A method of improving the low temperature toughness of a fastener

Abstract

The application discloses a method for improving low-temperature toughness of fasteners, which comprises the following steps: S1, adopting alloy structural steel 35CrMoA (meeting the requirement of GB / T 3077) or 40CrNi2MoA (meeting the requirement of ASME or ASTM standard A 320L43) as raw material for manufacturing fasteners, controlling the content of phosphorus, sulfur and manganese of the raw material and low-multiple structure defects of the raw material; S2, manufacturing fasteners by using the raw material; and S3, performing heat treatment on the fasteners, so that the fasteners after the heat treatment meet the requirements of hardness HRC 26-30, impact toughness A KV ≥27J and lateral expansion value ≥0.38mm. The method can effectively improve the low-temperature impact performance of the material by combining multiple technical means such as controlling the metallurgical quality of the raw material, reasonably matching the hardness, controlling the heat treatment process and modulating the composite of alloy powder, and can obtain products meeting the mechanical performance requirements.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of fastener materials, in particular to a method for improving the low-temperature toughness of fasteners. BACKGROUND

[0002] In recent years, low-temperature fasteners have been widely used in many major petrochemical engineering projects and major equipment plants. The so-called low-temperature fastener mainly refers to 35CrMoA-L studs produced according to the national standard (with -70℃, -80℃, -101℃ impact toughness requirements) and A320Gr.L43 studs produced according to the ASME / ASTM standard (with -101℃ impact toughness requirements) whose working temperature is lower than -20℃. The average impact toughness A KV of the three impact test samples at low temperature is ≥27J, and the impact toughness of one of the test samples is less than the specified value but not less than 70% of the specified value, and the lateral expansion value is ≥0.38mm. Such pressure pipe fasteners have harsh service conditions, both under high pressure and in a low-temperature state. In order to prevent the studs from brittle transition and instantaneous fracture failure in a low-temperature state, the relevant standards make relatively strict provisions for the materials and low-temperature impact performance of such fasteners, and users attach great importance to this. However, it has been a difficult problem in the fastener industry to meet the low-temperature impact performance of -101℃.

[0003] The content of carbon and the content of various alloying elements in the chemical composition of the material directly affect the mechanical properties of the part after heat treatment, and the content of impurities such as phosphorus and sulfur affects the number and distribution of inclusions in the material, and if the content of sulfur and phosphorus exceeds the standard, it will also affect the plasticity and toughness of the material under different use conditions, especially if the content of phosphorus is too high, the material will be cold brittle. There are many types of low-magnification organizational defects in steel, and common ones include porosity, segregation, bubbles, inclusions, white spots, and plastic and brittle inclusions. Severe low-magnification organizational defects can also reduce the plasticity and toughness of the steel, so when purchasing raw materials, the content of phosphorus and sulfur must be strictly controlled, and the low-magnification organizational defects of the steel must be controlled, and the mechanical properties of the material can be improved through appropriate heat treatment. The strength represents the load-carrying capacity of the part, the plasticity is the ability of the material to deform plastically, and the impact toughness is the ability of the material to resist impact load. For the same steel, after quenching and tempering, the strength and plasticity and toughness are in a trade-off relationship, and a reasonable hardness range must be determined to ensure good matching of the strength, plasticity and toughness of the material. In addition, the size of the grain after heat treatment has a great relationship with the strength and toughness of the steel, and refining the grain can improve the strength and toughness of the steel at the same time. Compared with ordinary heat treatment, critical zone heat treatment can significantly improve the toughness without reducing the strength, so when developing a heat treatment process, the quenching heating temperature is selected near the upper critical region, the holding time is sufficient but not too long to prevent the quenching martensite structure from being coarse, and the tempering holding time is sufficient to fully release the quenching internal stress. Through reasonable cooperation in several aspects, the low-temperature impact toughness of the fastener can be improved. However, there is currently a lack of reliable solutions to improve the low-temperature impact performance of the fastener. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a method for improving the low-temperature toughness of a fastener in view of the deficiencies in the prior art. The present application can effectively improve the low-temperature impact performance of the material and obtain a product that meets the mechanical performance requirements by controlling the metallurgical quality of the raw material, reasonably matching the hardness, controlling the heat treatment process, and adjusting the composite of alloy powder.

[0005] To solve the above technical problems, the technical scheme adopted by the present application is as follows: a method for improving the low-temperature toughness of a fastener, comprising the following steps:

[0006] S1, using alloy structural steel 35CrMoA or 40CrNi2MoA as a raw material for manufacturing a fastener, controlling the content of phosphorus, sulfur and manganese in the raw material and the low-magnification organizational defects of the raw material; wherein the chemical composition of 35CrMoA meets the requirements of GB / T 3077, and the chemical composition of 40CrNi2MoA meets the requirements of ASME or ASTM standard A 320L43;

[0007] S2, manufacturing the fastener by using the raw material;

[0008] S3, heat treating the fastener, so that the fastener after heat treatment satisfies: hardness HRC 26-30, impact toughness A KV ≥27J, lateral expansion value ≥0.38mm;

[0009] The step of heat treatment comprises:

[0010] S3-1, quenching heating: the quenching heating adopts step heating, the first heating is to 300-400℃, the second heating is to quenching temperature, the quenching temperature is 810-870℃, the quenching holding time is 15-40 minutes;

[0011] S3-2, cooling: the fastener is cooled by oil or by salt water solution;

[0012] S3-3, tempering: heating to tempering temperature, the tempering temperature is 590-670℃, the holding time is 90-120 minutes, then air cooling to room temperature.

[0013] Preferably, the step of heat treatment in step S3 comprises:

[0014] S3-1, quenching heating: the quenching heating adopts step heating, the first heating is by 30Kw oven, heating to 300-400℃; the second heating is by 50Kw medium temperature salt bath furnace, the furnace loading is ≤50Kg, heating to quenching temperature, the quenching temperature is 810-870℃, the quenching holding time is 15-40 minutes;

[0015] S3-2, cooling: the fastener is cooled by 10# or 20# mechanical oil or by salt water solution, the salt water solution contains sodium chloride with mass fraction 6-10%; the cooling medium temperature: the salt water solution is ≤40℃, the 10# or 20# mechanical oil is ≤80℃;

[0016] S3-3, tempering: the tempering is by 35Kw pit furnace, the furnace loading is ≤120Kg, heating to tempering temperature, the tempering temperature is 590-670℃, the holding time is 90-120 minutes, air cooling to room temperature.

[0017] Preferably, in step S1, the phosphorus content in the raw materials 35CrMoA and 40CrNi2MoA is controlled to be below 0.012% in mass fraction, and the sulfur content is controlled to be below 0.006% in mass fraction.

[0018] Preferably, in step S1, the manganese content in the raw material 40CrNi2MoA is 0.65-0.80% in mass fraction.

[0019] Preferably, the macrostructure defects of the raw material 35CrMoA and 40CrNi2MoA satisfy: general porosity and square segregation is 0.5 level or less, center porosity is 1.0 level or less; plastic inclusions and brittle inclusions are both not greater than 2.5 level, and the sum of the two is not greater than 4.5 level.

[0020] Preferably, the raw material is subjected to composite treatment by using the modulated alloy powder in step S2, and then the fastener is manufactured, and the specific steps are as follows: after the raw material 35CrMoA or 40CrNi2MoA is completely melted, the modulated alloy powder is added, smelting, casting, and machining are performed to obtain the fastener.

[0021] The modulated alloy powder is obtained by coating boron powder with an alloy.

[0022] Preferably, the mass ratio of the modulated alloy powder to the raw material is 0.05-0.24:99.95-99.76.

[0023] The chemical composition of the raw material 35CrMoA, by mass percentage, is: C: 0.32-0.40%, Mn: 0.40-0.70%, Si: 0.17-0.37%, Cr: 0.80-1.10%, Mo: 0.15-0.25%, P: ≤0.012%, S: ≤0.006%, and the balance is Fe and inevitable impurity elements.

[0024] The chemical composition of the raw material 40CrNi2MoA, by mass percentage, is: C: 0.38-0.43%, Mn: 0.65-0.80%, Si: 0.17-0.37, Cr: 0.70-0.90%, Ni: 1.65-2.00%, Mo: 0.20-0.30%, P: ≤0.012%, S: ≤0.006%, and the balance is Fe and inevitable impurity elements.

[0025] Preferably, the modulated alloy powder is prepared by the following method:

[0026] S2-1, B powder pretreatment;

[0027] S2-2, B powder sensitization treatment:

[0028] S2-3, B powder activation treatment:

[0029] S2-4, coating the activated B powder with an alloy:

[0030] S2-4-1, mixing nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate, and the activated B powder to obtain a precursor;

[0031] S2-4-2, the precursor is added into the ammonia water solution, then ammonium sulfate, sodium citrate and PdCl2 are added, and the obtained mixture is ultrasonically dispersed, then is transferred into a reaction kettle, hydrogen is introduced, and the reaction is carried out under stirring and heating, after the reaction is completed, the product is centrifuged, washed, vacuum dried, ground, and the modulated alloy powder is obtained.

[0032] Preferably, the modulated alloy powder is prepared by the following method:

[0033] S2-1, B powder pretreatment: the B powder with a particle size of 2-10 μm is boiled in a NaOH solution with a mass concentration of 5-20% for 5-30 min, then is washed with deionized water, immersed in a 30-50 wt% HNO3 solution for 2-10 min, then is washed with deionized water until neutral, and is dried to obtain the pretreated B powder;

[0034] S2-2, B powder sensitization treatment: 0.5-2 g of SnCl2·2H2O is dissolved in 25-100 mL of ethanol to obtain a sensitization solution; 0.5-2 g of the pretreated B powder prepared in step S1 is added into the sensitization solution, and is ultrasonically treated for 10-30 min, then is centrifuged, and the solid product is washed with ethanol and deionized water in sequence to obtain the sensitized B powder;

[0035] S2-3, B powder activation treatment: 0.5-2 g of the sensitized B powder is added into 25-100 mL of a PdCl2 hydrochloric acid solution with a mass fraction of 0.025-0.1%, and is ultrasonically treated for 10-40 min, then is centrifuged, and the solid product is washed with ethanol, and then is vacuum dried at 40-65 °C for 4-16 h to obtain the activated B powder;

[0036] S2-4, the B powder is coated with an alloy:

[0037] S2-4-1, nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate and the activated B powder are mixed to obtain a precursor, and the mass ratio of Ni: Fe: La: Y: B in the precursor is 5.5-7.5: 3.8-6: 1-4: 0.75-3: 1;

[0038] S2-4-2, 2.5-10 g of the precursor is added into 50-200 mL of an ammonia water solution with a mass concentration of 2.5-10%, and then 5-20 g / L of ammonium sulfate, 0.06-0.25 g / L of sodium citrate and 0.03-0.12 g / L of PdCl2 are added, and ultrasonic dispersion is performed for 5-20 min; the obtained mixture is transferred into a reaction kettle, hydrogen is introduced, and reaction is performed at 250-100 rpm of stirring and 150-170 ℃ for 0.5-2 h, wherein the hydrogen partial pressure is controlled to be 2-3 MPa during the reaction; after the reaction is completed, the product is cooled to below 50-65 ℃, and then the kettle is unloaded after being degassed; the product is centrifuged, washed with ethanol, and vacuum dried at 80-100 ℃ for 6-24 h; and then the product is ground to obtain the alloy powder.

[0039] Preferably, the mass ratio of Ni:Fe:La:Y:B in the precursor is 6:4.5:2:1.5:1.

[0040] The beneficial effects of the present application are:

[0041] The present application provides a method for improving the low-temperature toughness of fasteners, in which the metallurgical quality of raw materials is controlled, the hardness is reasonably matched, and the heat treatment process is controlled, so that the low-temperature impact performance and mechanical performance (-101 ℃) of the obtained product can meet the requirements, the method is simple and easy to produce, and has a wide application prospect; specifically, the metallurgical quality of raw materials is reasonably controlled, especially the content of trace elements and the grade of inclusions, and the reasonable hardness range is controlled, the quenching heating temperature is controlled to be near the upper critical region, the quenching holding time is appropriate (if too short, the free ferrite is not fully dissolved, and if too long, the grain will be coarse), and the tempering holding time is sufficient.

[0042] In the further optimized scheme of the present application, the raw materials are further compounded by using the alloy powder, B, La and Y elements which can be uniformly dispersed are added in the fastener raw material system, which can further improve the low-temperature toughness of the fastener; the alloy powder has a core-shell structure, B powder is used as the core, and NiFe alloy doped with La and Y is used as the shell, the B powder is coated by the alloy shell, which can solve the problems of poor wettability of B powder with the metal matrix of the fastener and difficult uniform dispersion, and also can realize the uniform dispersion of trace elements La and Y in the metal matrix of the fastener (La and Y have large density difference with the steel liquid, and trace La and Y are not easy to uniformly disperse in the steel liquid), so that B, La and Y have a synergistic effect in improving the low-temperature toughness. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 The XRD pattern of the alloy powder prepared in Example 3 is shown in the following figure:

[0044] Figure 2 The impact toughness test results of the fasteners with the specification of M20 x L prepared in the examples 1-4 and the comparative examples are shown in the following table. DETAILED DESCRIPTION

[0045] The application will be further described in connection with the following examples, which a person skilled in the art can implement with reference to the description and the examples.

[0046] It should be understood that the terms such as "have", "contain" and "include" used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0047] The test methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the following examples can be obtained commercially unless otherwise specified. The specific conditions are not specified in the following examples, and the conventional conditions or the conditions recommended by the manufacturer are used. The reagents or instruments used are not specified by the manufacturer, and are conventional products that can be obtained commercially.

[0048] In a first aspect of the application, a method for improving the low-temperature toughness of a fastener is provided, comprising the following steps:

[0049] S1, using alloy structural steel 35CrMoA or 40CrNi2MoA as a raw material for manufacturing the fastener, controlling the phosphorus, sulfur and manganese content of the raw material and the macrostructure defects of the raw material; wherein the chemical composition of 35CrMoA meets the requirements of GB / T 3077, and the chemical composition of 40CrNi2MoA meets the requirements of ASME or ASTM standard A 320L43;

[0050] In a preferred embodiment, in particular, the chemical composition of the raw material meets the requirements shown in the following table 1:

[0051] Table 1

[0052]

[0053] In a preferred embodiment, as shown in the following table 2, the macrostructure defects of the raw materials 35CrMoA and 40CrNi2MoA meet the following requirements: the general porosity and square segregation is not more than 0.5 level, the center porosity is not more than 1.0 level; the plastic inclusions and brittle inclusions are not more than 2.5 level, and the sum of the two is not more than 4.5 level.

[0054] Table 2

[0055]

[0056] S2, using raw materials to manufacture fasteners, the manufacturing process is a conventional process, not specifically limited in the present application; in the preferred embodiments, the fastener is selected from a bolt or a stud, which can be metric M20-M39 and American 0.75"-1.75" and the like;

[0057] S3, heat treatment of the fastener, so that the fastener after heat treatment meets: hardness HRC 26-30, impact toughness A at-101 ℃ KV ≥27J, lateral expansion value ≥0.38mm;

[0058] The step of heat treatment comprises:

[0059] S3-1, quenching heating: quenching heating adopts step heating, the first heating uses a 30Kw oven, heated to 300-400℃; the second heating uses a 50Kw medium temperature salt bath furnace, the loading capacity is ≤50Kg, heated to quenching temperature, the quenching temperature is 810-870℃, the quenching holding time is 15-40 minutes; the specific holding time can be calculated according to about 1 minute per millimeter of the bolt or stud blank section diameter;

[0060] S3-2, cooling: the fastener is cooled by using 10# or 20# mechanical oil or using a salt water solution containing 6%-10% sodium chloride by mass fraction; the cooling medium temperature: the salt water solution ≤40℃, the 10# or 20# mechanical oil ≤80℃;

[0061] S3-3, tempering: the tempering uses a 35Kw pit furnace, the loading capacity is ≤120Kg, heated to the tempering temperature, the tempering temperature is 590-670℃, the holding time is 90-120 minutes, and air cooling to room temperature. The obtained product is a fastener blank, which is further machined and treated according to the requirements.

[0062] The second aspect of the present application also provides a further improved method for improving the low temperature toughness of the fastener, which is different from the above in that: in step S2, the raw material is subjected to composite treatment by using a modulated alloy powder, and then is manufactured into a fastener, and the specific steps are: after the raw material 35CrMoA or 40CrNi2MoA is completely melted, the modulated alloy powder is added, smelted, cast, and machined to obtain the fastener.

[0063] The modulated alloy powder is obtained by coating boron powder with an alloy.

[0064] In the preferred embodiments, the mass ratio of the modulated alloy powder to the raw material is: 0.05-0.24:99.95-99.76.

[0065] In the preferred embodiments, the modulated alloy powder is prepared by the following method:

[0066] S2-1, B powder pretreatment: B powder with a particle size of 2-10 μm is boiled in a NaOH solution with a mass concentration of 5-20% for 5-30 min, then washed with deionized water, soaked in a 30-50 wt% HNO3 solution for 2-10 min, washed with deionized water until neutral, dried, and the pretreated B powder is obtained;

[0067] S2-2, B powder sensitization treatment: 0.5-2 g of SnCl2·2H2O is dissolved in 25-100 mL of ethanol to obtain a sensitization solution; 0.5-2 g of the pretreated B powder prepared in step S1 is added to the sensitization solution, ultrasonically treated for 10-30 min, centrifuged, and the solid product is washed with ethanol and deionized water in sequence to obtain the sensitized B powder;

[0068] S2-3, B powder activation treatment: 0.5-2 g of the sensitized B powder is added to 25-100 mL of a PdCl2 hydrochloric acid solution with a mass fraction of 0.025-0.1%, ultrasonically treated for 10-40 min, centrifuged, and the solid product is washed with ethanol, then vacuum dried at 40-65°C for 4-16 h to obtain the activated B powder;

[0069] S2-4, alloy-coated B powder:

[0070] S2-4-1, nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate, and the activated B powder are mixed to obtain a precursor, and the mass ratio of Ni: Fe: La: Y: B in the precursor is 5.5-7.5: 3.8-6: 1-4: 0.75-3: 1;

[0071] S2-4-2, 2.5-10 g of the precursor is added to 50-200 mL of an ammonia water solution with a mass concentration of 2.5-10%, and then 5-20 g / L of ammonium sulfate, 0.06-0.25 g / L of sodium citrate, and 0.03-0.12 g / L of PdCl2 are added, ultrasonically dispersed for 5-20 min, the obtained mixture is transferred to a reaction kettle, hydrogen is introduced, and the reaction is carried out at 250-100 rpm of stirring and 150-170°C for 0.5-2 h, the hydrogen partial pressure is controlled at 2-3 MPa during the reaction, after the reaction is completed, the product is cooled to below 50-65°C, then degassed, and discharged, centrifuged, washed with ethanol, vacuum dried at 80-100°C for 6-24 h, and ground to obtain the alloy powder.

[0072] In a preferred embodiment, the mass ratio of Ni: Fe: La: Y: B in the precursor is 6: 4.5: 2: 1.5: 1.

[0073] In the application, by adopting the modulated alloy powder to composite process the raw material, adding B, La and Y elements which can be uniformly dispersed in the fastener raw material system, the low temperature toughness of the fastener can be effectively improved; the modulated alloy powder has a core-shell structure, taking B powder as the core and NiFe alloy doped with La and Y as the shell, the B powder is coated by the alloy shell, on the one hand, the problem of poor wettability of B powder with the fastener metal matrix and difficult uniform dispersion can be solved, and at the same time, the uniform dispersion of trace elements La and Y in the fastener metal matrix can be realized (La and Y have large density difference with the steel liquid, and trace La and Y are not easy to be uniformly dispersed in the steel liquid), so that B, La and Y play a synergistic effect in improving low temperature toughness and the like.

[0074] The main preparation mechanism of the modulated alloy powder is as follows: firstly, the micron B powder is pretreated by alkali soaking and acid soaking in sequence, then is sensitized and activated in sequence, and then is alloy coated by the hydrothermal hydrogen reduction method after the activation treatment, in the coating process, nickel nitrate, yttrium nitrate, lanthanum nitrate and iron nitrate are used as raw materials of the alloy coating layer, hydrogen provides a reducing atmosphere, and sodium citrate is used as a complexing agent, which can effectively slow down the grain growth rate, so as to refine the grains and obtain a uniform and dense coating layer; PdCl2 is used as a catalyst to reduce the energy barrier in the reaction process and promote the deposition of the alloy.

[0075] In the alloy coating layer, Ni and Fe are the main coating elements, which can significantly improve the wettability between the modulated alloy powder and the steel matrix, and can make the density of the modulated alloy powder closer to the steel liquid, and further through the action of mechanical entrainment, the modulated alloy powder can be uniformly dispersed in the steel liquid.

[0076] Trace boron has been proved to be able to improve the low temperature toughness of steel, and the action mechanism includes: (1) improving the toughness by grain refinement; (2) grain boundary strengthening, the segregation of boron at the grain boundary can inhibit the segregation of sulfur and phosphorus along the grain and the low temperature brittle fracture caused thereby, and boron can improve the grain boundary bonding force, thereby improving the low temperature toughness (Zhong Hao, Li Zhanwei, Li Wenyin, et al. Effect of microalloying on low temperature impact resistance of alloy cold heading steel SCM440 [C] / / 2014 National Conference on Steel Production Technology. 0 [2025-01-23]. DOI: Conference Article / 5af171f6c095d71bc8c2a071.).

[0077] La and Y can play the role of desulfurization and deoxidation in purifying molten steel, Y can reduce the critical nucleation work, increase the crystallization core, inhibit the growth of coarse grains, refine the as-cast structure of steel, reduce alloy segregation and strengthen the grain boundary; the finer the grain size, the more the grain boundary, the more obvious the effect of hindering dislocation movement, resulting in improved toughness. The mixed addition of La and Y can obtain the effect of synergistic enhancement in improving the toughness of steel.

[0078] The above is the general idea of the present application, and the following provides detailed examples and comparative examples based thereon to further illustrate the present application.

[0079] Example 1

[0080] A method for improving the low-temperature toughness of a fastener, comprising the following steps:

[0081] S1, using 35CrMoA of Jiangyin Xingcheng Special Steel Co., Ltd. as a raw material for manufacturing a fastener, controlling the phosphorus, sulfur and manganese content of the raw material and the macrostructure defects of the raw material; wherein the chemical composition of 35CrMoA meets the requirements of GB / T 3077;

[0082] In this embodiment, the prepared fastener is a bolt, and its specifications include metric M20-M39 and American 0.75"-1.75"; the specific chemical composition is shown in Table 3, and the macrostructure defect condition is shown in Table 4:

[0083] Table 3

[0084]

[0085]

[0086] Table 4

[0087]

[0088] S2, using the raw material to manufacture a fastener through conventional processes such as smelting, casting, forging and machining;

[0089] S3, heat treating the fastener, so that the heat treated fastener meets: hardness HRC 26-30, impact toughness A KV ≥27J at-101℃, lateral expansion value ≥0.38mm;

[0090] The step of heat treatment comprises:

[0091] S3-1, quenching heating: the quenching heating adopts step heating, the first heating uses a 30Kw oven, heating to 300℃-400℃; the second heating uses a 50Kw medium temperature salt bath furnace, the furnace loading is ≤50Kg, the bolt diameter specification made of 35CrMoA is ≤M22 or 7 / 8", the quenching temperature is 850℃-870℃, the quenching temperature of the bolt diameter specification made of 35CrMoA is ≥M24 or 1", the quenching temperature is 830℃-850℃, the quenching holding time is 15-40 minutes, the holding time is calculated according to the bolt section diameter of about 1 minute per millimeter;

[0092] S3-2, cooling: the fastener is cooled with 20# mechanical oil or with a salt water solution containing 6%-10% by mass of sodium chloride; the cooling medium temperature: the salt water solution is ≤40℃, the 10# or 20# mechanical oil is ≤80℃;

[0093] S3-3, tempering: the tempering uses a 35Kw pit furnace, the furnace loading is ≤120Kg, heating to the tempering temperature, the tempering temperature is 590℃-670℃, the holding time is 90-120 minutes, air cooling to room temperature. The obtained product is a fastener blank (bolt), and subsequent further finishing and other treatments are carried out according to the requirements.

[0094] The specific heat treatment process parameters of bolts of various specifications are shown in Table 5:

[0095] Table 5

[0096]

[0097] Performance test:

[0098] The low temperature impact test is carried out on the furnace sample, the sampling position is along the axial direction and as close to the outer surface as possible according to the national standard GB / T 3098.1, and the sample is taken from the axial center line for products less than or equal to 1" according to the American standard A320, and the sample should be taken from the 1 / 2 radius for products greater than 1". In the spirit of strict principles, in this embodiment, the test sample is less than or equal to M24 or 1" from the center of the blank, and greater than M24 or 1" from the 1 / 2 radius, the sample meets the provisions of GB / T 229 or A 370, the test equipment is a PIT452D-2 impact test machine, the low temperature medium is liquid nitrogen plus alcohol, the holding is carried out in a self-made impact test low temperature tank, the test temperature is -101℃, the supercooling degree is -3℃, the holding time is 20 minutes, and the test results are shown in Table 6.

[0099] Table 6

[0100]

[0101] The impact toughness and lateral expansion values in the table are measured in three groups, and the average value is taken as the result.

[0102] Example 2

[0103] A method for improving the low-temperature toughness of fasteners is provided, comprising the following steps:

[0104] S1. 40CrNi2MoA from Jiangyin Xingcheng Special Steel Co., Ltd. is used as the raw material for manufacturing fasteners. The phosphorus, sulfur and manganese content of the raw material and the low magnification structural defects of the raw material are controlled. The chemical composition of 40CrNi2MoA meets the requirements of ASME or ASTM standard A 320L43.

[0105] In this embodiment, the fasteners prepared are bolts, with specifications including metric M20-M39 and US 0.75”-1.75”; the specific chemical composition is shown in Table 7 below, and the low-magnification structural defects are shown in Table 8 below:

[0106] Table 7

[0107]

[0108]

[0109] Table 8

[0110]

[0111] S2. Fasteners are manufactured from raw materials through conventional processes such as smelting, casting, forging, and machining.

[0112] S3. Perform heat treatment on the fasteners to ensure that the heat-treated fasteners meet the following requirements: hardness HRC26~30, and impact toughness A at -101℃. KV ≥27J, lateral expansion value ≥0.38mm; the heat treatment steps include:

[0113] S3-1. Quenching Heating: Quenching heating adopts stepped heating. The first heating uses a 30Kw oven to heat to 300℃~400℃; the second heating uses a 50Kw medium-temperature salt bath furnace with a furnace load ≤50Kg. For 40CrNi2MoA studs with a diameter ≤M22 or 7 / 8”, the quenching temperature is 830℃~850℃; for 40CrNi2MoA studs with a diameter ≥M24 or 1”, the quenching temperature is 810℃~830℃, and the quenching holding time is 15~40 minutes, calculated based on approximately 1 minute per millimeter of stud cross-sectional diameter.

[0114] S3-2 Cooling: Cool the fasteners with 20# machine oil or a brine solution containing 6% to 10% sodium chloride by mass; Cooling medium temperature: brine solution ≤40℃, 10# or 20# machine oil ≤80℃;

[0115] S3-3, tempering: the tempering is carried out in a 35Kw pit furnace, the loading capacity is ≤120Kg, heated to the tempering temperature, the tempering temperature is 590℃-670℃, the holding time is 90-120 minutes, air cooling to room temperature. The obtained product is a fastener blank (bolt), which is further machined and treated according to the needs.

[0116] The specific heat treatment process parameters of bolts of various specifications are shown in Table 9:

[0117] Table 9

[0118]

[0119] Performance test:

[0120] The test method is the same as that of Example 1, and the test results are shown in Table 10.

[0121] Table 10

[0122]

[0123] The impact toughness and lateral expansion values in the table are measured in three groups, and the average value is taken as the result.

[0124] Example 3

[0125] Based on the further improvement on Example 1, the difference of the method for improving the low-temperature toughness of the fastener provided by this embodiment is that: in step S2, the raw material is compounded by using a modulated alloy powder, and then is manufactured into a fastener, and the specific steps are as follows:

[0126] The raw material 35CrMoA is added into an electric melting furnace, melted at 1580℃ for 20min, then the modulated alloy powder is added, and the melting is continued for 5min, LF furnace refining, refining temperature is 1650℃, refining time is 10min, VD vacuum degassing, the obtained steel liquid is cast, the ingot is forged into shape, cooled, machined, and the fastener is obtained. The mass ratio of the modulated alloy powder to the raw material 35CrMoA is 0.1:99.9.

[0127] The modulated alloy powder is prepared by the following method:

[0128] S2-1, B powder pretreatment: the B powder with an average particle size of 5μm is boiled in a 10% NaOH solution for 15min, then washed with deionized water, soaked in a 40wt% HNO3 solution for 5min, then washed with deionized water until neutral, dried, and the pretreated B powder is obtained;

[0129] S2-2, Sensitization treatment: 1 g SnCl2·2H2O was dissolved in 50 mL of ethanol to obtain a sensitization solution; 1 g of the pretreated B powder prepared in step S1 was added to the sensitization solution, and ultrasonic treatment was performed for 15 min; centrifugal separation was performed, and the solid product was washed with ethanol and deionized water in sequence to obtain a sensitized B powder;

[0130] S2-3, Activation treatment: 1 g of the sensitized B powder was added to 50 mL of a 0.05% PdCl2 hydrochloric acid (35 wt% concentrated hydrochloric acid) solution, and ultrasonic treatment was performed for 20 min; centrifugal separation was performed, and the solid product was washed with ethanol and then vacuum dried at 50°C for 8 h to obtain an activated B powder;

[0131] S2-4, Alloy coating was performed on the B powder:

[0132] S2-4-1, nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate and the activated B powder were mixed to obtain a precursor, and the mass ratio of Ni: Fe: La: Y: B in the precursor was 6: 4.5: 2: 1.5: 1;

[0133] S2-4-2, 5 g of the precursor was added to 100 mL of a 5% ammonia water solution, and then 10 g / L of ammonium sulfate, 0.125 g / L of sodium citrate and 0.06 g / L of PdCl2 were added; ultrasonic dispersion was performed for 10 min; the obtained mixture was transferred to a reaction kettle, hydrogen was introduced, and reaction was performed at 500 rpm of stirring, in a hydrogen atmosphere, at 160°C for 1 h; during the reaction, the hydrogen partial pressure was controlled to be 2.5 MPa; after the reaction was completed, the product was cooled to below 60°C, and then the kettle was discharged after being degassed; the product was centrifuged, washed with ethanol, vacuum dried at 90°C for 12 h, and ground to obtain a modulated alloy powder.

[0134] Reference Figure 1 , and the XRD pattern of the modulated alloy powder is shown in FIG. 4.

[0135] As an example, a bolt with a specification of M20xL was prepared in this embodiment, the heat treatment method and sample size were the same as those of Example 1, and the impact toughness test was performed in the same manner as that of Example 1,

[0136] Example 4

[0137] Based on the further improvement in Example 2, the difference between the method for improving the low-temperature toughness of the fastener provided in this embodiment is that a modulated alloy powder is used to composite treat the raw material in step S2, and then the fastener is manufactured, and the specific steps are as follows:

[0138] The raw material 40CrNi2MoA is added into an electric melting furnace, melted at 1600℃ for 25 min, then the alloying powder is added, and the melting is continued for 5 min. The LF furnace is used for refining, the refining temperature is 1650℃, the refining time is 10 min, and the VD vacuum degassing is used. The obtained molten steel is cast, the ingot is forged into a shape, cooled, and machined to obtain the fastener. The mass ratio of the alloying powder to the raw material 40CrNi2MoA is 0.1:99.9.

[0139] The preparation method of the alloying powder is the same as that in Example 3, which is not described here.

[0140] Comparative Example Group A

[0141] The comparative example group is based on Example 3, and provides several examples for comparison, as follows.

[0142] Comparative Example A-1

[0143] The difference from Example 3 is that in this example, the alloying powder is prepared by the following method:

[0144] Under the protection of nitrogen, nickel, yttrium, lanthanum, and iron are mixed uniformly according to the mass ratio of Ni:Fe:La:Y of 6:4.5:2:1.5 to obtain the alloying powder.

[0145] Comparative Example A-2

[0146] The difference from Example 3 is that in this example, the preparation step S2-4 of the alloying powder is specifically:

[0147] S2-4-1, the nickel nitrate, lanthanum nitrate, iron nitrate, and activated B powder are mixed to obtain a precursor, and the mass ratio of Ni:Fe:La:B in the precursor is 6:4.5:2:1;

[0148] S2-4-2, 5g of the precursor is added into 100mL of 5% ammonia water solution, and then 10g / L of ammonium sulfate, 0.125g / L of sodium citrate, and 0.06g / L of PdCl2 are added. The obtained mixture is transferred into a reaction kettle, hydrogen is introduced, and the reaction is carried out at 500rpm stirring, hydrogen atmosphere, and 160℃ for 1h. The hydrogen partial pressure is controlled at 2.5MPa during the reaction. After the reaction is completed, the temperature is cooled to below 60℃, and then the gas is discharged to unload the kettle. The product is centrifuged, washed with ethanol, vacuum dried at 90℃ for 12h, and ground to obtain the alloying powder.

[0149] Comparative Example A-3

[0150] The difference from Example 3 is that in this example, the preparation step S2-4 of the alloying powder is specifically:

[0151] S2-4-1, mix nickel nitrate, yttrium nitrate, iron nitrate and activated B powder to obtain a precursor, the mass ratio of Ni:Fe:Y:B in the precursor is 6:4.5:1.5:1;

[0152] S2-4-2, take 5g of the precursor and add it into 100mL of 5% ammonia water solution, then add 10g / L of ammonium sulfate, 0.125g / L of sodium citrate and 0.06g / L of PdCl2, ultrasonic dispersion for 10min, then transfer the obtained mixture into a reaction kettle, pass in hydrogen, react at 500rpm of stirring, hydrogen atmosphere and 160℃ for 1h, control the hydrogen partial pressure to be 2.5MPa during the reaction, after the reaction, cool down to below 60℃, then release the gas and discharge the kettle, centrifuge the product, wash with ethanol, vacuum dry at 90℃ for 12h, grind to obtain the alloying powder.

[0153] Comparative Example A-4

[0154] The difference from Example 3 is only that in this example, the alloying powder is prepared by the following method:

[0155] Mix nickel, yttrium, lanthanum, iron and B powder according to the mass ratio of Ni:Fe:La:Y:B is 6:4.5:2:1.5:1 under the protection of nitrogen to obtain the alloying powder.

[0156] Comparative Example Group B

[0157] This comparative example group is based on Example 3, and provides several examples for comparison, as follows.

[0158] Comparative Example B-1

[0159] The difference from Example 4 is only that in this example, the alloying powder is prepared by the following method:

[0160] Mix nickel, yttrium, lanthanum and iron according to the mass ratio of Ni:Fe:La:Y is 6:4.5:2:1.5 under the protection of nitrogen to obtain the alloying powder.

[0161] Comparative Example B-2

[0162] The difference from Example 4 is only that in this example, the preparation step S2-4 of the alloying powder is specifically:

[0163] S2-4-1, mix nickel nitrate, yttrium nitrate, iron nitrate and activated B powder to obtain a precursor, the mass ratio of Ni:Fe:Y:B in the precursor is 6:4.5:1.5:1;

[0164] S2-4-2, 5 g of the precursor was taken and added into 100 mL of an ammonia water solution with a mass concentration of 5%, and then 10 g / L of ammonium sulfate, 0.125 g / L of sodium citrate, and 0.06 g / L of PdCl2 were added, and ultrasonic dispersion was performed for 10 min, the obtained mixture was transferred into a reaction kettle, hydrogen was introduced, and reaction was performed at 500 rpm of stirring, in a hydrogen atmosphere, at 160°C for 1 h, the hydrogen partial pressure was controlled to be 2.5 MPa during the reaction, after the reaction was completed, the product was cooled to below 60°C, and then the kettle was discharged after being degassed, the product was centrifuged, washed with ethanol, vacuum dried at 90°C for 12 h, ground, and a modulated alloy powder was obtained.

[0165] Comparative Example B-3

[0166] The difference from Example 4 is only that, in this example, the preparation step S2-4 of the modulated alloy powder is specifically as follows:

[0167] S2-4-1, nickel nitrate, yttrium nitrate, iron nitrate, and activated B powder were mixed to obtain a precursor, and the mass ratio of Ni:Fe:Y:B in the precursor was 6:4.5:1.5:1;

[0168] S2-4-2, 5 g of the precursor was taken and added into 100 mL of an ammonia water solution with a mass concentration of 5%, and then 10 g / L of ammonium sulfate, 0.125 g / L of sodium citrate, and 0.06 g / L of PdCl2 were added, and ultrasonic dispersion was performed for 10 min, the obtained mixture was transferred into a reaction kettle, hydrogen was introduced, and reaction was performed at 500 rpm of stirring, in a hydrogen atmosphere, at 160°C for 1 h, the hydrogen partial pressure was controlled to be 2.5 MPa during the reaction, after the reaction was completed, the product was cooled to below 60°C, and then the kettle was discharged after being degassed, the product was centrifuged, washed with ethanol, vacuum dried at 90°C for 12 h, ground, and a modulated alloy powder was obtained.

[0169] Comparative Example B-4

[0170] The difference from Example 4 is only that, in this example, the modulated alloy powder was prepared by the following method:

[0171] Under the protection of nitrogen, nickel, yttrium, lanthanum, iron, and B powder were uniformly mixed according to a mass ratio of Ni:Fe:La:Y:B of 6:4.5:2:1.5:1 to obtain a modulated alloy powder.

[0172] The impact toughness test results of Examples 3-4 and the comparative examples are as follows in Table 11, in order to facilitate comparison, the impact toughness test results of the specifications M20xL in Examples 1-2 are also incorporated into Table 11, and the details are as follows:

[0173] Table 11

[0174]

[0175]

[0176] According to the test results of the above table:

[0177] It can be seen from the comparison of Example 3 and Example 1 that the low-temperature toughness of the bolt fastener prepared by compounding the raw material 35CrMoA with the modulated alloy powder can be improved.

[0178] It can be seen from the comparison of Example 4 and Example 2 that the low-temperature toughness of the bolt fastener prepared by compounding the raw material 40CrNi2MoA with the modulated alloy powder can be improved.

[0179] The comparison of the results of Comparative Example A-1 and Example 3 can show that for the raw material 35CrMoA, the addition of trace boron powder can improve the low-temperature toughness, and the toughness of Comparative Example A-4 is lower than that of Example 3, which is mainly due to the difficulty of uniform dispersion of trace elements such as boron, yttrium and lanthanum in the modulated alloy powder obtained by direct mixing in the matrix.

[0180] The comparison of the results of Comparative Example A-2, Comparative Example A-3 and Example 3 can show that for the raw material 35CrMoA, the combined addition of yttrium and lanthanum has a significant improvement effect on the low-temperature toughness.

[0181] The comparison of the results of Comparative Example B-1 and Example 4 can show that for the raw material 40CrNi2MoA, the addition of trace boron powder can also improve the low-temperature toughness, and the toughness of Comparative Example B-4 is lower than that of Example 4, which is mainly due to the difficulty of uniform dispersion of trace elements such as boron, yttrium and lanthanum in the modulated alloy powder obtained by direct mixing in the matrix.

[0182] The comparison of the results of Comparative Example B-2, Comparative Example B-3 and Example 4 can show that for the raw material 40CrNi2MoA, the combined addition of yttrium and lanthanum also has a significant improvement effect on the low-temperature toughness.

[0183] Although the embodiments of the present application have been disclosed as above, they are not limited to the application listed in the specification and the embodiments, and can be fully applied to various fields suitable for the present application, and additional modifications can be easily realized by those skilled in the art, and therefore the present application is not limited to specific details, without departing from the general concept defined by the claims and the equivalent scope.

Claims

1. A method of improving the cryogenic toughness of a fastener, comprising: The steps include the following: S1, using alloy structural steel 35CrMoA or 40CrNi2MoA as a raw material for manufacturing fasteners, controlling the phosphorus, sulfur and manganese content of the raw material and the macrostructure defects of the raw material; wherein the chemical composition of 35CrMoA meets the requirements of GB / T 3077, and the chemical composition of 40CrNi2MoA meets the requirements of ASME or ASTM standard A 320 L43; S2, using the raw material to manufacture fasteners; S3, heat treating the fastener so that the fastener after the heat treatment satisfies: hardness HRC 26~30, impact toughness A at -101℃ ≥27J, lateral expansion value ≥0.38mm KV ≥27J, lateral expansion value ≥0.38mm; The steps of heat treatment include: S3-1, quenching heating: quenching heating adopts step heating, the first heating is to 300-400 DEG C, the second heating is to quenching temperature, the quenching temperature is 810-870 DEG C, and the quenching holding time is 15-40 minutes; S3-2, cooling: the fasteners are cooled with oil or with salt water solution; S3-3, tempering: heating to tempering temperature, the tempering temperature is 590-670 DEG C, the holding time is 90-120 minutes, and then air cooling to room temperature; In step S2, the raw material is treated by composite treatment of the alloy powder, and then the fasteners are manufactured, and the specific steps are as follows: after the raw material 35CrMoA or 40CrNi2MoA is completely melted, the alloy powder is added, smelted, cast and machined to obtain the fasteners; The alloy powder is obtained by coating boron powder with an alloy; The mass ratio of the alloy powder to the raw material is 0.05-0.24:99.95-99.76; The alloy powder is prepared by the following method: S2-1, B powder pretreatment; S2-2, B powder sensitization treatment: S2-3, B powder activation treatment: S2-4, coating the activated B powder with an alloy: S2-4-1, mixing nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate and activated B powder to obtain a precursor; S2-4-2, taking the precursor and adding it into an ammonia water solution, then adding ammonium sulfate, sodium citrate and PdCl2, ultrasonic dispersion, transferring the obtained mixture into a reaction kettle, passing hydrogen, reacting under stirring and heating, cooling after the reaction, centrifuging, washing, vacuum drying and grinding the product to obtain the alloy powder.

2. The method of improving cryogenic toughness of a fastener of claim 1, wherein, The steps of heat treatment in step S3 include: S3-1, quenching heating: quenching heating adopts step heating, the first heating is to 300-400 DEG C, the second heating is to quenching temperature, the quenching temperature is 810-870 DEG C, and the quenching holding time is 15-40 minutes; S3-2, cooling: the fasteners are cooled with oil or with salt water solution; S3-3, tempering: heating to tempering temperature, the tempering temperature is 590-670 DEG C, the holding time is 90-120 minutes, and then air cooling to room temperature; In step S2, the raw material is treated by composite treatment of the alloy powder, and then the fasteners are manufactured, and the specific steps are as follows: after the raw material 35CrMoA or 40CrNi2MoA is completely melted, the alloy powder is added, smelted, cast and machined to obtain the fasteners; The alloy powder is obtained by coating boron powder with an alloy; The mass ratio of the alloy powder to the raw material is 0.05-0.24:99.95-99.76; The alloy powder is prepared by the following method: S2-1, B powder pretreatment; S2-2, B powder sensitization treatment: S2-3, B powder activation treatment: S2-4, coating the activated B powder with an alloy: S2-4-1, mixing nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate and activated B powder to obtain a precursor; S2-4-2, taking the precursor and adding it into an ammonia water solution, then adding ammonium sulfate, sodium citrate and PdCl2, ultrasonic dispersion, transferring the obtained mixture into a reaction kettle, passing hydrogen, reacting under stirring and heating, cooling after the reaction, centrifuging, washing, vacuum drying and grinding the product to obtain the alloy powder.

3. The method of improving cryogenic toughness of a fastener of claim 1, wherein, In step S1, the phosphorus content in the raw materials 35CrMoA and 40CrNi2MoA is controlled to be less than or equal to 0.012% by mass, and the sulfur content is controlled to be less than or equal to 0.006% by mass.

4. The method of improving cryogenic toughness of a fastener of claim 1, wherein, In step S1, the manganese content in the raw material 40CrNi2MoA is 0.65% to 0.80% by mass.

5. The method of improving cryogenic toughness of a fastener of claim 1 wherein, The macrostructure defects of the raw materials 35CrMoA and 40CrNi2MoA satisfy that the general porosity and square segregation is less than or equal to 0.5 level, and the center porosity is less than or equal to 1.0 level; the plastic inclusions and the brittle inclusions are both less than or equal to 2.5 level, and the sum of the two is less than or equal to 4.5 level.

6. The method of improving cryogenic toughness of a fastener of claim 1, wherein, The chemical composition of the raw material 35CrMoA, by mass percentage, is: C: 0.32-0.40%, Mn: 0.40-0.70%, Si: 0.17-0.37%, Cr: 0.80-1.10%, Mo: 0.15-0.25%, P: ≤0.012%, S: ≤0.006%, and the balance is Fe and inevitable impurity elements; The chemical composition of the raw material 40CrNi2MoA, by mass percentage, is: C: 0.38-0.43%, Mn: 0.65-0.80%, Si: 0.17-0.37, Cr: 0.70-0.90%, Ni: 1.65-2.00%, Mo: 0.20-0.30%, P: ≤0.012%, S: ≤0.006%, and the balance is Fe and inevitable impurity elements.

7. The method of improving cryogenic toughness of a fastener of claim 6, wherein, The alloy powder is prepared by the following method: S2-1, B powder pretreatment: the B powder with a particle size of 2-10 μm is boiled in a NaOH solution with a mass concentration of 5-20% for 5-30 min, taken out, washed with deionized water, then soaked in a HNO3 solution with a mass fraction of 30-50% for 2-10 min, taken out, washed with deionized water until neutral, and dried to obtain the pretreated B powder; S2-2, B powder sensitization treatment: 0.5-2 g of SnCl2·2H2O is dissolved in 25-100 mL of ethanol to obtain a sensitization solution; 0.5-2 g of the pretreated B powder prepared in step S1 is added into the sensitization solution, ultrasonically treated for 10-30 min, centrifugally separated, and the solid product is washed with ethanol and deionized water in sequence to obtain the sensitized B powder; S2-3, B powder activation treatment: 0.5-2 g of the sensitized B powder is added into 25-100 mL of a PdCl2 hydrochloric acid solution with a mass fraction of 0.025-0.1%, ultrasonically treated for 10-40 min, centrifugally separated, the solid product is washed with ethanol, and then vacuum dried at 40-65 °C for 4-16 h to obtain the activated B powder; S2-4, the alloy is used to coat the B powder: S2-4-1, the nickel nitrate, yttrium nitrate, lanthanum nitrate, iron nitrate and the activated B powder are mixed to obtain a precursor, and the mass ratio of Ni: Fe: La: Y: B in the precursor is 5.5-7.5: 3.8-6: 1-4: 0.75-3: 1; S2-4-2, 2.5-10 g of the precursor is added into 50-200 mL of an ammonia water solution with a mass concentration of 2.5-10%, and then 5-20 g / L of ammonium sulfate, 0.06-0.25 g / L of sodium citrate, and 0.03-0.12 g / L of PdCl2 are added, and ultrasonic dispersion is performed for 5-20 min; the obtained mixture is transferred into a reaction kettle, hydrogen is introduced, and reaction is performed at 100-250 rpm of stirring and 150-170°C for 0.5-2 h, wherein the hydrogen partial pressure is controlled to be 2-3 MPa during the reaction; after the reaction is completed, the product is cooled to below 50-65°C, and then gas is discharged to unload the kettle; the product is centrifuged, washed with ethanol, vacuum dried at 80-100°C for 6-24 h, ground, and a modulated alloy powder is obtained.

8. The method of improving cryogenic toughness of a fastener of claim 7, wherein, The mass ratio of Ni:Fe:La:Y:B in the precursor is 6:4.5:2:1.5:1.

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