A UNS N08120 alloy and its preparation method

Abstract

The present invention provides a UNS N08120 alloy, comprising the following components by mass ratio: C: 0.040% - 0.060%; Mn: 0.60% - 0.80%; P: ≤0.020%; S: ≤0.010%; Si: 0.30% - 0.50%; Cr: 24.00% - 25.00%; Ni: 36.00% - 38.00%; Nb + Ta: 0.60% - 0.80%; Cu: ≤0.2%; N: 0.20% - 0.25%; Al: ≤0.05%; Ti: 0.03 - 0.05%; Mo: 0.90% - 1.10%; W: 0.80% - 1.20%, Co: 2.50% - 2.80%, B: 0.003% - 0.005%, and the balance is Fe and unavoidable impurities. The present invention also provides a preparation method of the above UNS N08120 alloy. By the preparation method and alloy composition control of the present invention, the problem of coarse mixed crystals in the UNS N08120 alloy is solved.

Description

Technical Field

[0001] The present invention relates to the field of special alloys, and more particularly, to a UNS N08120 alloy and a method for preparing the same. Background Art

[0002] The UNS N08120 alloy (hereinafter referred to as 8120) is a solution-strengthened heat-resistant alloy developed on the basis of 800H. This alloy has extremely high high-temperature strength and at the same time has the ability to resist carburization and sulfidation. The high-temperature oxidation resistance of 8120 is the best among Fe-Cr-Ni alloys, and its comprehensive performance is better than that of the 300 stainless steel series and the 800 series. Even compared with Ni-Cr series alloys, the strength of this alloy below 1095 °C is significantly higher. At the same time, the alloy has good hot and cold working formability, and the products include bars, flange forgings, pipes, plates, etc., and are widely used in fluidized bed components in the field of chemical gasifiers.

[0003] In order to achieve the high strength and high creep properties of 8120, the ASME specification requires that the grain size be coarser than grade 5. In actual production, the grain size is between 0 and 5 grades according to the specific form of the product, and the grade difference is large. In extreme cases, there is even an obvious division of large and small grains on the yin and yang sides. The chaotic and coarse grains after solution treatment affect the later flaw detection quality, and the clutter and abnormal waveform levels are relatively high, making it difficult to distinguish from the true defect waves. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a UNS N08120 alloy to solve the problems of coarse grains, unqualified strength and creep properties of conventional UNS N08120 alloys.

[0005] To solve the above problems, the present invention provides a UNS N08120 alloy, comprising the following components by mass ratio:

[0006] C: 0.040% - 0.060%; Mn: 0.60% - 0.80%; P: ≤0.020%; S: ≤0.010%; Si: 0.30% - 0.50%; Cr: 24.00% - 25.00%; Ni: 36.00% - 38.00%; Nb + Ta: 0.60% - 0.80%; Cu: ≤0.2%; N: 0.20% - 0.25%; Al: ≤0.05%; Ti: 0.03 - 0.05%; Mo: 0.90% - 1.10%; W: 0.80% - 1.20%, Co: 2.50% - 2.80%, B: 0.003% - 0.005%, and the balance is Fe and unavoidable impurities.

[0007] As a preferred solution, it comprises the following components by mass ratio:

[0008] C: 0.051%; Mn: 0.75%; P: 0.15%; S: 0.002%; Si: 0.45%; Cr: 24.58%; Ni: 36.89%; Nb+Ta: 0.75%; Cu0.03%; N: 0.21%; Al: 0.04%; Ti: 0.04%; Mo: 1.07%;

[0009] W: 1.10%, Co: 2.75%, B: 0.003%, and the balance is Fe and inevitable impurities.

[0010] Another problem to be solved by the present invention is to provide a method for preparing the above UNS N08120 alloy to solve the problem that the UNS N08120 alloy prepared by conventional methods has coarse grains and substandard strength and durability.

[0011] In order to solve the above problems, the present invention provides a method for preparing UNS N08120 alloy, comprising the following steps:

[0012] S1: preparing raw materials according to the standard composition range of N08120 in ASME SA-564, adding the raw materials into an induction furnace for smelting, and preparing an induction ingot;

[0013] S2: grinding the induction ingot prepared in step S1 and then electroslag remelting it to obtain an electroslag ingot;

[0014] S3: subjecting the electroslag ingot prepared in step S2 to a heating forging process, wherein the heating forging process comprises heating the electroslag ingot to 1000±10°C and 1220±10°C in sequence and keeping the temperature before forging, wherein the forging temperature is ≥1000°C;

[0015] S4: heat treating the forging after the treatment in step S3 to obtain UNS N08120 alloy; the heat treatment includes pretreatment and solution treatment, the pretreatment includes heating the forging to 1000±10°C and keeping it warm before air cooling; the solution treatment includes heating the forging after the pretreatment to 1000±10°C and 1200±10°C in sequence and keeping it warm before water cooling.

[0016] As a preferred solution, in step S3, the heat treatment of the heated forging includes: controlling the furnace temperature of the electroslag ingot to 400°C and heating for 2 hours; raising the furnace temperature to 1000±10°C for the first time, and the heating rate is ≤80°C / h, and the first insulation is performed; then the temperature is raised to 1220±10°C for the second time, and the heating rate is ≤80°C / h, and the second insulation is performed to end the heat treatment.

[0017] As a preferred solution, the time for the first heat preservation is 2 - 3 min / mm, and the time for the second temperature increase is 2 - 3 min / mm.

[0018] As a preferred solution, in step S3, the forging conditions are as follows: blooming is carried out using a 6000T press, the total deformation per heat is controlled to be ≥30%, the single-pass deformation is greater than the critical deformation and is controlled between 10% - 15%.

[0019] As a preferred solution, in step S4, the pre-treatment conditions are as follows: the forging is charged into the furnace at a temperature ≤300°C, the furnace temperature is increased to 1000 ± 10°C at a speed ≤100°C / h, after the forging is heated to 1000 ± 10°C, heat preservation is carried out, and then it is taken out of the furnace and air-cooled.

[0020] As a preferred solution, the time for heat preservation is 3 - 4 min / mm.

[0021] As a preferred solution, in step S4, the solution treatment conditions are as follows: the forging is charged into the furnace at a temperature ≤300°C, it is first heated to 500°C, and then continuously heated to 1000 ± 10°C for the second time, and the heating speed ≤100°C / h, and the first heat preservation is carried out; then it is heated to the furnace temperature of 1200 ± 10°C for the third time at a speed ≤80°C / h, after the forging is heated to 1200 ± 10°C, the second heat preservation is carried out, and then it is taken out of the furnace and water-cooled.

[0022] As a preferred solution, the time for the first heat preservation is 2 min / mm, and the time for the second heat preservation is 3 - 4 min / mm.

[0023] For the preparation method of a UNS N08120 alloy of the present invention, the heat treatment in the above step S4 adopts two processes of pre-treatment and final treatment. The pre-treatment heat preservation is set in the temperature range with the largest precipitation temperature of the second phase, and the precipitation peak temperatures of brittle phases such as LAVES and σ phases should be avoided. The purpose of the pre-treatment is to rapidly nucleate at positions such as grain boundaries and sub-grains. These particles play a pinning role to prevent abnormal grain growth at high temperatures in the later stage, and at the same time release the tissue stress left over from the forging process, making the energy fluctuation of the entire matrix tend to be balanced. In this way, the abnormal growth of local grains caused by uneven structural energy during the later high-temperature solution stage can be alleviated.

[0024] A preparation method of UNS N08120 alloy is based on the existing traditional smelting process. On the basis of optimizing the composition of UNS N08120 alloy, MC-type carbides and Laves phases are added to the alloy, and the elements of W, Mo, Co, and Ti added to the composition are all strong carbide-forming elements or elements that promote carbide formation. The precipitation of a large number of carbides can form particles between grains to hinder grain growth, playing a role in refining grains to a certain extent; and high-quality ingots of UNS N08120 alloy are produced by medium-frequency induction furnace + electroslag remelting, and then the control of the forged structure of 8120 alloy is realized through a special forging process. Finally, the control of grain size is realized through heat treatment. The alloy grains of the finally prepared UNS N08120 alloy are uniform and not coarse, and the strength and creep properties of N08120 alloy are improved. Description of the Drawings

[0025] Figure 1 It is the thermal trend phase diagram of Example 1;

[0026] Figure 2 It is the thermal trend phase diagram of the comparative example;

[0027] Figure 3 It is the trend diagram of the Young's modulus change of N08120;

[0028] Figure 4 It is the thermal expansion coefficient diagram of N08120;

[0029] Figure 5 It is the forging heating process diagram of the electroslag ingot;

[0030] Figure 6 It is the stress-strain curve diagram;

[0031] Figure 7 It is the trend diagram of the change of the second-phase particle content with temperature;

[0032] Among them, when the abscissa corresponds to 1000 °C, the curves from top to bottom are MC+MN, MC, and MN in turn;

[0033] Figure 8 It is the workpiece pretreatment process diagram;

[0034] Figure 9 It is the workpiece solution process diagram;

[0035] Figure 10 It is the grain schematic diagram of Example in State 1;

[0036] Figure 11 It is the grain schematic diagram of the comparative example in State 2;

[0037] Figure 12 It is the grain schematic diagram of the comparative example in State 3;

[0038] Figure 13 Schematic diagram of crystal grains for the embodiment of State 4;

[0039] Figure 14 Schematic diagram of crystal grains for the embodiment of State 5. Detailed implementation manners

[0040] The technical solutions of the present invention will be described clearly and completely hereinafter. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0041] The following detection equipment is used in the following embodiments: SPECTROMAX direct-reading spectrometer, LECO nitrogen-hydrogen-oxygen analyzer, Yingzhicheng carbon-sulfur analyzer, SANS pendulum impact tester with a low-temperature bath and SANS electronic universal tensile machine with an environmental chamber, Leica optical microscope with a ZEISS lens, Caikang optical Rockwell hardness tester, and self-made corrosion device.

[0042] The present invention provides an UNS N08120 alloy, comprising the following components by mass ratio:

[0043] C: 0.040% - 0.060%; Mn: 0.60% - 0.80%; P: ≤0.020%; S: ≤0.010%; Si: 0.30% - 0.50%; Cr: 24.00% - 25.00%; Ni: 36.00% - 38.00%; Nb + Ta: 0.60% - 0.80%; Cu: ≤0.2%; N: 0.20% - 0.25%; Al: ≤0.05%; Ti: 0.03 - 0.05%; Mo: 0.90% - 1.10%; W: 0.80% - 1.20%, Co: 2.50% - 2.80%, B: 0.003% - 0.005%, and the balance is Fe and inevitable impurities.

[0044] As a preferred solution, it comprises the following components by mass ratio:

[0045] C: 0.051%; Mn: 0.75%; P: 0.15%; S: 0.002%; Si: 0.45%; Cr: 24.58%; Ni: 36.89%; Nb + Ta: 0.75%; Cu 0.03%; N: 0.21%; Al: 0.04%; Ti: 0.04%; Mo: 1.07%;

[0046] W: 1.10%, Co: 2.75%, B: 0.003%, and the balance is Fe and inevitable impurities.

[0047] The present invention also provides a method for preparing UNS N08120 alloy, comprising the following steps:

[0048] S1: preparing raw materials according to the standard composition range of N08120 in ASME SA-564, adding the raw materials into an induction furnace for smelting, and preparing an induction ingot;

[0049] S2: grinding the induction ingot prepared in step S2 and then electroslag remelting it to obtain an electroslag ingot;

[0050] S3: subjecting the electroslag ingot prepared in step S2 to a heating forging process, wherein the heating forging process comprises heating the electroslag ingot to 1000±10°C and 1220±10°C in sequence and keeping the temperature before forging, wherein the forging temperature is ≥1000°C;

[0051] S4: heat treating the forging after the treatment in step S3 to obtain UNS N08120 alloy; the heat treatment includes pretreatment and solution treatment, the pretreatment includes heating the forging to 1000±10°C and keeping it warm before air cooling; the solution treatment includes heating the forging after the pretreatment to 1000±10°C and 1200±10°C in sequence and keeping it warm before water cooling.

[0052] As a preferred solution, in step S3, the heat treatment of the heated forging includes: controlling the furnace temperature of the electroslag ingot to 400°C and heating for 2 hours; raising the furnace temperature to 1000±10°C for the first time, and the heating rate is ≤80°C / h, and the first insulation is performed; then the temperature is raised to 1220±10°C for the second time, and the heating rate is ≤80°C / h, and the second insulation is performed to end the heat treatment.

[0053] As a preferred solution, the first insulation time is 2-3 min / mm, and the second heating time is 2-3 min / mm.

[0054] As a preferred solution, in step S3, the forging conditions are: using a 6000T press to open the blank, controlling the total deformation per fire to ≥30%, and controlling the deformation per pass to be greater than the critical deformation, and between 10% and 15%.

[0055] As a preferred solution, in step S4, the pretreatment conditions are: charging the forging into a furnace at a temperature of ≤300°C, raising the furnace temperature to 1000±10°C at a rate of ≤100°C / h, and keeping the forging warm to 1000±10°C, and then taking it out of the furnace and air cooling it.

[0056] As a preferred solution, the insulation time is 3-4 min / mm.

[0057] As a preferred solution, in step S4, the conditions for solution treatment are as follows: the forging is charged into the furnace at a temperature ≤300°C, heated to 500°C for the first time, and then continuously heated to 1000 ± 10°C for the second time, with a heating rate ≤100°C / h for the first heat preservation; then heated to the furnace temperature of 1200 ± 10°C at a rate ≤80°C / h for the third time. After the forging is heated to 1200 ± 10°C, the second heat preservation is carried out, and then it is taken out of the furnace for water cooling treatment.

[0058] As a preferred solution, the time for the first heat preservation is 2 min / mm, and the time for the second heat preservation is 3 - 4 min / mm.

[0059] Example 1:

[0060] This example provides a UNS N08120 alloy, including the following components by mass ratio:

[0061] C: 0.051%; Mn: 0.75%; P: 0.15%; S: 0.002%; Si: 0.45%; Cr: 24.58%; Ni: 36.89%; Nb + Ta: 0.75%; Cu 0.03%; N: 0.21%; Al: 0.04%; Ti: 0.04%; Mo: 1.07%;

[0062] W: 1.10%, Co: 2.75%, B: 0.003%, and the balance is Fe and inevitable impurities.

[0063] This example also provides a preparation method for a UNS N08120 alloy, including the following steps:

[0064] S1: According to the standard composition range of N08120 in ASME SA - 564, raw materials are proportioned, and the raw materials are added to an induction furnace for melting to prepare an induction ingot;

[0065] S2: The induction ingot prepared in step S2 is ground and then subjected to electroslag remelting to obtain an electroslag ingot;

[0066] S3: The electroslag ingot prepared in step S2 is subjected to hot forging treatment. The heat treatment of hot forging includes: controlling the furnace inlet temperature of the electroslag ingot to be 400°C and heating for 2 h; heating the furnace temperature to 1000°C for the first time, with a heating rate ≤80°C / h for the first heat preservation; then heating to 1220°C for the second time, with a heating rate ≤80°C / h for the second heat preservation to end the heat treatment; the time for the first heat preservation is 2.5 min / mm, and the time for the second heating is 2.5 min / mm;

[0067] The forging conditions are as follows: The ingot is bloomed using a 6000T press, and the total deformation per heat is controlled at ≥30%, and the single-pass deformation is greater than the critical deformation, controlled between 10% - 15%.

[0068] S4: The forging processed in step S3 is heat-treated to obtain UNS N08120 alloy; the heat treatment includes pre-treatment and solution treatment. The conditions for the pre-treatment are: The forging is charged into the furnace at a temperature ≤300°C, the furnace temperature is raised to 1000°C at a rate ≤100°C / h, after the forging is heated to 1000°C, it is held for 3 - 4 min / mm, and then taken out of the furnace and air-cooled; the conditions for the solution treatment are: The forging is charged into the furnace at a temperature ≤300°C, first heated to 500°C, and then continuously heated to 1000°C for the second time, and the heating rate ≤100°C / h, and the first holding is carried out; then it is heated to the furnace temperature of 1200°C for the third time at a rate ≤80°C / h, after the forging is heated to 1200°C, the second holding is carried out, and then taken out of the furnace and water-cooled. The time for the first holding is 2 min / mm, and the time for the second holding is 3.5 min / mm.

[0069] Example 2:

[0070] This example provides a UNS N08120 alloy, including the following components by mass ratio:

[0071] C: 0.040%; Mn: 0.60%; P: ≤0.020%; S: ≤0.010%; Si: 0.30%; Cr: 24.00%; Ni: 36.00%; Nb + Ta: 0.60%; Cu: ≤0.2%; N: 0.20%; Al: ≤0.05%; Ti: 0.03; Mo: 0.90%; W: 0.80%, Co: 2.50%, B: 0.003%, and the balance is Fe and unavoidable impurities.

[0072] This example also provides a preparation method for UNS N08120 alloy, including the following steps:

[0073] S1: According to the standard component range of N08120 in ASME SA - 564, raw materials are proportioned, and the raw materials are added to an induction furnace for melting to prepare an induction ingot.

[0074] S2: The induction ingot prepared in step S2 is ground and then subjected to electroslag remelting to obtain an electroslag ingot.

[0075] S3: Heat and forge the electroslag ingot prepared in step S2. The heat treatment for heat forging includes: controlling the furnace inlet temperature of the electroslag ingot at 400 °C and heating for 2 h; first raising the furnace temperature to 990 °C at a heating rate ≤ 80 °C / h for the first heat preservation; then raising the temperature to 1210 °C for the second time at a heating rate ≤ 80 °C / h for the second heat preservation to end the heat treatment. The time for the first heat preservation is 2 min / mm, and the time for the second temperature rise is 2 min / mm.

[0076] The forging conditions are as follows: Use a 6000T press for cogging, control the total deformation per heat at ≥ 30%, and the single-pass deformation is greater than the critical deformation, controlled between 10% - 15%.

[0077] S4: Heat-treat the forgings processed in step S3 to obtain UNS N08120 alloy. The heat treatment includes pre-treatment and solution treatment. The pre-treatment conditions are: Load the forgings into the furnace at a temperature ≤ 300 °C, raise the furnace temperature to 1000 ± 10 °C at a rate ≤ 100 °C / h. After the forgings are heated to 990 °C, conduct heat preservation for 3 min / mm, and then take them out of the furnace and air-cool. The solution treatment conditions are: Load the forgings into the furnace at a temperature ≤ 300 °C, first raise the temperature to 500 °C, then continue to raise the temperature to 990 °C for the second time at a heating rate ≤ 100 °C / h for the first heat preservation; then raise the temperature to the furnace temperature of 1190 °C at a rate ≤ 80 °C / h for the third time. After the forgings are heated to 1190 °C, conduct the second heat preservation, and then take them out of the furnace and water-cool. The time for the first heat preservation is 2 min / mm, and the time for the second heat preservation is 3 min / mm.

[0078] Example 3:

[0079] This example provides a UNS N08120 alloy, including the following components by mass ratio:

[0080] C: 0.060%; Mn: 0.80%; P: ≤ 0.020%; S: ≤ 0.010%; Si: 0.50%; 25.00%; Ni: 38.00%;

[0081] Nb + Ta: 0.80%; Cu: ≤ 0.2%; N: 0.25%; Al: ≤ 0.05%; Ti: 0.05%; Mo: 1.10%; W: 1.20%, Co: 2.80%, B: 0.005%, and the balance is Fe and unavoidable impurities.

[0082] This example also provides a preparation method for the UNS N08120 alloy, including the following steps:

[0083] S1: preparing raw materials according to the standard composition range of N08120 in ASME SA-564, adding the raw materials into an induction furnace for smelting, and preparing an induction ingot;

[0084] S2: grinding the induction ingot prepared in step S2 and then electroslag remelting it to obtain an electroslag ingot;

[0085] S3: subjecting the electroslag ingot prepared in step S2 to a heating forging treatment, wherein the heating forging heat treatment comprises: controlling the inlet temperature of the electroslag ingot to 400°C and heating for 2h; raising the temperature in the furnace to 1010°C for the first time, and the heating rate is ≤80°C / h, and performing the first heat preservation; then, raising the temperature to 1230°C for the second time, and the heating rate is ≤80°C / h, and performing the second heat preservation, and ending the heat treatment; the first heat preservation time is 3min / mm, and the second heat rise time is 3min / mm;

[0086] The forging conditions are: using a 6000T press to open the blank, controlling the total deformation of each fire to ≥30%, and controlling the deformation of a single pass to be greater than the critical deformation and between 10% and 15%;

[0087] S4: heat-treating the forging after the treatment in step S3 to obtain UNS N08120 alloy; the heat treatment includes pretreatment and solution treatment, and the pretreatment conditions are as follows: charging the forging at a temperature of ≤300°C, raising the furnace temperature to 1010°C at a rate of ≤100°C / h, and holding the forging for 4min / mm after the forging is heated to 1010°C, and then air-cooling the forging; the solution treatment conditions are as follows: charging the forging at a temperature of ≤300°C, heating the forging to 500°C for the first time, and then heating the forging to 1010°C for the second time at a heating rate of ≤100°C / h, and holding the forging for the first time; then heating the forging to 1210°C for the third time at a rate of ≤80°C / h, and holding the forging for the second time after the forging is heated to 1210°C, and then water-cooling the forging, the first holding time is 2min / mm, and the second holding time is 4min / mm.

[0088] Comparative Example:

[0089] The comparative example is an induction ingot, and its composition is shown in Table 1:

[0090] Table 1 Comparison of the internal control components and the components before and after control

[0091]

[0092]

[0093] After comparing the optimized and non-optimized compositions, Example 1 has more MC-type carbides and Laves phases than Comparative Example. Figure 1 and Figure 2 are the thermal trend phase diagrams of Example 1 and Comparative Example respectively.

[0094] This is consistent with the concept of composition design. W, Mo, Co, and Ti are all strong carbide-forming elements or elements that promote carbide formation. The precipitation of a large number of carbides can form particles at the grain boundaries to hinder grain growth, playing a role in refining the grains to a certain extent. A certain proportion of MC-type carbides still exist in the composition of Example 1 until 1250 °C.

[0095] In the preparation method of the present invention, forging controls the final forging reduction and the final forging reheat time to control the forging grain size.

[0096] Young's modulus is a physical quantity that describes the ability of a material to resist deformation. As Figure 3 shown, the deformation resistance of N08120 has an obvious cliff-like drop above 1180 °C. Therefore, the appropriate forging heating temperature is above 1180 °C. Considering the full melting temperature of the brittle phase Laves phase is about 990 °C, the final forging temperature is specified to be at least greater than 1000 °C considering the supercooling degree. The liquidus temperature is about 1310 °C. Considering the superheat degree and combining the general understanding of material overheating and burning, the heating temperature is specified not to be greater than 1240 °C. After the electroslag ingot is surface-treated, it is heated for forging in a chamber furnace. As Figure 4 shown, the thermal expansion coefficient of this grade is small. Therefore, the steel ingot is heated using a three-stage heating system of low-temperature preheating, medium-temperature soaking, and high-temperature holding to ensure uniform temperature inside and outside the steel ingot, relieve the thermal stress caused by the temperature difference inside and outside the steel ingot, and reduce the risk of thermal deformation cracking. The medium-temperature holding section is set at 1000 °C, mainly considering that the dissolution temperature of the Laves phase is 990 °C, and the heating temperature must avoid the formation temperature of this brittle phase. The heating process of the electroslag ingot is as Figure 5 shown.

[0097] As Figure 6 shown, according to the stress-strain curve of N08120, when the strain rate is 0.5 s -1 , the strain curve reaches a peak and then remains horizontal, belonging to a typical recovery type. When the strain rate is 5 s-1, the strain curve reaches a peak and then, due to the fast deformation rate and the recrystallization rate not being able to keep up with the work hardening rate, the curve is wavy. Obvious dynamic recrystallization occurs when the strain rate is below 0.05 s-1. The loading rate of the press crossbeam can be calculated by e ε = (dl / dt) / l 0 in reverse, where ε e is the strain rate, dl / dt is the loading rate, and l 0is the ingot length. During the actual production process, the loading rate should be properly controlled. On the premise of dynamic crystallization, the temperature rise at the core of the material should be minimized as much as possible, while maintaining the production rhythm to meet the deformation task of a single heat treatment.

[0098] The electroslag ingot is bloomed using a 6000T press. The total deformation per heat is controlled at ≥30%, and the deformation per single pass is greater than the critical deformation, controlled at about 10%-15%. During the rolling process, the temperature is always controlled at ≥1000°C, and the rolling speed is moderate. Excessive large deformation should be avoided to prevent excessive temperature rise at the core of the material, which may cause cracking.

[0099] During the hot deformation process, the second-phase particles are distributed at the subgrain boundaries and grain boundaries, which hinder the movement of dislocations and the migration of grain boundaries in the deformed matrix. The particles distributed at the subgrain boundaries play a greater role in the polygonization and stabilization of subgrains. Precipitated at the large-angle grain boundaries, they reduce the interface migration rate and prevent the growth of recrystallized grains.

[0100] The heat treatment process consists of two steps: pretreatment and final treatment. The pretreatment holding temperature is set in the temperature range with the highest precipitation temperature of the second phase, and the precipitation peak temperatures of brittle phases such as LAVES and σ phases should be avoided. The purpose of pretreatment is to quickly nucleate at grain boundaries, subgrains, etc. These particles play a pinning role to prevent abnormal grain growth at high temperatures in the later stage. At the same time, the residual tissue stress during the forging process is released, making the energy fluctuation of the entire matrix tend to be balanced, which can alleviate the abnormal growth of local grains caused by uneven structural energy during the later high-temperature solution stage.

[0101] Figure 7 The figure shows the trend of the change of nitride and carbide second phases with temperature from 1000-1250°C. It can be seen from the figure that as the temperature increases, the MC-type second phase continuously decreases and completely dissolves at 1250°C. The MN-type increases slightly with temperature. The total content of the second phase MC+MN decreases from 0.4% to 0.22% from 1000°C to 1250°C. Considering that the complete dissolution temperature of the LAVES phase is 990°C, the appropriate pretreatment temperature is set at 1000°C. At this temperature, the total amount of MN+MC is the highest and the number of nuclei is the largest.

[0102] The second-stage final heat treatment is carried out at 1200°C recommended by ASME to dissolve most of the precipitate phases. Since there are still about 3% of carbonitrides present, these second-phase particles play a certain role in hindering grain growth. The process curves of pretreatment and final solution treatment are shown in Figure 8 and Figure 9 .

[0103] Grain size test: Approximately 1.8 tons of raw materials are added to the intermediate-frequency induction melting furnace, and the batching is controlled according to the internal control composition in Table 1. The batching composition and measured composition of the induction ingot are shown in Table 1.

[0104] The grain size test results are asFigures 10 - 14 As shown, the thickness of the test ring is about 30 mm:

[0105] Table 2 Grain Size Detection Results

[0106]

[0107] From the above grain size detection results, it can be seen that for the UNS N08120 alloy prepared by the present invention, through the alloy composition control + heat treatment process of the present invention, the problem of mixed crystals in the alloy is effectively solved, thereby providing a UNS N08120 alloy with good strength and creep properties.

[0108] Although the present disclosure is disclosed as above, the protection scope of the present disclosure is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present disclosure, and these changes and modifications will all fall within the protection scope of the present invention.

Claims

1. A method for preparing UNS N08120 alloy, Features: The following steps are involved: S1: preparing raw materials according to the standard composition range of N08120 in ASME SA-564, adding the raw materials into an induction furnace for smelting, and preparing an induction ingot; S2: grinding the induction ingot prepared in step S1 and then electroslag remelting it to obtain an electroslag ingot; S3: subjecting the electroslag ingot prepared in step S2 to a heating forging process, wherein the heating forging process comprises heating the electroslag ingot to 1000±10°C and 1220±10°C in sequence and keeping the temperature before forging, wherein the forging temperature is ≥1000°C; S4: heat-treating the forged piece after the treatment in step S3 to obtain UNS N08120 alloy; the heat treatment includes pretreatment and solution treatment, the pretreatment includes heating the forged piece to 1000±10°C and keeping the temperature before air cooling; the solution treatment includes heating the forged piece after the pretreatment to 1000±10°C and 1200±10°C in sequence and keeping the temperature before water cooling; The UNS N08120 alloy includes the following components by mass ratio: C:0.040%-0.060%; Mn:0.60%-0.80%; P:≤0.020%; S:≤0.010%; Si:0.30%-0.50%; Cr:24.00%-25.00%; Ni:36.00%-38.00%; Nb+Ta:0.60%-0.80%; Cu:≤0.2%; N:0.20%-0.25%; Al:≤0.05%; Ti:0.03-0.05%; Mo:0.90%-1.10%; W:0.80%-1.20%, Co:2.50%-2.80%, B:0.003%-0.005%, the remainder is Fe and unavoidable impurities, the sum of the mass percentages of each component is 100%.

2. The method for preparing the UNS N08120 alloy according to claim 1, Features: The composition includes the following components in mass ratio: C: 0.051%; Mn: 0.75%; P: 0.15%; S: 0.002%; Si: 0.45%; Cr: 24.58%; Ni: 36.89%; Nb+Ta: 0.75%; Cu 0.03%; N: 0.21%; Al: 0.04%; Ti: 0.04%; Mo: 1.07%; W: 1.10%, Co: 2.75%, B: 0.003%, the remainder is Fe and unavoidable impurities, and the sum of the mass percentages of each component is 100%.

3. The preparation method of UNS N08120 alloy according to claim 1, Features: In the step S3, the heat treatment of hot forging includes: controlling the furnace inlet temperature of the electroslag ingot to be 400°C and heating for 2 h; first raising the furnace temperature to 1000 ± 10°C at a heating rate ≤ 80°C / h for the first heat preservation; then raising the temperature for the second time to 1220 ± 10°C at a heating rate ≤ 80°C / h for the second heat preservation, and ending the heat treatment.

4. The method for preparing UNS N08120 alloy according to claim 1, characterized in that: in the step S3, the forging conditions are: using a 6000T press for cogging, controlling the total deformation per heat to be ≥ 30%, and the single-pass deformation amount is greater than the critical deformation amount, controlled between 10% - 15%.

5. The method for preparing UNS N08120 alloy according to claim 1, characterized in that: in the step S4, the pre-treatment conditions are: loading the forging into the furnace at a temperature ≤ 300°C, raising the furnace temperature to 1000 ± 10°C at a rate ≤ 100°C / h, performing heat preservation after the forging is heated to 1000 ± 10°C, and then taking it out of the furnace and air-cooling.

6. The method for preparing UNS N08120 alloy according to claim 1, characterized in that: in the step S4, the solution treatment conditions are: loading the forging into the furnace at a temperature ≤ 300°C, raising the temperature to 500°C for the first time, and then continuing to raise the temperature to 1000 ± 10°C for the second time at a heating rate ≤ 100°C / h for the first heat preservation; then raising the temperature to the furnace temperature of 1200 ± 10°C at a rate ≤ 80°C / h for the third time, performing heat preservation after the forging is heated to 1200 ± 10°C, and then taking it out of the furnace and water-cooling.

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