Ultrasonic characterization of heat-treatment effects on SAE-1040 and -4340 steels

• Ultrasonic sound velocity and attenuation are sensitive to microstructural changes.
• Volume fraction of phases and percentage of sub-grain boundaries affect greatly ultrasonic testing measurements.
• The ultrasonic velocity and attenuation can be well correlated with the mechanical properties.
• Ultrasonic velocity and attenuation indicate good capacity to identify changes in microstructure.

In this work microstructural characterization and mechanical testing results were correlated with ultrasonic velocity and sound attenuation of steels SAE-1040 and SAE-4340. Both types were subjected to three types of heat treatment; the first was annealing at 850 °C, the second was austenitizing at 1000 °C followed by oil quenching and the third was similar austenitizing then water quenching. Treatments of SAE-1040 steel resulted in microstructures containing different ferrite and pearlite contents, different inter-lamellar spacing and also different grain size. Similar ferrite and pearlite content was obtained when annealing SAE-4340 whereas, oil and water quenching resulted into martensite. With SAE-1040, the sound velocity was reduced in the order of annealing–oil–water quenching due to the reduction of ferrite on the expense of pearlite. The same order in sound velocity reduction was also obtained with SAE-4340 due to the change in microstructural phases from pearlite to martensite. In comparison to pearlite, the martensite possessed higher crystal lattice distortion, higher dislocation density and lower elastic modulus all of which contribute in reducing sound velocity. Attenuation of SAE-1040 increased in the order of annealing–oil–water quenching because of higher pearlite content and the reduction in inter-lamellar spacing. Attenuation of SAE-4340 gave an opposite order due to the reduction of the extent of microstructural anisotropy. The mechanical properties and hardness were predominantly affected by the microstructural phases leading to the logical correlation with ultrasonic parameters.