In modern manufacturing, forging is an indispensable metalworking process. It is not merely intended to give metal parts a specific shape and size; more importantly, forging can significantly improve the performance of metals, enabling them to meet the requirements of various complex working conditions. This article will delve into the impact of forging on metals, from the selection of raw materials to the implementation of forging processes, and then to the performance of the final products, providing a comprehensive analysis of the secrets and importance of forging technology.
In forging production, the performance of metal parts is crucial. In addition to meeting basic shape and size requirements, metal parts must also possess a series of performance indicators to ensure their reliability and durability in practical use. These performance indicators mainly include strength, plasticity, impact toughness, fatigue strength, fracture initiation, and stress corrosion resistance. For parts operating in high-temperature environments, additional considerations include instantaneous tensile performance at high temperature, long-term performance, creep resistance, and thermal fatigue performance.
The level of these performance indicators directly determines the service life and safety of metal parts. For example, high-strength metal parts can maintain their shape under large loads; good plasticity means that parts can undergo a certain degree of deformation without breaking under external forces; parts with high impact toughness can withstand sudden impact forces; and parts with high fatigue strength can maintain stable performance under repeated stress. The realization of these performances relies on precise control of the forging process.
The raw materials used for forging mainly include cast ingots, rolled stock, extrusions, and billets. Among them, rolled stock, extrusions, and billets are all semi-finished products formed from cast ingots through different processing techniques. During the forging process, reasonable process design and parameters can significantly improve the structure and performance of the raw materials.
Breaking columnar crystals: During the solidification of metals, columnar crystal structures often form. This structure can affect the mechanical properties of metals to a certain extent. Through forging, the columnar crystals can be broken, making the metal structure more uniform.
Improving macrosegregation: During metal casting, due to differences in cooling rate, macrosegregation may occur, meaning that the chemical composition varies in different areas. Forging can homogenize the metal composition through deformation and diffusion.
Formation of forged-state structure: Forging can transform cast-state structures into forged-state structures. Cast-state structures often contain pores and looseness, while forged-state structures are denser and perform better.
Welding internal voids: Under appropriate temperature and stress conditions, forging can weld internal voids, increasing material density. A dense metal structure improves both strength and toughness.
After forging, cast ingots develop a fiber structure, which plays an important role in the mechanical properties of metals. Through further rolling, extrusion, and die forging, forged parts can achieve a reasonable fiber orientation distribution. Proper fiber orientation allows more uniform stress distribution when the parts are loaded, thereby improving strength and toughness.
The size and uniformity of grains have an important effect on metal performance. During forging, grain size and uniformity can be controlled by adjusting deformation conditions and cooling rates. Fine grains improve metal strength and toughness and also enhance fatigue resistance.
In some alloys, there exist second phases, such as alloy carbides in ledeburitic steel. Forging, combined with deformation and heat treatment, can make the distribution of these second phases more uniform. Uniform second-phase distribution improves hardness and wear resistance while reducing stress concentration and enhancing crack resistance.
The deformation during forging induces work hardening. Work hardening can increase metal strength but may reduce plasticity. Through reasonable process control, strength can be increased while minimizing the impact on plasticity.
The forging process generally includes material cutting, heating, forming, post-forging cooling, pickling, and post-forging heat treatment. The process parameters of each step significantly affect the final performance of the metal.
Heating processes include furnace loading temperature, heating temperature, heating rate, soaking time, and furnace atmosphere composition. Improper heating may cause a series of defects such as decarburization, overheating, and burning. Decarburization reduces the carbon content on the metal surface, lowering hardness and wear resistance; overheating and burning coarsen the grains, reducing toughness and fatigue performance.
For billets with large cross-sections, poor thermal conductivity, or low plasticity, excessively fast heating and insufficient soaking can lead to uneven temperature distribution, thermal stress, and billet cracking. Therefore, during heating, the heating rate and soaking time should be reasonably controlled according to the characteristics and size of the metal.
Forming processes include deformation method, degree of deformation, deformation temperature, deformation speed, stress state, tool conditions, and lubrication conditions. Improper forming can cause coarse or uneven grains, cracks, laps, folds, shear, whirls, residual cast structures, and other defects.
Different forming methods, due to their different stress conditions and stress-strain characteristics, may produce different main defects. For example, the main defect of upsetting billets is longitudinal or 45° cracks on the side surface; for elongating rectangular-section billets, the main defects are transverse surface cracks and corner cracks; open-die forging defects include underfilling, laps, and misalignment.
During post-forging cooling, improper procedures may cause cooling cracks, white spots, or network carbides. Different metal materials have different cooling requirements. For example, materials such as austenitic and ferritic heat-resistant stainless steels, high-temperature alloys, aluminum alloys, magnesium alloys, which do not undergo allotropic transformation during heating and cooling, as well as certain copper and titanium alloys, have defects formed during forging that cannot be remedied by heat treatment.
Materials that undergo allotropic transformation, such as structural steels and martensitic stainless steels, are greatly affected by forging-induced or pre-existing defects during heat treatment. Some defects can be improved by post-forging heat treatment; some require special heat treatment; others cannot be eliminated, resulting in decreased performance or unqualified parts.
High-quality raw materials are a prerequisite for ensuring the quality of forged parts. Defects in raw materials affect both the forming process and the final quality. Excessive chemical elements or impurity content in raw materials significantly impact forging and part quality.
For example, elements such as S, P, Cu, Sn easily form low-melting phases, making forged parts prone to hot brittleness. To obtain intrinsically fine-grain steel, the residual aluminum content in steel must be controlled within a certain range. Too little aluminum cannot control grain size; too much aluminum can cause wood-grain or torn fracture surfaces during deformation. In austenitic stainless steel, higher contents of Cr, Si, Al, Mo increase ferrite, making banded cracks more likely and imparting magnetism to parts.
Other defects, such as shrinkage cavities, sub-surface porosity, severe carbide segregation, large non-metallic inclusions, dendritic crystals, severe looseness, white spots, oxide films, segregation bands, or mixed metals in raw materials, can cause cracks or reduce forged part performance. Surface cracks, folds, scabs, and coarse-grain rings in raw materials can lead to surface cracks in forged parts.
Different materials, due to differences in composition and structure, undergo distinct microstructural changes and mechanical behavior during heating, forging, and cooling. Consequently, the types of defects that may occur during improper forging are also material-specific.
Forging defects mainly include coarse carbide particles, uneven distribution, and cracks. This material requires special attention to control carbide morphology and distribution to avoid cracks. Proper heating and cooling can improve carbide distribution, toughness, and crack resistance.
Forging defects mainly include coarse grains and cracks. High-temperature alloys operate under extreme temperatures, requiring very fine and uniform grains. Coarse grains reduce high-temperature strength and creep resistance, necessitating strict control of deformation temperature and cooling rate.
Forging defects mainly include intergranular chromium depletion, reduced intergranular corrosion resistance, ferrite banding, and cracks. Careful control of heating and cooling during forging improves corrosion resistance and crack resistance.
Forging defects mainly include coarse grains, folds, whirls, and penetration defects. Controlling deformation speed and cooling rate is crucial to avoid coarse grains. Proper process control ensures fine and uniform grains, improving strength and toughness.
To ensure high-quality metal parts, comprehensive optimization and control of forging processes is essential, from raw material selection, heating, forming, cooling, to final heat treatment.
Selecting high-quality raw materials is the foundation for quality forged parts. Chemical composition and microstructure must be strictly inspected. Defective materials should be treated or discarded.
Heating is critical in forging. Heating temperature, rate, soaking time, and furnace atmosphere should be controlled according to material and billet characteristics to prevent decarburization and overheating.
Forming optimization requires selecting appropriate deformation method, degree, temperature, and speed according to material and part shape. Tooling quality and lubrication must be ensured to reduce friction and stress concentration.
Cooling significantly affects final metal performance. Cooling method and rate must match material characteristics. For materials undergoing allotropic transformation, cooling must be carefully controlled to avoid defects.
Heat treatment improves microstructure and performance and eliminates forging-induced defects. Heat treatment parameters should be tailored to material properties and service requirements.
The impact of forging on metals is comprehensive, from microstructure to macroscopic performance. Every stage of the forging process significantly affects the final properties of metals. Through reasonable process control and optimization, the microstructure and performance of metals can be significantly improved, allowing them to meet the requirements of various complex working conditions. In modern manufacturing, optimization and control of forging processes are key to improving product quality and competitiveness. Continuous exploration and innovation are essential for perfecting forging technology and making greater contributions to the development of manufacturing.
