Ultrasonic Nondestructive Characterization Of Microstructures Of Alloy Steels

According to structural characteristics of multi-phased alloy steels and the properties of ultrasonic propagation, microstructures characterization with different heat treatment processes (normalizing, quenching/annealing, quenching plus low tempering, quenching plus high tempering) for three steels (40Cr, 38CrMoAl, GCrlSSiMn) was studied experimentally by using ultrasonic methods. Ultrasonic velocity, relative attenuation coefficient, and power spectral analyses were researched on the same samples respectively, on the basis of theory and experiment, the sensitivity and other characterizations of the three methods used to distinguish different microstructures were compared. The principle of multi-method and multi-parameter is suggested, which should be followed during characterizing microstructures of alloy steels by ultrasonic NOT method.Ultrasonic velocity (C,) of transformation products through different heat processes of three steels was measured by high-accuracy ultrasonic installation. The velocity of quenched microstructure of 38CrMoAl is the lowest, which gradually increases with increasing tempering temperature between 200ç™± and 650ç™± after quenching process. There is good correlation between HRC and C,. The velocity in normalizing structure is between the velocity in medium temperature and high temperature tempering production after quenching. The variation principle of wave velocity in different microstructures of 40Cr is similar with that of 38CrMoAl. The velocity in quenching and low temperature tempering structure of GCrlSSiMn is much lower than the velocity in normalizing, annealing and high temperature tempering structure. The difference in velocity among normalizing, annealing and high tempering structures are all approaching. The difference among the elasticity modulus of various microstructures is probably the main reason, which caused the experiment results above.Under the same experiment conditions, such as sample shape, size, surface roughness, establishment of testing system, pressure on the probe, and thickness of coupling medium, the variety of relative attenuation coefficient (or) in different heat treatment samples is measured and compared by measuring the attenuation obtained from the cylindrical surface of column. The result is as following: for 40Cr and 38CrMoAl, a MT