Critical Brittleness Temperature Research Articles

Corrosion damage in the material and the variation of the critical brittleness temperature

Corrosion damage in the material and the variation of the critical brittleness temperature

Evaluation of the brittle fracture susceptibility of steels on the basis of the spectrum of low strain rates

1. The temperature dependence of the rate of low plastic strains determined at the stresses below the yield stress is the strain characteristic of the structure of the material and the temperature spectrum of the strain rates determining the ‘mobility’ or relaxation capacity of the structure. 2. We detected the nonmonotonic nature of the\(\dot \varepsilon \left( {\rm T} \right)\) dependence, indicating that the processes of local plastic deformation are rapidly activated at certain temperatures. 3. It is shown that the form of the\(\dot \varepsilon \left( {\rm T} \right)\) dependence and the position of the maxima of\(\dot \varepsilon \left( {\rm T} \right)\) are only slightly sensitive to the surface state and the sign and magnitude of applied load but respond to the variation of the structure as a result of heat treatment. They also differ for steels of various grades. 4. The critical brittleness temperature can be determined on the basis of the temperature of the peak of\(\dot \varepsilon \left( {\rm T} \right)\) in the expected temperature range of the ductile-brittle transition. The estimation of this temperature by the method described above and by the existing methods for a number of cases where the existing methods confirmed their reliability indicates that\({\rm T}^{\dot \varepsilon } \) can be used as the characteristic of the brittle fracture resistance of the material.

Permissible crack size in the frame of a water-cooled, watermoderated reactor

The theoretical determination of the permissible crack size is outlined for the example of a semielliptical crack under the weld seam in the shell of the reactor frame. The crack is in the basic metal of the shell in the melting zone, and is oriented along the frame axis so that it opens under the action of axial stress. The ratio of the crack depth l to its length at the fusion boundary 2c is one third. The shell material is 15Kh2NMFA steel. The following shifts in critical brittleness temperature of the steel are determined: as a result of aging of the material ..delta..T/sub c/ = 5/sup 0/C; as a result of cyclic damage ..delta..T/sub N/ = 0/sup 0/C; and as a result of neutron irradiation (a/sub f/ = 23/sup 0/C neutron/sup -1/3 cm/sup -2/) ..delta..T/sub f/ = 92/sup 0/C; the critical brittleness temperature of the steel in the initial state ..delta..T/sub Co/ = -25/sup 0/C; the temperature margin ..delta..T/sub M/ = 30/sup 0/C.usin

A criterion of failure of steels susceptible to propagation of brittle microcracks along crystal boundaries

1. A structural criterion of the ductile-brittle transition of structural steels for the case of weakening of the cohesion strength of the crystal (grain) boundaries was proposed. The criterion takes into account the variation of the length of free passage of the brittle microcrack and effective surface energy of failure during the appearance of intercrystalline failure on the brittle fracture surfaces. 2. The variation of the true fracture stress during propagation of the brittle crack both through the body and along the boundaries of the crystals is closely linked with the fraction of intercrystalline failure. 3. For steels with weakened crystal boundaries we proposed a structural criterion of crack resistance K1c in the region of the lower shalf of the values of this criterion which does not depend on the size of the crystals and is closely linked with the fraction of intercrystalline failure on the brittle fracture surfaces. 4. For the steels with weakened crystal boundaries, the results revealed the resistance of a close relationship between the cracking resistance characteristics K1c in the range of the lower shelf of the values of this parameter, critical brittleness temperature Tc, and true fracture stress σF.

Effect of neutron irradiation on the properties of the repair welds of the 15Kh2MFA steel

The authors studied the effect of neutron irradiation on the tendency of the metal belonging to the heat affected zone of the weld toward brittle fracture (an increase in the critical temperature of brittleness). For comparison, the authors studied the radiation embrittlement of the original base metal (steel 15Kh2MFA) subjected to the conventional heat treatment of the reactor frames consisting of hardening and high-temperature tempering. Along with these materials, the radiational embrittlement of the base metal in the rehardened condition without tempering was studied. It was concluded that the presence of the regions repaired according to this technology and located in the frame at the level of the reactor core does not pose the problem of decreased resistance to brittle fracture.

Some recularities in the ductile-brittle failure of cast irons with spheroidal graphite

1. An increase in the pearlite content in the metal base and manganese in cast iron leads to an increase in critical brittleness temperatures. 2. There is no clear correspondence between critical brittleness temperatures with impact testing by GOST 9454-78, determined from 50% ductile component in a specimen fracture and from the value of failure energy. 3. Sections with a sharp change in the relationship\(K_{I_c ^{ - \sigma _{0.2} } } \) apparently correspond to a change in the material failure mechanism.

Microscopic and macroscopic plastic deformation as a criterion of the limiting state of a material during fracture

1. For bcc materials undergoing the ductile-brittle transition, we propose that the presence or absence of a slightly deformed macroscopic zone under the surface of impact and static fractures be regarded as a criterion of the realization of a plane stress or plane strain state, respectively, at the crack tip. 2. A relationship was established between the stress state of the material at the crack tip and the critical brittleness temperature for the given class of materials. 3. Within the range of the ductile-brittle transition there are temperature regions corresponding to different fractographic and structural criteria of the character of the fracture.

The rational alloying of chromium-nickel structural steels

1. The extremal dependence of the critical brittleness temperature on the chromium and nickel contents in steel with consideration given to data on the effect of chromium and nickel on hardenability and nickel (for a certain phosphorus content) on temper brittleness makes it possible to select the optimum composition of Cr-Ni-Mo-V steels. 2. Cr-Ni-Mo-V steels with an increased chromium content (more than 2.0%) and nickel content (more than 2.0%) are inclined to embrittlement during tempering in connection with the formation of special carbides that are segregated along the grain boundaries. Embrittlement intensifies with increasing chromium and nickel content in the steel. 3. Cr-Ni-Mo-V steels are prone to embrittlement during slow cooling owing to the grain-boundary segregation of phosphorus. Embrittlement during slow cooling from the tempering temperature increases with increasing nickel content owing to intensification of the grain-boundary segregation of phosphorus. 4. As compared with steel containing 1.0–1.5% Cr and 3.0–4.0% Ni, Cr-Ni-Mo-V steels with up to 2% Cr and up to 2% Ni have a lower critical brittleness temperature and adequately high hardenability, which ensures a bainite structure and a satisfactory set of mechanical properties in forgings up to 1850 mm in diameter when quenched in water, and in forgings up to 850 mm in diameter when quenched in oil.

Influence of sulfur on the microstructure and impact strength of 17G1S steel

1. Within the investigated limits (0.002–0.098%) sulfur has practically no influence on the static strength and ductility of steel at normal temperature. 2. With an increase in sulfur content from 0.007 to 0.098% the impact strength of 17G1S steel continuously decreases and there is a simultaneous drop in the critical temperature T50. With a reduction in sulfur content from 0.007 to 0.002% there is sharp decrease both in impact strength and in the share of fibrous structure in the fracture, which is related not only to the decrease in sulfur content but also to obtaining coarser ferrite grains after normalizing. 3. The use of a special heat treatment makes it possible to obtain a fine-grained structure in steel with 0.002% S and correspondingly to increase the impact strength. At the same time, the positive influence of sulfur on the critical brittleness temperature is maintained (T50 drops).

Evaluating the tendency toward brittle fracture of structural steels intended for use in large cross section parts

For structures of large dimensions operating at low temperatures, evaluation of the material quantity from the point of view of the tendency toward brittle fracture is important. Normally, this is done based on the results of impact tests of 10  10 mm cross section samples, on the basis of which the critical brittleness temperature and the impact strength with a change in service temperature of the structure are determined. However, experience has shown that fractures of actual structures undel" service conditions [1, 2] often occur at temperatures significantly higher than the critical temperature determined from standard impact strength curves. FuYther improvement in the impact test method and, in particular, increasing the sharpness of the notch, the use of side notches, and increasing the speed of the tests, does not lead to a substantial increase in the critical brittleness temperature, and the results obtained do not make it possible to predict the possibility of brittle fracture of heavy parts or structures. This has made it necessary to develop new methods of testing materials. At present to evaluate the tendency toward brittle fracture of structural material, determination of the fracture toughness of the material Kic, which characterizes the crack resistance in plane deformation, is recommended. In service, design elements are most frequently under conditions differing from the experimental ones. Therefore, for a material it is also necessary to detenaaine the critical temperature at which in a part of a given cross section the transition from ductile to brittle fracture occurs taking into consideration constraint of plastic deformation in the fracture zone. We have investigated the possibility of evaluating the tendency of structural steels toward brittle fracture on the basis of fracture toughness Kit and on the basis of the results of impact tests of large cross section samples. The experiments were made on 30KhMA and 35KhMFA steels (Table 1). The fracture toughness (Kie) was determined in off-center tension of compact samples with thicknesses of 60 mm (30KhMA) and 50 mm (35KhMFA) in accordance with tile British standard [3]. The tests were made on an Instron-1255 universal test machine with recording of the load vs displacement curve in the -50 to +100~ temperature range for 30KhMA steel and -80 to +170~ for 35 KhMFA steel. From Fig. 1 it may be seen that with an increase in temperature the fracture toughness KI~ of both types of steels steadily increases, although the absolute value of KI, is low. At temperatures above +20~ the fracture toughness of these steels is practically the same, while at temperatures below +20~ the I{ie of 30KhMA is somewhat higher than that of 35KhMFA steel. The critical brittleness temperature was determined in impact tests. To amplify the influence of constraint of plastic deformation appearing in large cross section parts, 30  30 X 160 mm impact samples with a notch with a width Of 3 mm to half of the cross section ending in an angle of 45 ~ with a tip curvature of r = 0.25 mm were prepared together with standard 10  10 mm samples. The standard samples were tested on an MK-30 impact testing machine in the -180 to +60~ range and the large samples on a pendulum impact machine with an energy of 2.5 kJ at temperatures of -180 to

Method and results of investigations into relationships governing fatigue crack propagation in plane bending at low and high temperatures

E. Norris and R. Wiley, "Investigations in the area of tests for determining the critical brittleness temperature of metals for main pipe lines," in: Impact Tests of Metals [Russian translation], Mir, Moscow (1973), pp. 199-228. A. Fuchs etal., Arch. Eisenhuttenw., 45, No. 2, 127-136 (1975). B. M. Ovsyannikov and V. G. Laz'ko, "The relationship of the fracture toughness of constructional steelsto chemical composition and structure," Spets. Stali Splavy, No. 2, 116-125 (1973). B. M. Ovsyannikov, V. G. Laz'ko, and A. G. Zakurdaev, "The influence of grain size and pearlite content on the resistance to failure of low-alloy steel," Metalloved. Term. Obrab. Met., No. 8, 44-47 (1978). A. Tetelman and A. Evili, "Failure of high-strength materials," in: Failure [in Russian], Vol. 6, lletallurgiya, l loscow (1976), pp. 160-164. V. H. Finkel', Retarding of Failure [in Russian], Metallurgiya, Moscow (1977). H. Berns, Z. Werkstoff., i, No. 6, 195-200 (June, 1978).

Statistical method of determining ductile-brittle transition temperature

1. By physical nature the critical brittleness temperature is a random variable and can be represented by the statistical distribution. 2. For steel 20L the type of fracture is a more accurate criterion for determining Tcri than the impact strength, and the standard deviation of % D increases sharply in the critical temperature range; near the limits of the critical temperature range the statistical distributions of % D change sharply, passing from bell-shaped to the J type. Also, the index of the type of fracture changes rapidly at the limits of the critical temperature range, changing from random to reliable or impossible, which does not occur in the case of the impact strength. 3. The critical temperature for steel 20L determined on the basis ofan=2 kg-m/cm2 is too low.

Determination of the critical brittleness temperatures of low-carbon steel sections

Determination of the critical brittleness temperatures of low-carbon steel sections

The Influence of Size on the Brittle Strength of Steel

Abstract In this paper the influence of size effect on the impact cold brittleness of carbon steel is discussed. Static bending of notched specimens shows the same size effect, so that an assumption of the special role played by high velocity must be rejected. The experiments on the determination of the critical temperature of brittleness for impact-tension tests also display the size effect and therefore exclude any possible influence of the stress gradient. Experiments on static tension and bending of cylindrical specimens of brittle phosphorous steel in liquid air reveal the statistical nature of the size effect and give a good qualitative verification of Weibull’s theory. Weibull’s homogeneity exponent m calculated from experiments with tension and bending is found to have nearly identical values (namely 23.5 and 25.4). The theoretical values of the brittle strength of small specimens calculated with this value of m differ from the experimental ones by about 3 per cent; the ratio of the strengths in the case of bending and tension is found experimentally to be 1.40 against 1.39, according to the theory. The “scatter” in particular values for large specimens, as predicted by theory, is smaller than for small ones.