Crack Resistance Evaluation for 38KhN3MFA-Sh Structural Steel According to Fracture Characteristics and Elastic Wave Velocities

Abstract

The results of studies on the effects of tempering temperature on the static strength and crack resistance of 38KhN3MFA-Sh steel are presented. The fractured surface texture has been studied after different heat treatment modes using electron fractography analysis. Relationships between the fracture characteristics and the critical stress intensity factor have been established. The effect of tempering temperature on the velocities of ultrasonic bulk waves has been investigated. A linear relationship between the velocity of elastic waves and the critical stress intensity factor of 38KhN3MFA-Sh steel has been found. The revealed relationship makes it possible to estimate the changes in crack resistance of steel using a nondestructive testing method with variations in the tempering temperature. The method of electron fractography has been used to analyze the fractures in the samples of structural high-quality 38KhN3MFA-Sh steel with a crack. The investigations of the fractured surface texture after different heat-treatment modes have shown that the microrelief is represented by flattened cone-shaped pits. The increase in the tempering temperature is accompanied by an increase in the diameter of flattened pit-cones on the fracture surface. A quadratic relationship between the crack resistance parameter and the diameter of the pits has been revealed. It has been shown that the contribution of the structural condition of the ferrite matrix to crack resistance is much more significant than the contribution of isolated carbides. The elastic wave velocities in steel have been measured; their values exhibit an increase with an increase in the tempering temperature. The strength and crack resistance parameters of the structural steel exposed to high-temperature tempering have been predicted on the basis of the values of transverse wave velocities. The deviations of the predicted values of crack resistance K1c and ultimate strength σu from the experimental values do not exceed 5.4 and 12.6%, respectively.