RU2420600C1 - Extra thin-wall tube from austenite boron-containing steel for cover of fuel element, and method for its manufacture - Google Patents

Abstract

FIELD: power industry.

SUBSTANCE: tube is made from steel containing components at their following ratio, wt %: carbon 0.05-0.09, silicium 0.3-0.6, manganese 1.0-2.0, sulphur not more than 0.010, phosphorus 0.010-0.025, chrome 15.0-16.5, nickel 18.0-25.0, molybdenum 1.9-2.5, titanium 0.25-0.45, niobium 0.1-0.4, vanadium 0.1-0.15, boron 0.001-0.005, cerium 0.15 is design; ferrum is the rest; at that, at 700°C at single-axis axial tension it has long-term strength of 14.0-15.0 kg/mm at test period of 10000 hours and steady-state creep at stress of 14 kg/mm², which is equal to (1.0-1.1)×10^-4%/h. Tube manufacturing method involves cycles of heating and hot deformation of ingot, tube workpiece and cold deformation of workpiece with further heat treatments and final cold deformation; at that, prior to hot deformation on the ingot and during tube conversion the homogenising annealing is performed at temperature which is by 40-110°C less than temperature of formation of boride eutectic, but higher than the temperature at which boron content in solid solution is not less than 20 ppm (20×10^-4 wt %).

EFFECT: improving long-term strength, reducing creep speed and deformation of covers of fuel elements at irradiation owing to improving swelling resistance.

3 cl, 3 dwg, 2 tbl, 1 ex

Description

The invention relates to nuclear technology, namely to the manufacture of shells of fuel elements of fast neutron reactors from radiation-resistant steel, and can be used as a method in hot and cold redistribution of pipe billets in the manufacture of pipes for core elements.

When working in the active zone of fast reactors, structural materials are exposed to intense radiation damage, which determines the life of a fuel assembly. The use of more radiation-resistant materials will increase the life of the core elements (increase fuel burnup), which, in turn, leads to an improvement in the technical and economic indicators of fast reactors.

Strict requirements are imposed on materials operating in a fast neutron reactor core in terms of resistance to swelling, interaction of the shells of fuel elements with fission products of nuclear fuel, embrittlement during prolonged and intense radiation damage, and corrosion resistance in sodium. These materials are also subject to increased requirements of high ductility, long-term strength, low creep rate at temperatures up to (700-850) ° C (in the area of hot spots on the inner surface of the fuel element shell), good resistance to low-cycle fatigue and thermal shock associated with changes cooling conditions, structural phase stability.

To the greatest extent, these requirements, when used as a structural material for the shells of the fuel elements of fast neutron reactors, correspond to austenitic chromium-nickel steels, which are most widely used in nuclear engineering.

Known austenitic chromium-nickel steel containing boron microadditives in amounts (0.002-0.008) wt.%, Used for the shells of fuel elements, allowing to achieve a burnup of 10% t.a. and damaging doses up to 75-80 displacements per atom [EP No. 0121630, IPC C22C 38/58, 1984], a method for manufacturing a shell is not specified. Steel has the following composition, wt.%:

carbon 0.02-0.08 silicon 0.5-1.0 manganese 1.5-2.5 sulfur 0.004-0.010 phosphorus 0.02-0.08 chromium 12.5-14.5 nickel 14.5-15.5 molybdenum 1.5-2.5 titanium 0.1-0.4 niobium 0.02-0.05 vanadium 0.01-0.05 tantalum 0.005-0.02 boron 0.002-0.008 nitrogen no more than 0,01 cobalt 0.02-0.05 aluminum 0.02-0.05 copper 0.01-0.04 arsenic no more than 0,03 oxygen no more than 0,01 zirconium no more than 0,01 the attitude Ti / C + N = 4-6 the amount of phosphorus, boron and sulfur Σ = P + B + S≤0.03 iron rest.

The disadvantage of steel is the insufficient resistance to swelling according to the criterion of the maximum permissible shape change at damaging doses of more than 90 displacements per atom, which does not allow for deeper burnup of fuel and increase the technical and economic performance of fast neutron reactors.

Also known is austenitic steel containing boron microadditives in amounts of 0.002-0.005 wt.%, Used for shells of fuel elements, which allows to achieve a burnup of 11-11.5% t. and damaging doses up to 92 displacements per atom [RF patent No. 2233906, IPC C22C38 / 58, 2003], a method of manufacturing a shell is not specified.

Steel has the following composition, wt.%:

carbon 0.05-0.08 silicon 0.3-0.6 manganese 1.0-2.0 sulfur no more than 0,012 phosphorus no more than 0,020 chromium 15,5-17,0 nickel 14.5-15.5 molybdenum 1.9-2.5 titanium 0.2-0.5 vanadium 0.1-0.3 boron 0.002-0.005 nitrogen no more than 0,02 cobalt no more than 0,02 aluminum no more than 0.1 magnesium 0.0001-0.005 calcium 0.0005-0.005 copper no more than 0,03 arsenic no more than 0,003 oxygen no more than 0,01 gallium and / or germanium no more than 0,0002 bismuth and / or lead and / or tin no more than 0,001 lanthanum and / or cerium and / or praseodymium, and / or neodymium and / or scandium no more than 0.05; titanium to carbon ratio Ti / C at least 4; phosphorus to boron ratio P / B 3-7; the amount of phosphorus, boron and sulfur Σ = P + B + S≤0.04; iron rest.

The disadvantage of this steel is the insufficient resistance to swelling according to the criterion of the maximum permissible shape change at damaging doses of more than 92 displacements per atom, which does not allow higher burnups of fuel (more than 11.5-12.0%, i.e.).

Also known for the shells of fuel elements of fast reactors austenitic chromium-nickel steel EP172, the chemical composition of which is given in technical specifications TU 14-1-3723-84, wt.%:

chromium 15.0-16.5 nickel 14.5-16.0 molybdenum 2.5-3.0 manganese 0.5-0.9 niobium 0.35-0.90 boron 0.003-0.008 carbon 0.05-0.09 silicon 0.3-0.6 nitrogen 0.02-0.04 phosphorus no more than 0,020 sulfur no more than 0,010 aluminum no more than 0.1 cerium calc. 0.015 iron rest.

The operational experience of the BN - 600 reactor with shells of fuel elements made of EP172 steel showed that its use allows burnout to reach ≈11.5%, i.e. (damaging dose ≈ 85 displacements per atom) without loss of fuel element tightness, but at the same time the shells have a shape change that is more than the maximum permissible. A method of manufacturing a shell is not specified.

Known austenitic steel EK164 for the shells of the fuel elements of fast neutron reactors, allowing to achieve fuel burnup above 11-12.0% t.a. and damaging doses of 95-100 displacements per atom [RF patent No. 2068022, IPC C22C 38/58, 1996].

Steel has the following composition, wt.%:

carbon 0.05-0.09 silicon 0.3-0.6 manganese 1.0-2.0 sulfur no more than 0,010 phosphorus 0.010-0.025 chromium 15.0-16.5 nickel 18.0-25.0 molybdenum 1.9-2.5 titanium 0.25-0.45 niobium 0.1-0.4 vanadium 0.1-0.15 boron 0.001-0.005 cerium 0.15 calculated iron rest provided the amount of phosphorus and boron Σ = P + B≤0.025 the ratio of the amount of titanium, vanadium and niobium to carbon Ti + V + Nb / C = 8-13.

An increase in the swelling resistance of this steel is achieved by the fact that 3 carbide-forming elements (titanium, niobium and vanadium), micro-additives of boron are added to its composition, the nickel content is increased, the phosphorus content is increased and regulated.

The aim of the invention is to increase the characteristics of long-term strength and resistance to shape change (swelling) of the shells of fuel elements from boron-containing chromium-nickel steel EK 164 during burnups of more than 12% and damaging doses of more than 100-110 displacements per atom due to the increase in boron content in the solid solution and its more homogeneous distribution.

The swelling of austenitic chromium-nickel steels is determined by many factors, including the degree of alloying of the solid solution with such elements as boron, carbon, and others, which, in turn, is determined by the temperature of treatment for the solid solution.

Steel EK164, like other boron-containing steels, is prone to dendritic segregation, which accordingly leads to uneven distribution of boron in the steel structure.

A known method of heat treatment of austenitic stainless steels of type 18 Cr - 10 Ni containing boron, which consists in heating to temperatures of at least 1200 ° C in order to convert carbon into solid solution and cooling at a rate that fixes carbon in austenite. Further, tempering is carried out in the temperature range 600-800 ° C, which leads to dispersion hardening of steel [French application No. 2175526, C21D 7/00, 1973].

The disadvantage of this method is that during subsequent tempering, the separation of excess phases occurs, which causes depletion of the solid solution (austenite) in carbon and boron. This, in turn, leads to a decrease in the radiation resistance of austenitic steel and, in particular, to a decrease in the resistance to swelling, accompanied by a change in shape and an increase in the volume of the product during its operation in a fast reactor.

Also known is a method of producing pipes from austenitic stainless steel that does not contain boron, which consists in heating at temperatures of 1050-1300 ° C, cooling at a regulated rate of 0.25 ° C / s and further stepwise deformation at 500 ° C [EPO application No. 1342807, IPC, C21D 8/10, 2003].

Steel has the following composition, wt.%:

carbon 0.03-0.12 silicon 0.1-0.9 manganese 0.1-2.0 chromium 15.0-22.0 nickel 8.0-15.0 titanium 0.002-0.05 niobium 0.3-1.5 aluminum 0.0005-0.03 nitrogen 0.005-0.2 oxygen 0.001-0.008 iron rest

The disadvantage of this method is that it is applicable to steels, which do not contain boron microadditives.

The closest in technical essence is the method of thermomechanical processing of chromium-nickel boron steel EK 164, including heating cycles for hot deformation at temperatures of 1100-1160 ° C for 4-8 hours, hot deformation of the ingot, intermediate workpieces (technological instructions for forging production No. TI 131 -K1-2002 and rolled at mills No. TI 131-P3-01-2000 of OJSC Metallurgical Plant Elektrostal), cold deformation cycles of pipe billets, conversion pipes with subsequent heat treatments at temperatures of 1010 -1060 ° С and a final cold deformation of ~ 20% on the finished size of the extra-thin-walled shell pipe (heat treatment technological instruction No. 25250.00018 of OAO MSZ).

The boron distribution in the structure is not uniform, the boron content in the solid solution of the metal of the billet at a size of ⌀ 65 mm is 5-10 ppm (5 ÷ 10 × 10 ^-4 wt.%) [See autoradiogram of the distribution of boron in figa]. In the subsequent pipe redistribution, including reusable cold deformation and heat treatment for solid solution at temperatures of 1000-1040 ° C, the distribution of boron is not uniform, its content in the solid solution increases slightly and in finished pipes austenitized at 1050-1060 ° C is 10-20 ppm (see autoradiogram of the distribution of boron on figb).

Thus, the specified processing method does not allow uniform distribution of boron in the structure of steel EC 164, significantly increase its content in solid solution, and accordingly increase the characteristics of long-term strength, creep and resistance to swelling.

The aim of the invention is to increase the characteristics of long-term strength and resistance to swelling of the shells of fuel elements of steel EK 164 when irradiated with fast neutrons due to a more homogeneous distribution of boron in the structure of the steel and enrichment of solid solution.

The technical result of the invention is to increase long-term strength, reduce creep rate and reduce the shape change of the shells of fuel elements during radiation exposure by increasing the resistance to swelling while maintaining a high level of manufacturability, weldability and other characteristics.

To achieve a technical result, an extra-thin-walled steel pipe contains components in the following ratio, wt.%: Carbon 0.05-0.09; silicon 0.3-0.6; manganese 1.0-2.0; sulfur not more than 0.010; phosphorus 0.010-0.025; chrome 15.0-16.5; nickel 18.0-25.0; molybdenum 1.9-2.5; titanium 0.25-0.45; niobium 0.1-0.4; vanadium 0.1-0.15; boron 0.001-0.005; cerium 0.15 calculated; the rest is iron, moreover, at 700 ° C under uniaxial axial tension, it has a long-term strength of 14.0-15.0 kg / mm ² at a test time of 10,000 hours and a steady-state creep rate of 14 kg / mm ² equal to (1.0 -1.1) × 10 ^-4 % / h.

To achieve a technical result, a method of obtaining a particularly thin-walled pipe for a shell of a fast-neutron reactor fuel element made of austenitic boron-containing steel containing components in the following ratio, wt.%: Carbon 0.05-0.09; silicon 0.3-0.6; manganese 1.0-2.0; sulfur not more than 0.010; phosphorus 0.010-0.025; chrome 15.0-16.5; nickel 18.0-25.0; molybdenum 1.9-2.5; titanium 0.25-0.45; niobium 0.1-0.4; vanadium 0.1-0.15; boron 0.001-0.005; cerium 0.15 calculated; iron, the rest includes cycles of heating and hot deformation of the ingot, tube billet and cold deformation of the billet with subsequent heat treatments and final cold deformation, and before hot deformation on the ingot and in the process of pipe redistribution, homogenizing annealing is carried out at a temperature of 40-110 ° C lower the temperature of formation of boride eutectic, but higher than the temperature at which the boron content in the solid solution is not less than 20 ppm (20 × 10 ^-4 wt.%.).

In a particular embodiment, the temperature of the homogenizing heating of the ingot and the pipe billet made of austenitic boron-containing steel is set in the range of 1180-1250 ° C.

This goal is achieved by the fact that before carrying out hot deformation at the stage of manufacturing a pipe billet (ingot, billet of intermediate size) and at the stage of manufacturing a particularly thin-walled pipe with reusable cold deformation, homogenization is carried out at temperatures 40-110 ° C below the temperature at which boride eutectic formation begins, but higher temperature at which the boron content in the solid solution is ≤20 ppm.

The essence of the method lies in the fact that the authors found temperature conditions that ensured a uniform distribution of boron and its significant enrichment of a solid solution of austenitic steel EK164 boron steel, necessary to increase the long-term strength and radiation resistance of products operating in the active zone of a fast neutron nuclear reactor .

At annealing temperatures of 1050–1160 ° С, the boron distribution in the structure of steel EK164 is uneven, the line-by-line nature of its distribution is observed, and the boron content in the solid solution is 5-10 ppm (5 ÷ 10 × 10 ^-4 wt.%) Heating of the ingot or tube above 1250 ° C for the specified steel is unacceptable due to the possibility of the formation of boride eutectic along the grain boundaries and the impossibility of further hot deformation (the temperature of formation of boride eutectic is 1300 ° C - see microstructure and autoradiogram of boron distribution on figa and 2b).

Significant differences of the proposed method are that the authors first determined the temperature conditions of processing, which provides a significant increase in the characteristics of long-term strength and resistance to swelling of steel EK164 due to the redistribution of boron in the steel structure and its significant enrichment of the solid solution.

On figa (known method) shows the distribution of boron in the structure of a pipe billet with a size of ⌀ 65 mm from steel EC164 after heating for forging at a temperature of 1160 ° C. The boron content in the solid solution is 8 ppm (8 × 10 ^-4 wt.%).

On figb (known method) shows the distribution of boron in the microstructure austenitized at 1050 ° C especially thin-walled pipes with a size of ⌀6.9 × 0.4 mm The boron content in the solid solution is 18 ppm (18 × 10 ^-4 wt.%).

On figa (the proposed method) shows the distribution of boron in the structure of the tube billet 065 mm from steel EK164 after homogenization of the ingot at a temperature of 1250 ° C for 5 hours and subsequent forging at 1160 ° C. The boron content in the solid solution is 30 ppm (30 × 10 ^-4 wt.%).

On figb (the proposed method) shows the distribution of boron in the microstructure austenitized at 1060 ° C especially thin-walled pipes with a size of 06.9 × 0.4 mm, made of metal ingot and conversion pipe subjected to homogenization. The boron content in the solid solution is 46 ppm (46 × 10 ^-4 wt.%).

On figa shows the formation along the grain boundaries of boride eutectic in steel EC164 at a temperature of 1300 ° C (exposure 5 hours) - microstructure.

On figb shows the distribution of boron along the grain boundaries in steel EK164 at the temperature of formation of boride eutectic (1300 ° C, 5 hours) - track autoradiogram.

An example of a specific implementation of the method

For processing, we took ingots with a diameter of ⌀380 mm from EK164 steel, obtained by the method of vacuum induction smelting followed by vacuum-arc remelting, containing 0.005 wt.% Boron.

The content of the remaining elements was also within the requirements of technical specifications TU 14-1-5380-99.

Ingots with a diameter of ⌀ 380 mm were loaded into a furnace and heated to a temperature of 1200 ° C, held for 8 hours, after which they were chilled to the temperatures of the onset of hot deformation for steel EK164, then forged to a size of 130 mm square, heated at a temperature of 1160 ° C for 8 hours and rolled on a “solid” calibration of 1 unit of the “600” mill to a ⌀95 mm billet with its subsequent continuous grinding. The ground billet of ⌀95 mm was heated and rolled on a 350/250 mill at temperatures of 1100–1160 ° C to the finished tube billet size of ⌀65 mm. Next, a ⌀65 mm pipe billet was deeply drilled to a size of ⌀57 × 8.5 mm, heat treated according to the regime of 1010 ° С in a vacuum furnace, then it was rolled on a KhPT-55 mill to a size of a conversion pipe ⌀42 × 4.8 mm and subjected to homogenizing annealing at 1180 ° C for 3 hours. Then, by cold deformation by rolling in 4 passes with intermediate annealing and final deformation by the method of short drawing, ~ 20% were used to obtain pipes used for cladding of fuel elements of experimental BN-600 fuel assemblies.

The method of track autoradiography was used to determine the distribution of boron in the metal of the pipe billet, as well as in conversion and finished pipes. The amount of boron in the solid solution was determined by the same method. The technique of track autoradiography is described in the work of A.A. Gusakov "Study of the distribution of light elements in metals and alloys by methods of track and activation autoradiography", author. dissertations for the degree of candidate of technical sciences, reg. No. 106-in-chipboard, Moscow, 1980

The results obtained on the distribution of boron and its content in a solid solution in the metal of a tube billet that underwent homogenizing annealing at 1250 ° C for 5 hours, and in the metal of especially thin-walled tubes made from this tube billet using homogenizing annealing of a conversion pipe ⌀ 42 × 4, 8 mm at 1180 ° C for 3 hours, are shown in figure 2A) and b).

For comparison, figure 1a) and b) shows the results obtained on the metal steel EK164, processed by a known method.

Thus, in comparison with the processing according to the prototype method, the processing according to the claimed method allows significantly more homogeneous distribution of boron in steel and ~ 2 times increase its content in solid solution.

The study of the characteristics of long-term strength and creep was carried out on samples of shell pipes ,96.9 × 0.4 mm from steel EK164-ID ch.d. made by the prototype method and the claimed method (see table 1).

The tests were carried out under uniaxial axial tension on standard testing machines AIMA-5-2 in accordance with GOST 3248-81, GOST 10145-81 and the technique 320.586.001 MVIS. The test procedure is described:

- GOST 3248-81 Metals. Creep test method, Standards Publishing, M., 1981, 9 pp .;

- GOST 10145-81 Metals. Creep test method, Standards Publishing, M., 1981, 9 pp .;

- Methodology for creep tests and long-term strength of shell pipes of austenitic and ferritic steels under uniaxial axial tension, 320.586.001 MVis, VNIINM. 2007, 13. The tests were carried out in air at a temperature of 700 ° C and axial tensile stresses of 14-22 kgf / mm ² .

The experimental data shown in Table 1 show that the use of homogenizing annealing in the temperature range of 1180–1250 ° C at the stage of manufacturing a pipe billet and during pipe redistribution increases the long-term strength and resistance to thermal creep. In this case, the shell pipes made by the proposed method have long-term strength limits based on 3000 and 10000 hours 1.2-1.4 times higher, and the rate of thermal creep is an order of magnitude lower than pipes made by the known method.

Thus, the application of the proposed method for thermomechanical treatment of boron-containing steel EK164 in combination with cold deformation by 20% on finished products will increase ~ 2 times the boron content in the solid solution, which leads to an increase in the characteristics of long-term strength and thermal creep.

When tested in reactor conditions, this will lead to a decrease in the amount of swelling, which will increase the service life of the shells of the fast-neutron reactor fuel elements.

Using EP172 steel as an example, the effect of homogenizing annealing on radiation swelling and embrittlement is shown (Table 2).

The study of radiation swelling and embrittlement of steel EP172 was carried out on samples that were processed by the prototype method and the claimed method, after irradiation in the material science assembly of the industrial fast reactor BN-350 with damaging neutron doses of 53-59 an.s. (displacements per atom) at temperatures of 480-490 ° C, corresponding to a maximum in the temperature dependence of swelling (see table 2).

Swelling (increase in volume ΔV / V) was evaluated by hydrostatic weighing by decreasing density or transmission electron microscopy.

Mechanical properties were determined by tensile testing of irradiated samples on a remote tensile testing machine.

The experimental data shown in Table 2 show that preliminary homogenizing annealing at a temperature of 1250 ° C for 0.5 hours reduces the swelling of samples of EP172 steel by ~ 4 times and increases the ductility (δ _о ) by 1.5-2 times.

Thus, the application of the proposed method for the treatment of boron-containing steel EP172 in combination with cold deformation by 20% on finished products will increase ~ 2 times the boron content in the solid solution, while significantly (~ 4 times) reduce its swelling and increase (~ 1.5-2 times) residual ductility, which will increase the service life of the cladding of fuel rods or other elements of the reactor core on fast neutrons.

Tests in the BN-600 reactor EK164 steel as the material of the shells of the fuel elements of the experimental fuel assembly irradiated to a damaging dose of 77 displacements per atom (maximum burnup 10.7%, i.e.), showed that an increase in the diameter of fuel rods with an E164 steel cladding in the cold-deformed state is almost 2 times less than the increase in the diameter of the fuel element with a shell made of steel ChS68 in the cold-deformed state under the same irradiation conditions.

Feasibility from the use of the invention for the materials of the active zones of fast industrial reactors is expressed in increasing fuel burnup.

table 2 Particularly thin-walled austenitic boron-containing steel pipe for fuel cladding and method for its preparation The effect of homogenizing treatment on the swelling and ductility of steel EP172 irradiated in a BN-350 reactor with damaging doses of 53-59 d.s.a. at temperatures of 480-490 ° C Swelling, ΔV / V,% Temperature fur. sv-° C Plasticity δ _about ,% Known method 500 4,5 austenization 1080 ° C, 30 600 3.0 min + cold 3.8 700 8.0 deformation 20% 750 9.5 The proposed method Homogenization 1250 ° C, 30 500 9.0 min + 600 8.0 austenization 1080 ° C, 30 0.7 700 11.5 min + cold deformation 20% 750 13.5

Claims (3)

1. Particularly thin-walled pipe for the shell of a fuel element of a fast neutron reactor made of austenitic boron-containing steel containing components in the following ratio, wt.%: Carbon 0.05-0.09, silicon 0.3-0.6, manganese 1.0 -2.0, sulfur not more than 0.010, phosphorus 0.010-0.025, chromium 15.0-16.5, nickel 18.0-25.0, molybdenum 1.9-2.5, titanium 0.25-0.45 , niobium 0.1-0.4, vanadium 0.1-0.15, boron 0.001-0.005, cerium 0.15 calculated, the rest of iron, having at uniaxial axial tension at 700 ° C the long-term strength is 14.0-15 0 kg / mm ² at 10,000 hours testing time and steady-state creep rate at a stress of 14 kg / mm, equal to (1.0-1.1) × 10 ^-4% / h. 2. A method of obtaining a particularly thin-walled pipe for a fast-neutron reactor fuel element shell made of austenitic boron-containing steel containing components in the following ratio, wt.%: Carbon 0.05-0.09, silicon 0.3-0.6, manganese 1 , 0-2.0, sulfur no more than 0.010, phosphorus 0.010-0.025, chromium 15.0-16.5, nickel 18.0-25.0, molybdenum 1.9-2.5, titanium 0.25-0 45, niobium 0.1-0.4, vanadium 0.1-0.15, boron 0.001-0.005, cerium 0.15 calculated, iron, the rest, including cycles of heating and hot deformation of the ingot, pipe billet and cold deformation of the billet followed by ter moobrabotkami and final cold deformation, the ingot before hot deformation and redistribution during tubular blanks homogenizing annealing is conducted at a temperature 40-110 ° C below the temperature of formation of the eutectic and boride above the temperature at which the boron content of at least 20 million solid solution ^{- 1} (20 · 10 ^-4 wt.%). 3. The method according to claim 2, in which the temperature of the homogenizing heating of the ingot and the pipe billet is set in the range of 1180-1250 ° C.

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