Critical Driving Forces for Formation of Bainite

An empirical equation for predicting bainite start temperatures of steels was recently derived by starting from binary Fe-C alloys and continuing with ternary Fe-C-M alloys. This result is now illustrated with a family of B_S lines in a T,C diagram for a series of constant Mn contents. The critical driving force for the formation of ferrite is calculated for diffusionless or diffusional processes, and these quantities are used as dependent variables with carbon content or temperature as independent variables. Negative critical driving forces are predicted for a diffusionless process in binary Fe-C alloys, showing that this process cannot apply to the formation of bainite. The critical driving force for a diffusional process increases strongly with decreasing temperature and increasing carbon content. Mn and Ni, contrary to Cr, Mo and Si, have remarkably small effects on this critical driving force. The results are discussed by imagining that the magnitude of the critical driving force is governed by the height of an energy barrier that must be surmounted during growth. It is modeled as completely determined by the alloy composition. It is represented with an equation evaluated by fitting to the recent empirical equation and describing the carbon dependence of the barrier.     

Bainite in the light of rapid continuous cooling information

Rapid continuous cooling of pure iron can produce three different transformations yielding acicular structures: Widmanstätten α, lath martensite, and lenticular martensite. The information on their extensions into binary systems with carbon, nickel, and chromium has been reviewed, and admittedly rough methods have been used for estimating growth rates in order to examine the role of diffusion. The effect of alloying elements on their plateau temperatures and growth rates indicates that Widmanstätten α in Fe-C alloys grows under conditions close to local equilibrium for carbon, and it is suggested that the same should hold for edgewise growth of bainite. In Fe-Ni alloys, there are indications that Widmanstätten α grows under a considerable solute drag, an effect which may also occur for bainite. In Fe-Cr alloys, the solute drag effect seems to be weaker but may increase with the carbon content.     

Spontaneous and Deformation‐Induced Martensite in Austenitic Stainless Steels with Different Stability

The fraction and microstructure of spontaneous and deformation‐induced martensite in three austenitic stainless steels with different austenite stability have been investigated. Samples were quenched in brine followed by cooling in liquid nitrogen or plastically deformed by uniaxial tensile testing at different initial temperatures. In‐situ ferritescope measurements of the martensite fraction was conducted during tensile testing and complemented with ex‐situ X‐ray diffractometry. The microstructures of quenched and deformed samples were examined using light optical microscopy and electron backscattered diffraction. It was found that annealing twins in austenite are effective nucleation sites for spontaneous α'‐martensite, while deformation‐induced α'‐martensite mainly formed within parallel shear‐bands. The α'‐martensite formed has an orientation relationship near the Kurdjumov‐Sachs (K‐S) relation with the parent austenite phase even at high plastic strains, and adjacent α'‐martensite variants were mainly twin related (<111> 60° or Σ3).     

Eutectoid Transformations in 4.12 Mass Pct Cr 0.88 Mass Pct C Steel

The sequence of eutectoid microstructures, obtained by lowering the temperature of the isothermal transformation, was studied in synthetic steel with 4.12 mass pct Cr 0.88 mass pct C. The results were compared with observations on plain carbon steels with 1.65 and 1.67 mass pct C. In both cases, the main features can be explained as an effect of a lowered temperature on the increasing supersaturation of cementite in austenite and an even stronger effect on that of ferrite. One distinction was a continuous change in the pearlite structure toward a more acicular structure. This structure is named acicular pearlite.     

Widening of Laths in Bainite

Units of bainite in Fe-C alloys from the upper temperature range inherit their shape from Widmanstätten plates of ferrite, which are lathlike. The thickness increases by long-range diffusion of carbon and the length by short-range diffusion of carbon from the advancing edge of the tip. Both have been studied extensively and are fairly well understood. Widening growth seems to have been much neglected, but a study of some aspects of widening is now presented. The present report is the last one in a series of four morphological studies of bainite, isothermally formed in Fe-C alloys with 0.3 or 0.7 mass pct carbon, mainly in the upper temperature range. It contains a number of morphological observations made on cross sections of packets of bainite, and it elucidated a number of interesting questions about bainite and resulted in some proposals. The ferrite plates in a packet are nucleated as a group on a grain boundary, not each one separately on the side of a prior plate. Lengthening occurs by advancement of a short edge that is formed in close contact to the grain boundary. Widening of laths does not start spontaneously. It is initiated by a modification of the structure of the long edge of the lath. When it then moves, the lattice of the new ferrite is rotated relative to the ferrite formed by lengthening and the habit plane is different. In a section through the length direction, it is difficult to recognize what part of ferrite has formed by widening growth. Furthermore, it is proposed that the individual plates in a microstructure, previously used to illustrate subunits formed by repeated nucleation, were nucleated on a hidden grain boundary.     

DICTRA, a tool for simulation of diffusional transformations in alloys

In the present paper, a general survey of the diffusion-controlled transformations (DICTRA) software is given. DICTRA is an engineering tool for diffusion simulations in multicomponent alloys. The simulations are based on multicomponent diffusion and thermodynamic data, both obtained by analyzing and assessing experimental information. This allows for many different cases to be studied as soon as the underlying data are available. DICTRA is not a complete simulation tool because only geometries that can be transformed into one space variable can be treated, but many well posed problems of practical interest may be solved. The program contains several different models, which are discussed in the present paper. Each model has its own applications and several examples from recent simulations are given in order to demonstrate the usage of the particular models.     

DICTRA, a tool for simulation of diffusional transformations in alloys

In the present paper, a general survey of the diffusion-controlled transformations (DICTRA) software is given. DICTRA is an engineering tool for diffusion simulations in multicomponent alloys. The simulations are based on multicomponent diffusion and thermodynamic data, both obtained by analyzing and assessing experimental information. This allows for many different cases to be studied as soon as the underlying data are available. DICTRA is not a complete simulation tool because only geometries that can be transformed into one space variable can be treated, but many well posed problems of practical interest may be solved. The program contains several different models, which are discussed in the present paper. Each model has its own applications and several examples from recent simulations are given in order to demonstrate the usage of the particular models.     

Deformation Microstructure and Deformation-Induced Martensite in Austenitic Fe-Cr-Ni Alloys Depending on Stacking Fault Energy

The deformation microstructure of austenitic Fe-18Cr-(10-12)Ni (wt pct) alloys with low stacking fault energies, estimated by first-principles calculations, was investigated after cold rolling. The ?-martensite was found to play a key role in the nucleation of α′-martensite, and at low SFE, ? formation is frequent and facilitates nucleation of α′ at individual shear bands, whereas shear band intersections become the dominant nucleation sites for α′ when SFE increases and mechanical twinning becomes frequent.     

Simulation of the Growth of Austenite from As-Quenched Martensite in Medium Mn Steels

As part of an ongoing development of third-generation advanced high-strength steels with acceptable cost, austenite reversion treatment of medium Mn steels becomes attractive because it can give rise to a microstructure of fine mixture of ferrite and austenite, leading to both high strength and large elongation. The growth of austenite during intercritical annealing is crucial for the final properties, primarily because it determines the fraction, composition, and phase stability of austenite. In the present work, the growth of austenite from as-quenched lath martensite in medium Mn steels has been simulated using the DICTRA software package. Cementite is added into the simulations based on experimental observations. Two types of systems (cells) are used, representing, respectively, (1) austenite and cementite forming apart from each other, and (2) austenite forming on the cementite/martensite interface. An interfacial dissipation energy has also been added to take into account a finite interface mobility. The simulations using the first type of setup with an addition of interfacial dissipation energy are able to reproduce the observed austenite growth in medium Mn steels reasonably well.