Bainite- A Microstructural Gem

There is a common saying that “Incidents happens for our own good”, for E.C. Bain maybe this came out to be true when he discovered the unique microstructure, Bainite, in 1930. Microstructure of any material as we know is the composition or spatial arrangements of the phases in it, or the defects which we usually observe by experimentation. They are important in the sense that they play vital role in deciding the materials properties like strength, hardness, ductility, toughness etc.

Pearlites are one of the few early discovered microstructures. Pearlites were composed of ferrite and cementite and they were adequately hard, strong and ductile. Not so longer, Martensite was discovered as one of the hardest phases in steels, much harder than pearlites but lacking ductility. Much later, Davenport and Bain reported the bainite microstructure possessing unique characteristics.

Although Bainitic microstructure had fine ferrite and carbides similar to the pearlites, they were treated separately. So, what makes bainite so special that it stood out of other microstructures. Is it worthy enough to be called as a microstructural gem? Don’t worry I will answer all your queries in a while and when we will be finished, you will most probably know how useful Bainitic microstructure is. We will also see how ambiguously they form and how we can utilise them in applications.

Formation:

After austenitizing the steels, they may either form pearlite or martensite depending on cooling rates. The pearlites were formed mostly in the temperature ranges above 550°C i.e. above the nose of T-T-T curve and martensite on very rapid cooling. But when the austenite decomposition is below the nose of the T-T-T curve in the temperature ranges of 250°C – 550°C, a new microstructure with fine aggregates of ferrite and carbide was found, this was Bainite. This was most prominently observed during the isothermal heat treatments but was also seen in the continuous cooling of the alloy steels. In continuous cooling the plain Carbon steels, however, due to the lesser time available for transformation, pearlite reactions overshadow it and we don’t see bainite. With time, several heat treatments like Austempering were developed to obtain bainitic microstructure.

Fig. 1: Austempering heat treatment process to obtain lower bainite (Google Images)

Bainite discovery was special because it had both the characteristics of pearlite and martensite. You can obtain it isothermally and also by athermal treatment at cooling rates too fast to form pearlites yet not rapid enough to form martensite. Bainite had its own C-type kinetics like pearlites. All these transformational features make bainite unique among microstructures.

Bainitic Morphology:

Bainite was similar to pearlites as it had a two-phase microstructure consisting ferrites and carbides; but what makes it different was their morphologies. These morphologies usually vary largely with the transformation temperature ranges they were formed, or how the ferrites and carbides are arranged. When considering the isothermal treatments, they can be broadly divided into two microstructural categories: upper bainite and lower bainite. They are obtained on various temperature ranges in the TTT diagram.

Fig 2: TTT diagram showing the Upper bainite region below the nose and lower bainite near
the Ms temperature. Credit: Cheggy.com

Bainite plates:

The upper and lower bainites have common clusters of ferrite plate called as sheaves. Each sheaf has plate within and called sub-units which are connected in three-dimension sharing common crystallographic orientation. Bainites crystals are however anisotropic in nature and their size is characterised by measuring the thickness on a random section in a direction normal to the long edges of the plates.

FIG 3: Transmission electron micrograph of a sheaf of upper bainite in a partially transformedFe-0.43C-2Si-3Mnwt%alloy (Bainite in steels by H.K.D.H Bhadeshia)

But what causes the formation of these plates?

The plates are actually formed by the rapid radial growth and high yield stress in the parent phase. We will learn later that the growth mechanism of bainites is displacive; this mechanism is adopted to minimize the strain energy associated with the displacement which causes the bainite growth in plate shape. As with the thickness of these plates it is found that temperature has very small effect in the thickness of bainitic plates. However, the dislocation density in austenite and high driving force affects them.

 1) Upper Bainite:

              This is usually observed in the temperature ranges of 550°C to 400°C. They have feathery texture and the fine ferrite plates grows as needle shapes called laths. As discussed earlier upper bainites have ferrite plate called sheaves; when the sub-units of sheaf are in lath shape, they are longest along the closed pack direction of ferrite.

The transformation starts with the nucleation of ferrites at the austenite grain boundary. But the growth part is accompanied with the shape change in near area. This shape change is seen as growing in the invariant plain strain (IPS) which seems similar to the martensite having large shear component. As discussed earlier this strain helps the ferrites to grow as thin plates. This growth however slows with time as the strain produced in the nearby ferrite region causes plastic deformation of the austenite there, so there is local increase in the dislocation density which hinders the ferrite growth.

The further mechanism is simple as the carbon nearby forms carbides most probably cementite near the ferrite-austenite boundary. This is either continuous if high carbon is there or discontinuous if low carbon is there. The important point is that high temperature is available for transformation so due to high diffusivity, Carbon easily escapes from the ferrites and diffuses in the partition between ferrites and austenite. So, we see cementite in between the ferrite plates. (See fig5).
After, the first plate and carbides form, there is still another mechanism going information of secondary ferrite (the next plate). The temperature is not high enough for proper diffusion of iron and substitutional atoms so after the depletion of region near carbide, the next ferrite plate grows like martensite i.e. shear displacive method with heavy twinning associated.
The sequence of reactions can be summarised as follows (α_s = secondary ferrite):

γ → γ + α_{b,supersaturated}

→α_{b,unsaturated} + γ_enriched

→α_{b,unsaturated} + α_s + θ.

Fig 5: Carbide precipitation process in Upper and lower bainite. (H.K.D.H. Bhadeshia, in Encyclopedia of Materials: Science and Technology, 2002)

Dislocation in Bainite:

Before you look in the other forms it’s better to discuss about the dislocations in bainite. By several experiments it is found that bainite ferrite has large dislocation density which is order of 10¹⁴ per m². I assume you know by now that shear mechanism is involved in bainitic ferrite formation which can be seen similar to martensite.

But why bainite has more dislocation density than martensite?

This is because of the non-elastic shear mechanism. Whenever the ferrites form it is accompanied by the plastic strain in nearby austenite so as to remove the plastic strain in it, this in turn increases the dislocations density nearby austenite regions. The austenite adjacent to the ferrite can accommodate the shape deformation by mechanical twinning or stack faulting so the density of defects increases as the transformation temperature decreases. This process is so intense that even a slight misorientation can affect the dislocation density.

Fig 6: Plastic strain and shape change during transformation (Google images)

Also, we know a similar ferrite morphology known as allotriomorphic ferrite formed at identical temperatures of bainite. But bainitic ferrite is still different than this because it has larger dislocation density. So, you must have started noticing now that Bainite is emerging as a unique microstructure. Let’s look at another morphology which forms very close to the martensite temperature ranges.

(2) Lower bainite:

This usually forms in the lower transformation temperature ranges like 400°C-250°C. The formation of the bainitic ferrite plates is similar to the upper bainite occurring as supersaturated α but the main difference comes in the carbide precipitation. The formation is seen similar to tempered martensite but is different as the carbides tends to adopt a single crystallographic variant in each plate of the lower bainite.

Due to lower temperature of transformation diffusivity of Carbon is low and some of the carbon precipitates in ferrite. As a result, Carbon percentage in austenite decreases and we observe small inter-plate carbides like cementite when austenite decomposes. (see fig 5)

Fig 7: The lower bainite microstructure. (Google images)

There is however an anomaly in lower bainite about the type of carbide formed. It is found that in hypoeutectoid bainitic steel initial carbides formed is ε-carbide that gradually changes to cementite on isothermal holding. The formation of ε-carbide is acceptable as the low diffusivity can cause quick formation of ε-carbide. This also emphasize that large excess Carbon must be trapped in the bainitic ferrite on initial forming stage. The rate at which the ε-carbide transforms to cementite increases with the increase in transformation temperature. Also, elements like Silicon, Nickel helps in ε-carbide formation.

In lower bainites, the dislocation density largely influences the conversion of carbides. If the dislocation density is high than Carbon gets trapped and is not available for ε-carbide formation hence we see more stable cementite being initially formed from supersaturated ferrite. Look at the reactions for more clarity.

Case 1: High dislocation density

γ → γ + α_{b,supersaturated}

   → θ_{in ferrite} + α_{b,supersaturated} + γ_enriched

   → α_{b,supersaturated} + α + θ_{between ferrite plates} + θ_{in ferrite}

Case 2: Low dislocation density

γ → γ + α_{b,supersaturated}

   → ε_{in ferrite} + α_{b,supersaturated} + γ_enriched

   → α_{b,supersaturated} + ε_{in ferrite} + α + θ_{between ferrite plate}

→ α_{b,supersaturated} + θ_{in ferrite} + θ_{between ferrite plates} + α

Also, in some cases we see other carbides like eta-carbide or chi-carbide which forms under different transformation criteria.

Apart from these two basic morphologies, there are other industrially important bainite morphologies like granular bainite, columnar bainite, coalesced bainite etc. Let’s discuss one of the important forms.

Granular Bainite:

         This morphology was more commonly observed in industries because it forms during the continuous cooling process in many low carbon steels. These were characterized by coarse plates which is basically sheaves of bainitic ferrite with very thin regions of austenite between the sub-units because of the low carbon concentration of the steels involved.

Fig 8: The TEM micrographs of granular bainite (Google images)

A characteristic (though not unique) feature of granular bainite is the lack of carbides in the microstructure. The carbon that is partitioned from the bainitic ferrite stabilises the residual austenite, so that the final microstructure contains both retained austenite and some high-carbon martensite.

So, you see now how unique the bainites can be ranging from simple ferrite and carbides but yet so ambiguous in their formation. Now let’s discuss how the bainites effect mechanical properties and how we can use them.

 Mechanical properties of bainite:

•   Strength: The strength of bainite depends on several factors like strength by annealed iron, solution strengthening, dislocation strengthening, strength due to Carbon, particles effects, grain size effects etc. They can have variety from low strength pearlites to high strength martensite. The bainitic ferrite has usually excess carbon left over after annealing, this carbon provides strength by carbide precipitation hindering dislocation motion. But most importantly it is found that main factor for bainite strength is its finer grain sizes.
• Hardness: We know that hardness increases with increasing carbon content. It is found that low Carbon alloys has almost same hardness for full bainite microstructure at all temperatures. However, High carbon alloys hardness decreases with temperature which is because of bainite growth instead of martensite. We can conclude that austenitizing temperature has no effect on bainite hardness until its high enough to dissolve all carbides. Also, study says that Austenite grain size has no effect on the hardness.
•  Tempering: Usually, the bainitic steels are not tempered. Tempering decreases hardness and tensile strength more profoundly of the lower bainite.
• Yielding effects: Bainitic steels usually has the problem of gradual yielding. In bainites the proof stress to UTS ration falls very less than 0.8 which means the mobile dislocations cause low proof stress. The bainites being a heterogenous microstructure with fine carbides have stress concentration leading to gradual yielding. This is not always an issue as some steels like Dual-Phase steels uses this property. So, the Bainitic DP steels have less strength than Martensite DP steel but with more ductility, formability and fatigue strength
• Ductility: The ductility of the low carbon bainitic steels are greater than the high carbon alloy. As compared to the tempered martensite the elongation of fully bainitic low carbon steels are better.
•  Toughness: The lower bainites usually have higher toughness than the upper bainite because of much finer grains
• Hardenability: The most prominent property of bainites is there good hardenability. To increase it further we usually use low carbon, low alloy steels with small amounts of B or Mo. The Boron is most important alloying element in increasing the hardenability of Bainite steels by decreasing the proeutectoid ferrite phases in low Carbon steels.

Thus, you can observe now that there are various properties related to these morphologies which controls their applications.

Application:

The bainitic steels have vast applications which depends on their toughness, weldability, formability, strength needed and cost associated. Due to this properties, low Carbon bainitic steels are widely used in applications as they good weldability, high strength and good formability.

• They can be used in the automobile industries in making cam shafts or crash reinforcement bars which readily replaced the use of Martensite because of low economy related.
• The creep resistant bainite steels are used in power generation industries.
• They are used as accelerated cooled steels because of their good formability.
• To decrease inclusions the bainite usually forms intragranular nucleated bainite, also called acicular ferrite. They are mor heterogeneous and decreases crack growth significantly.
• The bainitic low Carbon steels can be used as pressure vessels, boilers of light weight bridges, strong structural components in aircrafts and many more.
• The bainitic high carbon steels can be used as mandrel bars, back-up rolls, railways wheels or tyres and many more.

 Conclusion:

So, you made it till last. Well, you might have understood that bainite is not some usual microstructure one can see. They require several conditions to be formed, involve several morphologies each having their own unique transformation and uses. They have varying mechanical properties like range of strength, good ductility, toughness, weldability, hardenability etc. They capture a large market share in the industries production. The studies are increasing to use alloying elements wisely to increase the bainite use and remove the minor faults associated.  The new age nanostructured bainite are made to improve the properties and applications. They are widely accepted to be included in mixed microstructure of steels like DP steels, QP steels and others. Their vast applications in automobiles, manufacturing, daily use is just glimpse of its importance.

Bainites brings a new world of discoveries which we never know lead us to where. Surely, it is a microstructural gem because of its uniqueness, its morphologies, its ambiguous transformations, its properties and other factors. So, why don’t you decide if it’s worthy or not!

 References:

• • https://en.wikipedia.org/wiki/Bainite
  • Physical Metallurgy by Vijendra Singh.
  • Steels: Microstructure and Properties by H.K.D.H. Bhadeshia and R.W.K. Honeycombe.
  • Steel Heat Treatment by George E. Totten
  • Youtube (NPTEL and Bhadeshia lectures).

 Further Readings:

• • Bhadeshia, H. K. D. H., The lower bainite transformation and the significance of carbide precipitation,Acta Metallurgica 28, 1103, 1980.
  • Hehemann, R. F., The bainite reaction, Phase Transformations, American Society for Metals, Ohio, USA, p. 397, 1970.
  • Bhadeshia, H. K. D. H., Bainite in Steels, 2nd edition, Institute of Materials, London, UK,2001.
  • Takahashi, M., Kinetics of the bainite transformation, Current Opinion in Solid State and Materials Science 8, 213, 2004.
  • Christian, J. W. and Edmonds, D. V., The bainite transformation, Phase Transformations in Ferrous Alloys,TMS-AIME, Pennsylvania, USA, p. 293, 1984.