The Influence of Material Defects in Forgings

Manufacturing processes for various components in industrial settings involve intricate procedures, often prone to specific challenges that can impact product quality. From oblique cutting to end cracks and gas cutting cracks, each issue poses unique risks during manufacturing and forging operations. Addressing these challenges requires a comprehensive understanding of their causes and potential solutions to ensure the integrity and reliability of the final forged parts.

1. Oblique Cutting

Oblique cutting, also known as slant cutting, pertains to the angular deviation observed in the end face of a blank in comparison to its longitudinal axis during the loading and unloading process on sawing or punching machines. When severe slant occurs, there is a risk of the material folding during forging operations, potentially leading to defects in the final product. Thus, it is essential to mitigate slanting issues to ensure the integrity and quality of the manufactured components.

2. Burr Formation at the End of Blanks

During the feeding and unloading process on cutting or punching machines, the end of the blank may become bent and develop burrs. This occurs when there is excessive clearance between the scissors or cutting die edge, or when the edge is dull. Consequently, the blank bends before it is properly cut, causing some of the metal to be squeezed into the clearance of the blade or die. This results in the formation of protruding burrs at the end of the blank.

Blanks with burrs are susceptible to local overheating and excessive burning during heating processes. Additionally, they are prone to folding and cracking when subjected to forging operations. Therefore, it is crucial to address issues related to burr formation to prevent defects and ensure the quality of the manufactured parts.

3. Concave End Faces on Blanks

During the cutting process on the machining equipment, the end face of the blank may become concave. This occurs when there is insufficient clearance between the cutting blades, causing the metal section to experience cracks and misalignment. As a result, the metal at the end of the blank undergoes a double shearing action, leading to partial metal detachment and the formation of a concave-shaped end face.

Blanks with concave end faces are highly susceptible to folding and cracking during forging operations. It is imperative to address the root causes of concave end faces to mitigate defects and ensure the integrity of the forged components.

4. End Cracks in Blanks

End cracks frequently occur in large-section alloy steel and high-carbon steel bars shortly after cold shearing, typically within 3 to 4 hours post-shear. This issue arises due to excessive unit pressure applied by the blade, causing circular-section blanks to deform into an oval shape. Consequently, significant internal stress accumulates within the material.

As the compressed end surface attempts to revert to its original shape, the built-up internal stress often leads to crack formation hours after cutting. Additionally, materials with uneven hardness or severe segregation are more prone to shear cracks.

During forging operations, blanks with end cracks pose significant challenges, as the cracks tend to propagate further under the forging pressure. It is essential to address the root causes of end cracks, such as optimizing blade pressure and ensuring material uniformity, to prevent defects and ensure the integrity of the forged components.

5. Cracking from Gas Cutting

Gas cutting cracks typically manifest at the ends of billets, resulting from residual stresses and thermal stresses induced by cutting without preheating the raw material. These cracks have a tendency to propagate further during forging operations. To mitigate this issue, it is advisable to clean the billet thoroughly before forging to remove any surface imperfections.

When gas cut cracks are present in a blank, they pose a risk of expanding during forging, potentially compromising the integrity of the final product. Therefore, proactive measures, such as meticulous cleaning and inspection, should be taken to address these cracks prior to forging.

6. Cracking Due to Convex Core Formation

During lathe blanking processes, the end face of the bar often exhibits a convex core at its center. This convex core poses challenges during forging due to its reduced plasticity resulting from its smaller section and rapid cooling.

As a consequence, stress becomes concentrated at the junction of the sudden section change, exacerbating the discrepancy in plasticity between the convex core and surrounding material. Under the force of the hammer, this concentration of stress renders the area surrounding the convex core susceptible to cracking.

To mitigate the risk of convex core cracking during forging, careful consideration should be given to the material's plasticity, forging temperature, and forging technique. Additionally, optimizing the lathe blanking process to minimize the formation of convex cores can help alleviate this issue and improve the overall quality of forged components.