CN114574765A

CN114574765A - Preparation method of high-performance fastener for lead-based stack

Info

Publication number: CN114574765A
Application number: CN202210210872.5A
Authority: CN
Inventors: 燕春光; 严伟; 徐海涛; 史显波; 乔鹏瑞; 单以银; 韩丽青; 戎利建; 杨红义; 李依依
Original assignee: Institute of Metal Research of CAS; China Institute of Atomic of Energy
Current assignee: Institute of Metal Research of CAS; China Institute of Atomic of Energy
Priority date: 2022-03-04
Filing date: 2022-03-04
Publication date: 2022-06-03
Anticipated expiration: 2042-03-04
Also published as: CN114574765B

Abstract

The invention belongs to the field of high-temperature fasteners, and particularly relates to a high-performance fastener resistant to liquid lead (lead bismuth) corrosion for a high-temperature lead-based stack and a preparation method thereof. The fastener material comprises the following chemical components in percentage by weight: c: 0.06-0.12%; si: 2.0-3.0%; mn: 0 to 1.0 percent; s: 0 to 0.005 percent; p: 0 to 0.01 percent; cr: 13.0 to 17.0 percent; ni: 8.0-15.0%; cu: 0 to 1.0 percent; mo: 0.5-2.0%; nb: 8 is multiplied by 100C to 1.0 percent; o: 0 to 0.003%; n: 0 to 0.03 percent; the balance of Fe. The high-performance fastener integrates comprehensive performances of excellent lead and bismuth corrosion resistance, high strength, high endurance resistance, high fatigue performance, excellent stress relaxation resistance and the like, and can be used for connecting structural materials facing high-temperature lead (lead and bismuth) corrosion environments in the nuclear energy field.

Description

Preparation method of high-performance fastener for lead-based stack

Technical Field

The invention belongs to the field of high-temperature fasteners, and particularly relates to a high-performance fastener resistant to liquid lead (lead bismuth) corrosion for a high-temperature lead-based stack and a preparation method thereof.

Background

A lead (lead bismuth) cooling nuclear reactor, called lead-based reactor for short, is one of six main reactor types of a fourth generation nuclear reactor. The lead-based reactor can well meet the target requirements of safety, economy, persistence and nuclear non-diffusion, is focused internationally, and has wide development space in the future.

The fastener is one of the structural components in the lead-based stack, and is responsible for the extremely critical connection, which concerns the safe operation of the components in the stack. The bolt connection by adopting the fastener has the advantages of compact structure, easy assembly and disassembly, large connecting force, reusability and the like, and is widely applied to various mechanical structures. Bolt connection is also commonly adopted in the connection structure of the internal components of the improved three-loop pressurized water reactor (CPR1000) in China. The number of bolt connectors of the members in the first stage of the nuclear power plant in the Rev mountain Australia can be thousands of. It can be seen that the fastener has wide and important application in nuclear reactors.

However, the role of the fasteners within the nuclear reactor is critical. Historically, multiple bolt loosening and fracture accidents have occurred, which have caused great harm to the operation of the reactor. For example, in 1988, the secondary support of the lower reactor core internal component and the instrument sleeve assembly in the first stage of Qinshan, a plurality of instrument sleeves are broken, and the connecting screws between the instrument sleeves and the grid plate and the reactor core supporting plate are loosened and broken; in 1998, after the thermal functional test of the project of Pakistan C1, 5 bolts on the hanging basket of the internals were found to loosen.

Therefore, a fastener which is used in a lead-based pile with a more severe service environment must have good liquid metal corrosion resistance, and also have special performance requirements such as high heat strength (fracture resistance), high-temperature stress relaxation resistance (looseness resistance), fatigue fracture resistance (fracture resistance) and the like. At home and abroad, a high-performance fastener which integrates liquid lead (lead bismuth) corrosion resistance, stress relaxation resistance, fatigue fracture resistance and high heat strength is not available at present.

Disclosure of Invention

The invention aims to provide a high-performance fastener which is comprehensively integrated with liquid lead (lead bismuth) corrosion resistance, stress relaxation resistance, fatigue fracture resistance, long service life and high heat strength and a preparation method thereof, and meets the use requirements of high-temperature, strong corrosion, vibration and other severe service environments in a lead-based stack.

The technical scheme of the invention is as follows:

a high-performance fastener for a lead-based stack comprises the following chemical components in percentage by weight: c: 0.06-0.12%; si: 2.0-3.0%; mn: 0 to 1.0 percent; s: 0 to 0.005%; p: 0 to 0.01 percent; cr: 13.0 to 17.0 percent; ni: 8.0-15.0%; cu: 0 to 1.0 percent; mo: 0.5-2.0%; nb: 8 is multiplied by 100C to 1.0 percent; o: 0 to 0.003%; n: 0 to 0.03 percent; and the balance of Fe.

The preparation method of the high-performance fastener for the lead-based stack comprises the following steps:

(1) preparing raw materials: preparing materials according to the designed components;

(2) double vacuum melting: obtaining an ingot by adopting a vacuum induction smelting and vacuum consumable duplex vacuum smelting process;

(3) homogenizing: carrying out high-temperature homogenization treatment on the cast ingot;

(4) ingot casting and forging: hot forging the homogenized cast ingot to obtain a forged rod;

(5) heat treatment of steel bars: the forged steel bar is subjected to solution treatment, and the treatment process comprises the following steps: preserving the heat for 0.5 to 2 hours at the temperature of 1000 to 1150 ℃, and cooling the mixture to room temperature in air;

(6) cold drawing of the steel bar: carrying out cold drawing on the steel bar subjected to the solution treatment, wherein the cold drawing deformation is not less than 30%;

(7) and (3) secondary heat treatment of the steel bar: keeping the temperature of the steel bar at 800-900 ℃ for 2-4 hours after cold drawing, and cooling the steel bar to room temperature in air;

(8) planing and cutting a steel bar: cutting the steel bar subjected to the secondary cold drawing heat treatment into pieces with required lengths, slicing the sliced bolt bar, wherein the slicing amount of each pass is not more than 0.16mm, and the surface roughness of the sliced bolt is not more than Ra0.4 mu m;

(9) and (3) nut forming: heating one end of the planed bolt for 10-30 s by using an induction coil, and then putting the bolt into a steel die to be press-forged into a nut;

(10) thread rolling: and rolling threads on the processed bolt to form a finished fastener product.

In the preparation method of the high-performance fastener for the lead-based stack, in the step (1), the design components of the raw materials need to meet the following requirements: cr equivalent < 20; the Ni equivalent is greater than 14, and the formed alloy room-temperature structure is single austenite;

the chromium equivalent is calculated according to formula (1):

cr equivalent weight 100 × (Cr + Mo +1.5Si +0.5Nb) (1)

The nickel equivalent is calculated according to formula (2):

ni equivalent of 100 × (Ni +30 × C +0.5 × Mn +0.5 × Cu) (2).

The preparation method of the high-performance fastener for the lead-based stack comprises the following steps of (3) homogenizing: and (3) the charging temperature of the ingot is less than 700 ℃, the ingot is heated to 1200-1280 ℃ along with the furnace, the heat preservation time is not less than 12 hours, and the ingot is taken out of the furnace and cooled to room temperature by air.

The preparation method of the high-performance fastener for the lead-based stack comprises the following steps of (4): the charging temperature of the ingot is less than 700 ℃, the temperature is increased to 1150-1200 ℃ along with the furnace, and the heat preservation time is not less than 8 hours; the initial forging temperature is 1080-1180 ℃, the final forging temperature is 850-950 ℃, repeated high-reduction forging is carried out in the longitudinal-transverse-longitudinal three directions during forging, the repeated times are not less than 6, the single deformation is more than 10%, the total forging ratio is more than 20, and the forging is carried out by air cooling to room temperature.

According to the preparation method of the high-performance fastener for the lead-based stack, in the step (6), the cold-drawing deformation of each pass of the steel bar is not less than 10%, the intermediate annealing frequency is not more than 2, the annealing temperature is 900-1000 ℃, and the heat preservation time is 300-600 seconds.

The preparation method of the high-performance fastener for the lead-based stack has the following room temperature performance indexes: the yield strength is more than or equal to 300MPa, the tensile strength is more than or equal to 700MPa, and the impact energy is more than or equal to 160J; the high-temperature performance indexes at 550 ℃ are as follows: the yield strength is more than or equal to 200MPa, the tensile strength is more than or equal to 480MPa, and the elongation is more than or equal to 40.0 percent.

The preparation method of the high-performance fastener for the lead-based stack has the advantages that the thickness of an oxide film is not more than 20 mu m after the fastener is corroded in the liquid lead-bismuth alloy (45% Pb-Bi) with the saturated oxygen concentration and the 550 ℃ for 2000 hours, and the fastener has excellent liquid lead-bismuth corrosion resistance.

The preparation method of the high-performance fastener for the lead-based stack has the advantage that the durable fracture time is more than 2000 hours at the stress of 550 ℃ and 260 MPa.

The preparation method of the high-performance fastener for the lead-based stack has the advantages that the initial stress is 80MPa at 550 ℃, and the residual stress is more than 60MPa after the high-performance fastener is kept for 1000 hours; the cycle number of the fatigue test is not less than 17000 times under the conditions that the temperature is 550 ℃, the strain amplitude is +/-0.3%, the strain ratio is-1, the loading waveform is triangular wave, and the strain rate is 0.001 mm/s.

The design idea of the invention is as follows:

the premise of meeting the requirements of high-performance fasteners under severe working conditions such as high temperature, strong corrosion, vibration and the like in the lead-based stack is that the material for manufacturing the fasteners has excellent comprehensive performance. The material for the fastener is composed of key alloy elements such as Cr, Ni, Si, Mo, C, Nb, Cu and the like. Si can enhance the oxidation resistance of Cr in the alloy, so that a compact Cr-rich and Si-rich oxide layer is generated under the high-temperature condition, and the excellent liquid lead bismuth corrosion resistance of the steel is ensured; mo can improve the heat strength of the alloy and ensure higher initial high-temperature strength and long-term good endurance resistance of the steel; nb and Cu form a high-density nano-sized NbC and Cu-rich precipitated phase, so that excellent stress relaxation resistance and creep resistance of the steel are ensured; the alloy material with extremely high purity can be obtained by adopting double vacuum purification smelting combined with high temperature homogenization, heat and cold processing, so that the alloy material has excellent fatigue fracture resistance.

The tissue regulation of the fastener alloy material is a guarantee for further improving various performances. The structural parameters such as the grain size, the number density of carbides, the number density of Cu-rich phases and the dislocation density are regulated and controlled by means of high-temperature homogenization, heat treatment, cold working, heat treatment and the like, so that the further improvement of various properties of alloy materials used by the fastening piece is ensured.

In the preparation process of the fastener bolt, the processing streamline generated by slicing, hot heading and thread rolling which are subjected to the fastener bolt also has important influence on the comprehensive performance of the finished fastener bolt. The invention further improves the comprehensive performance of the fastener by the processes of precise slicing, high-temperature rapid hot forging and thread rolling, thereby improving the process guarantee.

The key element content of the component design in the alloy material used for the fastener of the invention is illustrated as follows:

C：0.06～0.12wt％

c can enlarge the austenite phase region and stabilize the austenite structure. The other important role of C in the alloy is to form nano-sized NbC with Nb, form high-density fine dispersed NbC particle pinning dislocation in the structure and improve the high-temperature stress relaxation resistance and creep strength of the alloy. The content of C and the content of Nb in the alloy follow the principle of ideal chemical proportion, and the content of Nb is ensured to be 8 times of the content of C. If the content of C or Nb is too low, the formed NbC has low density and small effect; and C contentToo high may form M early with Cr element in the alloy₂₃C₆Carbide, on the contrary, deteriorates the overall performance. Therefore, the C content of the alloy is 0.06-0.12 wt%.

Si：2.0～3.0wt％

The bonding force of Si and O is strong. Therefore, the thermal stability of the Si oxide is extremely strong. Si first combines with O in the ambient in an oxygen-containing ambient to form an oxide of Si. Si is added into the alloy, and the Si is preferentially oxidized under a high-temperature environment to form an oxide barrier containing Si, so that further corrosion of the external environment can be prevented. By utilizing the function of Si, a proper amount of Si is added into the alloy to play a role in excellent resistance to corrosion of liquid lead and bismuth. The solid solubility of Si in austenite is larger, more than 2.0 wt% of Si can be added to form a continuous compact corrosion-resistant 'barrier' with stronger binding force, but Si is a stronger ferrite forming element, and excessive Si can embrittle steel. Therefore, the Si content in the alloy is 2.0-3.0 wt% in comprehensive consideration.

Cr：13.0～17.0wt％

Cr can promote passivation of the alloy and keep the alloy in a stable passive state. Also, this action of Cr causes the formation of a continuous dense Cr on the surface of the alloy₂O₃The passive film can block ion migration and element dissolution to liquid lead and bismuth, so that the liquid metal corrosion resistance of the alloy is improved, the effect of Cr and Si are mutually enhanced, and the liquid lead and bismuth corrosion resistance is better. However, Cr and C easily form M₂₃C₆. Therefore, the Cr content in the alloy is controlled to be 13.0-17.0 wt%.

Ni：8.0～15.0wt％

Ni mainly has the functions of forming and stabilizing austenite, so that the alloy obtains a completely austenitic structure, and the thermodynamic stability of the alloy is improved. However, the solubility of Ni in the liquid lead-bismuth alloy is relatively high, and the liquid metal corrosion resistance is deteriorated due to excessively high content of Ni; at the same time, an increase in the Ni content in the steel decreases the solubility of C in the alloy, thereby increasing the tendency for carbide precipitation. Therefore, the Ni content in the alloy is controlled to be 8.0-15.0 wt% in comprehensive consideration.

Cu：0～1.0wt％

Cu is a non-carbide forming element, a nano-sized Cu-rich phase can be separated out in the heat treatment and long-term service processes when Cu is added into the austenitic alloy, the coarsening rate of the nano-sized Cu-rich phase is low, and the nano-sized Cu-rich phase can play a role in pinning dislocation so as to improve the stress relaxation resistance and the endurance strength. Meanwhile, the addition of Cu into the alloy can obviously reduce the cold hardening tendency of the alloy and improve the cold working forming performance. However, excessive Cu deteriorates the hot workability of the material. Therefore, the Cu content in the alloy is preferably 0 to 1.0 wt%.

Mo：0.5～2.0wt％

Mo is an element which forms and stabilizes and expands a ferrite phase region, and in order to maintain a single austenite structure, Mo is added to the alloy while increasing the content of Ni. The main role of Mo in the alloy is to improve the high temperature strength. As the Mo content in the alloy increases, the high temperature durability increases, but Mo promotes intermetallic phases in the alloy, such as: the precipitation of sigma phase and Laves phase reduces the stability of the structure. Therefore, the Mo content in the alloy is 0.5-2.0 wt% in comprehensive consideration.

Nb：8×100C～1.0wt％

Nb is a key element in the alloy and is the basis for ensuring excellent stress relaxation resistance. Nb and C in the alloy for the fastener form a high-density NbC nano-sized precipitated phase, and dislocation is pinned through the precipitated phase to prevent the elastic strain from transforming to the plastic strain, so that higher residual stress is maintained. According to a rough calculation, the Nb content required to fix all C in austenite to NbC was 7.78 times the C content. The minimum Nb content in the alloy is 8 times the C content, considering that Nb is also partially consumed by forming nitrides corresponding to the trace amount of N in the steel. Since Nb is an easily segregating element, and excessive Nb in the alloy forms Fe after long-term aging₂Nb type Laves phase, deteriorating performance. Thus, taken together, the maximum amount of Nb is no more than 1.0 wt.%.

The invention has the advantages and beneficial effects that:

1. the invention breaks through the technical barrier of the fastener for the lead-based stack, and obtains the high-performance fastener which integrates the comprehensive properties of excellent lead-bismuth corrosion resistance, high strength, high endurance resistance, high fatigue performance, excellent stress relaxation resistance and the like.

2. The fastener can be applied to the connection of structural materials in the nuclear energy field facing high-temperature lead (lead bismuth) corrosion environment.

Drawings

FIG. 1 is a microstructure diagram of example 1.

FIG. 2 is a microstructure view of example 2.

FIG. 3 is a graph of residual stress versus time at 550 ℃ for the steels of example 5 and comparative example 1, with an initial stress of 80 MPa.

FIG. 4 is a graph showing the morphology of an oxide film in comparative example 1 after corrosion for 2000 hours in a liquid lead bismuth alloy (45% Pb-Bi) having a saturated oxygen concentration and at 550 ℃.

FIG. 5 is a graph showing the morphology of the oxide film of example 5 after corrosion in a liquid lead-bismuth alloy (45% Pb-Bi) with a saturated oxygen concentration and 550 ℃ for 2000 hours.

Detailed Description

In practice, the main method of manufacture of the fasteners of the invention is as follows, with the differences from the main method of manufacture being additionally described in the respective examples and comparative examples for comparison. Notably, all experimental samples from which performance data were obtained were cut on the finished fastener.

(1) Preparing raw materials: the ingredients are mixed according to the design components.

(2) Double vacuum melting: carrying out vacuum induction smelting and pouring on the raw materials to obtain an ingot; removing surface oxide skin of the cast ingot obtained by vacuum induction smelting, and cutting two ends to prepare a consumable electrode bar; and further purifying and smelting the consumable electrode rod in a vacuum consumable smelting furnace to obtain a high-purity consumable ingot.

(3) Homogenizing: and (3) cold-charging the cast ingot into a furnace, heating the cast ingot to 1220 ℃ along with the furnace, carrying out high-temperature homogenization treatment, keeping the temperature for 20 hours, discharging the cast ingot from the furnace, and air-cooling the cast ingot to room temperature.

(4) Ingot casting and forging: cold loading the homogenized cast ingot into a furnace, heating to 1180 ℃ along with the furnace, preserving heat for 10 hours, then forging, wherein the initial forging temperature is 1125 ℃, the initial forging is circularly and repeatedly forged at a high reduction rate in the longitudinal direction, the transverse direction and the longitudinal direction, the circulation is repeated for 7 times, the forging deformation of each time is about 13%, and the total forging ratio is about 24; then forging into round bars, wherein the final forging temperature is about 935 ℃, and air cooling to room temperature after forging.

(5) Heat treatment of steel bars: the forged steel bar is subjected to solution treatment, and the treatment process comprises the following steps: keeping the temperature at 1080 ℃ for 1.5 hours, and cooling to room temperature in air.

(6) Cold drawing of the steel bar: and (3) carrying out cold drawing on the steel bar subjected to the solution treatment for 3 times, wherein the cold drawing deformation of the 1 st pass is about 18%, the cold drawing deformation of the 2 nd pass is about 14%, and after the 2 nd pass of drawing, carrying out heat preservation at 920 ℃ for 480 seconds for annealing for 1 time, and cooling to room temperature along with the furnace. The cold drawing deformation of the 3 rd pass is about 12 percent, and the total cold drawing deformation is about 44 percent.

(7) And (3) secondary heat treatment of the steel bar: and (3) keeping the temperature of the steel bar at 870 ℃ for 3 hours after cold drawing, and cooling the steel bar to room temperature in air.

(8) Planing and cutting a steel bar: cutting the steel bar subjected to the secondary cold-drawing heat treatment; and slicing the cut bolt bar on a lathe, wherein the slicing amount of each pass is 0.10mm, and the surface roughness of the sliced bolt is Ra0.1 mu m.

(9) And (3) nut forming: one end of the planed bolt is heated in an induction coil for 18s, and then the bolt is put into a steel die for manufacturing the bolt to be hot forged into a nut.

(10) Thread rolling: and (3) rolling the processed steel bar on an oil-cooled roller press to form a fastener finished product with the diameter of 30 mm.

Hereinafter, the present invention will be described by comparing various examples and comparative examples, which are for illustrative purposes only and the present invention is not limited to these examples.

Example 1

In the embodiment, the adopted alloy raw materials comprise the following chemical components: c: 0.065%; si: 2.17 percent; mn: 0.43 percent; s: 0.0017%; p: 0.009%; cr: 14.55 percent; ni: 9.5 percent; mo: 0.05 percent; nb: 0.68 percent; o: 0.002%; n: 0.005 percent; the balance of Fe. Wherein the Cr equivalent is 18.20<20, and the Ni equivalent is 11.96 no more than 14.

Example 2

In the embodiment, the adopted alloy raw materials comprise the following chemical components: c: 0.081%; si: 2.58 percent; mn: 0.60 percent; s: 0.0016 percent; p: 0.008 percent; cr: 14.70 percent; ni: 12.75 percent; cu: 0.64 of; mo: 0.56 percent; nb: 0.80 percent; o: 0.002%; n: 0.005 percent; the balance of Fe. Wherein the Cr equivalent is 19.53<20 and the Ni equivalent is 15.80> 14.

Example 3

In the embodiment, the adopted alloy raw materials comprise the following chemical components: c: 0.09%; si: 2.47 percent; mn: 0.58 percent; s: 0.0017%; p: 0.008 percent; cr: 14.60 percent; ni: 14.75 percent; mo: 1.06 percent; nb: 0.85 percent; o: 0.002%; n: 0.005 percent; and the balance of Fe. Wherein the Cr equivalent is 19.79<20 and the Ni equivalent is 17.74> 14.

Example 4

In the embodiment, the adopted alloy raw materials comprise the following chemical components: c: 0.12 percent; si: 2.57 percent; mn: 0.51 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.73%; ni: 10.26 percent; mo: 0.03 percent; nb: 0.96 percent; o: 0.0016 percent; n: 0.004%; the balance of Fe. Wherein the Cr equivalent is 19.09<20, and the Ni equivalent is 14.12> 14.

Example 5

In the embodiment, the adopted alloy raw materials comprise the following chemical components: c: 0.11 percent; si: 2.3 percent; mn: 0.81 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.2 percent; ni: 12.24 percent; cu: 0.90; mo: 1.53 percent; nb: 0.90 percent; o: 0.0016 percent; n: 0.004%; the balance of Fe. Wherein the Cr equivalent is 19.63<20 and the Ni equivalent is 16.39> 14.

Example 6

In the embodiment, the adopted alloy raw materials comprise the following chemical components: c: 0.11 percent; si: 2.44 percent; mn: 0.61%; s: 0.0014%; p: 0.007%; cr: 14.3 percent; ni: 11.22 percent; mo: 1.03 percent; nb: 0.42 percent; o: 0.0017%; n: 0.004%; the balance of Fe. Wherein the Cr equivalent is 19.2<20, and the Ni equivalent is 14.82> 14.

Comparative example 1

In this comparative example, the alloy material used was commercial 316 type austenitic stainless steel, and the chemical composition was: c: 0.022%; si: 0.34 percent; mn: 1.36 percent; s: 0.023%; p: 0.03 percent; cr: 18.24 percent; ni: 12.36 percent; mo: 2.53 percent; n: 0.12 percent; the balance of Fe.

Comparative example 2

In the comparative example, the adopted alloy raw materials comprise the following chemical components: c: 0.11 percent; si: 2.3 percent; mn: 0.81 percent; s: 0.0015 percent; p: 0.008 percent; cr: 14.2 percent; ni: 12.24 percent; cu: 0.90; mo: 1.53 percent; nb: 0.90 percent; o: 0.0016 percent; n: 0.004%; the balance of Fe. Wherein, the Cr equivalent is 19.63<20, and the Ni equivalent is 15.94> 14.

Comparative example 2 differs from example 5 in that comparative example 2 is not cold drawn and the other manufacturing processes are the same.

Comparative example 3

Comparative example 3 differs from example 5 in that comparative example 3 does not perform the homogenization process, and the other preparation processes are the same.

The room temperature and 550 ℃ mechanical properties of the above examples and comparative examples are shown in Table 1.

TABLE 1

The results in table 1 show that the steel of the invention obtains a single austenite structure and higher mechanical properties at room temperature and high temperature by regulating the Cr equivalent and the Ni equivalent, and the fastener prepared in example 1 has a dual-phase structure and reduced high-temperature strength because the Ni equivalent cannot meet the requirements. From the results of examples 2, 3 and 5, it can be seen that the required Mo addition to the fastener results in an increase in strength at 550 ℃ and that the strength increases with increasing Mo content. Comparative example 1, although containing a higher Mo content, did not contain C, Nb and Cu, and did not reach the strength level of the fastener of the present invention.

As shown in FIG. 1, the microstructure of example 1 was a two-phase microstructure.

As shown in fig. 2, the microstructure of example 2 is a single austenite structure.

The above examples and comparative examples show the endurance fracture times at 550 ℃ under a stress of 260MPa in Table 2.

TABLE 2

From the results of examples 2, 3 and 5, the fastener of the invention has the advantages that after the required Mo is added, the durable fracture time at 260MPa and 550 ℃ exceeds 2000 hours, and the fracture time is longer as the Mo content is increased; however, if cold drawing or high temperature homogenization is not used, or if the Nb content is too low, the permanent fracture time is greatly reduced (see example 5 and comparative examples 2 and 3). It can be seen that the optimum performance can only be achieved by a combination of the composition design and the texture control of the fastener stock itself.

The above examples and comparative examples have an initial stress of 80MPa at 550 ℃ and residual stress values after 1000 hours of holding are shown in Table 3.

TABLE 3

	Example 2	Example 5	Example 6	Comparative example 1	Comparative example 2	Comparative example 3
							Residual stress/MPa	49	62	35	17	26	22

The results in Table 3 show that the residual stress after the addition of Nb, C and Cu to the fastener of the present invention is significantly higher than that of the steel of comparative example 1 in which Nb, C and Cu were not added as required, and that the inventive steel exhibits excellent stress relaxation resistance. However, the best resistance to stress relaxation is not achieved with low Nb (example 6) or without cold drawing or high temperature homogenization (comparative examples 2 and 3).

As shown in FIG. 3, the initial stress of the steels of example 5 and comparative example 1 was 80MPa at 550 ℃, and from the graph of the residual stress versus time, it can be seen that the stress of example 5 decreased with time to a much lower degree than that of comparative example 1, i.e., example 5 had a higher residual stress and exhibited a more excellent stress relaxation resistance than comparative example 1.

The results of the fatigue tests of the above examples and comparative examples at 550 ℃ with a strain amplitude of. + -. 0.3%, a strain ratio of-1, a triangular wave as a loading waveform, and a strain rate of 0.001mm/s are shown in Table 4.

TABLE 4

	Example 5	Comparative example 2	Comparative example 3
				Cycle of the cycle	17621	11373	7853

The results in table 4 show that, because the fasteners prepared from the same raw material composition have different fatigue lives due to different preparation processes (comparative example 2 is not cold-drawn, and comparative example 3 is not homogenized), the influence of high-temperature homogenization and cold-drawing on the fatigue lives can be seen to be very significant, that is, the optimal fatigue life can be obtained only by combining the composition design and the structure regulation of the raw material of the fastener.

The values of the oxide film thicknesses after etching in a liquid lead-bismuth alloy (45% Pb-Bi) at 550 ℃ for 2000 hours at a saturated oxygen concentration in the above examples and comparative examples are shown in Table 5.

TABLE 5

As shown in fig. 4 and 5, from the appearance of the oxide film after the steels of comparative example 1 and example 5 were corroded for 2000 hours in the liquid lead bismuth alloy (45% Pb — Bi) with the saturated oxygen concentration and the temperature of 550 ℃, it can be seen that the oxide film of comparative example 1 has the thickest thickness, the corrosion resistance of the liquid lead bismuth alloy is poor, the oxide film of example 5 has the thinnest thickness, and the lead bismuth corrosion resistance of example 5 is optimal.

The results in Table 5 show that the lead-bismuth corrosion resistance of the fastener of the invention is obviously improved after Si is added (the comparative example 1 does not add Si raw material as required); besides being related to the alloying of Si element, the lead bismuth corrosion resistance of the fastener is found to play an important role in improving the liquid lead bismuth corrosion resistance for the first time. The addition of Mo has improved resistance to corrosion by liquid lead bismuth metal (examples 2 and 4), which makes example 5 the thinnest oxide layer, although the Si content is not the highest, exhibit the best resistance to lead bismuth corrosion.

The above embodiments are merely illustrative of the technical ideas and features of the present invention, and the purpose thereof is to enable those skilled in the art to understand the contents of the present invention and implement the present invention, and not to limit the protection scope of the present invention. All equivalent changes and modifications made according to the spirit of the present invention should be covered within the protection scope of the present invention.

Claims

1. A high-performance fastener for a lead-based stack is characterized in that the chemical composition of the fastener material is as follows in percentage by weight: c: 0.06-0.12%; si: 2.0-3.0%; mn: 0 to 1.0 percent; s: 0 to 0.005%; p: 0 to 0.01 percent; cr: 13.0-17.0%; ni: 8.0-15.0%; cu: 0 to 1.0 percent; mo: 0.5-2.0%; nb: 8 × 100C-1.0%; o: 0 to 0.003%; n: 0 to 0.03 percent; the balance of Fe.

2. A method for preparing a high-performance fastener for a lead matrix stack according to claim 1, comprising the steps of:

(4) casting ingot forging: hot forging the homogenized cast ingot to obtain a forged rod;

(8) planing and cutting a steel bar: cutting the steel bar subjected to the cold drawing and secondary heat treatment to the required length, slicing the sliced bolt bar, wherein the slicing amount of each pass is not more than 0.16mm, and the surface roughness of the sliced bolt is not more than Ra0.4 mu m;

3. The method for preparing a high-performance fastener for a lead-based stack according to claim 2, wherein in the step (1), the design components of the raw materials are as follows: cr equivalent < 20; the Ni equivalent is more than 14, and the formed alloy has a single austenite at room temperature;

the chromium equivalent is calculated according to formula (1):

cr equivalent weight 100 × (Cr + Mo +1.5Si +0.5Nb) (1)

The nickel equivalent is calculated according to formula (2):

ni equivalent of 100 × (Ni +30 × C +0.5 × Mn +0.5 × Cu) (2).

4. The method for preparing a high-performance fastener for a lead-based stack according to claim 2, wherein in the step (3), the homogenization treatment process comprises: and (3) the charging temperature of the ingot is less than 700 ℃, the ingot is heated to 1200-1280 ℃ along with the furnace, the heat preservation time is not less than 12 hours, and the ingot is taken out of the furnace and cooled to room temperature by air.

5. The method for preparing a high-performance fastener for a lead-based stack according to claim 2, wherein in the step (4), the forging process comprises: the charging temperature of the ingot is less than 700 ℃, the temperature is increased to 1150-1200 ℃ along with the furnace, and the heat preservation time is not less than 8 hours; the initial forging temperature is 1080-1180 ℃, the final forging temperature is 850-950 ℃, repeated high-reduction forging is carried out in the longitudinal-transverse-longitudinal three directions in the forging process, the repeated times are not less than 6, the single deformation is more than 10%, the total forging ratio is more than 20, and the forging is carried out by air cooling to the room temperature.

6. The method for preparing the high-performance fastener for the lead-based stack according to claim 2, wherein in the step (6), the cold-drawing deformation of the steel bar in each pass is not less than 10%, the intermediate annealing times are not more than 2, the annealing temperature is 900-1000 ℃, and the heat preservation time is 300-600 seconds.

7. The method for producing a high-performance fastener for a lead-based stack according to claim 2, wherein the room-temperature performance index is as follows: the yield strength is more than or equal to 300MPa, the tensile strength is more than or equal to 700MPa, and the impact energy is more than or equal to 160J; the high-temperature performance indexes at 550 ℃ are as follows: the yield strength is more than or equal to 200MPa, the tensile strength is more than or equal to 480MPa, and the elongation is more than or equal to 40.0%.

8. The method for producing a high-performance fastener for a lead-based stack as defined in claim 2, wherein the oxide film thickness after etching for 2000 hours in a liquid lead bismuth alloy (45% Pb-Bi) having a saturated oxygen concentration at 550 ℃ is not more than 20 μm, and has excellent resistance to corrosion by liquid lead bismuth.

9. The method of making a high performance fastener for a lead heap as defined in claim 2 wherein the permanent rupture time at 550 ℃ and 260MPa stress is greater than 2000 hours.

10. The method for producing a high-performance fastener for a lead nugget according to claim 2, wherein the initial stress is 80MPa at 550 ℃, and the residual stress after 1000 hours holding is more than 60 MPa; the cycle number of the fatigue test is not less than 17000 times under the conditions that the temperature is 550 ℃, the strain amplitude is +/-0.3%, the strain ratio is-1, the loading waveform is triangular wave, and the strain rate is 0.001 mm/s.