English Views: 0 Author: Eternal Tech Team Publish Time: 2026-09-18 Origin: Site
Stainless steel forgings, die steel forgings, and roll forgings are widely used in key fields such as power generation equipment, petrochemicals, shipbuilding, and heavy machinery. These large forgings are often the main load-bearing parts of the equipment. If there are hidden defects inside them, such as cracks, inclusions, or gas holes, they could cause a disastrous failure under high temperature and high pressure or repeated changing loads. Nondestructive testing (NDT) is like “X-ray vision” that finds these hidden defects without damaging the forging.
For forging manufacturers that focus on exports, knowing the NDT requirements in ASTM and other international standards is not only a necessary part of quality control, but also the key to winning the trust of overseas customers. This article will explain the core points of NDT for large forgings in plain language, from standards systems and practical methods to the latest technology trends.
In the international market, NDT of large forgings mainly follows these ASTM standards:
ASTM A388/A388M is the basic standard for ultrasonic testing of large forgings. It covers the operating rules for contact pulse-echo ultrasonic testing, including two technical paths: straight beam probes and angle beam probes. This standard requires the use of ultrasonic equipment with linear display capability. At least 75% of the height range on the screen should keep a linear response, so that defect signals can be read accurately. In straight beam probe testing, sensitivity calibration can use the reflection method, the reference block method, or the DGS (Distance-Gain-Size) method.
ASTM A418 is specifically for ultrasonic testing of steel forgings for turbine and generator rotors. It applies to rotor forgings such as A469, A470, A768, and A940. Rotor forgings work under high-speed rotation, so they can barely tolerate internal defects. That is why this standard is stricter.
ASTM A745/A745M is for ultrasonic testing of austenitic stainless steel forgings. Because austenitic stainless steel has coarse grains, ultrasonic waves scatter a lot as they travel through it. This makes testing much harder than for carbon steel and low-alloy steel forgings. This standard provides a special technical solution for NDT of stainless steel forgings.
In addition, ASTM A503/A503M (2025 edition) is an acceptance specification for forged crankshafts. It applies to forged steel crankshafts with a main journal or crankpin diameter of no less than 100 mm, and covers both continuous grain flow forging and solid forging processes. ASTM A275/A275M sets out the operating methods for magnetic particle testing of steel forgings.
Ultrasonic testing is the most commonly used method for checking the internal quality of large forgings. The idea is simple: a probe sends high-frequency sound waves into the forging. When the sound waves hit defects like cracks or inclusions, they bounce back as reflected signals. By analyzing the time and strength of these reflected signals, we can tell where the defect is and how big it is.
In practice, large forgings are usually tested by UT after rough machining. This makes sure the forging surface has a cylindrical or flat surface that lets the probe move smoothly. For round forgings, a cylindrical surface needs to be machined for radial testing. The end face needs to be machined into a flat surface perpendicular to the forging axis for axial testing. 100% volume coverage is the standard requirement for safety-critical parts. This means the sound beam needs to cover every cubic centimeter of the forging. This usually takes multiple scans from different directions working together to achieve.
Magnetic particle testing is used to find surface and near-surface defects in ferromagnetic material forgings, such as cracks, laps, and hairline cracks. ASTM A275 sets out the specific operating methods. For ferromagnetic materials like die steel forgings, magnetic particle testing is a useful supplement to UT.
For non-ferromagnetic materials like stainless steel forgings, penetrant testing is the first choice for surface defect testing. The testing process includes surface preparation, applying the penetrant, removing excess penetrant, drying, developing, and inspection. For acceptance, linear defects are usually not allowed. The maximum size of round defects is generally kept within 2 mm, and the number of defects in every 100 cm² should not exceed 3.
In the past two years, the most notable technical progress in NDT of large forgings is the gradual use of ultrasonic phased array (PAUT) and the full matrix capture method (TFM).
A study published in 2026 at ECNDT (the European Conference on Nondestructive Testing) looked at large forgings with a thickness of 100–300 mm used in power generation equipment. It systematically compared the testing ability of traditional manual UT with advanced methods like PAUT and TFM. The study found that when traditional manual UT is used to find small defects with an equivalent size of 0.5–1 mm, its reliability clearly drops because of scattering from the microstructure and sound attenuation. PAUT, however, uses electronic control to steer and focus the sound beam. This allows more flexible scanning and higher testing resolution.
It is worth noting that for austenitic stainless steel forgings, the latest industry practice recommends using low-frequency dual matrix array probes together with a longitudinal wave transmit-receive (TRL) setup. This is currently the most advanced testing solution for coarse-grained materials. In real projects, some companies in China have already used PAUT and digital radiography (DR) together for overall quality verification when delivering large-diameter flange forgings, and the results have been good.
In China, group standards released in 2025 now specifically cover phased array ultrasonic testing for ferritic-martensitic steel forgings and stainless steel forgings. These standards cover areas such as testing personnel qualifications, equipment requirements, test blocks, sensitivity settings, and quality levels.
Common defect types in NDT of large forgings include:
White spots (hydrogen-induced cracks): internal cracks caused by too much hydrogen during the cooling of large forgings. They have a huge impact on forging safety and are a key target for UT.
Non-metallic inclusions: inclusions that come from the steelmaking process of the ingot. They may appear as single particles or in clusters.
Residual shrinkage cavities: loose areas in the core that were not fully pressed solid during forging.
Coarse grains: especially in austenitic stainless steel, large grains increase ultrasonic scattering and lower the signal-to-noise ratio of testing.
For quality control, it is suggested to start with these steps. First, make sure testing personnel hold qualifications recognized by ISO 9712 or ASNT. NDT certification for forging products should cover methods such as ultrasonic, magnetic particle, and penetrant testing. Second, testing should be done after rough machining to make sure the surface roughness meets the coupling requirements of the probe. Third, choose the right testing method and acceptance level based on the forging material and use. For carbon steel and low-alloy steel forgings, 100% volume inspection according to ASTM A388 is recommended.
Based on the standard updates and technology developments of the past two years, NDT of large forgings shows a few clear trends:
Standards keep being updated. ASTM A388 was revised in both 2023 and 2026, and A418 and A745 also released their latest versions in 2024. Export companies need to track standard changes in time. This helps avoid acceptance disputes caused by using different versions of a standard.
Advanced ultrasonic technology is being put to use faster. PAUT and TFM are moving from lab research to industrial use. This is especially true in fields that demand very high testing reliability, such as nuclear power and power generation equipment. Advanced ultrasonic methods are gradually replacing traditional manual UT.
Digitalization and traceability requirements are going up. More and more international customers require NDT data to be digitally recorded and traceable. Test reports need to include complete scanning parameters, calibration records, and defect evaluation results.
For manufacturers of stainless steel forgings, die steel forgings, and roll forgings, building an NDT system that meets ASTM standards is not only the basis for meeting customer acceptance requirements. It is also an important safeguard for improving international competitiveness of their products.
