The Influence of Forging on the Microstructure and Properties of Forgings

The Influence of Forging on the Microstructure and Properties of Forgings

1. The Impact of Forging on Metal Microstructure and Properties

In forging production, besides ensuring the required shape and dimensions of forgings, it is crucial to meet the performance requirements of the parts during service. These requirements mainly include:

• Strength indicators: Yield strength, tensile strength, etc.
• Ductility indicators: Elongation, reduction of area, etc.
• Impact toughness
• Fatigue strength
• Fracture toughness
• Stress corrosion cracking resistance

For parts operating at high temperatures, additional requirements include:

• High-temperature instantaneous tensile properties
• Creep resistance
• Thermal fatigue resistance

Raw materials for forging include ingots, rolled stock, extruded stock, and forged billets. Rolled, extruded, and forged billets are semi-finished products formed from ingots through rolling, extrusion, and forging processes, respectively.

By adopting reasonable processes and parameters during forging production, the microstructure and properties of raw materials can be improved through the following aspects:

1. Breaking down columnar grains: This refines the grain structure, improves macrosegregation, transforms the as-cast structure into a forged structure, and heals internal pores under suitable temperature and stress conditions, ultimately enhancing material density.
2. Forming a fibrous structure: Forging the ingot creates a fibrous structure, which is further refined through rolling, extrusion, and die forging to achieve a reasonable fiber orientation distribution in the forging.
3. Controlling grain size and uniformity
4. Improving the distribution of the second phase: For example, the distribution of alloy carbides in hypereutectoid steel.
5. Strengthening the structure through work hardening

Due to the aforementioned microstructural improvements, the plasticity, impact toughness, fatigue strength, and creep resistance of forgings are also enhanced. Subsequent heat treatment of the parts can then achieve the desired comprehensive properties, such as hardness, strength, and plasticity.

However, if the quality of the raw materials is poor or the forging process is unreasonable, forging defects may occur, including surface defects, internal defects, or unqualified properties.

2. The Influence of Raw Materials on Forging Quality

Good quality raw materials are a prerequisite for ensuring forging quality. Defects in raw materials will affect the forming process and the final quality of forgings.

• Chemical composition: If the chemical elements in the raw materials exceed the specified range or the content of impurity elements is too high, it will significantly impact the forming and quality of forgings. For example, elements like S, B, Cu, and Sn easily form low-melting-point phases, making forgings prone to hot shortness.
• Aluminum content: To obtain fine-grained steel, the residual aluminum content in the steel needs to be controlled within a certain range, for example, 0.02% to 0.04% (mass fraction). If the content is too low, it cannot effectively control grain growth, often leading to unqualified grain size in forgings. Conversely, excessive aluminum content can easily lead to woody or tear-like fracture surfaces under the conditions of fiber structure formation during pressure processing.
• Austenitic stainless steel: In austenitic stainless steel, the higher the content of Si, Al, and Mo, the more ferrite phase will be present. This makes it more prone to banding during forging and can result in magnetism in the parts.

Defects in raw materials, such as shrinkage cavities, subcutaneous bubbles, severe carbide segregation, and coarse non-metallic inclusions (slag), can easily cause cracks in forgings during the forging process. Defects like dendritic crystals, severe looseness, non-metallic inclusions, white spots, oxide films, segregation bands, and inclusions of dissimilar metals can easily lead to a decline in the properties of forgings. Surface cracks, folds, scars, and coarse grain rings in raw materials can easily cause surface cracks in forgings.

3. The Influence of the Forging Process on Forging Quality

The forging process generally consists of the following steps: blanking, heating, forming, post-forging cooling, pickling, and post-forging heat treatment. Improper processes during forging can lead to a series of forging defects.

• Heating process: This includes charging temperature, heating temperature, heating rate, holding time, and furnace atmosphere. Improper heating, such as excessive heating temperature and prolonged heating time, will cause defects like decarburization, overheating, and burning. For large cross-section sizes and materials with poor thermal conductivity and plasticity, if the heating rate is too fast and the holding time is too short, it often leads to uneven temperature distribution, causing thermal stress and cracking of the forging billet.
• Forming process: This includes deformation mode, degree of deformation, deformation temperature, deformation speed, stress state, tooling conditions, and lubrication conditions. Improper forming processes can lead to coarse grains, uneven grain size, various cracks, folds, metal flow defects, eddy currents, and residual casting structures.
• Post-forging cooling: Improper processes during post-forging cooling can cause cooling cracks, white spots, and network carbides.

4. The Influence of Forging Microstructure on the Final Heat-Treated Microstructure and Properties

For materials that do not undergo allotropic transformation during heating and cooling, such as austenitic and ferritic heat-resistant stainless steels, superalloys, aluminum alloys, and magnesium alloys, as well as some copper alloys and titanium alloys, microstructural defects generated during the forging process cannot be improved by heat treatment.

For materials that undergo allotropic transformation during heating and cooling, such as structural steel and martensitic stainless steel, certain microstructural defects caused by improper forging processes or residual defects from raw materials have a significant impact on the quality of heat-treated forgings. Examples are provided below:

1. Improvable defects: Some microstructural defects in forgings can be improved during post-forging heat treatment, and satisfactory microstructure and properties can still be obtained after final heat treatment. For example, coarse grains and Widmanstätten structure in generally overheated structural steel forgings, and slight network carbides in hypereutectoid steel and bearing steel due to improper cooling.
2. Defects requiring special treatment: Some microstructural defects in forgings are difficult to eliminate with normal heat treatment and require measures such as high-temperature normalizing, repeated normalizing, low-temperature tempering, and high-temperature diffusion annealing to be improved.
3. Non-improvable defects: Some microstructural defects in forgings cannot be eliminated by general heat treatment processes, resulting in reduced properties or even unqualified properties in the final heat-treated forgings. For example, severe stone-like and rock candy fracture surfaces, burning, ferrite bands in stainless steel, and carbide networks and bands in ledeburitic high-alloy tool steel.
4. Worsening defects: Some microstructural defects in forgings will further develop during final heat treatment and even cause cracking. For example, if coarse grain structure in alloy structural steel forgings is not improved during post-forging heat treatment, it often leads to coarse martensite and unqualified properties after carbonitriding and quenching. Coarse banded carbides in high-speed steel often cause cracking during quenching.

Different forming methods, due to their different force conditions and stress-strain characteristics, have different main defects that may occur. For example, the main defect during billet upsetting is longitudinal or 45° cracks on the side surface, while ingot upsetting often leaves residual cast structure at the upper and lower ends. The main defects during the elongation of rectangular billets are transverse cracks and corner cracks on the surface, and diagonal cracks and transverse cracks inside. The main defects during open-die forging are underfilling, folding, and misalignment.

Different types of materials, due to their different compositions and microstructures, have different microstructural changes and mechanical behaviors during heating, forging, and cooling. Therefore, improper forging processes can lead to specific defects. For example, the defects in ledeburitic high-alloy tool steel forgings are mainly coarse carbide particles, uneven distribution, and cracks. The defects in superalloy forgings are mainly coarse grains and cracks. The defects in austenitic stainless steel forgings are mainly intergranular chromium depletion, reduced intergranular corrosion resistance, ferrite banding, and cracks. The defects in aluminum alloy forgings are mainly coarse grains, folds, eddy currents, and metal flow defects.