KR20240000854A - Heat-resistant steel with improved high-temperature properties and bolt-nut fastening member used for turbine casing for thermal power comprising the same - Google Patents

Abstract

The present invention provides 0.09 to 0.15% by weight of carbon (C); Silicon (Si) greater than 0% by weight and less than or equal to 0.18% by weight; Manganese (Mn) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Nickel (Ni) in an amount greater than 0% by weight and less than or equal to 0.2% by weight; 10.0 to 10.5% by weight chromium (Cr); 0.65 to 0.75% by weight molybdenum (Mo); 0.15 to 0.25% by weight of vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.06 to 0.12 weight percent niobium (Nb); 1.7 to 1.9 weight percent tungsten (W); 3.0 to 3.5% by weight of cobalt (Co); Phosphorus (P) greater than 0% by weight and less than or equal to 0.015% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.01% by weight; 0.003 to 0.008 weight percent boron (B); 0.01 to 0.035 weight percent nitrogen (N); The present invention relates to a heat-resistant steel having excellent high-temperature characteristics, including iron (Fe) residues and other inevitable impurities, and a bolt-nut fastening member for a turbine casing made of the heat-resistant steel.

Description

Heat-resistant steel with improved high-temperature properties and bolt-nut fastening members for thermal power turbine casings containing the same

The present invention relates to a novel heat-resistant steel, and more specifically, to a heat-resistant steel with high temperature characteristics optimized as a material for bolt and nut fastening members for fastening thermal power plant turbine casings.

The turbine casing is an airtight chamber that covers the outside of the turbine's blade rotor, providing space for steam to do work, and has a diaphragm and packing assembled inside. The turbine casing is structurally divided into high-pressure turbines, medium-pressure turbines, and low-pressure turbines. It is divided into upper and lower halves on the horizontal center plane for easy disassembly and assembly, and is joined with casing bolts to fasten them.

When servicing a turbine, the casing is disassembled to inspect and maintain the rotor, blades, diaphragm, and packing inside. For this, disassembly and assembly of the casing bolts must be preceded by disassembly and assembly of the bolts. It is designed to heat the bolt using a heater to thermally expand it and then tighten it tightly using contraction force when cooled.

Casing bolts, unlike regular bolts, have a length of up to 1,620 mm, and are mainly made of 12Cr steel and Inconel 718. The casing bolts are hollow inside the bolt, so the bolt heating part can be directly inserted into the hollow as shown in Figure 1. It is a heating structure.

Casing bolts and nuts used in such high-temperature environments have a damage mechanism that leads to damage or adhesion between parts due to thermal expansion when used for a long time. In particular, if casing bolts are damaged during operation of a power plant, it may result in long-term shutdown, resulting in maintenance man-hours and large losses in electricity production. Therefore, it can be said that further development of materials with excellent high-temperature characteristics is needed.

Republic of Korea Patent Publication No. 10-2019-0044154 (Publication date: 2019.04.30) Republic of Korea Patent Publication No. 10-2022-0025240 (Publication date: 2022.03.03)

The purpose of the present invention is to provide a new heat-resistant steel with excellent high-temperature properties suitable as a material for bolts and nuts for thermal power turbine casings, and a fastening member composed of bolts and nuts for thermal power turbine casings containing the same.

In order to solve the above problem, the present invention includes 0.09 to 0.15% by weight of carbon (C); Silicon (Si) greater than 0% by weight and less than or equal to 0.18% by weight; Manganese (Mn) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Nickel (Ni) in an amount greater than 0% by weight and less than or equal to 0.2% by weight; 10.0 to 10.5% by weight chromium (Cr); 0.65 to 0.75% by weight molybdenum (Mo); 0.15 to 0.25% by weight of vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.06 to 0.12 weight percent niobium (Nb); 1.7 to 1.9 weight percent tungsten (W); 3.0 to 3.5% by weight of cobalt (Co); Phosphorus (P) greater than 0% by weight and less than or equal to 0.015% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.01% by weight; 0.003 to 0.008 weight percent boron (B); 0.01 to 0.035 weight percent nitrogen (N); We propose a heat-resistant steel with excellent high-temperature characteristics that contains iron (Fe) residues and other unavoidable impurities.

In addition, the present invention provides a method for manufacturing the heat-resistant steel, including (a) 0.09 to 0.15% by weight of carbon (C); Silicon (Si) greater than 0% by weight and less than or equal to 0.18% by weight; Manganese (Mn) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Nickel (Ni) in an amount greater than 0% by weight and less than or equal to 0.2% by weight; 10.0 to 10.5% by weight chromium (Cr); 0.65 to 0.75% by weight molybdenum (Mo); 0.15 to 0.25% by weight of vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.06 to 0.12 weight percent niobium (Nb); 1.7 to 1.9 weight percent tungsten (W); 3.0 to 3.5% by weight of cobalt (Co); Phosphorus (P) greater than 0% by weight and less than or equal to 0.015% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.01% by weight; 0.003 to 0.008 weight percent boron (B); 0.01 to 0.035 weight percent nitrogen (N); Manufacturing an alloy by casting molten metal containing iron (Fe) residues and other unavoidable impurities; (b) plastic working the alloy; (c) a heat treatment step of heating the alloy to 1100°C, maintaining it for a predetermined time, and then rapidly cooling it; and (d) a tempering heat treatment step of maintaining the alloy at 650°C for a predetermined time.

And, the present invention is 0.12 to 0.16% by weight of carbon (C); Silicon (Si) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; 0.3 to 0.7% by weight of manganese (Mn); Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.5% by weight; 0.35 to 0.65% by weight of nickel (Ni); 10.0 to 11.0 weight percent chromium (Cr); 0.3 to 0.5% by weight molybdenum (Mo); 0.14 to 0.20 weight percent vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.05 to 0.11 weight percent niobium (Nb); 1.5 to 1.9 weight percent tungsten (W); Phosphorus (P) greater than 0% by weight and less than or equal to 0.02% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.015% by weight; 0.04 to 0.08 weight percent nitrogen (N); We propose a heat-resistant steel with excellent high-temperature characteristics that contains iron (Fe) residues and other unavoidable impurities.

In addition, the present invention provides a method for manufacturing the heat-resistant steel, including (a) 0.12 to 0.16% by weight of carbon (C); Silicon (Si) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; 0.3 to 0.7% by weight of manganese (Mn); Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.5% by weight; 0.35 to 0.65% by weight of nickel (Ni); 10.0 to 11.0 weight percent chromium (Cr); 0.3 to 0.5% by weight molybdenum (Mo); 0.14 to 0.20 weight percent vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.05 to 0.11 weight percent niobium (Nb); 1.5 to 1.9 weight percent tungsten (W); Phosphorus (P) greater than 0% by weight and less than or equal to 0.02% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.015% by weight; 0.04 to 0.08 weight percent nitrogen (N); Manufacturing an alloy by casting molten metal containing iron (Fe) residues and other unavoidable impurities; (b) plastic working the alloy; (c) a heat treatment step of heating the alloy to 1100°C, maintaining it for a predetermined time, and then rapidly cooling it; and (d) a tempering heat treatment step of maintaining the alloy at 650°C for a predetermined time.

Furthermore, the present invention proposes a fastening member composed of bolts and nuts as a fastening member for fastening the turbine casing made of the above-described heat-resistant steel.

The heat-resistant steel according to the present invention has high temperature characteristics optimized as a material for bolt and nut fastening members for fastening turbine casings in thermal power plants, enabling local production of turbine casing bolts and nuts, and further developing high-temperature component materials, heat treatment, and manufacturing process technology. It contributes to the establishment and has the potential to expand and apply its application area to other parts.

Figure 1 is a conceptual diagram showing the structure and disassembly process of a turbine casing bolt.
Figure 2 is an optical microscope photograph showing the microstructure of a heat-resistant steel (MTB10AA compatible material) specimen according to Composition 1 of Example 1 of the present application.
Figure 3 is an optical microscope photograph showing the microstructure of a heat-resistant steel (10705MBU compatible material) specimen according to Composition 2 of Example 1 herein.
Figure 4 shows the SEM-EDS Mapping results of a commercial material (MTB10AA) specimen.
Figure 5 is the SEM-EDS Mapping result of a heat-resistant steel (MTB10AA compatible material) specimen according to Composition 1 of Example 1 of the present application.
Figure 6 shows the SEM-EDS Mapping results of a commercial material (10705MBU) specimen.
Figure 7 shows the SEM-EDS Mapping results of a heat-resistant steel (10705MBU compatible material) specimen according to Composition 2 of Example 1 of the present application.

In describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

Since the embodiments according to the concept of the present invention can make various changes and have various forms, specific embodiments will be illustrated in the drawings and described in detail in the present specification or application. However, this is not intended to limit the embodiments according to the concept of the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, and substitutes included in the spirit and technical scope of the present invention.

The terms used herein are only used to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as “include” or “have” are intended to indicate the existence of a described feature, number, step, operation, component, part, or combination thereof, but are not intended to indicate the presence of one or more other features or numbers. It should be understood that this does not preclude the existence or addition of steps, operations, components, parts, or combinations thereof.

Hereinafter, the present invention will be described in detail.

The present invention relates to a novel heat-resistant steel with excellent heat-resistant properties suitable as a material for parts exposed to a high temperature environment for a long time, such as bolts and nuts for fastening the turbine casing of a thermal power plant.

The heat-resistant steel with improved high-temperature properties according to the present invention contains 0.09 to 0.15% by weight of carbon (C); Silicon (Si) greater than 0% by weight and less than or equal to 0.18% by weight; Manganese (Mn) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Nickel (Ni) in an amount greater than 0% by weight and less than or equal to 0.2% by weight; 10.0 to 10.5% by weight chromium (Cr); 0.65 to 0.75% by weight molybdenum (Mo); 0.15 to 0.25% by weight of vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.06 to 0.12 weight percent niobium (Nb); 1.7 to 1.9 weight percent tungsten (W); 3.0 to 3.5% by weight of cobalt (Co); Phosphorus (P) greater than 0% by weight and less than or equal to 0.015% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.01% by weight; 0.003 to 0.008 weight percent boron (B); 0.01 to 0.035 weight percent nitrogen (N); It is characterized by containing iron (Fe) residues and other inevitable impurities.

In addition, another heat-resistant steel having improved high-temperature properties according to the present invention includes 0.12 to 0.16% by weight of carbon (C); Silicon (Si) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; 0.3 to 0.7% by weight of manganese (Mn); Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.5% by weight; 0.35 to 0.65% by weight of nickel (Ni); 10.0 to 11.0 weight percent chromium (Cr); 0.3 to 0.5% by weight molybdenum (Mo); 0.14 to 0.20 weight percent vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.05 to 0.11 weight percent niobium (Nb); 1.5 to 1.9 weight percent tungsten (W); Phosphorus (P) greater than 0% by weight and less than or equal to 0.02% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.015% by weight; 0.04 to 0.08 weight percent nitrogen (N); It is characterized by containing iron (Fe) residues and other inevitable impurities.

Meanwhile, in the heat-resistant steel with excellent high-temperature characteristics according to the present invention, the reason for limiting the alloy elements and their contents (% means weight%) as described above is as follows.

① Carbon (C)

It is an element that affects the structure and strength of steel. Regarding creep characteristics, the C content and N content most suitable for creep rupture strength change depending on the amount of V, Nb, etc. added. C promotes martensite transformation and combines with Fe, Cr, Mo, V, Nb, and W in the alloy to form carbides and increase high-temperature strength. If the carbides are small, it forms (Fe, Cr)₂(Mo, W). Cohesion and coarsening of the Laves phase, an intermetallic compound, are promoted and high temperature creep strength is reduced. From this perspective, the range of 0.08 to 0.16% is appropriate.

② Chrome (Cr)

It is an element that improves corrosion resistance and oxidation resistance. It is necessary to increase the amount added along with the increase in the use temperature of steel. Cr, along with Re, is a major element that controls the decline in creep strength over a long period of time at 650°C, and by strictly limiting the amount of Cr added, it is possible to maintain creep strength at high temperatures over a long period of time.

In general, for 8-12% Cr ferritic heat-resistant steel, it is said that high Cr side is advantageous as long as δ-ferrite does not occur at 600℃ high stress and short time (1000-2000 hr), but if Cr exceeds 9.8%, martensite structure is formed. This softens and the high temperature creep strength decreases. When adding Re, it has the effect of suppressing the decrease in high-temperature creep strength, so it is good to set it at 8.5 to 10.2%.

The acceleration creep suppression parameter must be set to 0 or less to control the coarsening of the Laves phase precipitated in the matrix, and can be set to more than 50,000 hours, when creep deformation begins to accelerate discontinuously.

·Acceleration creep suppression parameter calculation formula

3[%Cr] + [%Mo] + [%W] - 15[%Re] - 31.5

The addition of Re is effective in improving the high-temperature properties of the material, but is very expensive. Therefore, this study aims to improve the high-temperature properties of the material by controlling the amount of Micro Alloy added, and Cr is limited to the range of 10-11%.

③ Molybdenum (Mo)

High-temperature strength is increased through solid solution strengthening and precipitation strengthening. Mo suppresses the cohesion and coarsening of carbides, strengthens the matrix through solid solution in the alloy, and improves high-temperature strength and high-temperature creep strength by precipitating in fine dispersion in the matrix in the form of Laves. If added in excess, it is easy to generate δ-ferrite and promotes coarsening of Laves phase agglomeration, so it is limited to 0.1 to 1.5%, more preferably 0.3 to 0.75%.

④ Tongue stem (W)

It suppresses the cohesion and coarsening of carbides, strengthens the matrix through solid solution in the alloy, and improves high-temperature strength and creep strength by finely dispersing and precipitating it as a lasbes phase in the matrix. If it exceeds 5%, it becomes easy to generate δ-ferrite and promotes coarsening of Laves phase agglomeration, so it is limited to 1.2 to 4%, and more preferably 1.5 to 1.9%.

⑤ Vanadium (V)

V forms fine carbides and carbonitrides and improves high-temperature creep strength, but if the carbide is excessive, it reduces high-temperature strength, so it is limited to 0.1 to 0.25%, and more preferably 0.14 to 0.25%.

⑥ Niobium (Nb)

Nb is an element that forms fine carbides and carbonitrides, improves high-temperature creep strength, and improves low-temperature toughness by promoting refinement of crystal grains. According to the experimental results so far, the creep rupture strength of 0.05% Nb is optimal. This Nb value exceeds the solubility limit, but the Nb that could not be dissolved becomes NbC, which is effective in suppressing the coarsening of austenite grains during annealing. In the present invention, the Nb content is limited to 0.05-0.12%.

⑦ Cobalt (Co)

Co suppresses the formation of δ-ferrite and improves high-temperature strength and high-temperature creep strength. To prevent the formation of δ-ferrite, the content is required to be 1.5% or more. If it exceeds 6%, ductility and high-temperature creep strength are reduced and costs are increased, so it is limited to 2.5% to 4.5%, and more preferably 3.0%. It is ~3.5%.

⑧ Copper (Cu)

Cu, like Mn and Ni, is an austenite stabilizing element, so it contributes to improving toughness and suppresses precipitation of δ ferrite phase and carbides. It also has the effect of lowering the Ac1 point and improving hardenability. In addition, the formation of a softened layer in the weld heat-affected zone is suppressed. If it is less than 0.1%, the above effect is not sufficient, and if it exceeds 1%, the high temperature creep strength and hot workability are reduced, so it can be added in an amount of 0.8% or less, more preferably 0.5% or less.

⑨ Boron (B)

The hardenability of steel is improved by adding about 0.005%. It is also known to be effective in improving strength and toughness as the structure becomes denser. This has the effect of suppressing the coarsening of austenite particles, martensite, precipitated carbides in martensite, sulfur nitrides, and precipitated Laves phases at high temperatures and over a long period of time. In addition, high-temperature creep strength is improved by complex addition with alloy elements such as W and Nb. On the other hand, if it exceeds 0.015%, a precipitated BN phase is formed, and high temperature creep ductility and toughness deteriorate. The appropriate usage range is 0.003~0.008%.

⑩ Nitrogen (N)

N combines with Nb, V, etc. to form nitride and improves high-temperature strength and high-temperature creep strength, and the appropriate amount is preferably 0.010 to 0.08%.

⑪ Nickel (Ni)

Ni, like Mn, is an austenite stabilizing element, and when it exceeds 0.8%, it promotes the cohesion and coarsening of carbides and Laves phases, reducing high-temperature creep strength. The scope of use is limited to 0.8% or less.

⑫ Silicon (Si)

Si is always used as a deoxidizer, but if there is too much of it, segregation increases in the steel, it is highly susceptible to temper embrittlement, and cutting toughness is lost. Aging at high temperatures for a long time causes a decrease in toughness. Considering so-called high purification to suppress brittleness of steel, it is better to have as few of these elements as possible. However, it is known that Si has the effect of suppressing water vapor oxidation, so it is good to secure a certain amount of Si in boiler materials. Commercially, it is limited to 0.25% or less, but more preferably, it is limited to 0.18%.

⑬ Manganese (Mn)

It is used as a deoxidizing and desulfurizing agent during the dissolution process. Mn combines with S to reduce toughness, and at the same time, accelerates the decline in toughness due to long-term aging at high temperatures and forms non-metallic inclusions that reduce high-temperature creep strength. In the present invention, the Mn content is limited to 0.7% or less.

⑭ Phosphorus (P)

P is an element that increases susceptibility to tempering embrittlement and accelerates the decline in toughness due to long-term aging at high temperatures. To improve this, it is best to reduce the content as much as possible. Considering refining technology, it is limited to 0.02% or less.

⑮ Hwang (S)

Since S forms sulfide that reduces toughness together with Mn, Fe, Nb, V, etc., it is better to keep S as small as possible. Considering the limitations of refining technology, it is limited to 0.015% or less.

The heat-resistant steel according to the present invention described in detail above has optimized mechanical properties and high-temperature characteristics as a material for bolt and nut fastening members for fastening thermal power plant turbine casings, enabling local production of turbine casing bolts and nuts, and furthermore, high-temperature component materials and It contributes to the establishment of heat treatment and manufacturing process technology and has the potential to expand and apply its application area to other parts.

Hereinafter, the present invention will be described in more detail with reference to preferred embodiments. The presented examples are only specific examples of the present invention and are not provided for the purpose of limiting the scope of the present invention.

<Example 1>

1. Production of heat-resistant steel specimens according to the present invention

Test materials were manufactured and analyzed to establish the manufacturing conditions for domestic materials, and the composition of heat-resistant steel according to the present invention corresponding to commercial materials (MTB10AA and 10705MBU) is shown in Tables 1 and 2 below.

[Table 1] Heat-resistant steel composition 1 (MTB10AA corresponding material)

[Table 2] Heat-resistant steel composition 2 (10705MBU corresponding material)

The material heat treatment conditions are the same for both types of materials and follow the following heat treatment conditions.

- Quenching: Heating above 1,100℃ (maintenance time: 0.5Hr/25mm) and then rapid cooling using oil.

- Tempering: Heating above 650℃ (maintenance time: 1Hr/25㎜) and then air cooling to room temperature

These final production conditions were modified through the latest update of MPP, and mass production of materials proceeded with the approval of Mujin Precision Co., Ltd. For the MTB10AA corresponding material, 8ton VIM was used, and for the 10705MBU corresponding material, the melting process was performed with 60ton EAF.

The MTB10AA compatible material undergoes re-melting using ESR for high purification after casting, and the 10705MBU compatible material undergoes casting after refining in VOD. Then, the material is cut to length and forged at a set forging ratio using a 2,200 ton press to form a slab.

Afterwards, the surface scale of the forged slab is removed through surface polishing and rolling is performed to shape it into a round bar. The rolled round bars are heat treated and ready for shipment through post-processing and inspection.

2. Material property analysis

Specimens were produced to analyze the characteristics of MTB10AA and 10705MBU corresponding materials. The material corresponding to MTB10AA is 145ϕ, and the material corresponding to 10705MBU is cut into a round bar with a diameter of 155ϕ. Since the diameter of the round bar for both types of materials is larger than 50 mm, specimens were collected at 1/2 the radius to reduce the variation in physical properties occurring on the surface and deep.

The analysis of chemical composition and mechanical properties was requested from the Korea Testing and Research Institute, and the results were received in the form of a report, and the microstructure was analyzed using an optical microscope, scanning electron microscope, and XRD. And thermochemical properties such as thermal expansion coefficient and thermal conductivity were analyzed through Korea University of Technology and Education.

① Analysis of chemical composition and mechanical properties

The chemical composition and mechanical property analysis results of the MTB10AA and 10705MBU corresponding materials are shown in Tables 3 and 4 below.

The Si component range of the MTB10AA compatible material was expanded from 0.15 to 0.18, and the amount of Micro Alloy input was increased compared to the test material to ensure stable mechanical properties. Even though the 145ϕ material, which was 20 mm larger than the 125ϕ produced as the test material, was analyzed, it was confirmed that the tensile strength was 968N/mm2, which was very good compared to the specification of 950N/mm2.

As a result of the component analysis, it satisfies the specifications in all component ranges, and the mechanical properties also meet the standards, so it is judged that the soundness of the MTB10AA domestically produced material is good, and the manufacturing process is also optimized. Considering the mass effect, it is expected that mechanical properties will improve as the diameter of the material decreases.

[Table 3] MTB10AA corresponding material composition and mechanical property analysis results

Because the test material analysis results for the 10705MBU compatible material were good, the material was mass-produced without changing the manufacturing conditions. However, since the test material was melted using a vacuum furnace, but the mass-produced material was melted using a large electric furnace, we wanted to check whether there was a decrease in mechanical properties due to changes in material cleanliness.

As a result of chemical composition analysis, all of the material's component specifications were satisfied. The content of Si, a deoxidizing agent, is within Spec. The content was confirmed to be stable and not close to the upper limit, and the impurities P and S also showed very good results. It is judged that the content of deoxidizer can be maintained at a good level because it is melted in an electric furnace and then goes through a refining process in a vacuum atmosphere (VOD (Vacuum Oxygen Decarburization)).

The mechanical property results also satisfy all specifications, and no decrease in mechanical properties of the mass-produced material compared to the test material was confirmed. Therefore, the chemical composition and manufacturing conditions for the 10705MBU corresponding material were confirmed to be good.

[Table 4] Result of analysis of material composition and mechanical properties for 10705MBU

② Analysis of microstructure characteristics

Cleanliness analysis was conducted on MTB10AA and 10705MBU corresponding materials. The cleanliness test of MTB10AA and 10705MBU materials is specified in the MHPS Spec. to follow JIS G 0555 'Microscopic testing Method for the non-metallic inclusions in steel', and domestically produced materials were tested using the same method. Non-metallic inclusions A is classified as sulfide, B is aluminate, and C is silicate, and the test is conducted by determining the fraction of the corresponding non-metallic inclusion within the microstructure.

As a result of the cleanliness analysis, both MTB10AA and 10705MBU corresponding materials satisfied the material cleanliness specifications. In the case of MTB10AA compatible material, it goes through a re-melting process for high purification, so the standards are more stringent than 10705MBU compatible material and shows better cleanliness. Both types of domestically developed materials are Spec. Since it has a cleanliness level of less than 13%, it is confirmed that there are no problems with cleanliness management during the process.

In the case of δ-ferrite structure, it reduces the toughness and creep ductility of heat-resistant steel, so it must be managed to less than 1%. δ-ferrite was not confirmed in both MTB10AA and 10705MBU corresponding materials.

[Table 5] Cleanliness analysis results of MTB10AA compatible materials

[Table 6] Results of cleanliness analysis of materials corresponding to 10705MBU

The MTB10AA and 10705MBU corresponding materials were micro-polished and etched with Vilella's Reagent, and the microstructure was observed through an optical microscope. Sampling conditions for microstructure observation were the same as those for imported material specimens.

In Figure 2, the microstructure of the MTB10AA corresponding material is listed by magnification. The base tissue is Tempered Martensite, and it is judged to be a healthy tissue as its size and distribution are uniform. The grain size is No. based on ASTM E112. It was analyzed as 6. Compared to imported materials, the grain size is one level finer, and generally the grain size of heat-resistant steel is ASTM No. It is 4 to 8, so it is very good. Compared to imported materials, no microstructural peculiarities are identified.

Figure 3 is a microstructure image of the material corresponding to 10705MBU, and the base structure is also Tempered Martensite. It is confirmed to be a good tissue with uniform size and distribution. The grain size is No. based on ASTM E112. It was analyzed as 7, and the grain size is one level larger than that of imported materials. It is confirmed that there are no microstructural peculiarities compared to the 10705MBU imported material.

The distribution of major chemical components was confirmed by mapping the microstructure observation specimen using EDS of a scanning electron microscope, and the analysis results are shown in Figures 4 to 7. When segregation or carbide is formed at the grain boundary, it appears as a dark dot or line in the mapping image because specific components are gathered in that area. These defects are mainly caused by unstable solidification process after dissolution or poor heat treatment.

As a result of analysis of imported and domestically produced materials, it was found that major elements were evenly distributed without segregation, carbides, or component deficiencies. The distribution of chemical elements in domestically produced materials was very good compared to imported materials, and no structural peculiarities were identified.

Specimens were collected from imported and domestically produced materials, and after fine polishing, crystal structure and tissue fraction analysis was performed using XRD.

According to the graph showing the XRD peak for the MTB10AA material and the 10705MBU material, the peak was confirmed at the same 2θ position for the imported raw material and the domestically produced material, showing that the crystal structures of the two materials were the same. If the crystal structures of the two materials are not the same, the positions of the peaks appear differently, and if stress exists due to heat treatment and processing, a peak shift appears.

The MTB10AA raw material was analyzed to have a tempered martensite fraction of 99.77%, and the domestically produced material was analyzed to be 99.79%. The 10705MBU raw material was analyzed to have a tempered martensite fraction of 99.93%, and the domestically produced material was analyzed to be 99.33%. It is believed that the raw material and the domestically produced material have very similar tissue fractions, and these results appear to be the same XRD peak.

③ Thermal characteristics analysis

In the case of materials used at high temperatures, expansion occurs as the temperature rises, and since turbine casing bolts are processed to the same drawing when manufacturing, problems with thermal expansion may occur if the thermal characteristics of the substitute material differ greatly from the existing material. Therefore, the thermal characteristics of imported raw materials and domestically produced materials were analyzed and compared.

Table 7 below shows data comparing the thermal expansion coefficients of imported and domestically produced materials. In the case of MTB10AA material, the difference between the domestically produced material and the imported material is about 0.08%, which means that there is almost no difference in thermal expansion coefficient between the two materials. The difference in thermal expansion coefficient of the 10705MBU material between domestically produced and imported materials is approximately 0.5%, and the difference in thermal expansion coefficient between the two materials is also considered to be very good. The difference in thermal expansion coefficient between the two materials is such that when 1m of material is heated to 500℃, the MTB10AA domestic material has a difference of 0.005 mm compared to the imported material, and the 10705MBU domestic material has a difference of 0.03 mm compared to the imported material. Turbine casing bolts Considering the processing tolerance, the effect is very minimal.

[Table 7] Analysis results of thermal expansion coefficient (25℃ ~ 550℃) of MTB10AA and 10705MBU materials

If thermal conductivity is low in a high temperature environment, the ability to dissipate heat to the outside decreases and the cooling efficiency of the system decreases. Therefore, alternative materials should be designed to have similar thermal conductivity to the reference material. Table 8 below shows data comparing the thermal conductivity of imported and domestically produced materials, and the measurement results were averaged after measuring the thermal conductivity of three specimens according to the type of material.

At 550℃, the difference in thermal conductivity between the imported and domestically produced materials of MTB10AA is about 0.8%, and the difference in thermal conductivity between the imported and domestically produced materials of 10705MBU is 0.07%. Considering the standard deviation generated from data measuring the same type of material three times, it can be seen that there is almost no difference in thermal conductivity between imported and domestically produced materials.

[Table 8] Thermal conductivity analysis results of MTB10AA and 10705MBU materials

Specific heat is the amount of heat required to raise 1g of a substance by 1℃ and is an inherent property of the material. When the same amount of heat is applied to two substances of the same mass, the material with a higher specific heat has the characteristic of having a smaller temperature change. Table 9 below shows the results of measuring the specific heat characteristics of imported and domestically produced materials, and it can be seen that the specific heat characteristics of imported and domestically produced materials are similar depending on the material type. In other words, comparing all analyzed thermal property results shows that imported materials and domestically produced materials have similar thermal properties at high temperatures and can be replaced.

[Table 9] Specific heat analysis results for MTB10AA and 10705MBU materials

3. Reliability test

① Creep test

A creep test was conducted to analyze the creep characteristics of imported and domestically produced materials. The 10705MBU material was tested at 525℃ and 550℃, and the MTB10AA material, which can be used at higher temperatures than 10705MBU, was tested at 550℃ and 650℃.

The creep test results of MTB10AA imported and domestically produced materials are shown in Table 10 below. When a stress of 355N/㎟ was applied at 550℃ for both imported and domestic materials, no fracture occurred even after 1,530 hours. Since the strain rate is less than 1%, it appears that the creep limit has not yet been reached. When a stress of 220N/㎟ was applied at 650℃, the imported material fractured after 457 hours, and the domestically produced material fractured after 578 hours, which was longer than the imported material.

[Table 10] MTB10AA material creep test results

Creep test results for 10705MBU imported and domestically produced materials are shown in Table 11 below.

For both imported and domestically produced materials, no fracture occurred even when tested for 810 hours under a stress condition of 415N/㎟ at 525℃ and a stress condition of 355N/㎟ at 550℃, and the strain rate was confirmed to be less than 1%.

Both imported and domestically produced materials, MTB10AA and 10705MBU, showed good creep characteristics at 550℃, the temperature for actual use in the field, and the domestically produced material showed better durability than the imported material at a higher temperature of 650℃.

[Table 11] Creep test results for 10705MBU material

<Example 2>

1. Prototype production

The bolts selected for demonstration are divided into three types according to size as follows. To manufacture turbine casing bolts, the material was cut to fit the dimensions as shown in Figure 57.

○ Bolt No. R/L 8, 10 (Outer Casing Bolt): Size 4·3/4″× 1575 - 10705MBU

○ Bolt No. R/L 32, 33 (Inner Casing Bolt): Size 3·1/2″× 795 - 10705MBU

○ Bolt No. R/L 42 (Inner Casing Bolt): Size 5·1/4″×1095 - MTB10AA

The material cut to size was processed using a CNC programmed according to the drawing to complete the production of the bolt. The manufactured bolts were inspected for dimensions and appearance, and as a result of the inspection, they were confirmed to be good products.

2. Field verification test

The 10 turbine casing bolts manufactured were assembled into the HP turbine of Dangjin Thermal Power Plant Unit 10 on May 10, 2021, and operation began with the completion of OH on June 10, 2021. Reliability was verified through on-site inspection and abnormalities such as vibration and temperature through turbine operation trend analysis for one year until May 27, 2022, when the project ends.

As of May 27, 2022, no abnormalities have been identified regarding the operation of the Dangjin Unit 10 HP turbine, and operation trends will continue to be monitored even after the end of the project. Afterwards, during the OH period of Unit 10 in the first half of 2023, the bolts will be disassembled and inspected to closely check whether there are any unusual issues during the two-year field installation.

Above, embodiments of the present invention have been described with reference to the attached drawings, but those skilled in the art will understand that the present invention can be implemented in other specific forms without changing the technical idea or essential features. You will understand that it exists. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

Claims (5)

0.09 to 0.15% by weight carbon (C);
Silicon (Si) greater than 0% by weight and less than or equal to 0.18% by weight;
Manganese (Mn) in an amount greater than 0% by weight and less than or equal to 0.15% by weight;
Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.15% by weight;
Nickel (Ni) in an amount greater than 0% by weight and less than or equal to 0.2% by weight;
10.0 to 10.5% by weight chromium (Cr);
0.65 to 0.75% by weight molybdenum (Mo);
0.15 to 0.25% by weight of vanadium (V);
Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight;
0.06 to 0.12 weight percent niobium (Nb);
1.7 to 1.9 weight percent tungsten (W);
3.0 to 3.5% by weight of cobalt (Co);
Phosphorus (P) greater than 0% by weight and less than or equal to 0.015% by weight;
Sulfur (S) of more than 0% by weight and less than or equal to 0.01% by weight;
0.003 to 0.008 weight percent boron (B);
0.01 to 0.035 weight percent nitrogen (N);
Heat-resistant steel containing iron (Fe) residues and other inevitable impurities. (a) 0.09 to 0.15% by weight of carbon (C); Silicon (Si) greater than 0% by weight and less than or equal to 0.18% by weight; Manganese (Mn) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; Nickel (Ni) in an amount greater than 0% by weight and less than or equal to 0.2% by weight; 10.0 to 10.5% by weight chromium (Cr); 0.65 to 0.75% by weight molybdenum (Mo); 0.15 to 0.25% by weight of vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.06 to 0.12 weight percent niobium (Nb); 1.7 to 1.9 weight percent tungsten (W); 3.0 to 3.5% by weight of cobalt (Co); Phosphorus (P) greater than 0% by weight and less than or equal to 0.015% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.01% by weight; 0.003 to 0.008 weight percent boron (B); 0.01 to 0.035 weight percent nitrogen (N); Manufacturing an alloy by casting molten metal containing iron (Fe) residues and other unavoidable impurities;
(b) plastic working the alloy;
(c) a heat treatment step of heating the alloy to 1100°C, maintaining it for a predetermined time, and then rapidly cooling it; and
(d) a tempering heat treatment step of maintaining the alloy at 650° C. for a predetermined time. 0.12 to 0.16 weight percent carbon (C);
Silicon (Si) in an amount greater than 0% by weight and less than or equal to 0.15% by weight;
0.3 to 0.7% by weight of manganese (Mn);
Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.5% by weight;
0.35 to 0.65% by weight of nickel (Ni);
10.0 to 11.0 weight percent chromium (Cr);
0.3 to 0.5% by weight molybdenum (Mo);
0.14 to 0.20 weight percent vanadium (V);
Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight;
0.05 to 0.11 weight percent niobium (Nb);
1.5 to 1.9 weight percent tungsten (W);
Phosphorus (P) greater than 0% by weight and less than or equal to 0.02% by weight;
Sulfur (S) of more than 0% by weight and less than or equal to 0.015% by weight;
0.04 to 0.08 weight percent nitrogen (N);
Heat-resistant steel containing iron (Fe) residues and other inevitable impurities. (a) 0.12 to 0.16 weight percent carbon (C); Silicon (Si) in an amount greater than 0% by weight and less than or equal to 0.15% by weight; 0.3 to 0.7% by weight of manganese (Mn); Copper (Cu) in an amount greater than 0% by weight and less than or equal to 0.5% by weight; 0.35 to 0.65% by weight of nickel (Ni); 10.0 to 11.0 weight percent chromium (Cr); 0.3 to 0.5% by weight molybdenum (Mo); 0.14 to 0.20 weight percent vanadium (V); Aluminum (Al) exceeding 0% by weight and not exceeding 0.04% by weight; 0.05 to 0.11 weight percent niobium (Nb); 1.5 to 1.9 weight percent tungsten (W); Phosphorus (P) greater than 0% by weight and less than or equal to 0.02% by weight; Sulfur (S) of more than 0% by weight and less than or equal to 0.015% by weight; 0.04 to 0.08 weight percent nitrogen (N); Manufacturing an alloy by casting molten metal containing iron (Fe) residues and other unavoidable impurities;
(b) plastic working the alloy;
(c) a heat treatment step of heating the alloy to 1100°C, maintaining it for a predetermined time, and then rapidly cooling it; and
(d) a tempering heat treatment step of maintaining the alloy at 650° C. for a predetermined time. A bolt and nut fastening member for a turbine casing made of the heat-resistant steel according to claim 1 or 3.