RU2235791C1 - Method for complex thermal processing of large-sized forged pieces - Google Patents

Abstract

FIELD: thermal processing of large-sized forged pieces and blanks for reactor casings of water-cooled nuclear power plants, petroleum-chemical reactors and other large-sized equipment.

SUBSTANCE: method involves thermal processing of large-sized forged pieces and blanks having weight of up to 250 t from Cr-Mo-V composition steels and steels of said composition with admixtures of Ni in an amount of ~ 2% for preventing development of defects, such as flakes and cracks, and providing for required strength and plasticity level, as well as for reducing level of initial critical fraility temperature T_cf and reducing of critical fraility temperature shift as a result of radiation ΔTf owing to elimination of large-sized and differently sized grains resulting from multiple heating procedures and different forging reduction ratios during plastic deformation; providing fine-grained tempered bainite-martensite structures of homogeneous section.

EFFECT: improved quality of steel structure resulting in increased service life, power and safety level of large-sized equipment manufactured from such steel.

2 dwg, 1 tbl

Description

The invention relates to a method for heat treatment of large-sized (up to ~ 250 t) forgings and blanks of reactor vessels of water-type atomic power plants (WWER), petrochemical reactors and other large-sized equipment made of Cr-Mo-V composition steels, as well as from steels of the specified compositions with Ni additives in an amount up to ~ 2%.

The most dangerous consequence of reactor irradiation as applied to the materials of nuclear power plants is radiation embrittlement, which contributes to the displacement of the critical brittle temperature to higher temperatures, reduces the work of fracture in the viscous zone of the temperature dependence of impact strength, which leads to a decrease in the safe operation time of the nuclear power plant / 1 /. The criterion for assessing the radiation embrittlement of steel is ΔТ _F , defined as the difference between the critical temperatures of the material after irradiation (T _F ) and the material in the initial state (T _KO ).

One of the reserves for increasing the stock of brittle strength of a material is to increase the initial level of resistance to brittle fracture and to reduce the shift of the critical temperature of brittleness as a result of irradiation (ΔТ _F ) due to grain refinement during heat treatment. Grain grinding leads to a weakening of the embrittlement effect of harmful impurities due to a decrease in their grain boundary segregations resulting from radiation-stimulated diffusion.

Known methods of heat treatment do not provide the necessary complex of mechanical and service properties of the material.

The pre-heat treatment modes used at different plants are somewhat different, but they are based on a single principle: creating conditions for the transformation of the austenitic structure into a ferritocarbide mixture with the removal of most of the hydrogen. The most commonly used modes of preliminary heat treatment of forgings are described in / 2, 3, 4 /. Exposure at a temperature below A ₁ when cooling from forging heating (when the diffusion rate in the α phase is greatest) ensures the removal of hydrogen to a level below the critical concentration. The process of coagulation of the carbide phase proceeding with this increases the ductility of the metal, relieves internal stresses and further reduces the hardness of the forgings.

For some especially large forgings (hot rolling rolls, shafts, rotors of turbogenerators and steam turbines, etc.), for the purpose of phase recrystallization of the metal, the heat treatment schedule includes an additional heating stage above A _C3 with cooling the forgings in air or in the furnace. The heat treatment of large forgings of especially critical purpose is sometimes performed with two or three heating levels above A _C3 and, accordingly, with two or three stages of subcooling between them, so that by repeated recrystallization to eliminate the inevitability that occurs during forging and to achieve the most complete grain refinement and increase the ductility of steel . However, for steels prone to structural heredity, it is not always possible to achieve coarse grain elimination in this way.

Final heat treatment of forgings, as a rule, consists of single or double hardening and high tempering / 3 /. The use of double hardening provides a more uniform structure after hardening. However, with high-temperature heating for the first quenching, a significant increase in grain occurs. Re-hardening from a lower temperature does not provide a finer grain due to the phenomenon of structural heredity when heating steels having a bainitic-martensitic structure. At the same time, neither the application of single hardening nor the use of double hardening ensures the absence of coarse-grained and heterogeneous grains resulting from repeated high-temperature heating during forging and varying degrees of bite in individual parts of a large forging.

The closest (prototype) to the claimed method of heat treatment is the currently used mode of heat treatment of large-sized blanks of power engineering from steels of Cr-Mo-V composition, as well as for steels of the specified composition with Ni additives in an amount of up to ~ 2.0% / 5 /. The prototype includes at the stage of preliminary heat treatment a long digging at a temperature of 600-650 ° C followed by slow cooling (with a furnace at a speed of ≤30 ° / hour) to a temperature of 250-300 ° C, heating to a temperature of 910-950 ° C for phase recrystallization, delayed cooling on the extended hearth to 250-300 ° C and high tempering at a temperature of 640-680 ° C, followed by cooling at a speed of ≤40 ° C / s to 400 ° C, at a speed of ≤20 ° C / s to 150 ° C, then in air (Fig. 1, a / 5 /) and at the stage of final heat treatment, double quenching in water from a temperature of 950-1000 ° C (1st hardening) and 920-950 ° C (2nd hardening) and high tempering at a temperature of 640-680 ° C (Fig. 1b / 5 /).

The disadvantage of this mode is that it does not eliminate the heterogeneity and coarse grains resulting from repeated high-temperature heating during forging and varying degrees of bite in some parts of a large forging, and does not provide a uniform cross-section of a fine-grained structure of the processed forgings.

The technical result of the invention is the development of a complex heat treatment technology for large forgings made of steel of the compositions in question, which ensures the prevention of defects (flocs, cracks), obtaining a uniform cross-section of a fine-grained structure of the processed forgings, lowering the initial level of critical brittleness temperature (T _KO ) and reducing the shift of critical temperature brittleness due to irradiation (ΔТ _F ).

The technical result is achieved due to the fact that the digging is carried out at a temperature below A _{C1 of} 20-50 ° C, the first austenitization is carried out at a temperature above A _{C3 of} 130-160 ° C, at the cooling stage, after the first austenitization, a step of isothermal aging is introduced at a temperature below A _C1 at 20-50 ° C for 12-20 hours, high tempering after the first austenitization is carried out at a temperature below A _C1 at 20-50 ° C, the second austenitization is carried out at a temperature above A _C3 at 130-160 ° C, followed by cooling to a temperature below A _C1 to 20-50 ° C and introduced into an intermediate article stump isothermal hold at this temperature for 12-20 hours, a third austenitization is carried out at a temperature above the A _C3 at 110-140 ° C.

The proposed complex mode of preliminary and final heat treatment (Fig. 2) completely solves the tasks posed before the preliminary and final heat treatment, namely, it prevents defects (flocs, cracks), the required level of strength and ductility with a decrease in the initial level of critical brittleness temperature (Т _KО ) and reducing the transition temperature shift as a result of irradiation (? T _F) by obtaining homogeneous over the cross section of the forging fine-grained tempered bainitic-March nsitnoy structure.

At preliminary heat treatment:

- linting (1) is carried out at a temperature of minimum austenite stability (20-50 ° C lower than A _C1 , namely 620-680 ° C) with the aim of the most complete and fast conversion of austenite to a ferritoperlite mixture, which contributes to a more complete removal of hydrogen during conversion A → F + P and preparing the structure for grinding grain;

- after subcooling to a temperature of ~ 250-300 ° C, the forgings are heated to the first austenitization temperature (3), corresponding to the austenite recrystallization temperature (130-160 ° C higher than A _C3 , namely ~ 1000 ° C), at which grain is crushed;

- after the first austenitization at the cooling stage, on the extended hearth, an isothermal exposure step (4) is introduced for 12-20 hours at a temperature of minimum austenite stability (20-50 ° C lower than A _C1 , namely 620-680 ° C) for further degassing hydrogen forgings as a result of γ → α transformation and additional grain refinement as a result of the formation of a ferritoperlite structure inside small recrystallized austenite grains formed at the stage (3);

- high tempering (6) is carried out at a temperature lower than A _C1 by 20-50 ° C (620-680 ° C) to ensure maximum equalization of the hydrogen content over the cross section of the forging, which reliably protects the forgings from the formation of flocs.

In the final heat treatment:

- heating under the second austenitization (7) is carried out to austenite recrystallization temperature (130-160 ° С higher than A _C3 , namely ~ 1000 ° С), as a result of which grain refinement and elimination of the grain size remaining after preliminary heat treatment are eliminated;

- after the second austenitization, an isothermal exposure step (8) is introduced for a period of 12-20 hours at a temperature of minimum austenite stability (20-50 ° C lower than A _C1 , namely 620-680 ° C), at which grain refinement occurs as a result of transformation A → Ф + П; the exposure time at a temperature of minimum austenite stability should be sufficient for the complete transformation A → Ф + П and not cause strong coagulation and stabilization of the carbide phase;

- the third austenitization (9) is carried out at a temperature higher than A _C3 by 110-140 ° C, providing the maximum dissolution of the carbide phase to achieve the required degree of austenite alloying and, as a consequence, the workpiece hardenability in thickness;

- high vacation (10) is carried out at a temperature below A _C1 at 110-120 ° C; it provides the required level of strength and ductility of the base metal and the possibility of carrying out additional technological post-welding holidays and repair holidays.

Grinding grain in a large forging and the absence of heterogeneity leads to a weakening of the embrittlement effect of harmful impurities (in particular, phosphorus) during operation due to a decrease in the concentration of phosphorus at grain boundaries, which leads to a decrease in the shift of the critical brittle temperature due to irradiation (ΔТ _F ). In addition, the energy intensity of intergranular fracture with small grains is much higher than with coarse grains, since the crack propagation path along grain boundaries is associated with a large number of sharp changes in direction absorbing energy, which provides a lower initial value of T _KO / 1 /.

The authors conducted an experimental testing of the proposed method of heat treatment. The study was conducted on metal templates cut from 3 pilot industrial shells with a thickness of 320 mm. The templates were heat treated, fully simulating the heat treatment conditions of industrial shells.

Comparative mechanical properties of steels after processing according to the known and claimed methods of heat treatment are given in the table.

As can be seen from the table, the use of the proposed method of heat treatment of large forgings from the considered steels for NPP housings provides the following advantages compared to the mode used:

- provides a fine-grained homogeneous structure over the entire cross section of the forgings and eliminates the heterogeneity of the cross section and the height of the workpiece (4-6 points over the entire cross section of the forgings compared to 1-4 points using the known method of heat treatment)

- leads to a decrease in the initial level of T _KO on all studied steel grades.

- provides a decrease in the shift of the critical temperature of brittleness as a result of irradiation.

According to the "Methodology for determining the resource of VVER nuclear reactor vessels during the operation of MRK-SHR-2000," a decrease in the initial level of T _KO by 1 ° C increases the resource by 1 year.

Thus, the use of the proposed method for the heat treatment of bulky forgings and blanks of reactor vessels of water-water type nuclear power plants (WWER), petrochemical reactors and other bulky equipment will allow to increase the life of the equipment while increasing its power and safety level.

LITERATURE

1. A.M. Parshin. ″ Radiation damage and performance of structural materials. "- St. Petersburg: Polytechnic, 1997. - 311 p.

2. Sklyuev P.V. "Hydrogen and flocs in large forgings." - M.: Mashgiz, 1963 .-- 187 p.

3. "Metallurgy and heat treatment of steel." Handbook Ed. Bernstein M.L. and Rakhstadt A.G. - M.: Metallurgy, 1983 .-- 214 p.

4. Bashnin Yu.A., Paisov I.V. and others. "Heat treatment of large forgings." - M.: Metallurgy, 1973. - 173 p.

5. Technological instructions for the heat treatment of workpieces and welded (deposited) assemblies and products of power equipment of nuclear power plants and other equipment made of steel grades 15X2NMFA, 15X2NMFAA, 15X2NMFA Cl. 1, 10GN2MFA, 10GN2MFAL and 09N2MFBA-A (TI No. 390.2503030.000) Izhora plants. ”

Claims (1)

The method of complex heat treatment of large forgings, including at the stage of pre-heat treatment of lances at a high tempering temperature, followed by cooling with the furnace to a temperature of 250 ÷ 300 ° C at a speed of ≤30 ° / h, first austenitization, cooling after the first austenitization on an extended hearth to a temperature of 250 ÷ 300 ° С, subsequent high tempering and cooling at a rate of ≤40 ° / h to 400 ° С, then at a speed of ≤20 ° / h to 150 ° С and further in air, and at the stage of final heat treatment, the second and third austenitization, cooling denie in water after the third austenitizing, high-temperature tempering and cooling in air, characterized in that Kopecz carried out at a temperature below A _C1 to 20 ÷ 50 ° C, the first austenitizing is conducted at a temperature above the A _C3 at 130 ÷ 160 ° C in the cooling step after the first austenitization, an isothermal exposure step is introduced at a temperature below A _{C1 of} 20 ÷ 50 ° C for 12 ÷ 20 hours, a high tempering after the first austenitization is carried out at a temperature below A _{C1 of} 20 ÷ 50 ° C, the second austenitization is carried out at a temperature above A _C3 130 ÷ 160 ° C, followed by cooling to those a temperature below A _C1 to 20 ÷ 50 ° C and introduced into an intermediate stage of the isothermal hold at this temperature for 12 ÷ 20 hours, a third austenitization is carried out at a temperature above the A _C3 at 110 ÷ 140 ° C.

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