Free forging is a highly flexible metalworking process in which metal billets undergo plastic deformation at high temperatures to achieve the desired shape. However, cracks are one of the most common defects in
free forging, directly affecting the performance and lifespan of forged parts. To address this issue, a comprehensive analysis of the causes, preventive measures, and treatment methods for cracks is essential to improve the quality of forged parts and ensure stable production.
Surface cracks are often observed during the initial stages of forging or throughout the process. They are typically caused by surface defects in the billet, improper forging processes, or uneven cooling.
Shallow Transverse Cracks
Causes
Subsurface gas bubbles in the ingot surface: If gas bubbles do not weld properly, they can expose during the initial forging process and form shallow cracks.
Excessive feed rate: Metals with poor plasticity, such as high-carbon steel or high-alloy steel, may crack under excessive deformation.
Prevention
Improve billet quality by selecting deoxidized and refined steel to reduce internal bubbles and impurities.
Optimize forging parameters by adjusting the feed rate and controlling the deformation speed to reduce surface stress concentration.
Treatment: Clean the crack areas with a flame when the crack is first visible to prevent further defect penetration.
Deep Transverse Cracks
Causes
Casting defects: Oscillations of the molten steel during pouring can delay the solidification of the outer skin, creating shrinkage cavities.
Mold issues: Rough or defective mold walls lead to concentrated shrinkage stresses.
Prevention
Control pouring speed to avoid oscillations that weaken the grain boundaries.
Regularly inspect the mold quality to ensure smooth inner surfaces.
Treatment: Use non-destructive testing methods (such as ultrasonic or magnetic particle testing) to detect crack depth. Remove defective areas or repair the forged part as necessary.
Surface Longitudinal Cracks
Causes: Excessive friction during forging leads to tensile stresses on the surface. Uneven heating or cooling causes coarse metal grains or surface fatigue.
Prevention: Ensure uniform heating to maintain consistent temperatures between the surface and the internal material. Apply lubricants to the billet surface to reduce friction.
Treatment: If cracks are detected, remove them promptly through re-heating or surface restoration to restore the forged part's properties.
Side Longitudinal Cracks
Cause: Uneven deformation during forging. Low forging temperature, resulting in insufficient plasticity of the billet.
Prevention: Optimize heating temperatures and forging sequences to avoid localized excessive stress.
Internal cracks are more dangerous because they are often hidden and difficult to detect directly, posing a greater threat to the quality of forged parts.
Causes
Improper riser design: Shrinkage cavities not concentrated in the riser area lead to internal cracks.
Insufficient cutting of the billet head: Incomplete removal of the shrinkage region.
Prevention
Increase the riser volume to ensure shrinkage cavities are removed effectively.
Adopt proper cutting strategies to ensure that residual shrinkage areas are removed.
Treatment: Use ultrasonic testing to detect shrinkage areas. If necessary, re-cut or re-forge the part.
Cause: Insufficient material plasticity: High-alloy steels tend to form stress concentration zones when repeatedly forged in the same area.
Prevention:
Increase forging temperature to avoid cooling in specific areas that reduce plasticity.
Optimize deformation paths to avoid repetitive processing of the same part.
Cause: Rapid heating or large temperature differences during cooling cause uneven internal stress release.
Prevention:
Control heating and cooling rates and use slow-cooling processes.
Pre-treat the material thermally to reduce residual stress.
In addition to surface and internal cracks, free-forged flanges can experience other types of cracks caused by material properties, improper operations, or environmental conditions. These require careful analysis and handling based on specific circumstances.
Cause: After cooling or heat treatment, micro-cracks formed during forging expand under residual stress.
Prevention and Treatment: Strengthen control over heat treatment processes to reduce cooling speed. Use magnetic particle or ultrasonic testing to detect micro-cracks early, preventing their expansion.
Cause: High levels of impurities such as copper (Cu) and tin (Sn) in the steel reduce grain boundary strength. At high temperatures, molten copper may penetrate the steel's grain boundaries, weakening their bond.
Prevention and Treatment:
Control material composition to ensure the purity of the metal meets standards.
Regularly clean furnaces to avoid copper slag contamination.
Strictly control heating and forging temperatures to reduce impact during initial forging.
To address the various types of cracks that may occur during the forging of flanges, effective preventive and repair strategies are essential to ensure the quality of the forged parts and improve production efficiency.
Use refined steel to reduce internal inclusions and bubbles. Regularly inspect steel ingots to ensure they are free of subsurface defects.
Adjust the initial feed rate and deformation amount to reduce surface and internal stress concentrations. Use staged heating or controlled cooling methods to evenly release stress.
Introduce ultrasonic, magnetic particle, or X-ray testing throughout the forging process to detect and address cracks early, preventing problems from escalating during subsequent processing.
Use suitable annealing, normalizing, or slow-cooling techniques for the forged parts to reduce internal stress. Eliminate work hardening to further improve the parts' resistance to cracking.
The causes of cracks in free-forged flanges involve multiple factors, from material quality to precise control of forging processes, as well as improvements in post-forging heat treatment and inspection methods. By implementing targeted preventive measures and timely crack repairs, companies can not only improve the quality of forged parts but also reduce production costs and enhance competitiveness.
