Heat-resistant alloys for fire grates and incinerator grates

Abstract

This invention provides a heat-resistant alloy for fire grates and incinerator grates that are highly hard and have large shock absorption energy. [Solution] The above problem was solved by a heat-resistant alloy for grates containing, by mass%, 0.86-0.93% C, 1.8-2.0% Si, 0.6-0.9% Mn, 23-25% Cr, 0.17-0.23% N, and 0.15-0.4% Ni, with the remainder being Fe and unavoidable impurities.

Description

Technical Field

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[0001] The present invention relates to a heat-resistant alloy for a fire grate having high hardness and large impact absorption energy, and a waste incinerator fire grate.

Background Art

[0002] The fire grate used in a waste incinerator is spread on the furnace floor of the waste incinerator equipment to stir and send out the waste upward for incineration. Such fire grates are mainly made of high-chromium cast steel excellent in heat resistance, wear resistance and corrosion resistance. Although this high-chromium cast steel has good corrosion resistance and oxidation resistance, when it is used for a fire grate and exposed to a high temperature of 500 to 600 °C or higher, the hardness decreases, the sliding surface wears, the fire grate itself thins and the strength becomes insufficient, resulting in a problem that the fire grate cannot move smoothly. In response to such problems, Patent Document 1 proposes a heat-resistant cast steel for a fire grate in a waste incinerator containing 0.7 to 1.3% by mass of C, 1.0 to 2.5% by mass of Si, 20 to 30% by mass of Cr, 0.82% by mass of Mn, and 0.2 to 0.3% by mass of N, with the balance being Fe.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] [[ID=�5]]In fact, for the fire grates used in waste incinerators, fire grates made of JIS heat-resistant cast steel such as SCH2, SCH11, and SCH13 are used. Recently, hardness has been required for fire grates, and conventional heat-resistant cast steels have not been satisfactory. Therefore, a heat-resistant cast steel with increased hardness by containing N as in Patent Document 1 has been developed, but the heat-resistant cast steel has only shown an effect within a narrow range of Mn 0.82%.

[0005] This invention was made to solve these problems, and its objective is to provide a heat-resistant alloy for grates and incinerator grates that have high hardness and large shock absorption energy. [Means for solving the problem]

[0006] The heat-resistant alloy for grates according to the present invention is characterized by containing, by mass%, 0.86 to 0.93% C, 1.8 to 2.0% Si, 0.6 to 0.9% Mn, 23 to 25% Cr, 0.17 to 0.23% N, and 0.15 to 0.4% Ni, with the remainder being Fe and unavoidable impurities.

[0007] In the heat-resistant alloy for grates according to the present invention, it is preferable that the sum of the N content and the Ni content is within the range of 0.40 to 0.63%.

[0008] The incinerator grate according to the present invention is characterized by being made of the heat-resistant alloy for grates described above. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a heat-resistant alloy for grates and incinerator grates that have high hardness and large shock absorption energy. In particular, it has the characteristics of having high toughness, being hard, and also being ductile and resistant to brittle fracture, making it a desirable heat-resistant alloy for grates. [Modes for carrying out the invention]

[0010] The heat-resistant alloy for grates and the incinerator grates according to the present invention will be described in detail based on their embodiments. The following embodiments are merely examples of the present invention, and the present invention is not limited to these embodiments. In this application, "%" is an abbreviation for mass% (weight%), and "quantity" is an abbreviation for content.

[0011] [Heat-resistant alloy for grate] The heat-resistant alloy for grates according to the present invention is characterized by containing, by mass%, 0.86 to 0.93% C, 1.8 to 2.0% Si, 0.6 to 0.9% Mn, 23 to 25% Cr, 0.17 to 0.23% N, and 0.15 to 0.4% Ni, with the remainder being Fe and unavoidable impurities. In this case, more preferably, the sum of the N content and Ni content is in the range of 0.40 to 0.63%.

[0012] The alloy of the present invention contains the above-mentioned component composition, and the γ generated by the peritectic reaction (α + L → γ) decomposes into α and carbides, causing film-like carbides and fine carbides to precipitate, resulting in a heat-resistant alloy with high strength and high toughness. In particular, it has high toughness, is hard, and exhibits elongation, making it resistant to brittle fracture, making it a desirable heat-resistant alloy for gratings. Furthermore, due to its high hardness and large impact absorption energy, it is expected to exhibit excellent high-temperature corrosion resistance, and especially excellent high-temperature sliding wear resistance.

[0013] Furthermore, it is cost-effective because it exhibits excellent high-temperature corrosion resistance even with a small amount of Ni. In addition, while Ni contributes to improved hardness, the amount of Ni that can be dissolved depends on the Cr content. When the Cr content exceeds 25%, the amount of Ni that can be dissolved increases, making gas defects more likely to occur during casting. In this invention, by keeping the Cr content below 25%, the amount of Ni that can be dissolved (content) is reduced to prevent gas defects, while a small amount of Ni (within the above range) that generates the γ austenite phase can be included, thereby maintaining a state of high hardness and high shock absorption energy. The γ austenite phase generated by including a small amount of Ni is the γ produced by the peritectic reaction (α + L → γ) described above, and its decomposition contributes to the precipitation of carbides that provide high strength and high toughness, which is preferable.

[0014] Furthermore, while a σ phase prone to embrittlement is more likely to form when the Cr content exceeds 25% by mass, such a σ phase does not form when the Cr content is 25% or less by mass. Therefore, by keeping the Cr content below 25% (where the σ phase does not form) and including the above-mentioned predetermined amounts of N and Ni, it is possible to achieve high toughness, hardness, elongation, and resistance to brittle fracture.

[0015] In this invention, N is preferably included to improve hardness, but if the Ni content is too high, the impact absorption energy will not be very large. According to this invention, based on the roles of Cr, N, and Ni described above, by setting the sum of the N content and Ni content to within the range of 0.40 to 0.63% when the Cr content is 23 to 25 mass%, gas defects can be prevented, and the impact absorption energy can be increased while maintaining hardness, thereby reducing brittleness.

[0016] Let's explain each component.

[0017] (C: 0.86~0.93%) Carbon (C) generates the γ phase through a peritectic reaction (α + L → γ). The generated γ phase is decomposed into α and carbides, and film-like carbides and fine carbides precipitate. This precipitation of carbides enables high strength and high toughness. In this invention, from these viewpoints, the C content is set in the range of 0.86 to 0.93%.

[0018] (Si: 1.8~2.0%) Silicon (Si) exhibits a deoxidizing effect during melting and refining, improving the flowability of molten metal during casting. Furthermore, the lower the Si content, the more easily γ→α+ carbide decomposition occurs. In this invention, the Si content that facilitates γ→α+ carbide decomposition is set to a range of 1.8 to 2.0%.

[0019] (Mn: 0.6~0.9%) Manganese (Mn) acts as both a deoxidizing and desulfurizing agent, and also improves castability and hardenability, while suppressing the formation of δ-ferrite. Furthermore, a higher Mn content facilitates the decomposition of γ→α+ carbides. In this invention, the Mn content that facilitates the decomposition of γ→α+ carbides was set to a range of 0.6 to 0.9%.

[0020] (Cr: 23-25%) Cr (chromium) is an essential element for improving high-temperature corrosion resistance. It reacts with oxygen in the atmosphere to form a protective chromium oxide film against corrosion on the alloy surface, suppressing the corrosion of the base material. When the Cr content is lower, the decomposition into γ→α + carbide progresses, and the precipitation of the σ phase that causes embrittlement can be avoided. Specifically, when the Cr amount exceeds 25% by mass, the σ phase that is prone to embrittlement is likely to form. On the other hand, when the Cr amount is 25% by mass or less, such a σ phase is less likely to form. On the other hand, when the Cr content is less than 20%, the chromium oxide film decreases and the corrosion resistance is likely to deteriorate. In the present invention, considering the formation of the σ phase and the corrosion resistance, the Cr content is set in the range of 23 - 25%.

[0021] (N: 0.17 - 0.23%) N (nitrogen) forms chromium carbonitride, contributes to the improvement of hardness, and can maintain high hardness from room temperature to high temperature. N contributes to the improvement of hardness, but the solid solution amount of N depends on the Cr content. When the Cr content exceeds 25%, the solid solution amount of N increases, and gas defects are likely to occur during casting. In the present invention, as described above, the Cr content is set to 25% or less from the viewpoint of avoiding the precipitation of the σ phase. At the same time, by setting the Cr content to 25% or less, the solid solution amount (content) of N is reduced so as not to cause gas defects. Also, as will be described later, by containing a small amount of Ni (within the above range) that forms the γ-austenite phase, together with the action of N, a state with high hardness and large impact absorption energy can be maintained. From these viewpoints, in the present invention, the N content is set to be as low as 0.19 - 0.23%. By doing so, a high-quality heat-resistant alloy casting is realized, and a sufficient hardness improvement effect is obtained within this range.

[0022] (Ni: 0.15% - 0.4%) Ni (nickel) is known as a heat-resistant alloy component. In the present invention, the amount of Ni was made a small amount of 0.15% to 0.4%. Even with a small amount of Ni, it has excellent high-temperature wear resistance, which is also advantageous in terms of cost. Also, by containing a small amount of Ni (within the above range), a γ-austenite phase is generated. The generated γ-austenite phase becomes γ generated by the above-described peritectic reaction (α + L → γ), and since it can contribute to the precipitation of carbides that bring high strength and high toughness by its decomposition, it is preferable. Note that if the Ni content is too high, the impact absorption energy does not increase so much, so it was contained as the minimum necessary amount.

[0023] Based on the roles of Cr, N, and Ni described above, when the Cr content is 23 to 25% by mass, by setting the sum of the N content and the Ni content within the range of 0.40 to 0.63%, gas defects can be prevented, and the impact absorption energy can be increased while maintaining the hardness, and brittleness can be reduced.

[0024] (Balance and inevitable impurities) Fe (iron) is the base metal constituting the balance of this heat-resistant alloy. The Fe content as the balance is the remaining content after subtracting the main elements (C, Si, Mn, Cr, N, Ni) and inevitable impurities. Inevitable impurities are elements that may inevitably be introduced depending on the situation of raw materials, materials, manufacturing equipment, etc. For example, P, S, Sn, As, Pb, Al, Ti, Mo, V, Nb, etc. can be mentioned, and all may be contained in the range of 0 (not included) to 0.05%, preferably 0 to 0.02%. It is premised that these inevitable impurities are elements and their contents that do not inhibit the effects of the present invention. Among these inevitable impurities, for P and S, although not particularly limited, for example, it is desirable to be within the usually mixed range of P: 0.04% or less and S: 0.04% or less in order to ensure castability. As with ordinary heat-resistant steels, for inevitable impurity elements other than P and S, the coexistence of impurity elements within the range usually mixed as inevitable impurities is acceptable in order to ensure castability.

[0025] The Fe-based heat-resistant alloy steel according to the present invention, as described above, is used as a heat-resistant alloy for grates, possessing high hardness and large shock absorption energy. In particular, it has high toughness, is hard, and exhibits elongation, making it resistant to brittle fracture, thus making it a desirable heat-resistant alloy for grates.

[0026] (Manufacturing method) The heat-resistant alloy for grates according to the present invention is obtained by preparing raw materials of the above composition, blending the prepared raw materials and melting them in the atmosphere, and casting the molten metal in the atmosphere. The raw materials can be prepared by preparing raw materials whose component compositions are known and weighing them to achieve the target component composition. Melting is performed by blending the prepared raw materials and melting them in the atmosphere. The conditions such as the melting temperature during melting are not particularly limited, and known methods appropriate to the heat-resistant alloy can be employed. Casting is performed by casting the molten metal in the atmosphere. The conditions such as the casting temperature, heat treatment, and slow cooling are not particularly limited, and known methods appropriate to the heat-resistant alloy can be employed.

[0027] [fire grate] The grate according to the present invention is manufactured from the heat-resistant alloy for grates according to the present invention. As explained in the background art section, the grate is used in incinerators, laid on the hearth, and used to agitate and send out waste from above while incinerating it. Examples of incinerator grates include those used in stoker-type waste incinerators. The shape and type of incinerator grates are not limited in any way and can be applied to various known types. For example, grates include not only general air-cooled types but also water-cooled types, and not only movable grates but also fixed grates. [Examples]

[0028] The present invention will be described in more detail by reference to examples and comparative examples, but the present invention is not limited in any way to these examples.

[0029] [Experimental sample] Multiple alloy raw materials with varying component compositions were prepared. These alloy raw materials were melted in the atmosphere and then cast in the atmosphere (casting temperature: 1500°C) to obtain alloy samples 1 to 6 with different component compositions, as shown in Table 1. Conventional samples 1 to 3 are commercially available JIS heat-resistant cast steel symbols and consist of samples SCH2 (conventional sample 1), SCH11 (conventional sample 2), and SCH13 (conventional sample 3).

[0030] Of the components of the obtained alloy, C, Si, Mn, Cr, and Ni were measured using an emission spectrometer. The N content was determined using an oxygen, nitrogen, and hydrogen analyzer (EMGA-930) manufactured by Horiba, Ltd., in accordance with JIS G1239 ("Iron and steel - Determination method for oxygen - Inert gas fusion - Near-infrared absorption method"), with a sample mass of 1.0 g.

[0031] [Table 1]

[0032] [Characteristic evaluation] Tensile tests, Brinell hardness tests, Charpy impact tests, and abrasion tests were performed on each sample. The results are shown in Table 2.

[0033] (Charpy impact test) The impact absorption energy (also called absorbed energy) was measured using a Charpy impact test apparatus in accordance with JIS Z2242 "Charpy impact test method for metallic materials".

[0034] (Corrosion test) The corrosion test was conducted in accordance with JIS Z2293 "High-Temperature Corrosion Test Methods for Salt Immersion and Salt Burial of Metallic Materials." Test specimens (10 mm long, 10 mm wide, 2 mm thick) of Sample 1 and Conventional Products 1-3 were buried in ash and heated (700°C, 120 hours) while a mixed gas was flowed through them. The results were then evaluated. The ash used was collected from an actual waste incinerator. After the test, the test specimens were cut in half, and the cross-sectional thickness was measured to calculate the amount of corrosion (μm). Table 2 shows the results from a general waste incinerator and an industrial waste incinerator.

[0035] [Table 2]

[0036] [result] As shown in Table 2, among samples 1 to 6, samples 1 to 3 showed high tensile strength and a reasonable elongation, as well as improved shock absorption energy, yielding favorable results as heat-resistant alloys for grates. In this invention, a heat-resistant alloy with a favorable alloy composition, obtained by mass%, containing 0.86 to 0.93% C, 1.8 to 2.0% Si, 0.6 to 0.9% Mn, 23 to 25% Cr, 0.17 to 0.23% N, and 0.15 to 0.4% Ni, with the remainder being Fe and unavoidable impurities, was proposed as a heat-resistant alloy for grates. On the other hand, samples 4 to 6 had lower tensile strength and considerably lower elongation (less than 1%) compared to samples 1 to 3, and their shock absorption energy did not show improvement.

[0037] Although the corrosion test results are only for Sample 1 and Conventional Products 1-3, Table 2 shows that Sample 1 performed better than Conventional Products 1-3 under general waste incineration conditions, and was comparable under industrial waste incineration conditions. Furthermore, since Sample 1 has a lower Ni content than Conventional Products 1-3, it has a significant cost advantage and can be considered a practical heat-resistant alloy for grates.

Claims

1. A heat-resistant alloy for grates, characterized by containing, by mass%, 0.86 to 0.93% C, 1.8 to 2.0% Si, 0.6 to 0.9% Mn, 23 to 25% Cr, 0.17 to 0.23% N, and 0.15 to 0.4% Ni, with the remainder being Fe and unavoidable impurities.

2. The heat-resistant alloy for a grate according to claim 1, wherein the sum of the N content and the Ni content is in the range of 0.40 to 0.63%.

3. A grate made of the heat-resistant alloy for grates described in claim 1 or 2.

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