A Brief Analysis of Ultrasonic Flaw Detection Application Techniques for Forging

Forgings and castings are important blanks for various mechanical equipment, boilers, and pressure vessels.
1. Ultrasonic flaw detection of castings is challenging due to the coarse grain size, poor sound transmission, and low signal-to-noise ratio of castings. It utilizes sound beams with high-frequency acoustic energy to propagate inside the casting, and detects defects by reflecting off internal surfaces or defects. The magnitude of the reflected acoustic energy is a function of the directionality and nature of the internal surface or defect, as well as the acoustic impedance of the reflector. Therefore, various defects or internal surface reflections can be used to detect the location, wall thickness, or depth of subsurface defects. Ultrasonic testing, as a widely used nondestructive testing method, has the following main advantages: high detection sensitivity, allowing for the detection of small cracks; and strong penetration capability, enabling the detection of thick-section castings. Its main limitations include: difficulty in interpreting the reflection waveform of disconnected defects with complex contour dimensions and poor directionality; undesirable internal structures, such as grain size, organizational structure, porosity, inclusion content, or small dispersed precipitates, also hinder waveform interpretation; in addition, reference standard test blocks are required during testing.
II. Ultrasonic Testing of Forgings (I) Forging Processing and Common Defects Forgings are formed by forging and deforming hot ingots. The forging process includes heating, deformation, and cooling. Forging defects can be divided into casting defects, forging defects, and heat treatment defects. Casting defects mainly include shrinkage cavities, porosity, inclusions, and cracks. Forging defects mainly include folding, flakes, and cracks. Heat treatment defects are mainly cracks.
Shrinkage cavity residue is the remnant of shrinkage cavity in the ingot that remains due to insufficient head cutting during forging, commonly found at the ends of forgings.
Porosity refers to the non-compactness and cavities formed in steel ingots during solidification and contraction. During forging, due to insufficient forging ratio, it is not fully dissolved and mainly exists in the center and head of the steel ingot.
There are inherent inclusions, external non-metallic inclusions, and metallic inclusions. Inherent inclusions are mainly concentrated in the center and head of the ingot.
Cracks include casting cracks, forging cracks, heat treatment cracks, etc. The axial intergranular cracks in austenitic steel are cracks caused by casting. Improper forging and heat treatment can lead to the formation of cracks on the surface or in the core of the forged piece.
White spots are caused by high hydrogen content in the forged piece, followed by rapid cooling after forging, which prevents the dissolved hydrogen in the steel from escaping in time, leading to excessive stress and subsequent cracking. These white spots are primarily concentrated in the center of the large cross-section of the forged piece. They always appear in clusters within the steel.
(2) Overview of flaw detection methods: Classified by flaw detection time, flaw detection of forgings can be divided into flaw detection of raw materials, flaw detection during manufacturing, product inspection, and in-service inspection.
The purpose of flaw detection in raw materials and during manufacturing is to detect defects early so that timely measures can be taken to prevent the defects from expanding and causing scrap. The purpose of product inspection is to ensure product quality. The purpose of in-service inspection is to monitor defects that may occur or develop after operation, mainly fatigue cracks.
1. Flaw Detection of Axle Forgings: The forging process of axle forgings primarily involves drawing, so most defects are oriented parallel to the axis. The best detection method for such defects is using a longitudinal wave straight probe for radial detection. Considering that defects may have other distributions and orientations, the flaw detection of axle forgings should also be supplemented with axial detection using a straight probe and circumferential detection using an angle probe.
2. Flaw detection of disc and bowl forgings: The forging process of disc and bowl forgings primarily involves upsetting. The distribution of defects is mainly parallel to the end face, so using a straight probe to detect on the end face is the best method to identify defects.
3. Flaw Detection of Cylindrical Forgings The forging process of cylindrical forgings involves upsetting, punching, and rolling. Therefore, the orientation of defects in cylindrical forgings is more complex than that in shaft forgings and disc forgings. However, since the central part of the ingot with the poorest quality has been removed during punching, the quality of cylindrical forgings is generally better. The main orientation of defects is still parallel to the outer cylindrical surface of the cylinder, so the flaw detection of cylindrical forgings mainly relies on straight probe detection of the outer cylindrical surface. However, for thick-walled cylindrical forgings, angle probe detection must be added.
(III) Selection of Detection Conditions 1. Selection of Probe When conducting ultrasonic flaw detection on forgings, longitudinal wave straight probes are primarily used, with wafer sizes ranging from Φ14 to Φ28mm, with Φ20mm being commonly used. For smaller forgings, considering the near-field region and coupling loss, small wafer probes are generally employed. Sometimes, to detect defects that form a certain inclination angle with the detection surface, angle probes with a certain K value can also be used for detection. For defects at close range, due to the influence of the blind zone and near-field region of the straight probe, dual-crystal straight probes are often used for detection.
The grain size of forgings is generally relatively fine, so a higher flaw detection frequency can be chosen, commonly ranging from 2.5 to 5.0 MHz. For a small number of forgings made of materials with coarse grains and severe attenuation, to avoid the appearance of "forest echoes" and improve the signal-to-noise ratio, a lower frequency should be selected, generally ranging from 1.0 to 2.5 MHz.