In this article we will discuss about:- 1. Introduction to Bainite 2. Morphology of Bainite 3. Crystallography 4. Characteristics 5. Reaction 6. Applications.
Contents:
- Introduction to Bainite
- Morphology of Bainite
- Crystallography of Bainite
- Characteristics of Bainite
- Kinetics of Bainite Reaction
- Applications of Bainitic Steels
1. Introduction to Bainite:
In 1930, Davenport and Bain were the first to report about bainite as a product of austenite decomposition. This generic term, bainite, has been so designated in the honour of E.C. Bain. The TTT diagram of eutectoid steel illustrates its isothermal formation between the nose of the curve and the Ms temperature. In this wide range of temperature usually 250 to 550°C, lath-shaped fine aggregates of ferrite and carbide form, whose morphology is distinctly different from that of fine lamellar pearlite.
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Bainite possesses some of the features which are similar to pearlite reaction and have some of the characteristics of martensite. Though, bainite is presumed to have its own ‘C’ type kinetics, the pearlite and bainite transformations overlap considerably in plain carbon and many low alloy steels.
Bainite may be made to form isothermally, or during athermal treatments at cooling rates too fast to form pearlite, yet not rapid enough to produce martensite. In carbon steels, however, the overhanging pearlite nose eclipses the bainite transformation.
The result is that austenite either fully transforms to pearlite, or (where the rate of cooling is sufficiently fast to suppress the formation of pearlite), it transforms to martensite (as the rate of cooling is also fast enough to miss the bainite range) as seen in CCT diagram of an eutectoid steel. Because of its eclipse in continuous cooling of carbon steels, the formation of bainite is perhaps the least understood and least explored decomposition of austenite.
Bainite forms easily in some alloy steels, containing elements like Cr, Mo, B, etc. Alloying elements which strongly retard the transformation to ferrite/pearlite, help in formation of bainite on continuous cooling. Ability to form bainite increases with greater hardenability of the steel.
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Bainitic transformation is quite complex as summarised, “the formation of bainite constitutes a complex problem in competitive reaction kinetics involving the allotropic transformation of γ → α, the partition of carbon between these phases, precipitation of cementite and other carbides and relaxation, of transformation strain”.
Bainite forms by a shear transformation of austenite, the first formed nucleus is ferrite. Bainite, as a two-phase microstructure, consists of ferrite and iron carbide. There is a variation in the morphology of bainite and in the type of carbide (Fe3C, or ε carbide or Fe2.4C) depending on the temperature of transformation and the composition of the steel as illustrated in Fig. 3.36. Bainitic microstructure can be divided into two broad categories based on the morphology and, this sharp change occurs between the two forms of bainite around a temperature of about 400°C.
There is a small temperature range in which both types of bainite are present:
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1. Upper bainite also called feathery bainite due to its appearance like a feather [in microstructure Fig. 3.37 (a)] and, forms in the temperature range of around 550° to 400°C. The small plates of carbides are parallel to the direction of growth of bainite (Fig. 3.38 a).
2. Lower bainite also called plate bainite has martensite-like acicular appearance, as it forms as individual plates (Fig. 3.37 b) and forms in the temperature range of about 400° to 250°C, The small plates of carbides are at an angle of 55° to 60° to the axis of the bainite plate (Fig. 3.38 b).
2. Morphology of Bainite:
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1. Upper Bainite:
Transmission electron microscope shows upper bainite to have lath, or needle-like shapes (that is, one dimension is longer than the other two). The cross-sections of these laths, perpendicular to the long axis, show wide variations of shapes like plate, elliptical, or blocky.
The ferrite laths run parallel to the longer axis with the carbide precipitation mainly at lath boundaries. These ferrite laths have a fine structure consisting of smaller sub-laths (also called sheaves) about 0.5 µm wide with high dislocation density, which increases with the decreasing transformation temperature, and even the minimum density of dislocations is greater than in widmanstatten ferrite.
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These sub-laths are only slightly disoriented from each other, such that longitudinal boundaries are low angle boundaries. In short, it may be said that the external morphology of upper bainite is lath, or needle-shaped, and internally, it consists of laths of ferrite running parallel to its long axis with cementite particles at lath boundaries- these ferrite laths in turn be sub-divided into sub-laths having high dislocation densities. As the temperature of transformation is lowered, the morphology of upper bainite shows laths to become finer and closer to cause closer spacing of the carbide particles between laths.
The essential feature of the upper bainite is that diffusivity of carbon is high enough to cause the partition of carbon between ferrite and austenite, i.e., when bainitic laths grow, the low carbon content (< 0.03%) of the product bainitic ferrite, causes enrichment of carbon in austenite.
Thus, the carbides do not precipitate within the laths, but in the austenite at the lath boundaries, when the carbon content becomes critical enough to cause their precipitation. In low carbon steels, the carbide is present as discontinuous stringers and isolated particles along the lath boundaries.
When the carbon content is high, the carbides are present as continuous stringers. Sometimes a film of retained austenite may be present in between the ferrite laths. The Ms temperature of this austenite, which has got enriched in carbon (during the growth of ferrite laths), is lowered causing this retention. On cooling further, the retained austenite may transform to high carbon martensite (a brittle phase), which seriously impairs the ductility of the steel (such a bainite is called granular bainite).
2. Lower Bainite:
In lower bainite, ferrite forms as individual but broader plates (and not as lath), adopting a lenticular shape, which is closer in morphology to martensite plates. When the nucleation of these plates occurs at the austenite grain boundaries, secondary plates form within the grain (called intragranular nucleation) from the primary plates. The bainitic ferritic plates have smaller sub-plates of about 0.5 µm width and slightly disoriented from each other, i.e. have low angle boundaries between them.
The ferritic plates in lower bainite have higher dislocation density than in upper bainite, but definitely lesser than in martensite. The carbides in lower bainite precipitate within the ferrite plates.
The carbide-precipitates are on a very fine scale normally having the shapes of rods or blades. The carbide rods or blades are more or less aligned parallel to each other, making an angle generally between 55° and 60° with the growth axis of the ferrite plate. Thus, the formation of lower bainite appears to be interface-controlled process. The precipitation of carbide leads to lowering of carbon content of austenite, which increases the driving force for further transformation.
The carbide in lower bainite in found to be either cementite, or ε-carbide, or a mixture of them depending on the temperature of transformation, and the composition of the steel. Alloying elements do not form significant amounts of their carbides in bainite as their diffusion does not occur. Silicon encourages the formation of ε -carbide as it retards the nucleation of cementite, but other elements allow cementite to be formed.
3. Crystallography of Bainite:
One of the fundamental characteristic of bainitic reaction is that both upper and lower bainite exhibit a definite surface-relief, similar to that displayed by martensite plates, or needles. Lower bainite exhibits a single uniform surface-relief across each plate. In upper bainite, each lath exhibits a multiple surface relief, i.e. each relief is associated with the formation of individual lath.
1. Upper Bainite:
The ferrite has Kurdjumov-Sach relationship with austenite. The cementite of upper bainite is related to austenite by Pitsch relationship-
2. Lower Bainite:
Lower bainite resembles in appearance to tempered martensite. As the carbides in lower bainite occur at a particular angle (≈ 55°) to the growth direction of bainite plates, the following epitaxial relationship between the carbide and ferrite plates has been reported for lower bainite-
4. Characteristics of Bainite:
1. Surface-Relief:
The surface-relief accompanied with bainite formation suggests some similarity with martensitic transformation. As the surface displacements appear to be uniform for individual plates in lower bainite reaction, there is every likely-hood of occurrence of martensite type shear with invariant plane strain type characteristics. However, the bainite plates grow with time, and new plates are also nucleated.
2. Bs – Bf Temperature:
Bainitic transformation has its own ‘C’ curve on its TTT diagram. There is quite an overlapping between the bottom of the pearlitic reaction curve and the top of the bainitic reaction curve as illustrated in Fig. 3.39 (a) and ultimately, for example, the TIT diagram of an eutectoid steel is a continuous single curve. Addition of certain alloying elements separates these reactions and thus, these reactions are represented as separate ‘C’ curves on TTT diagram as represented in Fig. 3.39 (b) for 0.5% C and 3% Cr steel.
Like the martensitic transformation, the bainite transformation does not occur until the temperature of isothermal reaction falls below a definite temperature designated as Bs (start of bainite formation) temperature and goes to completion at a temperature designated at Bf.
Fig. 3.39 (b) illustrates clearly the Bs temperature. For low alloy steels, the Bs temperature is given by:
BS(°C) = 830 – 270 (% C) – 90 (% Mn) – 37 (% Ni) – 70 (% Cr) – 83 (% Mo) …(3.29)
Carbon has the largest effect in lowering the Bs temperature. Above Bs, austenite does not form bainite except in presence of externally applied stress. Further, at temperatures below Bs, austenite does not transform completely to bainite. The amount of bainite formed increases as the isothermal reaction temperature is lowered as shown schematically in Fig. 3.40. Below a lower limiting temperature, Bf (bainite finish), it is thus possible to transform austenite completely to bainite.
In alloy steels, having separate pearlite and bainite reaction curves, the fraction of austenite which is unchanged when the steel is held between Bs and Bf is capable of remaining as austenite for indefinitely long periods of time. This is true if the steel is held at the transformation temperature as illustrated in Fig. 3.41. The temperature for 50% transformation, B50 and for complete transformation to bainite, Bf are related to Bs temperature by-
B50 (°Q = Bs – 60 …(3.30)
Bf (°C) = Bs – 120 …(3.31)
Since, the morphology of lower bainite is very much similar to that of martensite it is often difficult to fix a lower temperature limit, Bf for its formation, though to an approximation, equation 3.31 could be used. The bainitic region may extend into the martensite range and thus, its subsequent formation may be affected by the formation of martensite. Thus, Bf temperature may be above, or below the Ms temperature and if the latter is true, then it is difficult to obtain fully bainitic steels.
The transition temperature from upper bainite to lower bainite, as illustrated in Fig. 3.36 is effected by the carbon content of the steel in quite a complex way. It is the temperature at which diffusivity of carbon (in austenite) is too slow to allow diffusion of carbon away from γ/(α) interface. The extrapolated Acm line (Fig. 3.36) indicates that about above 0.5% C, cementite can directly form from austenite. The austenite thus, depleted of carbon, transforms to upper bainite, which leads to sharp drop in the transition temperature.
3. Absence of Macro-Diffusion:
The growth of bainite plates requires the local diffusion of carbon, which helps to nucleate the finely dispersed carbides in ferrite matrix. This constitutes one of the differences with the growth of martensitic needles, where no diffusion occurs (this difference is also the cause of slower rate of growth of bainite as compared to martensite).
As the diffusion of substitutional alloying elements is not immediately necessary in the bainitic transformation, and it does not occur, whereas the pardoning of the important substitutional alloying elements, occurs in the growth of pearlite. This is an important difference between pearlitic and bainitic reactions.
4. Effect of Alloying Elements:
That the bainitic reaction is kinetically shielded by ferrite and pearlite reactions in carbon steels so that such steels on continuous cooling do not show bainite in micro-structure. The substitutional alloying elements, when added in steels, retard ferritic and pearlitic reactions as well as lower the Bs temperature to yield a separate curve for bainite reaction (Fig. 3.39 b), but even then, fully bainitic steel may be difficult to obtain because martensite also forms. The addition of 0.002% boron to low carbon steels having 0.5% molybdenum can yield fully bainitic steels. The substitutional alloying elements do not redistribute (i.e., do not diffuse) during the bainitic reaction, but the presence of silicon encourages the formation of ε -carbide by retarding the nucleation of cementite in ferrite.
5. Kinetics of Bainite Reaction:
Bainite reaction has two important characteristics:
1. It has several basic features of the nucleation and growth process- For example- Bainite reaction occurs isothermally. It has an incubation period, during which no transformation occurs. It shows a typical sigmoidal curve in which the reaction kinetics starts slowly, after an incubation period, then the rate reaches to a maximum, and then slows down. In plain carbon steels, the activation energy for the formation of upper bainite corresponds to the energy of diffusion of carbon in austenite, whereas for lower bainite, it corresponds to the diffusion of carbon in ferrite.
2. It has several characteristics of martensitic reaction. Bainite plates possess habit planes that are characteristically irrational in nature just, in martensite. Habit planes have indices that are not simple whole numbers and which, vary with the temperature of reaction. Ferrite in bainite plates possesses basically a different orientation relationship relative to the parent austenite, which is similar to that found in simple ferrite nucleating directly from austenite, thus, ferrite is the nucleating phase for bainite.
It appears that both upper bainite and lower bainite form by successive nucleation of the individual plates, and the growth rates are controlled by multiple nucleation processes. It has been suggested that the growth of bainite is associated with the relief of transformation strains.
It is confirmed that lower bainitic ferrite appears to form by shear and it has been observed that the first formed ferrite lath is supersaturated with carbon to a much greater extent than in case of upper bainitic ferrite. In general, bainitic reaction, in particular lower bainite is a shear-type transformation whose growth rate is controlled by the diffusion of carbon in the austenite.
6. Applications of Bainite:
It is possible to produce in bainitic steel of a given composition, a range of strengths extending between the lower strength of the ferrite-pearlite structures and the highest strength of martensite, by varying the temperature at which the austenite transforms to bainite as illustrated in Fig. 3.42. Such steels can have adequate ductility without hardening and tempering. Boron can be added in steels (in low carbon, where it is effective) to obtain bainitic steel.
Fig. 3.43 illustrates effect of boron on the isothermal diagram of low-carbon 0.5% Mo steel:
Thus, it is possible to have bainitic structure in hot-rolled plates by having a carbon of about, 0.15% (good weldability) and retarding ferrite formation by adding elements like boron and molybdenum, and controlling the grain size by adding niobium. Such steels have yield strengths in the range of 450 to 900 MPa. The composition of the steel to obtain desired strength can be chosen based on the lower cost, high toughness, weldability and formability.
Fig. 3.44 illustrates that lower bainite has better toughness than upper bainite as in upper bainite, the large sized carbides crack to form supercritical defect, which propagates rapidly, whereas in lower bainite, the smaller carbides do not crack, or if they do, the defect is not of critical size, and thus, initiation of brittle failure is difficult. The alloying elements chosen are chromium, molybdenum and manganese.
More than 0.5% Mo makes the steel expensive. Low carbons steels with such elements and 0.002% boron are high strength steels, readily weldable, and have good formability. The heat- affected zone after welding, when forms bainite, which does not cause stresses and weld-cracks as compared to when martensite forms.
Low carbon (alloy) bainitic steels have widespread applications such as pressure vessels, boilers, cranes and lifting equipment, pipes for gas and oil transportation, earth-moving vehicles, structural members of light weight bridges, mine supports, fans, strong structural components in air craft engineering, engine mounts, plates for heavy engineering applications. High carbon bainitic steels are used for large die-blocks, mandrel bars, back-up rolls, drill rods, railway wheels and tyres.