Brittle fracture resistance of reactor pressure vessel steels in the initial state *

The authors investigate the influence of chemical and structural inhomogeneity on the brittle fracture resistance (BFR) of VVER vessel materials in the initial state (without irradiation). It is proposed to replace the brittle fracture resistance assessment using the critical brittleness temperature TC for the BFR assessment using the brittle-viscous transition temperature TT. Consideration was given to calibration charts used for studying the TT dependence on the grain size and heat treatment. A comparison of the TC and TT values in the experimental industrial 15H2NMFA-A steel billets shows that the TC values are significantly lower than the TT values: – at the lower level of conservatism, the difference between TC and TT is 22 °C; – at the upper level of conservatism, this difference is 24 °C. The array data on the critical brittleness temperature and the ductile-to-brittle transition temperature of impact test samples of 15H2NMFAA (for VVER-1000) and 15H2NMFA grade 1 (for VVER-1200) steels were statistically processed. The industrial shell samples were manufactured at the “Energomashspetsstal” plant (Kramatorsk, Ukraine). It was found that, in the metal of VVER-1000 vessel surveillance specimens with the copper content – less than 0.06%, heat treatment has a significant effect on the TT value, which changes from –99 to –28°C; – from 0.07 to 0.12%, heat treatment has a significant effect on the TT value, which changes from –60 to –40°C.

In accordance with the reliable operation requirements, the maximum value of the radiation embrittlement coefficient A F is chosen for benchmark tests of several vessel materials (Fig. 1).
At present, the statistical model of the approximating dose-time dependence (DTD) DT C = A F × F n + dT C as the upper envelope of the data scatter band (corresponding to the upper level of conservatism DT C ) is presented in the RD (RD EO 1.1.2.09.0789-2012 (2012) and RD EO 1.1.2.99.0920-2013(2013)).
The normative conservative margin in defining DT C (dT C = 38 °C) for the base metal of 15H2NMFAA and 15H2NMFA grade 1 steels at a fluence up to 100×10 22 m -2 is several times higher than the difference of normative DT C for these steels not exceeding 10 °C (see Fig. 1), which does not correspond to the assumption that the metal chemical inhomogeneity has a significant effect on dT C (Vishkarev et al. (1980a)).
The BFR of vessel materials, T C = T C0 + DT C , is determined by the non-additive summation of normative guaranteed shifts in the critical brittleness temperature with the normative guaranteed critical brittleness temperature of the metal in the initial state, T C0 , (TU 0893-013-00212179-2003(TU 0893-013-00212179- (2003))) of 15H2NMFA grade 1 steel and characterized by an overestimated conservative margin, dT C (Dub et al. 2016).
In A comparison of the T C and T T values of the experimental metal of industrial 15H2NMFAA steel billets shows that the T C values are significantly lower than the T T values.The critical brittleness temperature, T C , is a normative criterion for determining the BFR which is widely used in calculations; therefore, the application of the T T criterion is of considerable interest.
The critical brittleness temperature, T C , is defined as the temperature of the intersection of the temperature-impact strength curve with the level of determining the transition temperature by the steel yield strength PNAE G-7-008-89 (1987).
The transition temperature, T T , characterizes the steep rise position in the curve of the temperature dependence of the absorbed energy (GOST R ISO 148-1-2013ISO 148-1- (2014))).The test conditions are as specified in (GOST R ISO 148-1-2013ISO 148-1- (2014) ) and DIN EN ISO 148-1-2011(2010)).The temperature dependence of the absorbed energy is established by constructing a smooth curve over individual points.A specific value of the absorbed energy is obtained as a percentage of the value corresponding to the upper area KV, for example, 50%.The main difference between the methods (PNAE G-7-008-89 (1987)) and (GOST R ISO 148-1-2013ISO 148-1- (2014))) consists in the detection levels of the transition temperature, T T , and the critical brittleness temperature, T C , relative to the level of the upper area KV and KCV: for T T , the transition temperature detection level is 50% relative to the level of the upper area KV; -for T C , the transition temperature detection level, for example, is 26.2% relative to the level of the upper area KCV at the yield point of 15H2NMFAA steel = 490 MPа, which is accompanied by a shift to lower temperatures.

Studies of T C and T T in experimental industrial billets made of 15H2NMFAA stell
The experimental-standard VVER-1000 core shell made of experimental-industrial 15H2NMFAA steel billets was manufactured at the "Energomashspetsstal" plant by electric-arc melting with vacuum casting (forging 14308, melting Nos.37356 and 17501); it contained 1.31% Ni, 0.005% P and 0.04% Cu; its impurity content practically corresponded to 15H2NMFA grade 1 steel (Tab.2).
The experiments were aimed at determining the critical brittleness temperature, T C , and estimating errors in determining, T C , depending on the number of samples tested and the test pattern Kazantsev et al. (2015) and GOST 9454-78 (1994).To this end, a statistical analysis of 1120 results of impact-bending tests was carried out (see Tab. 2) (Dub et al. 2011a, Dub et al. 2016, Vishkarev et al. 1980).It was assumed that the lower conservative level of the experimental value is its arithmetic average A and the upper one is the sum A + s ST , where s ST is the standard deviation (Dub et al. (2016), Dub et al. (2011b)).
A comparison of the calculated data confirms that the main difference between the methods is in the different levels of KV and KCV in determining the transition temperature, T T , and the critical brittleness temperature, T C , with the difference T T -T C reaching 22 °C and 24 °C at the lower and upper conservative levels, respectively (Tab.2).
It seems statistically justified to use the transition temperature, T T , to determine the brittle fracture resistance of VVER vessel materials, since T T is located at the center of the normal probability distribution KV, depending on the temperature of impact-bending test samples (Fig. 2).
The dependence of the absorbed energy KV on the temperature of testing industrial 15H2NMFAA steel billets at the lower and upper conservative levels of T T was Table 2. Results of processing array data on impact bending test samples of industrial VVER-1000 core shell billets

Study of T T in the experimental calibration metal of 15H2NMFAA steel of the basic chemical composition
The VVER-1000 and VVER-1200 core shells made of industrial 15H2NMFAA and 15H2NMFA grade 1 steel billets are subjected to the final heat treatment according to the conditions developed in the JSC RPA "CNIIT-MASH" (Markov (2011)).
The weld fusion boundary between the base metal and the weld seam material after applying the anticorrosive cladding is not subjected to the final heat treatment by The obtained billets were heat treated according to the following conditions providing a certain structure and grain size of the metal (GOST 5639-82 (1982)): -the weld seam metal and the base metal of the fusion boundary after technological tempering have a tempered cast structure with a grain size of 90-120 μm (hereinafter "isothermal annealing"); -the metal after standard heat treatment in basic mode (normalizing, quenching, tempering) has an improved structure with a grain size of 40-60 μm (hereinafter "quenching"); -the metal after heat treatment in conditions including quenching combined with isothermal annealing and subsequent tempering has a structure with additional perlite-type phase recrystallization, effectively grinding the inherent austenite grain to a level of 15-25 μm (hereinafter "tempering").
The metal of industrial shells of 15H2NMFA steel (15H2NMFA grade 1 of the basic chemical composition), melting No. 132009, containing 1.16% Ni, 0.0025% P and 0.025% Cu with given structural changes, smelted on a high-quality pure charge, was used for the billets (Dub et al. (2012), Dub et al. (2011b)).
To study the heat treatment effects on the BFR of the local continuous remelting (LCR) laboratory metal, immediately after remelting by welding, the metal is subjected to high tempering, and experimental sets of small-sized impact specimens are manufactured which are notionally named "tempering" or "zero heat treatment level" of the metal.
The final heat treatment of the VVER vessel core shells is determined by the corresponding modes: -"quenching" for 15H2NMFAA steel of VVER-1000 vessels; -"isothermal annealing" for 15H2NMFAA and 15H2N-MFA grade 1 steels of VVER-1200 vessels.
In the course of the study, calibration curves were constructed that reflect the T T dependence on the structure (grain size) corresponding to different heat treatments.To confirm the T T dependence on the grain size for the LCR metal structures of 15H2NMFA grade 1 steel of the basic chemical composition, it is necessary to use the results of testing industrial billets of standard VVER-1200 vessel shells subjected to "isothermal annealing" and the metal of surveillance specimens of industrial VVER-1000 vessel shells subjected to "quenching" (Dub et al. (2011b), Dub et al. (2012)).

Study of T T in the industrial steel billets of VVER vessel shells
The standard upper and lower nozzle shells of 15X2N-MFAA steel and the lower (supporting) and upper core shells of 15H2NMFA grade 1 steel of the VVER-1200 reactor, and the elongated core shell of the VVER-TOI reactor of 15H2NMFA grade 1 steel are manufactured at the "Energomashspetsstal" plant.The vessel equipment components were subjected to "isothermal annealing".Tables 4-6 provide basic information on the shells.

Calibration of industrial billets made of 15H2NMFA grade 1 steel of the basic chemical composition
An increase in the phosphorus content from 0.0025 to 0.0050% in the industrial billets of the VVER-1200 ves-sel standard shells, subjected to "isothermal annealing", is accompanied by a rise of T T to the level corresponding to the grain size of "quenching" mode for 15H2NMFA grade 1 steel of the basic chemical composition of (Fig. 3), which confirms the negative effect of phosphorus on the properties of grain boundaries.
At the same time, the metal of the core supporting shell and elongated shell ingot top end, subjected to "isothermal annealing" and corresponding to the technical standards in (TU 0893-013-00212179-2003(TU 0893-013-00212179- (2003))), are characterized by T T above the level corresponding to the grain size of "quenching" mode for 15H2NMFA grade 1 steel of the basic chemical composition.
This metal, when the VVER-1200 vessel is assembled, is located in the upper part of the core, which is sufficiently far from the radiation source, guaranteeing safe operation.

Calibration of the T T dependence on the metal grain size of 15H2NMFAA steel in VVER-1000 vessel shell surveillance specimens
An increase in the nickel content from 1.00 to 1.35%, P from 0.0025 to 0.0100%, and Cu from 0.025 to 0.12% in the VVER-1000 vessel witness sample billets, subjected to "quenching" mode, is accompanied (see Fig. 3) by:   -a rise of T T above the level corresponding to "tempering" mode for 15H2NMFA grade 1 steel of the basic chemical composition; -a fall of T T below the level corresponding to "quenching" mode for 15H2NMFA grade 1 steel of the basic chemical composition; The effects of the copper content on T T is determined when the metal structure of the VVER-1000 vessel surveillance specimens (""tempering, "quenching" and "isothermal annealing" modes) corresponds to the LCR metal structure of 15H2NMFA grade 1 steel of the basic chemical composition (Fig. 4).
In the metal of VVER-1000 vessel surveillance specimens with the copper content -less than 0.06%, heat treatment has a significant effect on the T T value, which changes from -99 to -28C; -from 0.07 to 1.2%, heat treatment has a significant effect on the T T value, which changes from -60 to -40C.-the T T value is practically independent of the level of conservatism (see Fig. 4).

Conclusion
1.A comparison of the T C and T T values in the experimental metal of industrial 15H2NMFAA steel billets (see Tab. 2) shows that the T C values are significantly lower than the T T values: -at the lower level of conservatism, the difference between T C and T T is 22 °C; -at the upper level of conservatism, this difference is 24 °C.2. Significant differences in the results are related to the determination of T T directly at the inflection point of the normal standard probability distribution and T C at the point located much lower; therefore, it seems statistically reasonable to use the transition temperature, T T , in determining the brittle fracture resistance of VVER vessel materials.
3. The calibration chart reflecting the T T dependence on the structure (grain size) of the LCR metal of 15H2N-MFA grade 1 steel of the basic chemical composition, corresponding to "tempering", "quenching" and "isothermal annealing" modes, should be used for confirming the results of testing standard VVER vessel shells.4.An increase in the phosphorus content from 0.0025 to 0.0050% in the metal of industrial billets of VVER-1200 vessel standard shells, subjected to "isothermal annealing", is accompanied by a rise of T P to the level corresponding to the grain size of "quenching" mode of the LCR metal of 15H2NMFA grade 1 steel of the basic chemical composition (Fig. 3), which confirms the negative effect of phosphorus on the properties of grain boundaries. 5.An increase in the nickel content from 1.00 to 1.35, phosphorus from 0.0025 to 0.0100%, and copper from 0.025 to 0.12% in the VVER-1000 vessel surveillance specimen billets, subjected to "quenching", is accompanied (see Fig. 3) by: -a rise of T T above the level corresponding to "tempering" mode for 15H2NMFA grade 1 steel of the basic chemical composition; -a fall of T T below the level corresponding to "quenching" mode for 15H2NMFA grade 1 steel of the basic chemical composition; 6.In the metal of VVER-1000 vessel surveillance specimens with the copper content -less than 0.06%, heat treatment has a significant effect on the T T value, which changes from −99 to −28 °C; -from 0.07 to 1.2 %, heat treatment has a significant effect on the T T value, which changes from −60 to −40 °C.-the T T value is practically independent of the level of conservatism (see Fig. 4).The work was supported by the Ministry of Education and Science of the Russian Federation under the Grant Agreement No. 14.579.21.0116 (the unique identifier: RFMEFI57915X0116).

Figure 1 .
Figure 1.Standard dose-time dependences ΔT с of 15H2NMFAA and 15H2NMFA grade 1 steels a smooth curve completely corresponding to the normal standard distribution over separate points (Tab.2а) with the inflection point in T T .At the upper conservative level of T T , the smooth curve reflects the spread of points relative to the normal standard distribution (Fig.2b) with the inflection point in T T , taking into account the standard deviation: s ST = 12 °C.

Figure 2 .
Figure 2. Temperature distributions KV in experimental industrial billets made of 15H2NMFAA steel at the level of conservatism: a) upper; b) lower

Table 1 .
Radiation embrittlement coefficients AF and n for the base metal 15H2NMFAA and 15H2NMFA grade 1 irradiated at 290 °C

Table 3 .
Results of determining T T in the welding-remelted laboratory billets of 15H2NMFA grade 1 steel of the basic chemical composition

Table 4 .
Results of determining T T in the VVER-1200 vessel standard nozzle shells made of 15H2NMFAA steel

Table 5 .
Results of determining T T in the VVER-1200 vessel standard core shells made of 15H2NMFA grade 1 steel

Table 6 .
Results of determining T T in the ends of the VVER-1000 vessel standard core elongated shell made of 15H2NMFA grade 1 steel