Abstract
Ultrasonic nondestructive techniques can be used to characterize grain size and to evaluate mechanical properties of metals more practically than conventional destructive optical metallography and tensile tests. Typical ultrasonic parameters that can be correlated with material properties include ultrasonic velocity, ultrasonic attenuation coefficient, and nonlinear ultrasonic parameters. In this work, the abilities of these ultrasonic parameters to characterize the grain size and the mechanical properties of 304L stainless steel were evaluated and compared. Heat-treated specimens with different grain sizes were prepared and tested, where grain size ranged from approximately 40 to 300 μm. The measurements of ultrasonic velocity and ultrasonic attenuation coefficient were based on a pulse-echo mode, and the nonlinear ultrasonic parameter was measured based on a through-transmission mode. Grain size, elastic modulus, yield strength, and hardness were measured using conventional destructive methods, and their results were correlated with the results of ultrasonic measurements. The experimental results showed that all the measured ultrasonic parameters correlated well with the average grain size and the mechanical properties of the specimens. The nonlinear ultrasonic parameter provided better sensitivity than the ultrasonic velocity and the ultrasonic attenuation coefficient, which suggests that the nonlinear ultrasonic measurement would be more effective in characterizing grain size and mechanical properties than linear ultrasonic measurements.
Highlights
There have been increasing demands for quantitative characterization of material microstructures that correlate with mechanical properties of metals in many industrial fields, such as power plant, manufacturing, infrastructure, and so on [1,2,3]
The standard intercept count method of ASTM E112 was applied to the photographs, as shown in Figure 3, which were obtained with an optical microscope to measure average grain size of heat-treated specimens
The increase in grain size caused a decrease in ultrasonic velocity and an increase in ultrasonic attenuation coefficient, which was due to ultrasonic scattering by grains
Summary
There have been increasing demands for quantitative characterization of material microstructures that correlate with mechanical properties of metals in many industrial fields, such as power plant, manufacturing, infrastructure, and so on [1,2,3]. Reports on the correlation between grain size and nonlinear ultrasonic parameter are rare in the literature [7,22] This is because most previous nonlinear ultrasound studies have focused mainly on evaluating material degradation, such as fatigue damage [23,24,25], creep damage [26,27], thermal aging [28,29,30], and plastic deformation [31,32], in which dominant microstructural characteristics that affect the nonlinear ultrasonic parameter are dislocation densities [33,34], precipitates [35,36,37], and phase transformations [38].
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