Today, let’s look at some common metallographic structures in steel, including ferrite, cementite, pearlite, austenite, martensite, bainite, ledeburite, and Widmanstätten structure, and compare their mechanical properties.
1. Ferrite
Definition:
Ferrite is an interstitial solid solution formed by a small amount of carbon and alloying elements dissolved in body-centered cubic (BCC) α-Fe.
Microstructure:
Appears bright white under the microscope, in blocky, lamellar, granular, or network forms.
Properties:
Ferrite has the lowest strength and hardness among structural steels (~250–300 MPa, 80–100 HB), but exhibits excellent ductility and toughness.
2. Cementite (Carbides)
Left: Network Cementite, 200x
Middle: Needle-like Cementite (Weinstein structure), 200x
Right: Granular Cementite, 500x
Definition:
Carbides are metallic compounds formed when the carbon content exceeds the solubility limit in iron, forming complex lattices.
Microstructure:
White in appearance, with various morphologies such as lamellar (needle-like), granular, network, or semi-network.
Properties:
High hardness (~745–800 HB), very brittle, almost zero plasticity and impact toughness.
3. Pearlite
Left: Lamellar Pearlite, 200x
Middle: Annealed T8 Steel, 500x
Right: Spheroidized Pearlite, 500x
Definition:
Pearlite is the eutectoid product of austenite formed during cooling. It is a mechanical mixture of ferrite and cementite.
Microstructure:
Lamellar in form; the interlamellar spacing decreases as undercooling increases.
Properties:
Higher strength and hardness than ferrite; lower ductility and toughness than ferrite, but much better than cementite.
4. Austenite
Definition:
Austenite is an interstitial solid solution formed by carbon and alloying elements in face-centered cubic (FCC) γ-Fe.
Microstructure:
Characterized by twin lines or slip lines in austenitic steels; grain boundaries are relatively straight. Residual austenite may remain after quenching. Grains are equiaxed and polygonal, with twinning inside. During final stages of heating, grains are small and boundaries irregular. With extended heating, grains grow and boundaries become straight.
Properties:
High toughness and ductility (elongation δ5 around 40–60%), moderate strength and hardness (~170–200 HB).
5. Martensite
Left: Needle-like Martensite
Right: Lath Martensite
Definition:
Martensite is a supersaturated solid solution of carbon (and alloying elements) in α-Fe, formed by diffusionless transformation.
Microstructure:
Predominantly needle-like or lath structures.
Types:
- Lath martensite: found in low-carbon structural steels.
- Needle-like martensite: found in high-carbon steels.
6. Bainite
Definition:
Bainite is a product of austenite transformation at temperatures between pearlite and martensite formation. It forms by a combination of shear and short-range diffusion.
Lower Bainite: Formed at 230–350°C, appears as needle-like structures with carbide alignment at 55°–60° to the needle axis. It’s hard to distinguish from tempered martensite under optical microscopy, but easier under electron microscopy. Carbides precipitate in ferrite needles, without twinning and with high dislocation density.
Granular Bainite: Consists of polygonal ferrite with many irregular island-like structures. Carbides are distributed on the ferrite matrix (these islands are either decomposed carbon-rich austenite or retained austenite/martensite).
7. Ledeburite
Structure:
At room temperature, ledeburite is a mixture of pearlite, cementite, and eutectic cementite. Formed by eutectic transformation from molten Fe-C alloy, composed of austenite and cementite at 4.3% C.
Morphology:
At high temperatures, eutectic cementite is fishbone- or network-like along grain boundaries. After hot working, it breaks into blocks and aligns in chains along the rolling direction.
Temperature Influence:
- Above 727°C: Ledeburite = Austenite + Cementite (symbol: Ld)
- Below 727°C: Ledeburite = Pearlite + Cementite (symbol: Ld′, also called abnormal ledeburite)
Changes with Temperature:
Carbon content remains constant, but the proportion of austenite and cementite varies.
Properties:
- Very hard and brittle (due to cementite matrix).
- Low toughness.
- Poor thermal conductivity. High residual stresses during cooling make forging of martensitic steels challenging.
8. Widmanstätten Structure
Right: Coarse-grained Widmanstätten structure, 200x
Definition:
Widmanstätten structure forms in the overheated zone of welds, where large austenite grains rapidly grow and cool, creating a special overheated structure.
Formation:
Occurs when large austenite grains form under suitable cooling rates. Parallel ferrite (or cementite) needles form within these grains, while residual austenite transforms into pearlite.
Microstructure:
Ferrite/cementite plates are directionally aligned in a feather-like, triangular, or orthogonal pattern, or appear mixed with pearlite.
Composition:
Primarily consists of ferrite (or cementite) needles and pearlite.
Formation Causes:
- Materials: Easily forms in overheated low- or medium-carbon steels.
- Process: Overheating and rapid cooling during welding are primary causes.
Performance:
- Reduces mechanical properties of steel, especially impact toughness and ductility.
- Increases ductile-brittle transition temperature, increasing the risk of brittle fracture.
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