In coarse-grained steel, which frequency is generally preferred to maximize penetration?

• A. 0.5 MHz
• B. 1 MHz
• C. 3 MHz
• D. 10 MHz

Answer: (B)

In coarse-grained steel, attenuation and scattering of ultrasonic waves are stronger at higher frequencies because grain boundaries disrupt the wave more as the frequency increases. Lower frequencies have longer wavelengths and experience less scattering, so they travel farther before the signal dies away. To maximize penetration, you want a frequency that stays strong deeper in the material while still providing enough sensitivity to detect flaws.

Among the given options, 1 MHz hits a practical balance: it's low enough to improve depth of penetration in coarse-grained steel compared with the higher frequencies, yet high enough to maintain reasonable flaw detection sensitivity. The higher frequencies (3 MHz and 10 MHz) would attenuate much more quickly in this type of steel and thus offer less penetration. The lowest option could provide the deepest penetration, but it would come with reduced resolution and signal strength, making flaw detection less reliable. Therefore, 1 MHz is generally the preferred choice for maximizing penetration in coarse-grained steel.