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

Abstract

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