In the field of mechanical manufacturing, forgings serve as critical components in numerous mechanical devices, and their quality directly affects equipment performance and service life. However, various microstructure defects may occur during the production of forgings. These defects not only affect the appearance of the forgings but can also seriously impair their internal performance. This article will delve into the causes, hazards, and countermeasures of low-magnification and high-magnification forging defects, helping readers gain a comprehensive understanding of this important topic.
Low-magnification defects refer to abnormalities in the internal structure of forgings observed under low magnification. These defects typically manifest at the macro level and have a significant impact on the overall performance of the forging. Common low-magnification defects include coarse grains, flow line disorder, folds, mingling flows, and shear bands. The formation of these defects is closely related to forging process parameters, die design, raw material quality, and various operational factors.
Coarse grains are among the common low-magnification defects in forgings. They are usually caused by improper temperature control and unreasonable deformation during forging. For example, if the initial forging temperature is too high, the metal remains in an unstable state at high temperatures for a prolonged time, and increased atomic activity makes grain growth easy. Similarly, if the final forging temperature is too high, grains may fail to refine properly during cooling.
Additionally, when the final forging deformation falls within a critical range, insufficient to fully break and refine the grains, yet beyond the range where grains remain stable, coarse grains are likely to form. For different materials, such as aluminum alloys, excessive deformation may produce texture, while low deformation temperature in high-temperature alloys may form mixed deformation structures, both leading to coarse grains.
Coarse grains severely affect forging performance by reducing plasticity and toughness and significantly decreasing fatigue resistance. In practical applications, this means that forgings are more prone to cracking under repeated loads, greatly shortening their service life.
Flow line disorder is also an important form of low-magnification defect. When die design is improper, for instance, if the die cavity shape does not match the metal flow direction, or if forging methods are poorly chosen, the metal flow path of the preform becomes chaotic, resulting in flow line disorder. Improper operation during forging, such as uneven hammering or die wear causing uneven metal flow, can also lead to flow line interruption, backflow, and vortices in the forging's low-magnification structure.
Unreasonable flow line distribution can cause early failure in service. Flow lines represent the metal's internal fiber structure, and proper distribution optimizes mechanical properties along specific directions. Disordered flow lines reduce this effect, creating stress concentration points that make cracks more likely at weak spots, ultimately impacting the forging's service life.
Folds form due to metal flow and operational factors during forging. If the raw material and billet have large shape deviations, die design is improper, forming sequences are unreasonable, lubrication is insufficient, or forging operation is incorrect, the oxidized surface layers of the metal may merge during deformation to form folds. Folds are usually aligned with flow lines, and their tails initially appear rounded. However, subsequent deformation may cause the folds to crack, producing sharp tails. Severe oxidation or decarburization often occurs on both sides of folds, and in some cases, carbon enrichment may also appear.
Folds are extremely harmful because they reduce the effective load-bearing area and, due to stress concentration, become fatigue initiation sites, leading to crack propagation and eventual forging failure.
Mingling flows occur for similar reasons as folds, formed when two metal streams or a stream carrying another merges. The metal remains continuous but represents improper flow line convergence. In mingling areas, originally angled flow lines converge, often resulting in significant differences in grain size between inner and outer regions. Uneven grain size lowers mechanical properties, especially when the disparity is pronounced. In practice, this causes uneven stress distribution and weak points under load, reducing the forging's overall performance and service life.
Shear bands commonly occur in alloys and high-temperature alloys sensitive to rapid cooling. When difficult-to-deform zones near the billet surface expand and experience intense shear, wavy fine-grain regions appear in the forging's transverse low-magnification structure, forming shear bands. Shear bands impart strong directionality to the structure, disrupting uniformity and reducing performance. In practice, this leads to weak points along the shear bands under multi-directional stress, lowering the forging's overall reliability.
After discussing the impact of low-magnification defects, we now examine microscopic (high-magnification) defects. High-magnification defects are abnormalities in the forging structure observed under high magnification. These defects usually manifest at the micro level, which may not be obvious macroscopically but have profound effects on mechanical properties and service life. Common high-magnification defects include uneven grains, banded structures, decarburization layer accumulation, uneven carbide distribution, and retained as-cast structures. Their formation is also closely related to forging parameters, raw material quality, and heat treatment processes.
Uneven grains are common high-magnification defects, mainly caused by uneven deformation. During forging, if local deformation falls into a critical range or overall deformation is uneven, grains break irregularly. Some areas may have coarse grains while others have fine grains, resulting in uneven grain distribution throughout the forging. Heat-resistant steels and high-temperature alloys are also prone to coarse grains due to localized work hardening.
Uneven grains significantly reduce fatigue strength and long-term performance. In practice, forgings are more prone to cracks and fractures under cyclic or sustained loading, greatly shortening service life.
Banded structures form due to the coexistence of two phases. In hypoeutectoid steels, ferrite and pearlite appear as bands. In austenitic or semi-martensitic steels, ferrite may form bands with austenite, bainite, or martensite. Banded structures reduce lateral plasticity and impact toughness. Cracks often initiate along the boundaries of these bands due to stress concentration, lowering the forging's overall fracture resistance.
Decarburization layer accumulation is a localized defect caused by improper forging. For example, during elongation of round bars, excessive hammering and large reductions can form double-drum shapes. When these are further compressed and elongated, some metal flows outward while some flows inward, causing decarburization accumulation in the center. These areas have lower hardness than normal regions, making them prone to cracking and reducing overall strength and reliability.
Uneven carbide distribution is mainly seen in die steels. Severe carbide segregation in the raw material, combined with inadequate forging ratios or improper methods, can produce large or networked carbide clusters. Such defects disrupt the metal matrix continuity, concentrate stress in carbide-rich areas, increase quench cracking risk, and reduce wear resistance. Tools and dies may chip or fail prematurely due to uneven carbide distribution.
Retained as-cast structures occur mainly in forgings made from cast ingots, particularly in difficult-to-deform areas. If the forging ratio is insufficient or methods are improper, as-cast structures cannot be fully refined and remain in the forging. This reduces performance, especially impact toughness and fatigue resistance. Forgings are thus more susceptible to cracking or fracture under impact or cyclic loads, greatly shortening service life.
Having analyzed the causes and hazards of low- and high-magnification defects, the critical question is how to effectively address these defects to ensure high-quality, high-performance forgings. Key strategies include:
Strictly control initial and final forging temperatures to avoid excessively high or low temperatures. Select appropriate temperature ranges according to material characteristics. For example, avoid excessive deformation in aluminum alloys that may cause texture formation, and avoid too-low deformation temperatures in high-temperature alloys that may form mixed structures. Ensure uniform and reasonable deformation to prevent coarse or uneven grains. Use simulation analysis to optimize parameters, ensuring uniform metal flow and proper flow line distribution.
Design die cavities according to forging shape and size to ensure smooth metal flow and avoid flow line disorder. Die cavity shapes should match the final forging, reducing flow resistance. Ensure manufacturing precision and minimize die wear impact. Regularly inspect and maintain dies, repairing worn areas to maintain surface finish and dimensional accuracy.
Conduct rigorous inspections to ensure chemical composition and microstructure uniformity meet requirements. For raw materials with severe carbide segregation, apply appropriate heat treatment or corrective forging. Choose reputable suppliers and maintain stable material quality. Establish long-term cooperation and periodically evaluate suppliers to ensure material quality meets production standards.
Use non-destructive testing such as magnetic particle, ultrasonic, dye penetrant, and radiographic inspection to comprehensively detect forging defects. Conduct metallographic analysis to examine both high- and low-magnification structures. Identify uneven grains, banded structures, and uneven carbide distribution, and adjust production processes based on findings.
Establish strict quality control throughout the production process. Detect and correct issues promptly to ensure forgings meet standards. Train operators to improve skills and quality awareness. Ensure operators follow process parameters strictly, reducing defects caused by improper handling.
The formation of forging defects is a complex process involving multiple factors. By optimizing forging parameters, improving die design and manufacturing, strengthening raw material quality control, applying advanced detection methods, and enforcing rigorous in-process monitoring, the occurrence of forging defects can be effectively reduced, improving forging quality and performance. In practice, these measures should be applied comprehensively according to the specific conditions of each forging to ensure compliance with design requirements, enhancing overall machinery performance and service life.
