Proper ultrasonic probe frequency selection is critical for accurate NDT testing. The core rule is a trade-off: higher frequency = better resolution for tiny defects, while lower frequency = stronger penetration with less attenuation. Below is a concise, industry-standard selection guide for three common workpiece types.

1. Coarse-Grain Workpieces

Typical materials: Austenitic stainless steel, cast steel, high-temperature alloy, coarse-grain welds

Challenge: Severe ultrasonic scattering at grain boundaries causes high grass noise, easily covering defect echoes.

Recommended frequency: 0.5MHz – 2.5MHz

• 0.5–1MHz: For ultra-thick coarse-grain castings (>100mm)

• 1–2.5MHz: For medium-thick welds and forgings (30–100mm)

Note: 5MHz+ frequencies are forbidden, as they produce excessive noise and fail testing.

2. Thin-Wall Workpieces (≤10mm)

Typical parts: Thin plates, small-bore tubes, thin-wall pipes, liner plates

Challenge: Small wall thickness leads to short blind zone; defect waves easily overlap with bottom echoes.

Recommended frequency: 5MHz – 15MHz (fine-grain material)

• 10–15MHz: For ultra-thin workpieces (0.5–3mm) to ensure high near-field resolution

• 5–10MHz: For conventional thin-wall parts (3–10mm)

Special rule: Thin-wall coarse-grain workpieces are limited to max 2.5MHz.

3. Thick Workpieces (>30mm)

Typical parts: Heavy forgings, thick-wall vessels, large shaft parts, thick welds

Challenge: Long sound propagation path causes serious energy attenuation.

Recommended frequency: 1MHz – 5MHz

• 1–2MHz: For ultra-thick workpieces (>150mm) for maximum penetration

• 2–2.5MHz: For medium-thick structural parts (30–150mm, universal for weld inspection)

• 5MHz: Optional for fine-grain thick carbon steel within 80mm