Bainitic Transformation and Characteristics

Bainitic Transformation

• Bainite is a two-component microstructure that forms from austenite between 550 to 250°C, which is in the pearlite and martensite range.
• Bainite formation resembles pearlite when overlapping with its range and martensite when overlapping with the martensite range.
• Bainite's microstructural development transitions between pearlite and martensite ranges.
• Bainite was discovered in 1930 and has three definitions: microstructural, overall kinetics, and surface relief.

Bainite Characteristics

• Diffusional, non-cooperative, competitive edgewise growth of two precipitate phases that form during eutectoid decomposition, where the minority phase is in non-lamellar form.
• Bainite has its own C-curve on a TTT-diagram that lies partially below but extensively overlaps pearlite in Fe-C and Fe-C-X alloys and entirely below pearlite in appropriately alloyed steels.
• Bainite consists of precipitate transformation plates, resulting in an invariant plain strain (IPS) surface relief effect, which forms by shear martensitically at temperatures usually above the martensite start temperature Ms.

Definition Differences and Formation

• The main difference between the three definitions is that the first definition requires two product phases like ferrite and cementite, while definitions (2) and (3) allows bainite to consist entirely of ferrite.
• Although bainite shares characteristics with martensite in morphology and shape deformation, it involves partitioning of carbon, unlike martensite, which sparks discussions about diffusion-controlled or displacive transformation mechanisms.
• Bainitic phase transformations have 2 steps: γ → α transformation mostly by shear + C diffusion leading to C partitioning between γ and α or the precipitation of carbides.
• The sequence of these steps differs based on carbon content.
  • In low C steels, transformation occurs first, followed by C diffusion in bainitic ferrite, leading to small precipitates (e.g., ε carbide).
  • In high C steels, cementite precipitation occurs first in austenite, followed by transformation of the C-depleted austenite.

Variety of Bainite Morphologies

• Describing different bainite morphologies is difficult due to the variety of terms, but they are similar to martensite and ferrite.
• Various perspectives on bainite morphology include fine acicular, coarse acicular, and granular bainite.
• The terms signify the spacing and arrangement of carbides within the bainite, affecting mechanical properties.
• In acicular structures, bainitic ferrite appears as long, stretched laths in parallel groups.
• Microstructure is considered fine if carbon-rich components and bainitic ferrite are only partially distinguishable; otherwise, it is coarse.
• If no continuous needles appear in groups, the microstructure is classified as granular.

Upper Bainite vs Lower Bainite

• Bainite manifests in two distinct microstructures: upper and lower bainite.
• In upper bainite, carbon-rich components (carbide, martensite, retained austenite) form between bainitic ferrite laths.
• In lower bainite, carbides precipitate within the ferrite plates.
• Both types of bainite can occur simultaneously.
• Differentiation between these microstructures is difficult with an optical microscope, so Transmission Electron Microscopy (TEM) is used.

TEM Micrographs of Bainite in 100Cr6 Steel

• Lower bainite in 100Cr6 steel exhibits nano-sized carbides precipitating within bainitic ferrite plates at approximately 60° to the longitudinal direction of bainitic ferrite.
• Boundaries of bainitic ferrite subunits are indicated with dashed green lines.
• Upper bainite ferrite has a lath-type shape, unlike the plate-like morphology of lower bainite.
• Nano-sized carbides precipitate between bainitic ferrite laths in upper bainite, with several carbides precipitating from the grain boundary (GB).
• Carbides in upper bainite are longer and thicker than in lower bainite.
• TEM images show lower bainite carbides average about 30nm in width and 80nm in length, while upper bainite carbides are as large as 100nm in width and 0.5 - 1µm in length.

Nano-Sized Carbide Precipitation

• Nano-sized carbide precipitation within lower bainite tends to adopt a single crystallographic variant in a bainitic ferrite plate, unlike in tempered martensitic microstructures.
• The phase transformation kinetics and microstructure evolution during bainite formation can be simulated using a multi-phase field simulation approach at 260 °C in 100Cr6 steel.

Bainite Classification and Microstructures

• Bainite is classified into carbon-poor bainite (B1), upper bainite (B2), and lower bainite (B3).
• Carbon-poor bainite, also known as "acicular ferrite", comprises ferrite laths and carbon-enriched austenite islands that transform into martensite.
• During continuous cooling or isothermal transformation, mixtures of upper and lower bainite may be obtained.

Schematic Classification

• B1 (Low carbon bainite): Ferrite laths with carbon-enriched retained austenite or martensite fields.
• B2 (Upper bainite): Ferrite-cementite two-phase structure.
• B3 (Lower bainite): Ferrite plates with finely dispersed cementite precipitates.

Upper Bainite Formation

• Upper bainite forms in the upper temperature range of the bainite field (550-400°C).
• The transformation progression depends on the carbon content of the steel.
  • High Carbon Steels: Transformation starts with diffusion-controlled precipitation of cementite (Fe3C), depleting austenite of carbon. Ms-temperature is reached and transformation occurs through shearing (martensite mechanism).
  • Low Steel Carbon levels: Driving force for cementite formation is smaller, ferrite laths form, and austenite transforms according to the martensite mechanism. After this, a second, diffusion-controlled step of the transformation occurs. Cementite precipitates out between the ferrite plates due to low carbon solubility.

Lower Bainite Formation

• Lower bainite forms in the lower temperature range of the bainite field (400-250°C).
• Ferrite forms similarly to upper bainite.
• Carbon diffusion is restricted due to the decreased transformation temperature, so carbon insoluble in ferrite cannot diffuse out of the ferrite plates.
• The diffusion-controlled sub-step involves carbide particles precipitating within the growing ferrite plates at an angle of approximately 60° from the ferrite axis due to preferential nucleation.
• Instead of cementite, easily nucleated ε carbide may precipitate or precede Fe3C formation.

Atom Probe Micrographs

• Atom Probe Tomography (APT) images of lower bainite in 100Cr6 steel reveal the local carbon distribution in the bainitic ferrite matrix and carbides.
• Carbon-depleted regions with <1.2 at.% carbon represent bainitic ferrite, which is supersaturated with carbon.
• Carbon-rich regions correspond to cementite (Fe3C) and ε carbide (Fe2.4C).
  • A particle with approximately 25 at.% carbon is cementite.
  • A particle with approximately 29.4 at.% carbon is ε carbide (Fe2.4C).
• Atom probe data indicates co-existence of ε carbide and Fe3C precipitation.
• Substitutional element distribution across the bainitic ferrite αB/cementite and αB/ε carbide interfaces suggests that both cementite and ε carbide precipitates in lower bainite occur under paraequilibrium mode.

Phase transformation condition.

• The interstitial elements diffuse and the substitutional elements freeze in the sublattice. After a long holding period, ε carbides transform into Fe3C. The transition between ε carbide/iron and cementite (θ) is the obvious reaction in an iron matrix: ε-Fe2.4C + 0.6 Fe = θ-Fe3C.

Ferritic vs Austenitic

• Fe is either bcc iron in a bainitic-ferritic matrix at low temperatures or fcc iron in austenite at higher temperatures.
• Gibbs free reaction energies determine stabilities.
  • Positive values: ε is favored
  • Negative values: cementite is favored
• In lower bainite, θ Fe3C and ε Fe2.4C have similar formation probability.
• In upper bainite, cementite formation is preferred.

Bainite Chart Classification Scheme

• Bainite exhibits complex microstructures that contain a bainitic ferrite matrix and 2nd phases like alloyed carbides, cementite, and retained austenite.
• The 2nd phases precipitate from the bainitic ferrite matrix at different locations and shapes, affecting mechanical properties.
• The classification analyzes micro- and atomic morphological features.
• It classifies bainitic microstructures at micro- and nano-scales for industrial applications and nano precipitation mechanism study.

Bainite Chart Components

• The Bainite chart classification identifies crystal structure and form of the basic structure and the location, type, and form of the sub-structure.
• Bainite microstructure consists of a basic structure and substructures.
• The basic structure is bainitic ferrite (bcc crystal structure) with different forms, ie, polygonal, granular, Widmanstätten.
• Substructures are the 2nd phases that precipitate at different locations (alloyed carbides, θ-carbide (cementite, Fe3C), ε-carbide (Fe2.4C), etc.)

Bainite Chart Classifications

• The bainite morphology at 500 °C in 100Cr6 steel is classified as "B-L, S-I/θ-E" or "B-L, S-I/θ-L".
• The basic structure adopts a lath-like shape (B-L), with Intragranular substructures (S-I) between the bainitic ferrite laths.
• The 2nd phase substructure is identified as θ-carbide with an Elongated or Lath-like shape.
• Precipitates from the grain boundary with an Elongated or Lath-like shape (B-L, S-B/θ-E or B-L, S-B/θ-L). The 3D atom probe tomography (APT) reveals elemental distribution within bainitic ferrite and θ-carbide . Carbon atom map shows carbon clusters in bainite (C-C).

Bainite Chart Classifications at 260°C

• Bainite microstructure formed at 260 °C in 100Cr6 steel: exhibits a PLate-like shape (B-PL).
• The Substructure precipitates Intragranularly (S-I) or from the bainitic ferrite platelet boundaries (S-B).
• The 2nd phase substructures in the bainitic microstructure exhibit large varieties and forms.
• The substructures in the bainitic structure at 260°C include Cr carbide with a round shape (AC-R), θ-carbide with a plate shape (θ-P*), martensite with a plate shape (M-P), retained austenite with a film shape (RA-F).
• the e-carbide and ε-carbide precipitations in the 3D atom map are the 2nd phases with a plate like shape.
• The bainite carbon clusters (C-C) are similar to the carbon clusters with the Carbon clusters within the bainite formation at 500 °C.

Temperature Range of Bainite Formation

• Bainite formation exists between pearlite and martensite due to its transitional microstructure.
• Bainitic transformations can occur continuously or isothermally.
• Bainite forms at higher temperatures than martensite because less supercooling below equilibrium is required.

Upper and Lower Bainite Transformations

• Upper and lower bainite can be distinguished by formation mechanism, determined by temperature and carbon content.
• In steels with low carbon content (about 0.1%), the transition from upper to lower bainite occurs at about 450°C.
• As the carbon content increases, the temperature boundary rises to around 550°C for approximately 0.5% carbon.
• As carbon content increases further, the temperature boundary subsequently falls to around 350°C.
• The temperature drop begins at point B.
• Point B is the intersection between the boundary line and the extrapolated Accm line.

Formation and Temperature Influence

• Upper bainite is limited by the 350°C line, as below this temperature, C diffusion is so hindered that carbide precipitation no longer occurs in austenite.
• No bainite transformation begins above B corresponding to 550°C because the carbon concentration of austenite does not fall below the Bs line.
• BIII bainitic ferrite is supersaturated with carbon, and carbides form within the acicular ferrite, so the nucleation rate decides the first precipitation phase.

Influence of Growth Rate

• Growth curves have similar slopes, but the growth rate decreases with increasing carbon content due to decreasing formation temperature.

Influence of Alloying Elements

• Alloying elements like chromium and nickel reduce bainite growth rate,restricting mobility of phase boundary by accumulating alloying elements.

Alloying Elements and Bainite Formation

• Carbon influences the ranges in which upper and lower bainite forms.
• Alloying elements shift the Bs-temperature to lower temperatures and longer holding times.
• The empirical formula for the influence of alloying elements on the Bs-temperature
• Bs(°C) = 830 - 270 (%C) - 90 (%Mn) - 37 (%Ni) - 70 (%Cr) - 83 (%Mo)

The Solubility of Carbon

• The solubility of carbon is significantly greater in austenite than in ferrite, which delays the reaction kinetics.
• Increasing alloying content creates a zone where nucleation is heavily restricted or impossible.
• Cluster-like segregations on the metastable austenite/bainite phase boundary rich in alloying elements (Cr, Mo, C) block bainitic ferrite growth.
• At critical content levels of Cr, Mo, and Mn, diffusion is temporarily hindered.

Impact of Alloying

• Adding alloying elements delays ferritic and pearlitic reactions and shifts the bainitic reaction to lower temperatures, creating a zone without transformation.
• To obtain a fully transformed bainitic microstructure is difficult in the neighbouring martensitic transformation.
• Bainitic structures offer weldability due to low carbon content.
• The transformation diagrams for metastable austenite show that boron delays ferrite and pearlite by about 0.003%

Mechanical Properties

• The mechanical properties of bainite is determined mainly by bainite laths' size.
• To a lesser degree, mechanical properties are determined by the size of the bainite bundles.
• The transformation temperature decides the lath width of bainite
• Bainite lath size decreases at a decreasing transformation temperature.
• Bainite lath size is independent of initial austenite grain size and significantly influences toughness.

Bainite Microstructure

• The grain is coarser as the maximum notch-impact temperature curve is shifted to a higher temperature.
• Increased austenite grain size strengthens yield.

Bainite Strength Properties

• The test samples are quenched in water immediately after either various austenitisings or a 60% deformation of the austenite.
• The Bs-temperature of an austenite grain size of 4 yields 800 MPa and an austenite grain size of 8 yields up to 920 MPa.
• The strength properties of bainite are closely connected to the transformation temperature
• grain refinement increases upper bainite (lath bainite).
• As the formation temperature decreases, the carbides becomes smaller.
• The strength properties and toughness increase and the ferrite sub-structure becomes finer.

Steels Mechanical Application

• Austempering is only used in special cases
• A combination of martensitic quench-hardening and subsequent tempering results in a higher toughness at the same strength despite the danger of forming hardening cracks increases.
• Isothermal-bainite transformation is preferred when a certain strength-toughness ratio, and is expected to be tempered in a temperature range.

Bainite Temperature Sensitivities

• Transformation in the lower bainite range, which is above the martensite field, is useful for sensitive steels and components when temperature balances the diameter of the material.
• It reduces the thermal stress of metastable austenite for transformation purposes.
• Transformation in the bainite field (continuous cooling) signifies relation to the thermomechanical treatment of low-alloyed construction steels.

Increasing Strength

• Increase strength in greatly reduced carbon content steels and can use increasing dislocation density in austenite.
• Strengthening results from grain refinement, solid solution, and dispersion hardening through lower end-rolling temperatures.

Widmanstätten Ferrite

• The microstructural form discovered in metallic masses within meteorites.
• Widmannstätten ferrite, forms at carbon contents between 0.2 and 0.4%.
• The process uses Quick cooling from high austenitizing temperatures.
• It has ferritic plates that form and grow into the austenitic grain.
• The needles demonstrates an orientated relationship with the austenitic lattice.
• There is no substructure present due to the absence of minimal presence
• can be found in casting microstructures and welds.

Acicular Ferrite

• It is needle-shaped or rod-like.
• It appears in the microstructure as enclosed ferritic plates without any intra-granular carbides precipitation.
• It does not grow in single needles from the prior austenite grain boundaries or starting existing precipitations or inclusions.

Applications of Bainitic Steels

• Ofot railway in northern Norway
• Routes through a mountainous region along fjords.
• Tracks can be subject to loads.
• Very high braking forces and extremely high wear on the tracks.
• Until 1967 pearlitic steel of grade 900 was used, and has to be exchanged
• fine pearlitic Cr-V steel of grade 1200 has been exchanged, and has served
• Bainitic steels - grade 1400 and has lifespan

Commercial Strength

• Modern commercial vehicles are an important field of application.
• They balance manufacturing and use the steels.
• The The steel's ratio of load to dead weight is good processability.
• The steel grades used often contain chemical compositions.
• the best possible cost/performance ratio for each specific application is guaranteed.
• The main frame in freight trucks is a characteristic.
• They are usually made from thermomechanically rolled steels
• Heat treatment ensures values.

Steel requirements

• Water-quenched and tempered fine-grained steels and heavy plates.
• The the range maintained with steel treatment.
• Minimum yield strength is mainited.
• The structure consists of martensite and fine bainite.
• Hot strips, compared to ferritic-pearlitic strips, allow for minimum yield to be set.