Low-nickel high-strength iron-nickel-based heat-resistant alloy and preparation method thereof

Abstract

The invention relates to a low-nickel high-strength iron-nickel-based heat-resistant alloy and a preparation method thereof, and belongs to the technical field of high-temperature metal materials, and the alloy comprises the following chemical components in percentage by weight: 0.01-0.10% of C, less than or equal to 0.20% of Si, less than or equal to 0.50% of Mn, less than or equal to 0.020% of P, less than or equal to 0.020% of S, 18.00-24.00% of Cr, 22.00-28.00% of Ni, 1.50-2.80% of Ti, 0.80-2.00% of Al, 0.70-1.80% of Nb, 0.20-0.60% of V, less than or equal to 0.060% of Zr, less than or equal to 0.050% of Ce, less than or equal to 0.20% of Cu and the balance of Fe According to the alloy, through optimized component design, the nickel content is reduced, and meanwhile it is ensured that the alloy still has excellent high-temperature performance at 750 DEG C (the tensile strength is larger than or equal to 730 MPa, the yield strength is larger than or equal to 610 MPa, and the percentage elongation after fracture A is larger than or equal to 21.0%). Meanwhile, the invention provides a special grain display method suitable for the alloy, the grain boundary display problem caused by the high chromium content and the solid solution state of the alloy can be effectively solved, and clear and stable display of the grain size is achieved. The alloy is suitable for manufacturing high-temperature parts such as engine air valves.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-temperature metal materials, and particularly relates to a low-nickel high-strength iron-nickel-based heat-resistant alloy and a preparation method thereof. BACKGROUND

[0002] Heat-resistant alloy is a key material for high-temperature components in the fields of aviation, energy, chemical industry and the like, and the performance of the heat-resistant alloy directly determines the service temperature and service life of equipment. In recent years, to reduce the dependence on strategic metal nickel and control costs, it has become an important trend in the industry to develop low-nickel content and high-performance iron-nickel-based heat-resistant alloys. Such alloys add elements such as chromium (Cr), aluminum (Al) and titanium (Ti) to the iron-nickel matrix, and use the precipitation of the γ' phase (Ni3(Al, Ti)) for strengthening, so as to achieve a high-temperature strength similar to that of high-cost nickel-based alloys.

[0003] However, the component design and industrial application of such alloys face double challenges:

[0004] First, in terms of alloy performance, the component balance is difficult. In order to ensure high-temperature oxidation resistance, a sufficient amount of chromium (usually > 12%) must be added; and in order to obtain a stable γ' strengthening phase, the content of elements such as Al and Ti needs to be accurately controlled. At the same time, how to compensate for the strength and maintain excellent high-temperature endurance performance by adding micro-alloying elements such as vanadium (V) and niobium (Nb) under the premise of reducing the content of nickel (such as 30% or lower) is a core problem in material design. The tensile strength and yield strength of some existing low-cost alloys (such as Ni30) at temperatures of 750 DEG C and above often cannot meet the use requirements of higher load engine components.

[0005] Secondly, in terms of quality control and characterization of the alloy, new problems are derived due to its high-performance characteristics. On the one hand, the high chromium content leads to the formation of a dense Cr2O3 passivation film on the surface of the alloy; on the other hand, in order to facilitate subsequent processing, the material is often delivered in a solid solution state, at which time the grain boundaries are clean and lack precipitates. These two characteristics together lead to the complete failure of traditional grain display methods (such as nitric acid alcohol and picric acid corrosion), which brings great obstacles to the organization inspection and quality control of the material.

[0006] Therefore, there is an urgent need in the art for a new type of iron-nickel-based heat-resistant alloy that has low cost, excellent high-temperature mechanical properties, and whose microstructure can be effectively and conveniently characterized. SUMMARY

[0007] In view of the above analysis, the embodiments of the present application aim to provide a low-nickel high-strength iron-nickel-based heat-resistant alloy and a preparation method thereof, to at least solve one of the problems of insufficient high-temperature strength of low-cost iron-nickel-based heat-resistant alloys, difficulty in balancing components, and difficulty in grain display due to high chromium content and solid solution state in the prior art.

[0008] The present application provides an iron-nickel based heat-resistant alloy, which has a chemical composition in terms of percentage by weight as follows: C: 0.01-0.10%, Si: ≤0.20%, Mn: ≤0.50%, P: ≤0.020%, S: ≤0.020%, Cr: 18.00-24.00%, Ni: 22.00-28.00%, Ti: 1.50-2.80%, Al: 0.80-2.00%, Nb: 0.70-1.80%, V: 0.20-0.60%, Zr: ≤0.060%, Ce: ≤0.050%, Cu: ≤0.20%, and the balance being Fe and inevitable impurities.

[0009] Further, the content of V is 0.40-0.60%, and the sum of the percentage by weight of Ti and Al is 3.00%-4.50%.

[0010] Further, the high-temperature tensile properties of the alloy at 750°C satisfy: tensile strength ≥730 MPa, and yield strength ≥610 MPa.

[0011] Further, the elongation A after breaking of the alloy at 750°C is ≥21.0%.

[0012] Based on the above-mentioned iron-nickel based heat-resistant alloy, the present application provides a preparation method of an iron-nickel based heat-resistant alloy, which comprises the following steps in sequence:

[0013] Step 1, vacuum induction smelting or non-vacuum induction smelting to obtain an electrode rod;

[0014] Step 2, electroslag remelting of the electrode rod under a protective atmosphere, with a melting rate of 3-6 kg / min;

[0015] Step 3, forging breakdown of the electroslag ingot, with a breakdown temperature of 1150-1200°C;

[0016] Step 4, bar rolling of the billet, with a final rolling temperature not lower than 950°C;

[0017] Step 5, solid solution treatment at 1080-1150°C, with a holding time of 1-2 hours, followed by cooling to room temperature at a cooling rate not lower than 50°C / min.

[0018] Further, the microstructure of the alloy after solid solution treatment is a single austenite matrix, and the average grain size level number G satisfies 4.0-7.0 levels.

[0019] Further, the solid solution treatment in Step 5 is: holding at 1120°C±10°C for 1.5 hours, followed by water quenching.

[0020] The application further provides a method for displaying the grain size of the iron-nickel based heat-resistant alloy.

[0021] S1, sampling and polishing the alloy after solid solution treatment;

[0022] S2, etching the polished sample with an etching liquid, the etching liquid being composed of 90-100 ml of distilled water, 10-20 ml of H2SO4 and 1-2 g of KMnO4;

[0023] S3, placing the prepared etching liquid and the sample in a 50-55℃ water bath for heating for 2-3 hours;

[0024] S4, taking out the sample, cleaning the black film on the surface of the sample with an oxalic acid aqueous solution with a mass concentration of 5%-10%, then rinsing with clean water and drying.

[0025] Further, the polishing step comprises polishing with 200-mesh, 500-mesh, 800-mesh and 1000-mesh metallographic sandpaper in sequence until no visible scratches are present on the surface of the sample.

[0026] In addition, the application further provides an engine valve made of the iron-nickel based heat-resistant alloy.

[0027] Compared with the prior art, the application can achieve at least one of the following beneficial effects:

[0028] 1) The iron-nickel based heat-resistant alloy provided by the application greatly reduces the raw material cost compared with the traditional nickel-based high-temperature alloy (nickel content > 65%) by controlling the nickel content to be 22.00%-28.00%. Meanwhile, by adding Ti, Al, Nb and V elements, a stable γ' strengthening phase is formed in the matrix, so that the material still has excellent high-temperature strength and durability in the range of 600-750℃.

[0029] 2) The application effectively controls the precipitation behavior and volume fraction of the γ' strengthening phase (Ni3(Al,Ti)) by preferably controlling the sum of the weight percentages of Ti and Al to be 3.00%-4.50%. This composition design enables the γ' phase to maintain a sufficient number and stability at a high temperature of 750℃, so that excellent strength (tensile strength can reach 735-760 MPa) and softening resistance are obtained. Meanwhile, this content range effectively avoids the excessive precipitation of brittle phases, so that the alloy still has good plasticity (for example, the elongation after fracture can reach 21.0%-23.5%) at high temperatures, and the comprehensive high-temperature mechanical properties are significantly better than those of similar comparative materials.

[0030] 3) The present application provides a special grain display method for the above-mentioned high chromium (Cr≥18%) content iron-nickel-based heat-resistant alloy (especially in the solid solution state). The use of sulfuric acid-potassium permanganate etching solution combined with water bath heating in a specific temperature range can effectively damage the dense chromium oxide passivation film on the surface of the alloy and clearly show the grain profile that is difficult to display due to its "clean" grain boundaries, solving the industry problem that traditional chemical etching methods are completely ineffective for such materials.

[0031] 4) The grain display method of the present application requires fewer types of reagents and simple proportioning, only three basic reagents of distilled water, sulfuric acid and potassium permanganate are needed, and stable and reproducible etching effect can be achieved by controlling the water bath heating temperature and time. This method avoids the safety hazards brought by the use of dangerous and explosive reagents such as picric acid in traditional metallographic methods, and the operation process is safer and more controllable, which is very suitable for popularization and application in industrial production and detection.

[0032] In the present application, the above technical solutions can be combined with each other to realize more preferred combination schemes. Other features and advantages of the present application will be described in the subsequent specification, and some advantages will become apparent from the specification, or will be understood by implementing the present application. The purpose and other advantages of the present application can be achieved and obtained from the contents specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF DRAWINGS

[0033] The accompanying drawings are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the principles of the application. In the drawings:

[0034] Figure 1 The grain size metallographic photo displayed after the grain display method of the present application is used for Example 1 (corresponding to 1# alloy);

[0035] Figure 2 The grain size metallographic photo displayed after the grain display method of the present application is used for Example 2 2# alloy;

[0036] Figure 3 The grain size metallographic photo displayed after the grain display method of the present application is used for Example 2 3# alloy;

[0037] Figure 4 The grain size metallographic photo displayed after the grain display method of the present application is used for Example 2 4# alloy;

[0038] Figure 5 The grain size metallographic photo displayed after the grain display method of the present application is used for Example 2 5# alloy;

[0039] Figure 6The grain size metallographic photo of the 6# alloy in Example 2 after being displayed by the grain display method of the present application;

[0040] Figure 7 The grain size metallographic photo of the 7# alloy in Example 2 after being displayed by the grain display method of the present application;

[0041] Figure 8 The grain size metallographic photo of the alloy in Example 3 after being displayed by the grain display method of the present application;

[0042] Figure 9 The photo obtained by metallographic examination of Comparative Example 1;

[0043] Figure 10 The photo obtained by metallographic examination of Comparative Example 2;

[0044] Figure 11 The photo obtained by metallographic examination of Comparative Example 5;

[0045] Figure 12 The photo obtained by metallographic examination of Comparative Example 6. DETAILED DESCRIPTION

[0046] The preferred embodiments of the present application will be described in detail below with reference to the drawings, which form a part of this application. The drawings and the associated descriptions are provided to illustrate the preferred embodiments of the present application and to explain the principles of the present application, but are not intended to limit the scope of the present application.

[0047] To meet the dual challenges of high-end equipment on high-temperature material performance and cost, the development of low nickel content and excellent high-temperature strength of iron-nickel-based heat-resistant alloy has become an urgent need in the industry. The embodiments of the present application aim to provide such a low-cost, high-performance iron-nickel-based heat-resistant alloy and its preparation method. Through component design, the alloy significantly reduces the nickel content to 22-28%, while ensuring excellent mechanical properties at temperatures up to 750°C and below through multi-element composite strengthening. In addition, due to the organizational characterization problem brought about by its high alloying characteristics, the present application also provides a special grain display technology.

[0048] In one aspect, a specific embodiment of the present application discloses a heat-resistant alloy, the chemical components of which are as follows in terms of percentage by weight: C: 0.01-0.10%, Si: ≤0.20%, Mn: ≤0.50%, P: ≤0.020%, S: ≤0.020%, Cr: 18.00-24.00%, Ni: 22.00-28.00%, Ti: 1.50-2.80%, Al: 0.80-2.00%, Nb: 0.70-1.80%, V: 0.20-0.60%, Zr: ≤0.060%, Ce: ≤0.050%, Cu: ≤0.20%, and the rest is Fe and inevitable impurities.

[0049] The heat-resistant alloy belongs to a high-performance iron-nickel-based high-temperature alloy, by significantly reducing the nickel content to 22-28%, the raw material cost is greatly reduced compared with the traditional nickel-based high-temperature alloy (nickel content > 65%). And the ratio of Ti, Al, V, Nb and other strengthening elements is accurately controlled to form stable γ' strengthening phase in the matrix, and the balance of low cost and high performance is successfully realized. Experiments show that the alloy still has excellent mechanical properties at 750°C high temperature, the tensile strength is ≥730MPa, the yield strength is ≥610MPa, and the elongation after fracture is ≥21.0%. Its comprehensive performance is significantly better than that of traditional material Ni30.

[0050] The combination of high yield strength and good plasticity of the alloy at 750°C means that it has excellent anti-deformation and anti-fracture ability under high stress and high temperature conditions, which is crucial for the design goal of long service life and high reliability of key components such as engine valves.

[0051] The above excellent high-temperature performance is due to the alloy composition design described below, wherein the role and ratio of each element are as follows:

[0052] C (0.01% to 0.10%): Carbon is a traditional interstitial solid solution and carbide forming element, which can provide certain strengthening effect by forming MC and M 23 C6 type carbide in the grain and grain boundary. However, too high carbon content will significantly reduce the plasticity and weldability of the alloy, and may form coarse primary carbides as crack sources. This range aims to take advantage of its benefits while strictly avoiding its adverse effects.

[0053] Si (≤0.20%): Silicon is necessary as a deoxidizer during smelting. However, as a strong solid solution strengthening element and a promoting element of topologically close-packed phases (such as σ phase, Laves phase), its solid solubility in the austenitic matrix is very low. Excessive Si content will sharply deteriorate the hot working plasticity, impact toughness and long-term durability of the alloy, and also adversely affect the welding performance of the material. Therefore, it must be limited to a low level.

[0054] Mn (≤0.50%): Manganese also has certain deoxidizing ability and can form MnS by combining with sulfur (S) to reduce the harmful effect of sulfur on the grain boundary and improve the hot working performance. However, manganese will reduce the stacking fault energy of the austenitic matrix, which may promote adverse deformation mechanisms, and excessive manganese will also slightly reduce the oxidation resistance and high-temperature strength of the alloy. The present invention controls it within this range, aiming to take its advantages and avoid its disadvantages.

[0055] P (≤0.020%): Phosphorus is one of the most harmful trace impurity elements, which has a very high tendency to segregate at the grain boundaries in austenitic alloys. Even at very low concentrations, it will strongly segregate at the grain boundaries, significantly reducing the binding energy and cohesion of the grain boundaries, leading to grain boundary embrittlement. This will seriously deteriorate the hot working plasticity and become the nucleation and expansion point of high temperature creep micro-cracks, posing a fatal threat to the long-term service life of the components. Therefore, it must be controlled at a very low level.

[0056] S (≤0.020%): Sulfur is another impurity element that must be strictly controlled. Its solubility in nickel-based or iron-nickel-based austenite is extremely low, and it is extremely easy to form low-melting eutectic phase at the grain boundaries (such as Ni-Ni3S2, eutectic temperature only 645℃). These low-melting phases will be in a molten state within the temperature range of hot working (such as forging, rolling), directly leading to the loss of grain boundary bonding, causing serious "hot brittleness" cracking. Ultra-low sulfur content is a prerequisite for ensuring good hot working performance and stable high-temperature grain boundary strength.

[0057] Cr (18.00% ~ 24.00%): Chromium is a key element to ensure the service life of the alloy in high temperature environment. Within this content range, the alloy can form a dense and strongly adhered Cr2O3 oxide film on the surface during high temperature service, effectively preventing the inward diffusion of oxygen elements, thereby imparting the alloy excellent high temperature oxidation resistance and corrosion resistance.

[0058] Ni (22.00% ~ 28.00%): Nickel is the basis for forming the austenitic matrix and the γ' strengthening phase (Ni3(Al, Ti)). The content of this design is precisely controlled within this range, aiming to ensure the formation of sufficient number of γ' phase to guarantee high temperature strength, while making its content significantly lower than traditional nickel-based high temperature alloys (such as Nimonic 80A with Ni content > 65%), thereby achieving the best balance between material performance and raw material cost.

[0059] Ti (1.50% ~ 2.80%) and Al (0.80% ~ 2.00%): Titanium and aluminum are the core elements of the γ' strengthening phase (Ni3(Al, Ti)). The aluminum content directly affects the intrinsic strength of the γ' phase, while titanium can partially replace aluminum to improve the solid solution temperature and anti-coarsening ability of the γ' phase.

[0060] Synergistic effect of Ti and Al: As a preferred solution, the sum of the weight percentage of Ti and Al is 3.00% to 4.50%. This range directly determines the volume fraction of the γ' strengthening phase (Ni3(Al, Ti)), which is the core to achieve a balance between excellent high-temperature strength (especially a tensile strength ≥ 730 MPa at 750°C) and good plasticity (an elongation after fracture ≥ 21%). Too low a content will result in insufficient strengthening phase and a decrease in high-temperature strength; too high a content may cause γ' phase coarsening or the precipitation of harmful topologically close-packed phases (such as σ phase), which will damage the plasticity and long-term stability. This preferred range is the key to obtaining a suitable volume fraction and size-stable γ' phase.

[0061] Nb (0.70% to 1.80%): Niobium is a multifunctional strengthening element. It is not only an effective γ' phase forming element that can partially replace Ti and Al to improve the stability of the γ' phase, but also a strong carbide forming element that can form fine MC-type carbides to provide additional intracrystalline and grain boundary strengthening. This content range can fully exert its synergistic strengthening effect and avoid the adverse effects of excessive addition on hot workability.

[0062] V (0.20% to 0.60%): Vanadium mainly plays a role in solid solution strengthening and grain refinement. V solid solution in the matrix can effectively hinder dislocation movement; at the same time, it can participate in the formation of carbides or interact with the γ' phase to further improve the organizational stability. When the V content is less than 0.20%, the strengthening effect is not significant; when it is higher than 0.60%, coarse carbides are easily formed, which will damage the hot ductility and toughness.

[0063] In a preferred embodiment, the V content is controlled to be 0.40 to 0.60%. Within this preferred range, the solid solution strengthening and grain refinement effect of V element is more significant, which can more stably improve the yield strength and creep resistance of the alloy in the range of 600°C to 750°C. The yield strength of the alloy at 750°C in this V content range can reach or exceed 620 MPa.

[0064] Zr (≤0.060%) and Ce (≤0.050%): Zirconium and cerium are effective grain boundary strengthening elements. They can capture oxygen and sulfur in the steel to form high-melting-point stabilized compounds (such as ZrO2, Ce2O3, CeS), thereby purifying the grain boundaries and significantly improving the hot working plasticity, long-term durability and creep resistance of the alloy. Their content is precisely controlled to maximize the benefits and avoid excessive formation of low-melting-point eutectic phases.

[0065] Cu (≤0.20%): As a residual element, the content of copper needs to be strictly limited to prevent hot brittleness due to the formation of low-melting-point phases during hot working.

[0066] The preparation method of the alloy comprises the following steps in sequence:

[0067] Step 1, vacuum induction melting or non-vacuum induction melting to obtain an electrode rod;

[0068] Step 2, electroslag remelting of the electrode rod under a protective atmosphere, with a melting rate of 3-6 kg / min;

[0069] Step 3, forging breakdown of the electroslag ingot, with a breakdown temperature of 1150-1200℃;

[0070] Step 4, bar rolling of the billet, with a final rolling temperature of no less than 950℃;

[0071] Step 5, solid solution treatment at 1080-1150℃, with a holding time of 1-2 hours, followed by cooling to room temperature at a cooling rate of no less than 50℃ / min.

[0072] The preparation method provided by the present application is a complete technical solution aimed at ensuring the purity of the alloy, the uniformity of the structure, and excellent comprehensive performance. Through the synergistic control of the following processes, the problems of composition segregation and thermal processing sensitivity in the preparation of high-performance iron-nickel-based alloys are successfully solved, enabling the alloy to have the advantages of high strength (tensile strength ≥ 730 MPa) and high plasticity (elongation after fracture ≥ 21%) at a high temperature of 750℃.

[0073] Specifically, in Step 1, vacuum or non-vacuum induction melting is used to accurately control the content of main components and trace elements of the alloy. In particular, for the accurate proportioning of Ti, Al, V, Nb, and other easily oxidized elements in the present application, precise control of the smelting process in a vacuum environment or under the atmosphere effectively reduces element loss and ensures accurate input of strengthening phase forming elements, laying a precise composition foundation for material performance.

[0074] Specifically, in Step 2, electroslag remelting is carried out under a protective atmosphere, which can effectively remove inclusions and further purify the steel. The melting rate is controlled at 3-6 kg / min, which is different from the traditional electroslag remelting that often pursues high melting rate (generally 3-10 kg / min) to improve efficiency. For the high Al and Ti content alloy described in the present application, too high a melting rate (> 6 kg / min) will intensify the flow of liquid metal in the molten pool, which may instead cause macro-segregation of Al and Ti, and is not conducive to the full play of the purification and strengthening effect of Zr and Ce grain boundary strengthening elements. Too low a melting rate (< 3 kg / min) will result in low production efficiency. Controlling the melting rate within this optimized range can ensure production efficiency while obtaining an electroslag ingot with extremely uniform composition and high purity, effectively avoiding the tendency of thermal processing cracks and subsequent uneven structure caused by composition segregation.

[0075] Specifically, in step 3, the forging breakdown temperature is set to 1150-1200℃. Since the alloy of the present application is strengthened by reducing Ni, increasing Cr and adding V, Nb and the like, the control of the high-temperature deformation resistance and plastic balance interval is particularly critical. When the temperature is lower than 1150℃, the deformation resistance of the alloy increases sharply, and for the alloy rich in Nb and V carbides, micro-cracks are easily induced during deformation; and when the temperature is higher than 1200℃, the grains may be excessively coarse. The present application determines this breakdown temperature window, which precisely matches the recrystallization and softening behavior of the alloy, so that the initial as-cast structure is effectively broken up while avoiding the risk of cracking caused by improper deformation temperature, providing a defect-free billet for subsequent hot working.

[0076] Specifically, in step 4, the rod rolling requires a finish rolling temperature of not less than 950℃. This requirement is set in view of the feature of the alloy of the present application that the deformation resistance increases rapidly at lower temperatures. If the finish rolling temperature is too low, dynamic recrystallization is insufficient, and work-hardened structures will be retained, which not only increases the internal stress before subsequent solid solution treatment, but also is not conducive to obtaining uniform and fine starting grains after solid solution treatment. Controlling the finish rolling temperature above this level ensures that the material can still undergo sufficient dynamic recrystallization at the end of hot working, thereby obtaining a uniform and fine pre-grain structure, which is crucial for ultimately obtaining an ideal grain size of 4.0-7.0.

[0077] Specifically, in step 5, the solid solution treatment is carried out at 1080-1150℃ for 1-2 hours, followed by cooling at not less than 50℃ / min. This temperature range is determined based on the dissolution behavior of the γ' phase forming elements (Al, Ti, Nb) in the alloy of the present application and the systematic study of grain growth tendency. It is sufficient to fully dissolve various types of strengthening phases to form a supersaturated solid solution, preparing for aging precipitation, while effectively inhibiting excessive grain growth. The subsequent rapid cooling (≥50℃ / min) has a significant advantage over air cooling or slow cooling processes. The present application aims to achieve strengthening by aging a large number of nanoscale γ' phases (Ni3(Al, Ti)), and rapid cooling can maximize the inhibition of alloying elements (especially Al, Ti) from precipitating in the form of coarse phases at grain boundaries and the like during cooling, creating conditions for subsequent aging to precipitate fine, uniform and high-volume fraction strengthening phases. If a slower cooling method is used, the strength, especially the yield strength, will be significantly lost.

[0078] Preferably, in step 5, the solid solution treatment is performed at 1120℃±10℃ for 1.5 hours, followed by water quenching. This preferred scheme is based on the control of the balance point between the dissolution of γ' phase and the grain growth. In this temperature and time window, both the sufficient resolubilization of Al, Ti, Nb and other elements to reserve solute for the subsequent precipitation of high volume fraction nanoscale γ' phase and the effective inhibition of excessive grain growth to stabilize the grain size in the ideal range of 4.0-7.0 can be ensured, while the production efficiency and microstructure stability are taken into account.

[0079] After the above process, the microstructure of the alloy presents a typical single austenitic matrix, the grain boundary is clean and clear, and there is no obvious precipitate. Through systematic detection of multiple batches of sample, the average grain size level number is stably in the range of 4.0-7.0. The uniform and fine grain structure and the optimized alloy composition synergistically work together to ensure the excellent high-temperature strength and plasticity matching of the material.

[0080] The above heat-resistant alloy provided by the present application has the problems of unclear grain display in metallographic analysis due to the presence of 18.00-24.00% chromium and the clean grain boundary in the solid solution state. In order to accurately characterize and evaluate the grain structure state of the alloy of the present application, another embodiment of the present application correspondingly provides an efficient grain display method. The method comprises the following steps:

[0081] S1, sampling and polishing from the alloy after solid solution treatment;

[0082] S2, etching the polished sample with an etching liquid, the etching liquid being composed of 90-100ml distilled water, 10-20ml H2SO4 and 1-2g KMnO4;

[0083] S3, placing the prepared etching liquid and the sample in a 50-55℃ water bath for heating for 2-3 hours;

[0084] S4, taking out the sample, cleaning the black film on the surface of the sample with an oxalic acid aqueous solution with a mass concentration of 5%-10%, then rinsing with clean water and drying.

[0085] In practice, the heat-resistant alloy sample is usually cut from a silver bright material after solid solution treatment. Since the Cr content of such alloy is high (≥ 18%), a dense Cr2O3 oxide film is easily formed on the surface, and the grain boundary lacks precipitated phase as a corrosion starting point in the solid solution state, resulting in that the traditional corrosion agents such as nitric acid alcohol and picric acid are difficult to effectively display the grain boundary. The present application uses an aqueous solution of sulfuric acid-potassium permanganate system, and provides the necessary activation energy by water bath heating, which can effectively destroy the surface oxide film and selectively corrode the grain boundary, so as to clearly show the grain morphology. The method only uses three common reagents, the ratio is simple, and the display effect that is difficult to achieve by traditional methods can be achieved by mild heating, which is safe and convenient to operate, and has good reproducibility.

[0086] In order to obtain a flat mirror surface that meets the requirements of metallographic observation, in S1, the polishing includes polishing by using 200 mesh, 500 mesh, 800 mesh and 1000 mesh metallographic sandpaper in sequence until the sample surface is free of visible scratches. The polishing process of this step-by-step polishing is a standard process for preparing a metallographic sample, and the purpose is to eliminate the damage layer caused by cutting and rough grinding, and to obtain a flat and scratch-free observation surface, which is the basis for obtaining a clear grain boundary image.

[0087] Considering the safety and uniformity of the preparation of the corrosion solution, in S2, the preparation method of the corrosion solution is to dissolve KMnO4 in distilled water first, and then slowly add H2SO4 and stir. This sequence is crucial: never pour water directly into concentrated sulfuric acid to prevent the risk of violent heat release and splashing. Potassium permanganate has strong oxidizing properties in a strong acid environment. This preparation sequence can avoid safety hazards, and stirring can help potassium permanganate disperse and dissolve uniformly in the acidic solution, forming a uniform corrosion solution.

[0088] The present application optimizes the ratio of each component of the corrosion solution. Among them, 90-100ml of distilled water as a solvent, its main role is to ensure that each reaction component is fully dissolved and mixed, and to provide a suitable liquid environment; too little water will cause the solution to be supersaturated, which can easily cause precipitation and affect the uniformity of corrosion, and too much water will dilute the effective ingredients and weaken the corrosion ability.

[0089] 10-20ml of H2SO4 is used to provide a strong acidic environment, and the amount is crucial: insufficient acid will make it difficult to destroy the Cr2O3 passivation film on the alloy surface and start effective corrosion; too much acid may cause the reaction to be too violent, causing over-corrosion of the grain boundary or even overall corrosion, losing the significance of displaying the grain boundary.

[0090] 1-2g of KMnO4 as oxidant, in an acidic environment, produces nascent oxygen, selectively attacking the irregular atomic arrangement, higher energy area at the grain boundary; its amount needs to match the amount of acid, too low, the oxidation ability is insufficient, the grain boundary contrast is poor, too high, the reaction by-product (such as MnO2) is too much, forming a thick black film to interfere with observation.

[0091] The present application determines the above ratio range through repeated experiments, so that the three work together to effectively destroy the passivation film of high chromium alloy and clearly display the grain boundary under mild heating conditions.

[0092] The corrosion liquid should be prepared immediately, and if stratification or precipitation occurs after preparation, it needs to be prepared again to ensure the uniformity and stability of the corrosion effect.

[0093] In order to eliminate the influence of temperature fluctuation on the initial corrosion rate, in S3, the prepared corrosion liquid is preferably placed at room temperature before water bath heating. If the temperature of the prepared corrosion liquid is too different from the target water bath temperature, direct placement in the water bath for heating will lead to unstable initial heating rate, which may affect the uniformity of corrosion. Placing at room temperature can make the corrosion liquid start heating from a relatively consistent initial state.

[0094] In order to accurately control the corrosion reaction rate and effect, in step S3, the temperature of the water bath heating is controlled at 50-55℃. Temperature is a key factor affecting the corrosion kinetics process: if the temperature is too low, the reaction rate is too slow, which leads to insufficient corrosion and makes it difficult to clearly display the grain boundary; if the temperature is too high, the reaction is too violent, which easily leads to excessive corrosion of the grain boundary, blurred outline, and even damage to the intracrystalline structure. Precise control of the temperature within this range is an important guarantee for achieving clear, continuous and moderate contrast corrosion effect of the grain boundary.

[0095] Preferably, the components of the corrosion liquid are limited to the range of 95-100ml distilled water, 10-15ml H2SO4 and 1-1.5g KMnO4, which can obtain better implementation effect.

[0096] This preferred range moderately increases the proportion of water phase and controls the upper limit of the amount of acid and oxidant. Slightly abundant water helps to dissipate reaction heat and product diffusion, making the corrosion process more controllable; at the same time, moderate acid and oxidant concentration is sufficient to destroy the surface oxidation film and display the grain boundary, but it can significantly reduce the risk of blurred grain boundary outline or intracrystalline structure erosion caused by too violent reaction. This specific combination significantly improves the reproducibility and operation tolerance of the corrosion process under the premise of ensuring the display effect, and easily obtains stable and consistent test results in different laboratories or production sites.

[0097] Preferably, in S3, the temperature of the water bath is controlled at 52-55℃. Within this narrower preferred range, the etching liquid can be ensured to have the best and stable activity, both effectively overcoming the resistance of passivation film formed due to high chromium content and providing sufficient reaction kinetics conditions, so that the perfect appearance of grain boundaries can be most reliably achieved within 2-3 hours of heating time, while the risk of over-etching to the sample structure is minimized.

[0098] In order to effectively remove the interfering black film formed on the sample surface during the etching process, in S4, the mass concentration of the aqueous oxalic acid solution is preferably 5%-10%. The black film formed on the sample surface after etching is mainly the reaction product of high-valence oxides of manganese, which can cover and interfere with the observation of grain boundaries. As a reducing acid, oxalic acid can effectively dissolve such oxide film. Using an aqueous oxalic acid solution within this concentration range, both the surface black film can be effectively removed to expose clear grain boundaries, and the possibility of over-etching or damage to the etched grain boundaries due to excessively high concentration is avoided.

[0099] Based on the excellent performance of the alloy, the application further provides an engine valve made of the alloy. The valve is particularly suitable for intake valves and exhaust valves of high-power passenger cars and commercial vehicles. The manufacturing process mainly includes: taking the silver bright material prepared by the above process as raw material, machining the valve blank through upsetting and grinding, and finally obtaining the finished valve through aging treatment.

[0100] The valve fully utilizes the high-temperature strength, oxidation resistance and microstructure stability of the alloy of the application, and can work reliably for a long time in the harsh environment of high temperature and high pressure of the engine. At the same time, due to the significant reduction of the nickel content in the alloy, the valve has more competitive manufacturing cost while maintaining excellent performance.

[0101] In summary, the application successfully develops a low-cost and high-performance iron-nickel-based heat-resistant alloy through unique component design. The nickel content (22-28wt%) of the alloy is significantly lower than that of traditional nickel-based alloys (>65%), but through γ' phase strengthening, the tensile strength at 750℃ is still as high as 735-760MPa, which is significantly better than that of the comparative material Ni30 (695MPa). More importantly, the special grain display method provided by the application successfully solves the problem of grain display caused by high chromium (≥18%) and solid solution, so that the grain boundaries can be clearly displayed. The two complement each other and together form a complete technical solution from material manufacturing to quality control, providing solid technical support for the low-cost and localization of key components of high-end equipment.

[0102] The application will be described in more detail below through specific examples. The examples are only a description of the best embodiment of the application and do not have any limitation on the scope of the application.

[0103] Example 1

[0104] The chemical composition (wt.%) of the heat-resistant alloy of this example is: C: 0.055%, Si: 0.17%, Mn: 0.38%, P: 0.011%, S: 0.006%, Cr: 22.55%, Ni: 27.36%, Ti: 2.55%, Al: 1.55%, Nb: 0.95%, V: 0.45%, Zr: 0.008%, Ce: 0.032%, Cu: 0.08%, and the balance of Fe and unavoidable impurities. The sum of Ti and Al contents is 4.10%.

[0105] The alloy is obtained through the following preparation steps:

[0106] Step 1, smelting is performed using a vacuum induction furnace to obtain an electrode bar;

[0107] Step 2, electroslag remelting is performed on the electrode bar under an argon protective atmosphere, with a controlled melting rate of 5 kg / min, to obtain a high-quality electroslag ingot;

[0108] Step 3, the electroslag ingot is subjected to forging breakdown at a breakdown temperature of 1180°C;

[0109] Step 4, bar rolling is performed on the forged billet, with a final rolling temperature controlled at 980°C;

[0110] Step 5, solid solution treatment is performed on the rolled bar: heat treatment at 1120°C for 1.5 hours, followed by water quenching (cooling rate much higher than 50°C / min), to obtain a solid solution silver bright material.

[0111] High-temperature tensile property testing is performed on the alloy, which exhibits significantly better tensile strength and yield strength at 600°C-750°C than the comparative material Ni30 (see 1# data in Table 2), demonstrating excellent high-temperature mechanical properties.

[0112] The grain display method is as follows:

[0113] S1, a sample is cut from the above silver bright material and polished using 200-mesh, 500-mesh, 800-mesh, and 1000-mesh metallographic sandpaper in sequence until the surface is free of visible scratches.

[0114] S2, the sample is etched using an etching solution composed of 100 ml distilled water, 10 ml H2SO4, and 1 g KMnO4.

[0115] S3, the container containing the etching solution and the sample is heated in a 50°C water bath for 3 hours.

[0116] S4, the sample is removed, the surface black film is cleaned with an 8% oxalic acid aqueous solution, and then rinsed with clean water and dried.

[0117] After this treatment, clear austenite grain morphology as shown in Fig. 1 can be obtained under optical microscope, with continuous grain boundaries and high contrast; the average grain size level of the microstructure is 6.0. Figure 1

[0118] Example 2

[0119] This example aims to verify the applicability of the heat-resistant alloy composition range described in the present application, and the feasibility of the grain display method parameters matched therewith.

[0120] Seven heats (heats 1-7) of heat-resistant alloy were smelted in a vacuum induction furnace, with the specific chemical composition (wt%) strictly controlled within the range described in the present application, as shown in Table 1. The seven heats of alloy were all made into silver bright materials for air valve alloy according to the preparation method of steps 1-5 described in Example 1.

[0121] The high-temperature tensile properties of the silver bright materials of the seven heats of alloy were tested, and the results are shown in Table 2. As can be seen from Table 2, the heat-resistant alloy (heats 1-7) prepared by the method of the present application and meeting the composition range of the present application has achieved an excellent combination of high strength and good plasticity in high-temperature mechanical properties, which is superior to the comparative material Ni30:

[0122] At 600℃, the tensile strength (R m ) of the alloy of the present application is as high as 1095-1120 MPa, and the yield strength (R p0.2 ) is as high as 860-895 MPa, which is about 50-225 MPa higher than that of Ni30 (R m : 1040 MPa, R p0.2 : 670 MPa).

[0123] With the increase of temperature, the strength advantage is more significant. At 750℃, the R m of the alloy of the present application can still be maintained at 735-760 MPa, and the R p0.2 is maintained at 615-645 MPa, which is about 40-65 MPa higher than that of Ni30 (R m : 695 MPa, R p0.2 : 585 MPa), showing more excellent high-temperature softening resistance.

[0124] ​The alloy of the present application has good high-temperature plasticity while maintaining high strength. In the range of 600-750℃, the elongation after fracture (A) of all the alloys of the present application is maintained at 15.5-23.5%, and the reduction of area (Z) is maintained at 27.0-33.5%. Particularly important is that the plasticity of the alloy of the present application (A: 21.0-23.5%, Z: 32.0-33.5%) is significantly superior to that of Ni30 (A: 10.0%, Z: 12.0%) at high temperature of 750℃, which indicates that the alloy has more reliable toughness and deformation capacity under conditions close to the service temperature.

[0125] To verify the universality of the grain display method, metallographic samples were cut from the silver bright materials of the seven alloys, and each sample was polished by using metallographic sandpaper of 200 mesh, 500 mesh, 800 mesh and 1000 mesh in turn. Subsequently, different etching liquid ratios and heating parameters were used to display the grains of the seven samples (see Table 3 for details). After etching, the samples were cleaned with an 8% oxalic acid aqueous solution, rinsed with water and dried.

[0126] Under optical microscope observation, the grain boundaries of all the seven samples were clearly and completely displayed. Among them, the effect of the 1# alloy has been shown in Example 1 and Figure 1 , and the grain size photos of the 2# to 7# alloys correspond to Figures 2 to 7 respectively. The above microstructure was evaluated, and the results showed that the average grain size level of the seven alloys was between 4 and 7.

[0127] Table 1 Chemical composition of the present application (wt%)

[0128]

[0129]

[0130] Table 2 High-temperature tensile properties of the present application and comparative materials

[0131]

[0132]

[0133] Table 3 Etching liquid ratio in the present application

[0134]

[0135] Example 3

[0136] The embodiment selects commercial nickel-based superalloy GH4080A (which mainly contains Cr ≈ 19.0%, Ni ≥ 65%, Ti ≈ 2.5%, Al ≈ 1.5% by weight percentage, and the rest is Fe and alloying elements) as the test object. After standard solid solution treatment, the microstructure of the alloy is austenitic matrix, and a dense oxide film is formed on the surface due to the high chromium content.

[0137] The grain display method is as follows:

[0138] S1, a metallographic sample is cut from the solid solution treated GH4080A alloy bar, and is polished to a mirror surface by using 200 mesh, 500 mesh, 800 mesh and 1000 mesh metallographic sandpaper in turn.

[0139] S2, the sample is etched by using an etching liquid composed of 100 ml distilled water, 10 ml H2SO4 and 1 g KMnO4.

[0140] S3, the container containing the etching liquid and the sample is heated in a 50°C water bath for 3 hours.

[0141] S4, the sample is taken out, the black film on the surface of the sample is cleaned with an oxalic acid aqueous solution with a mass concentration of 8%, and then washed with clean water and dried with a hair dryer.

[0142] After the above treatment, the grain boundaries of the GH4080A alloy can be clearly and continuously observed under an optical microscope (as shown in Figure 8 ).

[0143] Comparative Example 1

[0144] The comparative example uses the same alloy sample as Example 1 (1# alloy in Table 1), but the etching method is changed to traditional saturated picric acid aqueous solution (add a few drops of new jieer) for room temperature immersion etching. After etching, the surface of the sample has almost no change. Under the observation of an optical microscope, no clear grain boundary profile can be observed, and the metallographic photograph is as shown in Figure 9 .

[0145] Comparative Example 2

[0146] The comparative example uses the same alloy sample as 3# alloy in Example 2 (3# alloy in Table 1), but the etching method is changed to commonly used nitric acid alcohol solution (concentration of 5%) for room temperature immersion etching. After etching, the surface of the sample only has slight discoloration. Under the observation of an optical microscope, the grain boundaries still cannot be distinguished, and the metallographic photograph is as shown in Figure 10 .

[0147] Comparative Example 3

[0148] The comparative example uses the same 1# alloy sample as Example 1, but the corrosion solution is adjusted to: 150 ml of distilled water, 5 ml of H2SO4, and 0.5 g of KMnO4. After the same steps as Example 1, under the optical microscope, due to the serious lack of chemical activity of the corrosion solution, the sample surface only presents a uniform light gray tone, and it is completely impossible to observe any grain boundary profile, and the microstructure is the same as the uncorroded polished state.

[0149] Comparative Example 4

[0150] The comparative example uses the same 1# alloy sample as Example 1, but the oxidizing agent in the corrosion solution is replaced with 1 g of sodium dichromate (Na2Cr2O7). After the same steps as Example 1, under the optical microscope, the replacement of the oxidizing agent causes the corrosion reaction to lose specificity, and the sample surface appears large areas of uneven color spots and point corrosion pits, and the grain boundaries are only visible in a few areas, but the overall coherence is completely lost, and cannot form a complete grain network that can be used for grain size rating.

[0151] Comparative Example 5

[0152] The comparative example uses the same 1# alloy sample and corrosion solution ratio as Example 1, but the water bath heating temperature is set to 40℃, and the heating time is extended to 4 hours. After processing, due to insufficient reaction temperature, the corrosion process is difficult to proceed uniformly, resulting in a serious uneven corrosion morphology on the sample surface, presenting a patchy state where some areas are over-corroded and others are under-reacted. See Figure 11 After oxalic acid cleaning, the grain boundaries are intermittent and have poor contrast, and a complete and clear grain morphology cannot be obtained.

[0153] Comparative Example 6

[0154] The comparative example uses the same 1# alloy sample, corrosion solution ratio and heating parameters as Example 1. However, in the S4 step, instead of using an aqueous oxalic acid solution, an aqueous hydrochloric acid (HCl) solution with a mass concentration of 5% is used for cleaning. After processing, the strong acidity of the hydrochloric acid solution severely damages the grain boundaries that have been slightly corroded, resulting in excessive overall corrosion, with blurred, widened or even completely disappeared grain boundaries, making accurate rating impossible. See Figure 12 .

[0155] In summary, the present application successfully develops a low-cost, high-strength iron-nickel-based heat-resistant alloy and a grain display method. The alloy, through component optimization, ensures excellent high-temperature strength (such as a tensile strength of 735-760 MPa at 750℃) while reducing the nickel content to 22-28%; the grain display method effectively solves the problem of grain characterization of high-chromium solid solution alloys. Both provide reliable technical support for the manufacture and application of high-temperature key components (such as engine valves).

[0156] The above description is only the preferred embodiment of the present application, but the protection scope of the present application is not limited to this. Any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the present application, which should be covered in the protection scope of the present application.

Claims

1. An iron-nickel based heat resistant alloy, characterized by, The chemical composition in percentage by weight is: C: 0.01-0.10%, Si: ≤0.20%, Mn: ≤0.50%, P: ≤0.020%, S: ≤0.020%, Cr: 18.00-24.00%, Ni: 22.00-28.00%, Ti: 1.50-2.80%, Al: 0.80-2.00%, Nb: 0.70-1.80%, V: 0.20-0.60%, Zr: ≤0.060%, Ce: ≤0.050%, Cu: ≤0.20%, and the rest is Fe and inevitable impurities.

2. The iron-nickel based heat resistant alloy according to claim 1, characterized in that, The content of V is 0.40-0.60%, and the sum of the weight percentages of Ti and Al is 3.00%-4.50%.

3. The iron-nickel based heat resistant alloy according to claim 2, characterized in that, The high-temperature tensile properties of the alloy at 750 ℃ satisfy: tensile strength ≥730 MPa, yield strength ≥610 MPa.

4. The iron-nickel based heat resistant alloy according to claim 3, characterized in that, The elongation A after breaking of the alloy at 750 ℃ is ≥21.0%.

5. A method of producing the iron-nickel-based heat-resistant alloy as claimed in any one of claims 1 to 4, characterized by, The method comprises the following steps in sequence: Step 1: vacuum induction smelting or non-vacuum induction smelting to obtain an electrode rod; Step 2: electroslag remelting of the electrode rod under a protective atmosphere, with a melting rate of 3-6 kg / min; Step 3: forging breakdown of the electroslag ingot, with a breakdown temperature of 1150-1200 ℃; Step 4: bar rolling of the billet, with a final rolling temperature not lower than 950 ℃; Step 5: solid solution treatment at 1080-1150 ℃, with a holding time of 1-2 hours, followed by cooling to room temperature at a cooling rate not lower than 50 ℃ / min.

6. The production method according to claim 5, wherein After the solid solution treatment, the microstructure of the alloy is a single austenite matrix, and the average grain size level number G satisfies 4.0-7.0 levels.

7. The preparation method according to claim 5, characterized in that, The solid solution treatment in step 5 is: holding at 1120 ℃±10 ℃ for 1.5 hours, followed by water quenching.

8. A method for displaying the grain size of a ferronickel-based heat-resistant alloy as claimed in any one of claims 1 to 4, characterized by, The method comprises the following steps: S1: sampling and polishing from the alloy after the solid solution treatment; S2: etching the polished sample with an etching liquid composed of 90-100 ml distilled water, 10-20 ml H2SO4 and 1-2 g KMnO4; S3: placing the prepared etching liquid and the sample in a 50-55 ℃ water bath for heating for 2-3 hours; S4: taking out the sample, cleaning the black film on the surface of the sample with an oxalic acid aqueous solution with a mass concentration of 5%-10%, and then rinsing with clean water and drying.

9. The method of claim 6, wherein, The polishing step comprises polishing in sequence with 200-mesh, 500-mesh, 800-mesh and 1000-mesh metallographic sandpaper until there is no visible scratch on the surface of the sample.

10. An engine valve characterized by The iron-nickel-based heat-resistant alloy is made of any one of claims 1-4.

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