Molybdenum alloy plug as well as preparation method and application thereof

Abstract

The invention provides a molybdenum alloy plug and a preparation method and application thereof, and particularly relates to the technical field of powder metallurgy. According to the preparation method of the molybdenum alloy plug, a molybdenum source and doping phase powder are mixed through material mixing to obtain molybdenum alloy powder, then the molybdenum alloy powder is pressed into a blank through a staged pressure increasing system and a staged pressure relieving system in the cold isostatic pressing process, the blank is subjected to low-temperature pre-sintering and high-temperature densification treatment after shaping treatment, and the molybdenum alloy plug is obtained. And later-stage annealing is assisted, so that the molybdenum alloy plug is obtained. The preparation method is simple in technological process and low in cost, and the prepared molybdenum alloy plug product has the characteristics of high density, excellent hardness and toughness, long service life and safe and reliable strength. The invention further provides the molybdenum alloy plug prepared through the preparation method and application of the molybdenum alloy plug.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy technology, specifically relating to a molybdenum alloy mandrel, its preparation method, and its application. Background Technology

[0002] Molybdenum alloy mandrels are high-performance, high-temperature resistant, and wear-resistant key components, primarily used in the hot rolling piercing process of seamless steel pipes. Based on molybdenum (Mo), they incorporate alloying elements such as titanium (Ti), zirconium (Zr), and carbon (C) to form reinforcing phases, significantly improving the material's high-temperature mechanical properties and resistance to thermal fatigue. Compared to ordinary molybdenum alloys, molybdenum alloy mandrels exhibit superior overall performance under extreme conditions, maintaining high-temperature strength even at 1200℃ and withstanding instantaneous thermal shocks of up to 1300℃ during steel pipe piercing. However, due to the extremely harsh working conditions of piercing mandrels—experiencing high temperature and pressure, significant axial, radial, and tangential stresses, deformation friction, thermal stress from temperature differences during cooling, and thermal fatigue stress—mandrels are highly susceptible to damage. Therefore, improving the wear resistance and service life of molybdenum mandrels is a pressing issue that needs to be addressed.

[0003] Chinese patent CN104911428B discloses a wear-resistant molybdenum alloy mandrel and its preparation method. The molybdenum mandrel is composed of the following percentages of raw materials: Mo 64.84%, Al2O3 8%, Y2O3 12%, Ti 8%, Zr 5%, C 2%, and B 0.16%. Through multi-element doping and ceramic particle reinforcement, and ball milling, a uniformly mixed alloy powder is obtained, achieving the goal of reducing the brittleness of the molybdenum alloy and improving the service life of the molybdenum alloy mandrel. The recrystallization temperature of this invention is 300-400℃ higher than that of pure molybdenum, and its strength at high temperatures is higher than that of other molybdenum materials, with a hardness greater than 500 HV and better wear resistance. However, on the one hand, this invention uses a variety of raw materials such as molybdenum, Ti, Zr, Y2O3, and Al2O3, resulting in a relatively high cost; on the other hand, the limitations of the ball milling process prevent industrial-scale mass production.

[0004] In view of this, the present invention is hereby proposed. Summary of the Invention

[0005] To address the aforementioned shortcomings in existing technologies, this invention aims to provide a molybdenum alloy mandrel and its preparation method. This method involves pressing the molybdenum alloy mandrel blank through a staged pressurization and depressurization process, followed by low-temperature pre-firing and high-temperature densification treatment of the molybdenum mandrel blank, and then supplemented by subsequent annealing to ensure the formation of the second phase. The resulting molybdenum alloy mandrel product has the characteristics of high density, simple process, low cost, long service life, and excellent hardness and toughness.

[0006] To achieve the above objectives, the following technical solution is adopted: The primary objective of this invention is to provide a method for preparing a molybdenum alloy mandrel, comprising the following steps: (a) Mixing process: Titanium hydride, zirconium hydride, cerium oxide, molybdenum powder and carbon powder are pre-alloyed according to the chemical composition of the molybdenum alloy mandrel to obtain molybdenum alloy powder. (b) Forming process: The molybdenum alloy powder mixed in step (a) is loaded into a conical rubber sleeve and cold isostatically pressed to obtain a blank; Among them, a staged pressurization system is adopted in the pressurization stage of cold isostatic pressing, and a staged depressurization system is adopted in the depressurization stage of cold isostatic pressing. (c) Shaping process: The blank obtained in step (b) is turned and shaped to flatten the end face; (d) Sintering treatment: The blank after the shaping treatment in step (c) is subjected to low-temperature pre-sintering treatment, high-temperature densification treatment and post-stage annealing to obtain a molybdenum alloy mandrel.

[0007] Furthermore, based on the above-mentioned technical solution of the present invention, in step (a), the molybdenum alloy mandrel is made of the following raw materials in the following mass percentages: TiH2 0.65-1.5%, ZrH2 0.15-0.3%, CeO2 1.3-1.8%, C 0.4-0.6%, with the balance being molybdenum powder; And / or, the molybdenum powder has a Fisher particle size of 2.8~3.2μm, the TiH2 has a Fisher particle size of 10~30μm, the ZrH2 has a Fisher particle size of 5~20μm, the CeO2 has a Fisher particle size of 10~50μm, and the C powder has a Fisher particle size of 25~40nm; wherein the purity of the carbon powder is above 98%, and the purity of the other raw materials is above 99.95%.

[0008] Furthermore, based on the above-mentioned technical solution of the present invention, in step (a), titanium hydride, zirconium hydride, cerium oxide, molybdenum powder and carbon powder are pre-alloyed and mixed in a plow mixer. And / or, the pre-alloying mixture treatment time in step (a) is 2-6 hours.

[0009] Furthermore, based on the above technical solution of the present invention, in step (b), the molding pressure is up to 180~210MPa, and the total holding time during the staged pressure increase process is 9~18min; And / or, the staged pressurization system includes sequentially performing a first-stage pressurization, a second-stage pressurization, a third-stage pressurization, a fourth-stage pressurization, and a fifth-stage pressurization, and holding the pressure for a certain period of time after reaching the pressurization pressure of each stage; Preferably, the staged boosting system includes the following: First-stage pressurization: pressurize to 50~80MPa but not including 80MPa, and hold the pressure for 1~2 minutes; Secondary pressurization: pressurize to 80~120MPa but not including 120MPa, and hold the pressure for 1~2 minutes; Three-stage pressurization: pressurize to 120~150MPa but not including 150MPa, and hold the pressure for 1~2 minutes; Fourth-stage pressurization: pressurize to 150~180MPa but not including 180MPa, and hold the pressure for 1~2 minutes; Five-stage pressurization: pressurize to 180~210MPa and hold for 5~10 minutes; The pressurization rate during each pressurization stage is 4~6 MPa / min.

[0010] Furthermore, based on the above technical solution of the present invention, in step (b), the staged depressurization system includes performing first-level depressurization and second-level depressurization in sequence, and maintaining pressure for a certain period of time after reaching the depressurization pressure of each stage. Preferably, the phased depressurization system includes the following: First-stage pressure relief: Depressurize from the highest pressure to 180~120MPa, but not including 120MPa, and maintain the pressure for 1~2 minutes; Secondary depressurization: Depressurize to 120~80MPa and maintain the pressure for 1~2 minutes; The depressurization rate during each stage of depressurization is 4~6 MPa / min.

[0011] Furthermore, based on the above technical solution of the present invention, in step (c), the low-temperature pre-firing treatment includes staged heat preservation within the range of 800~1400℃ but not including 1400℃, and the total heat preservation time is 6~12h. And / or, the high-temperature densification treatment includes staged heat preservation within the range of 1400~2150℃, with a total heat preservation time of 10~18h; And / or, the subsequent annealing stage involves high-temperature densification treatment followed by furnace shutdown and cooling to 1300~1600℃ and holding at that temperature for 2~5 hours, then air cooling to below 100℃ before unloading from the furnace.

[0012] Furthermore, based on the above-mentioned technical solution of the present invention, in step (c) of the entire sintering process, the process of heating from room temperature to the highest sintering temperature and then cooling down is carried out in a staged sintering manner, specifically including the first stage sintering, the second stage sintering, the third stage sintering, the fourth stage sintering and the subsequent annealing; wherein, the first stage sintering and the second stage sintering are low-temperature pre-sintering treatments, and the third stage sintering and the fourth stage sintering are high-temperature densification treatments. Preferably, the sintering process includes: First stage sintering: The temperature is raised from room temperature to 800-1000℃ (excluding 1000℃) over 3-6 hours, and then held for 2-4 hours. Second stage sintering: After 3-6 hours, the temperature is raised from the holding temperature of the first stage sintering to 1000-1200℃ (excluding 1200℃), and held for 2-4 hours; then after another 2-4 hours, the temperature is raised from the previous holding temperature to 1200-1400℃ (excluding 1400℃), and held for 2-4 hours. The third stage of sintering: After 4 to 6 hours, the temperature is raised from the highest holding temperature of the second stage sintering to 1400 to 1600℃ (excluding 1600℃), and held for 2 to 4 hours; then after another 4 to 6 hours, the temperature is raised from the previous holding temperature to 1600 to 1800℃, and held for 2 to 4 hours. Fourth stage sintering: After 4-6 hours, the temperature is raised from the highest holding temperature of the third stage sintering to 2000-2150℃, and held for 6-10 hours; Post-stage annealing: The furnace is shut down and cooled from the holding temperature of the fourth stage sintering to 1300~1600℃, held for 2~5 hours, and then cooled to below 100℃ before being removed from the furnace.

[0013] Furthermore, based on the above-described technical solution of this invention, the hydrogen gas regime used in step (c) of the sintering process is as follows: before the holding temperature for the first stage of sintering rises from room temperature, the hydrogen inlet flow rate is controlled to be 0~0.3m³. 3 / h, lower intake flow rate controlled at 4~6m 3 / h; From the holding temperature of the first stage sintering to the highest holding temperature of the third stage sintering, the hydrogen inlet flow rate is controlled to be 0~0.3m. 3 / h, lower intake flow rate controlled at 3~5m 3 / h; From the highest holding temperature of the third stage sintering until the end of sintering, the hydrogen inlet flow rate is controlled at 0~0.3m. 3 / h, lower intake flow rate controlled at 2~4m 3 / h.

[0014] The second objective of this invention is to provide a molybdenum alloy mandrel, which is prepared using the method for preparing a molybdenum alloy mandrel provided in the first objective of this invention; The molybdenum alloy mandrel has a grain size of 7.0~7.8 and a hardness of 178~185 HV. 30 .

[0015] The third objective of this invention is to provide an application of the above-mentioned molybdenum alloy mandrel in the field of hot rolling piercing of seamless steel pipes.

[0016] Compared with the prior art, the technical solution of the present invention has at least the following technical effects: (1) This invention provides a method for preparing a molybdenum alloy mandrel. First, a molybdenum source and doped phase powder are mixed to obtain molybdenum alloy powder. Then, during cold isostatic pressing, the molybdenum alloy powder is pressed into a blank through a staged pressurization and depressurization process. After shaping, the blank is subjected to low-temperature pre-firing and high-temperature densification treatment, followed by a subsequent annealing stage to ensure the stable formation of the second phases TiC and ZrC, thereby obtaining the molybdenum alloy mandrel. This preparation method has a simple process flow and low cost. The prepared molybdenum alloy mandrel product has high density and excellent hardness and toughness, long service life, and reliable strength.

[0017] (2) The present invention provides a molybdenum alloy mandrel, which is prepared by the above-mentioned molybdenum alloy mandrel preparation method provided by the present invention. The molybdenum alloy mandrel has high density and takes into account both excellent hardness and toughness, and has a long service life. Attached Figure Description

[0018] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0019] Figure 1 A physical image of a molybdenum alloy mandrel according to one embodiment of the present invention is provided. Figure 2 Metallographic image of the molybdenum alloy mandrel obtained in Example 1 of this invention; Figure 3 Metallographic image of the molybdenum alloy mandrel obtained in Example 2 of this invention; Figure 4 Metallographic image of the molybdenum alloy mandrel obtained in Example 3 of this invention; Figure 5 Metallographic image of the molybdenum alloy mandrel obtained in Example 4 of this invention; Figure 6 Metallographic image of the molybdenum alloy mandrel obtained in Example 5 of this invention; Figure 7 Metallographic image of the molybdenum alloy mandrel obtained in Example 6 of this invention; Figure 8 Metallographic image of the molybdenum alloy mandrel prepared in Comparative Example 2 of this invention; Figure 9 Metallographic image of the molybdenum alloy mandrel prepared in Comparative Example 3 of this invention; Figure 10 Metallographic image of the molybdenum alloy mandrel prepared in Comparative Example 4 of this invention; Figure 11Metallographic image of the molybdenum alloy mandrel prepared in Comparative Example 5 of this invention; Figure 12 Metallographic image of the molybdenum alloy mandrel prepared in Comparative Example 6 of this invention; Figure 13 Metallographic image of the molybdenum alloy mandrel prepared in Comparative Example 7 of this invention; Among them, Figures 2-13 In the figures, the scale size is 100μm. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. Process parameters not specifically specified in the following embodiments are generally performed under conventional conditions.

[0021] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0022] According to a first aspect of the present invention, a method for preparing a molybdenum alloy mandrel is provided, comprising the following steps: (a) Mixing process: Titanium hydride (TiH2), zirconium hydride (ZrH2), cerium oxide (CeO2), molybdenum powder (Mo) and carbon powder (C) are mixed according to the raw material composition of the molybdenum alloy mandrel to obtain molybdenum alloy powder; (b) Molding process: The molybdenum alloy powder mixed in step (a) is loaded into a conical rubber sleeve of the corresponding specification, and a blank is obtained after cold isostatic pressing. Among them, a staged pressurization system is adopted in the pressurization stage of cold isostatic pressing, and a staged depressurization system is adopted in the depressurization stage of cold isostatic pressing. (c) Shaping process: The blank obtained in step (b) is turned and shaped according to the given drawings and the end face is flattened; (d) Sintering treatment: The blank after the shaping treatment in step (c) is subjected to low-temperature pre-firing, high-temperature densification treatment and post-stage annealing (post-stage heat treatment) to obtain the molybdenum alloy mandrel. The actual product picture is as follows. Figure 1 As shown.

[0023] Specifically, this invention provides a method for preparing a molybdenum alloy mandrel, which mainly involves mixing raw materials such as titanium hydride, zirconium hydride, cerium oxide, molybdenum powder, and carbon powder, followed by cold isostatic pressing and shaping, and then sintering the shaped blank to obtain the molybdenum alloy mandrel.

[0024] Unlike conventional cold isostatic pressing (COP) processes, this invention employs a staged pressurization and depressurization process. During the staged pressurization, slowing down the pressurization rate allows for more thorough and complete deformation of the particle contact area during the gradual pressure increase. Simultaneously, extending the holding time allows the high-pressure gas in the compact to dissipate. During the staged depressurization, a slow depressurization speed is preferable. Sudden pressure release causes the compressed high-pressure air in the compact's voids to expand abruptly, potentially cracking the mandrel. The elastic aftereffect further exacerbates the mandrel's rupture. Using this staged pressurization and depressurization process results in a compact with a smooth surface, free of pores, edge chipping, and cracks, and with a high density that can be uniformly controlled at 6-6.8 g / cm³. 3 .

[0025] In this invention, a molybdenum source and doped phase powders (titanium hydride, zirconium hydride, cerium oxide, and carbon powder) are mixed to obtain molybdenum alloy powder. The molybdenum alloy powder is then pressed into a blank (molybdenum alloy mandrel blank) using a staged pressurization and depressurization process. After shaping, the blank undergoes low-temperature pre-sintering and high-temperature densification treatment, followed by a subsequent annealing stage to ensure the stable formation of the second phases TiC and ZrC, thereby obtaining the molybdenum alloy mandrel. This preparation method has a simple process flow, low cost, and produces a high-density molybdenum alloy mandrel product (≥9.5 g / cm³). 3 It also features excellent hardness and toughness, long service life, reliable strength, and high yield.

[0026] As an optional embodiment of the technical solution of the present invention, in step (a), taking the mass fraction of molybdenum alloy powder used to prepare the molybdenum alloy mandrel as 100%, the molybdenum alloy mandrel is mainly made of the following raw materials in the following mass percentages: TiH2 0.65-1.5%, ZrH2 0.15-0.3%, CeO2 1.3-1.8%, C 0.4-0.6%, with the balance being molybdenum powder.

[0027] In this invention, TiH2 activates the surface of molybdenum particles during sintering, improving the alloying degree and uniformity of the molybdenum alloy mandrel during sintering. Typical, but not limiting, mass fractions of TiH2 are 0.65%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or 1.5%, and any range between any two points.

[0028] The role of ZrH2 is similar to that of TiH2. On the one hand, the active element Zr forms compounds with the interstitial elements O and N in metallic molybdenum, reducing the harmful effects of the interstitial elements. On the other hand, the carbides formed by the active element Zr and carbon play an important role in improving the strength of the alloy by being dispersed in the molybdenum matrix. Typical, but not limiting, mass fractions of ZrH2 are 0.15%, 0.2%, 0.25%, or 0.3%, and any range between any two points.

[0029] CeO2 can refine crystal structure and inhibit the formation of Mo2C, thereby improving the plasticity of alloys, especially the high-temperature performance of molybdenum alloy mandrels. Typical, but not limiting, mass fractions of CeO2 are 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, or 1.8%, and any range between any two points.

[0030] On the one hand, carbon reacts with hydrogen in the hydride to form hydrocarbons during sintering, maximizing the removal of hydrogen from the hydride to achieve the best activation sintering effect. On the other hand, some carbon reacts with active metal elements to form second-phase refractory metal carbides TiC and ZrC, which strengthen the molybdenum matrix. In this invention, the typical but non-limiting mass fraction of carbon is 0.4%, 0.5%, or 0.6%, or any value range between any two points.

[0031] The present invention further specifies the particle size of each of the above-mentioned raw materials. As an optional embodiment of the technical solution of the present invention, in step (a), the Fisher particle size of the molybdenum powder is 2.8~3.2 μm (e.g., 2.8 μm, 2.9 μm, 3.0 μm, 3.1 μm, or 3.2 μm, etc.), the Fisher particle size of TiH2 is 10~30 μm (e.g., 10 μm, 15 μm, 20 μm, 25 μm, or 30 μm, etc.), and the Fisher particle size of ZrH2 is 5~20 μm (… For example, 5μm, 10μm, 15μm or 20μm, etc.); the Fisher particle size of CeO2 is 10~50μm (for example, 10μm, 20μm, 30μm, 40μm or 50μm, etc.); the Fisher particle size of toner is 25~40nm (for example, 25nm, 30nm, 35nm or 40nm, etc.); among them, the purity of toner is above 98%, and the purity of the other raw materials is above 99.95%.

[0032] Based on the above limitations, fine molybdenum powder has a high surface energy and a short atomic diffusion path during sintering, making it easier to achieve densification and obtain a high-density sintered blank. At the same time, the carbon powder has a finer particle size because nano-carbon powder has a very large specific surface area, which allows it to react more fully and quickly with the oxides on the surface of molybdenum powder during sintering. This effectively removes oxygen impurities and improves the purity and toughness of molybdenum. In addition, nano-powder is easy to diffuse further during subsequent sintering processes, which can improve the uniformity of the sintered structure.

[0033] In the process of mixing the above-mentioned raw materials, the existing technology usually uses a V-type mixer or a double-motion mixer. The present invention optimizes this mixing method. As an optional embodiment of the technical solution of the present invention, in step (a), titanium hydride, zirconium hydride, cerium oxide, molybdenum powder and carbon powder are mixed in a plow mixer (plow-type mixer).

[0034] The mixing method was adjusted, replacing the V-type mixer or dual-motion mixer with a plow mixer. The plow mixer features a single-shaft, multi-rake tooth design, with each rake tooth arranged in a different geometric shape. This causes the material to be thrown into a back-and-forth moving curtain within the mixer body, thus achieving cross-mixing between materials. For mixing materials with significantly different specific gravities (such as molybdenum powder and carbon powder, titanium hydride, zirconium hydride, etc.), the plow mixer exhibits better mixing performance.

[0035] As an optional embodiment of the technical solution of the present invention, in step (b), the molding pressure is up to 180~210MPa, and / or the total holding time (the sum of the holding times of each stage) is 9~18min.

[0036] As an optional implementation of the technical solution of the present invention, the staged pressurization system includes sequentially performing first-stage pressurization, second-stage pressurization, third-stage pressurization, fourth-stage pressurization, and fifth-stage pressurization, and maintaining the pressure for a certain period of time after reaching the pressurization pressure of each stage.

[0037] As a preferred embodiment of the technical solution of the present invention, the staged boosting system includes the following: First-stage pressurization: pressurize to 50~80MPa (excluding 80MPa), such as 50 MPa, 60 MPa, 70 MPa, 75MPa or 79 MPa, and hold the pressure for 1~2 minutes, such as 1 minute, 1.5 minutes or 2 minutes. Secondary pressurization: Pressurize to 80~120MPa (excluding 120MPa), such as 80MPa, 90MPa, 100MPa, 110MPa or 118MPa, and hold the pressure for 1~2 minutes, such as 1 minute, 1.5 minutes or 2 minutes. Three-stage pressurization: pressurize to 120~150MPa (excluding 150MPa), such as 120MPa, 130MPa, 140MPa, 145MPa or 148MPa, and hold the pressure for 1~2 minutes, such as 1 minute, 1.5 minutes or 2 minutes. Level 4 pressurization: Pressurize to 150~180MPa (excluding 180MPa), such as 150MPa, 160MPa, 170MPa, 175MPa or 178MPa, and hold the pressure for 1~2 minutes, such as 1 minute, 1.5 minutes or 2 minutes. Five-stage pressure boosting: boost the pressure to 180~210MPa, for example, 180MPa, 190MPa, 200MPa, 205MPa or 210MPa, and hold the pressure for 5~10 minutes, for example, 5 minutes, 6 minutes, 8 minutes or 10 minutes; The pressurization rate during each stage of pressurization is 4~6 MPa / min, for example, 4 MPa / min, 4.5 MPa / min, 5 MPa / min, 5.5 MPa / min or 6 MPa / min.

[0038] As an optional implementation of the technical solution of the present invention, in step (b), the staged depressurization system includes performing first-level depressurization and second-level depressurization in sequence, and maintaining pressure for a certain period of time after reaching the depressurization pressure of each stage.

[0039] In a preferred embodiment of the technical solution of the present invention, step (b) includes the following staged depressurization process: First-stage pressure relief: Depressurize to 180~120MPa (excluding 120MPa), such as 180MPa, 170MPa, 160MPa, 150MPa, 140MPa or 125MPa, and hold the pressure for 1~2 minutes, such as 1 minute, 1.5 minutes or 2 minutes. Secondary depressurization: Depressurize to 120~80MPa, such as 120MPa, 110MPa, 100MPa, 90MPa or 80MPa, and hold the pressure for 1~2 minutes, such as 1 minute, 1.5 minutes or 2 minutes. After the second stage of depressurization, the pressure can be directly reduced to atmospheric pressure.

[0040] The depressurization rate during each stage of depressurization is 4~6 MPa / min, for example, 4 MPa / min, 4.5 MPa / min, 5 MPa / min, 5.5 MPa / min or 6 MPa / min.

[0041] As an optional embodiment of the technical solution of the present invention, in step (d), the low-temperature pre-sintering treatment includes staged heat preservation in the range of 800~1400℃ (but not including 1400℃), with a total heat preservation time of 6~12h (i.e. the sum of the heat preservation time of the first stage sintering and the second stage sintering), the high-temperature densification treatment includes staged heat preservation in the range of 1400~2150℃, with a total heat preservation time of 10~18h (i.e. the sum of the heat preservation time of the third stage sintering and the fourth stage sintering), and the subsequent annealing is to shut down the furnace and cool down to 1300~1600℃ after the high-temperature densification treatment and keep it at that temperature for 2~5h, and then air-cool it to below 100℃ before unloading.

[0042] Unlike conventional processes, the sintering process of this invention employs a multi-stage approach. As an optional embodiment of the technical solution of this invention, step (c) involves a phased sintering process, from heating at room temperature to the maximum sintering temperature and then cooling down. Specifically, this includes a first-stage sintering, a second-stage sintering, a third-stage sintering, a fourth-stage sintering, and a final annealing stage. The first and second stages of sintering are low-temperature pre-sintering treatments, while the third and fourth stages are high-temperature densification treatments.

[0043] As a preferred embodiment of the technical solution of the present invention, the sintering process includes: First stage sintering: The temperature is raised from room temperature to 800-1000℃ (excluding 1000℃, such as 800℃, 850℃, 900℃, 950℃, or 980℃) for 3-6 hours (e.g., 3 hours, 4 hours, 5 hours, or 6 hours), and held at that temperature for 2-4 hours (e.g., 2 hours, 3 hours, or 4 hours). Second stage sintering: After 3-6 hours (e.g., 3h, 4h, 5h, or 6h), the temperature is increased from the holding temperature of the first stage sintering (i.e., 800-1000℃ but excluding 1000℃) to 1000-1200℃ (excluding 1200℃, e.g., 1000℃, 1050℃, 1100℃, 1150℃, or 1180℃), and held for 2-4 hours (e.g., 2h, 3h, or 4h); then after another 2-4 hours (e.g., 2h, 3h, or 4h), the temperature is increased from the previous holding temperature (i.e., 1000-1200℃ but excluding 1200℃) to 1200-1400℃ (excluding 1400℃, e.g., 1200℃, 1250℃, 1300℃, 1350℃, or 1380℃), and held for 2-4 hours (e.g., 2h, 3h, or 4h). The third stage of sintering involves raising the temperature from the highest holding temperature of the second stage sintering (i.e., 1200~1400℃ excluding 1400℃) to 1400~1600℃ (excluding 1600℃, e.g., 1400℃, 1450℃, 1500℃, 1550℃, or 1580℃) and holding it for 2~4 hours (e.g., 2 hours, 3 hours, or 4 hours). Then, after another 4~6 hours (e.g., 4 hours, 5 hours, or 6 hours), the temperature is raised from the previous holding temperature (i.e., 1400~1600℃ excluding 1600℃) to 1600~1800℃ (e.g., 1600℃, 1650℃, 1700℃, 1750℃, or 1800℃) and held for 2~4 hours (e.g., 2 hours, 3 hours, or 4 hours). Fourth stage sintering: After 4 to 6 hours (e.g., 4 hours, 5 hours, or 6 hours), the temperature is increased from the highest holding temperature of the third stage sintering (i.e., 1600 to 1800℃) to 2000 to 2150℃ (e.g., 2000℃, 2050℃, 2100℃, or 2150℃), and held for 6 to 10 hours (e.g., 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours). Post-stage annealing: The furnace is shut down and cooled from the holding temperature of the fourth stage sintering (i.e., 2000~2150℃) to 1300~1600℃ (e.g., 1300℃, 1400℃, 1500℃ or 1600℃, etc.), held for 2~5 hours (e.g., 2 hours, 3 hours, 4 hours or 5 hours, etc.), and then cooled to below 100℃ before being removed from the furnace.

[0044] Specifically, the sintering process of this invention is carried out in multiple stages. The purpose of the first stage, low-temperature sintering, is to completely decompose TiH2 and ZrH2, while the C, N, and O impurities distributed in the grain boundaries of the blank volatilize, which is beneficial to the volatilization of adsorbed gases and moisture. The purpose of the second stage sintering is to homogenize the blank. Since heat diffusion takes time, holding the temperature can make the heat diffusion uniform. At the same time, the low-melting-point impurities K, Ca, and Na volatilize. During the process, the particles begin to make point contact and initially form sintering necks. The third stage sintering can generate strengthening phases TiC and ZrC, while a small amount of high-melting-point impurities Mg and Al volatilize. During the process, the particles further form sintering necks, forming a sintering skeleton, and the voids exhibit spheroidization. The fourth stage can complete the migration and growth of particles, and the blank gradually achieves densification. At the same time, the internal pores are closed until there are no through holes. In the final stage, annealing is carried out. Since this stage is the generation stage of dispersed phases TiC and ZrC, holding the temperature in this range for a certain period of time can promote the formation of TiC and ZrC, and improve the hardness and wear resistance of the molybdenum mandrel.

[0045] As an optional embodiment of the technical solution of this invention, this invention uses an induction sintering furnace for sintering molybdenum alloy products. The induction coil is used for heating, and the gas intake method involves simultaneous intake through upper and lower pipes, with pressure balance maintained only through exhaust from the furnace sidewalls. The hydrogen system for high-temperature sintering is as follows: from room temperature to the holding temperature of the first stage sintering, the hydrogen intake flow rate is controlled at 0~0.3m³. 3 / h, lower intake flow rate controlled at 4~6m 3 / h; From the holding temperature of the first stage sintering to the highest holding temperature of the third stage sintering, the hydrogen inlet flow rate is controlled at 0~0.3m. 3 / h, lower intake flow rate controlled at 3~5m 3 / h; From the highest holding temperature of the third stage sintering until the end of sintering, the hydrogen inlet flow rate is controlled at 0~0.3m. 3 / h, lower intake flow rate controlled at 2~4m 3 / h.

[0046] During the sintering of the molybdenum alloy mandrel, the main gas and moisture emissions occur before the holding temperature of the first stage sintering (800~1000℃, excluding 1000℃). This process is also the main stage for the complete decomposition of TiH2 and ZrH2. Therefore, the hydrogen flow rate should be appropriately large during this stage to ensure that the gas flow throughout the sintering furnace carries away gaseous impurities. From the holding temperature of the first stage sintering to the highest holding temperature of the third stage sintering (1600~1800℃, excluding 1800℃), low-melting-point impurities K, Ca, and Na in the molybdenum powder, as well as a small amount of high-melting-point impurities Mg and Al, begin to volatilize. During this stage, a certain amount of hydrogen flow rate is needed to carry away volatile metallic impurities and improve the purity of the sintered body. After the highest holding temperature of the third stage sintering until the end of sintering, the main process is the further sintering and densification of the billet. A small amount of hydrogen is needed to dilute the residual oxygen or nitrogen in the furnace and maintain a stable reducing environment.

[0047] The method for preparing molybdenum alloy mandrels provided by this invention can be used to prepare molybdenum alloy mandrels of various specifications, such as molybdenum alloy mandrels with specifications from D265*400 to D305*500, to meet different application requirements.

[0048] According to a second aspect of the present invention, a molybdenum alloy mandrel is also provided, which is prepared by the method for preparing the molybdenum alloy mandrel described in the first aspect of the present invention; The molybdenum alloy mandrel has a grain size of 7.0~7.8 and a hardness of 178~185 HV. 30 .

[0049] According to a third aspect of the invention, the application of the above-described molybdenum alloy mandrel in the field of hot rolling piercing of seamless steel pipes is also provided.

[0050] The molybdenum alloy mandrel is used for the piercing and rolling of 316L seamless steel pipes on site. Before each steel pipe is pierced and rolled, the surface condition of the molybdenum alloy mandrel must be carefully observed. If micro-cracks are found on the surface, the mandrel is further machined and modified to pierce and roll smaller seamless steel pipes. The total service life after completion is 170 to 210 pieces.

[0051] The present invention will now be described in further detail with reference to specific embodiments and comparative examples.

[0052] Example 1 This embodiment provides a molybdenum alloy mandrel made from the following percentages of raw materials: TiH2: 1.1%, ZrH2: 0.2%, CeO2: 1.3%, C: 0.4%, with the balance being molybdenum powder.

[0053] The molybdenum powder has a Fisher particle size of approximately 2.8 μm, TiH2 has a Fisher particle size of approximately 15 μm, ZrH2 has a Fisher particle size of approximately 10 μm, CeO2 has a Fisher particle size of approximately 30 μm, and the carbon powder has a Fisher particle size of approximately 25 nm. The carbon powder has a purity of over 98%, and the purity of the other raw materials is over 99.95%.

[0054] This embodiment describes a method for preparing a high-performance molybdenum alloy mandrel in the hot rolling piercing process of seamless steel pipes, comprising the following steps: (1) Mixing treatment: According to the raw material composition of the molybdenum alloy mandrel, titanium hydride, zirconium hydride, cerium oxide, carbon powder and molybdenum powder are pre-alloyed in a plow mixer for 4 hours to obtain molybdenum alloy powder.

[0055] (2) Molding process: Weigh the uniformly mixed molybdenum alloy powder with a balance and put it into a conical black rubber sleeve mold of the corresponding specification. Use an iron hammer to make it have a uniform tap density. Then press it with a cold isostatic press to obtain a molybdenum alloy conical blank. The cold isostatic pressing pressurization process is as follows: First-stage pressure boost: Boost pressure to 50MPa and hold for 60 seconds; Secondary boost: Boost pressure to 100MPa and hold for 60 seconds; Three-stage pressure boost: Boost pressure to 140MPa and hold for 60 seconds; Fourth-stage pressure boost: Boost pressure to 170MPa and hold for 60 seconds; Five-stage pressure boost: Boost pressure to 210MPa and hold for 300 seconds; The pressurization rate during each pressurization stage is 5 MPa / min.

[0056] The cold isostatic pressure relief system is as follows: First-stage pressure relief: Depressurize to 150MPa and hold for 60 seconds; Secondary depressurization: Depressurize to 80MPa and hold for 60s; The depressurization rate during each depressurization process is 5 MPa / min.

[0057] The density of the molybdenum alloy conical compact is 6.8 g / cm³. 3 .

[0058] (3) Shaping process: The molybdenum alloy conical blank is machined and its end face is flattened on a vertical lathe according to the given drawings; (4) Sintering treatment: The molybdenum alloy billet is subjected to low-temperature pre-sintering treatment (first-stage sintering and second-stage sintering) and high-temperature densification treatment (third-stage sintering and fourth-stage sintering) in an intermediate frequency furnace, followed by subsequent annealing (post-stage heat treatment). The specific operation steps are as follows: First stage sintering: The temperature is raised from room temperature to 900℃ over 4 hours and held for 2 hours; The second stage of sintering: After 4 hours of heating to 1100℃ and holding for 2 hours, the temperature is raised to 1300℃ for another 3 hours and held for 2 hours. The third stage of sintering: After 4 hours of heating to 1500℃ and holding for 2 hours, the temperature is raised to 1700℃ and held for 2 hours. Fourth stage sintering: heating to 2100℃ for 6 hours and holding at that temperature for 6 hours; Post-stage annealing: After cooling to 1600℃ in the furnace, hold for 2 hours, then stop the furnace and cool to 80℃ before unloading.

[0059] The corresponding hydrogen flow rate is: 0.3m³ / h for top inlet before heat preservation at 900℃. 3 / h, lower air intake 4m 3 / h; 0.3m of air intake at the top during the 900℃ heat preservation period and before the 1700℃ heat preservation period. 3 / h, lower air intake 3m 3 / h; After heat preservation at 1700℃, the air intake is 0.3m at the top. 3 / h, lower air intake 2m 3 / h.

[0060] After sintering the billet, a molybdenum alloy mandrel with dimensions D265*400 is obtained. A picture of the actual product is shown below. Figure 1 As shown.

[0061] Example 2 This embodiment provides a molybdenum alloy mandrel, made from the following percentages of raw materials: TiH2: 1.3%, ZrH2: 0.3%, CeO2: 1.3%, C: 0.6%, with the balance being molybdenum powder.

[0062] The molybdenum powder has a Fisher particle size of approximately 2.8 μm, TiH2 has a Fisher particle size of approximately 15 μm, ZrH2 has a Fisher particle size of approximately 10 μm, CeO2 has a Fisher particle size of approximately 30 μm, and the carbon powder has a Fisher particle size of approximately 25 nm. The purity of the carbon powder is above 98%, and the purity of the other raw materials is above 99.95%.

[0063] The preparation method of the high-performance molybdenum alloy mandrel used in the hot rolling piercing process of seamless steel pipe in this embodiment is the same as that in Embodiment 1, and will not be repeated here.

[0064] Example 3 This embodiment provides a molybdenum alloy mandrel made from the following percentages of raw materials: TiH2: 1.1%, ZrH2: 0.2%, CeO2: 1.3%, C: 0.4%, with the balance being molybdenum powder.

[0065] The molybdenum powder has a Fisher particle size of approximately 2.9 μm, TiH2 has a Fisher particle size of approximately 15 μm, ZrH2 has a Fisher particle size of approximately 15 μm, CeO2 has a Fisher particle size of approximately 20 μm, and the carbon powder has a Fisher particle size of approximately 35 nm. The purity of the carbon powder is above 98%, and the purity of the other raw materials is above 99.95%.

[0066] The preparation method of the high-performance molybdenum alloy mandrel used in the hot rolling piercing process of seamless steel pipe in this embodiment is the same as that in Embodiment 1, and will not be repeated here.

[0067] Example 4 This embodiment provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Embodiment 1, and will not be repeated here.

[0068] The preparation method of the high-performance molybdenum alloy mandrel used in the hot rolling piercing process of seamless steel pipe in this embodiment is the same as that in Example 1, except that the cold isostatic pressing pressure raising system and pressure releasing system used in step (2) forming process are different from those in Example 1.

[0069] In this embodiment, the cold isostatic pressing pressurization process is as follows: First-stage pressure boost: Boost pressure to 50MPa and hold for 60 seconds; Secondary boost: Boost pressure to 120MPa and hold for 60 seconds; Three-stage pressure boost: Boost pressure to 140MPa and hold for 60 seconds; Fourth-stage pressure boost: Boost pressure to 160MPa and hold for 60 seconds; Five-stage pressure boost: Boost pressure to 200MPa and hold for 500 seconds; The pressurization rate during each pressurization stage is 6 MPa / min.

[0070] The cold isostatic pressure relief system is as follows: First-stage pressure relief: Depressurize to 150MPa and hold for 90 seconds; Secondary depressurization: Depressurize to 100MPa and hold for 90 seconds; The depressurization rate during each depressurization process is 6 MPa / min.

[0071] Example 5 This embodiment provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Embodiment 1, and will not be repeated here.

[0072] This embodiment describes a method for preparing a high-performance molybdenum alloy mandrel in the hot rolling piercing process of seamless steel pipes. Except for step (4), the sintering process differs from that in Embodiment 1; all other steps and process parameters are the same as in Embodiment 1. The specific sintering process in this embodiment is as follows: First stage sintering: The temperature is raised from room temperature to 800℃ over 4 hours and held for 3 hours; The second stage of sintering: After 4 hours of heating to 1000℃ and holding for 4 hours, the temperature is then raised to 1200℃ for 3 hours and held for 2 hours. The third stage of sintering: After 4 hours of heating to 1400℃ and holding for 4 hours, the temperature is raised to 1600℃ and held for 2 hours. Fourth stage sintering: heating to 2000℃ for 5 hours and holding at that temperature for 6 hours; Post-stage annealing: After cooling to 1600℃ in the furnace, hold for 2 hours, then stop the furnace and cool to 80℃ before unloading.

[0073] The corresponding hydrogen flow rate is: 0.3m³ / h intake before heat preservation at 800℃. 3 / h, lower air intake 4m 3 / h; 0.3m of air intake at the top during the 800℃ heat preservation period up to the 1600℃ heat preservation period. 3 / h, lower air intake 3m 3 / h; After heat preservation at 1600℃, the air intake is 0.3m at the top. 3 / h, lower air intake 2m 3 / h.

[0074] Example 6 This embodiment provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Embodiment 1, and will not be repeated here.

[0075] The preparation method of the high-performance molybdenum alloy mandrel in the hot rolling piercing process of seamless steel pipe in this embodiment is the same as that in Example 1, except that the hydrogen flow rate used in the sintering treatment in step (4) is different.

[0076] The hydrogen system used during sintering in this embodiment is as follows: the corresponding hydrogen flow rate is 0.2 m³ / s at the top before holding at 900℃. 3 / h, lower air intake 4m 3 / h; 0.2m of air intake at the top before maintaining a temperature from 900℃ to 1700℃. 3 / h, lower air intake 3m 3 / h; after heat preservation at 1700℃, the air intake is 0.2m. 3 / h, lower air intake 2m 3 / h.

[0077] Comparative Example 1 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0078] The preparation method of this comparative molybdenum alloy mandrel is the same as that of Example 1, except that the staged pressure increase in step (2) of cold isostatic pressing is changed to direct pressure increase, that is, direct pressure increase to 210MPa and pressure holding for 300s, and the staged pressure release in Example 1 is changed to direct pressure release, without additional pressure holding operation in the middle. The pressure increase rate during the pressure increase process and the pressure release rate during the pressure release process are both 5MPa / min.

[0079] Comparative Example 2 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0080] The preparation method of this comparative molybdenum alloy mandrel is the same as that of Example 1, except that the staged pressure increase in the cold isostatic pressing stage of step (2) is adjusted.

[0081] Specifically, the comparative cold isostatic pressing pressurization process is as follows: First-stage pressure boost: Pressure boosted to 140MPa, pressure held for 60s; Secondary pressurization: pressurize to 210MPa and hold for 300 seconds; The pressurization rate during each pressurization stage is 5 MPa / min.

[0082] Comparative Example 3 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0083] The preparation method of the comparative molybdenum alloy mandrel is the same as that of Example 1, except that the staged pressure increase in step (2) of cold isostatic pressing is adjusted to direct pressure increase, that is, direct pressure increase to 210MPa and pressure holding for 300s, without additional pressure holding operation in the middle, and the pressure increase rate during the pressure increase process is 5MPa / min.

[0084] Comparative Example 4 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0085] The preparation method of the molybdenum alloy mandrel in this comparative example is the same as that in Example 1, except that the staged depressurization in step (2) of cold isostatic pressing is changed to direct depressurization, and no additional pressure holding operation is performed in the middle, and the depressurization rate during the depressurization process is 5MPa / min.

[0086] Comparative Example 5 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0087] The preparation method of the molybdenum alloy mandrel in this comparative example is the same as that in Example 1, except that the staged sintering of the high-temperature sintering process in step (4) of Example 1 is adjusted to directly raise the temperature from room temperature to 2100℃ after 25 hours and hold it for 6 hours.

[0088] Comparative Example 6 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0089] The preparation method of this comparative molybdenum alloy mandrel is the same as that of Example 1, except that the process parameters of the staged sintering in step (4) of Example 1 are adjusted.

[0090] Specifically, the sintering process in this comparative example is completed in a hydrogen-filled medium-frequency furnace, as follows: First stage sintering: The temperature is raised from room temperature to 1400℃ over 8 hours and held for 4 hours. The second stage of sintering: After 8 hours of heating to 1800℃ and holding for 4 hours, the temperature is further increased to 2100℃ and held for 6 hours. The third stage of sintering: after cooling to 1600℃ in the furnace, hold for 2 hours, then stop the furnace and cool to 80℃ before unloading.

[0091] Comparative Example 7 This comparative example provides a molybdenum alloy mandrel, whose raw material composition is the same as that of Example 1.

[0092] The preparation method of the molybdenum alloy mandrel in this comparative example is the same as that in Example 1, except that the post-stage annealing process in step (4) of sintering is deleted, that is, after holding at 2100°C for 6 hours in the fourth stage of sintering, the furnace is directly stopped and cooled to 80°C before being taken out of the furnace.

[0093] To illustrate the technical effects of the various embodiments and comparative examples, the following experimental examples are provided.

[0094] Experimental Example 1 Metallographic examination was performed on the molybdenum alloy mandrels prepared in each embodiment and comparative example. Specific metallographic images are shown below. Figures 2-13 As shown. From Figures 2-7As can be seen from the data, the molybdenum alloy mandrels prepared in each embodiment have relatively uniform grain size, and there is no obvious agglomeration of doped phases within the field of view. Furthermore, from... Figures 8-13 As can be seen from the metallographic images of the molybdenum alloy mandrels prepared in Comparative Examples 2 to 7, the metallographic images all show large black agglomerated phases. The dopant phases are enriched but not uniformly distributed in the molybdenum matrix. This phenomenon will seriously affect the performance and tube life of the molybdenum alloy mandrel.

[0095] Additionally, it should be noted that the green text in the upper left corner of each figure is only an explanation of the objective lens magnification and field of view size, which may not be very clear, but does not affect the expression of metallographic technical information.

[0096] Experiment Example 2 The density, grain size, hardness, and service life of the molybdenum alloy mandrels prepared in each embodiment and comparative example were tested. Density was tested according to GB / T 1423-1996; grain size according to GB / T 6394-2017; Vickers hardness according to GB / T 4340.1-2024; and service life was defined as the effective number of 316L steel plates used in the corresponding embodiment or comparative example. The service life of the molybdenum alloy mandrel can, to some extent, represent its toughness. Specific test results are shown in Table 1.

[0097] Table 1. Data Detection of Molybdenum Alloy Mandrels in Each Embodiment and Comparative Example Note: Comparative Example 1 showed signs of blank tipping and severe surface indentation, and no subsequent sintering experiment was conducted.

[0098] As can be seen from the data in Table 1, the density of the molybdenum alloy mandrels prepared in each embodiment of the present invention is ≥9.5 g / cm³. 3 The molybdenum alloy mandrel has a grain size of 7.0~7.8 and a hardness of 178~185 HV. 30 The service life is ≥170 pieces, indicating that the manufacturing parameters of the molybdenum mandrel designed in this invention are stable. By rationally designing the composition ratio, pressure boosting and depressurization system and sintering process of the molybdenum alloy mandrel, the performance of the molybdenum alloy mandrel at high temperature can be significantly improved.

[0099] Comparative Example 1, employing a direct pressurization and depressurization process, experienced billet decapitation and severe surface indentation during demolding, and subsequent sintering experiments were not conducted. Comparative Example 2, using a two-stage pressurization process, yielded a molybdenum alloy mandrel with a density of 9.66 g / cm³. 3 The grain size is 7.48, and the hardness is 179 HV. 30However, the service life was relatively short, only 136 pieces; Comparative Example 3 suffered from insufficient deformation due to excessively rapid pressurization, while Comparative Example 4 employed a direct depressurization process, causing the high-pressure air in the compact's voids to suddenly expand. Therefore, both products had relatively low densities (9.35 g / cm³, respectively). 3 and 9.33 g / cm 3 Furthermore, the service life of both samples was relatively low, at 119 and 127 pieces respectively. Comparative Example 5 omitted the low-temperature holding stage during sintering, directly raising the temperature from room temperature to 2100℃ over 25 hours. During sintering, the billet did not undergo sufficient impurity removal and densification, resulting in a lower product density of only 9.27 g / cm³. 3 Comparative Example 6 used a two-stage heating process for the billet, resulting in a molybdenum alloy mandrel with finer grains and higher hardness. However, due to its poor toughness, the corresponding service life was insufficient, with only 125 pieces produced. Comparative Example 7 omitted the final annealing treatment, resulting in a molybdenum alloy mandrel with excessively low hardness, failing to meet the requirements of subsequent steel pipe insertion tests. Therefore, this invention, through a staged pressure increase and depressurization process combined with a multi-stage heating and holding system, can significantly improve the performance of the molybdenum alloy mandrel, thereby extending its service life.

[0100] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A method for preparing a molybdenum alloy mandrel, characterized in that, Includes the following steps: (a) Mixing process: Titanium hydride, zirconium hydride, cerium oxide, molybdenum powder and carbon powder are pre-alloyed according to the chemical composition of the molybdenum alloy mandrel to obtain molybdenum alloy powder. (b) Forming process: The molybdenum alloy powder mixed in step (a) is loaded into a conical rubber sleeve and cold isostatically pressed to obtain a blank; Among them, a staged pressure increase system is adopted in the pressure increase stage of cold isostatic pressing, and a staged pressure release system is adopted in the pressure release stage of cold isostatic pressing. (c) Shaping process: The blank obtained in step (b) is turned and shaped to flatten the end face; (d) Sintering treatment: The blank after the shaping treatment in step (c) is subjected to low-temperature pre-sintering treatment, high-temperature densification treatment and post-stage annealing to obtain a molybdenum alloy mandrel.

2. The method for preparing the molybdenum alloy mandrel according to claim 1, characterized in that, In step (a), the molybdenum alloy mandrel is made from the following raw materials in the following mass percentages: 0.65-1.5% titanium hydride, 0.15-0.3% zirconium hydride, 1.3-1.8% cerium oxide, 0.4-0.6% carbon powder, and the balance being molybdenum powder; And / or, the molybdenum powder has a Fisher particle size of 2.8~3.2μm, the titanium hydride has a Fisher particle size of 10~30μm, the zirconium hydride has a Fisher particle size of 5~20μm, the cerium oxide has a Fisher particle size of 10~50μm, and the carbon powder has a Fisher particle size of 25~40nm; wherein, the purity of the carbon powder is above 98%, and the purity of the other raw materials is above 99.95%.

3. The method for preparing the molybdenum alloy mandrel according to claim 1, characterized in that, In step (a), titanium hydride, zirconium hydride, cerium oxide, molybdenum powder and carbon powder are pre-alloyed and mixed in a plow mixer; And / or, the pre-alloying mixture treatment time in step (a) is 2~6h.

4. The method for preparing the molybdenum alloy mandrel according to claim 1, characterized in that, In step (b), the molding pressure is up to 180~210MPa, and the total holding time during the staged pressure increase process is 9~18min; And / or, the staged pressurization system includes sequentially performing a first-stage pressurization, a second-stage pressurization, a third-stage pressurization, a fourth-stage pressurization, and a fifth-stage pressurization, and holding the pressure for a certain period of time after reaching the pressurization pressure of each stage; Preferably, the staged boosting system includes the following: First-stage pressurization: pressurize to 50~80MPa but not including 80MPa, and hold the pressure for 1~2 minutes; Secondary pressurization: pressurize to 80~120MPa but not including 120MPa, and hold the pressure for 1~2 minutes; Three-stage pressurization: pressurize to 120~150MPa but not including 150MPa, and hold the pressure for 1~2 minutes; Fourth-stage pressurization: pressurize to 150~180MPa but not including 180MPa, and hold the pressure for 1~2 minutes; Five-stage pressurization: pressurize to 180~210MPa and hold for 5~10 minutes; The pressurization rate during each pressurization stage is 4~6 MPa / min.

5. The method for preparing the molybdenum alloy mandrel according to claim 1, characterized in that, In step (b), the phased depressurization system includes performing first-level depressurization and second-level depressurization in sequence, and maintaining pressure for a certain period of time after reaching the depressurization pressure of each level; Preferably, the phased depressurization system includes the following: First-stage pressure relief: Depressurize from the highest pressure to 180~120MPa, but not including 120MPa, and maintain the pressure for 1~2 minutes; Secondary depressurization: Depressurize to 120~80MPa and maintain the pressure for 1~2 minutes; The depressurization rate during each stage of depressurization is 4~6 MPa / min.

6. The method for preparing the molybdenum alloy mandrel according to claim 1, characterized in that, In step (c), the low-temperature pre-firing treatment includes staged heat preservation within the range of 800~1400℃ but not 1400℃, with a total heat preservation time of 6~12h; And / or, the high-temperature densification treatment includes staged heat preservation in the range of 1400~2150℃, with a total heat preservation time of 10~18h; And / or, the subsequent annealing stage involves high-temperature densification treatment followed by furnace shutdown and cooling to 1300~1600℃ and holding at that temperature for 2~5 hours, then air cooling to below 100℃ before unloading from the furnace.

7. The method for preparing the molybdenum alloy mandrel according to claim 1, characterized in that, Step (c) Throughout the sintering process, the process of heating from room temperature to the highest sintering temperature and then cooling down is carried out in stages, specifically including the first stage sintering, the second stage sintering, the third stage sintering, the fourth stage sintering and the final stage annealing; wherein, the first stage sintering and the second stage sintering are low-temperature pre-sintering treatments, and the third stage sintering and the fourth stage sintering are high-temperature densification treatments. Preferably, the sintering process includes: First stage sintering: The temperature is raised from room temperature to 800-1000℃ (excluding 1000℃) over 3-6 hours, and then held for 2-4 hours. Second stage sintering: After 3-6 hours, the temperature is raised from the holding temperature of the first stage sintering to 1000-1200℃ (excluding 1200℃), and held for 2-4 hours; after another 2-4 hours, the temperature is raised from the previous holding temperature to 1200-1400℃ (excluding 1400℃), and held for 2-4 hours. The third stage of sintering: After 4 to 6 hours, the temperature is raised from the highest holding temperature of the second stage sintering to 1400 to 1600℃ (excluding 1600℃), and held for 2 to 4 hours; then after another 4 to 6 hours, the temperature is raised from the previous holding temperature to 1600 to 1800℃, and held for 2 to 4 hours. Fourth stage sintering: After 4-6 hours, the temperature is raised from the highest holding temperature of the third stage sintering to 2000-2150℃, and held for 6-10 hours; Post-stage annealing: The furnace is shut down and cooled from the holding temperature of the fourth stage sintering to 1300~1600℃, held for 2~5 hours, and then cooled to below 100℃ before being removed from the furnace.

8. The method for preparing the molybdenum alloy mandrel according to claim 7, characterized in that, The hydrogen system used in step (c) of the sintering process is as follows: the hydrogen inlet flow rate is controlled at 0~0.3m³ before the holding temperature for the first stage of sintering rises from room temperature. 3 / h, lower intake flow rate controlled at 4~6m 3 / h; From the holding temperature of the first stage sintering to the highest holding temperature of the third stage sintering, the hydrogen inlet flow rate is controlled to be 0~0.3m. 3 / h, lower intake flow rate controlled at 3~5m 3 / h; From the highest holding temperature of the third stage sintering until the end of sintering, the hydrogen inlet flow rate is controlled at 0~0.3m. 3 / h, lower intake flow rate controlled at 2~4m 3 / h.

9. A molybdenum alloy mandrel, characterized in that, The molybdenum alloy mandrel is prepared by the method described in any one of claims 1-8; The molybdenum alloy mandrel has a grain size of 7.0~7.8 and a hardness of 178~185 HV. 30 .

10. The application of the molybdenum alloy mandrel prepared by the method of any one of claims 1-8 or the molybdenum alloy mandrel of claim 9 in the field of hot rolling piercing of seamless steel pipes.

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