Bainite

Bainite (named after the U.S. metallurgists Edgar C. Bain ) is a structure which can be caused by isothermal transformation or continuous cooling in the heat treatment of carbon steel.

Synonym to bainite is used in German-speaking countries, the term intermediate structure. Is formed at temperatures and cooling rates, which are between those for the formation of martensite or pearlite. Unlike the formation of martensite here Umklappvorgänge are coupled in the crystal lattice, and diffusion processes, thereby converting different mechanisms are possible. Due to the function of cooling rate, carbon content, alloying elements and the resulting formation temperature of bainite has no characteristic structure. Bainite, as well as perlite, from the phases ferrite and cementite ( Fe3C ), but differs in shape, size and distribution of the pearlite. A distinction is made in two main structural forms, the upper bainite (also granular bainite ) and lower bainite.

Austempering or isothermic conversion in the bainite is a austenitization followed by quenching to temperatures above the martensite start temperature Ms. The cooling rate must be chosen so that there can be no transformation in the pearlite. When holding at the temperature above Ms, the austenite transforms as completely as possible to bainite.

Through a slow flip-over of the austenite arise from the grain boundaries or impurities, highly supersaturated in carbon ferrite crystals with cubic raumzentriertem crystal lattice ( bcc lattice ). The carbon is deposited in the lower bainite due to the higher rate of diffusion in the bcc lattice in the form of spherical or ellipsoidal cementite within the ferrite grain from. When upper bainite, the carbon can diffuse into the austenite and form carbides.

The upper bainite is formed in the upper temperature range of bainite, he has a needle-like structure, which is strongly reminiscent of martensite. The favorable conditions for the diffusion of the carbon diffuses to the grain boundaries of the Ferritnadeln. It caused irregular and broken cementite here. Because of the irregular distribution of the structure often has a granular appearance. In case of insufficient metallographic analysis, the structure can be easily confused with perlite or even the Widmanstatten structure.

The lower bainite formed during isothermal and continuous cooling in the lower temperature range of bainite. Through the formation of ferrite, the austenite carbon accumulates, on further cooling, the Austenitbereiche in ferrite, cementite, acicular bainite and martensite transform. By austempering residual stresses are reduced and the toughness increases, so that offer this method for crack- sensitive steels and complex shaped components.

• 7.1 kinetics
• 7.2 carbon allocation in the transformation front
• 7.3 carbide
• 7.4 orientation relationship
• 7.5 Restaustenitstabilisierung

• 9.1 kinetics
• 9.2 carbon allocation in the transformation front
• 9.3 carbide
• 9.4 orientation relationship
• 9.5 Restaustenitstabilisierung

Bainitmorphologien

Bainite is a structure which is formed from the austenite at temperatures below the pearlite to martensite through both isothermally and in a continuous cooling. One distinguishes upper and lower bainite. Upper bainite is composed of acicular ferrite, which is arranged in packets. Between the individual Ferritnadeln more or less continuous film of carbides parallel to the needle axis in front. Contrast, Lower bainite consists of ferrite within which form the carbides at an angle of 60 ° to the needle axis. Under certain conversion conditions other Bainitmorphologien as inverse, granular or langnadeliger bainite may arise, as is illustrated in Figure 1 ( Bainitmorphologien ).

Definitions of bainite

Currently found in the literature, three different definitions of bainite, which lead to considerable misunderstanding. A distinction

• The micro- structural definition,
• The kinetic definition and
• The surface relief definition,

Build on the special phenomena of the phase transition, so that can not be decided without further about their general validity or non- validity.

The micro- structural definition

Then in iron-based materials bainite is considered as consisting nichtlamellares product of eutectoid decomposition of ferrite and carbide. The two product phases form a diffusion-controlled time in succession, the carbides to precipitate first formed in the ferrite or at the interface either. Lacks the secretion of the second phase from the thermodynamic or kinetic reasons, as it is possible in the conversion of silicon-containing steels, we expected according to this definition is actually no longer speak of bainite. The definitions made allow, however, to speak in non-ferrous metals of bainitic transformations.

The kinetic definition

This definition assumes that in the isothermal and continuous TTT diagram for the beginning and end of the bainite pearlite distinguishable from those of the curves occur and thus a separate formation kinetics of the bainite exists. The bainite transformation is through a sluggish area whose extent is strongly influenced by alloying elements, be separated from the pearlite transformation. Since some steels can be detected despite the absence of conversion contracts range bainite, this definition turns out to be unsuitable.

The surface relief definition

The relationship of the bainitic transformation with the martensitic shows itself in a surface relief appearance. This is compatible therewith to view bainite plate-shaped phase, which occurs above the martensite start temperature Ms by shearing from the austenite lattice. The conversion is done by a coordinated, non-thermally activated atom transfer across the moving phase boundary. The kinetics of the conversion is determined by the diffusion in the austenite Interstitionsatomen, which can take place both before and after shearing. This surface relief definition is currently the most common Bainitfestlegung.

Nucleation

Bainitnadeln ( Sheaves ) are elongated plates begin their thicker ends at grain boundaries. They comprise ferritic subunits (subunits ), which more or less completely, as indicated in figure 2, are separated by carbides or austenite. The abutting subunits are separated by small angle grain boundaries and show themselves an elongated slats or plate form, as it is the cheapest by Nabarro for educated in a field of tension phases ( see also the electron microscopy shown in Figure 3). It is currently agreed that in unalloyed hypoeutectoid and silicon-containing steels hypereutectoid the formation of the lower and upper bainite with a kohlenstoffübersättigtem Ferritkeim begins. Only in non-silicon alloyed steels hypereutectoid may be the first phase formed at higher transition temperatures also cementite. This is called inverse bainite.

The Ferritkeimbildung of the bainite takes place mostly at the austenite grain boundaries due to thermal lattice vibrations by a cooperative Gitterscherung and less frequently in other lattice defects. After a critical radius is exceeded, the nucleus is capable of growth and forms a subunit ( subunit ). New nucleation sites ( sympathetic nucleation ) are formed at the boundary surfaces of the first Bainitkeims. A nucleation in the austenite is possible despite there elevated carbon content, since a high-energy α - γ - interface is replaced by a low-energy α - α - interface, so that this is the necessary energy for nucleation available. The nucleation rate increases with increasing undercooling below the equilibrium temperature. For the sub-units are smaller and more numerous because of the growth of the sub-units stops when new nuclei at their phase boundaries. The size of the subunits is independent of the austenite grain size and the Bainitnadelgröße. The latter is bounded by the austenite grain boundaries and existing needles. On the other hand go Olson, Bhadeshia and Cohen in a recent paper assumes that the nucleation of the bainite is, like that of the martensite on the presence of preformed germs. Are viable embryos critical size is accepted, that the problem of nucleation is reduced to the start of microbial growth. The sympathetic nucleation is explained by the fact that it comes through the growing Bainitnadeln to adapt deformations in the austenite with dislocation arrangements in the vicinity of the growing needle that match preformed germs.

Microbial growth

In the temperature range of bainitic transformation takes place virtually no diffusion of atoms of the matrix, while at the same time a high permeability of the carbon and nitrogen atoms is added. The phase boundary between austenite and ferrite is partially coherent, and may be viewed as made ​​up of the interface dislocations. The conversion is done by thermally activated glide of this interface by the atomic lattice, with larger movements of the matrix atoms without exchange processes take place. This scherungsbedingte, martensitähnliche conversion is controlled by the diffusion of the Interstitionsatome that occur slowly in comparison with the speed of a moving interface.

Bhadeshia considered the coupled processes of carbon diffusion and Gitterscherung associated with the thermally activated motion converting front. During the waiting time of the conversion front of an obstacle to the next activating event diffusion processes can occur, lower the free energy of the product phases, thereby increasing the driving force for the interfacial motion. After the obstacle is the conversion front running free again, without being hindered by diffusion processes, until it encounters the next obstacle.

This idea contrasts with a diffusion model in which the growth of bainitic ferrite of the diffusion- controlled movement of stages ( Ledges ) is attributed to the α - γ interface, so the same mechanism, which is also associated with the formation of the proeutectoid ferrite with Widmann sites - structure is discussed. Sandvik notes, however, that the deformation twins occur in the deformed austenite are overrun by growing Bainitnadeln and find themselves as lattice dislocations in the ferrite. A conversion by diffusion-controlled movement of steps but would have to stop at twin boundaries, since there the necessary lattice coherence is disturbed. The takeover of the lattice defect in the ferrite contradicts a diffusion-controlled transformation. It is important in accordance with the detection conducted by Dahmen that a surface relief can also occur by a diffusion controlled transformation and is therefore no clear indication of a scherungskontrollierten conversion.

Thermodynamics

The driving force of a conversion is given by the difference of the free energies of the initial phases and the product phases. This need not necessarily set the equilibrium phases that have the largest difference in the free enthalpy of the output phases. Both the martensitic and the bainitic transformation lead to a metastable state. These states lie with their energy content above the equilibrium state to a relative minimum, and can be moved under certain conditions while releasing energy in the direction of balance. Such metastable states can be reached in the bainitic transformation, for example, carbon-rich ferrite in equilibrium with ε - carbide. The occurrence of concentration gradients, by the differences in the free energy within the phases can be very different, leading to metastable states.

Figure 4 shows the dependence of the free energy of the phases α and γ from their carbon content. An equilibrium reaction of γ with the carbon concentration is effected to Xγ α with the carbon concentration Xγα and γ with the carbon concentration Xαγ. The two equilibrium concentrations lie on a tangent with the equation

Which bears both the α - and on the γ - parabola.

Into α and Xαγ to reach the equilibrium concentration of Xγα in γ is required a strong carbon diffusion. The free enthalpy of the γ - phase some of Gγ drops to Gαγ, while the free energy of a converted into α volume element is greatly reduced on Gγα. The overall system thereby decreases from its free energy by the amount? G. The driving force for the transformation is given by ΔGα.

In case of modified conversion conditions provided a sufficient driving force to drain, set in the product phases of Xγα or Xαγ different carbon concentrations, a non-equilibrium reaction. In Figure 5, it is assumed that the austenite transforms with concentration Xγ > Xγα in ferrite with the concentration Xα. Was the conversion diffusion-controlled pure, so the driving force ΔGα would be dissipated solely by the movement of the diffusion field before converting Front (? G = ΔGα ), in which then the concentration would set Xm < Xαγ. However, if a share ΔGs for the movement of the phase boundary and the scherungsbedingten, cooperative atom transfer across the moving interface is required, there arises the concentration Xi < Xm one.

The distribution of ΔGα in ΔGd and ΔGs arises from the condition that the diffusion must proceed at the same speed as the shear. Through this coupling of diffusion and shear, the carbon concentrations form dynamically from in front of the moving interface, as Figure 14 shows. The highest carbon concentration of the austenite Xi arises in the interface. The carbon diffused from there into the austenite, whereby the carbon content of austenite increases Xγ (dashed line in Figure 14). Xγ is approaching the value Xm, an additional reaction is impossible, since no Enthalpieabsenkung of the overall system? G occurs more. The bainitic transformation ceases and can only by lowering of Xγ eg be continued by carbide formation, or by lowering the temperature.

Austenite

Prerequisite for a fully running bainitic transformation is the formation of carbides from the austenite. Since Carbide absorb large amounts of carbon, they represent a carbon sink, vacuum the carbon from the austenite. Carbon enrichment in austenite, which - as shown above - would bring the conversion to a standstill, are no longer possible. The carbide is prevented or delayed, for example, by silicon as an alloying element, larger austenite amounts are not converted. They are then quenching to room temperature before in whole or in part as austenite. The austenite is dependent on how far the martensite has lowered the remaining austenite.

Lower bainite

The lower bainite is formed at relatively low conversion temperatures below the transition temperature to the upper bainite, to below the martensite starting temperature. Theoretically, lower bainite can form to martensite. Figure 6 shows the microstructure of lower bainite in silicon steel 80Si10.

Formation kinetics

Vasudevan, Graham and axon provide for the formation of bainite detects a change of the transformation kinetics when falling below a temperature of 350 ° C and identified the conversion product as lower bainite. This grows with an activation energy of 14 000 cal / mol ( 0.61 eV), which is discussed in the context of carbon diffusion in supersaturated ferrite as the rate-determining process. Because of the increasing carbon content increases the volume jump at the α → γ transformation with falling transition temperatures.

Radcliffe and Rollason give as activation energy for the formation of lower bainite values ​​7500-13000 cal / mol ( 0.33 to 0.56 eV), J. Barford such 14500-16500 cal / mol ( 0.63 to 0.72 eV ) to. In this case, it is assumed that a separate conversion mechanism for the lower bainite.

Carbon allocation in the transformation front

At the low transition temperatures can diffuse out into the austenite due to the low diffusivity of the carbon in austenite and the measured high conversion speeds, no significant portion of the carbon from the ferrite. Thus, first there occurs a martensitic transformation of austenite at near full carbon supersaturation, so that the carbon content of the ferrite formed is almost equal to that of austenite. Figure 7 illustrates this fact. The high carbon content in ferrite may decrease after conversion either by the formation of carbide in the ferrite, or by diffusion away in the austenite.

Carbide

Initially it was thought that, in the formation of lower bainite, the carbides are directly excreted at the interface from the austenite so that minimizes the interfacial energy. However Bhadeshia could prove that the carbides precipitate during the conversion from the ferrite.

Similar to the tempered martensite, the carbides form within the Ferritnadeln in the same crystallographic directions with angles to the needle axis of about 60 ° (see Figure 8). It is usually first order ε - carbide ( Fe2, 4C ), which merges into cementite after long conversion times. The precipitation of the carbides of transition behind the front lowers the carbon saturation of the ferrite and thus the free energy of the structure. The Carbidform corresponds to the state of minimum strain energy. The number and distribution of fine carbides are responsible for the good mechanical properties of the lower bainite.

In connection with the situation of the excreted ε - carbides at an angle of 60 ° to Ferritnadelachse was assumed that the precipitates form of deformation twins. But it was no correspondence between the orientation of the carbide precipitates and the twin planes in the ferrite are found. Therefore, it was assumed that the carbide precipitation occurs oriented energy reasons.

But you proved that arise from the growing ferrite during deformation of austenite twins. These twins in the austenite are sheared by the conversion front and " converted " into the bcc lattice. These lattice defects are formed in the course of conversion Carbide. How can it be explained that the habit plane of the carbide precipitation corresponds to no twin plane in ferrite.

In the model developed by Spanos, fishing and Aaronson mechanism of carbide formation, as outlined in Figure 9, from long Ferritkeimen (1 ) is assumed, that creates in the second step by sympathetic nucleation more Ferriteinheiten (2). The area enclosed between the Ferriteinheiten austenite accumulates by diffusion from the ferrite strongly to carbon, until it comes to the formation of carbides in the austenite ( 3). The final step is to close the gaps around the carbides, since now more ferrite in the - can be Austenitbereichen - now carbon impoverished. Wandering small-angle grain boundaries of existing same orientation differences between the Ferriteinheiten so that their former boundaries with almost disappear (4).

Orientation relationship

After Bhadeshia occurs between austenite and ferrite of lower bainite primarily on the Kurdjumov -Sachs orientation relationship.

( 2.6)

At the same orientation relationships exist after Nishiama Aquarius.

( 2.7)

The two orientation relationships differ only by about 5 °. As an orientation relationship between ferrite and cementite is true for the lower bainite

( 2.8)

In a recent study, one finds, however, the orientation relationship after Bagaryatski

( 2.9)

Met. Finally, it is not possible Shackleton and Kelly to demonstrate an orientation relationship between cementite and austenite for the lower bainite. It is concluded that the cementite is precipitated in the lower bainite within the ferrite, and not from the austenite.

The ε - carbides demonstrate Dorazil, Podrabsky and Švejcar orientation relationships on the austenite to ferrite as well, which by itself

Can be described. After that can not be decided for the ε - carbide, whether it is excreted from the bainitic ferrite, or from the austenite.

Restaustenitstabilisierung

Since the temperatures in the lower bainite, there is hardly carbon partitioning, the bainitic reaction can usually proceed to completion, so that little or no residual austenite remains. If the reaction is, however, ended prematurely by quenching, then the not yet converted to bainite austenite transforms to martensite and it can vary depending on carbon content and alloy composition austenite remain.

Alloying of silicon carbide formation is suppressed in the C- supersaturated ferrite. Therefore, the carbon diffuses into the not yet converted austenite where it increases the carbon content, to the bainitic transformation comes to a standstill. The then still unconverted Austenite is so heavily enriched with carbon, that it is present at room temperature austenite after quenching.

Transition temperature from the lower to upper bainite

Another aspect of the controversial bainite is the transition temperature from the lower to the upper bainite. It is believed that this - as shown in Figure 10 - with increasing carbon contents of 400 ° C. to about 550 ° C at 0.5 mass % C increases. With further increase in carbon content arises at constant conversion speed a higher supersaturation of the ferrite formed, so that the carbon diffuses more slowly in the austenite. Correspondingly higher transition temperatures for a sufficient carbon diffusion are always needed in the austenite, so that there can form carbide precipitates. Exceeds the state of the alloy, the extrapolated Acm line of the Fe - Fe3C - chart, the alloy is almost übereutektoid and it leads to precipitation of carbides in the austenite, which is the formation of upper bainite. Therefore, the transition temperature decreases above 0.7 mass % C. to 350.degree. Below this temperature, the precipitation of carbides in the austenite is slower than that of the ferrite and forms lower bainite.

The increase in the transition temperature for small carbon contents, as has been observed it, but stems from the definition, that the transition temperature is the highest temperature at which carbide still leaves the ferrite. Since in the course of formation of upper bainite, especially after long conversion times, because of the carbon enrichment in austenite, thereby increasing supersaturation of the ferrite, also quite carbide may be eliminated in the ferrite, the advance curve does not represent the transition of the formation mechanisms. Rather, it returns the transition from the upper to the lower bainite of the hypothetical Fe ε carbide state diagram. Figure 11 shows that leaves the ferrite transformation temperature below 350 ° C. ε - carbide. Accordingly, it is the transition temperature constant regardless sets at 350 ° C determined by the carbon content. The excretion of ε - carbide according to this theory, the most important mechanism of the formation of lower bainite. The metastable ε - carbide then transformed after long conversion times in stable cementite to.

Another way of looking at the transition temperature is proposed as follows: It is assumed that falls below the transition temperature, a change of the conversion mechanism takes place, which has its own kinetics and its own operating temperature, the demonstrated capacity to between the bainite and martensite ( see Figure 12). The transition temperature increases as the other two curves with decreasing carbon content, because the required driving force and thus the undercooling decreases also for the formation of lower bainite with the carbon content. The experimentally observed decrease of the transition temperature at low carbon contents is assessed here as Härtbarkeitsproblem. The Austenitzerfall begins after a short time, so that already is on upper bainite transformation temperature on cooling. Only at lower transformation temperatures, the samples cool quickly enough. The excretion of ε - carbide from the supersaturated ferrite is represented as a race of elimination against the diffusion away of the carbon in the austenite. According to the existing carbon in the ferrite to ε - carbide formation is only sufficient at higher carbon steels, which was confirmed experimentally.

Upper bainite

In transition temperatures below the range of pearlite formation and above the region of formation of lower bainite is formed of upper bainite. The carbon diffusion in the austenite phase transformation it is of crucial importance. Figure 13 shows the microstructure of upper bainite in silicon steel 80Si10.

Formation kinetics

In the temperature range between 350 ° C and 400 ° C are found for the conversion of an activation energy of 34,000 cal / mol ( 1.48 eV), which corresponds approximately to the carbon diffusion into γ - iron (1.34 eV). Above 350 ° C, a constant carbon content of 0.03% is observed in the ferrite, which is close to the equilibrium concentration. At the same time, a linearly decreasing with increasing conversion temperature change in length of the sample is observed.

Alternatively, the activation energy is found for the formation of upper bainite values ​​18000-32000 cal / mol ( 0.78 to 1.39 eV), or such a 22,000 to 30,000 cal / mol (0.95 to 1 30 eV ).

Carbon allocation in the transformation front

The ferrite of the upper bainite containing a lower carbon content than the austenite from which it originated, but it is still oversaturated. This supersaturation decreases with increasing transformation temperature due to the increasing diffusion away in the austenite, which greatly enriched by this mechanism of carbon. At low transformation temperatures a carbon content of Xm is the speed limit near the boundary is reached ( see Figure 14), since the carbon diffusion runs delayed in the austenite. The bainitic reaction comes to a grinding halt and thus can continue only by renewed sympathetic nucleation. This allows the decreasing with decreasing transition temperature width and growing number of Bainitaggregate explain. The high enrichment of carbon in austenite is reduced by carbide formation. Is carbide impossible, for example, by high Si contents, so quantities remain austenite in the microstructure.

Carbide

If the austenite enclosed by growing Ferritnadeln, he accumulates so strong that carbides may precipitate from the austenite. It is always to cementite, which is secreted directly from the enriched austenite. The carbides of the upper bainite are always in the form of more or less continuous Carbidfilme between Ferritnadeln (cf. Figure 15). With increasing carbon content of the alloy, the Ferritnadeln be thinner, the Carbidfilme discontinuous and occur more frequently. It is found that the nucleation of carbides by the tensions that arise in the surrounding austenite by the shaping of the growing Ferritnadeln, is facilitated. From studies of the orientation relationship of carbide, austenite and ferrite, it is concluded that the carbides are also formed by a Gitterscherung upper bainite. This theory contradicts Aaronson and shows that both the formation of bainitic ferrite and carbides can be explained by a diffusion-controlled transformation.

Orientation relationship

Is observed between austenite and ferrite of the upper bainite, the orientation relationship after Nishiyama - Wassermann, which is also valid for the lower bainite. Within the accuracy of the diffraction patterns produced can also be valid for the Kurdjumov -Sachs relationship. For the orientation between cementite and austenite Pitsch suggests the relationship

Pickering, however,

Before.

After Pickering no orientation relationship between ferrite and cementite are observed, from which he concludes that the cementite precipitates not from the ferrite, but from the austenite.

Restaustenitstabilisierung

Reichert the austenite is highly-carbon, so, if the enrichment is not reduced by the formation of carbides, bainite come to a standstill. This phenomenon is referred to as " phenomenon of incomplete conversion " in the definition of kinetic bainite. In the temperature range of this incomplete conversion, the nucleation of cementite is disabled. This can be achieved by the addition of chromium or silicon. In both cases, the enriched austenite is proving to be very stable against quenching to room temperature, so that may be left behind significant amounts of retained austenite, which significantly influence the mechanical properties of the alloy.

Influence of alloying elements on the formation of bainite

The estimation of the influence of alloying elements on the formation of bainite is relatively complex, since the kinetics of the reactions occurring often does not change in proportion to the amounts of alloying additions. To make matters worse, that affect the elements against each other in their effect. Alloying components which form a substitutional solid solution with the iron phase, the bainitic transformation influence indirectly, since in this temperature range of bainite formation no substitution atomic diffusion occurs. Thus, the growth kinetics of the bainite can be changed by influencing the rate of diffusion of carbon due to the alloying element. Qualitatively viewed reduce the elements manganese, nickel, chromium and silicon from the Bainitstarttemperatur and lengthen the conversion time. The elements chromium, molybdenum, vanadium and tungsten lead in the TTT diagram for a separation of the perlite and bainite by the formation of a sluggish transformation range.

• Carbon is the main influencing factor with respect to the morphology of the bainite. With increasing carbon content, the width of the growth comes Bainitnadeln earlier account of the difficulty carbon diffusion to a halt. Accordingly, the Bainitnadeln become thinner and more numerous. Increasing a carbon content also promotes the formation of carbides from the ferrite, both ( the lower bainite ), as well as from the austenite ( the upper bainite ). With an increasing carbon content, the incubation time is extended and lowered the bainite starting temperature (Bs).

• The addition of chromium, the incubation period is also extended and lowered Bs. The increase in Austenitbeständigkeit can go so far that in certain temperature ranges for long periods there is no more conversion and a carrier conversion range occurs.

• Silicon raises the AC1 and AC3 temperature of the metastable Fe - Fe3C diagram and shifts the eutectoid concentration at lower carbon contents. The kinetics of the pearlite and of bainite is not greatly influenced by silicon. Silicon is practically insoluble in cementite.

• Manganese greatly increases the austenite stability in both the pearlite and bainite in which can result in manganese steels to high Restaustenitgehalten and the conversion times in the bainite can be relatively long. This is also for the bainitic transformation, the quenching and tempering improved. Manganese is soluble in cementite and forms with carbon Mn3C with a cementite isomorphic to the structure.

• The addition of nickel leads such as chromium or manganese to a lowering of BS. At high nickel contents, the range of the full bainitic transformation builds a strong, for example in the temperature range up to 10 ° C above the martensite start temperature upon addition of 4% nickel.

• Molybdenum increases the AC3 temperature without the AC1 temperature influence. It delays especially the proeutectoid ferrite precipitation and the formation of pearlite. This makes it easier for large components, the cooling to the temperatures of the bainitic transformation without preliminaries of ferrite or pearlite.

• The ferrite and pearlite are also greatly delayed by boron. The pearlite shifts to longer transition periods during bainite formation is unaffected. Especially in the continuous conversion so pure bainitic microstructure can be generated. It is important that the nitrogen present is bound by aluminum or titanium, as the resulting boron nitride otherwise cause embrittlement.

The bainitic transformation in silicon steels

In silicon-containing steels result, compared to the already described mechanism of bainitic transformation in silicon steels, some special features owing silicon suppresses the formation of cementite. Since the carbide formation is a prerequisite for a complete bainitic transformation, it comes in silicon steels to incomplete conversions with high Restaustenitgehalten. Studies on silicon steels can provide important clues for elucidating the formation of bainitic ferrite, since the transformation products are not changed by a subsequent carbide formation.

Silicon is practically insoluble in cementite. The growth of a Zementitkeimes presupposes the diffusion away from silicon, which can take place only very slowly in the transformation temperatures of bainite. To the Zementitkeim a Siliziumgradient builds up the local activity of the carbon greatly increased (see Figure 16). Wherein the carbon flow is reduced to Zementitkeim so that the seed does not grow further.

The transformation in the region of the upper bainite runs in silicon steels because of difficult carbide formation in two phases. In the first phase is formed bainitic ferrite with a relatively high rate of formation, with the surrounding austenite is heavily enriched with carbon. In the second phase, which began in silicon steels only after very long times, then carbides form from this enriched austenite. By lowering the carbon content in the austenite ferrite formation can continue to run, and Sekundärferrit is formed by lateral growth of the existing Ferritnadeln. In the area of the lower bainite ε - carbides are deposited after short conversion times within the ferrite, since silicon has little influence the ε - carbide formation. Only the transformation of the ε - carbide in cementite is suppressed by the presence of silicon. Due to the existing carbide formation, the lower bainite Restaustenitmengen lower than the upper bainite. Carbides found can not be identified as cementite, because they contain substantial amounts of silicon. Also Roehrig and Dorazil report the occurrence of Silicocarbiden after a long conversion in the temperature range of the upper bainite.

At higher silicon content and transition temperatures between 350 ° C and 400 ° C large Restaustenitbereiche may arise, which are only slightly enriched with carbon, and have a negative effect on the mechanical properties of the alloy. In the austenite which is trapped between growing Ferritnadeln, there are deformation twins, which point to the locally high carbon content of the austenite between the Ferritnadeln.

Phenomenon of incomplete conversion

It is observed that the bainitic transformation when approaching BS runs always incomplete until it comes to a standstill at BS. After some time, in which nothing is done, is a pearlite. If now by adding alloying elements, the temperature range of the pearlite to bainite higher or shifted to lower temperatures, the result is a temperature range in which conversions, if ever run, only after very long times. It explains this phenomenon with the oppressed carbide formation at higher temperatures. The austenite is enriched with carbon quickly, so that the conversion after a short time comes to a standstill.

Also on the phenomenon of incomplete transformation ignited the controversy surrounding the mechanism of bainite formation. Bradley and Aaronson lead the transformation inertial range back to a " Solute Drag Effect Like " ( SDLE ). This model assumes that substitution atoms can not diffuse freely through the atomic lattice in the temperature range of bainite, but accumulate in the moving phase boundary. If it is to elements which lower the carbon activity, the driving force for diffusion away the carbon from the ferrite to the austenite decreases. This effect lowers the conversion speed and, in extreme cases, the moving during the conversion phase boundary after a short time, by the formation of carbides within this interface, bring to a standstill. Add a comment directly Bhadeshia and Edmonds argue, because there are examples of alloying elements, the lower the carbon activity, but do not cause a sluggish transformation range. Furthermore, can the SDLE only the slow conversion range between bainite and pearlite explain, but not the second conversion sluggish area was found between lower bainite and upper bainite.

Mechanical properties of bainitic iron -based alloys

Strengthening Mechanisms

The most important occurring in the bainitic microstructure strengthening mechanisms are grain boundary strengthening, transfer solidification, the solid solution strengthening and dispersion strengthening.

In the grain boundary strengthening, the question arises how a particle size in the bainitic microstructure is defined. One possibility is the former austenite grain size, which indirectly the length of Bainitnadeln and the size of the packets determines which are composed of several needles. Edmonds and Cochrane found for austenitic no relation to the strength properties, while for the packet size, the relationship

Find.

One defines the width of each Bainitnadeln as grain size and determined

The Hall - Petch relationship is the same. Since the Ferritnadeln lower conversion temperature are smaller and more numerous, the observed increase in strength can be justified.

Depending on the transition temperature are in the bainitic ferrite high dislocation densities 109-1010 cm -2 before. The dislocation density increases due to the decreasing indentation from the ferrite with increasing transition temperatures. It is all the higher, the more carbides are present.

Only some of these dislocations participate as glide dislocations to the plastic deformation. Their movement through the metal mesh is hindered by the spatial structure of the nonslip dislocations, the dissolved impurities, the carbides, and by grain and phase boundaries. The proportion of dislocation hardening can be achieved by quantitatively

Estimate. α1 is a constant, G is the shear modulus, b the magnitude of the Burgers vector and ρ the total dislocation density.

The interaction between slip dislocations and lying in the slip planes Interstitions or substitution atoms lead to a voltage component

Wherein α2 and M are constants, and C is the impurity concentration. Dissolved in the bainitic ferrite increases carbon conversion with decreasing temperature, leading to increased solid-solution strengthening.

The carbides in upper bainite affect the strength properties only to the extent that they favor the crack formation and propagation. With the glide dislocations do not interact because they are at the Ferritnadelgrenzen. In lower bainite, the carbides formed in the ferrite cause precipitation strengthening, the voltage component of the

Supplies. This is ne, the number of carbide particles per mm 2, and A, B are constants.

For determining the strength properties of mixtures of different phases of the mixture is usually

Proposed. Here Vi is the volume fraction of the microstructure and i? I the characteristic strength value dar. This estimate has proved for the mixture of upper bainite and martensite as appropriate. In the mixture of lower bainite with martensite but there is a larger inaccuracies. The mixture of bainite with retained austenite can be judged according to this formula, as long as the austenite does not transform to martensite.

Influence of residual austenite on the mechanical properties

It is found that the austenite and morphology strongly affects the toughness properties of different high silicon content steels due to the high ductility and conversion ability of retained austenite. During the deformation of states with a higher carbon concentration in the austenite transforms martensite, while the deformation of conditions with a lower carbon content twinning is observed in the austenite. The austenite in which the largest elongation at break occurs is indicated with 33 to 37 % by volume. Samples with higher austenite (up to 50 % by volume) again have inferior toughness properties. The reason for this lies in the morphology of the retained austenite. At lower Restaustenitgehalten the austenite is like a film between the Ferritnadeln and acts as a lubricating film between the harder Ferritaggregaten, whereby the deformability of the structure is improved. The contribution of residual austenite to the total deformation is due to the strain-induced martensite is very high, so that an enlargement of the austenite to improve the elongation at break is equal. At higher Restaustenitmengen the austenite orders blocky and its deformation mechanism changes from the strain-induced martensite to deform by twinning. Since further increasing austenite content of the block- shaped arrangement proportion of retained austenite increases, this results from an austenite of 37 vol% back to sinking elongations. This relationship is also responsible for the conversion decreasing with increasing temperature KIC value.

Deformation and strength behavior

The isothermal bainite transformation offers a number of advantages. In the area of the lower bainite in addition to high strength excellent toughness properties are achieved, as is shown for steels with a carbon content of 0.1 to 1%. In this case, the chromium content is from 0 to 1 % and the silicon content of 0.1 to 0.6% was varied. At conversion temperatures between 400 and 600 ° C, a yield ratio of 0.6 to 0.8 was determined. For tensile strength over 850 N/mm2 the converted in the bainite steels showed a superior ductility compared to normal tempered steels. This very good mechanical properties of bainite are retained down to the lowest temperatures. Furthermore, the elongation at fracture, reduction of area and impact strength is higher than that of comparable strength after normal remuneration. The creep rupture strength, fatigue strength and fatigue life are favorably influenced by this heat treatment process.

The transition from lower to upper bainite causes a jump in the transition temperature impact resistance. Upper bainite shows the higher transition temperatures, which is attributed to the different carbide. The facet size of the cleavage fracture surfaces is consistent with the size of the Bainitkolonien. Possibly Present martensite leads to a reduction of the facet size.

Sometimes bainitic steels show a rather low yield strength. Schaaber makes for an incompletely implemented conversion charge. According to his investigations, the yield strength reaches a maximum only when the highest possible conversion rate is achieved. In addition to the yield strength, the fatigue strength is particularly sensitive to incomplete conversion.

Materials with bainitic microstructure states are successfully used for valve springs and plates, as the bainitic microstructure bring benefits in the fatigue strength and fatigue strength of these components. It can be shown that the fatigue strength is higher than the bainite transformed samples of hardened and tempered samples of comparable tensile strength. It is to make sure that a possible bainitic transformation. Here, the bainitic microstructure is characterized by the fact that it can reduce effectively by internal or external notches and cracks generated voltage spikes.

The transformation in the bainite is not only due to the good mechanical properties of interest, but also from the aspect of low distortion and virtually crack-free hardening heat treatment. Due to the relatively high transition temperatures of both the quench and the transformation voltages are much lower than in the conventional curing. In addition, the transformation in the bainite is associated with significantly smaller changes in volume than the martensitic transformation.

Cyclic deformation behavior at room temperature

In cyclic stress of steels can be distinguished four stages of fatigue after Macherauch: The elastic-plastic cyclic deformation stage, the micro-cracking stage, the stage of stable crack propagation and finally the fatigue fracture. In hardened steels, the cyclic deformation stage and micro-cracking predominates only occurs shortly before the stress fracture. When normalized or quenched and tempered steels yet highly stable crack propagation can comprise a significant part of life depending on the stress level.

For elastic-plastic cyclic deformation provides the stress- strain relation Total hysteresis loops, which at sufficiently stabilized material behavior different parameters may be taken in accordance with Figure 17. In voltage- controlled trial management can be the total strain amplitude εa, t and determine the plastic strain amplitude εa, p as a function of number of load cycles N. Cyclic hardening ( softening ) is associated with a decrease (increase) of εa, p, and therefore also of εa, t connected. In total strain controlled trial leadership to face, however, the stress amplitudes σa and the plastic strain amplitude εa, p as the dependent variables. A cyclic hardening ( softening ) is an increase (decrease) in σa and a decrease (increase) of εa, p linked. If the dependent variables as a function of the logarithm of the number of cycles at a given stress amplitude, the result is called cyclic deformation curves. Taking out these related pairs of values ​​of σa and εa, p and εa, t and transmits them against each other, we obtain the cyclic stress - strain curve. This may such as a stress-strain curve of the tensile test, cyclical stretching and proof stress are removed.

The cyclic deformation curves allow conclusions on the material behavior during cyclic loading. Normalized steels usually show after a quasi- elastic incubation period a number of load cycles interval strong Wechselentfestigung to which a lifetime range connects with exchange consolidation. The observed Wechselentfestigung is due to the occurrence of Dehnungsinhomogenitäten that run as fatigue Lüders bands on the test section.

Even tempered steels show after an incubation period a strong Wechselentfestigung that continues to crack initiation. With increasing voltage amplitude thereby decreases both the incubation time as well as the service life. Since a formation of dislocations is very unlikely because of the present high dislocation density, which occur plastic deformation to be generated by rearrangement of the existing dislocation structure. For hardened material states offer enhanced opportunities for the dislocations to the elastic interaction with the solute in an elevated non-equilibrium concentration of carbon atoms, which leads to a change of solidification. Since it is low by tempering the proportion of the dissolved carbon, the interaction possibilities of the dislocations with the carbon atoms and the dislocation structure transformations reduce lead to Wechselentfestigungen.

For stable crack propagation, the cyclic plastic deformation at the crack tip are crucial. The crack propagation is determined by the stress range of stress intensity DELTA.K. The crack length growth per load cycle is

Described, c and n are constants. For double- logarithmic plot of da / dN on DELTA.K results in a linear relationship. Below a threshold value of DELTA.K occurs no more crack growth. At very high values ​​DELTA.K unstable crack propagation leads to breakage.