Method for improving creep resistance of IVB group alloy

Abstract

The invention discloses a method for improving creep resistance of an IVB group alloy. The method comprises the following steps: firstly, carrying out annealing treatment on an IVB group alloy workpiece; the IVB group alloy workpiece is heated to a given temperature for heat preservation, then a given load is applied and kept, or the load is gradually applied to the IVB group alloy workpiece while the IVB group alloy workpiece is heated until the given temperature and the given load are reached, the given temperature range is 25-400 DEG C, and the load keeping time is 10 min to 1 h; and the load is unloaded after the IVB group alloy workpiece reaches the environment temperature. According to the method, the steady-state creep rate of the IVB group alloy workpiece can be effectively reduced, and the service life of the IVB group alloy workpiece under the high-temperature load condition is prolonged.

Description

Technical Field

[0001] The present invention belongs to the field of metal processing, and particularly relates to a method for improving the creep resistance of group IVB alloys. Background Art

[0002] Titanium, zirconium, and hafnium in group IVB of the periodic table have similar physical and chemical properties. These three metals are in a close-packed hexagonal crystal structure in the temperature range from room temperature to 800 °C, and have excellent mechanical properties and corrosion resistance. Therefore, they are widely used in fields such as aerospace engines, nuclear reactor fuels and control rods, and reaction vessels in the chemical industry. These materials generally withstand complex and harsh service environments such as high temperature, corrosion, irradiation, and high stress. Improving creep resistance is a common requirement for these materials in the above-mentioned application scenarios. Patent CN103608475A discloses a solution for improving the creep performance of zirconium alloys through heat treatment. However, in this technical solution, the zirconium alloy needs to be heated to a fully recrystallized or 80%-95% recrystallized state, and the process is complex and prone to thermal deformation of zirconium alloy parts. Therefore, providing a more simple method for improving the creep resistance of group IVB alloys has positive significance for improving the safety and economy of reactors. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for improving the creep resistance of group IVB alloys, and to improve the reliability of group IVB alloy parts under high temperature and high pressure conditions.

[0004] According to an embodiment of one aspect of the present invention, a method for improving the creep resistance of group IVB alloys is provided. The method includes the following steps:

[0005] Step a): Anneal the group IVB alloy part.

[0006] Step b): Apply a given load to the group IVB alloy part at a given temperature and hold the load; wherein, the given load is not lower than the creep load of the group IVB alloy part and does not exceed the yield strength of the group IVB alloy part at the given temperature, and the holding time is not less than 10 min;

[0007] Step c): Unload the load after reaching the holding time.

[0008] Through this method, the creep performance of group IVB alloy parts can be effectively enhanced, and the steady-state creep rate of group IVB alloy parts can be reduced.

[0009] Further, in some embodiments, the group IVB alloy includes titanium alloy, zirconium alloy, and hafnium alloy.

[0010] Further, in some embodiments, the composition of the Group IVB alloy workpiece by weight percentage includes 0 < Sn ≤ 1.7%, 0 < Nb ≤ 3%, no less than 95% of Ti or Zr or Hf, and inevitable impurities.

[0011] Further, in some embodiments, the Group IVB alloy workpiece is a zirconium alloy workpiece. In step b), when loading, it further includes the step of heating the zirconium alloy workpiece to a given temperature, and the given temperature does not exceed 400°C.

[0012] Further, in some embodiments, in step b), while heating the zirconium alloy workpiece, a load is gradually applied to the zirconium alloy workpiece, so that when the zirconium alloy workpiece reaches the given temperature, the applied load reaches the given load.

[0013] Further, in some embodiments, in step b), the ways of applying load include tension, compression, torsion or bending.

[0014] Further, in some embodiments, the Group IVB alloy workpiece is an alloy tube.

[0015] Further, in some embodiments, in step b), the way of loading the Group IVB alloy workpiece is to fill high-pressure gas into the alloy tube.

[0016] Further, in some embodiments, before step b), the alloy tube is straightened.

[0017] Further, in some embodiments, the structure of the Group IVB alloy workpiece is a recrystallized structure or a stress-relieved structure, and the matrix is a close-packed hexagonal α phase. Description of the Drawings

[0018] Figure 1 is the metallographic structure photograph of the first alloy sample;

[0019] Figure 2 is the metallographic structure photograph of the second alloy sample;

[0020] Figure 3 is the creep curve of the comparative example and the example of the first alloy sample;

[0021] Figure 4 is the creep curve of the comparative example and the example of the second alloy sample.

[0022] The purpose of the above drawings is to make a detailed description of the present invention, so that those skilled in the art can understand the technical concept of the present invention, rather than aiming to limit the present invention. Detailed Embodiments

[0023] The following further elaborates on the present invention through specific embodiments in conjunction with the accompanying drawings.

[0024] As used herein, the mention of "embodiment" means that the specific features, structures, or characteristics described in connection with the embodiment may be included in at least one embodiment herein. The phrase appearing in various positions in the specification does not necessarily refer to the same embodiment, nor is it limited to mutually exclusive independent or alternative embodiments. Those skilled in the art should be able to understand that the embodiments herein can be combined with other embodiments without structural conflicts.

[0025] In the description herein, terms such as "first" and "second" are only used to distinguish different objects, and cannot be construed as indicating relative importance or limiting the quantity, specific order, or primary-secondary relationship of the described technical features. In the description herein, the meaning of "a plurality" is at least two.

[0026] Group-IVB alloys are widely used as structural materials in the construction of nuclear reactors due to their excellent high-temperature mechanical properties, high-temperature corrosion resistance, and small thermal neutron capture cross-section, etc. For example, fuel rod cladding is one of the important applications of zirconium alloys in the nuclear power field. As fuel rod cladding, zirconium alloys enclose and seal radioactive fuel and are immersed in a coolant for heat exchange. Due to long-term service under high-temperature and high-pressure conditions, the creep performance of zirconium alloys has an important impact on the safety and reliability of reactors. Creep of zirconium alloy fuel rod cladding is a key mechanism for nuclear fuel damage and failure, affecting the service life of fuel rods. If the creep rate of zirconium alloy is too high, it will lead to premature contact between the fuel rod cladding and the fuel pellets, bending and deformation failure of the fuel skeleton, etc., posing risks to fuel integrity and subsequent operations and affecting the service life of fuel rods.

[0027] Currently, in order to reduce the creep rate of zirconium alloys and improve the safety and economy of nuclear reactors, the main technical measures include adjusting the composition of zirconium alloys and heat-treating zirconium alloys. However, due to the special service environment of zirconium alloys, the addition of alloying elements in zirconium alloys is subject to certain limitations. At the same time, for thin-walled pipe fittings such as zirconium alloy fuel rod cladding, too high a heat treatment temperature will also increase the risk of thermal deformation. Similarly, titanium alloys and hafnium alloys, which also belong to Group-IVB alloys and are widely used in the aerospace industry and the nuclear industry, also have an urgent need to improve creep performance. Therefore, it is of positive significance to provide a simple and effective method for improving the creep resistance of Group-IVB alloys.

[0028] An embodiment of the present invention provides a method for improving the creep resistance of Group-IVB alloys, the method comprising the following steps:

[0029] Step a): Anneal the workpiece made of Group-IVB alloy to be treated.

[0030] In some embodiments, the Group IVB alloy workpiece to be processed is made of zirconium alloy and is prepared by the following method: First, the raw materials are melted; then, forging is carried out at 950°C - 980°C, controlling the final forging temperature > 700°C and the maximum deformation amount of about 70%; next, extrusion treatment is carried out at 550°C - 780°C, with an extrusion ratio of 4 - 25 (deformation amount of 80% - 96%); multi-pass cold rolling is carried out at 25°C, controlling the rolling deformation amount of 50% - 80%, and the intermediate annealing temperature during rolling is 500°C - 750°C.

[0031] In different embodiments, the zirconium alloy workpiece can be a zirconium alloy plate, a zirconium alloy rod or a zirconium alloy tube, and can also be manufactured into other forms according to design requirements; the zirconium alloy workpiece can also be replaced with a titanium alloy workpiece or a hafnium alloy workpiece. Here, zirconium alloy, titanium alloy, and hafnium alloy respectively refer to alloys based on single-phase Zr, Ti, and Hf. In the range from room temperature to 800°C, Ti, Zr, and Hf are all in a close-packed hexagonal crystal structure, and their physical properties are highly similar. In a preferred embodiment, the weight ratio of Ti or Zr or Hf in the Group IVB alloy workpiece is not less than 95%, and in addition, 0 < Sn ≤ 1.7% and 0 < Nb ≤ 3% are contained. The matrix is a close-packed hexagonal α phase. In one embodiment, the zirconium alloy workpiece can be made of a Zr-Sn-Nb alloy, in which, by weight ratio, 0 < Sn ≤ 1.7%, 0 < Nb ≤ 3%, Zr is not less than 95%, and inevitable impurities, and the total content of Fe, Cr, and Si in the alloy does not exceed 0.5%. The structure of the annealed zirconium alloy workpiece reaches the recrystallized structure state as shown in Figure 1 shown, or the stress-relieved structure state as shown in Figure 2 shown, and its matrix is the α-Zr phase with a close-packed hexagonal structure.

[0032] Step b): At a given temperature, a given load is applied to the Group IVB alloy workpiece and kept loaded, where the applied load is not lower than the creep load of the Group IVB alloy workpiece and does not exceed the yield strength at this given temperature, and the loading time is not less than 10 min.

[0033] In different embodiments, the loading can be carried out at room temperature not lower than 25°C, or the Group IVB alloy workpiece can be heated and then kept loaded, or the load can be gradually applied while heating and raising the temperature; in a preferred embodiment, the heating temperature does not exceed 400°C. In different embodiments, the loading methods include tensile, compressive, torsional or bending loading, etc. In a preferred embodiment, the heating and temperature-raising rate does not exceed 30°C / min, and the loading rate of the applied load does not exceed 1 MPa / min.

[0034] Step c): After the temperature of the Group IVB alloy workpiece reaches the ambient temperature, the load is unloaded to complete the treatment.

[0035] After the IVB group alloy parts are processed through the above process, a creep loading test is carried out. The steady-state creep rate does not exceed 70% of the untreated IVB group alloy parts, effectively improving the creep resistance performance and being beneficial to the optimization of the zirconium alloy fuel rod cladding life.

[0036] In a preferred embodiment, a zirconium alloy tube made of Zr-Sn-Nb alloy is used as the zirconium alloy part for the creep test. The specific composition of the zirconium alloy is: by weight ratio, Sn accounts for 1%, Nb accounts for 1%, and Zr accounts for 98%. The cold-rolled zirconium alloy tube is annealed to the fully recrystallized structure as shown in Figure 1 as the first alloy; the cold-rolled zirconium alloy tube is annealed to the stress-relieved structure as shown in Figure 2 as the second alloy. The outer diameter of the zirconium alloy tube is 9.5 mm and the wall thickness is 0.5 mm.

[0037] In Example 1, the first alloy sample is kept at 25 °C, pressurized to 16 MPa by filling an inert gas into the zirconium alloy tube, cooled and unloaded after holding the pressure for 15 min.

[0038] In Example 2, the first alloy sample is heated at a rate of 20 °C / min, and at the same time pressurized by filling an inert gas at a rate of 0.5 MPa / min. The pipe material is heated to 380 °C, the pressure load is increased to 16 MPa, cooled and unloaded after holding the load for 15 min.

[0039] In Comparative Example 1, an untreated first alloy sample is used.

[0040] In Example 3, the second alloy sample is kept at 25 °C, pressurized to 16 MPa by filling an inert gas into the zirconium alloy tube, cooled and unloaded after holding the pressure for 15 min.

[0041] In Example 4, the second alloy sample is heated at a rate of 20 °C / min, and at the same time pressurized by filling an inert gas at a rate of 0.5 MPa / min. The pipe material is heated to 380 °C, the pressure load is increased to 16 MPa, cooled and unloaded after holding the load for 15 min.

[0042] In Comparative Example 2, an untreated second alloy sample is used.

[0043] The above samples are respectively installed on an internal pressure creep testing machine to carry out an internal pressure creep test. The test temperature is 380 °C, the load is applied by filling an inert gas into the zirconium alloy tube, the test internal pressure is 16 MPa, and the test period is 10 days. By recording the data of the outer diameter of the pipe material changing with time in real time, the creep curves as shown in Figure 3 and Figure 4 are obtained. A linear segment is taken on the creep curve, and the long-term steady-state creep rate is obtained by fitting. The results are as follows:

[0044] The steady-state creep rate of Comparative Example 1 was 2.6×10 -5 / h;

[0045] The steady-state creep rate of Example 1 was 1.8×10 -5 / h;

[0046] The steady-state creep rate of Example 2 was 1.0×10 -5 / h.

[0047] It can be seen that for the recrystallized structure with the same low dislocation density, the steady-state creep rate of Example 1 was about 70% of that of untreated Comparative Example 1, and the service life could reach 1.4 times that of Comparative Example 1; the steady-state creep rate of Example 2 was about 39% of that of Comparative Example 1, and the service life could reach 2.5 times that of Comparative Example 1.

[0048] The steady-state creep rate of Comparative Example 2 was 8.6×10 -5 / h;

[0049] The steady-state creep rate of Example 3 was 6.7×10 -5 / h;

[0050] The steady-state creep rate of Example 4 was 2.1×10 -5 / h.

[0051] It can be seen that for the stress-relieved structure with the same strip structure, the steady-state creep rate of Example 3 was about 78% of that of untreated Comparative Example 2, and the service life could reach 1.3 times that of Comparative Example 2; the steady-state creep rate of Example 4 was about 24% of that of Comparative Example 2, and the service life could reach 4.2 times that of Comparative Example 2.

[0052] From the comparison between the above-mentioned examples and comparative examples, it can be known that by using the method for improving the creep resistance of Group IVB alloys provided in the examples of the present invention, the steady-state creep rate of Group IVB alloy parts can be effectively reduced, and the service life of Group IVB alloy parts under high-temperature load conditions can be improved. At the same time, this method has a simple process, is easy to implement, does not require special equipment, and has a low processing cost.

[0053] The purpose of the above examples is to further elaborate on the present invention in combination with the accompanying drawings, so that those skilled in the art can understand the technical concept of the present invention. Within the scope disclosed by the present invention, optimizing or equivalently replacing the involved method steps, and combining the implementation manners in different examples without conflict in structure and principle all fall within the protection scope of the present invention.

Claims

1. A method for improving creep resistance of IVB group alloys, characterized in that: It includes the following steps: Step a): Anneal the Group IVB alloy workpiece. Step b): Apply a given load to the Group IVB alloy workpiece at a given temperature and hold the load; wherein, the given load is not lower than the creep load of the Group IVB alloy workpiece and does not exceed the yield strength of the Group IVB alloy workpiece at the given temperature, and the load holding time is not less than 10 min. Step c): Unload the load after reaching the load holding time.

2. The method for improving creep resistance of Group IVB alloys according to claim 1, characterized in that: The Group IVB alloy includes titanium alloy, zirconium alloy or hafnium alloy.

3. The method for improving creep resistance of Group IVB alloys according to claim 2, characterized in that: The composition of the Group IVB alloy workpiece by weight ratio includes 0 < Sn ≤ 1.7%, 0 < Nb ≤ 3%, not less than 95% of Ti or Zr or Hf, and inevitable impurities.

4. The method for improving creep resistance of Group IVB alloys according to claim 1, 2 or 3, characterized in that: The Group IVB alloy workpiece is made of a zirconium alloy workpiece. In step b), when loading, it further includes the step of heating the zirconium alloy workpiece to a given temperature, and the given temperature does not exceed 400°C.

5. The method for improving creep resistance of Group IVB alloys according to claim 4, characterized in that: In step b), while heating the zirconium alloy workpiece, gradually apply a load to the zirconium alloy workpiece so that the load reaches the given load while the zirconium alloy workpiece reaches the given temperature.

6. The method for improving creep resistance of Group IVB alloys according to claim 1, 2 or 3, characterized in that: In step b), the way of applying the load includes tension, compression, torsion or bending.

7. The method for improving creep resistance of Group IVB alloys according to claim 1, 2 or 3, characterized in that: The Group IVB alloy workpiece is an alloy tube.

8. The method for improving creep resistance of Group IVB alloys according to claim 7, characterized in that: In step b), the way of loading the Group IVB alloy workpiece is to fill the alloy tube with pressurized gas.

9. The method for improving creep resistance of Group IVB alloys according to claim 7, characterized in that: Before step b), straighten the alloy tube.

10. The method for improving creep resistance of Group IVB alloys according to claim 1, 2 or 3, characterized in that: The structure of the Group IVB alloy workpiece is a recrystallized structure or a stress-relieved structure, and the matrix is a close-packed hexagonal α phase.

Images