Bainite — Upper, Lower and Granular Bainite Formation in Steel
Bainite is a non-lamellar, acicular microstructural constituent that forms in steel when austenite is transformed at temperatures between the pearlite and martensite regimes. First described by E.C. Bain and N.Y. Davenport in 1930, it occupies a critically important position in physical metallurgy: its three morphological variants — upper bainite, lower bainite, and granular bainite — offer engineers a tunable combination of strength, hardness, and toughness that neither pearlite nor martensite alone can match.
Key Takeaways
- Bainite forms isothermally or on continuous cooling between approximately 250–550°C, bracketed by the pearlite and martensite regimes on the TTT/CCT diagram.
- Upper bainite (400–550°C) has cementite at interlath boundaries; lower bainite (250–400°C) has intralath carbides at ~55° to the habit plane, giving superior toughness.
- Granular bainite forms on continuous cooling in low-carbon, high-alloy steels and contains islands of martensite-austenite (M-A) constituent instead of discrete carbide films.
- The bainite transformation is displacive for the ferrite component (invariant-plane-strain shape change) but requires short-range carbon diffusion for carbide precipitation — a mixed-mode mechanism.
- Austempered ductile iron (ADI) exploits the bainite transformation window to produce ausferrite, achieving tensile strengths of 800–1600 MPa with significant elongation.
- Lower bainite generally outperforms upper bainite in Charpy toughness at equivalent hardness because intralath carbides do not provide the continuous crack-propagation paths that interlath cementite films do.
Thermodynamics and Kinetics of the Bainite Transformation
Bainite forms when austenite is cooled to, or held isothermally at, temperatures between the bainite start temperature (Bs) and the martensite start temperature (Ms). The driving force for transformation is the reduction in Gibbs free energy when austenite converts to bainitic ferrite plus carbides:
ΔG = G_product − G_parent < 0 (thermodynamically favourable)
B_s (°C) ≈ 830 − 270(%C) − 90(%Mn) − 37(%Ni) − 70(%Cr) − 83(%Mo)
[Bhadeshia empirical equation for low-alloy steels]Unlike the purely diffusional pearlite transformation, bainite forms by a displacive mechanism analogous to martensite: iron and substitutional alloying atoms move cooperatively, producing an invariant-plane-strain (IPS) shape change measurable by surface relief experiments. This places bainite firmly in the displacive (or military) transformation class. Carbon, however, being an interstitial atom with high diffusivity, is mobile enough at bainite temperatures to redistribute after the displacive event — either diffusing to inter-lath boundaries (upper bainite) or precipitating within the plate (lower bainite).
Incomplete Reaction Phenomenon
A diagnostic feature of the bainite transformation is that it stops before the austenite is fully consumed — even during prolonged isothermal holding. The transformation halts when the free energy of bainitic ferrite formation from the carbon-enriched austenite (enriched by prior bainite formation) equals zero. This is the T0‘ criterion: transformation ceases when the austenite carbon content reaches the T0‘ curve on the Fe-C phase diagram. Retained austenite stabilised in this way may subsequently transform to martensite on cooling, forming the martensite-austenite (M-A) constituent prevalent in granular bainite.
T₀' curve: locus of compositions where ΔG(γ→α) = 0
accounting for stored strain energy (~400 J/mol)
Bainite reaction stops when: γ carbon content ≥ T₀'(T)Upper Bainite (400–550°C): Mechanism and Microstructure
Upper bainite forms when austenite is isothermally held between approximately 400°C and 550°C. At these temperatures, carbon diffusivity in austenite is high enough (~10-13 to 10-15 m²/s) that carbon rejected from the supersaturated bainitic ferrite laths can migrate to interlath regions and precipitate as cementite (Fe3C) before the next sub-unit forms.
Sub-unit and Sheaf Growth
Growth occurs by the sequential nucleation of sub-units — individual ferrite laths typically 0.2–0.5 μm wide and 10–20 μm long. Each sub-unit nucleates at the tip of the preceding sub-unit or at austenite grain boundaries, giving an autocatalytic cascade that builds a sheaf of parallel laths. The sheaf grows until it encounters another sheaf, a grain boundary, or transforms all available austenite within the T0‘ constraint.
Carbide Distribution in Upper Bainite
As each ferrite lath forms, carbon is expelled into the adjacent austenite. When the local austenite carbon content exceeds the cementite precipitation limit, cementite films or rods nucleate between adjacent laths. This produces the characteristic upper bainite morphology:
- Ferrite lath width: 0.2–0.5 μm (coarser than lower bainite)
- Cementite at interlath boundaries: film or rod morphology, typically 0.02–0.1 μm thick
- Cementite orientation: approximately parallel to the ferrite lath long axis
- Prior austenite grain boundaries visible as sheath boundaries between sheaf groups
Lower Bainite (250–400°C): Mechanism and Microstructure
Lower bainite forms at temperatures between approximately 250°C and 400°C, where carbon diffusivity is sufficiently reduced that carbon cannot escape the supersaturated ferrite plate before the plate has completed its growth. Carbon therefore precipitates within the ferrite plate as fine carbides, rather than at the boundaries.
Intralath Carbide Precipitation
The carbides in lower bainite typically precipitate at a characteristic angle of approximately 55–60° to the plate habit plane, a feature that distinguishes lower bainite unambiguously from upper bainite and from martensite. The precipitating carbide is often epsilon-carbide (Fe2.4C) at lower transformation temperatures, transitioning to cementite (Fe3C) at higher temperatures within the lower bainite regime.
Carbide orientation in lower bainite:
≈ 55–60° to the plate habit plane (diagnostic feature)
Burgers vector: a/2[111]α ∥ [100] of epsilon-carbide
Diffusion distance for carbon during lower bainite formation:
x ≈ 2√(D·t) ≈ 10–50 nm at 300°C, t ~ 100 s
→ carbon cannot reach interlath boundary (typically >200 nm away)Plate Morphology and Size
Lower bainite plates are typically finer than upper bainite laths: plate width 0.1–0.3 μm, length 5–15 μm. The finer scale and the absence of coarse boundary carbides are the principal reasons lower bainite has superior fracture toughness compared to upper bainite at equivalent hardness. The Charpy transition temperature for lower bainite may be 50–100°C lower than for upper bainite in the same steel composition.
Crystallographic Relationships
Both upper and lower bainitic ferrite maintain the Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (N-W) orientation relationship with the parent austenite grain, identical to lath martensite:
Kurdjumov-Sachs (K-S):
{111}γ ∥ {110}α and <110>γ ∥ <111>α
→ 24 orientation variants per prior austenite grain
Nishiyama-Wassermann (N-W):
{111}γ ∥ {110}α and <112>γ ∥ <110>α
→ 12 orientation variants per prior austenite grainGranular Bainite
Granular bainite was first characterised in low-carbon, high-alloy steels undergoing continuous cooling (not isothermal transformation). It lacks the clearly defined parallel lath or plate morphology of classical upper or lower bainite, instead presenting as irregular or equiaxed bainitic ferrite with dispersed islands of martensite-austenite (M-A) constituent.
Formation Mechanism
During continuous cooling through the bainite temperature range at moderate rates, bainitic ferrite forms without complete carbide precipitation. The residual, carbon-enriched austenite between bainite regions is stabilised to low temperatures; on further cooling below Ms, part of it transforms to martensite, leaving a final mixture of fresh martensite and retained austenite that appears as dark-etching blocky islands in the optical micrograph — the M-A constituent. Because there is no time for the systematic sheaf-growth sequence characteristic of isothermal transformation, the overall morphology is granular or acicular-irregular rather than classically sheaf-like.
Industrial Relevance of Granular Bainite
Granular bainite is widely encountered in:
- Thermomechanically controlled processed (TMCP) pipeline steels (API 5L X65–X80)
- High-strength low-alloy (HSLA) structural steels
- Coarse-grained heat-affected zones (CGHAZ) of high-heat-input welds in these grades
- Large-section forgings cooled in still air from normalising temperature
The M-A islands in granular bainite can be detrimental to HAZ toughness if they are large (>3 μm) or numerous, as they act as brittle crack initiators under impact loading. Compositional controls — reducing carbon content and limiting the carbon equivalent — and reduced heat input in welding are the primary mitigation strategies. Refer to the HAZ microstructure guide and the hydrogen-induced cracking article for further context on controlling HAZ constituent phases.
Comparison of Bainite Morphologies
| Feature | Upper Bainite | Lower Bainite | Granular Bainite |
|---|---|---|---|
| Temperature range | 400–550°C | 250–400°C | Typically 350–500°C (continuous cooling) |
| Ferrite morphology | Parallel laths in sheaves | Plates, slightly coarser in outline | Equiaxed/irregular ferrite blocks |
| Carbide location | Interlath boundaries (films/rods) | Intralath (55–60° to habit plane) | Largely absent; M-A islands instead |
| Carbide type | Cementite (Fe3C) | Epsilon-carbide (lower T) or cementite | None discrete; austenite retained |
| Typical hardness (0.4%C steel) | 30–45 HRC | 45–55 HRC | 25–40 HRC (lower-C grades) |
| Charpy toughness | Moderate; limited by interlath carbides | Good; best of the three morphologies | Variable; M-A islands critical |
| Optical etch appearance | Dark acicular sheaves; upper martensite-like | Fine dark needles, harder to resolve | Dark blocky islands (M-A) in lighter ferrite |
| Distinguishing technique | Nital etch, optical; SEM for carbides | Klemm’s tint etch; TEM for carbide angle | LePera etch reveals M-A as white |
Mechanical Properties of Bainitic Steels
Strength and Hardness
The yield and tensile strength of bainite increase with decreasing transformation temperature due to three concurrent strengthening mechanisms:
- Hall-Petch strengthening: finer lath/plate width at lower temperatures increases dislocation boundary density.
- Dislocation density: the IPS shape change introduces a high density of geometrically necessary dislocations (~1014 m-2), contributing directly to flow stress.
- Carbide precipitation hardening: fine intralath carbides in lower bainite act as Orowan obstacles to dislocation motion.
Hall-Petch contribution to yield strength:
ΔσḾ = kḾ · d⁻½
where d = effective ferrite lath width (m), kḾ ≈ 0.6 MPa·m½ for bainitic ferrite
Dislocation strengthening:
Δσν = MαGb√ρ
M = Taylor factor (3.06), α ≈ 0.25, G = 80 GPa, b = 2.52×10⁻¹⁰ mToughness
Toughness in bainite is primarily controlled by the effective crack propagation unit — the prior austenite grain size modulates the maximum sheaf length, while the misorientation between adjacent packets determines the likelihood that a propagating crack deflects at each boundary. Lower bainite, with its higher packet boundary misorientation distribution (more variants active per grain) and absence of coarse boundary carbides, typically exhibits a Charpy transition temperature 50–100°C lower than upper bainite at the same tensile strength. For context on how HAZ microstructure affects weld toughness, see the HAZ microstructure article.
| Property | Effect of Finer Scale | Effect of Higher Carbon | Effect of Alloying (Mo, Ni, Cr) |
|---|---|---|---|
| Yield strength | Increases (Hall-Petch) | Increases (solid solution + precipitation) | Mo and Cr increase; Ni has modest effect |
| Tensile strength | Increases | Increases significantly | Generally all increase |
| Elongation (%) | Usually improves slightly | Reduces | Ni improves; Mo broadly neutral |
| Charpy toughness | Improves (shorter crack path) | Reduces (more carbides) | Ni improves; V can reduce at high levels |
| Hardness (HRC) | Increases | Increases significantly | Cr, Mo, V all increase |
Identifying Bainite in Optical Metallography
Metallographic identification of bainite morphology is a practical skill required for failure analysis, weld procedure qualification, and heat treatment audit work. The following protocol applies to carbon and low-alloy steels:
Specimen Preparation
- Mount, grind through 240–1200 grit SiC, then polish with 6 μm, 3 μm, and 1 μm diamond.
- Final polishing with 0.05 μm colloidal silica (OPS) for 2–5 minutes on a vibratory polisher.
- Rinse with ethanol and dry with warm air. Do not touch the polished face.
Etching
- Nital (2% HNO3 in ethanol): General-purpose. Reveals grain boundaries and phase contrast. Upper bainite appears as dark acicular regions; prior austenite grain boundaries visible as faint lines.
- Klemm’s reagent (tint etch): Colours bainite brown-grey, martensite white-yellow, ferrite blue. Best for distinguishing mixed bainite-martensite microstructures.
- LePera reagent (sodium metabisulphite + picric acid): Colours M-A islands white, bainite and ferrite blue-grey. Essential for granular bainite identification in pipeline and HSLA steels.
Electron Microscopy Confirmation
For definitive identification, scanning electron microscopy (SEM) in back-scattered electron (BSE) mode reveals cementite films (bright due to atomic number contrast) at interlath boundaries in upper bainite. Transmission electron microscopy (TEM) is required to confirm the ~55° carbide orientation in lower bainite and to distinguish epsilon-carbide from cementite by selected-area diffraction. Refer to ASM Handbook Volume 9 (Metallography and Microstructures) for reference micrographs and standard procedures, and to the martensite formation article for distinguishing bainite from lath martensite in optical micrographs.
Austempered Ductile Iron (ADI) and the Ausferrite Microstructure
The most commercially significant application of the bainite transformation in cast alloys is austempered ductile iron (ADI). The austempering heat treatment exploits the bainite transformation window in spheroidal graphite (SG) cast iron to produce a unique microstructure called ausferrite — a mixture of bainitic ferrite and high-carbon retained austenite.
Austempering Process
- Austenitisation: 850–950°C for 1–4 hours to fully austenitise the matrix and dissolve pearlite.
- Rapid transfer and quench to the austempering bath (molten salt or fluidised bed): 260–400°C.
- Isothermal hold: 1–4 hours within the bainite window. Ausferrite forms; Stage I reaction.
- Quench to room temperature: must be completed before Stage II begins (where the stabilised austenite starts to decompose to bainite + carbide, degrading properties).
ADI Austempering Window:
Tᵣᵗᵗᵗᵗ = 260–400°C (bainite formation range in SG iron)
Stage I: austenite → bainitic ferrite + C-enriched austenite
Stage II: C-enriched austenite → bainite + carbide [AVOID]
Retained austenite carbon ≈ 1.8–2.2 wt% after Stage IADI Mechanical Properties
ADI grades are specified in ISO 17804 and ASTM A897. The austempering temperature controls the microstructure and properties:
| ADI Grade (ISO 17804) | Austempering T (°C) | UTS (MPa) | 0.2% YS (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|---|---|
| Grade 800/10 | 380–400 | ≥800 | ≥500 | ≥10 | 260–320 |
| Grade 1000/5 | 330–360 | ≥1000 | ≥700 | ≥5 | 300–360 |
| Grade 1200/2 | 300–330 | ≥1200 | ≥850 | ≥2 | 340–400 |
| Grade 1400/1 | 260–300 | ≥1400 | ≥1100 | ≥1 | 380–440 |
Industrial Applications and Significance
Bainitic microstructures are deliberately produced — or their accidental formation must be anticipated — in a wide range of engineering applications:
Structural and Pressure Vessel Steels
In quenched and tempered (Q&T) pressure vessel and structural steels, bainite forms in thick sections where the centre cooling rate is insufficient to produce full martensite. A mixed bainite/martensite microstructure in thick-section ASME Section VIII or EN 10028-2 pressure parts is generally acceptable if the bainite is lower bainite and if Charpy impact requirements at the test temperature are met. Upper bainite in the centre of thick forgings is a quality concern that must be confirmed by mechanical testing and metallographic coupon examination. For heat treatment of steel components, see the quenching and tempering guide and the annealing and normalising article.
Rail Steel
Bainitic rail steels (e.g., 0.3–0.4%C, 1.5–2%Mn, 0.3–0.5%Si, 0.1–0.3%Mo) are used in high-speed and heavy-haul applications where pearlitic rail is prone to rolling contact fatigue (RCF). The fine bainitic microstructure, particularly lower bainite, offers superior wear resistance and RCF resistance due to its higher hardness and more homogeneous hardness distribution compared to pearlite.
Bainitic Gear and Bearing Steels
Certain gear applications use bainite-hardened steels (e.g., 17CrNiMo6, 20MnCr5) where the combination of bainite core toughness and carburised case hardness provides a superior fatigue-resistant component compared to through-hardened martensite, which can be more susceptible to brittle fracture at notches under impact loading.
Weld Heat-Affected Zone
Bainite frequently forms in the subcritical and intercritical HAZ of medium-carbon structural steel weldments. The heat-affected zone microstructure guide covers the specific zones in detail. From a practical welding engineering standpoint, a bainitic HAZ is generally preferred over a martensitic HAZ because it typically requires no mandatory post-weld heat treatment (PWHT) in P-number 1 carbon steels up to moderate carbon equivalents, though hardness limits (e.g., 350 HV per EN ISO 15614-1) must be satisfied. Hydrogen-assisted cold cracking (HACC) risk increases significantly when the HAZ hardness exceeds these limits; see the hydrogen-induced cracking article for crack susceptibility assessment. Refer also to Charpy impact testing for HAZ toughness qualification procedures.
Nanostructured Bainite
A significant research-to-application development since the 2000s is nanostructured bainite, also called superbainite, produced by transformation at temperatures as low as 125–200°C over transformation times of 2–100 days. Compositions such as 0.78%C-1.5%Si-2%Mn-1%Cr-0.25%Mo are designed to suppress cementite precipitation (Si retards Fe3C nucleation) and produce plates of bainitic ferrite only 20–40 nm wide, giving tensile strengths up to 2500 MPa with reasonable elongation (~5–10%). Current commercial applications include armour plate and high-wear tooling.
Relationship to the Fe-C Phase Diagram and TTT/CCT Diagrams
The bainite transformation window on the TTT diagram is bounded above by the Ae1 line (727°C) and the eutectoid nose, and below by the Ms temperature. The Bs temperature, as given by the Bhadeshia equation above, can be calculated from composition and lies in the range 350–550°C for most engineering steels.
On the CCT diagram, the bainite C-curve generally lies to the right of the pearlite C-curve in alloy steels — meaning pearlite forms first on moderate cooling, and bainite forms at faster cooling rates that bypass pearlite. In plain carbon steels, the two C-curves may nearly overlap, making clean bainite formation by continuous cooling difficult. Isothermal austempering (holding in a salt bath or fluidised bed at a fixed temperature within the bainite window) is the reliable method for producing a fully bainitic microstructure in these steels.
For a thorough treatment of Fe-C phase equilibria, see the iron-carbon phase diagram guide. For the bainite sub-domain of the phase diagram, the eutectoid reaction context is covered in the eutectoid reaction article, and grain boundary effects on nucleation are addressed in the grain boundaries guide.
For comparison with the two flanking microstructures, the martensite formation article describes the diffusionless transformation below Ms, while the pearlite colony growth article covers the cooperative ferrite-cementite lamellar growth mechanism above the bainite range. Hardness verification of heat-treated bainitic components follows the methods described in the hardness testing guide.
Frequently Asked Questions
What is the temperature range for upper bainite formation?
What is the temperature range for lower bainite formation?
How does upper bainite differ from lower bainite in microstructure?
What is granular bainite?
Does bainite form by a diffusionless or diffusion-controlled mechanism?
How is bainite identified in optical metallography?
Why does lower bainite generally have better toughness than upper bainite at equivalent hardness?
What is austempered ductile iron (ADI) and how does bainite relate to it?
What is the significance of bainite in welding heat-affected zones?
Recommended Technical References
Further Reading
