Advanced Ultrasonic Testing of Castings and Forgings: FMC/TFM Phased Array UT (PAUT) Inspection

Introduction

Ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) are essential nondestructive testing methods for evaluating the internal integrity of metallic forgings and castings. These techniques provide reliable detection and characterization of internal discontinuities, including inclusions, porosity, shrinkage cavities, cracks, and other manufacturing-related defects that may compromise component performance. Industry standards such as ASTM A388/A388M for steel forgings and ASTM A609/A609M for carbon, low-alloy, and martensitic stainless steel castings establish standardized ultrasonic examination practices and evaluation requirements. While conventional UT provides proven methods for detecting internal discontinuities, advanced PAUT technology offers enhanced inspection coverage, electronic beam steering, and detailed cross-sectional imaging to improve flaw detection, characterization, and sizing. Together, these technologies provide manufacturers and asset owners with valuable tools for verifying material quality, supporting quality assurance programs, and evaluating the fitness of critical metallic components.

Figure 1: Example FMC/TFM data from a steel casting

Ultrasonic Testing of Steel Forgings – ASTM A388/A388M

ASTM A388/A388M establishes standardized practices for ultrasonic examination of steel forgings using longitudinal-wave and, where applicable, shear-wave techniques. Conventional ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) can be used to evaluate the internal integrity of forged components, identifying discontinuities such as inclusions, cracks, voids, and other manufacturing-related defects. Examinations typically employ straight-beam scanning to evaluate volumetric integrity, with angle-beam examinations performed when required by component geometry or inspection specifications. ASTM A388/A388M is primarily an examination practice and does not establish universal acceptance criteria for all forgings. Instead, acceptance requirements are generally defined by the purchaser, applicable material specification, or governing construction code. Discontinuities are evaluated based on their ultrasonic response, including indication amplitude, location, extent, and associated loss of backwall reflection, as required by the applicable acceptance specification.

Ultrasonic Testing of Steel Castings – ASTM A609/A609M

ASTM A609/A609M specifically addresses ultrasonic examination of carbon, low-alloy, and martensitic stainless steel castings, where manufacturing processes may produce shrinkage cavities, porosity, inclusions, and other internal discontinuities. Unlike forgings, castings frequently exhibit complex geometries, variable grain structures, and increased acoustic attenuation, requiring careful selection of examination frequencies, calibration techniques, and scanning strategies. ASTM A609/A609M provides two examination approaches: Procedure A, using amplitude-based quality levels, and Procedure B, using flat-bottom-hole reference standards and specified evaluation criteria. Under Procedure A, indications are evaluated against established reference sensitivity and applicable quality-level requirements, including the extent of indications exceeding the evaluation threshold. Procedure B evaluates indications against responses established using reference reflectors and the applicable specified acceptance requirements. The appropriate procedure and acceptance level must be established by the purchase specification or governing engineering requirements.

Figure 2: Example PAUT data from a steel casting.

Advanced PAUT and FMC/TFM Applications

Phased array ultrasonic testing (PAUT), including Full Matrix Capture (FMC) and Total Focusing Method (TFM) imaging, provides advanced capabilities for examining complex steel castings and forgings. Unlike conventional single-element UT, PAUT can electronically control beam angles and focal characteristics, while FMC/TFM techniques collect and process multiple transmit-receive combinations to generate detailed ultrasonic images of internal discontinuities. These technologies can improve inspection coverage, flaw visualization, and characterization, particularly in components with challenging geometries or complex internal reflectors. However, the use of PAUT or FMC/TFM does not automatically establish compliance with ASTM A388/A388M or ASTM A609/A609M. The examination technique must be demonstrated and qualified as applicable to the governing specification, and acceptance decisions must remain consistent with the specified reference sensitivity, evaluation methodology, and acceptance criteria. When properly implemented, these advanced technologies provide valuable additional information for evaluating the quality and integrity of critical castings and forgings.

Phased Array Ultrasonic Testing (PAUT) vs. Total Focusing Method (TFM)

Conventional Phased Array Ultrasonic Testing (PAUT) utilizes multiple transducer elements that are electronically pulsed with precisely controlled time delays to steer, focus, and sweep ultrasonic beams through a material. This technology enables inspectors to examine components from multiple angles and focal depths without physically repositioning the transducer for each beam angle. PAUT typically produces sectorial (S-scan) or linear (E-scan) images that provide real-time visualization of internal discontinuities, making it particularly effective for examining metallic castings and forgings. However, conventional PAUT generally relies on predefined focal laws, meaning the ultrasonic beam is optimized for selected angles and focal depths. Consequently, image resolution and sensitivity can vary throughout the inspection volume, particularly outside the designated focal regions.

Total Focusing Method (TFM) is an advanced ultrasonic imaging technique that typically utilizes Full Matrix Capture (FMC) data, in which individual transducer elements transmit sequentially while multiple elements receive and record the resulting ultrasonic signals. Unlike conventional PAUT, which physically focuses the ultrasonic beam during transmission, TFM computationally reconstructs the inspection volume by applying appropriate time-of-flight delays to the recorded signals, effectively focusing at every image pixel within the selected region of interest. This approach can provide improved spatial resolution, more consistent focusing, and enhanced visualization of closely spaced or irregularly shaped discontinuities. For castings and forgings, FMC/TFM can be particularly valuable for characterizing shrinkage cavities, inclusions, cracks, and other complex internal reflectors. However, TFM generally requires greater data acquisition and processing resources, and its performance remains dependent on material acoustic properties, probe configuration, inspection geometry, and the selected reconstruction algorithm. While both technologies use phased array transducers, TFM differs fundamentally in how ultrasonic data are acquired, focused, and reconstructed into an image.

Case Study

Example FMC/TFM data for a steel casting is shown below using a 0 degree 32-element dual element linear array. The top left view shows the resulting C-scan with the vertical green index axis on the left and the horizontal light blue scan axis on the bottom. The bottom left view shows the cross-sectional thickness view of the scan axis plane. The top right view shows the cross-sectional thickness view of the index axis plane. The bottom right view is the A-scan selected by the blue data cursors in the scan and index axes directions. The red and green horizontal and vertical are using in the scan and index axes to measure indication scan axis length, index axis length, and depth.

In this example a casting defect is locates along the scan index at 6.2” and the index axis at 7.05” The gain was adjusted to set the defect response to approximately 80% FSH. The red and green measurement cursors were positioned at the defect edges using 6 dB sizing. The defect length in the scan and index axes was approximately 0.200” x 0.060” for a 0.120 square inch area. Comparing these values against ASTM A609 Table 2 Rejection Level value, this defect passes all ultrasonic testing quality levels by area and maximum length.

Figure 3: Example FMC/TFM data from steel casting illustrating how to use data cursors to size defects in scan and index axes. The selected indication passes ASTM A609A Quality Level 1 requirements

Summary

This case study demonstrates the effectiveness of FMC/TFM phased array ultrasonic testing for detecting, imaging, and characterizing internal discontinuities in steel castings. Using advanced ultrasonic imaging and 6 dB sizing techniques, a casting indication was located and measured at approximately 0.200 × 0.060 inches, representing a projected area of 0.0120 square inches. These measurements provide valuable information for evaluating discontinuities against applicable ASTM A609/A609M acceptance criteria. FMC/TFM technology offers enhanced flaw visualization and characterization, making it a valuable tool for quality assurance and nondestructive inspection of critical castings and forgings.