Nomenclature

Aam Minimum detectable increase in austenite layer thickness A Mean areal intercept in stereology Ac3 Temperature at which a sample becomes fully austenitic during Ae3 Temperature separating the a + 7 and 7 phase fields for a specific Ar3 Temperature at which an austenitic sample begins to transform to ferrite during cooling Af Temperature at which the transformation to austenite is complete As Temperature at which the transformation to austenite begins As Mean free slip area in statistical...

260

Tures, the amount of acicular ferrite decreases with the austenite grain size, as grain boundary nucleated phases such as allotriomorphic ferrite become more dominant (Barbaro et al., 1988). The dependence of the volume fraction of acicular ferrite on the austenite grain size becomes less pronounced as the cooling rate (from the austenite phase field) is increased, since at slow cooling rates, much of the austenite is consumed during the higher temperature formation of allotriomorphic ferrite....

372

(Heitmann and Babu, 1987 Grassl et al., 1989). Such steels are intended to replace quenched and tempered martensitic alloys. The compositions are chosen to avoid the formation of carbides during bainitic transformation, carbides which can be detrimental to toughness. The toughness turns out to be better than that of ferrite-pearlite microstructures at comparable strength levels. However, at higher carbon concentrations the bainitic steels compare unfavourably with martensitic steels which have...

152Notes Related to Table 151

Nucleation and growth reactions are of first order in the Ehrenfest classification in all such reactions, the parent and product phases can coexist, and are separated by well-defined interfaces. Martensitic transformations, although they can be rapid, still involve a nucleation and growth process. It is significant that all of the ferrite crystals which grow in the form of plates cause an invariant-plane shape deformation which is dominated by shear. The ferrite within pearlite does not have a...

377

Fig. 13.24 Charpy impact toughness data for Fe-0.43C-2Mn-3Si wt alloy transformed to a mixture of bainitic ferrite and carbon-enriched retained austenite. The data from the untempered microstructure are from Fig. 13.9. The other data were obtained after tempering the microstructure at 500 C for 1 h to induce the decomposition of the austenite into a mixture of ferrite and carbides. Ausforming is the deformation of austenite at temperatures well below Ae3, followed by transformation to...

13 Bainitic Steels Industrial Practice

In spite of the early optimism about the potential of bainitic steels, commercial exploitation took many years to become established. The steels were not better than quenched and tempered martensitic steels, partly because of the coarse cementite particles associated with bainite and because the continuous cooling heat treatments which were popular in industry, could not in practice produce fully bainitic steels. The use of lean alloys gave mixed microstructures whereas intense alloying led to...

287

Fig. 12.2 Variation in hardness as a function of the isothermal transformation temperature (after Lyman and Troiano, 1946.) microstructure is almost fully bainitic for all of the temperatures studied. This contrasts with higher carbon alloys, where the hardness first decreases as the transformation temperature is reduced this is because the fraction of bainite increases at the expense of residual phases like martensite and degenerate pearlite.'' The microhardness of bainite, in a mixed...

1296 Tungsten Strengthened Steels

The heat associated with the welding of steels introduces a heat-affected zone (HAZ) in the solid metal adjacent to the weld the heating and cooling cycle within this zone may induce undesirable microstructures such as untempered martensite. Post-weld heat treatments are then used to render any martensite in the HAZ harmless. However, this is not practical for large constructions, which also cannot be heated prior to welding in order to avoid the formation of brittle martensite. A new bainitic...

344 Interphase Precipitation

An alternative view is that the cementite of lower bainite nucleates and grows at the austenite-ferrite interface, a process which is well established in the high temperature precipitation of carbides and is described as interphase precipitation (Honeycombe and Pickering, 1972). The carbon that is necessary to sustain the growth of cementite can be absorbed from the adjacent austenite and it is then not necessary for the ferrite to be supersaturated. It is argued that during nucleation, the...

Ab

Fig. 6.22 (a) Constant temperature free-energy curves showing the quantities G and Gn for the case where the interface compositions are as illustrated in (b). Note that the net free energy available for interfacial motion after allowing for strain energy and interface energy contributions is AG, which is the sum of the two dissipations Gdd and Gid. of solute ahead of the interface. The concentrations xa and Xj are not independent because the choice of either fixes the value of the other...

307

In fact, one of the tests for the susceptibility of bainitic microstructures to impurity-controlled embrittlement involves a comparison of the toughness of samples which are water quenched from a high tempering temperature (680 C) with those cooled slowly to promote impurity segregation (Bodnar et al., 1989). Studies of creep resistant bainitic steels show that phosphorus and tin, and to a lesser extent manganese and silicon, are all embrittling elements (Bodnar et al., 1989). Manganese is...

724 Comparison of Theory and Experimental Data

The general behaviour indicated by the calculations for plain carbon steels, is found to be that observed experimentally. Some interesting work by Oka and Okamoto (1986) proves that there is no upper bainite in plain carbon steels with more than 0.8 wt of carbon the only bainite observed in their experiments was classical lower bainite at all temperatures above the Ms temperature (Fig. 7.6a). Ohmori and Honeycombe (1971), in a study of plain carbon steels, showed that during isothermal...

390

The isothermal heat treatment which leads to the formation of bainite is known as austempering and the resulting cast iron, which is known as austempered ductile cast iron, has much improved ductility and toughness (Dorazil et al., 1962 Blackmore and Harding, 1984 Moore et al., 1985a Ueda and Takita, 1986 Shea and Ryntz, 1986 Franetovic et al., 1987a,b Rundman et al., 1988), together with improved abrasion wear resistance (Shepperson and Allen, 1987 Lu and Zhang, 1989). The austempered ductile...

108

Fig. 4.14 Equilibrium fractions of carbides at 565 C (838 K) in some common power plant steels, the first two of which frequently are bainitic. The remaining alloys are essentially martensitic. The detailed chemical compositions are given in Table 12.2. Small fractions of vanadium and niobium carbonitrides are present in some steels but are not shown. Thus, the modified 9Cr1Mo contains 0.0009 NbN and 0.003 VN, the 9CrMoWV steel contains 0.0008 NbN and 0.0032 VN. Fig. 4.14 Equilibrium fractions...

105Effect of Inclusions on the Austenite Grain Size in Welds

A microstructure with large austenite grains has a better chance of transforming to acicular ferrite because the number density of grain boundary nucleation sites is reduced. It is sometimes assumed that the austenite grain size is determined by Zener pinning by inclusions. This analogy is, however, not justified since the austenite grains form by the transformation of 8-ferrite grains which evolve during solidification, whereas Zener pinning deals with the hindrance of grain boundaries during...

361

A triple phase steel containing some martensite in addition to bainite and ferrite, to promote gradual yielding. Sudo and coworkers (1981, 1982) have demonstrated that triple phase steels (Alloy 6, Table 13.3) do have the best combination of mechanical properties for press forming applications. Notice that their triple phase steel is leaner in alloy content than the dual phase alloys the development of the required microstructure therefore depends on more difficult heat treatment practice....

108 Stress Affected Acicular Ferrite

Welded fabrications are prone to the development of residual stresses whose magnitudes may approach the yield stress. This may have consequences on the development of the acicular ferrite microstructure during the cooling of the weld to ambient temperature. Dallum and Olson (1989) have shown that stress has little influence on the overall volume fraction of acicular ferrite. Nevertheless, an externally applied stress accelerates transformation and alters the morphology of acicular ferrite as...

133 Bainitic Steels

Paraequilibrium ferrite & paraequilibrium cementite 4. Bainite and paraequilibrium cementite 6. Mechanically alloyed ODS metal with a description of the key industrial processes in the context of bainite. The range of bainitic alloys currently in use is summarised in Fig. 13.2 the inoculated acicular ferrite steels have already been dealt with in Chapter 10.

240

Mole fraction of carbon in austenite Mole fraction of carbon in austenite Fig. 10.4 Data from experiments in which the austenite is transformed iso-thermally to acicular ferrite, showing that the reaction stops when the carbon concentration of the austenite reaches the TQ curve (Strangwood and Bhadeshia, 1987). that of bainite because it nucleates intragranularly in steels containing a greater number density of inclusions than austenite grain surface nucleation sites (Yang and Bhadeshia, 1986)....

426

JISI 172 (1952) 307-313. Kocks, U. F. Phil. Mag. 13 (1966) 541. Kocks, U. F. Can. J. Phys. 45 (1967) 737. Kocks, U. F., Argon, A. S. and Ashby, M. F. Prog. Mater. Sci. 19 (1975) 1. Koistinen, D. P. Trans. ASM 50 (1958) 227. Korenko, M. K. PhD thesis entitled 'Martensitic Transformations in High Magnetic Fields', Massachusetts Institute of Technology (1973). Kostic, M. M., Hawbolt, E. B. and Brown, L. C. Metall. Trans. 7A (1976) 1643. Kostic, M. M., Hawbolt, E. B. and...

336

It is believed that in ferritic steels, irradiation induces the formation of two kinds of dislocations, one of which is strongly biased whereas the other is neutral. This provides sinks for both interstitials and vacancies, resulting in a smaller excess of void-forming vacancies. There are differences in the void swelling resistance of creep-resistant ferritic steels because of variations in their microstructures. For a given vacancy concentration, a larger number density of vacancy traps leads...

463 Effect of Carbon on Carbide Enrichment

There are two effects which depend on the carbon concentration of the steel. The ternary Fe-Cr-C phase diagram on the M3C a field shows that an increase in the carbon concentration is accompanied by a decrease in the equilibrium concentration of chromium in the carbide. Thus, the carbide enrichment rate is expected to decrease. A further effect is that the volume fraction of cementite increases, in general leading to an increase in particle thickness and volume fraction. The thickness increase...

71 The Matas and Hehemann Model

The transition between upper and lower bainite is believed to occur over a narrow range of temperatures. It is possible for both forms to occur simultaneously during isothermal transformation near the transition temperature (Pickering, 1967). Matas and Hehemann (1961) proposed that the difference between upper and lower bainite comes from a competition between the rate at which carbides can precipitate from ferrite and the speed with which carbon is partitioned from supersaturated ferrite into...

113Columnar Bainite

'Columnar bainite' is a description of a non-lamellar aggregate of cementite and ferrite, the overall shape of which is like an irregular and slightly elongated colony (Fig. 11.3). The distribution of cementite particles within the colony is rather peculiar, the majority of needle-shaped particles being aligned to the longer dimension of the colony. This latter region is surrounded by a layer of a different microstructure, in which the coarse cementite particles meet the austenite ferrite...

85 The Effect on Microstructure

An applied stress will tend to favour the development of crystallographic variants which comply with that stress. This is analogous to the selective operation of a few of the available slip systems in a crystal under stress it is the systems with the largest Schmid factors which are favoured. Assuming that variant selection is similarly controlled by the interaction of the applied stress with the shape deformation, the stress should cause an alignment of the plates at roughly 45 to the tensile...

50

The atomic force microscopy has also shown that the low values of s measured in early work using light microscopy arise because the low resolution causes an averaging of the shape strain over the sheaf. The plastic relaxation is of course, ultimately responsible for the arrest in the growth of the bainite plates, giving the sub-unit and sheaf hierarchies in the microstructure of bainite as discussed in section 2.2.1, it also leads to an increase in the dislocation density of the bainite.

205

The application of a tensile or compressive stress assists the transformation by boosting the overall driving force AG with the term AGMEcH, so that the Bs temperature rises continuously with the magnitude of the applied stress. Consider a temperature Ba corresponding to an applied stress a, where a becomes greater than the yield strength of the austenite. It is difficult to justify a thermodynamic analysis when the austenite undergoes plastic deformation prior to transformation. The...

298

Fig. 12.7 Schematic illustration of impact transition curves (a) and of the cause of the ductile brittle transition temperature (b) in body-centred cubic metals where the plastic flow stress is much more sensitive to temperature than the cleavage stress. Fig. 12.7 Schematic illustration of impact transition curves (a) and of the cause of the ductile brittle transition temperature (b) in body-centred cubic metals where the plastic flow stress is much more sensitive to temperature than the...

662 Growth Rate of Sheaves of Bainite

After nucleating at austenite grain surfaces, sheaves of bainite propagate by the repeated formation of sub-units, each of which grows to a limited size. New sub-units are favoured near the tips of existing platelets nucleation in adjacent positions occurs at a much lower rate. Therefore, the overall shape of the sheaf is also that of a plate in three dimensions with growth limited only by austenite grain or twin boundaries. Most direct observations have used optical microscopy and hence...

124 Ductility

It was noticed as early as 1957 by Irvine and Pickering, that low-carbon bainitic or martensitic steels always show superior tensile ductility when compared with their high-carbon counterparts, even when the comparison is made at identical strength. Their subsequent work (1965) confirmed that ductility can be improved by reducing the carbon concentration of a fully bainitic microstructure while maintaining its strength using substitutional solid solution strengthening. Ductile fracture in good...

106

Fig. 4.12 (a) Solute concentration profile that develops during enrichment of cementite. c6a is the concentration in cementite which is in equilibrium with ferrite, (b) Size dependence of the cementite chemical composition, for particles extracted from a bainitic microstructure aged for 4 weeks at 565 C (Wilson, 1991). Detailed analysis shows that the scatter in the data is a consequence of the microanalysis technique. Fig. 4.12 (a) Solute concentration profile that develops during enrichment...

345

Fig. 13.2 The range of bainitic steels currently available on a commercial basis. TRIP stands for transformation-induced plasticity. Fig. 13.2 The range of bainitic steels currently available on a commercial basis. TRIP stands for transformation-induced plasticity. The tendency to soften depends on how far the original microstructure deviates from equilibrium (Table 13.3). Large deviations naturally lead to greater rates of softening because the driving force for tempering is the energy stored...

1297 Regenerative Heat Treatments

Cavitation and other irreversible creep damage occurs at the late stages in the life of creep-resistant steels. During that period, any loss in properties is due Fig. 12.24 Comparison of the allowable stress, as a function of temperature, for the tungsten-strengthened steel, 2 Cr1Mo and a 9Cr1Mo martensitic steel. After Komai et al. (1999). Fig. 12.24 Comparison of the allowable stress, as a function of temperature, for the tungsten-strengthened steel, 2 Cr1Mo and a 9Cr1Mo martensitic steel....

1286 Stress Corrosion Resistance

Cleavage fracture occurs when a critical stress is exceeded over a region ahead of the crack tip, such as to stimulate the growth of an existing microcrack. This critical stress can be reduced by environmental effects. Cleavage fracture then occurs at a critical stress intensity KISCC which is about a third of KIC. This means that stress corrosion can severely limit the effective use of high strength steels. The effect of corrosion manifests primarily via hydrogen embrittlement, the hydrogen...

151Key Characteristics of Transformations in Steels

Table 15.1 lists the key characteristics of phase transformations in steels. The nomenclature used for the transformation products is as follows martensite (a'), lower bainite (alb), upper bainite (aub), acicular ferrite (aa), Widmanstatten ferrite (aw), allotriomorphic ferrite (a), idiomorphic ferrite (ai), pearlite (P), substitutional alloying elements (X). Consistency of a comment with the transformation concerned is indicated by ( ), inconsistency by ( ) cases where the comment is only...

261

10.6.1 Some Specific Effects of Allotriomorphic Ferrite We now proceed to consider a particular role of allotriomorphic ferrite formation in influencing the development of acicular ferrite in mixed microstructures. The effect is especially prominent in chromium- and molybdenum-containing steels. At relatively high concentrations of chromium (> 1.5 wt ) or molybdenum (> 0.5 wt ), the columnar austenite grains of steel weld deposits transform into bainite instead of acicular ferrite. The...

331 Partitioning and Distribution of Carbon

The carbon concentration of bainitic ferrite during transformation is of major importance in determining the kinetics of carbide precipitation. The transformation, however, occurs at high temperatures so excess carbon in the ferrite can be removed by precipitation or by partitioning into austenite. These two processes occur simultaneously, although one or the other may dominate depending on temperature. They can both be rapid because of the high mobility of carbon in iron. The partitioning of...

189

1968a), and in a Fe-4.08Cr-0.3C wt alloy which has a Bs temperature of 490 C. Ohmori and Honeycombe (1971) have shown that in a series of high purity Fe-0.16-0.81C wt alloys, lower bainite is not obtained when the carbon concentration is less than about 0.4 wt . Tsuzaki et al. (1991) found a similar result for Fe-Si-C alloys only upper bainite formed in a Fe-2Si-1Mn-0.34C wt steel, whereas both upper and lower bainite could be observed when a higher carbon variant (0.59 wt ) was examined. A...

12922Cr1Mo Type Steels

This is a popular steel which has been used reliably for many decades. Its creep resistance, like that of many low-alloy steels used in the power generation industry, comes from fine and stable dispersions of alloy carbides together with contributions from solid solution strengthening. The latter is especially important in the long term when the carbide dispersions have become ineffective due to coarsening. Molybdenum is particularly effective as a solid solution strengthening element (Lundin...

41 Introduction

Tempering is a term historically associated with the heat treatment of martensite in steels. It describes how the microstructure and mechanical properties change as the metastable sample is held isothermally at a temperature where austenite cannot form. The changes during the tempering of martensite can be categorised into stages. During the first stage, excess carbon in solid solution segregates to defects or forms clusters within the solid solution. It then precipitates, either as cementite...

611 Simultaneous Transformations

A simple modification for two precipitates (a and ft) is that equation 6.43 becomes a coupled set of two equations, ( Va + V3 ( Va + Vft dva il--V-) dva and dV13 il--V-) dV (6.50) This can be done for any number of reactions happening together (Robson and Bhadeshia, 1997 Jones and Bhadeshia, 1997). The resulting set of equations must in general be solved numerically, although a few analytical solutions are possible for special cases which we shall now illustrate (Kasuya et al., 1999).

722Kinetics of Cementite Precipitation

It is not yet possible to estimate the rate of cementite precipitation from supersaturated ferrite as a function of time, temperature and chemical composition. Fig. 7.2 Calculated time for the decarburisation of supersaturated ferrite plates (of thickness 0.2 m) in plain carbon steels with 0.1, 0.2 and 0.4 wt carbon respectively. The calculated martensite-start and bainite-start temperatures are also indicated. However, for plain carbon steels, and in some cases for alloy steels, martensite...

142 The Weldability of Bainitic Steels

The region which is adjacent to the fusion zone of a weld is influenced by heat diffusion from the fusion zone. This region is the heat-affected zone (HAZ). Its boundaries need not be precisely defined because the definition depends on purpose. The heat dissipated into the HAZ can be detected as the temperature at any point rises to a maximum and then drops gently towards the far-field temperature. The severity of the heating or cooling cycles, and the peak temperature, depends on the location...

329

Extensive decarburisation with the dissolution of carbides as the system attempts to maintain an equilibrium carbon concentration in the ferrite. Thick steel plates (300-400 mm) are required in applications such as coal conversion plant. Conventional steels do not have adequate hardenability so attention has been focussed on improving the popular bainitic 2 Cr1Mo steel, with the aim of extending the temperature range over which the alloy can be utilised, whilst maintaining the bainitic...

136 Steels with High Formability

The oil crisis of the seventies led to the development of the dual phase steels in an effort to reduce the weight of cars and make them fuel efficient. Fig. 13.10 (a) A light micrograph illustrating the effect of chemical segregation along the mid-thickness of heavy gauge plate. (b) Distribution of carbon concentration in the segregated zone for conventional control-rolled and rapidly cooled steel plates (Tamehiro et al., 1985c). Fig. 13.10 (a) A light micrograph illustrating the effect of...

118

Fig. 5.1 The chemical potentials ja and jb for components A and B respectively, in a solution containing a mole fraction x of B and 1 x of A. The potentials are given by the intercepts on the vertical axes of the tangent drawn at x to the curve representing the solution free energy. jA and jB are the molar Gibbs free energies of pure A and B respectively. Fig. 5.1 The chemical potentials ja and jb for components A and B respectively, in a solution containing a mole fraction x of B and 1 x of A....

239

Fig. 10.3 Nomarski interference contrast micrograph illustrating the displacements associated with the formation of acicular ferrite (Strangwood and Bhadeshia, 1987). Fig. 10.3 Nomarski interference contrast micrograph illustrating the displacements associated with the formation of acicular ferrite (Strangwood and Bhadeshia, 1987). et al.(1988) reported that the apparent plate thickness and length changed from about 1 to 2 m as the weld cooling rate was reduced.

247

Table 10.2 List of ceramics found to be chemically active in experiments designed to test for ferrite nucleation at ceramic steel bonds. Chemically Active Chemically Inactive Ti02 TiO, Ti203, TiC, TiB2, TiN in turn stimulates the nucleation of bainite. One such mineral is Ti02. Structural and behavioural analogues of Ti02 (Sn02, Mn02 and Pb02) are also found to stimulate bainite in the same manner. Ti02 and related minerals tend to form oxygen vacancy defects at elevated temperatures, thus...

2

Fig. 1.1 Microstructures in a eutectoid steel (a) Pearlite formed at 720 C (b) bainite obtained by isothermal transformation at 290 C (c) bainite obtained by isothermal transformation at 180 C (d) martensite. The micrographs were taken by Vilella and were published in the book The Alloying Elements in Steel (Bain, 1939). Notice how the bainite etches much darker than martensite, because its microstructure contains many fine carbides. Fig. 1.1 Microstructures in a eutectoid steel (a) Pearlite...

13

Fig. 1.6 Surface effects observed during the transformation of pre-polished samples of austenite (Ko and Cottrell, 1952) (a) Surface relief due to the formation of bainite (b) Line traces obtained by traversing a stylus across the surface of a pearlitic and a bainitic sample. Notice the severe upheavals caused by bainite, which contrast with the negligible relief due to pearlite. Fig. 1.6 Surface effects observed during the transformation of pre-polished samples of austenite (Ko and Cottrell,...

193

The yield strength of martensite, ay, is expressed as a combination of the intrinsic yield strength, the effect of the dislocation cell structure, and precipitation hardening by cementite (Daigne et al., 1982) ay a0 + k 1 + kpA_1, MPa (7.5) where Oq is the intrinsic strength of martensite (including solid solution strengthening due to carbon), e1 is the average transverse thickness of the cell structure, and A is the average distance between a particle and its two or three nearest neighbours....

274

Which is less stable at high temperatures. The steelmaking involves deoxidation with titanium, whilst avoiding other strong deoxidisers such as Al, Ca or the rare earth elements. The oxygen concentration in the molten steel should be between 60 and 120 p.p.m., depending on application. High toughness levels demand a small inclusion (and hence oxygen) content. The steel must otherwise be clean with a minimal concentration of sulphur. The active inclusions form in the melt or during the...

14

The growth direction of the macroscopic plate of bainite was supposed to be normal to the plane of the lamellae. Although this particular mechanism has since been shown to be incorrect, they identified clearly the condition necessary for cementite precipitation to occur from residual austenite during the bainite transformation. Cementite precipitates from austenite if the carbon concentration of the latter exceeds that given by the extrapolated 7 7 + d phase boundary. Although many of the...

35 Chemical Composition of Bainitic Carbides

It has long been established, using magnetic, chemical and X-ray methods on extracted carbides, that the cementite associated with upper bainite has a substitutional solute content which is close to, or slightly higher than that of Fig. 3.8 (a) The partition coefficient for chromium in cementite, when the cemen-tite is a part of bainite or pearlite, together with equilibrium data (Chance and Ridley, 1981). The partition coefficient is the ratio of the concentration in cementite to that in the...

109 Effect of Strain on the Acicular Ferrite Transformation

The distinguishing feature of acicular ferrite is that it must nucleate intragra-nularly on inclusions. The amount of acicular ferrite is reduced if the number density of grain boundary nucleation sites is increased relative to the number density of inclusions. The effect of deforming austenite prior to its transformation is to increase the nucleation potency and number density of the austenite grain boundaries. This is not helpful in promoting acicular ferrite. This is why the thermomechanical...

192

Where t is the volume fraction of cementite normalised by its equilibrium volume fraction at the reaction temperature, t is the time, and kA and n are rate constants determined from the experimental data. Since it is assumed that t is related at any time to the hardness of the martensite, H t , it follows that m (H0 - H t ) (H0 - HF) (7.2) H0 is the hardness of the as-quenched virgin martensite, HF is its hardness when all the carbon has precipitated but before any significant recovery,...

211

Fig. 8.9 Optical micrographs showing the large effect of mechanical stabilisation in refining the microstructure and in reducing the amount of bainite (a) transformation from undeformed austenite (b) transformation from plastically deformed austenite (Shipway and Bhadeshia, 1995). Fig. 8.9 Optical micrographs showing the large effect of mechanical stabilisation in refining the microstructure and in reducing the amount of bainite (a) transformation from undeformed austenite (b) transformation...

1295Enhanced CrMo Bainitic Steels

Many attempts have been made to improve on the properties of the low-alloy creep-resistant steels, especially in the context of pressure vessels in hydrogen environments. A higher concentration of alloy carbide-forming elements can reduce the stability of cementite, which is prone to hydrogen problems (Ritchie et al., 1984 George et al., 1985). Those alloys containing unstable carbides such as cementite and Mo2C react with ingressed hydrogen, leading to decarbur-isation, cavitation and to the...

348 Chi Carbide

X-Carbide is another transition carbide which is metastable with respect to cementite. It is found during the tempering of martensite, where highresolution electron microscopy has demonstrated that what at first sight appears to be faulted cementite in fact consists of interpenetrating layers of Fig. 3.7 Lattice resolution transmission electron micrographs showing the inter-growth of layers of cementite and -carbide (Ohmori, 1986). (a) Carbide particle which precipitated in lower bainitic...

42Tempering Kinetics

It is astonishing that there is as yet no quantitative model for the kinetics of tempering, certainly not of the kind that could be used in the design of alloys or heat-treatments. Figure 4.2 illustrates an empirical method of expressing tempering data using a time-temperature parameter, useful because it permits interpolation between experimental data and a method of estimating the effect of anisothermal heat treatments which are common in industrial practice. The method has its origins in...

95Summary

Microstructures containing a mixture of bainitic ferrite and austenite when heated do not require the nucleation of new austenite. Nevertheless, they have to be superheated over a large temperature range before the austenite begins to grow. This is because the bainite reaction stops before equilibrium is achieved so that the fraction of austenite in the initial microstructure is greater than required by equilibrium. The nucleation of austenite is necessary when the original microstructure does...

355

Fig. 13.7 The effect of the temperature at which rolling is completed, on the variation in hardness of a Fe-0.16C-0.63Mn wt accelerated cooled steel (Tamukai et al., 1981). Fig. 13.7 The effect of the temperature at which rolling is completed, on the variation in hardness of a Fe-0.16C-0.63Mn wt accelerated cooled steel (Tamukai et al., 1981). Some of the steels processed using accelerated cooling have a high harden-ability which introduces martensite into the microstructure with an...

6113 Time TemperatureTransformation TTT Diagrams

Transformation curves on TTT diagrams tend to have a C shape because reaction rates are slow both at high and at low temperatures. The diffusion of atoms becomes difficult at low temperatures whereas the driving force for transformation is reduced as the temperature is raised. The phase diagram thus sets the thermodynamic limits to the decomposition of austenite (Fig. 6.31). Most TTT diagrams can be considered to consist essentially of two C curves, one for high temperatures representing...

357

C + Mn 6 + (Cr+Mo+V) 5 + (Ni+Cu) 15 wt Fig. 13.9 Relationship between the carbon equivalent and tensile strength for conventionally produced control-rolled steels and accelerated cooled steels the latter have a mixed microstructure of ferrite and bainite (Tamehiro et al., 1985a). Control-rolled steels are cast continuously so they contain pronounced chemical segregation along the mid-thickness of the plate. For example, the manganese concentration at the centre can reach twice the average...

106Influence of Other Transformation Products

In weld deposits, acicular ferrite is one of the last transformation products to form after the growth of allotriomorphic and Widmanstatten ferrite. As a consequence, it is bound to be influenced by prior transformation products. Indeed, its volume fraction during continuous cooling transformation of such welds can in many cases be estimated simply by calculating the volume fractions of allotriomorphic and Widmanstatten ferrite, and assuming that the remainder of the austenite transforms to...

269

Fig. 10.20 (a) Dilatometric data monitored along orthogonal directions, showing the displacive character of the acicular ferrite reaction, and the acceleration of transformation by the applied stress. (b) The microstructure obtained in the absence of stress. (c) The aligned microstructure generated by the formation only of those acicular ferrite variants which are favoured by the applied stress. The transformation conditions for (b) and (c) are otherwise identical (after Babu). Fig. 10.20 (a)...

401

Diffuse than those from reconstructive reactions (allotriomorphic ferrite, pearlite). For example, Radcliffe and Rollason demonstrated a larger lattice strain with martensite and bainite than with pearlite. The diffusion that occurs during reconstructive transformation help accommodate the volume change, preventing the development of stresses. The residual stresses develop mainly because transformation does not usually occur uniformly in all regions of the sample. This can be exploited for...

134 Controlled Rolling of Bainitic Steels

The strengthening of iron via a reduction in grain size is an attractive option because a small grain size leads also to an improvement in toughness. This simple fact has led to the development of impressive thermomechanical processing technology capable of refining the austenite grain structure prior to its transformation to ferrite (Fig. 13.3). A fine austenite grain size leads to a correspondingly refined ferrite grain structure. The controlled-rolling process Fig. 13.3 Flow chart...

6104 Anisothermal Transformation Kinetics

A popular method of converting between isothermal and anisothermal transformation data is the additive reaction rule of Scheil (1935). A cooling curve is treated as a combination of a sufficiently large number of isothermal reaction steps. Referring to Fig. 6.29, a fraction 0.05 of transformation is achieved during continuous cooling when with the summation beginning as soon as the parent phase cools below the equilibrium temperature. The rule can be justified if the reaction rate depends...

92Anisothermal Transformation

Heat treatments are rarely isothermal in commercial practice. A continuous heating curve can be expressed as a series of small isothermal steps i, each occurring at a successively higher temperature, with a time interval ti associated with each step. With Scheil's rule, a specified increment of transformation is achieved during continuous heating when the sum of all the ratios of time steps to incubation periods equals unity where t is the time required to reach the specified fraction of...

91

Fig. 4.1 The time scales associated with a variety of tempering phenomena for bainite. Fig. 4.1 The time scales associated with a variety of tempering phenomena for bainite. steels with a bainitic microstructure are slower than with martensite, because the coarser cementite particles take longer to dissolve (Woodhead and Quarell, 1965). Secondary hardening involves the replacement of metastable cementite with substitutional-solute-rich alloy carbides. When compared with martensite, bainite...

303

Theless be measured and if it is shown to be constant, then aF itself can be used as a measure of 'toughness' (Bowen et al., 1986), although it is not clear how possible variations in rc can be accounted for. A constant aF indicates that the critical step in the fracture process is the propagation of a microcrack. Bowen et al. used this approach, together with KIC studies to explain the toughness of tempered martensite and bainite in a low-alloy steel. In all cases, KIC values were found to...

6112 Precipitation in Secondary Hardening Steels

Whereas the analytical cases described above are revealing, it is unlikely in practice for the phases to be related in the way described. This is illustrated for secondary hardening bainitic and martensitic steels of the kind used commonly in the construction of power plant. The phases interfere with each other not only by reducing the volume available for transformation, but also by removing solute from the matrix and thereby changing its composition. This change in matrix composition affects...

27

Fig. 2.9 Intense dislocation debris both at, and in the vicinity of the bainite austenite transformation front (Bhadeshia and Edmonds, 1979a). Fig. 2.9 Intense dislocation debris both at, and in the vicinity of the bainite austenite transformation front (Bhadeshia and Edmonds, 1979a). isothermal transformation to bainite at 300, 360 and 400 C gave dislocation densities of 6.3 x 1015, 4.7 x 1015 and 4.1 x 1015m2 respectively (Fondekar et al, 1970). 2.2.1 Quantitative Estimation of Dislocation...

351

Fig. 13.5 Sections along 45 of the crystal orientation distribution functions showing transformation textures of control-rolled steels with a variety of microstructures (Yutori and Ogawa, 1979). For steels the most important features of the orientation distribution function are in the section of Euler space at 45 because it contains orientations of the form 100 < uvw > and the fibre textures hkl < 110 > and 111 < uvw > . (a) Allotriomorphic ferrite pearlite (b) acicular ferrite (c)...

11 The Discovery of Bainite

During the late 1920s, in the course of these pioneering studies on the isothermal transformation of austenite at temperatures above that at which martensite first forms, but below that at which fine pearlite is found, Davenport and Bain (1930) discovered a new microstructure consisting of an 'acicular, dark etching aggregate' which was quite unlike the pearlite or martensite observed in the same steel (Fig. 1.1). They originally called this microstructure 'martensite-troostite' since they...

88 Transformation under Constraint Residual Stresses

Residual stresses are mostly introduced unintentionally during fabrication. They are of particular importance in welded structures where they have a detrimental effect. Jones and Alberry (1977a,b) conducted an elegant series of experiments to illustrate the interaction between transformations and residual stress. Using bainitic, martensitic and stable austenitic steels, they demonstrated that transformation plasticity during the cooling of a uniaxially constrained sample from the austenite...

285

Normalising (i.e. air cooling from the austenitising temperature), and permitted the characterisation of the mechanical properties of bainite in isolation. Many investigations of the mechanical properties fail to recognise that the microstructures studied were not fully bainitic. In the discussion that follows, attention is restricted to cases where the microstructure has been characterised thoroughly, and where it plays a significant role in determining the mechanical properties.

257

Fig. 10.14 Calculations showing how the components of inclusions in welds change as the chemical composition is altered. Manganese and silicon oxides are progressively replaced by titanium oxide. When the oxygen has reacted completely with titanium, the latter begins to combine with nitrogen and helps to liberate boron. Fig. 10.14 Calculations showing how the components of inclusions in welds change as the chemical composition is altered. Manganese and silicon oxides are progressively replaced...

I2 14 16 18 20 22 24 Tempering parameter T20 logt

Fig. 4.7 Secondary hardening peak in a vanadium-containing bainitic steel (after Irvine and Pickering, 1957). The tempering parameter is defined with the absolute temperature T and the time t in hours. 1957). Whether or not peaks are observed in the tempering curves, the data are all consistent with secondary hardening because the tempering resistance is improved relative to plain carbon steels. It would be interesting to see whether it is possible to design a steel in which the bainite...

253 The Shape Change Further Considerations

In talking about the application of the phenomenological theory of martensite to bainite, the classical view (Hull, 1954 Bilby and Christian, 1956 Christian, 1962) that the experimentally observed invariant-plane strain shape deformation implies a coordinated movement of at least the iron and substitutional atoms was implicitly accepted. Given that there has been some confusion in the literature about the interpretation of this shape change, it is worth presenting an assessment of the...

316

For a high strength steel transformed isothermally to a mixed microstructure of bainite, martensite and retained austenite, Ritchie (1977a) found that the deformation-induced transformation of retained austenite to martensite reduced the reversibility of plastic strain during cyclic deformation, causing the cyclic yield strength to exceed the ordinary yield strength and consequently leading to a reduction in AK0 (Fig. 12.17). Later work on metastable austenitic stainless steel...

663 Growth Rate of Sub Units of Bainite

The growth rate of martensite can be so fast as to be limited only by the speed of sound in the metal. Although bainite grows rapidly, the lengthening rate is much smaller than that for martensite. The interface moves relatively slowly even though it is glissile. This is probably because of the plastic work that is done as the bainite grows. A good analogy is to compare brittle failure in a glass where cracks propagate rapidly, with cleavage failure in metals which is not as rapid because of...

84 Plastic Deformation and Mechanical Stabilisation

It has been emphasised that displacive transformations involve the coordinated movement of atoms and that such movements cannot be sustained against strong defects such as grain boundaries. Thus, martensite plates, which form by a displacive mechanism, cannot cross austenite grain bound- Fig. 8.5 The influence of internal stresses on the rate of transformation at 410 C, in a wt alloy. Curve A represents isothermal transformation to upper bainite curve B is for a sample which was first partially...

194

(b) upper & lower bainite (c) no upper bainite (b) upper & lower bainite (c) no upper bainite 200 300 400 500 600 700 Temperature C 200 300 400 500 600 700 Temperature C 200 300 400 500 600 700 Temperature C 200 300 400 500 600 700 Temperature C 200 300 400 500 600 700 Temperature C 200 300 400 500 600 700 Temperature C Fig. 7.4 Illustration of how differences in the relative variation of the decarbur-isation time td and the precipitation time te can lead to (a) a steel which is incapable...

238

Fig. 10.2 Scanning transmission electron micrograph of a nonmetallic inclusion in a steel weld metal. The inclusion surface is very irregular, and it features many phases (after Barritte, 1982). Fig. 10.2 Scanning transmission electron micrograph of a nonmetallic inclusion in a steel weld metal. The inclusion surface is very irregular, and it features many phases (after Barritte, 1982). resulting dislocations are inherited by the acicular ferrite as it grows, giving a dislocation density which...

9 From Bainite to Austenite

Many commercial processes cause the steel to revert into the austenitic condition. The transformation of low-temperature ferrite into high-temperature austenite differs from the case where the latter transforms during cooling. Transformation during cooling follows a C curve kinetics in which the overall transformation rate goes through a maximum as a function of the undercooling below the equilibrium temperature. This is because diffusion coefficients decrease but the driving force increases as...

369

Fig. 13.18 Soluble niobium in ULCB steel (Hulka et al., 1988). (a) Variation in the soluble niobium concentration as a function of the finish rolling temperature (b) variation in microstructure as a function of the finish rolling temperature. Fig. 13.18 Soluble niobium in ULCB steel (Hulka et al., 1988). (a) Variation in the soluble niobium concentration as a function of the finish rolling temperature (b) variation in microstructure as a function of the finish rolling temperature. temperatures...

39

Fig. 2.19 (a) Nomarski differential interference contrast micrograph showing the general surface displacements due to upper bainite. (b) Higher magnification Nomarski image showing identical surface relief for all the sub-units within a given sheaf. (c) Sandvik's experiment showing the displacement of twin boundaries (parallel to the black line) caused by individual sub-units of bainite. The ferrite variants b1 and b2 belong to separate sheaves. Fig. 2.19 (a) Nomarski differential interference...

366

The drawing reduction is determined by the ability of the thinner, work-hardened section which leaves the die, to sustain the drawing force without further deformation. An alternative process achieves the reduction in section using inductive heating. The region of the rod which passes through the hot zone softens and is extended by the drawing force. It stops deforming on leaving the hot zone. The process avoids all the difficulties associated with die erosion and requires a smaller drawing...

44

Fig. 2.22 (a) and (b) show the effect of the Bain strain on austenite, which when undeformed is represented as a sphere of diameter wx yz in three-dimensions. The strain transforms it to an ellipsoid of revolution. (c) shows the invariant-line strain obtained by combining the Bain strain with a rigid body rotation. Fig. 2.22 (a) and (b) show the effect of the Bain strain on austenite, which when undeformed is represented as a sphere of diameter wx yz in three-dimensions. The strain transforms...

123 Ratio of Proof Stress to Ultimate Tensile Strength

If a material does not exhibit a sharp yield point, then it is necessary to define a proof stress which is the stress needed to produce a specified amount of plastic strain (usually 0.2 ). The strain rate of the test should also be defined but this is usually neglected because for steels there is only a 10 increase in the flow stress with an order of magnitude change in strain rate (Knott, 1981). Sharp yield points are not observed in stress-strain curves of bainite so it is usual to specify...

681 Stability

In Fig. 6.20, xm represents the maximum concentration of carbon that can be tolerated in ferrite which precipitates from austenite of composition x. A higher concentration cannot be sustained because there would be an increase in free energy on transformation. Growth with partial supersaturation, such as the case where the interface compositions are given by x xm and x7 x is expected to be unstable to perturbations since the concentration field must tend to adjust towards lower free energy...

611 Transformation Start Temperature

Widmanstatten Ferrite

It is a common observation that the Widmanstatten ferrite-start (Wg) and bainite-start (Bs) temperatures are more sensitive to the steel composition than is the Ae3 temperature. This indicates that the influence of solutes on the nucleation of Widmanstatten ferrite and bainite is more than just thermodynamic (Fig. 6.3a). Some clues to this behaviour come from studies of time-temperature-transformation diagrams, which consist essentially of two C-curves. The lower C-curve has a characteristic...

62 Possible Mechanisms of Nucleation

Widmanstatten Ferrite

Phase fluctuations occur as random events due to the thermal vibration of atoms. An individual fluctuation may or may not be associated with a reduction in free energy, but it can only survive and grow if there is a reduction. There is a cost associated with the creation of a new phase, the interface energy, Fig. 6.6 (a) Free energy curves for the nucleation of Widmanstatten ferrite and bainite in a low alloy steel for which the AGm and GN curves exhibit a double intersection, (b) Calculated...

8

Civil Term With Diagram

Assumed that bainite growth is diffusionless, any carbon supersaturation in bainitic ferrite being relieved subsequent to growth, by partitioning into the residual austenite. The atomic mechanism of bainite growth was not discussed in detail, but he believed that unlike martensite, there is no strain energy associated with the growth of bainite. Thus bainite should form at a temperature just below T0, where the austenite and ferrite of the same composition have identical free energies (Fig....

321 Precipitation within Lower Bainitic Ferrite

There are many observations that reveal the precipitation of carbides from supersaturated lower bainite in a process identical to the tempering of martensite. In situ hot-stage transmission electron microscopy has shown that the lower bainitic ferrite remains supersaturated with carbon some time after the completion of the ferrite growth (Kang etal., 1990). Unlike the microstructure of tempered martensite, the carbides tend to adopt a single crystallographic variant in a given plate of lower...

255 The Structure of the Interface

It has already been pointed out that any atomic height steps in the bainitic austenite interface are transformation dislocations, with strain fields whose character can be specified by assigning a Burgers vector to each such dislocation. The motion of these steps (or coherency dislocations) which are in forced coherency, leads to phase change there is continuity of planes and vectors across the steps so that regions of the parent lattice are homogeneously deformed into that of the product as...

139 High Strength Bainitic Steels without Carbides

We have seen in Chapters 2 and 3 that an interesting microstructure results when a silicon or aluminium-alloyed steel is transformed into upper bainite. The carbon that is partitioned into the residual austenite does not precipitate as cementite, but remains there to make the austenite stable at ambient temperature. The microstructure obtained consists of fine plates of bainitic ferrite separated by carbon-enriched regions of austenite (Fig. 13.21). The potential advantages of this mixed...

449

Carbide enrichment 107-8 concentration 9 distribution 71-2 equivalent 388, 398-9 partitioning 71-2, 150-2 redistribution 8 cast irons 388-96 CCT see continuous cooling transformation bainitic ferrite heating 234-5 coarsening 98-100 composition 101-8 cooperative growth 161-2 habit planes 77 orientation relationship 76-7 precipitation kinetics 191-4 Charpy test 298-300 chemical composition alloying elements 29-35 carbides 85-8 chemical driving forces 202-4 chemical potential 118-22 chemical...