Nickel-based casting alloy, manufacturing method thereof and fire grate using nickel-based casting alloy

Abstract

The invention provides a nickel-based casting alloy, a manufacturing method thereof and a fire grate using the nickel-based casting alloy. The nickel-based casting alloy contains 0.15 to 0.35 mass% of C, 1.5 to 3.0 mass% of Si, 1.5 mass% or less of Mn, 50.0 to 55.0 mass% of Ni, 20.0 to 23.0 mass% of Cr, 2.0 to 6.0 mass% of Mo, 4.0 to 5.0 mass% of W, and 3.7 to 5.4 mass% of Nb, with the balance being Fe and other unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a nickel-based cast alloy, a method for manufacturing the same, and a grate using the nickel-based cast alloy. Background Art

[0002] Heat-resistant alloys typified by nickel-based cast alloys are used as materials for grates of waste incinerators and the like. The grate is laid on the hearth part of a waste incineration facility. The hearth part is the part that bears the waste input into the waste incinerator, and the burned waste moves on the grate. Therefore, the grate needs to have heat resistance to withstand the high temperature generated by the combustion of the waste. Moreover, the grate needs to have both abrasion resistance to withstand abrasion by hard substances such as metals or incineration ash contained in the waste, and corrosion resistance to withstand corrosion by chlorides and sulfides contained in the waste.

[0003] As a heat-resistant alloy having abrasion resistance and corrosion resistance while ensuring heat resistance, austenitic stainless steel disclosed in Japanese Patent Laid-Open No. 60-162757 (hereinafter, Patent Document 1) is known. Summary of the Invention Problems to be Solved by the Invention

[0004] However, the austenitic stainless steel of Patent Document 1 was developed for uses such as thermal power generation boilers. The austenitic stainless steel in thermal power generation boilers and the like is different from the grate for waste incinerators, and the situation of abrasion by hard substances occurs less frequently. Therefore, the austenitic stainless steel of Patent Document 1 aims to improve corrosion resistance at high temperatures rather than abrasion resistance.

[0005] Under such circumstances, the austenitic stainless steel described in Patent Document 1 is a forged material that has been solution-treated under a temperature condition of 1050°C and does not have abrasion resistance to the extent of being used as a grate. Therefore, in the grate using the austenitic stainless steel of Patent Document 1, there is a problem of being worn out in a short period due to abrasion by hard substances contained in the waste.

[0006] Therefore, a heat-resistant alloy suitable for a corrosion environment at high temperatures and an environment where abrasion by hard substances occurs, especially an environment such as a waste incinerator, is required. Therefore, an object of the present invention is to provide a nickel-based cast alloy having improved corrosion resistance and abrasion resistance compared to the prior art, a method for manufacturing the same, and a grate using the nickel-based cast alloy for use in such an environment. Means for Solving the Problems

[0007] In order to solve the above problems, the nickel-based cast alloy of the first invention is characterized in that it contains 0.15 to 0.35% by mass of C, 1.5 to 3.0% by mass of Si, 1.5% by mass or less of Mn, 50.0 to 55.0% by mass of Ni, 20.0 to 23.0% by mass of Cr, 2.0 to 6.0% by mass of Mo, 4.0 to 5.0% by mass of W, and 3.7 to 5.4% by mass of Nb, and the balance contains Fe and other inevitable impurities.

[0008] The nickel-based cast alloy of the second invention is characterized in that the volume fraction of the δ phase (Ni3Nb) precipitated due to Nb is 10% or more and 15% or less.

[0009] The manufacturing method of the nickel-based cast alloy of the third invention is the manufacturing method of the nickel-based cast alloy of the first invention or the second invention, and includes: a step of casting the nickel-based cast alloy; and a step of heat-treating the cast nickel-based cast alloy at a temperature in the range of 745 °C to 765 °C for 10 hours or more.

[0010] The grate of the fourth invention is characterized in that it is used in an incinerator and uses the nickel-based cast alloy of the first invention or the second invention. Advantages of the Invention

[0011] According to the nickel-based cast alloy, its manufacturing method, and the nickel-based cast alloy of the present invention, corrosion resistance and wear resistance can be improved. Brief Description of the Drawings

[0012] Figure 1 It is a schematic diagram showing a stoker-type waste incinerator equipped with a grate using the nickel-based cast alloy of the embodiment of the present invention. Figure 2 It is a diagram showing the calculation results of the carbide precipitation amount when determining the amount of Mo contained in the nickel-based cast alloy of the embodiment of the present invention. Figure 3 It is a diagram showing the calculation results of the δ phase precipitation amount when determining the amount of Nb contained in the nickel-based cast alloy of the embodiment of the present invention. Figure 4 It is a diagram showing the metal structure of the sample before heat treatment in the example. Figure 5 It is a diagram showing the metal structure of the sample after heat treatment in the example. Figure 6 It is the result of measuring the hardness of the samples of the example and the comparative example, and is a diagram showing the relationship between the content of W in each sample and the hardness. Figure 7It is a graph showing the relationship between the area ratio of the δ-phase of the test piece of the example and the hardness at that area ratio. Figure 8 It is a graph showing the relationship between the area ratio of the δ-phase of the test piece of the example and the impact value at that area ratio. Figure 9 It is a graph of the results of a corrosion test conducted using the specimens of the examples and the specimens of the comparative examples. Figure 10 It is a graph showing the relationship between the heat treatment temperature range of the test piece of the example and the hardness within that temperature range. Figure 11 It is a graph showing the relationship between the heat treatment time of the test piece of the example and the area ratio of the δ-phase at that heat treatment time. Figure 12 It is a graph showing the relationship between the heat treatment time of the test piece of the example and the hardness at that heat treatment time. Figure 13 It is a graph showing the relationship between the heat treatment time of the test piece of the example and the impact value at that heat treatment time. Detailed implementation mode

[0013] Hereinafter, with reference to Figure 1 the grate 11 of the waste incinerator 1 using the nickel-based cast alloy of the embodiment of the present invention will be described. In Figure 1 as an example of the grate, the grate 11 used in the mechanical grate type waste incinerator 1 is shown.

[0014] Figure 1 The mechanical grate type waste incinerator 1 shown includes: a hopper 2 into which waste G is thrown; a furnace bed part 3 that sends out the waste G thrown into the hopper 2 and burns the waste G to become incineration ash; and a discharge port 7 that discharges the incineration ash obtained by incinerating the waste G. The furnace bed part 3 includes a drying section 4 that dries the waste G starting from the upstream side along the waste G sending direction, a combustion section 5 that burns the waste G at a position downstream of the drying section 4, and a post-combustion section 6 that further burns the fixed carbon residue of the waste G. A plurality of grates 11 are laid on the furnace bed part 3. On the plurality of grates 11 thus laid on the furnace bed part 3, the waste G containing chlorides and hard substances burns to become ash. Therefore, the grate 11 becomes hot in a corrosive environment and further wears due to hard substances.

[0015] The grate 11 laid on the combustion section 5 becomes particularly hot among the plurality of grates 11 laid on the furnace bed part 3. Specifically, there are cases where the temperature of the grate 11 laid on the combustion section 5 reaches 600 °C or higher. Therefore, in the conventional grate 11 used in such a high-temperature environment, the loss caused by high-temperature corrosion and wear is large, which is a problem from the perspective of lifespan.

[0016] The nickel-based cast alloy of the present invention has improved wear resistance and corrosion resistance in a high-temperature environment compared to the past. By means of precipitation strengthening caused by the precipitation of the δ phase (Ni3Nb) through the inclusion of Nb (niobium), and solid solution strengthening caused by the inclusion of W (tungsten), the wear resistance in a high-temperature environment is improved. In addition, by containing Mo (molybdenum) in an amount that does not reduce the corrosion resistance in addition to Ni (nickel) and Cr (chromium), the oxide film formed on the surface of the nickel-based cast alloy is stabilized, thereby improving the corrosion resistance in a high-temperature environment.

[0017] Hereinafter, the composition of the nickel-based cast alloy, which is the gist of the present invention, will be described in detail.

[0018] The nickel-based cast alloy of the present invention contains 0.15 to 0.35% by mass of C (carbon), 1.5 to 3.0% by mass of Si (silicon), 1.5% by mass or less of Mn (manganese), 50.0 to 55.0% by mass of Ni (nickel), 20.0 to 23.0% by mass of Cr (chromium), 2.0 to 6.0% by mass of Mo (molybdenum), 4.0 to 5.0% by mass of W (tungsten), and 3.7 to 5.4% by mass of Nb (niobium), and the balance contains Fe (iron) and other inevitable impurities.

[0019] Although C is effective in improving wear resistance, it reduces corrosion resistance. Therefore, for the content of C, in order to improve wear resistance, 0.15% by mass is set as the lower limit value, and in order to prevent the reduction of corrosion resistance, 0.35% by mass is set as the upper limit value.

[0020] Si is effective in improving castability and corrosion resistance, but causes embrittlement. Therefore, for the content of Si, in order to improve castability and corrosion resistance, 1.5% by mass is set as the lower limit value, and in order to prevent embrittlement, 3.0% by mass is set as the upper limit value.

[0021] Mn is effective in improving castability and eliminating the harm caused by impurities such as S (sulfur), but reduces corrosion resistance. Therefore, for the content of Mn, in order to prevent the reduction of corrosion resistance, 1.5% by mass is set as the upper limit value.

[0022] Ni stabilizes the austenite phase and improves oxidation resistance and high-temperature strength. Therefore, for the content of Ni, in order to improve oxidation resistance and high-temperature strength, 50.0% by mass is set as the lower limit value, and considering the cost performance of oxidation resistance and high-temperature strength, 55.0% by mass is set as the upper limit value.

[0023] Cr forms a stable spinel-type oxide mainly composed of Cr2O3, improving the corrosion resistance. On the other hand, Cr reacts with chlorides contained in waste G to form chlorides such as chromium chloride (CrCl2). Therefore, Cr reduces the corrosion resistance of the material in a high-temperature environment with a large amount of chlorides. Therefore, for the content of Cr, in order to improve the corrosion resistance, the lower limit value is set to 20.0 mass%, and in order to prevent the reduction of corrosion resistance in a high-temperature environment with a large amount of chlorides, the upper limit value is set to 23.0 mass%.

[0024] By containing Mo in a metal containing Ni and Cr, the corrosion resistance is improved. On the other hand, Mo precipitates carbides. The excessive precipitation of these carbides reduces the corrosion resistance. Therefore, in order to improve the corrosion resistance, the lower limit value of the Mo content is set to 2.0 mass%. In addition, in order to prevent the reduction of corrosion resistance, the upper limit value of the Mo content is determined so that the volume fraction of the above-mentioned carbides precipitated due to Mo is 10% or less. By using the equilibrium state calculation software (Thermo-Calc version 2022a) and the equilibrium state calculation database (Thermo-Calc Software TCNI10 Ni-based Superalloys Database), the amount of carbide precipitation is calculated to obtain the Mo content for making the volume fraction of the precipitated carbides 10%.

[0025] Figure 2 is a graph showing the calculation results of the amount of carbide precipitation when determining the Mo content of the nickel-based cast alloy according to the embodiment of the present invention. In Figure 2 the relationship between the Mo content and the volume fraction of the precipitated carbides under the condition of 600 °C (an example of a high-temperature environment) of the nickel-based cast alloy according to the embodiment is shown. According to Figure 2 the calculation results shown, if the Mo content is 6.0 mass% or less, the volume fraction of the carbides precipitated due to Mo is 10.0% or less. Therefore, the upper limit value of the Mo content is set to 6.0 mass%.

[0026] The Mo content is preferably 2.0 mass% or more and 4.0 mass% or less. This is because, although the upper limit value of the Mo content only needs to be 6.0 mass%, by setting the upper limit value of the Mo content to 4.0 mass%, a further effect can be achieved. The further effect is to ensure the stability during mass production. Hereinafter, with reference to Figure 2 the case of ensuring the stability during mass production will be described. According to Figure 2The calculation results of the carbide precipitation amount shown are as follows. If the Mo content is 2.0 mass% or more and 4.0 mass% or less, the increase amount of carbide with respect to the increase in the Mo content is small. On the other hand, if the Mo content is greater than 4.0 mass%, the increase amount of carbide with respect to the increase in the Mo content becomes larger compared to the case where the Mo content is 2.0 mass% or more and 4.0 mass% or less. Therefore, when mass-producing the nickel-based cast alloy of the present invention, if the Mo content is greater than 4.0 mass%, the deviation in the carbide precipitation amount among individuals due to the deviation in the Mo content among mass-produced individuals becomes larger. However, if the Mo content is 2.0 mass% or more and 4.0 mass% or less, when mass-producing the nickel-based cast alloy of the present invention, the deviation in the carbide precipitation amount among individuals due to the deviation in the Mo content among mass-produced individuals can be suppressed to be small.

[0027] W solid-solution strengthens the alloy containing W and stabilizes the metal structure of the alloy. Thereby, the wear resistance of the alloy containing W can be improved. On the other hand, if the W content is above a certain amount, the effect of improving the wear resistance brought by W saturates. Therefore, from the viewpoint of the cost performance of wear resistance, the W content was determined based on the results of measuring the hardness of multiple specimens with different W contents. According to this result, for the W content, in order to improve the wear resistance, 4.0 mass% was set as the lower limit value, and 5.0 mass%, at which the effect of improving the wear resistance saturates, was set as the upper limit value.

[0028] Nb improves the wear resistance by precipitation strengthening caused by the precipitation of the δ phase. On the other hand, if the δ phase precipitates excessively, the toughness decreases. Therefore, the Nb content was determined based on the volume fraction of the δ phase precipitated due to Nb. In the nickel-based cast alloy of the embodiment, in order to ensure wear resistance equal to or higher than that of the conventional material used as a grate, the lower limit value of the volume fraction of the δ phase in a high-temperature environment was set to 10%, and in order to prevent a decrease in toughness, the upper limit value of the volume fraction of the δ phase was set to 15%. The Nb content at which the volume fraction of the δ phase is 10 - 15% was also calculated by calculating the precipitation amount of the δ phase using the above-mentioned equilibrium state calculation software and the above-mentioned equilibrium state calculation database.

[0029] Figure 3 is a graph showing the calculation results of the δ phase precipitation amount when determining the amount of Nb contained in the nickel-based cast alloy of the embodiment of the present invention. In Figure 3 the relationship between the Nb content and the volume fraction of the precipitated δ phase under the condition of 600 °C (an example of a high-temperature environment) of the nickel-based cast alloy of the embodiment is shown. According to Figure 3Regarding the calculated results shown, if the content of Nb is 3.7 mass% or more, the volume fraction of the δ-phase precipitated due to Nb is 10% or more. Therefore, the lower limit value of the content of Nb is set to 3.7 mass%. Further, if the content of Nb is 5.4 mass% or less, the volume fraction of the δ-phase precipitated due to Nb is 15% or less. Therefore, the upper limit value of the content of Nb is set to 5.4 mass%.

[0030] For example, similar to ordinary nickel-based cast alloys, inevitable impurity elements are elements mixed in to ensure castability. As the above-mentioned inevitable impurities, for example, there are P (phosphorus), S, etc.

[0031] Hereinafter, a preferred manufacturing method of the nickel-based cast alloy of the present invention will be described in detail.

[0032] The manufacturing method of the nickel-based cast alloy according to the embodiment preferably includes a step of casting the nickel-based cast alloy and a step of heat-treating the cast nickel-based cast alloy in a temperature range of 745°C to 765°C (heat treatment step). If this is done, since the precipitation of the δ-phase is promoted, the wear resistance of the nickel-based cast alloy according to the embodiment can be improved.

[0033] The heat treatment time in the heat treatment step is preferably 10 hours or more. The nickel-based cast alloy according to the embodiment heat-treated under such conditions has improved wear resistance.

[0034] Furthermore, the heat treatment time in the heat treatment step is preferably 20 hours or more. In the nickel-based cast alloy according to the embodiment heat-treated under such conditions, the volume fraction of the precipitated δ-phase is 10% or more. Such a nickel-based cast alloy can ensure wear resistance equal to or higher than that of a conventional grate even when not used in a high-temperature environment.

[0035] Moreover, the heat treatment time in the heat treatment step is preferably 24 hours or more and 30 hours or less. If the heat treatment time is 24 hours or more, the change amount of the volume fraction of the δ-phase with respect to the change in the heat treatment time is small. That is, if the heat treatment time is 24 hours or more, the influence of the change in the heat treatment time on the wear resistance is small. Therefore, when mass-producing the nickel-based cast alloy according to the embodiment, the deviation in wear resistance between individual products caused by the deviation in the heat treatment time among individual products in mass production becomes small. On the other hand, if the upper limit value of the heat treatment time is 30 hours, toughness higher than that of a conventional material used as a grate can be ensured, and the nickel-based cast alloy according to the embodiment can be manufactured within a time with good manufacturing efficiency. Examples

[0036] As an example, a Y-shaped sample (a specimen of Invention 1) of the present invention was cast. The Y-shaped sample refers to a sample having a shape corresponding to Shape a of JIS G 0307:2014. The composition of the specimen of Invention 1 is shown in Table 1 below. In addition, the nickel-based cast alloy of the present invention promotes the precipitation of the δ phase by becoming a high temperature after casting. In order to confirm the precipitation amount of the δ phase precipitated by becoming a high temperature, a specimen for heating was collected from the Y-shaped sample of Invention 1. The specimen for heating was heated at 750 °C for 24 hours. The specimen for heating after heating was used as the specimen of Invention 2. Then, test pieces were collected from the specimen of Invention 1 and the specimen of Invention 2, respectively, and material tests for investigating the precipitation amount of carbides, the precipitation amount of the δ phase, wear resistance, toughness, and corrosion resistance were carried out.

[0037] [Table 1] [Mass %] C Si Mn P S Ni Cr Mo W [[ID= ​ ​ 0.23 1.98 0.67 0.021 0.011 51.83 21.79 3.56 4.52 4.73 ​

[0038] The precipitation amount of carbides was measured by observing the microstructure of the cross section of the test piece collected from the specimen of Invention 1, and the precipitation amount of the δ phase was measured by observing the microstructure of the cross section of the test piece collected from the specimen of Invention 2. When observing the microstructures of the specimens of Invention 1 and Invention 2, the cross sections of the objects observed in the test pieces collected from the specimens of Invention 1 and Invention 2 were electrolytically etched with an aqueous solution containing 10 mass% chromic acid after polishing. Then, the cross sections of the test pieces collected from each specimen thus treated were observed with an optical microscope.

[0039] The precipitation morphology of carbides in the microstructure observation of the cross sections of the test pieces collected from the specimens of Invention 1 and Invention 2 is as ​ shown, and the precipitation morphology of the δ phase is as ​ shown. The precipitation amounts of carbide B and δ phase D were measured by their respective area ratios.

[0040] ​ The area ratio of carbide B in the specimen of Invention 1 shown as ​ is 5.9%. The area ratio of carbide B was measured at any cross section of the specimen of Invention 1, and it is considered that the area ratio is approximately equivalent to the volume ratio. Here, in the nickel-based cast alloy of the present invention, the content of Mo was determined by calculation so that the volume ratio of carbides is 10% or less. From

[0041] ​The area ratio of the δ-phase D of the specimen of the inventive article 2 shown is 13.7%. The volume ratio of the δ-phase was measured at any cross-section of the specimen of the inventive article 2, and it is considered that the area ratio and the volume ratio are approximately equivalent. Here, in the nickel-based cast alloy of the present invention, the content of Nb is determined so that the volume ratio of the δ-phase is 10 to 15%. From ​ the results shown, it can be confirmed that in the case of the nickel-based cast alloy of the present invention, the volume ratio of the δ-phase is actually in the range of 10% to 15%.

[0042] Next, for abrasion resistance, toughness, and corrosion resistance, in order to compare with the inventive article 1, Y-shaped samples (specimens of Comparative Example 1 and Comparative Example 2) and a grate block (specimen of Comparative Example 3) of Comparative Example 3 were respectively cast. The compositions of the specimens of Comparative Example 1 to Comparative Example 3 are shown in Table 2 below. As shown in Table 2, the specimens of Comparative Example 1 and Comparative Example 2 differ only in the content of W from the nickel-based cast alloy of the present invention. That is, the content of W in the specimen of Comparative Example 1 is less than the lower limit value of W contained in the nickel-based cast alloy of the present invention. In addition, the content of W in the specimen of Comparative Example 2 is more than the upper limit value of W contained in the nickel-based cast alloy of the present invention. The specimen of Comparative Example 3 is an example of a conventional material for a grate.

[0043] [Table 2] [mass%] C ​ ​ P S ​ Cr ​ W ​ ​ N ​ 0.23 2.00 0.66 0.019 0.012 52.10 21.12 3.63 3.29 4.73 ​ - ​ 0.24 2.08 0.71 0.018 0.012 51.60 21.28 3.64 5.42 4.73 ​ - ​ 0.93 1.85 0.47 0.027 0.011 1.09 28.74 - - - ​ 0.27

[0044] In order to evaluate the abrasion resistance of the nickel-based cast alloy of the present invention, the hardness of the specimen of the inventive article 1 and the specimens of Comparative Example 1 and Comparative Example 2 was measured. The test force was set to 49 N, and the hardness of each specimen was measured using a Vickers hardness tester. The results of measuring the hardness of the specimen of the inventive article 1 and the specimens of Comparative Example 1 to Comparative Example 2 are shown in ​ . The relationship between the content of W in each specimen and the hardness is shown in ​ . According to the measurement results shown in ​ , the hardness of the specimen of the inventive article 1 is a value greater than the hardness of the specimen of Comparative Example 1 and a value less than the hardness of the specimen of Comparative Example 2. In addition, it can be confirmed that if the content of W is greater than 5 mass%, the effect of increasing the hardness, i.e., the abrasion resistance, saturates.

[0045] In order to further evaluate the abrasion resistance of the nickel-based cast alloy of the present invention, the relationship between the volume ratio of the δ-phase precipitated in the nickel-based cast alloy and the abrasion resistance of the nickel-based alloy was confirmed. For this purpose, a plurality of test pieces having different area ratios of only the δ-phase were manufactured, and the hardness of each test piece was measured. The hardness of each test piece was measured using the above-mentioned Vickers hardness tester. The area ratio of the δ-phase of each test piece and the hardness of each test piece are shown in ​ . From​ It was confirmed that the hardness of the nickel-based cast alloy of the present invention with a δ-phase area ratio of 10% or more is a larger value compared to the hardness of 250 HV of the conventional material used as a grate. That is, it was confirmed that the nickel-based cast alloy of the present invention with a δ-phase volume ratio of 10% or more has higher abrasion resistance compared to the conventional material used as a grate.

[0046] In addition, in order to evaluate the toughness of the specimen of the present invention product 1, Charpy impact tests were carried out using test pieces collected from the specimens of the present invention product 1 and the specimens of Comparative Examples 1 to 3. When carrying out the Charpy impact test, a U-shaped notch with a depth of 2 mm was provided on the test pieces collected from each specimen. Table 3 shows the results of the Charpy impact tests of the test pieces collected from the specimens of the present invention product 1 and the specimens of Comparative Examples 1 to 3. According to the test results shown in Table 3, the impact values measured using the specimens of the present invention product 1 and the specimens of Comparative Examples 1 and 2 are larger values than the impact value measured using the specimen of Comparative Example 3. Here, the impact value of each test piece measured by the Charpy impact test is regarded as the toughness of each test piece. Therefore, according to the results of the Charpy impact test shown in Table 3, it was confirmed that the toughness of the nickel-based cast alloy of the present invention is higher than that of the conventional material used as a grate.

[0047] [Table 3] <![CDATA[Impact value (J / cm 2 )]]> ​ 8.0 ​ 7.0 ​ 5.1 ​ 2.2

[0048] In order to further evaluate the toughness of the specimen of the present invention product, the relationship between the volume ratio of the δ-phase precipitated in the nickel-based cast alloy and the toughness of the nickel-based cast alloy was confirmed. For this purpose, a plurality of test pieces with different δ-phase area ratios were manufactured, and the impact values of each test piece were measured. The impact values of each test piece were measured by the above-mentioned Charpy impact test. ​ shows the δ-phase area ratio of each test piece and the impact value of each test piece. From ​ it was confirmed that, compared to the impact value of 2.2 J / cm 2 (the impact value of Comparative Example 3 in Table 3) of the conventional material used as a grate, the impact value of the nickel-based cast alloy of the present invention with a δ-phase area ratio of 15% or less becomes a larger value. That is, it was confirmed that the nickel-based cast alloy of the present invention with a δ-phase volume ratio of 15% or less has higher toughness compared to the conventional material used as a grate.

[0049] In order to evaluate the corrosion resistance of the nickel-based cast alloy of the present invention at high temperatures, high-temperature corrosion tests based on JIS Z2293:2004 were carried out using plate-shaped test pieces respectively collected from the specimens of Invention Product 1, the specimens of Invention Product 2, and the specimens of Comparative Example 3. For the shape of the plate-shaped test pieces collected from each specimen, the length was set to 15 mm, the width was set to 10 mm, and the thickness was set to 2 mm. In the high-temperature corrosion test, while the plate-shaped test pieces collected from each specimen were buried in the incineration ash collected from a waste incinerator, the overall temperature was raised and maintained. Table 4 shows the results of EDX (Energy Dispersive X-ray Spectroscopy) elemental analysis of the incineration ash in which each test piece was buried. Differential thermal analysis was performed on the incineration ash in advance to study the holding temperature of the high-temperature corrosion test. Although the details of the differential thermal analysis are not described, it was found from the results of the differential thermal analysis that the incineration ash shown in Table 4 showed signs of melting at 660°C. Here, the incineration ash containing Cl (chlorine) and S promotes the corrosion of the test pieces in the molten state. Therefore, the temperature maintained in the high-temperature corrosion test was set to 660°C.

[0050] [Table 4] [Mass%] O ​ ​ Al ​ P S Cl K ​ ​ ​ ​ ​ 32.6 3.7 1.7 3.5 4.3 0.9 2.1 8.8 3.0 30.8 1.8 3.2 1.0 1.8

[0051] In the high-temperature corrosion test, while each test piece was buried in the incineration ash shown in Table 4, it was maintained at a temperature of 660°C for 100 hours. After that, the amount of thinning (mm) of each test piece was measured. The results of the high-temperature corrosion test are shown in ​ . According to the results of the high-temperature corrosion test, no significant difference was found in the amount of thinning per 100 hours between the test pieces collected from the specimens of Invention Product 1 and the test pieces collected from the specimens of Invention Product 2. In other words, it was confirmed that the corrosion resistance of the nickel-based cast alloy of the present invention was not different with or without heat treatment. In addition, the amount of thinning of the test piece collected from the specimen of Invention Product 1 was 0.2 times or less the amount of thinning of the test piece collected from the specimen of Comparative Example 3. That is, the specimen of Invention Product 1 had a corrosion resistance more than 5 times that of the specimen of Comparative Example 3.

[0052] In the above, the wear resistance and corrosion resistance of the nickel-based cast alloy of the present invention were evaluated using the specimens of Invention Product 1 and Invention Product 2, and the specimens of Comparative Examples 1 to 3. Since the nickel-based cast alloy of the present invention is used under high-temperature conditions, it can ensure wear resistance equal to or higher than that of conventional materials used as grate bars. In addition, it was confirmed that the nickel-based cast alloy of the present invention has higher corrosion resistance than conventional materials used as grate bars.

[0053] In order to confirm the relationship between the heat treatment temperature and the wear resistance of the nickel-based cast alloy of the present invention manufactured by a manufacturing method including a process of casting a nickel-based cast alloy and a process of heat-treating the cast nickel-based cast alloy, after casting, test pieces heat-treated at a temperature range of 730°C to 770°C for 10 hours or 24 hours were manufactured, and the hardness of each was measured. Specifically, as ​ shown, after casting, test pieces heat-treated at each temperature condition of 730°C, 740°C, 745°C, 750°C, 760°C, 765°C, or 770°C for 10 hours or 24 hours were manufactured. The hardness of the test pieces heat-treated at each temperature condition for 10 hours or 24 hours was measured using a Vickers hardness tester under a test force of 49 N.

[0054] In ​ in addition to showing the hardness measured with each of the above test pieces, the hardness of the sample of Invention Product 1 is also shown. As ​ shown, the hardness of the test pieces heat-treated at a temperature range of 745°C to 765°C for 10 hours becomes a value greater than the hardness of the sample of Invention Product 1. That is, the wear resistance of the test pieces heat-treated at a temperature range of 745°C to 765°C for 10 hours is improved compared to the case where no heat treatment is performed. Similarly, the wear resistance of the test pieces heat-treated at a temperature range of 745°C to 765°C for 24 hours is improved compared to the case where no heat treatment is performed.

[0055] Furthermore, in order to confirm the volume fraction of the δ-phase precipitated in the nickel-based cast alloy of the present invention manufactured by a manufacturing method including a process of casting a nickel-based cast alloy and a process of heat-treating the cast nickel-based cast alloy, after casting, a plurality of test pieces with only different heat treatment times were manufactured. Specifically, test pieces heat-treated at a heat treatment temperature of 750°C for 10 hours, 20 hours, 24 hours, or 30 hours were manufactured. Moreover, the area fraction of the test pieces manufactured under each condition was measured.

[0056] In ​ in addition to showing the area fraction of the δ-phase of each of the above test pieces, the area fraction of the δ-phase of the sample of Invention Product 1 is also shown. As ​ shown, the area fraction of the δ-phase of the test piece heat-treated at 750°C for 20 hours is a value greater than 10%.

[0057] Moreover, in order to confirm the relationship between the heat treatment time and the wear resistance, and the relationship between the heat treatment time and the toughness of the nickel-based cast alloy of the present invention manufactured by a manufacturing method including a process of casting a nickel-based cast alloy and a process of heat-treating the cast nickel-based cast alloy, the hardness and the impact value of the test pieces heat-treated at 750 °C for 10 hours, 20 hours, 24 hours, or 30 hours were measured. The hardness of these test pieces was measured using a Vickers hardness tester in the same manner as described above, and the impact value of the test pieces was measured using a Charpy impact tester in the same manner as described above.

[0058] In ​ in addition to showing the Vickers hardness of each of the above test pieces, the Vickers hardness of the sample of Invention Product 1 is also shown. As ​ shown, the hardness of the test piece heat-treated at 750 °C for 24 hours is equal to the hardness of the test piece heat-treated at 750 °C for 30 hours. In addition, in ​ in addition to showing the impact value of each of the above test pieces, the impact value of the sample of Invention Product 1 is also shown. As ​ shown, there is a tendency that the longer the heat treatment time, the lower the impact value. Moreover, the impact value of the test piece with a heat treatment time of 30 hours is higher than the impact value of the sample of Comparative Example 3.

[0059] From the above, it can be confirmed that the nickel-based cast alloy of the present invention improves the wear resistance by performing heat treatment for 10 hours or more within the temperature range of 745 °C to 765 °C, and can ensure wear resistance equal to or higher than that of conventional materials used for grate bars by performing heat treatment for 24 hours or more. During the heat treatment for 24 hours or more and 30 hours or less, the change in the heat treatment time has little effect on the wear resistance.

Claims

1. A nickel-based cast alloy, characterized in that, the nickel-based cast alloy contains 0.15 to 0.35% by mass of C, 1.5 to 3.0% by mass of Si, 1.5% by mass or less of Mn, 50.0 to 55.0% by mass of Ni, 20.0 to 23.0% by mass of Cr, 2.0 to 6.0% by mass of Mo, 4.0 to 5.0% by mass of W, and 3.7 to 5.4% by mass of Nb, and the balance contains Fe and other inevitable impurities.

2. The nickel-based cast alloy according to claim 1, characterized in that, the volume fraction of the δ phase (Ni3Nb) precipitated due to Nb is 10% or more and 15% or less.

3. A method for manufacturing a nickel-based cast alloy, which is the method for manufacturing the nickel-based cast alloy according to claim 1 or 2, characterized in that, It includes: a step of casting the nickel-based cast alloy; and a step of heat-treating the cast nickel-based cast alloy at a temperature in the range of 745°C to 765°C for 10 hours or more.

4. A grate used in an incinerator, characterized in that, the nickel-based cast alloy according to claim 1 or 2 is used.

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