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Bainite in Steel | Guide for Upper and Lower Bainite | Bainitic Transformation

Bainite in Steel is a platelike microstructure formed between 150-500oC. This microstructure is explained by edger Bain and requires controlled cooling in-between pearlite and martensitic formation. The heat treatment process used for bainite formation is termed Austempering. Bainitic steel has several applications in auto and rail parts.

What is Bainite?

Bainite in steel is a platelike non-lamellar mixture of ferrite and cementite formed between 150oC – 450oC. The microstructure is divided into upper bainite and lower bainite with a better combination of mechanical properties. The most common applications of bainite include lightweight car bodies, auto, and rail parts, and more.

“Bainite steel is non-lamellar microstructure opposite to pearlite that evolves when two phases i.e. ferrite and iron carbide grow at different rates”.

Definition of Bainite

History of Bainitic Steel

Bainite in steel intrigues researchers as it provides an amazing combination of mechanical properties without lots of alloying addition. A lot of research has been carried out within the bainitic structure in the 1900s. 

Bain in 1930 started experimenting with the isothermal cooling in-between pearlitic transformation range and martensitic transformation range. Within this region, experiments by Bain revealed acicular, dark etched structures different from martensite and pearlite. 

The structure revealed at that point was similar to the below figure; 

Bainite microstructure

Toriano referred to this microstructure as Austempering structure in 1940. 

Researchers and colleagues from Bain’s lab started calling this dark-etched bainite to commemorate the efforts of Bain in presenting the first bainitic microstructure. 

Later researchers postulated the microstructure as initially flat plates of ferrites nucleates along specific crystallographic planes within austenite.

With ferrite ejecting from carbon, carbide particles are formed within ferrite plates. The formation of cementite and ferrite plates is random and non-lamellar opposite to the lamellar structure of pearlite. 

The high and low range of bainite steel is later on termed as Upper and Lower bainite with a subsequent difference in properties that you will found below.

Morphology and Characteristics – Bainite Microstructure

Pearlitic growth occurs at common nucleation or high energy points like triple points and grain boundaries. This pearlitic transformation involves coupled growth of ferrite and cementite and the composition of both phases compliments each other’s growth. This means extra carbon from ferrite is accommodated within cementite. 

In the case of bainite, the process of ferrite and cementite nucleation and growth takes place inseparable stages. Firstly, ferrite growth takes place and later on, precipitation of carbide takes place. 

We will explain both processes in form of separable stages. Stages are termed as follows;

1. Growth of Ferrite

2. Precipitation of Carbide

Growth of Ferrite

Whether it’s upper or lower side of said phase, it is an aggregate of plates of ferrite that are separated by martensite, untransformed austenite, or cementite. The aggregation of those ferritic plates is termed as sheaves. A single ferritic plate in the sheave is called a sub-set. 

Sub-sets are not isolated rather they are interconnected in 3 dimensions. All sub-sets are crystallographically aligned with each other.

The shape of sheaves is wedge type with a thicker side starting from the austenitic nucleation site. Sub-sets have lathe-type morphology that is more prominent near the austenite site. 

Subsets are nucleated to a specific size and have limited growth. Every new sub-set is nucleated over a previously nucleated one. The exact morphology can be seen below figure;

Lathe type ferrite is the main type of structure within bainitic microstructure where subsets have untransformed austenite and martensite present in between them. This lathe type of ferrite formation is complex proves and involves lots of variables to be processed.

One of the major factors to consider during shape transition within the structure is the yield strength of the parent phase and rapid radial growth of sub-sets.

How is Bainite formed in Eutectoid Steel?

The formation of a bainite plate involves lattice shear resulting in surface distortion mainly surface tilts and resulting accommodations. As opposed to the martensite plate where the formation is fast, bainitic plates forms slowly and continuously and the growth is retarded by the time required for the diffusion process. 

Incubation Period

At the start of the reaction, there is an incubation period where no transformation occurs after which the reaction is initiated.  

Transformation Step

During the transformation, the uniformly dispersed carbon atoms in austenite concentrate in localized regions leaving a carbon-free matrix. It is not an athermal reaction.  

The reaction involves compositional changes and requires the diffusion of carbon at a certain time at the transition temperature. 

The difference in Formation of Upper Bainite and Lower Bainite

In plain carbon steels, diffusion of carbon in austenite provides the required activation energy for the formation of upper bainite, while the diffusion of carbon in ferrite activates the formation of lower bainite.  

The formation of both upper and lower bainite occurs due to successive nucleation of individual plates and the multiple nucleation processes control the growth rate of these plates. Some theories suggest that the relief of transformational strains controls the growth of the bainite phase and the transformation is shear-type transformation.  

The formation of lower bainite is controlled by shear stresses as it was observed that the first ferrite lath formed was supersaturated with carbon much more than the upper bainite ferrite. 

Difference between upper bainite and lower bainite

Upper bainite

Upper bainite is formed at a temperature ranging from 300-500 ͦC. The lath-like ferrite elements are arranged in packets or sheaves with layers of carbides between ferrite plates. The higher temperature permits the excess carbon to partition before it can precipitate in ferrite. The carbide precipitates are formed from austenite grains high in carbon and the ferrite plates are free from any carbide precipitates. The carbide precipitates have around 6.7% carbon. 

Structure Evolution in Upper Bainite

When a bainite lathe grows in upper bainite, the high diffusivity of carbon allows partitioning of carbon between ferrite and austenite, hence, formed the low carbon content (< 0.03%) ferrite, results in the enrichment of carbon in austenite. 

Thus, instead of precipitation inside the laths, carbides having enough carbon precipitate out at the lath boundaries in austenite. In low carbon steels, the carbides are present as discrete particles at the lath boundaries. While in high carbon steels, these carbides form continuous stringers. 

You can see the exact difference in the picture given below, where upper and lower bainite structures are discussed in combination. 

what is the difference between pearlite and bainite microstructure?

The structure of upper bainite is quite fine and resembles pearlite, however, in pearlite alternate plates of ferrite and carbides are formed while in upper bainite, pockets of carbide precipitates are formed by the rejection of excess carbon by ferrite in between ferrite plates.

Lower Bainite   

In lower bainite formed at a temperature ranging from300-500 ͦC, the structure is relatively coarse and carbide precipitates have a hexagonal structure having high carbon ~ 8.4%. As opposed to upper bainite, lower bainite has two kinds of carbide precipitates.  

Due to reduce transformation temperature the diffusion is slower providing enough time for carbon o precipitate inside supersaturated ferrite. Some of the carbides precipitate out from carbon-rich austenite in between the ferrite plates also the same as upper bainite. 

Bainite VS Martensite

Difference between Lower Bainite and Martensite

The structure of lower bainite resembles martensite but the studies show that the carbides in lower bainite have the same crystallographic orientation while in the tempering of martensite variants of cementite with different crystallographic orientation are formed. 

Structure evolution in Lower Bainite

The ferrite forms as plates not lath and have a broader structure and have a higher dislocation density. When the ferrite plates nucleate at austenite grain boundaries, secondary plates are also formed within grains from primary plates. The carbides are precipitated out within the ferrite plates during the transformation. 

The carbides in lower bainite have a rod or blade-like morphology and are aligned almost parallel to each other. As the reaction continues the precipitation and consequent, lowering of carbon content in austenite provides a driving force for transformation. 

What quenching method produces 100% bainite?

The process uses to achieve a fully bainitic structure in steels is called austempering. It is an isothermal heat treatment process different from conventional heat treatment where a part is heated above 843°C and then quenched in oil or water at room temperature.

Heat treatment defects may occur when we quench steel from austenitizing temperature, we have provided common remedies for those defects.

In austempering, the part is heated to a temperature of 843°C to 954°C and then quenched in a molten salt bath kept at 232°C to 399°C. In a salt bath, heat is transferred by conduction combined with convection, which results in the formation of a fully bainitic structure throughout the whole section thickness. The bainitic transformation temperature is higher than the martensite start (Ms) temperature and hence the temperature of the salt bath is higher than the Ms temperature. 

TTT Diagram for bainite - Austempering process for bainite - bainite formation - Bainite cooling process

The immersion time in the bath depends on the thickness of the part, hardness and material chemistry, and section thickness. Generally, immersion time decreases with increasing transformation temperature, and an increase in carbon content increases the transformation time at the same transformation temperature. 

Bainite Properties

The bainitic steels have general high strength and good toughness attributed to the distribution and morphology of carbide particles in the matrix. For very wide sections, the formation of bainite structure is ideal to achieve superior properties as a constant transformation temperature permits the formation of similar microstructure and properties over a wide range of cooling rates.

  • Strength: Bainitic steel has a variety of strength range from lower strength like pearlite to higher strength like martensite. After annealing, ferrite of bainitic steel has excess leftover carbon which responsible for high strength due to more carbide precipitation. However, the fine grain structure is the main factor responsible for high strength in bainitic steels.
  • Hardness: The hardness of upper and lower bainite is like that of pearlite and tempered martensite, respectively. For hardness measurement, Rockwell, Vicker and Brinell hardness tester are commonly employed.
  • Toughness: Lower bainite has higher toughness than upper bainite due to the presence of finer cementite particles. The small carbides are not cracked, and if they crack the critical size of the defect is not possible to achieve and hence brittle failure is prevented

Bainite VS Martensite

The microstructure of martensite and bainitic steel appears to be the same and even the properties of tempered martensite and bainite. However, the morphology of both plates of steel is different when observed using a transmission electron microscope.

Difference in Microstructure

When observed under a light microscope, due to low reflectivity the microstructure of bainite appears darker than martensite. Similarly, the properties of both plates of steel are different from each other when studied in detail. Scaling of microstructure is required before you start studying it. Check our guide on microstructure scaling that can help you standardize microstructure and make them easy to understand for everyone.

Martensitic Structure Evolution

The heat treatment process for achieving martensitic structure requires heating of steel in austenite region and then quenching rapidly in oil or water or salt bath through martensite start (Ms) to finish (Mf) temperature. The transition is continuous and quenching is performed rapidly to avoid all other transformations. The tempering process is done by reheating at a low temperature where transformation is avoided but the stresses are reduced by the formation of small carbides and carbide plates at a higher temperature.

How to make Bainite?

On contrary, bainite form by an austempering process in which steel is heated to austenite range quenched to an intermediate temperature in a molten salt bath and held rather than cooling to room temperature. It is an athermal process, which results in a bainitic structure having excellent toughness. The martensite forms by diffusionless transformation whole in bainite carbides are formed by diffusion of carbon atoms.

Martensite is more prone to heat treatment defects than Bainite in steel

The formation of bainite by austempering process avoids the risk of quench cracking as stresses are less severe to cause cracking. When the bainitic transformation starts the part has more likely to have an equal distribution of temperature throughout leading to a homogenous structure at once.

As martensite formation is diffusionless transformation and rapid quenching to room temperature may cause uneven distribution of stresses, so it is difficult to avoid quench cracking in martensitic steels. 

Comparison of Properties of Bainite with Martensite

Bainite steels have somewhat superior wear resistance and toughness as compared to martensite at the same hardness may be due to the presence of a large fraction of carbides or some retained austenite.

Absence of Temper Embrittlement in Bainite

Similarly, the temper embrittlement phenomenon is not present in bainitic transformation unlike martempering. In a range of transition temperatures, plates of martensite are formed instead of lath, which results in a brittle structure. Whereas, during bainitic transformation, the structure formed is composed of laths instead of plates, even in high carbon steels, which prevents embrittlement.  

why pearlite bainite and martensite dont appear on diagram?

While I was researching for Bainite on Internet, I came across this question. So, Lets discuss this.

Pearlite is product of equilibrium reaction that involves slow and uniform cooling. While Martensite and bainite requires non-equilibrium conditions that are discussed above. Since, Iron-Iron carbide diagram is equilibrium reaction-based diagram, so we can not present martensite and bainite over it. However, pearlite is very much visible there. For martensite and bainitic phase, diagram followed in TTT diagram in steel.

What are 3 differences between martensite and bainite?

First difference is microstructure within both phases. Second difference is absence of temper embrittlement in case of bainite. Third difference is in properties. Bainitic region has higher wear resistance and toughness, while martensite has higher hardness. Detail is discussed above.

Why are bainite, spheroidite and tempered martensite not in the fe-c diagram?

Bainite, Spheroditie, and tempered martensite are the product of a non-equilibrium reaction that requires fast cooling and athermal heat transfer. While the Fe-C diagram is based on equilibrium reactions, that’s why these phases can not be presented in that diagram.

Application of Bainitic Steels

Due to their excellent mechanical properties, bainitic steels have widespread applications.

  • They have good weldability, good formability and high strength, and high toughness. 
  • They are readily used in the automobile industries to replace martensite for the fabrication of camshafts or crash reinforcement bars as they are more economical.
  • The low carbon bainite is also used in strength structural components in the aviation industry and for making, pressure vessels, and boilers.
  • The good creep resistance enables bainitic steels to use in power generation industries.
  • Due to higher strength, high carbon bainitic steels are used as mandrel bars, railway wheels or tires, back-up rolls, and many more.

If you want to learn about high carbon steels, we recommend checking out 1095 steel, 1080 steel and 1095 cro-van steel.

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