Bainitic steel


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Bainitic steel

  1. 1. Inspector Knowledge Series 01-0 An Introduction Pictorial Basic– Metallurgy Bainitic Steel - Descriptive approach Mok Chek MinRIG/PLANTIn house training ADDITIONAL CODE SDRL CODE TOTAL PGS 24REMARKSMAIN TAG NUMBER DISCIPLINE MetallurgyCLIENT PO NUMBERCLIENT DOCUMENT NUMBER CMM NDT Services Pg: 1/ 31
  2. 2. HISTORY 0 15.08.2008 For Approval CMM Rev Date Reason for issue Prep Check ApprCHANGE DESCRIPTIONRevision Change description 01 For Approval Pg: 2/ 31
  3. 3. to Bainitic Steels: Part One Bainitic Steels: Part Two Effects on the martensite, pearlite and bainite formation Pg: 3/ 31
  4. 4. alloy design of new ultra low carbon bainitic steels (ULCB-Ni) combined with developed thermo-mechanicaland heat treatment procedures allowed for production of heavy plates with YS 650÷690 MPa and guarantiedCharpy 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 requirementsfor ship, mobile and offshore constructions as well as for structural plates for cryogenic applications has beendeveloped.Before we go further, let us refresh on theunderstanding 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) Pg: 4/ 31
  5. 5. The TTT& CCT diagram for AISI 1080 steel Pg: 5/ 31
  6. 6. of Iron-Iron Carbide Phase Transformations on the TTT Diagram Figure 1. Fe-Fe3C T-T-T Diagram, Adapted from Callister pg. 295, Fig. 10.6The time-temperature transformation curves correspond to the start and finish of transformationswhich extend into the range of temperatures where austenite transforms to pearlite. Above550°C, austenite transforms completely to pearlite. Below 550°C, both pearlite and bainite areformed and below 450°C, only bainite is formed. The horizontal line C-D that runs between thetwo curves marks the beginning and end of isothermal transformations. The dashed line that runsparallel 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 indifferent phase transformations and hence different microstructures. Pg: 6/ 31
  7. 7. 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. Pg: 7/ 31
  8. 8. 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. Pg: 8/ 31
  9. 9. Pg: 9/ 31
  10. 10. Pg: 10/ 31
  11. 11. Steels: Part OneAbstract:The region in which lath-shaped fine aggregates of ferrite and cementite are formed, which possess some of the properties ofthe 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 duringtheir pioneer systematic studies of the isothermal decomposition of austenite. Bainite also occurs during thermal treatmentsat cooling rates too fast for pearlite to form, yet not rapid enough to produce martensite. The nature of bainite changes asthe transformation temperature is lowered. Two main forms can be identified, upper and lower.The Bainite ReactionExamination of the TTT diagram for a eutectoid carbon steel, Fig. 1, bearing in mind the fact that the pearlitereaction is essentially a high temperature one occurring between 550°C and 720°C and that the formation ofmartensite is a low temperature reaction, reveals that there is a wide range temperature, usually 250-550°C, whenneither 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 someof the properties of the high temperature reactions involving ferrite and pearlite as well as some of thecharacteristics of the martensite reaction. Pg: 11/ 31
  12. 12. generic term for these intermediate structures is bainite after Edgar Bain who with Davenport first found themduring their pioneer systematic studies of the isothermal decomposition of austenite. Bainite also occurs duringthermal treatments at cooling rates too fast for pearlite to form, yet not rapid enough to produce martensite. Thenature of bainite changes as the transformation temperature is lowered. Two main forms can be identified, upperand lower.Morphology and Crystallography of Upper BainiteBoth lower and upper bainite consist of aggregates of platelets or laths of ferrite, separated by regionsof residual phases consisting of un-transformed austenite or of phases such as martensite or cementiteThe morphology of upper bainite (temperature range 550-400°C) bears a close resemblance to Widmanstattenferrite, 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 ispredominantly acicular ferrite or bainite. A small grain sized sample has a relatively large number density of grainboundary nucleation sites and hence bainite dominates, whereas a relatively large number density of intragranularnucleation sites leads to a microstructure consisting mainly of acicular ferrite. Pg: 12/ 31
  13. 13. 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 structurecomprising smaller laths about 0.5 μm wide. These laths all possess the same variant of the Kurdjumov-Sachsrelationship, so they are only slightly disoriented from each other. The longitudinal boundaries are, therefore, lowangle boundaries.A typical austenite grain will have numerous sheaves of bainitic ferrite exhibiting the several variants of theKurdjumov-Sachs orientation relationship, so large angle boundaries will occur between sheaves. The dislocationdensity of the laths increases with decreasing transformation temperature, but even at the highest transformationtemperatures 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 itforms, consequently as the bainitic laths grow, the remaining austenite is enriched in carbon. This is an essentialfeature of upper bainite which forms in the range 550-400°C when the diffusivity of carbon is still high enough toallow 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. Inlow carbon steels, the carbide will be present as discontinuous stringers and isolated particles along the lathboundaries, while at higher carbon levels the stringers may become continuous. With some steels, the enrichedaustenite does not precipitate carbide, but remains as a film of retained austenite. Alternatively, on cooling it maytransform to high carbon martensite with an adverse effect on the ductility. This type of bainite is often referred toas granular bainite. Pg: 13/ 31
  14. 14. reading- Upper bainiteThe microstructure of upper bainite consists of fine plates of ferrite, each of which is about 0.2 micrometer thickand about 10 micrometers long. The plates grow in clusters called sheaves. Within each sheaf the plates areparallel and of identical crystallographic orientation, each with a well-defined crystallographic habit. The individualplates in a sheaf are often called the `sub-units of bainite. They are usually separated by low mis-orientationboundaries 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 grainboundaries. 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, virtuallyidentical to that observed during martensitic transformation. However, bainite grows at relatively high temperatureswhen compared with martensite. The large strains associated with the shape change cannot be sustained by theaustenite, the strength of which decreases as the temperature rises. These strains are relaxed by the plasticdeformation of the adjacent austenite. The local increase in dislocation density caused by the yielding of theaustenite blocks the further movement of the glissile transformation interface ( Figure). This localised plasticdeformation therefore halts the growth of the ferrite plate so that each sub-unit only achieves a limited size which ismuch less than the size of an austenite grain. Pg: 14/ 31
  15. 15. 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 orientationrelationship.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 habitplane determined is not that of an individual sub-unit. It corresponds instead to some average value depending onthe number, size and distribution of sub-units within the sheaf. All of these factors can vary with the transformationtemperature, 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 theremaining austenite in carbon. Eventually, cementite precipitates from the residual austenite layers in between theferrite sub-units. The amount of cementite depends on the carbon concentration of the alloy. High concentrationslead 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 Pg: 15/ 31
  16. 16. variants of carbide may precipitate from the austenite, each particle being indirectly related to the ferrite viathe ferrite/austenite orientation relationship.If sufficient quantities of alloying elements (such as silicon or aluminium) which retard the formation of cementiteare added to the steel, then it is possible to suppress the formation of cementite altogether. An upper bainiticmicrostructure consisting of just bainitic ferrite and carbon-enriched retained austenite is obtained instead. Themicrostructure may also contain martensite if the residual austenite decomposes on cooling to ambienttemperatureFigure 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. Pg: 16/ 31
  17. 17. and Crystallography of Lower BainiteLower bainite (temperature range 400-250°C) appears more acicular than upper bainite, with more clearly definedindividual plates adopting a lenticular habit. Viewed on a single surface they misleadingly suggest an acicularmorphology.However, two-surface optical microscopy of lower bainite indicates that the ferrite plates are much broader than inupper bainite, and closer in morphology to martensite plates. While these plates nucleate at austenitic grainboundaries, there is also much nucleation within the grains, i.e. intragranular nucleation, and secondary platesform 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 martensiticreaction. However, there is no reason to expect the transformations to be identical, and anyway theinhomogeneous shear would be expected to occur by slip in lower bainite, whereas twinning is the mode adoptedin higher carbon martensites.However, in contrast to tempered martensite the cementite particles in lower bainite exhibit only one variant of theorientation relationship, such that they form parallel arrays at about 60° to the axis of the bainite plate. Thisfeature of the precipitate suggests strongly that it has not precipitated within plates supersaturated with respect tocarbon, but that it has nucleated at the γ/α interface and grown as the interface has moved forward. It thusappears that the lower bainite reaction is basically an interface-controlled process leading to cementiteprecipitation, which then decreases the carbon content of the austenite and enhances the driving force for furthertransformation. Pg: 17/ 31
  18. 18. reading- Lower bainiteLower bainite has a microstructure and crystallographic features which are very similar to those ofupper bainite. The major distinction is that cementite particles also precipitate inside the plates offerrite Figure. There are, therefore, two kinds of cementite precipitates: those which grow from thecarbon-enriched austenite which separates the platelets of bainitic ferrite, and others which appearto precipitate from supersaturated ferrite. These latter particles exhibit the "tempering" orientationrelationship which is found when carbides precipitate during the heat treatment of martensite, oftendescribed 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. Intempered martensite, the carbides tend to precipitate in Widmanstatten arrays. This peculiar modeof precipitation in lower bainitic ferrite may arise because the carbides nucleate at theferrite/austenite interface, and hence attempt to adopt a unique variant of the orientationrelationship, one which gives an optimum match to both the austenite and ferrite with which theyare in contact. Pg: 18/ 31
  19. 19. 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 finercementite particles precipitate between the ferrite plates, when compared with an upper bainiticmicrostructure. An important consequence is that lower bainite is usually found to be much tougherthan upper bainite, in spite of the fact that it also tends to be stronger. The coarse cementiteparticles in upper bainite are notorious in their ability to nucleate cleavage cracks and voids. Lower Bainite Lower Bainite Pg: 19/ 31
  20. 20. Bainite Upper bainite Enchant : Nital Bainitic Structure Mag : 500X Lower Bainite Tempered martensite. Material with this Bainite with coarse prior austenite grain size.microstructure has an optimum combination Material is strong but brittle - an unacceptable of component strength and toughness microstructure which could result from incorrect heat treatment Pg: 20/ 31
  21. 21. + pearliteFerrite + bainite (Ferrite + martensite will look similar)Pearlite Martensite Pg: 21/ 31
  22. 22. Steels: Part TwoAbstract:In plain carbon steels, it is often difficult to separate the bainite reaction from the ferrite and pearlite reactions, becausethese phases can form under similar continuous to bainitic. For example, the TTT diagram for a 0.8% C steel is a continuouscurve although there is both a pearlite and bainite reaction occurring, but it is difficult to disentangle the reactions sufficientlyto 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. Themicrostructure is predominantly martensite but also has allo-triomorphic ferrite, Widmanstatten ferrite, bainite andpearlite. Notice that the spherical shape of a pearlite colony is obvious in this sample because of the lack ofimpingement. Notice also that pearlite, unlike bainite, grows across the austenite grain boundaries. TheWidmanstatten ferrite plates are white because of the lack of structure within the plates, whereas bainite etchesrelatively dark.Austenite γ Allotriomorphic ferrite α Idiomorphic ferrite αI Pearlite P Widmanstatten ferrite αwUpper bainite αb Lower bainite αlb Acicular ferrite αa Martensite α Cementite θ Pg: 22/ 31
  23. 23. Kinetics of Bainite FormationIn plain carbon steels, it is often difficult to separate the bainite reaction from the ferrite andpearlite 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 andbainite reaction occurring, but it is difficult to disentangle the reactions sufficiently to study theirkinetics.However, the addition of certain alloying elements separates the reactions to the extent that theycan be represented as individual curves on the TTT diagram, which then takes on a more complexform 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 isa well defined temperature Bs, above which no bainitic will form, which has been confirmed for awide range of alloy steels and has been correlated with the structural transition from Widmanstattenferrite to upper bainitic laths. Second, below Bs temperature and time dependent process take placeover 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 placeisothermally, starting with an incubation period during which no transformation occurs, followed byan 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 theprogress of the bainite reaction. It has been found that upper bainitic plates lengthen and thickenduring transformation by the movement along the plate boundaries of small steps which appear tobe diffusion-controlled.The plates grow at a constant rate edgewise, which leads to a model for the reaction in which thedriving force is provided by partition of the carbon from the ferrite to the austenite, the actualgrowth rate being determined by the rate of diffusion of carbon in austenite away from the γ/αinterface.Role of Alloying ElementsCarbon 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 theferrite and pearlite reactions which commence at higher temperatures and shorter times, so that incontinuously cooled samples bainitic structures are difficult to obtain. Even using isothermaltransformation difficulties arise if, for example, the ferrite reaction is particularly rapid the additionof metallic alloying elements usually results in retardation of the ferrite and pearlite reactions. Inaddition, the bainite reaction is depressed to lower temperatures.This often leads to greater separation of the reactions, and the TTT curves for many alloy steelsshow much more clearly separate C-curves for the pearlite and bainitic reactions. However, it is stilldifficult 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 byadding about 0.002% soluble boron to a 0.5% Mo steel. While the straight molybdenum steelencourages the bainite reaction, the boron markedly retards the ferrite reaction, probably bypreferential segregation to the prior austenite boundaries. This permits the bainite reaction to occurat shorter times. Consequently, by use of a range of cooling rates, fully bainitic steels can beobtained. Pg: 23/ 31
  24. 24. of Bainitic SteelsBainite frequently occurs in alloy steels during quenching to form martensite. The cooling ratetowards the centre of a steel bar is lower than the outside, so in large sections bainite can form inthe inner regions with martensite predominating towards the surface.However, low carbon fully bainitic steels have been developed, as described, using 0.5% Mo andvery 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, Mnwhich depress the temperature of maximum rate of formation of bainite. As the transformationtemperature 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 withadequate ductility can be obtained without further heat treatment, after the bainite reaction hastaken place. The steels are readily weldable, because bainite rather than martensite will form in theheat-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 reducesstresses arising from transformation.Chapter Authors: Andrzej Kazimierz Lis, Jadwiga Lis, Andrzej BochenekSeries: EUROMAT 99SummaryThe alloy design of new ultra low carbon bainitic steels (ULCB-Ni) combined with developedthermo=mechanical and heat treatment procedures allowed for production of heavy plates with YS 650÷690MPa and guarantied Charpy V impact energy 80 J at - 100°C and KIC > 100 MPaVm. The ultra low carbonbainitic steel grade HN5MVNb which meets Det Norske Veritas requirements for ship, mobile and offshoreconstructions 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 CharpyV impact toughness 80 J/cm2 at 77 K for heavy plates. The investigated steel may be economical substitute forNV20-2 and 10N9 grades of 9÷10% Ni steels due to higher (~200 MPa) yield strength and reduced to 5% Nicontent in the chemical composition. The materials selection in mechanical design of ships and pressurevessels based on Ashbys maps shows the dependence of stress intensity factor KIC and J integral values for theinvestigated ULCB-Ni steels on the thermo=mechanical and heat treatment procedures. Pg: 24/ 31
  25. 25. 1. Optical micrograph of allotriomorphic ferrite in Fe-0.5W-0.23C wt% alloy (after Sahay). The allotriomorphgrows 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 Pg: 25/ 31
  26. 26. 6: An idiomorph of ferrite in a sample which is partially transformed into α and then quenched so that theremaining 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 platesare martensite, the white bits are the untrasformed austenite. The contrast is from viewing the etched surface (nital)using differential interference contrast. Pg: 26/ 31
  27. 27. on the martensite, pearlite and bainite formationAbstract:All alloying elements with the possible exception of Co, lower temperature of the start of the martensite formation, as well asthe finish of the martensite formation, i.e. at 100% martensite. All alloying elements except Co delay the formation of ferriteand cementite. It is very difficult to formulate any general rules regarding the influence exerted by the various alloyingelements. However, it has definitely been found that some elements affect the bainite transformation more than the pearlitetransformation, while other elements act in the opposite mannerEffect on the temperature of martensite formationAll alloying elements with the possible exception of Co, lower Ms the temperature of the start of themartensite 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. Msmay be calculated from the equation given below, by inserting the percentage concentration of eachalloying element in the appropriate term. The equation is valid only if all the alloying elements arecompletely dissolved in the austenite.Ms = 561 - 474C - 33Mn - 17Ni - 17Cr - 21MoFor high-alloy and medium-alloy steels Stuhlmann has suggested the following equation:Ms( ) = 550 - 350C - 40Mn - 20Cr - 10Mo - 17Ni - 8W - 35V - 10Cu + 15Co + 30AlIt can be noted that carbon has the strongest influence on the Ms temperature. Figures 1 and 2show diagrams with an example of experimental results of the effect of Mn and Ni on the Mstemperature 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) Pg: 27/ 31
  28. 28. 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 duringthe isothermal transformationAll alloying elements except Co delay the formation of ferrite and cementite. It is very difficult toformulate 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 morethan the pearlite transformation, while other elements act in the opposite manner.Certain elements will, paradoxically, accelerate the transformations if their concentration increasesbeyond a certain limiting value, this limit been affected by other alloying elements present. Forcase-hardening and tool steels the time taken to initiate the pearlite-bainite transformation isreduced 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 elementby some few percent and the basic carbon content being kept about 0,50%, only a relatively smallretardation of the transformation rates is noticed. For plain carbon steels a successive increase in Cfrom 0,30% to 1% produces but a negligible effect. It is only in conjunction with several alloyingelements 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. Theshortest transformation time for this steel is less than 1/8th second. Note that the time scale islogarithmic; hence there is no zero time. As has been mentioned previously, both pearlite andbainite form simultaneously in this steel at about 550 °C. Since the curves overlap it is customaryto draw only one curve. With increasing contents of certain alloying elements, however, the nosesof 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 for10 min and quenching in a salt bath at various temperatures. After holding them for predeterminedtimes at various temperatures they are finally quenched in water. Before the salt-bath quenchingthe steel contains undissolved carbides but in view of the composition of the austenite the steel maybe regarded as an eutectoid one. The diagram should be studied with the aid of the explanatory textbelow. Pg: 28/ 31
  29. 29. 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 = pearlite1. 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. Pg: 29/ 31
  30. 30. StructuresFerriteFerrite (α), is the crystal arrangement for pure iron. This form exists as part of the structure in most steels and can usefullyabsorb carbides of iron and other metals by diffusion in the solid state. Ferrite takes a body centered cubic (bcc) form and issoft and ductile.AusteniteAustenite (γ), is a solid solution, that is, the component elements are arranged as if in solution (it also exists as an allotrope ofpure iron). All steel exists in this form at sufficiently high temperatures (see figure 1). Some alloy steels stabilise this singularphase and it is present even at room temperatures. The crystal arrangement is face centred cubic (fcc) and, like ferrite, it is softand ductile.CementiteCementite is iron carbide (Fe3C), When carbon atoms can no longer be accommodated in solution in ferrite and austenite (dueto an increase in carbon content or reduction in temperature), cementite forms, as it can accommodate more carbon in itscrystal structure. Like other carbides, it is hard and brittle.PearlitePearlite is a phase mixture consisting of alternating platelets of ferrite and cementite (α + Fe3C), which grows by conversionfrom austenite. A steel containing 0.77 wt% carbon can consist solely of pearlite if cooled sufficiently slowly from austenite (seefigure 1).Under the microscope it can have an iridescent mother of pearl appearance, hence the name.MartensiteMartensite is commonly found in steel that has been rapidly cooled (quenched) from austenite. It is a particularly hard, brittlearrangement. Essentially it forms because any carbon in solid solution in the austenitic phase at high temperatures does nothave enough time to be incorporated into cementite when cooled rapidly. The austenite crystals undergo a transformationinvolving the shearing of atom planes over each other. Martensite does not appear on the phase diagram (figure 1), as it is notan 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 inthe light microscope. In high carbon steels, plate martensite forms. For certain steels, the rapid cooling necessary to produce amartensitic 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 developmentof equilibrium phases is suppressed and martensite can be formed with less drastic cooling, such as oil quenching.BainiteIf the steel is cooled such that the formation of pearlite by the short range diffusion of iron atoms is not possible, bainite can beproduced. The bainite that forms at temperatures just below those at which pearlite forms is termed upper bainite. At lowertemperatures, lower bainite forms. Both lower and upper bainite consist of aggregates of platelets or laths of ferrite, separatedby regions of residual phases consisting of un-transformed austenite or of phases such as martensite or cementite Pg: 30/ 31
  31. 31. Figure 1. Part of the equilibrium diagram for the Fe-C system Pg: 31/ 31