I. Ultrasonic Testing of Plates

Ultrasonic testing of thick and medium-thick plates—shear-wave method.

To detect defects that are oriented at a certain angle to, or even perpendicular to, the surface of the plate—such as cracks—it is necessary, when appropriate, to supplement the inspection with shear-wave testing.

The determination of testing conditions for shear-wave inspection of plate materials, including the selection and calibration of the incident angle or refraction angle (or K-value), is conducted in the same manner as for conventional shear-wave testing. Sensitivity adjustment, however, is performed using reference test blocks made of the same grade of material, with the same metallurgical condition and surface condition, and featuring notches or cylindrical holes as reflectors.

Given that defect orientations can be highly variable, it is standard practice during inspection to perform grid-pattern scanning with the ultrasonic beam directed in four different directions—along both ends of the plate’s rolling direction and perpendicular to the rolling direction—resulting in a total of four scan passes to minimize the risk of missed defects.

When upper- and lower-surface grooves (with groove orientations perpendicular to the rolling direction of the plate) are used as reflectors, a distance–amplitude curve can be plotted in advance (by connecting the echo amplitudes from the upper and lower grooves) to facilitate the evaluation of defect echo amplitude during inspection.

The reference test blocks for shear-wave testing of plate materials are shown in Figure 4-4-4.

1. Ultrasonic Testing of Thin Plates

Sheets with a thickness of 0.5 to 4 mm—where the thickness is comparable to the ultrasonic wavelength—are generally referred to as thin plates.

In automated ultrasonic testing, methods such as water-immersion, overflow, and water-column spraying are commonly employed, with multi-channel pulse-echo ultrasonic transmission used for inspection. When the inspection parameters are adjusted to satisfy the resonance condition—specifically, when the plate thickness is an integer multiple of half the ultrasonic wavelength—the maximum penetration is achieved. However, if defects, particularly delaminations, are present in the plate, they disrupt the original resonance condition, leading to a substantial reduction in penetration. This manifests as a marked decrease in the amplitude of the transmitted wave, thereby enabling the detection of defects.

Under normal circumstances, manual contact flange wave testing (also known as plate wave testing) is employed.

2. Ultrasonic Testing of Profiles

So-called structural profiles refer to long, bar-shaped rolled products whose cross sections are rectangular, square, I-shaped, T-shaped, π-shaped, or other shapes, such as square steel, angle steel, I-beams, channel sections, and rails. For these profiles, the commonly used ultrasonic testing methods are longitudinal-wave testing and transverse-wave testing, with basic requirements that are essentially the same as those for ultrasonic testing of forgings, as shown in Figure 4-5-2.

For square steel bars, longitudinal wave straight probes are typically used, with the two adjacent side surfaces serving as the inspection surfaces; once a defect is detected, it should also be verified by inspecting the opposite side surface.

When high near-surface resolution is required, and given that the upper blind zone of a straight probe is typically larger than that of a low-blind-zone probe, it is necessary to perform four-sided inspection, or alternatively to use a combined dual-element straight probe for four-sided inspection.

Some metals exhibit insufficient ductility and are prone to cracking along strain lines during rolling and forging; in square or rectangular cross sections, this manifests as diagonal cracking. Therefore, it is also necessary to employ transverse shear-wave testing with a refraction angle close to the crack orientation angle (where the sound-beam direction is perpendicular to the longitudinal axis of the square steel).

During rail inspection, both longitudinal and transverse wave testing are employed based on the potential locations of defects within the rail—such as the web, tread, base, and fishplate bolt holes—and the orientation of these defects. Currently, most rail inspections utilize multi-channel instruments that deploy multiple probes at various positions along the rail to perform simultaneous inspections, thereby enhancing inspection efficiency.

For thin-section profiles—such as flat bars, angle bars, and strip steel—it is common to use combined dual-element straight-beam probes or shear-wave testing; in automated inspection, multi-channel ultrasonic pulse-echo testing is often employed.

For thin-section profiles—such as flat bars, angle bars, and strip steel—it is common to use combined dual-element straight-beam probes or shear-wave testing; in automated inspection, multi-channel ultrasonic pulse-echo testing is often employed.

3. Ultrasonic Testing of Large-Diameter Round-Bar Stock

Ultrasonic testing magnification for large-diameter circular-bar stock includes both contact-method and immersion-method inspections, as well as longitudinal-wave testing with circumferential radial incidence and transverse-wave testing with circumferential chordal incidence.

Basic ultrasonic testing procedure for longitudinal wave inspection using a single straight-beam probe in the contact method with circumferential radial incidence:

(1) Adjust the instrument to its normal operating state.

(2) Calibration: As with longitudinal-wave testing of forgings, calibration can be performed directly using the bar stock itself or using reference test blocks with acoustic properties identical or similar to those of the bar being inspected, employing the two-echo method.

(3) Adjustment of the initial sensitivity.

① Comparison Test Block Method: The comparison test block for ultrasonic longitudinal wave inspection of bar stock is shown in Figure 4-5-3.
           

② The fundamental wave method.

(4) Scanning: For bar inspection, scanning can be performed using circumferential scanning combined with linear translation, axial linear scanning combined with axial translation, or helical scanning. The first two methods are relatively easy to implement in manual contact testing, while the latter is more commonly used in automated inspection (where the bar rotates and the probe moves axially).

(5) Defect Evaluation: In ultrasonic testing of bar stock, defect location and evaluation are conducted in the same manner as for forgings; the burial depth of the defect can be directly read from the calibrated horizontal scale.

Quantitative evaluation of defects is generally performed using the equivalent-length method. When the bottom-wave method is employed, the quantitative procedure is identical to that used for forging inspection.

4. Water-immersion testing of bar stock

For ultrasonic testing of bar stock, immersion coupling is the most commonly used method; localized immersion coupling is also employed in some cases.

(1) Longitudinal wave inspection with circumferential radial incidence.

When a straight probe is used for immersion testing in water, the sound velocity in water is significantly lower than that in metal bars; consequently, as the sound beam passes from water through the convex curved surface into the bar, it undergoes substantial divergence, leading to both energy dispersion and increased susceptibility to interference.

To this end, water-immersion testing of bar stock typically employs water-immersion focused transducers, with line-focused types being preferred to enhance inspection efficiency. However, in order to ensure adequate coverage of the inspection area, the sound velocity in the water must still allow for a certain degree of beam divergence; therefore, the location of the underwater focal point of the water-immersion focused transducer is a critical consideration. Figure 4-5-6 illustrates three such scenarios:

a) When the focal point in the water is located above the center of the bar, the sound beam, upon entering the bar, first converges and then diverges, resulting in an uneven sound-field distribution and degrading the distance–amplitude characteristic of the sound field, which is detrimental to the quantitative evaluation of defects.
b) When the focal point in the water is located at the center of the bar, the sound beam does not diverge after entering the bar but instead converges at the center, resulting in the best detection performance for central defects;
c) When the underwater focal point is located in the lower half of the bar’s center, the sound beam diverges appropriately after entering the bar, ensuring adequate coverage and facilitating the detection of defects with various orientations and at different burial depths. Therefore, the water path length should be adjusted to achieve either condition b) or c), with the optimal setting being when the underwater focal point lies at or below the bar’s center.

(2) Shear wave inspection with circumferential and axial incidence

When performing shear-wave testing with circumferential, chordal incidence using the immersion method, the probe’s underwater focal point should be positioned on the diameter that is perpendicular to the sound axis and passes through the center of the bar. Control of the refraction angle can be achieved by adjusting the eccentricity—the distance between the sound axis and the sound axis passing through the center of the bar—as shown in Figure 4-5-7.

The eccentricity X is given by X = R · (C_water / C_s) · sin β, where:

R is the radius of the bar; Cs is the shear-wave velocity in the bar; β is the angle of refraction.

5. Inspection of Small-Diameter Round-Bar Stock

Small-diameter round-bar stock has a relatively small radius of curvature (generally defined as bars with a diameter less than 60 mm); when conventional single straight-beam transducers are used in the contact method for circumferential radial incidence of longitudinal waves, the limited near-surface resolution often places the entire inspection zone within the near field. Moreover, surface-reflected waves can introduce strong interference, to the extent that the initial pulse width may exceed half the bar’s diameter. For these reasons, the conventional flat transducer in the contact method with circumferential radial incidence of longitudinal waves is not suitable for inspecting small-diameter round-bar stock.

For such bar stock, in addition to immersion focused-beam ultrasonic testing, the following inspection methods are also commonly employed:

(1) Circumferential radial incidence inspection using a horseback-type combined dual-element probe.

As shown in Figure 4-5-8(a).

The flat bottom of the delay block in a conventional dual-element straight-beam probe is ground into a concave cylindrical surface to match the curved surface of the bar, with the probe’s acoustic isolation layer aligned axially with the bar. After grinding the delay block into a curved surface, the probe’s original angle of incidence is altered, and the height of the delay block also changes; therefore, experimental trials are required to determine the optimal inspection conditions. (Based on testing, for steel bars with diameters ranging from φ40 to 60 mm, a dual-element straight-beam probe with an original angle of incidence of 3° to 6° delivers the best results.)

Both of the above methods involve positioning the probe’s acoustic isolation layer along the axial direction of the bar, which is conducive to detecting the most common macroscopic defects in bars as well as microscopic defects such as linear segregation and banded microstructures, making it a highly sensitive inspection technique.

Some also adopt the method shown in Figure 4-5-8(d).

Two transducers are mounted on a saddle-shaped acrylic support, and the instrument operates in a “single-transmit–single-receive” mode, with the angle between the acoustic axes of the two crystals typically set at 70°.

2) Inspection using a standard commercial dual-element straight-beam probe.
As shown in Figure 4-5-8(b).

The flat bottom of the delay block in a conventional dual-element straight-beam probe is ground into a concave cylindrical surface to match the curved surface of the bar, with the probe’s acoustic isolation layer aligned axially with the bar. After grinding the delay block into a curved surface, the probe’s original angle of incidence is altered, and the height of the delay block also changes; therefore, experimental trials are required to determine the optimal inspection conditions. (Based on testing, for steel bars with diameters ranging from φ40 to 60 mm, a dual-element straight-beam probe with an original angle of incidence of 3° to 6° delivers the best results.)

Both of the above methods involve positioning the probe’s acoustic isolation layer along the axial direction of the bar, which is conducive to detecting the most common macroscopic defects in bars as well as microscopic defects such as linear segregation and banded microstructures, making it a highly sensitive inspection technique.

Some also adopt the method shown in Figure 4-5-8(d).

Specifically, a standard commercial dual-element straight-beam probe is used, with its acoustic isolation layer positioned perpendicular to the longitudinal axis of the bar stock and its delay block’s flat bottom machined into a concave cylindrical surface that conforms to the bar’s curved surface. This approach maintains the probe’s original angle of incidence and is relatively simple to machine; however, due to the unfavorable angular relationship between the sound beam and the orientation of defects, as well as suboptimal reflection conditions, the detection sensitivity is low. In particular, it is difficult to detect microstructural features such as banded structures and elongated macroscopic defects.

(3) Water-immersion line-focused dual-probe inspection.

As shown in Figure 4-5-8(c).

This method is primarily used for automated inspection and requires that the focused sound beam exhibit a certain degree of divergence within the bar to ensure adequate coverage. The principle for selecting the water distance is essentially the same as that described earlier for immersion longitudinal-wave testing of large-diameter bars, while the angle between the acoustic axes of the two transducers is broadly similar to that used with a straddle-type dual-element probe.

This method achieves detection performance comparable to that of a horse-riding-type dual-crystal probe, but it is essentially a longitudinal-wave inspection. In addition, adjustments to the spacing between the two probes, the water-path distance, and the angle of incidence are all made on a physical reference test block made from the same bar stock—specifically, a radial flat-bottom-hole test block machined from bar stock with the same curvature, material, and metallurgical condition as the workpiece. Notably, adjusting the sensitivity can be quite time-consuming; however, once properly calibrated, the inspection efficiency is relatively high.

(4) Single straight-beam probe inspection with a delay block.
As shown in Figure 4-5-8(e).

A delay block—made of acrylic, polymethyl methacrylate, or other metals—is attached to the front of a conventional straight-beam transducer, with its front surface machined into a concave cylindrical surface that conforms to the curved surface of the rod.

When using this method, it is important to ensure that the thickness of the delay block is sufficient to position the second interface echo after the first bottom-surface echo from the bar stock; moreover, the material of the delay block should not introduce excessive attenuation of the ultrasonic energy, so as not to compromise detection sensitivity.

The shape design of the delay block must be given particular attention to ensure that multiple reflected waves within the block do not cause interference. To achieve this, the delay block should have a certain width, and acoustic traps should be machined or installed on its outer surface to absorb internal echoes.

When this method is used for inspection, its sound-field distribution is relatively uniform, resulting in inspection performance that is essentially equivalent to that of single straight-beam surface-probe testing on large-diameter bar stock, while also offering superior near-surface resolution.
 

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