CONTROL OF PARAMETERS OF LOCAL FRACTURE PROCESS IN DELAYED AND ACTIVE FRACTURE OF MARTENSITIC STEEL
A new approach for quantitative studies of the impact of structural factors on the local strength of steels based on the control parameters of the process at the local destruction slowed and active destruction of martensitic steel. The basis of the method is: 1) the use of con-centralizers stress on the samples to create the exact coordinates of the process of local fracture; 2) calculation of the local stresses in the local destruction zone with the help of computer simulation by finite element method; 3) the establishment of pre-planned with-standing structural steel. As a model form is selected delayed fracture failure of hardened steel. Delayed fracture of steel due to the presence of residual internal microstresses in steel. The residual internal microstresses are reaching-out their maximum values to the tops of the largest martensite crystals entering the boundaries of the original austenite grains become that determines the location of crack. Using the method of mathematical modeling of stress condition (finite element method) of specimens with stress raiser allowed to determine the local stresses in the crack birthplace area. Test for the slow destruction of notched specimens given the ability to control the crack birthplace and stimulate the crack nucleation at the original crystal boundary. There was discovered for the first time, using finite elements method, a quantitative dependence of the strength of the grain boundary martensite steel 18X2H4BA in conditions of active and delayed fracture at different levels of residual internal microstresses as a result of various relaxation time after quenching. It was shown for the first time that the local strength of the boundaries of the original austenite crystal of martensitic steel depends on the type of the local fracture mechanism: the cleavage fracture at quasi-static loading or destruction mechanism of thermally activated delayed fracture.