During the forging process, various types of cracks may appear on the surface or inside the billet. These cracks not only affect the quality of the forged parts but may also lead to difficulties in subsequent processing. The formation of cracks is usually closely related to factors such as forging techniques, material properties, and operating conditions. By employing reasonable detection methods and timely treatment measures, the occurrence of cracks can be effectively reduced, ensuring the quality of forged parts. Below is a detailed analysis of common types of cracks and their treatment methods.
Surface Transverse Cracks
Causes: Surface transverse cracks often appear during the first heating in the forging process. These cracks are closely related to improper casting and forging operations of the ingot. Shallow transverse cracks usually result from unhealed bubbles on the ingot surface, with crack depths potentially exceeding 10 mm. Additionally, metals with poor plasticity are prone to such defects when the feed rate is excessive. Deeper transverse cracks are typically caused by defects in the ingot mold wall or improper casting methods, such as high-temperature, high-speed casting or improper cooling operations.
Treatment: Once surface transverse cracks, especially deeper ones, are detected, they should be immediately removed. Flame cleaning can be used to eliminate the cracks and prevent their expansion during subsequent forging. After cleaning, forging can usually continue without issues.
Internal Transverse Cracks
Causes: Internal transverse cracks are often caused by uneven heating or rapid cooling. These cracks are usually invisible to the naked eye and require magnetic particle or ultrasonic testing for detection. When heating a cold ingot too quickly, significant tensile stress is generated at the ingot's center, leading to cracks. High-carbon and high-alloy steels with poor plasticity are particularly prone to such cracks if the feed rate is too small during forging.
Treatment: To prevent internal transverse cracks, it is essential to control the heating temperature and rate, ensuring uniform heating of the material. Additionally, the feed rate should be adjusted according to the material properties to avoid cracks caused by insufficient feeding.
Surface Longitudinal Cracks
Causes: Surface longitudinal cracks commonly occur during the first forging or upsetting process. Their formation is closely related to defects in the ingot mold wall, unannealed new ingot molds, or improper casting operations. For example, high-temperature, high-speed casting can cause surface rupture, while improper cooling or premature demolding may also lead to cracks. Excessive reduction during chamfering or longitudinal scratches during rolling are also potential causes.
Treatment: When surface longitudinal cracks are detected, oxygen blowing should be used to remove the cracks promptly, preventing their expansion during subsequent forging. Strict control of the casting and cooling processes, along with regular inspection of forging equipment, is necessary to ensure the ingot mold wall is defect-free.
Side Surface Longitudinal Cracks
Causes: During free upsetting, friction between the billet and the anvil can cause uneven deformation, resulting in a "bulging" shape. Under tensile stress, irregular longitudinal cracks may form on the billet surface. Such cracks often appear on billets upset at low temperatures, especially in materials with poor plasticity.
Treatment: To avoid side surface longitudinal cracks, temperature control during upsetting should be optimized to ensure uniform billet temperature, avoiding upsetting at low temperatures.
Internal Longitudinal Cracks
Internal longitudinal cracks can be categorized as follows:
Cracks near the riser end: Caused by insufficient shrinkage cavity concentration during ingot solidification or insufficient riser end cropping.
Hollow longitudinal cracks: Result from tensile stress at the billet center during elongation or insufficient heating, leading to low temperatures.
Longitudinal cross cracks: Common in high-alloy steels, caused by excessive feed rates or repeated elongation at the same location.
Treatment: Precise forging techniques should be employed to prevent shrinkage cavities or uneven heating. Ensure the billet is uniformly heated before elongation, avoiding insufficient or uneven heating.
Bursting
Causes: Bursting refers to cracks that form on the surface or inside the billet during heating or after cooling and heat treatment. This is typically caused by significant residual stress in the billet, combined with rapid heating or improper cooling.
Treatment: To prevent bursting, control the heating rate to avoid rapid heating or cooling. Additionally, proper annealing before forging can help eliminate residual stress.
Spontaneous Cracking of Forged Parts
Causes: Spontaneous cracking usually occurs after forging or heat treatment, or after a prolonged period. It is often caused by micro-cracks formed during forging, which are exacerbated by cooling or heat treatment, or by significant residual stress inside the forged part.
Treatment: Strict control of the forging process is necessary to avoid excessive stress accumulation. Optimizing cooling and heat treatment processes can prevent the expansion of existing micro-cracks.
Crazing
Causes: Crazing refers to shallow, tortoise-shell-like cracks on the surface. It is usually caused by excessive copper (Cu) or tin (Sn) content in the steel, or by incomplete removal of slag during heating, allowing molten copper to penetrate the steel grain boundaries. It may also result from excessively high initial forging temperatures or heavy hammering.
Treatment: Crazing is generally shallow and can be prevented from expanding by timely removal. After removal, forging can usually continue without issues.
Conclusion
The formation of various types of cracks during forging is closely related to factors such as material plasticity, heating temperature, and forging pressure. By implementing reasonable process control, strict operating procedures, and timely crack detection, the occurrence of cracks can be effectively reduced, ensuring the quality and performance of forged parts. For detected cracks, appropriate treatment methods should be applied to prevent their expansion during subsequent forging.
