Patent Application
20190360080
The present application claims priority to Korean Patent Application No. 10-2018-0059199, filed May 24, 2018, the entire contents of which is incorporated herein for all purposes by this reference.
The present invention relates to a high chromium cast iron and, more particularly, to a highly abrasive and corrosive wear-resistant high chromium cast iron suitable for parts (for example, a throat bush) used in a highly corrosive and abrasive environment such as operating conditions of a flue gas desulfurization (FGD) apparatus of a thermoelectric power plant.
Air pollution arises from a variety of materials including dust, hydrocarbon, sulfur oxides (SOx), and nitrogen oxides (NOx) contained in exhaust flue gases of fossil-fuel power plants, industrial boilers, trash incinerators, etc. Among them, SOx and NOx emissions are targets of stringent environmental regulations in many countries.
Unlike other air pollutants, emissions of sulfur oxides (SOx), which are generated during the combustion of fossil fuels such as coal and oil, vary depending on the amount of sulfur contained in the fuel. Sulfur oxides (SOx) include mainly SO2 and a trace amount of SO3. SOx emissions cause environmental problems such as acid rain and smog.
For this reason, a variety of flue gas desulfurization technologies have been developed. Among them, a flue gas desulfurization apparatus which is the most widely used technology removes SOx contained in exhaust flue gases using principles of absorption, adsorption, oxidation, and reduction. Korean large-scale thermoelectric power plants mainly use a limestone-gypsum wet flue gas desulfurization method which is evaluated as the most commercially successful technology.
The limestone-gypsum wet flue gas desulfurization method first absorbs SOx gas using an absorbent such as water or an alkaline solution and dehydrates sludge produced from the reaction between SOx and the absorbent, thereby producing gypsum as a byproduct. This method has a removal efficiency of 90% or higher.
Sludge generated during the operation of the desulfurization apparatus contains various corrosive substances and a large amount of rigid solid particles. Therefore, pumps and pipes for transporting the sludge suffer from damages such as pitting, corrosive wear, and abrasive wear.
Among the parts of a desulfurization apparatus, a throat bush is a part of a recirculation pump for a desulfurization absorption tower. Throat bushes are consumable parts that are typically required to be replaced biennially. Nevertheless, in Korea, the whole domestic demand for throat bushes entirely relies on imports. This causes problems of high cost and long delivery time in obtaining throat bushes. Therefore, supply of domestic throat bushes is urgently required in terms of cost reduction and quick and stable delivery.
To achieve this purpose, development of an abrasive and corrosive wear-resistant material for throat bushes is urgently required because throat bushes are typically used in slurry pumps which operate in extremely abrasive and corrosive conditions.
The present invention has been made in view of the problems occurring in the related art and an objective of the present invention is to provide a highly abrasive and corrosive wear-resistant ferrous material suitable for a part used in highly abrasive and corrosive conditions such as an operating condition of a flue gas desulfurization apparatus of a thermoelectric power plant, a method of preparing the same material, and a part of a flue gas desulfurization apparatus of a thermoelectric power plant, which is made of the same material.
In order to accomplish the objectives of the present invention, according to one aspect of the present invention, there is provided an abrasive and corrosive wear-resistant high chromium cast iron having a composition of: 1.0 to 3.0 wt % of carbon (C); 0.5 to 2.0 wt % of silicon (Si); 0.5 to 2.0 wt % of manganese (Mn); 25.0 to 36.0 wt % of chromium (Cr); 0.5 to 3.0 wt % of nickel (Ni); 0.3 to 2.0 wt % of molybdenum (Mo); 0.5 to 1.5 wt % of copper (Cu); 0.2 to 2.0 wt % of vanadium (V); 0.03 to 0.3 wt % of titanium (Ti); more than 0 wt % but not more than 0.03 wt % of niobium (Nb); more than 0 wt % but not more than 0.03 wt % of zirconium (Zr); more than 0 wt % but not more than 0.3 wt % of nitrogen (N); more than 0 wt % but not more than 0.03 wt % of cobalt (Co); and balance iron (Fe) and other unavoidable impurities.
The composition of the high chromium cast iron may further include: more than 0 wt % but not more than 0.1 wt % of boron (B); and 0.2 to 1.0 wt % of tungsten (W).
In order to accomplish the objectives of the present invention, according to another aspect of the present invention, there is provided a method of preparing an abrasive and corrosive wear-resistant high chromium cast iron, the method comprising (a) preparing a cast iron by casting a molten iron containing: 1.0 to 3.0 wt % of carbon (C); 0.5 to 2.0 wt % of silicon (Si); 0.5 to 2.0 wt % of manganese (Mn); 25.0 to 36.0 wt % of chromium (Cr); 0.5 to 3.0 wt % of nickel (Ni); 0.3 to 2.0 wt % of molybdenum (Mo); 0.5 to 1.5 wt % of copper (Cu); 0.2 to 2.0 wt % of vanadium (V); 0.03 to 0.3 wt % of titanium (Ti); more than 0 wt % but not more than 0.03 wt % of niobium (Nb); more than 0 wt % but not more than 0.03 wt % of zirconium (Zr); more than 0 wt % but not more than 0.3 wt % of nitrogen (N); more than 0 wt % but not more than 0.03 wt % of cobalt (Co); and balance iron (Fe) and other unavoidable impurities, and (b) heat-treating the cast iron at a temperature within a range of 950 to 1100° C.
In the method, the molten iron may further contain: more than 0 wt % but not more than 0.1 wt % of boron (B); and 0.2 to 1.0 wt % of tungsten (W).
The method may further include (c) tempering the heat-treated cast iron at a temperature within a range of 200 to 300° C., wherein the (c) tempering is performed after the (b) heat-treating.
In order to accomplish the objectives of the present invention, according to a further aspect of the present invention, there is provided a part of a flue gas desulfurization apparatus for a thermoelectric power plant, the part being made of any one of the above-mentioned high chromium cast irons.
The part may be a throat bush.
The abrasive and corrosive wear-resistant high chromium cast iron according to the present invention is composed of a chromium carbide having a rigid compact structure being capable of preventing abrasive wear attributable to friction with corrosive slurry particles in operating conditions of a desulfurization recirculation pump and a highly anti-corrosive matrix structure supporting the chromium carbide. Therefore, the high chromium cast iron is suitable for parts such as a throat bush, which are typically used in an extremely abrasive and corrosive environment.
The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
In describing embodiments of the present invention, well-known functions or constructions will not be described in detail when they may obscure the gist of the present invention.
Embodiments in accordance with the concept of the present invention can undergo various changes to have various forms, and only some specific embodiments are illustrated in the drawings and described in detail in the present disclosure. While specific embodiments of the present invention are described herein below, they are only illustrative purposes and should not be construed as limiting to the present invention. Thus, the present invention should be construed to cover not only the specific embodiments but also cover all modifications, equivalents, and substitutions that fall within the concept and technical spirit of the present invention.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “includes”, or “has” when used in the present disclosure specify the presence of stated features, regions, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, regions, integers, steps, operations, elements, components and/or combinations thereof.
Hereinafter, embodiments of the present invention will be described.
The present disclosure provides a novel high chromium cast iron with excellent abrasive and corrosive wear resistance. The high chromium cast iron is suitable for centrifugal slurry pump parts such as a throat bush of a wet-type flue gas desulfurization apparatus used in a thermoelectric power plant.
The abrasive and corrosive wear-resistant high chromium cast iron according to the present invention contains: 1.0 to 3.0 wt % of carbon (C); 0.5 to 2.0 wt % of silicon (Si); 0.5 to 2.0 wt % of manganese (Mn); 25.0 to 36.0 wt % of chromium (Cr); 0.5 to 3.0 wt % of nickel (Ni); 0.3 to 2.0 wt % of molybdenum (Mo); 0.5 to 1.5 wt % of copper (Cu); 0.2 to 2.0 wt % of vanadium (V); 0.03 to 0.3 wt % of titanium (Ti); more than 0 wt % but not more than 0.03 wt % of niobium (Nb); more than 0 wt % but not more than 0.03 wt % of zirconium (Zr); more than 0 wt % but not more than 0.3 wt % of nitrogen (N); more than 0 wt % but not more than 0.03 wt % of cobalt (Co); and iron (Fe) balance and other unavoidable impurities.
The high chromium cast iron according to the present invention may further contain more than 0 wt % but not more than 0.1 wt % of boron (B) and 0.2 to 1.0 wt % of tungsten (W).
The composition of the high chromium cast iron according to the present invention is determined for the reasons described below.
In white cast iron, carbon (C) plays a key role in determining mechanical properties such as strength and hardness of the iron. The lower the carbon content, the better the corrosion resistance. The higher the carbon content, the better the abrasion resistance. White cast iron has a hypoeutectic structure or a hypereutectic structure according to the content of carbon. In the present invention, taking into account the required corrosive wear resistance for slurry pump parts, the content of carbon is preferably limited to a range of 1.0 to 3.0 wt % to obtain a cast iron having a hypoeutectic structure. More preferably, the content is limited to a range of 1.5 to 1.6 wt %.
Silicon (Si) is added to increase the fluidity of molten iron. Silicon reduces oxygen in molten iron and increases tensile strength and elasticity. However, excessive silicon forms delta ferrite, resulting in decrease in toughness. Therefore, the content of silicon is preferably limited to a range of 0.5 to 2.0 wt % and more preferably to a range of 0.6 to 0.9 wt %.
Manganese imparts fluidity to molten iron like silicon and is typically added in a content of 1 wt %. Manganese is a powerful austenite stabilizing component that stabilizes carbides in cast iron and increases abrasive wear resistance of the cast iron. When the content of manganese is 2 wt % or more, the structure of the cast iron is fragile, leading to a risk of cracking. Therefore, the content of manganese is preferably limited to a range of 0.5 to 2 wt % and more preferably to a range of 1.1 to 1.3 wt %.
In addition to iron and carbon, chromium is also one of the main components of white cast iron. Chromium is a strong carbide forming component. It is possible to form various carbides depending on the overall composition of these main components and the carbon content. White cast iron with a chromium content of wt % or less is suitable for abrasive wear-resistant applications which do not require corrosive wear resistance. When the content of chromium is 10 to 28 wt %, a matrix with a martensitic structure is formed when the cast iron undergoes heat treatment. Chromium typically forms (FeCr)7C3-type complex carbides by reacting with iron. Chromium is also a ferrite stabilizing component. When the content of chromium is 30 to 35 wt %, the matrix has a ferrite structure. The content of chromium is preferably limited to a range of 25 to 36 wt % and more preferably to a range of 31 to 35 wt %.
Nickel is a strong austenite stabilizing component in white cast iron. Nickel in a content of 0.5 wt % or more has an effect of improving a chemical resistance and dramatically increases toughness. When the content of nickel is more than 3 wt %, the austenite structure and the martensite structure will coexist in the matrix, resulting in decrease in strength and abrasive wear resistance. Therefore, the content of nickel is preferably limited to a range of 0.5 to 3.0 wt %.
Molybdenum has two functions in iron-chromium alloys. When the content of molybdenum is less than 1 wt %, it increases high temperature strength, breaking strength, and toughness. When the content of molybdenum is 1 wt % or more, complex carbides are formed at grain boundaries to reduce toughness. In the present invention, the content of molybdenum is preferably limited to a range of 0.3 to 2.0 wt % and more preferably to a range of 0.5 to 1.5 wt %.
Copper is a component that improves a corrosion resistance and weatherability. Copper is used as a corrosion inhibiting component for crude oil containing a weak acid or sulfur. In the present invention, the content of copper is limited to a range of 0.5 to 1.5 wt % and more preferably 1.0 to a range of 1.0 to 1.3 wt %.
Vanadium has a strong affinity for carbon and nitrogen to form a stable complex carbide and has an effect of refining the crystal grains, thereby improving high temperature strength and hardness. Vanadium contributes to enhancement of strength when its content is 0.2 wt % or more. However, when the content of vanadium is 2.0 wt % or more, it causes a metal protective coating to be dissolved and oxidized. Therefore, in the present invention, the content of vanadium is limited to a range of 0.2 to 2 wt % and more preferably to a range of 0.1 to 0.3 wt %.
Titanium in the form of TiC acts as a nucleus to form the pro-eutectic austenitic structure with a fine structure and uniformly precipitate carbides, thereby contributing to enhancement of tensile strength and abrasive wear resistance. In the present invention, the content of titanium is limited to a range of 0.03 to 0.3 wt %.
Niobium forms a complex carbide to inhibit grain growth and refines the structure to improve high temperature strength and high temperature corrosion resistance. Thus, niobium is used as a material for turbines of thermal power plants. However, when the content of niobium is excessive, it deteriorates high temperature ductility and causes surface cracks. In the present invention, the content of niobium is limited to a range of 0.03 wt % or less.
Like titanium, zirconium precipitates ZrC carbides, thereby contributing to enhancement of tensile strength and abrasive wear resistance. The content of zirconium is preferably limited to a range of 0.03 wt % or less.
Nitrogen is an austenite stabilizing component and has an effect of increasing the tensile strength like carbon. The content of nitrogen is preferably limited to a range of 0.3 wt % or less.
Cobalt in austenite-phase iron functions to strengthen the matrix structure and to increase high temperature strength. Cobalt has an effect of causing oxidative wear and relatively reducing abrasive wear under low speed friction conditions. However, under high speed friction conditions, even when a large amount of cobalt is added, there is no significant difference in the wear amount. Therefore, in the present invention, the content of cobalt is preferably limited to a range of 0.03 wt % or less.
When boron (B) is added in a small amount, it makes the crystal grains finer and bonds with carbon to form B2C carbides, thereby contributing to enhancement of abrasive wear resistance.
In the present invention, the content of boron is preferably limited to a range of 0.1 wt % or less.
Tungsten (W) improves breaking strength and acid resistance, and forms complex carbides in conjunction with carbon and chromium to improve high temperature strength and abrasive wear resistance. Tungsten is a strong perlite forming component. When tungsten is added in a content of 2 wt % or more, a large amount of perlite is produced to deteriorate impact strength and machinability. Therefore, in the present invention, the content of tungsten is preferably limited to a range of 0.2 to 1.0 wt % and more preferably to a range of 0.2 to 0.5 wt %.
The highly abrasive and corrosive wear-resistant high chromium cast iron can be prepared by a method including (a) preparing a cast iron by casting a molten iron containing: 1.0 to 3.0 wt % of carbon (C); 0.5 to 2.0 wt % of silicon (Si); 0.5 to 2.0 wt % of manganese (Mn); 25.0 to 36.0 wt % of chromium (Cr); 0.5 to 3.0 wt % of nickel (Ni); 0.3 to 2.0 wt % of molybdenum (Mo); 0.5 to 1.5 wt % of copper (Cu); 0.2 to 2.0 wt % of vanadium (V); 0.03 to 0.3 wt % of titanium (Ti); more than 0 wt % but not more than 0.03 wt % of niobium (Nb); more than 0 wt % but not more than 0.03 wt % of zirconium (Zr); more than 0 wt % but not more than 0.3 wt % of nitrogen (N); more than 0 wt % but not more than 0.03 wt % of cobalt (Co); and iron (Fe) balance and other unavoidable impurities, and (b) heat-treating the cast iron at a temperature within a range of 950 to 1100° C.
In the step (a), the molten iron may further contain more than 0 wt % but not more than 0.1 wt % of boron (B) and 0.2 to 1.0 wt % of tungsten (W).
Alternatively, the method further includes (c) of tempering the cast iron at a temperature within a range of 200 to 300° C. as necessary, in which the step (c) is performed after the step (b) is finished.
The highly abrasive and corrosive wear-resistant high chromium cast iron according to the present invention is composed of a chromium carbide having a rigid compact structure capable of preventing abrasive wear caused by corrosive slurry particles in operating conditions of a flue gas desulfurization recirculation pump and a highly anti-corrosive matrix structure supporting the chromium carbide. Therefore, the high chromium cast iron can be suitably used as a material for a part such as a throat bush that is typically used in extremely abrasive and corrosive conditions.
Hereinafter, the present invention will be described in more detail with reference to examples. The examples presented here are only for illustrative purposes and are not intended to limit the scope of the present invention.
In order to prepare casted Y-block specimens having alloy compositions according to Examples 1 to 3 listed in Table 1, pig iron and recovered iron, Fe—B, W, Mo, and Fe—Cr—N weighed by respectively predetermined weight ratios were charged into each of multiple furnaces, the internal temperature of the furnaces was increased to 1300° C., and scrap steel of STS 410 was further charged into each of the multiple furnaces. Next, the temperature of the furnaces was increased to 1600° C., Fe—S0 and Fe—Mn were added for the purpose of decarbonization, composition analysis was performed, the temperature of the furnaces was cooled to 1500° C., and the molten irons in the furnaces were discharged. At the time of discharging the molten iron with a ladle, 0.03% of Nb, 0.15% of Fe—V, 0.03% of Zr, and 0.03% of Co were injected into the molten iron in the ladle, and the injection-completed molten iron was charged into a product. The injection was performed in a manner of filling one-third of the total volume of the ladle with the molten iron, injecting the injection elements into the molten iron in the ladle, and sufficiently stirring the molten iron in the ladle.
In order to prepare a test specimen, an annealing heat treatment was performed. That is, the temperature of a furnace was increased to 950° C. at a rate of 100° C./hr, the temperature was maintained at 950° C. for 1.5 hours, and the furnace is cooled. Next, the test specimen was machined to a size suitable for test for erosion resistance and corrosion resistance. In this way, a plurality of test specimens was prepared.
The machined test specimens were heated to 600° C. at a rate of 100°/hr using an electric heating treatment device, maintained at a temperature of 600° C. for one hour, heated to 800° C. at a rate of 120° C./hr, maintained at a temperature of 800° C. for one hour, and heated to a temperature range of 950 to 1100° C. at a rate of 200° C./hr. The machined test specimens were maintained at a temperature within a temperature range of 950 to 1100° C. for two hours, were intensively cooled with a fan, and were air-cooled to a temperature range of 350 to 400° C. for the purpose of inhibiting precipitation of carbides.
After performing the destabilization heat treatment, the temperature was increased to a temperature range of 200 to 300° C. at a rate of 100° C./hr, tempering heat treatment was performed at a temperature within a temperature range of 200 to 300° C. for four hours, and air-cooling was performed.
(1) First Erosion Resistance Test
As an erosion and corrosion tester for testing erosion and corrosion resistance in wear-inducing conditions similar to slurry pump operating conditions, a tester illustrated in
As an abrasive for use in a test vessel of the erosion and corrosion tester, an alumina abrasive of #240 grade was selected and was mixed with water to implement more extreme wear-inducing conditions than limestone slurry of #320 grade. In this case, the content of the abrasive was 30 wt %. The acidity of the abrasive solution was adjusted to a pH of about 3 to 4 using an industrial sulfuric acid. The pH of the abrasive solution was periodically checked and the sulfuric acid was replenished as necessary. Erosion resistance test conditions and properties of alumina are shown in Table 2.
The erosion resistance test was performed for test specimens that were prepared according to Examples 1 to 3 and which were heat-treated at 1000° C. or 1050° C. and for a comparative specimen that was taken from a foreign-made throat bush (A49) which is widely used in Korean thermoelectric power plants. The test was primarily conducted for 40 hours, and the same test was secondarily conducted for 40 hours for reproduction confirmation.
For reference, MPY is the unit that is broadly used in U.S. to represent a corrosion speed. MPY can be represented in meters as described below according to the SI unit system.
1 MPY=0.0254 mm/yr=25.4 μm/yr=2.9 nm/h
MPY: 534 W/DAT
W: weight loss(mg), D: density(g/cm3), A: area(in2), T:time (hr)
Referring to
(2) Corrosion Resistance Test
A corrosion resistance test was performed using a corrosive solution called green death solution (4.2 vol % H2SO4+1.8 vol % HCl+0.6 wt % FeCl3+0.6 wt % CuCl2+Water). Test specimens were dipped in the corrosive solution for 24 hours at room temperature and the amount of corrosive wear for each test specimen was measured. The corrosive solution was weighed in a volume of 500 mL, and each test specimen with a size of 9.8 mm3 was subjected to polishing. In order to compare corrosion resistance between test specimens and comparative specimens, a comparative specimen taken from a foreign-made throat bush (A49, Warman® throatbush), which is widely used in Korean thermoelectric power plants, and another comparative specimen taken from STS 316L were tested for corrosion resistance.
Referring to
(3) Second Erosion Resistance Test
The erosion resistance test was further carried out on the composition according to Example 2, which had showed excellent properties in the first erosion resistance test. The second erosion resistance test was performed at the same rpm and temperature (400 rpm and 80° C.) as the first erosion resistance test. However, in the second erosion resistance test, pH was changed. The test time was 40 hours and MPY was calculated.
(a) First Test Condition (pH 4.5 to 5.5, 400 rpm, 80° C., #240 Alumina, 40 hours)
Referring to
(b) Second Test Condition (pH 1.5 to 2.5, 400 rpm, 80° C., #240 Alumina, 40 hours)
Referring to
On the other hand, the composition of Example 2 exhibited excellent erosion resistance compared to the STS 316L and the A49 prototype in the erosion test which was performed within a pH range of 1 to 4, regardless of the heat treatment temperatures.
While exemplary embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art will appreciate that the present invention can be implemented in other different forms without departing from the technical spirit or essential characteristics of the exemplary embodiments. Therefore, it can be understood that the exemplary embodiments described above are only for illustrative purposes and are not restrictive in all aspects.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2018-0059199 | May 2018 | KR | national |
