Ultrasonic Testing: Alloy Steel Bars

Cheatsheet Content

1. Principles of Ultrasonic Testing (UT) Definition: Non-destructive testing (NDT) method using high-frequency sound waves ($0.5\,MHz - 20\,MHz$) to detect flaws or characterize materials. Wave Propagation: Longitudinal (compression) waves: Particles oscillate parallel to wave direction. Primary for flaw detection. Shear (transverse) waves: Particles oscillate perpendicular to wave direction. Used for thinner materials or specific flaw orientations. Acoustic Impedance ($Z$): $Z = \rho \cdot c$, where $\rho$ is density and $c$ is sound velocity. Discontinuities cause reflections due to impedance mismatch. Snell's Law: Governs refraction at interfaces: $\frac{\sin\theta_1}{c_1} = \frac{\sin\theta_2}{c_2}$. Critical angles lead to mode conversion. 2. Equipment for UT of Bars Ultrasonic Flaw Detector: Generates and receives ultrasonic pulses. Displays A-scan, B-scan, or C-scan. A-scan: Amplitude vs. time-of-flight. Most common for manual inspection. Transducers (Probes): Convert electrical energy to sound and vice versa. Normal Beam (Straight Beam): $0^\circ$ incident angle, used for detecting flaws parallel to the surface (e.g., laminations, inclusions). Angle Beam: $30^\circ - 70^\circ$ incident angle, used for detecting flaws perpendicular or at an angle to the surface (e.g., cracks, lack of fusion). Often uses shear waves. Couplant: Material (gel, water, oil) to eliminate air gap between transducer and bar surface for efficient sound transmission. Reference Standards: Blocks with known artificial discontinuities (e.g., flat bottom holes, side-drilled holes) for calibration. 3. Testing Methods for Bars Contact Testing: Transducer directly contacts the bar surface with couplant. Manual: Operator moves transducer. Semi-automated: Transducer mounted on a fixture. Immersion Testing: Bar and transducer submerged in a water tank. Provides uniform coupling and allows automated scanning. Preferred for high-volume inspection or complex geometries. Eliminates couplant application issues. Automated/Semi-automated Systems: Often used for high-throughput bar inspection. Multiple transducers, squirters, or rotating bar/transducer setups. Helical scanning is common for round bars. 4. Flaw Detection in Alloy Steel Bars Common Flaws: Internal Flaws: Inclusions (oxides, sulfides), porosity, shrinkage, hydrogen flakes, internal cracks. Surface/Near-Surface Flaws: Longitudinal cracks, transverse cracks, laps, seams. Detection Principle: Ultrasonic waves reflect off discontinuities. The amplitude and time-of-flight of the reflected echoes provide information about the flaw's size, location, and orientation. Interpretation of A-scan: Initial Pulse: Transmitted wave. Backwall Echo: Reflection from the opposite surface of the bar. Flaw Echo: Reflection from an internal discontinuity. Its position relative to initial and backwall echoes indicates depth. Its amplitude relates to flaw size/type. 5. Procedure for Alloy Steel Bar UT Surface Preparation: Clean bar surface to remove scale, rust, grease, and loose paint. Smoothness affects coupling and signal quality. Equipment Setup: Select appropriate transducer (frequency, type) and flaw detector settings (gain, range, pulse repetition frequency). Calibration: Sensitivity Calibration: Adjust gain using a reference standard to ensure detectable flaws produce a specific amplitude (e.g., 80% Full Screen Height). Distance/Range Calibration: Use reference standard (or bar thickness) to set time base for accurate depth measurement. DAC/AVG Curves: Optional, but common for quantitative flaw sizing. Scanning: Apply couplant. Move transducer systematically across the bar surface, ensuring full coverage. For round bars, often involves rotating the bar or using multiple transducers in a helical pattern. Observe A-scan for flaw indications. Evaluation: Analyze flaw echoes (amplitude, time-of-flight, shape). Compareindications to acceptance/rejection criteria (e.g., ASTM E114, E2375, specific customer specifications). Characterize flaws (type, size, location). Documentation: Record all relevant data: equipment, settings, calibration, flaw locations, sizes, and evaluation results. 6. Considerations for Alloy Steel Grain Structure: Coarse grain structures (common in some alloy steels, especially after certain heat treatments) can cause high attenuation and scattering of ultrasonic waves, leading to "grass" or noise on the A-scan. Velocity Differences: Sound velocity can vary slightly with alloy composition and temperature. Calibration on the actual material is crucial. Attenuation: Higher in some alloy steels compared to carbon steels, requiring higher gain or lower frequency transducers. Flaw Types: Hydrogen flakes are a specific concern in some high-strength alloy steels; UT is effective for their detection. 7. Acceptance Criteria (Example - refer to specific standards) Standard/Specification Typical Criteria ASTM E114 Standard practice for UT of straight-beam contact method. Defines calibration and evaluation. ASTM E2375 Standard practice for UT of wrought products. Often referenced for bar inspection. Customer Specs Often specify maximum allowable flaw size, number of indications per length, or comparison to a specific reference reflector (e.g., $1/64''$ FBH). Rejection Indication amplitudes exceeding a specified DAC/AVG level, or exceeding specific flaw size/type limits.