes a metal material that contains the following constituents in mass-%: not more than 0.2% carbon, 0.01-4% silicon, 0.05-2% manganese, not more than 0.04% phosphorus, not more than 0.015% sulfur, 10-35% chromium, 30-78% nickel, not less than 0.005% aluminum, but less than 4.5% aluminum, 0.005-0.2% nitrogen and one or both of 0.015-3% copper and 0.015-3% cobalt, wherein the rest is mainly iron. Therein the value of 40 Si+Ni+5 Al+40 N+10 (Cu+Co) is not less than 50, wherein the symbols of the elements denote the alloy content of the respective elements. The metal material has an excellent corrosion resistance in an environment in which metal dusting may occur and it can therefore be used in furnace tubes, piping plants, heat-exchanger tubes, etc. in a petroleum refinery or in petrochemical plants. The durability and safety of the plant can be significantly improved.
U.S. Pat. No. 3,833,358 A describes an iron-base fireproof alloy that offers high resistance to creep, thermal shock, thermal fatigue and intercrystalline oxidation, as well as good weldability and consists mainly of the following elements (in proportions by weight):
U.S. Pat. No. 3,865,581 A describes a heat-resisting alloy having hot formability, containing 0.01 to 0.5% C, 0.01 to 2.0% Si, 0.01 to 3.0% Mn, 22 to 80% Ni and 10 to 40% Cr as main components together with one or both of 0.0005 to 0.20% B and 0.001 to 6.0% Zr and further one or more of 0.001 to 0.5% Ce, 0.001 to 0.2% Mg and 0.001 to 1.0% Be, the rest iron and unavoidable impurities. It is suitable for use in furnace construction (burner tips, protective housings, protective tubes for thermocouples, etc.).
DE 1024719 A describes a method for addition of cerium and/or lanthanum to a nickel-iron alloy. It relates to a hot-workable alloy, characterized by the following composition: 0 to 0.5% carbon, 10 to 60% of one or more of the elements chromium, molybdenum and tungsten, wherein the proportion of each individual one of these elements does not exceed 30%, 0 to 73% iron, 0.02 to 1.10% cerium or lanthanum or both, the rest being 4 to 70% nickel including impurities, with the proviso that the content of the rare earth metals is matched in the following way with the nickel content:
In EP 0 812 926 A1, a nickel-base alloy is described, the strength of which increases during use and which consists of 0.06-0.14% carbon, 35-46% nickel, 22.5-26.5% chromium, 0-1.5% manganese, 0.5-2% silicon, 0.1-1% titanium, 0.05-2% aluminum, 1-3% molybdenum, 0.2% niobium, 0.1-1% tantalum, 0-0.3% tungsten, 0-0.008% boron, 0-0.05% zirconium and the rest iron and incidental impurities.
WO 2007/124996 A1 describes a reaction vessel for the use in the manufacture of hydrogen sulfide by reaction between sulfur and hydrogen, wherein the reaction vessel and if applicable connecting lines as well as fittings and measuring and control devices consist partly or completely of an aluminum-containing material that is resistant to the reaction mixture. In particular, the material contains the constituents 0-0.3% C, 0-2.5% Si, 0-2.5% Mn, 0-0.1% P, 0-0.3% S, 15.0-28.0% Cr, 0-1.0% Cu, 0—the rest % Fe, 1.0-5.0% Al, 0-2.5% Co, 0-1.5% Ti, 0-0.4% Y as well as up to 70% Ni (% in wt-%).
DE 10 2007 005 605 A1 describes an iron-nickel-chromium-silicon alloy containing (in wt-%) 34 to 42% nickel, 18 to 26% chromium, 1.0 to 2.5% silicon and additions of 0.05 to 1% Al, 0.01 to 1% Mn, 0.01 to 0.26% lanthanum, 0.0005 to 0.05% magnesium, 0.01 to 0.14% carbon, 0.01 to 0.14% nitrogen, max. 0.01% sulfur, max. 0.005% boron, the rest iron and the usual process-related impurities. This alloy is used in heating elements.
U.S. Pat. No. 5,021,215 A discloses a high-strength, heat-resisting steel that has improved formability and consists mainly of (wt-%):
JPS 56163244 A describes the improvement of the hot workability and oxidation resistance of an austenitic steel by addition of a defined proportion of C, Si, Mn, Ni, Cr, Al, B, a rare earth element and Ca to the steel. This is achieved by an austenitic steel containing the following composition in wt-%: <0.2% C, 1.5-3.5% Si, <2% Mn, 8-35% Ni, 15-30% Cr, <2% Al, 0.0005-0.005% B, 0.005-0.1% of a rare earth element and 0.0005-0.02% Ca or additionally introduced 0.0005-0.03% Mg, if necessary. The austenitic steel obtained in this way is refined in a standard steelmaking furnace and this molten steel is formed to a billet, which is then hot-rolled.
U.S. Pat. No. 7,118,636 B2 describes a nickel-iron-chromium alloy that contains a strengthening phase, which is able to retain a fine grain structure during forging and during processing of the alloy at high temperatures. The alloy contains a sufficient proportion of titanium, zirconium, carbon and nitrogen that fine titanium and zirconium nitrides are formed, even though these are close to their solubility limit in the molten state of the alloy. In the manufacture of an article from such an alloy by thermomechanical working, a dispersion of the fine titanium and zirconium carbonitride precipitates is formed during solidification of the melt and remains in the alloy during subsequent processing steps (at high temperatures), thus inhibiting austenitic grain growth. The nickel-iron-chromium alloy contains less than 0.05 wt-% niobium, at least 0.05% zirconium, at least 0.05% carbon, at least 0.05% nitrogen, a carbon-to-nitrogen weight ratio of at least 1 to 2 up to less than 1 to 1, sufficient titanium, zirconium and/or aluminum to be free of chromium carbides, and titanium, zirconium, carbon and nitrogen in sufficient proportions to form a uniform dispersion of fine titanium and zirconium carbonitrides to obtain [(TixZr1-x) (CyN1-y)] in a sufficient proportion close to the solubility limit of the titanium and zirconium carbonitride precipitates in a molten state of the alloy. Moreover, this nickel-iron-chromium alloy consists of approx. 32 wt-% to approx. 38 wt-% iron, approx. 22 wt-% to 28% chromium, approx. 0.10% to approx. 0.60% titanium, approx. 0.05% to approx. 0.30% zirconium, approx. 0.05% to approx. 0.30% carbon, approx. 0.05% to approx. 0.30% nitrogen, approx. 0.05% to approx. 0.5% aluminum, up to 0.99% molybdenum, up to approx. 0.01% boron, up to approx. 1% silicon, up to approx. 1% manganese, the rest nickel and incidental impurities.
JPS 57134544 A describes the improvement of the resistance to stress corrosion cracking of oil drilling pipes by addition of specified proportions of Mo, W, etc. to a high Cr—Ni-containing steel as material for pipes. In this case an alloyed steel is used that has a composition of <0.10% C, <1.0% Si, <2.0% Mn, <0.030% P, <0.005% S, <0.5% Al, 22.5-30% Cr, 25-60% Ni and Mo and/or W and that satisfies the equations
The steel is used for a pipe for an oil borehole drilled in the highly corrosive, harsh environment of an oil source, a natural gas source, etc. It is possible to add to the alloy <1% Cu and/or <2% Co and/or <0.10% of one or more among the rare earth elements, <0.20% Y, <0.10% Mg, <0.10% Ca and <0.5% Ti. Pipes can be manufactured for oil boreholes with superior stress corrosion cracking resistance in the highly corrosive environment of an oil source containing H2S, CO2 and Cl.
The task underlying the invention therefore consists in designing the use of a nickel-iron-chromium alloy that
The task underlying this invention is accomplished by the use, as a powder, of a nickel-iron-chromium-alloy with excellent high-temperature corrosion resistance, wherein the powder consists of spherical particles having a size of 5 to 250 μm and wherein this alloy contains (in mass-%):
Advantageous further developments of the subject matter of the invention can be inferred from the associated dependent claims.
The nickel content lies between 35.0 and 38.0%, wherein preferred contents may be adjusted within the following ranges of values:
The range of values for the element chromium lies between 26.0 and 30.0%, wherein preferred ranges may be adjusted as follows:
The silicon content lies between >0.70 and 1.50%, Preferably, silicon can be adjusted within the range of values as follows in the alloy:
The aluminum content lies between 0.40 and 1.30%, wherein, here also, preferred aluminum contents may be specified as follows:
The same is true for the element manganese, which may be present in proportions of 0.0 to 1.0% in the alloy. Alternatively, the following range of values is also conceivable:
Magnesium and/or calcium is also present in contents of 0.0001 to 0.05%. Preferably, the possibility exists of adjusting these elements as follows in the alloy:
The alloy contains 0.015 to 0.12% carbon. Preferably, this may be adjusted within the range of values as follows in the alloy:
This is true in the same way for the element nitrogen, which is present in contents between 0.001 and 0.150%. Preferred contents may be specified as follows:
Furthermore, the alloy contains phosphorus in contents between 0.001 and 0.030%. Preferred contents may be specified as follows:
Furthermore, the alloy contains oxygen in contents between 0.0001 and 0.100%.
The element sulfur is present to at most 0.010% in the alloy. Preferred contents may be specified as follows:
Molybdenum is present with a content of less than 1.0% in the alloy. Beyond this, the molybdenum content may be limited as follows:
Furthermore, less than 1.0% cobalt is present in the alloy. Beyond this, the cobalt content may be limited as follows:
Furthermore, less than 0.5% copper may be present in the alloy. Beyond this, the content of copper may be limited as follows:
Tungsten is present in the alloy with a content of at most 1.0%. Beyond this, the tungsten content may be limited as follows:
The rest in the alloy consists of iron and the usual manufacturing-related impurities. Beyond this, the iron content may be limited as follows:
The following relationship between nickel, silicon and aluminum must be satisfied to ensure that a sufficient resistance exists in carburizing, sulfidizing and chlorinating environments.
wherein Ni, Si and Al and Si are the concentrations of the elements in question in mass-%.
Preferred ranges may be adjusted as follows:
Additions of oxygen-affine elements such as cerium, lanthanum, yttrium, zirconium and hafnium improve the corrosion resistance. They do this by being incorporated in the oxide layer, where they block the paths of diffusion of the oxygen to the grain boundaries.
If necessary, the alloy may contain 0.001 to 0.20% respectively of one or more of the elements cerium, lanthanum, yttrium, zirconium and hafnium, wherein the following formula must be satisfied:
wherein Ce, La, Y, Zr and Hf are the concentrations of the elements in question in mass-%.
Preferably, FRE may be adjusted as follows if at least one of the elements cerium, lanthanum, yttrium, zirconium and hafnium is present
Optionally, in case of simultaneous presence of cerium and lanthanum, cerium mixed metal (abbreviation CeMM) may also be used in contents of 0.001 to 0.20%, wherein FRE must be modified as follows:
wherein CeMM, Y, Zr and Hf are the concentrations of the elements in question in mass-%.
Preferably, FRE may be adjusted as follows in case of addition of cerium mixed metal:
Preferably, cerium, lanthanum, cerium mixed metal zirconium and hafnium may be present within the range of values as follows in the alloy:
Preferably, yttrium may be present within the range of values as follows in the alloy:
Optionally, the element titanium may be present in contents of 0.0 to 0.50% in the alloy. Preferably, titanium may be present within the range of values as follows in the alloy:
Optionally, the element niobium may be adjusted to contents of 0.0 to 0.2% in the alloy. Preferably, niobium may be present within the range of values as follows in the alloy:
Optionally, 0.0 to 0.20% tantalum may also be present in the allow. Preferred contents may be specified as follows:
Optionally, the element boron may be present in contents of 0.0001-0.008% in the alloy. Preferred contents may be specified as follows:
Furthermore, at most 0.50% vanadium may be present in the alloy.
Finally, as impurities, the elements lead, zinc and tin may also be specified in contents as follows:
Then the element beryllium may be specified as follows:
The powder according to the invention is preferably produced in a Vacuum induction melting Inert Gas Atomization system (VIGA). For this purpose, the alloy is first melted, if necessary openly or in vacuum, if necessary with subsequent ESR and/or VAR remelting. Then the powder is produced by means of atomization of the alloy melt in the Vacuum induction melting Inert Gas Atomization system (VIGA). In this system, the alloy is melted in a vacuum induction melting furnace (VIM), passed into a casting funnel that leads to a gas nozzle, in which the molten metal is atomized to metal particles with inert gas at high pressure of 5 to 100 bar. The melt is heated in the melting crucible at 5 to 400° C. above the melting point. The metal flow rate during atomization is 0.5 to 80 kg/min and the gas flow rate up to 150 m3/min. Due to the rapid cooling, the metal particles solidify in the form of balls (spherical particles). The inert gas used for atomization may contain 0.01 to 100% nitrogen if necessary. The gas phase is then separated from the powder in one cycle, after which the powder is packed.
In the process, the particles have a particle size of 5 to 250 μm, gas inclusions of 0.0 to 4% pore area (pores >1 μm) relative to the total area of evaluated objects, a bulk density of 2 up to the density of the alloy of approx. 8.5 g/cm3 and are packed airtightly under a shield-gas atmosphere containing argon.
The range of values for the particle size of the powder lies between 5 and 250 μm, wherein preferred ranges lie between 5 and 150 μm, or 10 and 150 μm. The preferred ranges are implemented by separation of too-fine and too-coarse particles by means of sieving and sifting processes. These processes are performed under shield-gas atmosphere and may be carried out one time or several times.
The powder has gas inclusions of 0.0 to 4% pore area (pores >1 μm) relative to the total area of evaluated objects, wherein preferred ranges are
The powder has a bulk density of 2 up to the density of the alloy of approx. 8.5 g/cm3, wherein preferred ranges may lie at the following values
The proportion of gas inclusions of the powder permits a low residual porosity of the manufactured parts.
The inert gas for the powder production may optionally be argon or a mixture of argon with 0.01 to <100% nitrogen. Possible limitations of the nitrogen content may be
Alternatively, the inert gas may optionally be helium.
The inert gas should preferably have a purity of at least 99.996 vol-%. In particular, the nitrogen content should have 0.0 to 10 ppmv, the oxygen content 0.0 to 4 ppmv and an H2O content≤5 ppmv.
In particular, the inert gas may preferably have a purity of at least 99.999 vol-%. In particular, the nitrogen content should have of 0.0 to 5 ppmv the oxygen content of 0.0 to 2 ppmv and an H2O content of ≤3 ppmv.
The dew point in the system lies in the range of −10 to −120° C. It preferably lies in the range of −30 to −100° C.
The pressure during powder atomization may preferably be 10 to 80 bar.
The powder produced in this way from the alloy may be used for any desired powder-using fabrication method for the manufacture of components or layers on components.
The powder produced in this way may be used for the additive manufacturing of components or layers on components.
By additive manufacturing, terms such as Generative Manufacturing, Rapid Technology, Rapid Tooling, Rapid Prototyping or the like are also understood.
In general, the following distinctions are made here:
The components or layers on components produced by additive manufacturing are built up to layer thicknesses of 5 to 600 μm and have, directly after production, a textured microstructure containing grains of a mean grain size of 2 to 1000 μm, elongated in buildup direction. The preferred range lies between 5 and 600 μm.
The powder produced from the alloy may be used for binder jetting methods. In these methods, components are built up in layers. In comparison with laser-melting methods, however, an organic binder that ensures that the powder particles are held together is used locally. After the curing of the binder, the so-called green part is freed from the unbound powder and subsequently debindered and sintered.
For the powder produced from the alloy, the methods and extra appliances for preheating and postheating may be of advantage. As an example, electron beam methods (EBM) may be considered. The powder bed is melted selectively in layers by the electron beam. The process takes place under high vacuum. Therefore this process is suitable in particular for hard materials, which have lower ductility, and/or for reactive materials. The preheating and/or postheating appliance may be implemented similarly in laser-based methods.
Moreover, the powder produced from the alloy may be used if necessary for the manufacture of components by means of HIP (hot isostatic pressing) or conventional sintering-press and extrusion-press processes. Furthermore, a combination method comprising additive manufacturing and subsequent HIP treatment is possible. If necessary, it is also possible to subsequently carry out a hot forming and/or if necessary cold forming or an alternation of hot and cold forming. For the hot forming, the component may be annealed if necessary at temperatures between 800° C. and 1290° C. for 0.1 hours to 70 hours, followed by hot-forming, if necessary with intermediate annealings between 800° C. and 1290° C. for 0.05 hours to 70 hours. The surface of the material may if necessary be chemically and/or mechanically stripped for cleaning intermediately (even several times) and/or at the end of the hot forming. For a cold working, a cold forming with reduction ratios up to 98% may be carried out, if necessary with intermediate annealings between 800° C. and 1250° C. for 0.05 minutes to 70 hours, if necessary under shield gas, such as, for example, argon or hydrogen, followed by a cooling in air, in the agitated annealing atmosphere or in the water bath.
The components or layers on components produced from the powder with the various methods may optionally be subjected to a solution annealing in the temperature range of 700° C. to 1250° C. for 0.1 minutes to 70 hours, if necessary under shield gas, such as, for example, argon or hydrogen, followed by a cooling in air, in the agitated annealing atmosphere or in the water bath. Thereafter the surface may be optionally cleaned or machined by pickling, abrasive blasting, grinding, turning, peeling, milling. Such a machining may optionally be carried out partly or completely even as early as before the annealing.
The components or layers on components produced from the powder have, after an annealing, a mean grain size of 2 μm to 2000 μm. The preferred range lies between 20 and 600 μm.
The components or layers on components produced from the powder according to the invention are preferably to be used in areas in which highly corrosive conditions prevail, such as, for example, carburizing or sulfidizing or chlorinating environments, or carburizing and chlorinating environments or carburizing and chlorinating environments or carburizing and sulfidizing and chlorinating environments, especially atmospheres. These environments occur, for example, in components in refuse-incineration plants, in pyrolysis plants, in refinery furnaces, in the chemical industry, in coal-gasification plants and in industrial furnace construction, for active carbon filters, refuse pyrolysis and recovery of precious metals.
The assessment of the high-temperature corrosion resistance in highly corrosive conditions was made on the example of a carburizing and sulfidizing and chlorinating environment, via the resistance of the material in a flowing synthetic gas atmosphere having these properties at high temperatures (at the Dechema).
For this purpose, specimens with the dimension of 20×8×4 mm3 were cut out of the semifinished product of the respective alloys, then provided with a bore of 3 mm and thereafter wet-ground with SiC paper to 1200 grit (grain size ˜15 μm). The specimens were degreased and cleaned in an ultrasonic bath containing isopropanol. By means of this bore, each specimen was suspended in the reaction vessel above a ceramic crucible, so that any spalled corrosion products were captured and that the mass of the spallings can be determined by weighing the crucible containing the corrosion products. The sum of the mass of the spallings and of the change in mass of the specimens is the gross change in mass of the specimen. The specific change in mass is the change in mass relative to the surface area of the specimens. These are denoted in the following as mnet for the specific net change in mass, mgross for the specific gross change in mass, mspall for the specific change in mass of the spalled oxides.
A gas mixture of 60% CO, 30% H2, 4% CO2, 1% H2S, 0.05% HCl and 3.95% H2O was passed through the space of the reaction vessel. This mixture has a carburizing (60% CO), sulfidizing (1% H2S) and chlorinating (0.05% HCl) action. Tests were performed at 500° C. The test duration was respectively 1056 hours, divided into 11 cycles of 96 hours each. Two specimens per alloy were exposed in each test. The indicated values are the mean values of these two specimens.
In the following investigation, an alloy is deemed to be resistant in carburizing and sulfidizing and chlorinating environments if after 1056 hours it exhibits
This is the case when the following relationship between nickel, silicon and aluminum is satisfied:
wherein Ni, Si and Al and Si are the concentrations of the elements in question in mass-%.
The assessment of the weldability is made via the extent of formation of hot cracks during welding. The greater the danger of hot-crack formation, the poorer is the weldability of a material.
For quantification of the susceptibility to hot cracks, the various alloys were tested with the MVT (Modified Varestraint Transvarestraint) test at the BAM (German Federal Institute for Materials Research and Testing). For this purpose, a specimen with the dimensions of 100 mm×40 mm×10 mm was made from the alloy. In the MVT test, a TIG weld (TIG: Tungsten Inert Gas) is made fully mechanically with constant feed speed longitudinally on the upper side of this specimen. When the arc passes the middle of the specimen, a defined bending strain is applied to the specimen. For this purpose, the specimens are bent longitudinally relative to the welding direction (Varestraint mode). In this phase of the bending, hot cracks are formed in a locally limited test zone on the MVT specimen.
The tests were performed with 4% bending strain, a die speed of 2 mm/s, with an energy per unit length of 7.5 KJ/cm, respectively under pure argon 4.8.
For the evaluation, the lengths of all solidification cracks and remelting cracks that are visible on the specimen in an optical microscope at 25× magnification are determined and summed. On the basis of these results, the material may then be classified into the category “hot-cracking safe” (range 1), “increasing hot-cracking tendency” (range 2) and “at risk of hot cracking” (range 3) as shown in Table 2.
In the following investigations, the alloys lying in range 1 “hot-cracking safe” and in range 2 “increasing hot-cracking tendency” in the MVT test are deemed to be acceptably weldable, since alloy AC66, which is weldable according to the prior art, lies in range 2. Alloys that lie in the at risk of hot cracking (range 3) are usually difficult to weld. In particular, welding with an intrinsic weld filler (having a composition comparable with that of the material to be welded) is more difficult or impossible.
The assessment of the hot strength was determined by hot tension tests. This is determined in a tension test according to DIN EN ISO 6892-2 at the desired temperature. In the process, the offset yield strength Rp0.2, the tensile strength Rm and the elongation to break A are determined. The tests were performed on round specimens with a diameter of 6 mm in the measurement region and a starting gauge length L0 of 30 mm. The offset yield strength Rp0.2 or the tensile strength Rm at 500° C. should attain at least the minimum values for the alloy AC66 according to the prior art:
It would be desirable for them to be better than the minimum values of the alloy 45™ according to the prior art.
The grain size is determined by means of a linear intercept method.
For establishment of the properties of the components that are manufactured from the powder, alloys melted on the laboratory scale in a vacuum furnace were used.
Tables 3a and 3b show the analyses of the batches melted on the laboratory scale together with, for comparison, some batches of AC66 (1.4877) and 45™ (2.4889) melted on the industrial scale according to the prior art. The batches according to the prior art are identified with a T and those according to the invention with an E. The batches melted on the laboratory scale are marked with an L, the batches melted on the industrial scale with a G.
The ingots of the alloys in Table 3a and 3b, melted on the laboratory scale in vacuum, were annealed between 900° C. and 1270° C. for 8 hours and hot-rolled to a final thickness of 13 mm and 6 mm by means of hot rolling and further intermediate annealings between 900° C. and 1270° C. for 0.1 to 1 hours. The sheets produced in this way were solution-annealed between 800° C. and 1250° C. for 1 hour. The specimens needed for the measurements were manufactured from these sheets.
For the alloys melted on the industrial scale, a sample was taken from the industrial-scale fabrication of a commercially fabricated sheet having appropriate thickness. The specimens needed for the measurements were manufactured from these sheets.
All alloy variants typically had a grain size of 50 to 190 μm.
For the exemplary batches in Table 3a and b), the following properties are compared:
The summary of the results is presented in Table 4.
Table 4 shows the results of the corrosion tests in the form of gross change in weight and spallings at 500° C. after 1056 hours in an atmosphere of 60% CO, 30% H2, 4% CO2, 1% H2S, 0.05% HCl and 3.95% H2O. All tested alloys have a chromium content of approximately 27 to 28%. The alloy AC66 according to the prior art with only 0.2% silicon exhibits by far the largest gross change in mass of 10.92 mg/cm2. The alloy 45™ according to the prior art, with 2.6% silicon, and all tested batches melted on the laboratory scale and having a silicon content higher than 1.0%, exhibit a gross change in mass of smaller than or equal to 2.0 mg/cm2 (2209, 250098, 250101, 250105, 250102 and 250107). If, additionally, the aluminum content is greater than 0.40%, a batch with a silicon content lower than or equal to 1.0% may also have a gross change in mass smaller than or equal to 2.0 mg/cm2, if simultaneously the formula (1a) Fc≤2.5 is satisfied. This is the case for batches 250084 (Si=0.59% and Al=0.95%), 250085 (Si=0.90% and Al 0.98%), 250106 (Si=0.98% and Al 0.80%) and 250108 (Si=0.70% and Al=0.86%).
Batches 250084, 250106, 250105, 250108 and 250107 are in accordance with the invention, while batch 2209 with a silicon content of higher than 1.50% and batch 250098 with a nickel content of 44.0% are not. Batch 250098 (Si=1.20% and Al=0.85%) exhibits, in comparison to batches 250106 (Si=0.98% and Al=0.80%) and 250101 (Si=1.01% and Al=0.75%), a comparable or larger gross increase in mass, despite a significantly increased silicon content of 1.2%. Batch 250098 (Ni=44.0%) has a significantly increased nickel content in comparison to batches 250106 (Ni=35.6%) and 250101 (Ni=38.2%). This shows that a higher nickel content worsens the corrosion. The upper limit for nickel is therefore set at a maximum of 40%.
In batch 250100 (Ni=38.2%, Si=0.99% and Al=0.43%), which is not in accordance with the invention and has an gross increase in mass (3.43 mg/cm2) of significantly greater than 2.0 mg/cm2, the aluminum content is somewhat too low, so that formula (1a) is not satisfied, in contrast to batch 250101 (Ni=38.2%, Si=1.01% and Al=0.75%). In batches 250103 (Ni=38.2, Si=0.36% and Al=0.82%) and 250099 (Ni=38.4%, Si=1.00% and Al=0.20%), which are not in accordance with the invention and likewise have a gross increase in mass (8.01 mg/cm2 and 5.35 mg/cm2 respectively) of significantly greater than 2.0 mg/cm2, the silicon and the aluminum contents are outside the claimed limits and in addition formula (1a) is not satisfied.
The alloys 250084, 250106 according to the invention also exhibit spalling. If, in addition, formula (1c) Fc≤1.0 is satisfied, these alloys no longer exhibit any spalling (250107) and moreover surprisingly have, at moderate silicon contents, a very low gross change in mass of significantly smaller than 1.0 mg/cm2, which is on the order of magnitude of 45™ with 2.6% silicon and 0.16% aluminum.
Table 4 shows the classification of the weldability of the alloys by means of the MVT test. The weldable alloy AC66 according to the prior art is in range 2. The alloy 45™ is classified in range 3 (at risk of hot cracking) and thus tends strongly to crack formation, which makes the welding difficult and welding with an intrinsic weld filler more difficult or impossible.
Batches that are not in accordance with the invention and have a silicon content higher than or equal to 1.50% higher than 1.50% (45™, batches 2091, 2099, 2100, 2200, 2203, 2207, 2208, 2209) all lie in range 3. Among the batches with a silicon content around 1.4%, the batches with an aluminum content lower than 0.1% lie in range 2 (batches 2093, 2101), while those with a higher aluminum content already lie in range 3 (batches 2103, 2096, 2097, 2098). The batches with a silicon content lower than 1.3% all lie in range 1 or 2 1 or 2 (AC66, batches 2095, 2102, 250084 to 250108). All laboratory batches according to the invention lie in range 1 (batches 250084, 250106, 250105, 250108 and 250107) or range 2 (batch 250102).
The results of the hot tension tests at 500° C. in Table show that, in all alloys melted on the laboratory scale, the offset yield strength Rp0.2 is greater than or equal to 153 MPA and thus they significantly exceed the minimum of 95 MPa of AC66. They also exceed even the minimum of 45™ of 150 MPa, albeit not significantly (See formula 5a and 6a). Similarly, the tensile strength Rm of all alloys according to the invention is greater than or equal to 192 MPa and thus likewise significantly greater than the minimum of 115 MPa of AC66 (see formula 5b). All hot tension tests at 500° C. had an elongation of greater than 35%.
The claimed limits for the alloys “E” according to the invention as powder can therefore be justified individually as follows:
A relatively low nickel content (with simultaneously higher iron content (the rest)) favors a lesser corrosion in a highly corrosive environment such as, for example, in a carburizing and sulfidizing and chlorinating atmosphere. Therefore a content of 40% is the upper limit for nickel. A too low nickel content (simultaneously too high iron content (the rest)) favors formation of the sigma phase, especially at high chromium content and silicon content. Therefore a nickel content of 35% is the lower limit.
Chromium improves the corrosion resistance in a highly corrosive environment such as, for example, in a carburizing and sulfidizing and chlorinating atmosphere. Too low chromium contents mean that the chromium concentration during use of the alloy in a highly corrosive environment decreases very rapidly below the critical limit, so that a closed chromium oxide can no longer be formed. Therefore 26% chromium is the lower limit for chromium in the case of use in carburizing and sulfidizing and chlorinating environments. Too high chromium contents promote the formation of the sigma phase of the alloy, especially at high chromium contents. Therefore 30% chromium is to be regarded as the upper limit.
Silicon improves the corrosion resistance in a highly corrosive environment such as, for example, in a carburizing and sulfidizing and chlorinating atmosphere. A minimum content of 0.40% is therefore necessary. Too high contents in turn impair the weldability and promote the formation of sigma phase, especially at high chromium contents. The silicon content is therefore limited to 1.50%.
A certain content of aluminum improves the corrosion resistance in a highly corrosive environment such as, for example, in a carburizing and sulfidizing and chlorinating atmosphere. A minimum content of 0.40% is therefore necessary. Too high contents in turn impair the weldability, especially at high chromium and silicon contents. The aluminum content is therefore limited to 1.30%.
Manganese is useful for improvement of the processability. Manganese is limited to 1.0%, since this element reduces the high-temperature corrosion resistance.
Even very low magnesium contents and/or calcium contents improve the processing by the binding of sulfur, whereby the occurrence of low-melting NiS eutectics is avoided. For magnesium and/or calcium, therefore, a minimum content of 0.0001% is necessary. At too high contents, intermetallic Ni—Mg phases or Ni—Ca phases may occur, which again greatly worsen the processability. The magnesium and/or calcium content is therefore limited to at most 0.05%.
A minimum content of 0.015% carbon is necessary for a good creep resistance. Carbon is limited to at most 0.12%, since above such a content this element reduces the processability by the excessive formation of primary carbides.
A minimum content of 0.001% nitrogen is necessary, whereby the processability and the hot strength of the material are improved. Nitrogen is limited to at most 0.150%, since this element reduces the processability due to the formation of coarse carbonitrides.
The content of phosphorus should be lower than or equal to 0.030%, since this surface-active element impairs the high-temperature corrosion resistance. A too low phosphorus content increases the costs. The phosphorus content is therefore ≥0.001%.
The oxygen content must be lower than or equal to 0.100%, in order to ensure the manufacturability of the alloy. A too low oxygen content increases the costs. The oxygen content is therefore ≥0.0001%.
The contents of sulfur should be adjusted as low as possible, since this surface-active element impairs the high-temperature corrosion resistance. Therefore max. 0.010% sulfur is specified.
Molybdenum is limited to lower than 1.0%, since this element reduces the high-temperature corrosion resistance.
Tungsten is limited to lower than 1.0%, since this element likewise reduces the high-temperature corrosion resistance.
Cobalt may be present in a content lower than 1.0% in this alloy. Higher contents reduce the high-temperature corrosion resistance.
Copper is limited to lower than 0.5%, since this element reduces the high-temperature corrosion resistance.
The following relationship between nickel, silicon and aluminum must be satisfied to ensure that a sufficient resistance exists in a highly corrosive environment such as, for example, in a carburizing and sulfidizing and chlorinating atmosphere.
wherein Ni, Si and Al and Si are the concentrations of the elements in question in mass-%. The limit for Fc has been justified in detail in the foregoing text.
If necessary, the high-temperature corrosion resistance may be further improved with additions of oxygen-affine elements. They do this by being incorporated in the oxide layer, where they block the paths of diffusion of the oxygen to the grain boundaries.
For one or more of the elements cerium, lanthanum, cerium mixed metal, yttrium, zirconium and hafnium, a minimum content of respectively 0.001% is necessary to obtain the effect that increases the high-temperature corrosion resistance. For cost reasons, the upper limit for the respective element is set to 0.20%. In this case, the following formula must be satisfied:
wherein Ce, La, Y, Zr, and Hf are the concentrations of the elements in question in mass-%. The total content of elements such as cerium, lanthanum, yttrium, zirconium and hafnium is limited by this formula. Contents with FRE >1.0 may increase the corrosion rates once again and impair the processability.
If necessary, titanium may be added. Titanium increases the high-temperature strength. At 0.50% and above, the high-temperature corrosion behavior may be impaired, which is why 0.50% is the maximum value.
If necessary, niobium may be added, since niobium also increases the high-temperature strength. Higher contents very greatly increase the costs. The upper limit is therefore set at 0.20%.
If necessary, the alloy may also contain tantalum, since tantalum also increases the high-temperature strength. Higher contents very greatly increase the costs. The upper limit is therefore set at 0.20%. A minimum content of 0.001% is necessary in order to achieve an effect.
If necessary, boron may be added to the alloy, since boron improves the creep resistance. Therefore a content of at least 0.0001% should be present. At the same time, this surface-active element worsens the high-temperature corrosion resistance. Therefore at most 0.008% boron is specified.
If necessary, vanadium is limited to at most 0.50%, since this element reduces the high-temperature corrosion resistance.
If necessary, lead is limited to at most 0.002%, since this element reduces the high-temperature corrosion resistance. The same is true for zinc and tin.
A too-small particle size of smaller than 5 μm impairs the flow behavior and is therefore to be avoided, a too-large particle size larger than 250 μm impairs the behavior during additive manufacturing.
A too-low bulk density of 2 g/cm3 impairs the behavior during additive manufacturing. The highest possible bulk density of approximately 8 g/cm3 is imposed by the density of the alloy.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2022 110 384.6 | Apr 2022 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/DE2023/100283 | 4/20/2023 | WO |
