RU2582626C1 - Method of making welded composite column type sample for testing crack resistance of irradiated metal - Google Patents

Abstract

FIELD: test equipment.

SUBSTANCE: invention relates to methods for testing of metals on crack resistance, in particular to production of welded composite sample of CT type for testing metal irradiated crack resistance of standard techniques. Holder is made from unirradiated metal and insert from irradiated metal fragment previously tested witness sample for housings of VVER. At first stage insert is made. At second stage metal for manufacture of a shell is selected, to do so yield point of insert irradiated metal is defined and by diagram "yield point of metal insert-yield point of metal cartridge" metal yield point is determined and metal cartridge elements are made from selected metal. With help of electron-beam or laser welding is performed by welding in certain sequence of separate elements cartridge to insert. First, welded front element holder, then welded side elements and then last weld seam welded rear element holder. At that, condition are made that temperature in centre of irradiated metal insert during welding does not exceed temperature of radiation. Then rear element of cartridge is cut before insertion and then after cyclic loading and fatigue crack growth to middle of insert. Then test sample is tested for crack resistance of composite welded by standard technique.

EFFECT: higher reliability of test results on crack resistance of irradiated metal by testing of the proposed welded composite CT type sample due to reduction of residual welding strains with preservation of properties of irradiated metal.

2 cl, 2 dwg, 3 tbl

Description

The invention relates to methods for testing metals for crack resistance [1-4], in particular, to a method for manufacturing a welded composite specimen of type ST for testing the crack resistance of irradiated metal, and can be used in research organizations to determine the properties of metal in exploited water-hydrous energy buildings Reactors (VVER).

To reliably assess the current state and determine the life of VVER hulls, it is necessary to obtain direct experimental data on the fracture toughness of metal in reactor vessels, which are directly used in calculating the resistance to brittle fracture of reactor vessels. At present, most of the data on the fracture toughness of metal in the hulls of reactors of operating WWERs have been obtained in tests of small-sized (10 × 10 mm cross-section) Charpy-type witness specimens with a crack. It is known that the crack resistance determined on small samples of this type is overestimated in comparison with the crack resistance determined on samples of the ST type, which can lead to inadequate evaluations when calculating the resistance to brittle fracture of reactor vessels. In addition, the number of witness samples for existing VVER is quite limited, which may lead to additional errors in determining the fracture toughness of the metal.

A known method of manufacturing a welded composite specimen of type ST, described in [5] and pleasant for our prototype, the essence of which is that first an insert is made from the wreckage of a previously tested irradiated witness specimen, then a ferrule is made of non-irradiated metal in shape and size, corresponding to the standard CT model, then a through socket is cut in the center of the cage according to the dimensions corresponding to the size of the insert, after that the insert is welded to the cage along the perimeter using electron beam or laser welding, then an incision is cut and two holes are made symmetrically for the grips of the test machine, then, by applying an alternating load to the specimen, the fatigue crack is grown to the middle of the insert, followed by testing of the obtained specimen in accordance with the standard crack resistance test procedure.

A disadvantage of the known method of manufacturing the sample is that during welding of the insert to the rigid structure of the cage, tensile residual welding stresses arise in the metal of the insert, which subsequently leads to a significant underestimation of the determined fracture toughness of the test metal of the insert when tested for crack resistance.

The technical result of the invention is to increase the reliability of the test results for crack resistance of irradiated metal by testing the proposed welded composite specimen type ST by reducing residual welding stresses while maintaining the properties of the irradiated metal.

The technical result is achieved due to the fact that in the proposed method for manufacturing a welded composite specimen of type ST for testing the crack resistance of irradiated metal, which includes fabricating an insert from an irradiated metal a fragment of a previously tested witness specimen and a cage of unirradiated metal, creating a through socket in size in the cage, the appropriate dimensions of the insert, placement of the insert in it and welding by electron beam or laser welding insert to the holder, cutting on the holder of the notch sample, the holes in the grip of the test machine are arranged symmetrically with respect to the notch; then, by applying an alternating load to the specimen, the fatigue crack is grown to the middle of the insert, followed by the crack resistance test of the manufactured specimen in accordance with the standard procedure, according to the invention, the yield strength of the irradiated insert metal is preliminarily determined and then on the diagram "yield strength of the metal of the insert - yield strength of the metal of the cage" choose metal for the clip, and the clip itself is made of a composite consisting of individual elements that are sequentially welded to the irradiated insert, first the front clip element is welded to it, then both side clips of the clip are welded to the insert, and then the back clip element is welded to the last weld, in this case, conditions are created so that the temperature in the center of the insert of the irradiated metal during welding does not exceed the irradiation temperature, then the rear element of the clip is cut before insertion, then after a cycle nical loading and fatigue crack growing to mid insert further test composite sample weld crack resistance is carried out by standard methods.

The proposed method for manufacturing a welded composite specimen of type CT using electron beam or laser welding and the proposed sequence of welds shown in FIG. 1 provides the following.

1. The values of the residual welding stresses and their gradient in the center of the insert are practically zero and, as a result, the absence of their influence on the value of crack resistance, determined during testing of the proposed welded composite sample. This result is a consequence of the fact that in the proposed sample, the clip is not integral, as in the prototype sample, but composite, and the welding of individual elements of the clip with an insert is carried out using straight-line seams separately and sequentially one after another. This leads to the fact that when welding one weld, the parts to be welded have free shrinkage compared to the prototype sample, in which welding is performed in a rigid circuit, as a result of which significant tensile residual stresses appear. In addition, the proposed weld sequence shown in FIG. 1, leads to the fact that, as a result of summing the alternating longitudinal and transverse residual welding stresses from each weld, the resulting residual welding stresses in the center of the insert are practically zero.

2. In the manufacture of a sample using welding, it is necessary to ensure conditions under which the temperature of the metal in the center of the insert during welding does not exceed the irradiation temperature. This temperature limitation is necessary in order to avoid "annealing" of radiation defects in the irradiated metal in the center of the insert, since upon overheating of the metal during welding, the determined crack resistance on the welded composite samples will be overestimated due to the restoration of the properties of the irradiated metal. The use in the proposed method of high-speed electron beam or laser welding for welding seams with complete cooling after each seam is performed allows the temperature of the metal in the center of the insert during welding to not exceed the irradiation temperature.

будет значительно ниже, чем предел текучести металла вставки $${\sigma }_{\mathrm{0,2}}^{вставки}$$

, то при нагружении образца деформироваться будет в основном обойма. В этом случае значение определяемой трещиностойкости K_JC будет завышено из-за завышенного значения перемещений по линии действия нагрузки. В противоположном случае, когда $${\sigma }_{\mathrm{0,2}}^{обоймы}>{\sigma }_{\mathrm{0,2}}^{вставки},$$

трещиностойкость K_JC будет занижена. В предлагаемом способе изготовления сварного составного образца выбор значения предела текучести металла обоймы выполняется по диаграмме «предел текучести металла вставки - предел текучести металла обоймы», представленной в фиг. 2. Данная диаграмма была получена авторами на основе трехмерных численных расчетов напряженно-деформированного состояния методом конечных элементов для стандартного и сварного составного образца типа СТ. Построение диаграммы осуществлялось путем определения таких сочетаний значений пределов текучести металла вставки и обоймы, которые обеспечивают идентичность напряженно-деформированного состояния рассмотренных образцов. Поэтому значение предела текучести металла обоймы, определенное по этой диаграмме, обеспечивает одинаковое деформирование металла вставки у вершины трещины предлагаемого сварного составного СТ образца и стандартного цельного образца типа СТ, изготовленного целиком из металла вставки, что приводит к практически одинаковым результатам при испытаниях на трещиностойкость этих образцов.3. An important factor affecting the determined fracture toughness of the insert metal is the mechanical properties of the ferrule metal. In fact, if the yield strength of metal clips $${\sigma }_{0.2}^{\mathrm{about}b\mathrm{about}\mathrm{th}ms}$$

will be significantly lower than the yield strength of the metal insert $${\sigma }_{0.2}^{\mathrm{at}\mathrm{from}t\mathrm{but}\mathrm{at}\mathrm{to}\mathrm{and}}$$

, then when loading the sample will be deformed mainly clip. In this case, the value of the determined crack resistance K _JC will be overestimated due to the overestimated value of displacements along the load line. In the opposite case, when $${\sigma }_{0.2}^{\mathrm{about}b\mathrm{about}\mathrm{th}ms}>{\sigma }_{0.2}^{\mathrm{at}\mathrm{from}t\mathrm{but}\mathrm{at}\mathrm{to}\mathrm{and}},$$

crack resistance K _JC will be underestimated. In the proposed method for manufacturing a welded composite sample, the choice of the yield strength of the metal of the casing is carried out according to the diagram "The yield strength of the metal of the insert - the yield strength of the metal of the casing", presented in FIG. 2. This diagram was obtained by the authors on the basis of three-dimensional numerical calculations of the stress-strain state by the finite element method for a standard and welded composite sample of type ST. The construction of the diagram was carried out by determining such combinations of the yield strengths of the insert metal and the cage that ensure the identity of the stress-strain state of the considered samples. Therefore, the value of the yield strength of the metal of the cage, determined from this diagram, provides the same deformation of the insert metal at the crack tip of the proposed welded composite CT specimen and a standard solid CT type specimen made entirely of insert metal, which leads to almost identical results when tested for crack resistance of these specimens .

4. The geometric dimensions of the proposed welded composite CT specimen correspond to the dimensions of standard CT specimens and the experimental determination of fracture toughness is carried out according to standard methods of fracture toughness testing.

5. In the proposed method for manufacturing a welded composite specimen of type ST, it is recommended to use inserts with a typical size - L × 10 mm, where L = 10 ÷ 20 mm. These dimensions are determined by the size of the fragments of Charpy cracked specimens from which the inserts are made. The steps for manufacturing a welded composite specimen of type CT with an insert of L × 10 mm are shown in FIG. one.

6. The application of the proposed method for the manufacture of welded composite samples of type ST will allow three times to increase the amount of experimental data for the investigated irradiated metal of the reactor vessel, since from the fragments of one previously tested Charpy-type witness specimen with a crack, two samples of type ST can be additionally made.

The experimental verification of the proposed method for manufacturing a welded composite CT specimen was performed by comparing the value of the reference fragility temperature, T ₀ , calculated on the basis of experimental data on crack resistance obtained using welded composite CT specimens and solid standard CT specimens. As the studied materials, we used the metal of VVER-type reactor vessels in various states: in the initial (unirradiated) and irradiated. As non-irradiated metal, 15Kh2NMFA-A steel was used, from which VVER-1000 reactor vessels are made. The chemical composition of the investigated metal steel grade 15X2NMFA-A are presented in table 1.

As the irradiated metal, the main metal (25Kh3NM grade steel) of the WWER reactor vessel, which has been in operation for 30 years, was used. The chemical composition of the investigated metal steel grade 25X3HM are presented in table 2.

From non-irradiated metal of grade 15X2NMFA-A steel and irradiated metal of grade 25X3NM steel, integral standard specimens of the ST type were made. The manufacture of solid standard CT samples was performed according to ASTM E 399 [2]. 12 one-piece standard ST samples were manufactured from steel grade 15X2NMFA-A and 11 from steel grade 25X3NM.

The manufacture of welded composite samples of type ST was performed according to the above method of manufacturing welded composite samples of type ST. Initially, 3 smooth cylindrical specimens (diameter 3 mm, length of the working part 15 mm) were made and tested for tension to determine the yield strengths of the investigated materials. Fabrication and tensile tests were carried out according to GOST 1497-84 [6]. The average yield strengths of the investigated materials at 20 ° C, determined by tensile tests, were: for non-irradiated steel grade 15X2NMFA-A σ _0.2 = 550 MPa, for irradiated metal steel grade 25X3NM σ _0.2 = 596 MPa. Given the obtained values of yield strengths, according to the diagram in FIG. 2, the acceptable values of yield strengths for the metal of the cage were determined: σ _0.2 ± Δσ = 530 ± 165 MPa for unirradiated metal and σ _0.2 ± Δσ = 580 ± 170 MPa for irradiated metal. Given these values, 15Kh2NMFA-A case reactor steel with a yield strength of 550 MPa was chosen as the metal for the manufacture of the holder for the sample with both irradiated and non-irradiated inserts.

From the selected materials, cage elements and inserts were made with dimensions ensuring the production of a welded composite specimen ST with dimensions of a standard specimen ST - 30 × 31 mm with a thickness of B = 10 mm. Inserts of two typical sizes L × 10 mm were made: with insert lengths L = 10 mm and L = 20 mm. Inserts of irradiated material were made from halves of previously tested Charpy impact bending samples. Welding of the insert and ferrule elements was carried out according to the method for manufacturing composite welded samples described above. First, using the electron beam welding, the front element of the composite holder was welded to the insert, then two side elements of the holder were alternately and then the rear element of the holder was welded. During the welding process, it was controlled that the temperature of the metal in the center of the insert did not exceed the irradiation temperature of 270 ° C. After welding of the rear cage element, an incision was made in it prior to insertion and two holes were drilled symmetrically for the grips of the testing machine. A fatigue crack was grown from the top of the notch by applying alternating loads. The initial fatigue cracks were grown on standard and welded composite samples on a Rumul Mikrotron resonance testing machine (Switzerland) with a power of 5 kN in the automatic mode of monitoring the load values, the number of cycles, and the crack length.

According to the method described above, welded composite CT samples were made: from steel grade 15X2NMFA-A with an insert L = 20 mm - 12 pcs., From an insert L = 10 mm - 8 pcs., From steel grade 25X3NM with an insert L = 20 mm - 12 pcs., With an insert L = 10 mm - 8 pcs.

Tests for crack resistance, determination of the values of K _JC and the reference temperature of fragility, T _o , was carried out according to [4]. The crack resistance tests of type ST samples were performed on a Zwick / Roell Z100 universal testing machine with a capacity of 100 kN (Germany). The control modules of the plants are equipped with personal computers (PCs) and the corresponding software that allows for work in the automated control mode. To create the required test temperature, the installation is equipped with a thermocryocamera, which is installed on the loading frame of the testing machine. Temperature control was carried out using calibrated chromel-alumel type thermocouples and a digital thermometer. Samples were tested in the mode of control of displacement along the axis of loading. In this case, a loading diagram was recorded in coordinates "displacement - load" in digital form on a PC. The loading was brought up to the moment of discontinuous destruction of the sample. The displacement was recorded by the crack opening sensor. As a crack opening sensor, a SANDNER high temperature transducer was used. Processing of test diagrams and calculation of the characteristics of K _JC and T _about was carried out according to the formulas of the document [4].

The values of the reference temperature of brittleness, T _about , obtained on the basis of processing the results of fracture toughness tests K _{JC of} standard solid CT samples and the proposed welded composite CT samples of steel grades 15X2NMFA-A and 25X3NM, are presented in Table 3.

As can be seen from table 3, for steel grade 15X2NMFA-A, the value of the reference temperature of brittleness, Т _о , for standard solid samples is minus 124 ° С, and for welded composite samples it is minus 139 ° С (inset 10 × 10) and minus 125 ° С (insert 20 × 10). The maximum difference in the parameter, T _o , for these samples is 12%. For steel grade 25KH3NM, the value of the reference embrittlement temperature, Т _о , for standard solid samples is minus 84 ° С, and for welded composite samples it is minus 86 ° С (insert 10 × 10) and minus 94 ° С (insert 20 × 10). The maximum difference in the parameter, T _o , for these samples is 12%. The obtained close values of the reference embrittlement temperature, T _o , indicate good agreement between the experimental data obtained using the proposed welded composite samples of the CT type with respect to the experimental data obtained with standard solid CT samples.

Technical and economic results compared with the prototype:

The proposed method for manufacturing a welded composite specimen of type ST ensures that the residual welding stresses do not affect the crack resistance while maintaining the properties of the irradiated metal, which improves the reliability of the test results for the crack resistance of the irradiated metal.

The introduction of this method of manufacturing a welded composite specimen of type ST using metal fragments of previously tested irradiated witness samples is necessary to directly determine the fracture toughness of the metal of the VVER-type reactor vessels in operation, while the amount of experimental data obtained as a result of testing the proposed samples using fragments of witness samples, tripled. The application of this approach will increase the safety of operation and increase the service life of VVER reactor vessels, which will lead to a reduction in the cost of the energy generated by these reactors.

Information sources

1. GOST 25.506-85 Methods of mechanical testing of metals. Determination of the characteristics of crack resistance (fracture toughness) under static loading. USSR State Committee for Standards, Moscow, 1985.

2. ASTM E 399-09 Standard Test Method for Plane-Strain Fracture Toughness of Metallic Materials. Annual Book of ASTM Standard. Vol. 01/03.

3. ASTM E 1921-10 Standard Test Method for Determination of Reference Temperature, T _o , for Ferritic Steels in the Transition Range. Annual Book of ASTM Standard. Vol. 01/03.

4. RD EO 1.1.2.09.0789-2012 Methodology for determining the fracture toughness from the test results of witness samples to calculate the strength and resource of VVER-1000 reactor vessels. Rosenergoatom Concern OJSC, Moscow, 2012.

5. van Walle E. Reconstruction: where do we stand? // Effects of Radiation on Material: 17 ^th International Symposium. ASTM STP 1270, 1996.

6. GOST 1497-84 Metals. Tensile test methods. USSR State Committee for Standards, Moscow, 1984.

Claims (2)

1. A method of manufacturing a welded composite specimen of type ST for testing the crack resistance of irradiated metal, comprising making an insert from an irradiated metal, a fragment of a previously tested witness specimen and a cage of non-irradiated metal, creating a through socket in the cage with the dimensions corresponding to the size of the insert, placing the insert in it and using electron beam or laser welding, the insert is welded to the ferrule, the notch is cut on the ferrule, the holes in the ferrule are made for the grips of the testing machine, p positioned symmetrically relative to the notch, then by applying an alternating load to the specimen, a fatigue crack is grown to the middle of the insert, followed by a crack test of the fabricated specimen in accordance with the standard method, characterized in that the yield strength of the irradiated metal of the insert is preliminarily determined and then the yield strength diagram is used metal insert - yield strength of the metal of the cage "choose the metal for the cage, and the cage itself is made composite, consisting of individual elements that are sequentially welded to the irradiated insert, first the front ferrule is welded to it, then both side ferrules are successively welded to the insert, and then the last ferrule is welded with the last weld, thereby creating conditions so that the temperature in the center of the insert the irradiated metal during welding did not exceed the irradiation temperature, then the rear element of the clip is cut before insertion, then after cyclic loading and growing the fatigue crack to the middle us insert the subsequent trial weld crack resistance of the composite sample is performed by standard methods. 2. The method according to p. 1, characterized in that the last weld is performed in two stages, while welding starts from an incision and is carried out in different directions.

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