1. Mechanical Properties: Strength, Toughness, and Fatigue Resistance
This is the core performance difference between the two, directly impacting the product’s load-bearing capacity and service life.
Ø Castings:
Lower Strength: When liquid metal solidifies, the atomic arrangement easily forms defects such as porosity, pores, and segregation (compositional inhomogeneity), resulting in low material density. The tensile strength and yield strength are typically 10%-30% lower than forgings made of the same material. For example, the tensile strength of ordinary cast steel is approximately 400-600 MPa, while comparable forged steel can reach 600-800 MPa.
Poor Toughness: Internal defects (such as pinholes and shrinkage cavities) become stress concentration points, making them susceptible to fracture under impact. Impact toughness (energy absorbed by impact) is typically only 1/2-2/3 that of forgings. For example, cast iron castings (such as gray cast iron) have almost no plasticity and easily fracture under impact.
Weak fatigue resistance: Due to its loose microstructure, cracks easily form and propagate at defects under repeated alternating loads (such as in rotating mechanical parts), resulting in a fatigue life far lower than that of forged parts.
Ø Forged parts:
Higher strength: During plastic deformation, the metal’s internal grains are broken down and refined, and continuous “fiber streamlines” (similar to the grain of wood) form along the direction of force, increasing the material’s density and significantly improving its strength. For example, forged automotive crankshafts achieve tensile strength over 20% higher than cast crankshafts.
Greater toughness: The refined grain size and uniform microstructure allow the material to withstand greater impact and deformation without breaking. For example, forged alloy steel cutting tools are both strong and resistant to chipping.
Higher fatigue resistance: The dense microstructure and continuous fiber streamlines effectively disperse stress, resulting in a fatigue life 2-5 times that of castings under alternating loads (such as in gears and connecting rods).
2. Internal Quality: The Difference Between Defects and Microstructure Uniformity
Ø Castings:
Common Defects: When liquid metal solidifies, uneven cooling rates and delayed gas discharge can easily lead to defects such as porosity (internal bubbles), sand holes (sand inclusions), shrinkage cavities (cavities formed by solidification shrinkage), and cracks (caused by cooling stress). These defects directly impair product performance.
Microstructure Inhomogeneity: “Segregation” (local compositional differences) may occur within the casting. For example, alloying element content differs between the edge and center of the casting, resulting in inconsistent performance. Furthermore, the grain size is coarse and non-directional.
Ø Forgings:
Fewer and More Controllable Defects: Forging uses external force to “compact” the metal billet, eliminating defects such as porosity and pores in the raw material (similar to kneading dough to remove bubbles), resulting in more stable internal quality.
More Uniform Microstructure: During plastic deformation, inclusions and second-phase particles within the metal are broken down and distributed along streamlines, resulting in refined and uniform grain size, thus avoiding segregation problems associated with casting.
3. Usage Characteristics: Differences in Vibration Damping, Wear Resistance, and Machinability
Ø Castings:
Good Vibration Damping: Some castings (such as gray cast iron) contain flake graphite, which absorbs vibration energy and is suitable for parts requiring vibration reduction, such as machine tool beds and engine blocks.
Moderate Wear Resistance: Surface hardness is relatively low (unless subsequently heat treated), but some casting alloys (such as wear-resistant cast iron) can improve wear resistance through compositional design, making them suitable for low-load wear parts (such as manhole covers).
Large Machining Allowance: Castings have a high surface roughness (typically Ra12.5-25μm) and may exhibit dimensional deviation, requiring extensive subsequent machining.
Ø Forgings:
Poor Vibration Damping: Their dense structure and high toughness make them more rigid, making them less likely to absorb vibration energy and unsuitable for vibration-damping parts.
Improved Wear Resistance: Subsequent heat treatment (such as quenching) can achieve higher surface hardness (for example, bearing steel forgings can reach a hardness of HRC60 or above after quenching). Supported by internal toughness, these forgings are wear-resistant and non-flaking, making them suitable for manufacturing high-speed wear parts such as gears and bearings.
Low Machining Allowance: Forgings have a smooth surface (typically Ra3.2-6.3μm) and high dimensional accuracy (especially for die forgings), which reduces machining effort and reduces costs.
Summary
Forgings, thanks to their dense structure, refined grains, and continuous fiber flow lines achieved through “solid-state plastic deformation,” offer comprehensive advantages over castings in key mechanical properties such as strength, toughness, and fatigue resistance. They are suitable for core structural components subjected to high loads and impacts. While castings may have lower performance, their complex shapes, adaptability, and vibration damping properties make them suitable for manufacturing low-stress, complex-shaped parts. In actual applications, the process needs to be selected according to the stress environment and functional requirements of the product. For example, the cylinder block of a car engine (complex shape, low impact) is cast, while the connecting rod connecting the cylinder block and piston (high impact, high load) must be forged.
