Critical Brittleness Temperature Research Articles

Investigation of the Effect of Ageing Temperature on a Set of Steel 15NiCuMoNb5 (WB36) Service Properties. Part III. Effect of Thermal Aging on Mechanical Properties and Critical Brittleness Temperature

Investigation of the Effect of Ageing Temperature on a Set of Steel 15NiCuMoNb5 (WB36) Service Properties. Part III. Effect of Thermal Aging on Mechanical Properties and Critical Brittleness Temperature

A Physically-Based Criterion for Determining the Critical Brittleness Temperature from Charpy Impact PRV Tests for Reactor Steels and Their Welds

A Physically-Based Criterion for Determining the Critical Brittleness Temperature from Charpy Impact PRV Tests for Reactor Steels and Their Welds

Thermal resistance of steels with increased strength properties for pressure vessels of advanced VVER reactors of various designs

The paper considers the results of structural studies and mechanical tests after a long-term thermal exposure of laboratory heats of the metallurgically improved 15Kh2NMFA steel and steel with an increased content of nickel considered as materials for the pressure vessels of advanced VVER-type reactors of various designs. It has been shown that, both for the improved 15Kh2NMFA steel and the high-nickel steel, there are no signs of grain boundary embrittlement after an segregation provoking embrittlement heat treatment. This is explained by the extremely low grain boundary segregation of phosphorus in the initial state caused by a high degree of the structure dispersity as well as by rather a low content of impurities. Besides, no changes have been found in the yield strength value for the improved 15Kh2NMFA steel, which agrees with the structure investigation results. For the high-nickel steel, a tendency towards a minor yield strength decrease by 5 to 10% and a regular reduction of the critical brittleness temperature has been revealed. A decrease in the mechanical properties has been caused by a relatively low temperature of tempering for the high-nickel steel and, accordingly, by the potential occurrence of the structure recovery during long-term thermal exposure, as evidenced by the results of an X-ray diffraction analysis. Despite the structure recovery in the high-nickel steel under the long-term thermal exposure, the main strengthening carbide phases remain stable. Due to this, the yield strength value remains at a relatively high level that exceeds the values for the modern VVER-type vessel steels, even in the case of a thermal exposure much in excess of the expected operating conditions for advanced VVER reactors. The observed decrease of critical brittleness temperature during the long-term thermal exposure contributes to an increase in the steel resistance to brittle fracture.

Study of mechanical properties and brittle fracture resistance for weld metal of WWER RPV

The results of the study of mechanical properties and brittle fracture resistance (BFR) are presented for weld metal of WWER RPV performed by automatic arc welding with use welding wire Sv-15CrNiMoTiA and ceramic flux 48AF-71. Mechanical properties are determined on the basis of test results of tensile smooth round bar. BFR are determined from impact strength tests and fracture toughness tests. The anisotropy of mechanical properties and BFR is investigated by testing the specimens with different orientations. Tests are conducted for specimens of two orientations: first orientation corresponds to the position of the specimen, in which the fracture surface is perpendicular to the axis of the weld; second orientation corresponds fracture surface parallel to the axis of t he weld. It i s shown that the weld metal performed according to above mentioned technology has no anisotropy both in mechanical properties and in BFR. An explanation of the significant scatter of BFR on the basis of the results of metallographic studies is proposed. The obtained experimental results on mechanical properties for investigated weld metal allow to use tensile smooth round bar with 3 mm diameter with transverse orientation instead of specimens with 6 mm diameter with longitudinal orientation as the scale factor and anisotropy are negligible. The correlation dependence between the values of reference temperature T0 determined by the Master Curve method and reference temperature T100 determined by the Advanced Unified Curve method and the value of critical brittleness temperature TK0 for the studied weld metal in the initial state is established.

Structural features ensuring the increase of service characteristics of high-nickel steels for pressure vessels of prospective energy-generation reactors

Complex structure studies and mechanical tests of low carbon Ni–Cr–Mo–V steels with low and high nickel content (low-Ni and high-Ni steels) were performed.Mechanical tests showed that high-Ni steels in initial state are characterized by higher strength properties and lower values of critical temperature of brittleness in comparison with low-Ni steels.Estimation of input of various structural elements in ensuring yield strength and brittle fracture stress (which is connected with critical temperature of brittleness) was performed. It was shown that increased strength characteristics of high-Ni steels are determined mostly by higher density of nanoscale carbide precipitations and dislocations. It was also shown that lower critical temperature of brittleness of high-Ni steels is associated with lower width of substructural blocks and, consequently, with higher number of barriers to brittle transcrystalline crack propagation.

Influence of macrostructure and phase composition on performance characteristics of welded joints of WWER type reactor vessels

The influence of the grain structure and phase composition of welded seams of WWER-type reactors on their operational characteristics in the initial state is analyzed. Comprehensive studies of welds with various structures formed as a result of welding using various technologies have been carried out. It is shown that differences in the phase composition and morphology of the grain structure of the weld metal in the initial state, which is characterized by an increased proportion of columnar grains and a higher density of carbide phases, lead to an increase in the yield strength and critical brittleness temperature of welded joints.

Study of the metal of the irradiated weld of the WWER-440 reactor body after 45 years of operation

This work presents the results of a study of the mechanical characteristics and chemical composition of metal samples cut from a non-irradiated weld seam of a WWER-440 reactor vessel after 45 years of operation. The calculated distribution of the critical temperature of brittleness over the thickness of the irradiated weld seam of the WWER-440 reactor vessel (140 mm) is obtained taking into account the distribution of the initial properties, the content of phosphorus and copper, and the density of the fast neutron flux over the thickness of the seam. Since all the circumferential welds connecting the shells in the WWER-440 reactor vessel are manufactured using the same technology, the results of the study of the non-irradiated weld can be used to assess the distribution of properties in the irradiated weld. At the same time, it is assumed that the effect of thermal aging at a temperature of 270°C (operating temperature of a non-irradiated weld) is small and can be neglected.

О методах оценки сопротивления сталей хрупкому разрушению

The problem of assessing steels durability, their resistance to brittle fracture in the structures used in various branches of industry, remains relevant at the development of the new equipment and control of the old equipment. Standard strength calculations do not give a complete picture of the ability of steels due to their physical and mechanical properties to resist the formation and further development of defects and cracks under the action of actual stresses. The correct choice of a structural material for technical devices operating at low temperatures, knowledge of its characteristics of resistance to brittle fracture, understanding of brittle fracture mechanism at low temperatures and research in this area are of great importance. Since 1980s, JSC IrkutskNIIkhimmash systematically conducts the research works related to impact strength characteristics. These works are carried out in connection with the need for assessing the resistance to brittle fracture of metal of the equipment for storing oil products, vessels and pipelines, drill pipes and tubing, etc. As a result of many years of research, the authors accumulated significant experimental material in the form of temperature dependences of impact strength - both for the material of various types of blanks (sheets, pipes, forgings) in the state of delivery, and for the metal of technical devices elements after their operation of various duration. The main objective of the article is to assist specialists in establishing the correct choice of steel for the new or old structures in terms of their resistance to brittle fracture without conducting special studies. Specialists can select the closest steel analogue (on chemical composition, mechanical properties, and the type of product without making cuts from the structure or blanks for additional research), and set the numerical value of the impact strength, critical temperature of brittleness and the stress intensity factor for any temperature in the range from –40 to 20° С in order to use them in further calculations.

Effect of Structural Factors on Difference in Structural Steel Longitudinal and Transverse Impact Specimen Embrittlement after Controlled Rolling and Thermomechanical Treatment

Using correlation-regression analysis, the effects of structure and chemical composition on KCV impact strength anisotropy and difference in critical brittleness temperatures for longitudinal and transverse specimens of ferrite-pearlite and high-strength (ferrite + bainite) steels after thermomechanical rolling are studied. The significant effect of pearlite content and sulfide inclusions on impact strength anisotropy and difference in critical brittleness temperatures for longitudinal and transverse specimens of ferrite-pearlite steels is demonstrated. For high strength ferrite-bainite steels prepared using thermomechanical rolling technology the effect of carbon and sulfur on degree of embrittlement for transverse specimens is considered that also depends on the content of titanium carbonitride and Al2O3-based aluminum oxides.

A novel methodology for the evaluation of low temperature failure properties of asphalt binders

This paper presents the results of an experimental investigation that focused on the failure properties of asphalt binders at low temperatures. A novel test method was developed, based on the use of the dynamic shear rheometer equipped with 4 mm parallel plates. The method entails the application of constant shear strain rates at various temperatures until failure. Test data are modelled by means of the elastic-viscoelastic correspondence principle and by thereafter referring to the shear stress at failure, to a purposely defined brittleness index and to a critical brittleness temperature. Results discussed in the paper, which refer to a preliminary set of experimental data, indicate that the proposed methodology may be very effective in evaluating and comparing low temperature failure properties of asphalt binders of various types and origins.

Metal Examination and an Assessment of a Feedwater Pipeline’s Reliability after Prolonged Operation

The operational reliability of feed pipelines made of 15GS steel used at thermal power plants is considered. The reason for the study consists in an incident at a Mongolian TPP, constructed according to a Soviet project, consisting in the detection of cracks in one of the bends of the feed pipeline. The zone of metal damage was removed, after which three fragments of the bend were sent for investigation to the All-Russia Thermal Engineering Institute (JSC VTI). After a nondestructive testing of the bend under laboratory conditions demonstrated the absence of defects in the metal and, thereby, indicated the locality of damage, the metal of the bend was examined in samples. The studies on the chemical composition and microstructure of the bend metal have confirmed its overall compliance with engineering specifications in these characteristics. According to the results of mechanical tests, the strength and plastic properties of the bend metal also met the requirements of engineering specifications. Furthermore, the metal of the stretched zone of the central part of the bend, wherein damage was detected, has exhibited a higher level of strength characteristics and a lower plasticity compared to other sections of the bend. In addition, the metal of this zone exhibited a lower impact strength and an increased value of the ratio between yield strength and tensile strength, which indicates brittle conditions. This has been confirmed by the determination of the critical brittleness temperature, whose value for the metal of the stretched zone of the central bend fragment is rather high amounting to at least 90°С. The results of strength calculations have confirmed compliance with the standard strength conditions for pipeline bends and, at the same time, they have demonstrated a very high level of local tensile stresses on the outer surface of the stretched bend zone. The calculation of the tendency of bends for the brittle fracture has shown that, taking into account the dimensions of surface’s technological defects allowed by the engineering specifications, the boundary of the brittle fracture of the bend is significantly lower than the pipeline operation temperature, but the hydraulic testing of the pipeline should be carried out at a temperature not lower than 60°C according to the conditions of brittle strength.

Improvement of the cold resistance of the 09G2S steel by programmed hardening combined with additional medium-temperature tempering

The article discusses a method of improving the cold resistance of steels of a ferrite-pearlite class. It has been established that the process of programmed hardening, combined with an additional medium temperature tempering, has a positive effect on the cold resistance of thermally improved 09G2S steel. The mechanical properties of steel grade 09G2S were determined before and after additional tempering at mediumtemperature. It is shown that the new technology allows the critical temperature of brittleness, determined by the criterion of the energy intensity of failure, to be shifted to lower temperatures by 15°C, and at the same time to increase the impact strength of 09G2S steel three times more at a temperature of minus 90°C without changing its strength properties.

Use of Programmed Hardening During Tempering in Order to Improve Steel Brittle Fracture Resistance

A new method for improving ferritic-pearlitic steel brittle fracture resistance is considered. The favorable effect of thermal improvement and subsequent programmed strengthening hardening during additional medium-temperature tempering is demonstrated on the example of steel 09G2S. The new technology makes it possible to lower the steel 09G2S critical brittleness temperature Tc0 determined in terms of the criterion of fracture energy intensity by 20°C, and to increase steel impact strength at –90°C by about a factor of three with retention of the specified strength properties.

Characteristics of the resistance to brittle fracture of the elements of light steel thin-walled structures determined on samples with a sharp notch groove

The structure and mechanical properties of 350 steel (GOST R 52246) are studied under tensile and impact bending in the zones of bend of galvanized light-gage steel structure elements. Batch sheet samples consisting of two and three samples with a V-shaped notch were used for profiles 1.5 and 2.0 mm thick, respectively. It is shown that a visco-brittle transition occurs in the temperature range of 20 – 90°C in galvanized samples subjected to plastic deformation. The critical temperature of brittleness T 34 of steel 350 decreases with a decrease in the thickness of rolled products from 3.0 to 1.5 mm due to a decrease in the rigidity of the stress-strain state under tension. At the same time, the size of ferrite grains in the studied ferrite-pearlite steels (1.17 – 0.22 C; 0.11 – 0.14 Si; 0.49 – 0.54 wt.% Mn) changes slightly and amounts to 8.0 – 8.9 μ m. A crystalline fracture surface oriented perpendicular to the axis of the sample is formed in the zones of bending of 3-mm light-gage steel structure elements in the region of the critical temperature of brittleness in the central zones of the fracture. In the samples 2 and 1.5 mm thick, the fracture surface is formed by two shear planes oriented at an angle of 45° to the sample axis at the base of which the colonies of facets of trans-crystallite cleavage are located. Galvanized steel 350 profiles (GOST R 52246) with a ferrite grain size 8.8 – 8.9 μ m can be used in light steel thin-walled (1.5 – 3.0 mm) structures in the operation temperature range of up to – 65°C.

Express evaluation of welded joints cool resistance

Low-temperature instrumented indentation tests of welded joints are carried out. The effect of metal cooling on indentation diagrams and metal hardness is investigated. Impact strength tests with a gradual decrease in temperature with the identification of the cold brittleness threshold and determination of the critical brittleness temperature for several structural steels are performed. For an express evaluation of a metal cold resistance, the parameter γ is proposed, which is equal to the ratio of hardness at a given low temperature to hardness at room temperature. The relation between the parameter γ and the temperature coefficient of hardness, which is included in the equation describing low-temperature dependence of hardness, is established. For several grades of carbon and alloy steels, the relation between the parameter γ and the critical brittleness temperature T0.4, determined by the criterion value of impact strength KCV = 0.4 MJ/m2, is established. For welded joints obtained by electron beam welding and arc welding, the values of T0.4 are determined by the parameter γ in various local zones, including parent metal, heat-affected zone and weld metal. The possibility of establishing the acceptable operation temperature of welded joints using the obtained distributions of the critical brittleness temperature T0.4 is shown.

Optimization of Restorative Heat Treatment of Turbine Housings Made from Low Alloy Steel

Metal of steel 20KhMFL housings after restorative heat treatment with a different amount of normalizing and high-temperature tempering (1N + T, 2N + T, 3N + 2T) has been studied by experiment for the first time as applied to modernizing turbines. Data for the short-term and stress-rupture strength, ductility in tension, critical brittleness temperature, critical notch opening, and hot hardness of metal, and the amount and number of defects for repair are analyzed. The advantage of heat treatment by a regime with single-stage normalizing and high-temperature tempering (1N + T) is demonstrated.

The Influence of Regimes of Thermomechanical Treatment on the Structural-Phase State, Mechanical Properties, and Fracture Toughness of Low-Alloy Steel API 5L Grade X52

Possible improvement of mechanical properties and fracture toughness of low-alloy steel API 5L X52 after helical rolling under the conditions of cooling rate variations is analyzed. Upon helical rolling and air cooling (Regime I), the average ferrite grain size in this steel is observed to decrease from 13.5 μm in the initial state to 3.5 μm, the volume fraction of pearlite also decreases, the banded orientation disappears, and the structure becomes more homogeneous. The structural changes provide an increase in the yield stress value from 270 to 430 MPa and strength from 480 to 630 MPa, with the plasticity remaining at the initial level (e = 25%). Simultaneously, the impact toughness value (KCV) increases in the entire temperature interval by about 30%, compared to that in the initial state, and the critical temperature of brittleness Т50 is shifted towards the region below –70°С. After rolling of low-alloy steel X52 and water quenching (Regime II), a ferrite-bainite structure is formed. Regime II favors an increase in the yield strength of the steel by a factor of 1.6, while the level of KCV slightly decreases.

Application of Probabilistic Leak-Before-Break for WWER-1000 Unit

In this paper a probabilistic Leak-Before-Break analysis for WWER-1000 unit was performed based on the Failure Assessment Diagram (FAD), treating strength and crack morphology parameters as stochastic values. In order to perform probabilistic calculations Critical temperature of brittleness and Yield (Ultimate) Stress, were fitted by normal distribution, and based on experimental data taken from the manufacture documentation found at the Ukrainian NPP. The statistical behavior of the leak rate (LR) and Critical crack length was examined. The failure probability was calculated using Monte-Carlo simulation, for different types of cracks. It was proven, then crack morphology parameters highly affects the leak rate, among the mechanical characteristic a Fracture toughness has more influence rather than Ultimate of Yield strength.

Conservative estimations of irradiation embrittlement of reactor pressure vessel steels for WWER-1000 lifetime prediction

PurposeThe purpose of this paper is to consider a procedure of water-water energetic reactor (WWER) reactor pressure vessel (RPV) lifetime prediction at the stages of design and lifetime extension using the standard irradiation embrittlement parameters as defined in regulatory documents. A comparison is made of the brittle fracture resistance (BFR) values evaluated using two criteria: shift in the critical brittleness temperature ΔTc or shift in the brittle-to-ductile transition temperature ΔTp and without shifts (Tc and Tp).Design/methodology/approachThe radiation resistance was determined using the following three approaches: calculation based on standard values ΔTc and Tc0 or ΔTp and Tp0 (a level of excessive conservatism); calculation based on standard value ΔTc and actual value Tc0 or actual values ΔTp and Tp0 (the level of realistic conservatism); or calculation based on actual values of Tc and Tc0 or Tp and Tp0 (the level of actual conservatism). The BFR was evaluated based on the results of testing the specimens subjected to irradiation in research reactors as well as surveillance specimens subjected to irradiation immediately under operating conditions.FindingsThe excessive conservatism in determining the actual lifetime of nuclear reactor vessel materials can be eliminated by using the immediate values of critical brittleness temperature and ductile-to-brittle transition temperature.Originality/valueObtained results can be applied to extend WWER vessel operating time at the stages of designing and operation due to substantiated decrease in conservatism. And it will allow carrying out a statistical substantiated assessment of the resistance to brittle fracture of the RPV steels.

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.