Mar 31,2026
Ultrasonic damage detection of mold steel: damage-free detection of internal defects
Ultrasonic damage detection of mold steel: damage-free detection of internal defects
Ultrasonic testing, Non-destructive testing, Internal defects, Inclusions, Material inspection
The internal quality of the steel in the mold is a “invisible world” that cannot be observed with the naked eye. Internal defects such as shrinking pores, air holes, interlocking, cracks, and so on can lie dormant deep in the steel and suddenly appear during the mold processing or service process, resulting in catastrophic failure. Therefore, ultrasound damage detection of the steel at key points in the mold manufacturing, especially large modules or key projects, is necessary. This is the most widely applied damage-free detection technique, which is like conducting a deep “B-ray” inspection of the steel to ensure its internal purity.
The basic principle of ultrasonic damage detection is that when ultrasonic waves are propagated in materials, reflections occur when they encounter interfaces (such as defects, bottoms). The probe emits high-frequency sound waves (typically 2-10 MHz) and receives reflected echoes. By analyzing the amplitude, time, and location of the echoes, the depth, size, and nature of the defects can be determined. In the molding industry, the most commonly used method is the Type A pulse reflection method.
The application of ultrasonic damage detection permeates the entire mold manufacturing process. When steel enters the factory, 100% ultrasonic damage detection is performed on large modules to screen out raw materials with internal defects, eliminating potential hazards from the source. For welded-repair molds, damage detection is used to check the internal quality of the weld seam, ensuring that there are no air holes, clots, and non-melting. After heat treatment of the mold, damage detection can evaluate whether tempered cracks have been generated. In mold failure analysis, damage detection is used to find the germination location and expansion path of fatigue cracks.
The ability of ultrasonic probes to detect defects depends on several factors. The type, direction, size, and depth of defects all affect the detection results. Typically, plate-shaped defects that are perpendicular to the direction of the sound beam (such as cracks) are most sensitive, spherical defects (such as air holes) are second, and defects that are parallel to the sound beam are harder to detect. Therefore, probes often need to be scanned from multiple directions to cover defects with different orientations. The choice of probes is also crucial: higher frequency, higher resolution, but weaker penetration ability; lower frequency, stronger penetration, but lower resolution. For large moulded steel, probes of 2.5 MHz or 5 MHz are often chosen.
The determination of injury detection results needs to be based on relevant standards and experience. The detection personnel determine the grade of defects based on echo amplitude and equivalent size, with reference standards such as GB/T 4162, SEP 1921. For cast steel, defects with equivalent diameters greater than a certain threshold are generally not allowed. In the injury detection report, the location, depth, equivalent size, and grade of defects are recorded in detail. The customer can decide whether to accept the steel based on the report.
Limitations of Ultrasonic Detection and Their Compensation. Ultrasonic detection is not sensitive to surface and near-surface defects (there are blind spots), detects complex-shaped components with difficulty, and results interpretation depends on the operator‘s experience. Therefore, it is sometimes necessary to combine other damage-free detection methods, such as magnetic powder detection (detecting surface cracks), permeation detection (detecting opening defects), to obtain comprehensive quality information.
Ultrasonic damage detection is invaluable in the molding industry. It uses scientific means to expose the “hidden corners” inside steel to sunlight, building the first line of defense for the reliability of the mold. Choosing a mold supplier with ultrasonic damage detection capabilities means having higher confidence in the quality of the mold.
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