Steel Microstructure Explained: Key Phases in Iron–Carbon Alloys

Modern materials can be categorized into four major groups—metals, polymers, ceramics, and composites.

Despite the rapid advancement of polymer materials, steel remains the most widely used and crucial material in engineering technology today.

So what factors determine steel’s dominant position? Let’s delve into the details for our readers.

Steel is refined from iron ore, offering abundant sources and low cost.

Also known as iron-carbon alloys, steel consists of iron (Fe) combined with carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), and trace elements like chromium (Cr) and vanadium (V).

By adjusting the content of various elements in steel and applying heat treatment processes (the four key steps: quenching, annealing, tempering, and normalizing), a wide variety of microstructures can be achieved, endowing steel with distinct physical properties.

When a steel sample is ground, polished, etched with a specific corrosive agent to reveal its structure, and then observed under a metallurgical microscope, the resulting pattern is known as the steel’s microstructure.

The secrets of steel materials lie within these microstructures.

Within the Fe-Fe₃C system, numerous iron-carbon alloys with varying compositions can be formulated.

Their equilibrium structures differ at different temperatures but are composed of several fundamental phases: ferrite (F), austenite (A), and cementite (Fe₃C).

These basic phases combine in a mechanical mixture to form the diverse microstructures found in steel. Common microstructures include the following eight types:

Ferrite  

The interstitial solid solution formed by carbon dissolving into the α-Fe lattice is termed ferrite.

It possesses a bcc structure and exhibits equiaxed polygonal grain distribution, denoted by the symbol F.

Its microstructure and properties resemble those of pure iron, featuring good plasticity and toughness, but with relatively low strength and hardness (30-100 HB).

In alloy steels, ferrite represents the solid solution of carbon and alloying elements within α-Fe.

Carbon’s solubility in α-Fe is very low; at the AC1 temperature, the maximum solubility is 0.0218%, but this decreases to 0.0084% as temperature drops.

Consequently, under slow cooling conditions, tertiary carbides form at ferrite grain boundaries.

Austenite

Austenite is a face-centered cubic (fcc) structure formed by carbon dissolving into the interstitial sites of the γ-Fe lattice.

It is a high-temperature phase denoted by the symbol A.

Austenite exhibits maximum carbon solubility of 2.11%C at 1148°C and can dissolve 0.77%C at 727°C.

It possesses higher strength and hardness than ferrite, along with good plasticity and toughness, and is non-magnetic.

Specific mechanical properties correlate with carbon content and grain size, typically ranging from 170 to 220 HBS and 40 to 50% elongation.

TRIP steel (Transformation-Residual-Inclusion-Properity steel) is developed based on the excellent plasticity and toughness of austenite.

It enhances plate plasticity and improves formability by utilizing strain-induced phase transformation in retained austenite and phase transformation-induced plasticity.

In carbon or alloy structural steels, austenite transforms into other phases during cooling.

Only in high-carbon steels and carburized steels subjected to carburizing and high-temperature quenching can austenite remain trapped within the interstices of martensite.

This microstructure appears white due to its resistance to corrosion.

Carbide  

Carbide is a metallic compound formed by the combination of carbon and iron in a specific ratio, represented by the molecular formula Fe₃C.

It contains 6.69% carbon and forms (Fe,M)₃C in alloys.

Carbide is hard and brittle, with virtually no plasticity or impact toughness, exhibiting extreme brittleness and a hardness of 800 HB.

In steel, it commonly appears as network, semi-network, flake, needle-flake, or granular distributions.

Pearlite

A mechanical mixture composed of ferrite and cementite is termed pearlite, denoted by the symbol P.

Its mechanical properties lie between those of ferrite and cementite, exhibiting relatively high strength, moderate hardness, and a degree of plasticity.

Pearlite is the eutectoid transformation product of steel.

Its morphology features alternating layers of ferrite and cementite resembling fingerprints, arranged in a layered pattern.

Based on the distribution pattern of carbides, it can be further classified into two types: lamellar pearlite and spheroidal pearlite.

(1) Flaky pearlite: Further subdivided into coarse-flaky, medium-flaky, and fine-flaky types.

(2) Spheroidal pearlite: Obtained through spheroidizing annealing, where carbides form spheroidal particles distributed within a ferritic matrix.

The size of these carbide spheroids depends on the spheroidizing annealing process, particularly the cooling rate.

Spheroidal pearlite can be classified into four types: coarse-spheroidal, spheroidal, fine-spheroidal, and punctate pearlite.

Bainite

Bainite is the product of the transformation of austenite in steel within the intermediate temperature range below the pearlite transformation zone and above the Ms point.

The microstructure consisting of ferrite and cementite forms an intermediate phase between pearlite and martensite, referred to as bainite (symbol B).

Based on the formation temperature, it is classified into granular bainite, upper bainite (B upper), and lower bainite (B lower).

Fibrous bainite exhibits lower strength but superior toughness; lower bainite possesses both high strength and good toughness; granular bainite demonstrates the poorest toughness.

Bainite morphology is highly variable; based on shape characteristics, it can be classified into three types: feathery, acicular, and granular.

Types and Microstructural Characteristics of Bainite in Steel

(1) Upper bainite: Characterized by elongated ferrite grains arranged largely parallel to each other, interspersed with fine, feather-like carbide particles (or short, rod-like carbides) oriented parallel to the ferrite grain axes.

(2) Lower bainite: Exhibits fine needle-like or lamellar structures with distinct orientation.

It is more susceptible to erosion than quenched martensite and closely resembles tempered martensite.

Distinction is extremely difficult under optical microscopy but readily achievable under electron microscopy.

Carbides precipitate within the needle-like ferrite, oriented at 55–60 degrees to the long axis of the ferrite plates.

Lower bainite contains no twins but exhibits numerous dislocations.

(3) Granular bainite: Appears as polygonal ferrite containing numerous irregular island-like structures.

When austenite in steel is cooled to slightly above the upper bainite transformation temperature, some carbon atoms migrate from ferrite through the ferrite/austenite phase boundary into the austenite.

This creates uneven carbon enrichment in the austenite, thereby inhibiting its transformation into ferrite.

These austenitic regions typically appear as isolated islands, granular or elongated in shape, distributed across the ferritic matrix.

Transformation Behavior and Microstructural Features of Granular and Lower Bainite in Steel

During continuous cooling, depending on the composition of the austenite and cooling conditions, the austenite within the granular pearlite may undergo the following transformations:

1) Complete or partial decomposition into ferrite and carbides.

Under electron microscopy, diffusely oriented granular, rod-shaped, or small block-like carbides are observable;

2) Partial transformation into martensite, appearing yellowish-brown under light microscopy;

3) Persistence as carbon-rich austenite.

In granular bainite, the ferritic matrix is embedded with granular carbides (island structure originally carbon-rich austenite, decomposed into ferrite and carbides during cooling, or transformed into martensite, or remaining as carbon-rich austenite grains).

Feather-like bainite features a ferritic matrix with strip-like carbides precipitating along the edges of ferrite plates.

Lower bainite exhibits needle-like ferrite grains covered with small plate-like carbides, oriented at approximately 55–60 degrees relative to the long axis of the ferrite grains.

Martensite  

It is a superheated microstructure composed of ferrite needles embedded in the steel matrix at approximately 60-degree angles to each other.

Coarse martensite reduces the plasticity and toughness of steel while increasing its brittleness.

In hypoeutectoid steel, overheating during heating causes coarse grain formation.

During rapid cooling, ferrite precipitates not only in a network pattern along austenite grain boundaries but also forms independently within grains.

This occurs through a shear mechanism, with ferrite needles aligning parallel to each other.

This distribution pattern is termed Widmanstätten structure.

In overheated hypereutectoid steels, carbides also form needle-like structures extending from grain boundaries into grains during cooling, resulting in Widmanstätten structure.

Martensite  

A supersaturated solid solution of carbon in α-Fe is termed martensite.

Martensite exhibits high strength and hardness but extremely poor plasticity, nearly zero, denoted by the symbol M.

It cannot withstand impact loads.

Martensite forms as the product of rapid cooling of undercooled austenite, where the shear mode transforms between the Ms and Mf points.

At this stage, carbon (and alloying elements) lacks sufficient time to diffuse, merely transitioning from the γ-Fe lattice (face-centered) to the α-Fe lattice (body-centered).

This signifies the transformation of carbon’s solid solution in γ-Fe (austenite) into its solid solution in α-Fe.

Thus, the martensitic transformation is “diffusion-free.”

Based on metallographic morphology, martensite can be classified into lamellar martensite (low-carbon) and acicular martensite.

(1) Lamellar Martensite: Also known as low-carbon martensite.

Fine martensite plates of roughly equal size are oriented and arranged in parallel, forming martensite bundles or martensite domains.

Between domains, the orientation difference is large; within a single original austenite grain, several domains with different orientations can form.

Since lamellar martensite forms at higher temperatures, self-tempering inevitably occurs during cooling, resulting in carbide precipitation within the formed martensite.

Consequently, it is prone to corrosion and darkening.

(2) Needle Martensite: 

Also known as plate martensite or high-carbon martensite, its fundamental characteristic is that the first plate of martensite formed within an austenite grain is relatively coarse, often traversing the entire grain.

This divides the austenite grain, limiting the size of subsequently formed martensite.

Consequently, plate martensite varies in size and exhibits irregular distribution.

Needle martensite forms in specific orientations.

Within the martensite needles, a central ridge plane becomes more pronounced with increasing carbon content, resulting in sharper needle tips.

Additionally, white retained austenite is present between the martensite needles.

(3) The martensite formed after quenching can develop into three special microstructures during tempering:

1) Tempered Martensite: Refers to the lamellar martensite (body-centered tetragonal crystal structure) formed during quenching, which undergoes decomposition during the first stage of tempering—where carbon precipitates as transitional carbides—resulting in a composite microstructure.

This structure features extremely fine transitional carbide flakes (with coherent interfaces to the matrix) dispersed within a solid solution matrix (whose crystal structure has transformed to body-centered cubic).

This structure cannot be resolved at maximum magnification under a metallographic (optical) microscope; only its overall appearance as black needles is visible (the shape of these black needles is essentially identical to the white needles of lamellar martensite, also known as “α-martensite,” formed during quenching).

These black needles are termed “tempered martensite.”

2) Tempered bainite: The product of quenched martensite undergoing medium-temperature tempering.

Its characteristics include: the martensitic needle morphology gradually disappears but remains faintly discernible (in chromium-alloyed steels, where the recrystallization temperature of alloy ferrite is higher, the needle shape is retained).

The precipitated carbides are fine and difficult to distinguish clearly under optical microscopy; only electron microscopy can reveal the carbide particles.

This structure is highly susceptible to corrosion, causing the microstructure to turn black.

If the tempering temperature is near the upper limit or the holding time is slightly prolonged, the needles appear white.

At this point, carbides segregate at the needle edges, resulting in slightly reduced hardness and decreased strength of the steel.

3) Tempered sorbite: The product formed when quenched martensite undergoes high-temperature tempering.

Its characteristic features include fine granular carbides distributed throughout the sorbite matrix, clearly discernible under optical microscopy.

This microstructure, also known as quenched and tempered structure, exhibits a favorable combination of strength and toughness.

The finer the granular carbides on the ferrite matrix, the higher the hardness and strength, though toughness is slightly reduced.

Conversely, coarser carbides yield lower hardness and strength but higher toughness.

Ledeburite

A eutectic mixture in iron-carbon alloys, specifically a liquid iron-carbon alloy with a carbon mass fraction (carbon content) of 4.3%.

At 1480°C, a mechanical mixture of austenite and cementite crystallizes simultaneously from the liquid phase.

This mixture is termed ledeburite and denoted by the symbol Ld.

Since austenite transforms into pearlite at 727°C, pearlite and cementite constitute ledeburite at room temperature.

For distinction, ledeburite above 727°C is termed high-temperature ledeburite (Ld), while that below 727°C is called low-temperature ledeburite (L’d).

Ledeburite exhibits properties similar to cementite, featuring high hardness and poor plasticity.