In the manufacturing process of forgings, flakes are tiny defects that severely disrupt the continuity of steel. Their presence significantly reduces the mechanical properties of steel, leading to decreased plasticity in structural components and potential brittle fractures during use. Therefore, in most ultrasonic testing standards, flakes are explicitly defined as internal defects that must not exist. To ensure the quality of forgings, the identification and prevention of flakes are particularly crucial in daily inspection work.

1. Detection of Flakes – Ultrasonic Testing

Ultrasonic testing is a common method for detecting flakes. During ultrasonic testing, if the forging shows no back echo and the defect echoes are very strong, interconnected, and vary in height, with the defect echoes becoming active as the probe moves slightly, these phenomena are typical characteristics of flakes. Ultrasonic testing can effectively identify flake defects in forgings.

2. Cause Analysis of Flakes

The formation of flakes is primarily due to the combined effect of excessive hydrogen content and internal stress in steel. Different materials have varying sensitivities to flakes, with certain alloy steels such as chromium steel, chromium-molybdenum steel, manganese steel, manganese-molybdenum steel, chromium-manganese steel, chromium-manganese-molybdenum steel, chromium-nickel-molybdenum steel, and chromium-nickel-tungsten steel being more prone to flakes. If the hydrogen content is high but the structural stress is low, flakes generally do not occur. For example, single-phase austenitic and ferritic steels rarely develop flakes due to the absence of phase transformation stress. It is important to note that flakes often appear several hours or even dozens of hours after the forging has cooled to room temperature. Therefore, flake inspection should be conducted after a certain period following cooling.

3. Preventive Measures for Flakes

Since flakes are mainly caused by the combined effect of hydrogen and structural stress in steel, eliminating hydrogen and structural stress is key to preventing flakes. First, the melting process should be strictly controlled, and methods such as vacuum pouring should be used to reduce hydrogen content. If the hydrogen content cannot be controlled below 2cm³/100g during steelmaking, a reasonable hydrogen removal cooling procedure must be applied after forging, and direct air cooling to room temperature must be avoided. To further eliminate flakes, the main principle of the cooling procedure is to maintain the steel at the temperature range where hydrogen diffusion is fastest for an extended period, minimizing various stresses (such as phase transformation stress, deformation residual stress, and cooling temperature stress) to allow hydrogen to fully diffuse out of the ingot. Specific measures include isothermal annealing.

Through scientific detection methods and effective preventive measures, the occurrence of flakes in forgings can be significantly reduced, ensuring the quality and safety of steel.

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