Bainitic steel

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An Introduction Pictorial Basic– Metallurgy Bainitic Steel 材料基础-贝氏体钢铁图文简易教材 Descriptive approach

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Content: Introduction to

Bainitic Steels: Part One

Bainitic Steels: Part Two

Effects on the martensite, pearlite and bainite formation


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Summary: The alloy design of new ultra low carbon bainitic steels (ULCB-Ni) combined with developed thermo-mechanical and heat treatment procedures allowed for production of heavy plates with YS 650÷690 MPa and guarantied Charpy V impact energy 80 J at - 100°C and KIC > 100 MPaVm. For example the ultra low carbon bainitic steel grade HN5MVNb which meets Det Norske Veritas requirements for ship, mobile and offshore constructions as well as for structural plates for cryogenic applications has been developed.

Before we go further, let us refresh on the understanding of phase diagrams. Isothermal transformations-TTT Diagram

Fig.1: Time-Temperature-Transformation (TTT) diagram for a 0.89 carbon steel

(US Steel Co., Atlas of Isothermal Diagrams)


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The TTT& CCT diagram for AISI 1080 steel


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Examples of Iron-Iron Carbide Phase Transformations on the TTT Diagram

Figure 1. Fe-Fe3C T-T-T Diagram, Adapted from Callister pg. 295, Fig. 10.6

The time-temperature transformation curves correspond to the start and finish of transformations which extend into the range of temperatures where austenite transforms to pearlite. Above 550°C, austenite transforms completely to pearlite. Below 550°C, both pearlite and bainite are formed and below 450°C, only bainite is formed. The horizontal line C-D that runs between the two curves marks the beginning and end of isothermal transformations. The dashed line that runs parallel to the solid line curves represents the time to transform half the austenite to pearlite. Below we have listed some simple examples as an exercise at other temperatures that result in different phase transformations and hence different microstructures.


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Fig 2. Time-Temperature Paths on Isothermal Transformation Diagram

1. Given Fig. 2, describe what transformations happen in:

a. Path 1 (Red line) b. Path 2 (Green line) c. Path 3 (Blue line) d. Path 4 (Orange line)

Solution

a. (Red) The specimen is cooled rapidly to 433 K and left for 20 minutes. The cooling rate is too rapid for pearlite to form at higher temperatures; therefore, the steel remains in the austenitic phase until the Ms Temperature is passed, where martensite begins to form. Since 433 K is the temperature at which half of the austenite transforms to martensite, the direct quench converts 50% of the structure to martensite. Holding at 433 K forms only a small quantity of additional martensite, so the structure can be assumed to be half martensite and half retained austenite.

b. (Green) The specimen is held at 523 K for 100 seconds, which is not long enough to form bainite. Therefore, the second quench from 523 K to room temperature develops a martensitic structure.

c. (Blue) An isothermal hold at 573 K for 500 seconds produces a half-bainite and half-austenite structure. Cooling quickly would result in a final structure of martensite and bainite.

d. (Orange) Austenite converts completely to fine pearlite after eight seconds at 873 K. This phase is stable and will not be changed on holding for 100,000 seconds at 873 K. The final structure, when cooled, is fine pearlite.


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Iron-carbon equilibrium diagram:

Note: 0.83% Carbon content on eutectoid composition.

Question: Why 0.83% and not 0.77%

Eutectic: A eutectic system occurs when a liquid phase transforms directly to a two-phase solid.

Eutectoid: A eutectoid system occurs when a single-phase solid transforms directly to a two-phase solid.


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Bainitic Steels: Part One

Abstract: The region in which lath-shaped fine aggregates of ferrite and cementite are formed, which possess some of the properties of the high temperature reactions involving ferrite and pearlite as well as some of the characteristics of the martensite reaction. The generic term for these intermediate structures is bainite after Edgar Bain who with Davenport first found them during their pioneer systematic studies of the isothermal decomposition of austenite. Bainite also occurs during thermal treatments at cooling rates too fast for pearlite to form, yet not rapid enough to produce martensite. The nature of bainite changes as the transformation temperature is lowered. Two main forms can be identified, upper and lower.

The Bainite Reaction

Examination of the TTT diagram for a eutectoid carbon steel, Fig. 1, bearing in mind the fact that the pearlite reaction is essentially a high temperature one occurring between 550°C and 720°C and that the formation of martensite is a low temperature reaction, reveals that there is a wide range temperature, usually 250-550°C, when neither of these phases forms.

Fig.1: Time-Temperature-Transformation (TTT) diagram for a 0.89 carbon steel

(US Steel Co., Atlas of Isothermal Diagrams) This is the region in which lath-shaped fine aggregates of ferrite and cementite are formed, which possess some of the properties of the high temperature reactions involving ferrite and pearlite as well as some of the characteristics of the martensite reaction.


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The generic term for these intermediate structures is bainite after Edgar Bain who with Davenport first found them during their pioneer systematic studies of the isothermal decomposition of austenite. Bainite also occurs during thermal treatments at cooling rates too fast for pearlite to form, yet not rapid enough to produce martensite. The nature of bainite changes as the transformation temperature is lowered. Two main forms can be identified, upper and lower.

Morphology and Crystallography of Upper Bainite

Both lower and upper bainite consist of aggregates of platelets or laths of ferrite, separated by regions of residual phases consisting of un-transformed austenite or of phases such as martensite or cementite

The morphology of upper bainite (temperature range 550-400°C) bears a close resemblance to Widmanstatten ferrite, as it is composed of long ferrite laths free from internal precipitation.

Optical micrographs showing white-etching (nital) wedge-shaped Widmanstatten ferrite plates in a matrix quenched to martensite. The plates are coarse (notice the sacle) and etch cleanly because they contain very little substructure Two-surface optical micrography decisively reveals that the ferrite component of upper bainite is composed of groups of thin parallel laths with a well-defined crystallographic habit. Like Widmanstatten ferrite, the bainitic ferrite laths exhibit the Kurdjumov-Sachs relationship with the parent austenite, but the relationship is less precise as the transformation temperature is lowered.

Figure: An illustration of the effect of austenite grain size in determining whether the microstructure is predominantly acicular ferrite or bainite. A small grain sized sample has a relatively large number density of grain boundary nucleation sites and hence bainite dominates, whereas a relatively large number density of intragranular nucleation sites leads to a microstructure consisting mainly of acicular ferrite.


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Figure 2: Replica transmission electron micrograph showing the structure of bainite. A needle of bainite passes from lower left to upper right-hand corners. The phase surrounding the bainite needle is martensite (Callister, 1994). Martensite is obtained by quenching an austenitized iron-carbon alloy to a low temperature. The details of the process involved A widely-accepted view is that the crystallography of upper bainite is very similar to that of low-carbon lath martensite. However, a detailed examination of the crystallography reveals that there are significant differences, and that upper bainite ferrite formation cannot be understood in terms of the crystallographic theory of martensite-formation. Electron microscopy shows that upper bainite laths have a fine structure

comprising smaller laths about 0.5 μm wide. These laths all possess the same variant of the Kurdjumov-Sachs relationship, so they are only slightly disoriented from each other. The longitudinal boundaries are, therefore, low angle boundaries.

A typical austenite grain will have numerous sheaves of bainitic ferrite exhibiting the several variants of the Kurdjumov-Sachs orientation relationship, so large angle boundaries will occur between sheaves. The dislocation density of the laths increases with decreasing transformation temperature, but even at the highest transformation temperatures the density is greater than that in Widmanstatten ferrite.

The upper bainitic ferrite has a much lower carbon concentration (<0.03% C) than the austenite from which it forms, consequently as the bainitic laths grow, the remaining austenite is enriched in carbon. This is an essential feature of upper bainite which forms in the range 550-400°C when the diffusivity of carbon is still high enough to allow partition between ferrite and austenite. Consequently, carbide precipitation does not occur within the laths, but in the austenite at the lath boundaries when a critical carbon concentration is reached.

The morphology of the cementite formed at the lath boundaries is dependent on the carbon content of the steel. In low carbon steels, the carbide will be present as discontinuous stringers and isolated particles along the lath boundaries, while at higher carbon levels the stringers may become continuous. With some steels, the enriched austenite does not precipitate carbide, but remains as a film of retained austenite. Alternatively, on cooling it may transform to high carbon martensite with an adverse effect on the ductility. This type of bainite is often referred to as granular bainite.


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More reading- Upper bainite The microstructure of upper bainite consists of fine plates of ferrite, each of which is about 0.2 micrometer thick and about 10 micrometers long. The plates grow in clusters called sheaves. Within each sheaf the plates are parallel and of identical crystallographic orientation, each with a well-defined crystallographic habit. The individual plates in a sheaf are often called the `sub-units' of bainite. They are usually separated by low mis-orientation boundaries or by cementite particles, Figure1.

1. The microstructure of upper bainite.

This is in a steel which is rich in silicon, which suppresses the precipitation of cementite. Instead of cementite we have films of austenite between the bainitic ferrite platelets. (a) Optical micrograph; (b) bright-field transmission electron micrograph; (c) dark-field image of retained austenite; (d) transmission electron micrograph montage of a sheaf of bainite (this sheaf looks like a single dark plate in the optical micrograph). After Bhadeshia and Edmonds, Acta Metallurgica, volume 28 (1980) 1265-1273.

Upper bainite evolves in distinct stages beginning with the nucleation of ferrite plates at the austenite grain boundaries. The growth of each plate is accompanied by a change in the shape of the transformed region ( Figure), a change which can be described precisely as an invariant-plane strain with a large shear component, virtually identical to that observed during martensitic transformation. However, bainite grows at relatively high temperatures when compared with martensite. The large strains associated with the shape change cannot be sustained by the austenite, the strength of which decreases as the temperature rises. These strains are relaxed by the plastic deformation of the adjacent austenite. The local increase in dislocation density caused by the yielding of the austenite blocks the further movement of the glissile transformation interface ( Figure). This localised plastic deformation therefore halts the growth of the ferrite plate so that each sub-unit only achieves a limited size which is much less than the size of an austenite grain.


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The intense tangles of dislocations which form at the bainite (light) austenite (dark) interface, due to deformation induced by the shape change accompanying transformation. The tangles of dislocations render the interface immobile by "work-hardening", leading to a loss of coherency and a halt to the growth process. This is responsible for the limited size of each platelet of bainite in a sheaf. After Bhadeshia and Edmonds, Metallurgical Transactions A, 10A (1979) 895-907. As with martensite, the shape change implies that the mechanism of growth of bainitic ferrite is displacive. It is the minimization of the strain energy associated with the displacements that ensures that bainite grows in the form of thin plates. Since the crystal structure of bainite is generated by a coordinated movement of atoms, it follows that there must exist an orientation relationship between the austenite and bainite. This relationship is found experimentally to be of the type where a pair of the most densely packed planes of the two lattices are approximately parallel, as are corresponding close-

packed directions within those planes. This is loosely described by a Kurdjumov-Sachs type orientation relationship.

Bainite forms on particular crystallographic planes, but the indices of the habit plane show considerable scatter ( Figure). This is because most of the measurements are made using light microscopy, in which case the habit plane determined is not that of an individual sub-unit. It corresponds instead to some average value depending on the number, size and distribution of sub-units within the sheaf. All of these factors can vary with the transformation temperature, time and chemical composition.

The irrational habit planes of bainite sheaves and of martensite plates [Greninger and Troiano, Trans. AIMME, 140 (1940) 307-336]. Notice the emphasis on the term sheaves. This is because the measurements are made using light microscopy and hence refer to the bainite sheaf as a whole rather than the individual sub-unit. It was emphasized earlier that upper bainite forms in two distinct stages, the first involving the formation of bainitic ferrite which has a very low solubility for carbon (< 0.02 wt.%). The growth of the ferrite therefore enriches the

remaining austenite in carbon. Eventually, cementite precipitates from the residual austenite layers in between the ferrite sub-units. The amount of cementite depends on the carbon concentration of the alloy. High concentrations lead to microstructures in which the ferrite platelets are separated by continuous layers of cementite. Small, discrete particles of cementite form when the alloy carbon concentration is low.

The cementite particles have a "Pitsch" orientation relationship with the austenite from which they precipitate: [0 0 1]Fe3C || [ -2 2 5]gamma [1 0 0]Fe3C || [ 5 -5 4]gamma [0 1 0]Fe3C || [ -1 -1 0]gamma


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Many variants of carbide may precipitate from the austenite, each particle being indirectly related to the ferrite via the ferrite/austenite orientation relationship.

If sufficient quantities of alloying elements (such as silicon or aluminium) which retard the formation of cementite are added to the steel, then it is possible to suppress the formation of cementite altogether. An upper bainitic microstructure consisting of just bainitic ferrite and carbon-enriched retained austenite is obtained instead. The microstructure may also contain martensite if the residual austenite decomposes on cooling to ambient temperature

Figure 34: Upper bainite; the phase between the platelets of bainitic ferrite is usually cementite. Figure 35: Lower bainite, with cementite inside the platelets and also between the platelets of bainitic ferrite.

Summary of the mechanism of the bainite reaction.


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Morphology and Crystallography of Lower Bainite

Lower bainite (temperature range 400-250°C) appears more acicular than upper bainite, with more clearly defined individual plates adopting a lenticular habit. Viewed on a single surface they misleadingly suggest an acicular morphology. However, two-surface optical microscopy of lower bainite indicates that the ferrite plates are much broader than in upper bainite, and closer in morphology to martensite plates. While these plates nucleate at austenitic grain boundaries, there is also much nucleation within the grains, i.e. intragranular nucleation, and secondary plates form from primary plates away from the grain boundaries.

Electron microscopy shows that the plates have a similar lath substructure to upper bainite, with the ferrite subunits about 0.5 µm wide and slightly disoriented from each other. The plates possess a higher dislocation density than upper bainite, but not as dense as in martensites of similar composition.

The crystallography of the plates seems to depend both on the temperature of transformation, and on the carbon content of the steel. Moreover they showed that the phenomenological theory of martensite could be used for lower bainite to give satisfactory agreement between theory and experiment.

Ohmori and coworkers have found that, in a 0.1% C steel, bainite formed near the Ms has a {011}α habit plane and a <111>α growth direction similar in behavior to low carbon lath martensite. However, on increasing the carbon to 0.6-0.8% C, the habit of the bainitic ferrite plates changes to {122}α//{496}γ, which is not the same as for martensite of the same composition which has a

{225}γ habit plane. Because of such variations, it has been suggested that lower bainite is not a true martensitic reaction. However, there is no reason to expect the transformations to be identical, and anyway the inhomogeneous shear would be expected to occur by slip in lower bainite, whereas twinning is the mode adopted in higher carbon martensites.

However, in contrast to tempered martensite the cementite particles in lower bainite exhibit only one variant of the orientation relationship, such that they form parallel arrays at about 60° to the axis of the bainite plate. This feature of the precipitate suggests strongly that it has not precipitated within plates supersaturated with respect to carbon, but that it has nucleated at the γ/α interface and grown as the interface has moved forward. It thus appears that the lower bainite reaction is basically an interface-controlled process leading to cementite precipitation, which then decreases the carbon content of the austenite and enhances the driving force for further transformation.


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More reading- Lower bainite Lower bainite has a microstructure and crystallographic features which are very similar to those of upper bainite. The major distinction is that cementite particles also precipitate inside the plates of ferrite Figure. There are, therefore, two kinds of cementite precipitates: those which grow from the carbon-enriched austenite which separates the platelets of bainitic ferrite, and others which appear to precipitate from supersaturated ferrite. These latter particles exhibit the "tempering" orientation relationship which is found when carbides precipitate during the heat treatment of martensite, often described as the Bagaryatski orientation relationship:

[0 0 1] Fe3C || [ -1 0 1]alpha [1 0 0] Fe3C || [ 1 1 1]alpha [0 1 0] Fe3C || [ -1 2 -1]alpha

The microstructure of lower bainite. Notice the precipitation of several variants of carbide particles within the plate of lower bainitic ferrite. Lower bainite otherwise also consists of fine platelets organized in sheaves, with each platelet separated partially by films of carbon-enriched retained austenite or carbides. After Bhadeshia and Edmonds, Metallurgical Transactions A, volume 10A (1979) 895-907.

The carbides in the ferrite need not always be cementite. Depending on the chemical composition and the transformation temperature, other transition carbides may precipitate first. For example, in high-carbon steels containing more than about 1 wt.% silicon (which retards cementite formation), epsilon carbide is commonly observed to precipitate in the bainitic ferrite.

In contrast to tempered martensite, the cementite particles in lower bainite frequently precipitate in just one variant of the orientation relationship ( Figure), such that they form parallel arrays at about 60 ° to the axis of the bainite plate. In

tempered martensite, the carbides tend to precipitate in Widmanstatten arrays. This peculiar mode of precipitation in lower bainitic ferrite may arise because the carbides nucleate at the ferrite/austenite interface, and hence attempt to adopt a unique variant of the orientation relationship, one which gives an optimum match to both the austenite and ferrite with which they are in contact.


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A transmission electron micrograph of lower bainite showing a single variant of carbide particles in each plate. Single variants tend to form when the driving force for cementite precipitation is small, i.e. in low carbon steels or at high temperatures where the carbon can escape rapidly from supersaturated ferrite. After Bhadeshia, Acta Metallurgica, volume 28 (1980) 1103-1114.

Another plausible explanation is that the carbide precipitation is influenced by the stresses associated with the displacive growth of lower bainite. The effect would be less pronounced during the tempering of martensite because the driving force for carbide precipitation is larger.

The carbides in the lower bainite are extremely fine, just a few nanometres thick and about 500 nm long. Because they precipitate within the ferrite, a smaller amount of carbon is partitioned into the residual austenite. This in turn means that fewer and finer

cementite particles precipitate between the ferrite plates, when compared with an upper bainitic microstructure. An important consequence is that lower bainite is usually found to be much tougher than upper bainite, in spite of the fact that it also tends to be stronger. The coarse cementite particles in upper bainite are notorious in their ability to nucleate cleavage cracks and voids.

Lower Bainite

Lower Bainite


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Upper bainite

Bainite

Lower Bainite

Enchant : Nital Bainitic Structure Mag : 500X

Tempered martensite. Material with this

microstructure has an optimum combination of component strength and toughness

Bainite with coarse prior austenite grain size. Material is strong but brittle - an unacceptable

microstructure which could result from incorrect heat treatment


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Microstructures

Ferrite + pearlite

Ferrite + bainite (Ferrite + martensite will look similar)

Pearlite

Martensite


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Bainitic Steels: Part Two

Abstract: In plain carbon steels, it is often difficult to separate the bainite reaction from the ferrite and pearlite reactions, because these phases can form under similar continuous to bainitic. For example, the TTT diagram for a 0.8% C steel is a continuous curve although there is both a pearlite and bainite reaction occurring, but it is difficult to disentangle the reactions sufficiently to study their kinetics. There are two important features of bainite kinetics which can be shown by a variety of techniques, e.g. dilatometry, electrical resistivity, magnetic measurements and by metallographic.

Figure 47: This is taken from the heat-affected zone of a weld in the coarse-austenite grain region. The microstructure is predominantly martensite but also has allo-triomorphic ferrite, Widmanstatten ferrite, bainite and pearlite. Notice that the spherical shape of a pearlite colony is obvious in this sample because of the lack of impingement. Notice also that pearlite, unlike bainite, grows across the austenite grain boundaries. The Widmanstatten ferrite plates are white because of the lack of structure within the plates, whereas bainite etches relatively dark.

Austenite γ Allotriomorphic ferrite α Idiomorphic ferrite αI Pearlite P Widmanstatten ferrite αw Upper bainite αb Lower bainite αlb Acicular ferrite αa Martensite α' Cementite θ


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Reaction Kinetics of Bainite Formation

In plain carbon steels, it is often difficult to separate the bainite reaction from the ferrite and pearlite reactions, because these phases can form under similar continuous to bainitic. For example, the TTT diagram for a 0.8% C steel is a continuous curve although there is both a pearlite and bainite reaction occurring, but it is difficult to disentangle the reactions sufficiently to study their kinetics.

However, the addition of certain alloying elements separates the reactions to the extent that they can be represented as individual curves on the TTT diagram, which then takes on a more complex form than the familiar C-curve.

There are two important features of bainite kinetics which can be shown by a variety of techniques, e.g. dilatometry, electrical resistivity, magnetic measurements and by metallography. First, there is a well defined temperature Bs, above which no bainitic will form, which has been confirmed for a wide range of alloy steels and has been correlated with the structural transition from Widmanstatten ferrite to upper bainitic laths. Second, below Bs temperature and time dependent process take place over a wide temperature range (up to 150°C), which does not go to completion.

The bainitic reaction has several basic features of a nucleation and growth process. It takes place isothermally, starting with an incubation period during which no transformation occurs, followed by an increasing rate of transformation to a maximum and then a gradual slowing down.

Using techniques such as thermionic emission microscopy it has been possible to study directly the progress of the bainite reaction. It has been found that upper bainitic plates lengthen and thicken during transformation by the movement along the plate boundaries of small steps which appear to be diffusion-controlled.

The plates grow at a constant rate edgewise, which leads to a model for the reaction in which the driving force is provided by partition of the carbon from the ferrite to the austenite, the actual growth rate being determined by the rate of diffusion of carbon in austenite away from the γ/α interface.

Role of Alloying Elements

Carbon has a large effect on the range of temperature over which upper and lower bainite occur. The Bs temperature is depressed by many alloying elements but carbon has the greatest influence, as indicated by the following empirical equation:

Bs(°C) = 830 - 270(%C) - 90(%Mn) - 37(%Ni) - 70(%Cr) - 83(%Mo)

Other alloying elements. In plain carbon steels, the bainitic reaction is kinetically shielded by the ferrite and pearlite reactions which commence at higher temperatures and shorter times, so that in continuously cooled samples bainitic structures are difficult to obtain. Even using isothermal transformation difficulties arise if, for example, the ferrite reaction is particularly rapid the addition of metallic alloying elements usually results in retardation of the ferrite and pearlite reactions. In addition, the bainite reaction is depressed to lower temperatures.

This often leads to greater separation of the reactions, and the TTT curves for many alloy steels show much more clearly separate C-curves for the pearlite and bainitic reactions. However, it is still difficult to obtain a fully bainitic structure because of its proximity to the martensite reaction.

A very effective means of isolating the bainite reaction in low carbon steels has been found by adding about 0.002% soluble boron to a 0.5% Mo steel. While the straight molybdenum steel encourages the bainite reaction, the boron markedly retards the ferrite reaction, probably by preferential segregation to the prior austenite boundaries. This permits the bainite reaction to occur at shorter times. Consequently, by use of a range of cooling rates, fully bainitic steels can be obtained.


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Use of Bainitic Steels Bainite frequently occurs in alloy steels during quenching to form martensite. The cooling rate towards the centre of a steel bar is lower than the outside, so in large sections bainite can form in the inner regions with martensite predominating towards the surface.

However, low carbon fully bainitic steels have been developed, as described, using 0.5% Mo and very small concentrations of boron, which allow bainite to form over a wide range of cooling rates. Further control of the reaction is obtained by use of metallic alloying elements such as Ni, Cr, Mn which depress the temperature of maximum rate of formation of bainite. As the transformation temperature is lowered, for a constant cooling rate, the strength of the steel increases substantially. For a series of steels with 0.2% C the tensile strength can be varied between 600 and 1200 MPa. However, this increase in strength is accompanied by a loss of ductility.

The practical advantage of bainitic steels is that relatively high strength levels together with adequate ductility can be obtained without further heat treatment, after the bainite reaction has taken place. The steels are readily weldable, because bainite rather than martensite will form in the heat-affected zone adjacent to the weld metal, and so the incidence of cracking will be reduced. Furthermore, the steels have a low carbon content, which improves the weldability and reduces stresses arising from transformation.

Bainitic Steels having High Toughness and Impact Energy Absorption for

Transportation and Low Temperature Structural Applications

Chapter Authors: Andrzej Kazimierz Lis, Jadwiga Lis, Andrzej Bochenek

Series: EUROMAT 99

Summary

The alloy design of new ultra low carbon bainitic steels (ULCB-Ni) combined with developed thermo=mechanical and heat treatment procedures allowed for production of heavy plates with YS 650÷690 MPa and guarantied Charpy V impact energy 80 J at - 100°C and KIC > 100 MPaVm. The ultra low carbon bainitic steel grade HN5MVNb which meets Det Norske Veritas requirements for ship, mobile and offshore constructions as well as for structural plates for cryogenic applications has been developed.

The special heat treatment procedures were applied to achieve guarantied yield strength 690 MPa and Charpy V impact toughness 80 J/cm2 at 77 K for heavy plates. The investigated steel may be economical substitute for NV20-2 and 10N9 grades of 9÷10% Ni steels due to higher (~200 MPa) yield strength and reduced to 5% Ni content in the chemical composition. The materials selection in mechanical design of ships and pressure vessels based on Ashby's maps shows the dependence of stress intensity factor KIC and J integral values for the investigated ULCB-Ni steels on the thermo=mechanical and heat treatment procedures.


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Fig 1. Optical micrograph of allotriomorphic ferrite in Fe-0.5W-0.23C wt% alloy (after Sahay). The allotriomorph grows at an austenite grain surface and its shape does not reflect its internal crystalline symmetry.

Fig 2. Optical micrograph of Widmanstatten ferrite in an Fe-Ni-Si-C low-alloy steel


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Figure 6: An idiomorph of ferrite in a sample which is partially transformed into α and then quenched so that the remaining γ undergoes martensitic transformation. The idiomorph is crystallographically facetted.

A mixed microstructure of martensite and bainite. Red needles are bainite in this micrograph, larger brown plates are martensite, the white bits are the untrasformed austenite. The contrast is from viewing the etched surface (nital) using differential interference contrast.


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Effects on the martensite, pearlite and bainite formation

Abstract: All alloying elements with the possible exception of Co, lower temperature of the start of the martensite formation, as well as the finish of the martensite formation, i.e. at 100% martensite. All alloying elements except Co delay the formation of ferrite and cementite. It is very difficult to formulate any general rules regarding the influence exerted by the various alloying elements. However, it has definitely been found that some elements affect the bainite transformation more than the pearlite transformation, while other elements act in the opposite manner

Effect on the temperature of martensite formation

All alloying elements with the possible exception of Co, lower Ms the temperature of the start of the martensite formation, as well as Mf, the finish of the martensite formation, i.e. at 100% martensite. For the majority of steels containing more than 0,50% C, Mf lies below room temperature.

This implies that after hardening these steels practically always contain some residual austenite. Ms may be calculated from the equation given below, by inserting the percentage concentration of each alloying element in the appropriate term. The equation is valid only if all the alloying elements are completely dissolved in the austenite.

Ms = 561 - 474C - 33Mn - 17Ni - 17Cr - 21Mo

For high-alloy and medium-alloy steels Stuhlmann has suggested the following equation:

Ms(癈) = 550 - 350C - 40Mn - 20Cr - 10Mo - 17Ni - 8W - 35V - 10Cu + 15Co + 30Al

It can be noted that carbon has the strongest influence on the Ms temperature. Figures 1 and 2 show diagrams with an example of experimental results of the effect of Mn and Ni on the Ms temperature of various types of steel.

Figure 1. Effect of Mn on the Ms - temperature (after Russel and

McGuire, Payson and Savage, Zyuzin, Grange and Stewart)


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Figure 2. Effect of Ni on the Ms - temperature (after Russel and

McGuire, Payson and Savage, Zyuzin, Grange and Stewart)

Effect on the formation of pearlite and bainite during

the isothermal transformation

All alloying elements except Co delay the formation of ferrite and cementite. It is very difficult to formulate any general rules regarding the influence exerted by the various alloying elements. However, it has definitely been found that some elements affect the bainite transformation more than the pearlite transformation, while other elements act in the opposite manner.

Certain elements will, paradoxically, accelerate the transformations if their concentration increases beyond a certain limiting value, this limit been affected by other alloying elements present. For case-hardening and tool steels the time taken to initiate the pearlite-bainite transformation is reduced as the carbon content exceeds about 1%. For tool steels and constructional steels Si-concentrations of 1,5% and above have been found to promote pearlite formation.

As a general principle it may be stated that by increasing the concentration of one alloying element by some few percent and the basic carbon content being kept about 0,50%, only a relatively small retardation of the transformation rates is noticed. For plain carbon steels a successive increase in C from 0,30% to 1% produces but a negligible effect. It is only in conjunction with several alloying elements that a more noticeable effect is produced.

The diagram in Figure 3, applicable to steel W 1 (l% C) will serve as a basis for this discussion. The shortest transformation time for this steel is less than 1/8th second. Note that the time scale is logarithmic; hence there is no zero time. As has been mentioned previously, both pearlite and bainite form simultaneously in this steel at about 550 °C. Since the curves overlap it is customary to draw only one curve. With increasing contents of certain alloying elements, however, the noses of the pearlite and bainite curves will separate.

The structures shown in Figure 3 are obtained by austenitizing samples of steel W 1 at 780 °C for 10 min and quenching in a salt bath at various temperatures. After holding them for predetermined times at various temperatures they are finally quenched in water. Before the salt-bath quenching the steel contains undissolved carbides but in view of the composition of the austenite the steel may be regarded as an eutectoid one. The diagram should be studied with the aid of the explanatory text below.


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Figure 3. TTT diagram for isothermal transformation of steel W 1

(1% C) A = austenite, B = bainite, Ms = start of martensite transformation, M50 = 50% M, P = pearlite

1. Quenching in a liquid bath at 700 °C; holding time 4 min. During this interval the C has separated out, partly as pearlite lamellae and partly as spheroidized cementite. Hardness 225 HV.

2. Quenching to 575 °C, holding time 4 s. A very fine, closely spaced pearlite as well as some bainite has formed. Note that the amount of spheroidized cementite is much less than in the preceding case. Hardness 380 HV.

3. Quenching to 450 °C, holding time 60 s. The structure consists mainly of bainite. Hardness 410 HV.

4. Quenching to 20 °C (room temperature). The matrix consists of, roughly, 93% martensite and 7% retained austenite. There is some 5% cementite as well which has not been included in the matrix figure. Hardness 850 HV.


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Glossary: Crystal Structures Ferrite Ferrite (α), is the crystal arrangement for pure iron. This form exists as part of the structure in most steels and can usefully absorb carbides of iron and other metals by diffusion in the solid state. Ferrite takes a body centered cubic (bcc) form and is soft and ductile.

Austenite Austenite (γ), is a solid solution, that is, the component elements are arranged as if in solution (it also exists as an allotrope of pure iron). All steel exists in this form at sufficiently high temperatures (see figure 1). Some alloy steels stabilise this singular phase and it is present even at room temperatures. The crystal arrangement is face centred cubic (fcc) and, like ferrite, it is soft and ductile.

Cementite Cementite is iron carbide (Fe3C), When carbon atoms can no longer be accommodated in solution in ferrite and austenite (due to an increase in carbon content or reduction in temperature), cementite forms, as it can accommodate more carbon in its crystal structure. Like other carbides, it is hard and brittle.

Pearlite Pearlite is a phase mixture consisting of alternating platelets of ferrite and cementite (α + Fe3C), which grows by conversion from austenite. A steel containing 0.77 wt% carbon can consist solely of pearlite if cooled sufficiently slowly from austenite (see figure 1).

Under the microscope it can have an iridescent mother of pearl appearance, hence the name.

Martensite Martensite is commonly found in steel that has been rapidly cooled ('quenched') from austenite. It is a particularly hard, brittle arrangement. Essentially it forms because any carbon in solid solution in the austenitic phase at high temperatures does not have enough time to be incorporated into cementite when cooled rapidly. The austenite crystals undergo a transformation involving the shearing of atom planes over each other. Martensite does not appear on the phase diagram (figure 1), as it is not an equilibrium phase. The strain energy involved in the martensitic reaction is enormous and a large undercooling is necessary. In low and medium carbon alloys, the martensite tends to form in lath shaped crystals that are generally too fine to resolve in the light microscope. In high carbon steels, plate martensite forms. For certain steels, the rapid cooling necessary to produce a martensitic structure (e.g. water or brine baths) introduces large surface tensile stresses and may cause quench cracking. However, when medium carbon steels are alloyed with elements such as nickel, chromium and molybdenum, the development of equilibrium phases is suppressed and martensite can be formed with less drastic cooling, such as oil quenching.

Bainite If the steel is cooled such that the formation of pearlite by the short range diffusion of iron atoms is not possible, bainite can be produced. The bainite that forms at temperatures just below those at which pearlite forms is termed upper bainite. At lower temperatures, lower bainite forms. Both lower and upper bainite consist of aggregates of platelets or laths of ferrite, separated by regions of residual phases consisting of un-transformed austenite or of phases such as martensite or cementite


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Figure 1. Part of the equilibrium diagram for the Fe-C system

Bainitic steel

Technology

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