The Transformation of Steel During Cooling
Cooling is an indispensable step in the heat treatment process.
After a steel part is heated and held at a certain temperature to obtain austenite with fine and uniform grains, cooling is then carried out.
I. Transformation Products and Transformation Process of Supercooled Austenite
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Supercooled Austenite: Austenite that remains untransformed (in terms of structure) below the critical point A₁.
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At this point, supercooled austenite does not transform immediately; instead, it is in a thermodynamically unstable state (as an unstable structure) and will eventually undergo transformation.
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Depending on the degree of supercooling (i.e., the different transformation temperatures), supercooled austenite undergoes three types of transformation:
- Pearlite transformation
- Bainite transformation
- Martensite transformation
1. Pearlite Transformation
- Transformation Condition: Supercooled austenite transforms into a pearlite-type structure within the temperature range of A₁ → 550°C.
- Transformation Product: A mechanical mixture structure consisting of alternating lamellae of ferrite and cementite.
- Pearlite is one of the five most fundamental structures in iron-carbon alloys. It is denoted by the letter "P" (from "Pearlite"). The name originates from its pearl-like luster.
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Classification: Based on the Thickness of Lamellae
Pearlite (P)
Formation temperature: A₁ ~ 650°C; it is a type of pearlite with relatively thick lamellae. Under an optical microscope, the lamellar structure of ferrite and cementite can be clearly distinguished, with a lamellar spacing of approximately 150 ~ 450 nm.
Sorbite (S)
Formation temperature: 650 ~ 600°C; it has relatively thin lamellae, with a thickness of approximately 80 ~ 150 nm. The lamellae are difficult to distinguish under an optical microscope and can only be identified as the lamellar structure of ferrite and cementite under a high-magnification optical microscope (at 800 ~ 1500× magnification).
Troostite (T)
Formation temperature: 600 ~ 550°C; it has extremely thin lamellae, with a thickness of approximately 30 ~ 80 nm. The lamellar characteristics cannot be distinguished at all under an optical microscope and can only be identified under an electron microscope.
Austenitizing temperature and austenite grain size before transformation only affect the size of pearlite colonies, but have no impact on the lamellar spacing.
From pearlite (P) to sorbite (S) and then to troostite (T), the lower the temperature, the smaller the lamellar spacing, and the higher the strength and hardness. They only differ in lamellar fineness and properties, with no essential distinction.
Similar to the austenitization process during heating, the pearlite transformation process during cooling is also a process of nucleation and growth in the solid state.
Similarly, due to the irregular atomic arrangement at grain boundaries, along with more defects such as vacancies and dislocations, atomic rearrangement easily occurs, so cementite first nucleates at the austenite grain boundaries.
After cementite nucleates, it begins to grow. During the growth process, the carbon content of the austenite on both sides of the cementite decreases, which promotes the nucleation of ferrite. The two nucleate and grow alternately, forming multiple lamellar structures composed of ferrite and Fe₃C.
At the same time, nucleation and growth also start simultaneously in other parts of the grain boundaries, forming multiple pearlite colonies with different orientations.
These pearlite colonies grow and merge into a continuous mass, and finally, the entire structure is transformed into pearlite; thus, the transformation of supercooled austenite to pearlite is completed.
Since iron and carbon atoms diffuse sufficiently due to the high temperature during the transformation of austenite to pearlite, this process is called a diffusion-type transformation.
2. Bainite (B) Transformation
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Transformation Condition: Supercooled austenite transforms within the temperature range of 550°C ~ Ms. For eutectoid steel, the Ms temperature is 230°C.
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Transformation Product: A two-phase mechanical mixture of Fe₃C (cementite) and carbon-supersaturated ferrite, denoted by the letter "B".
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In 1930, E.S. Davenport and E.C. Bain first observed the metallographic structure of the transformation product in steel after medium-temperature isothermal transformation. Later, to honor Bain's contributions, this structure was named "Bainite".
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Based on the differences in their microstructural morphologies, bainite can be classified into:
- Upper Bainite (B_u)
- Lower Bainite (B_l)
Upper Bainite (B₍upper₎ / Bᵤ)
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Morphology: Feather-like.
Discontinuous rod-shaped cementite (Fe₃C) is distributed between parallel ferrite laths that grow from the austenite grain boundaries into the grain interior.
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Lower Bainite (B₍lower₎ / Bₗ)
Morphology: Bamboo leaf-like. Fine flaky carbides (Fe₃C) are distributed on the ferrite needles.
Performance Characteristics of Lower Bainite:
The carbides in lower bainite are fine and uniformly distributed. In addition to high strength and hardness, it also has good plasticity and toughness, making it a commonly used structure in industrial production. Obtaining the lower bainite structure is one of the methods to strengthen steel materials.
Under the condition of the same hardness, the wear resistance of the lower bainite structure is significantly better than that of martensite, which can reach 1 to 3 times that of martensite. Therefore, obtaining lower bainite as the matrix structure in iron and steel materials is a goal pursued by researchers and engineers.
1) Formation Process of Upper Bainite
When the transformation temperature is relatively high (550 ~ 350°C), ferrite nuclei are preferentially formed in the low-carbon regions of austenite. These nuclei then grow parallelly from the austenite grain boundaries into the grain interior. Meanwhile, as the ferrite grows, the excess carbon atoms diffuse into the surrounding austenite. Finally, short rod-like or small flaky Fe₃C (cementite) precipitates between the ferrite laths, distributed discontinuously among the parallel and dense ferrite laths, thereby forming feather-like upper bainite.
2) Formation Process of Lower Bainite
Ferrite nuclei first form at the grain boundaries of austenite, then grow in a needle-like manner along specific crystal planes. Due to the relatively low transformation temperature of lower bainite, the excess carbon atoms cannot diffuse over long distances; instead, they can only precipitate as extremely fine carbides (Fe₃C) along specific crystal planes within the ferrite. This process results in the formation of bamboo leaf-like lower bainite.
3. Martensite (M) Transformation
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Transformation Condition: The temperature range is below the Ms point.
Supercooled austenite cannot transform at a constant temperature in this temperature range; instead, it undergoes transformation during continuous cooling with a very large degree of supercooling.
Transformation Product: A supersaturated interstitial solid solution of carbon in α-Fe (ferrite), denoted by the symbol "M".
In the 1890s, martensite was first discovered in a hard mineral by the German metallurgist Adolf Martens (1850-1914). In 1895, the Frenchman F. Osmond named this structure "Martensite" in honor of the German metallurgist A. Martens.
Classification of Martensite
The most common types of martensite are two:
lath martensite and
acicular martensite.
The type of martensite formed depends on the carbon content in austenite:
When the carbon content is greater than 1.0%, acicular martensite is obtained; When the carbon content is less than 0.2%, lath martensite is obtained; When the carbon content is between 0.2% and 1.0% (0.2% < C% < 1.0%), a mixed structure of the two types is obtained.
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