Steel forging defects are rarely discussed due to the reluctance of forging companies to highlight them. These imperfections can arise from the initial materials, the closed die forging process, or post-forging operations.
Defects are defined as imperfections exceeding certain limits. Some imperfections may not be classified as true "defects" because they fall within acceptable tolerances in relevant specifications.
Below is information about common and less common defects in steel forgings, stemming from closed die forging operations or typical post-forging processes in many forge plants. The aim is to familiarize readers with these defects, understand their impact on forging performance, and explore methods to eliminate them in future production.
(1) Incomplete Filling: This defect occurs when some sections of the forging remain unfilled, often due to poor die design or forging techniques. It can also result from insufficient raw material or inadequate heating. Proper die design, adequate raw material supply, and precise heating can rectify this issue.
(2) Cold Shut: Cold shut defects manifest as small cracks at corners due to improper forging die design, sharp corners, or excessive cooling. Increasing the fillet radius of the die can mitigate these defects.
(3) Surface Scale Pits: Scale pits arise from inadequate cleaning of the forged surface, particularly common in open forging environments. They appear as irregular depressions on the forging surface and can be eliminated through thorough surface cleaning.
(4) Die Misalignment: Die shift occurs when the upper and lower dies are misaligned, resulting in improperly dimensioned forged products. Proper alignment, achieved by providing matching notches on the upper and lower dies, can resolve this defect.
(5) Internal Flakes: Flakes are internal cracks caused by rapid cooling of the forged product, reducing its strength. Proper cooling methods can prevent the occurrence of these cracks and ensure the integrity of the forged product.
(6) Incorrect Grain Growth: Improper metal flow during casting can alter the predetermined grain structure of the product, leading to this defect. Proper die design can help alleviate this issue.
(7) Insufficient Forging Penetration: Incomplete forging, often caused by light or rapid hammer blows, results in this defect. Precise control over the forging press can rectify this issue.
(8) Surface Fractures: Surface cracking occurs when excessive force is applied to surfaces at low temperatures, resulting in numerous cracks. Controlling the working temperature can prevent this defect.
(9) Residual Stresses in Forging: Improper cooling of forged parts leads to residual stresses, mainly due to excessively rapid cooling. This defect can be mitigated by implementing a slower cooling process.
