Kinetics of martensitic transformation in partially bainitic 300M steel

H. K. D. H. Bhadeshia

This guide examines the research conducted by S. A. Khan and H. K. D. H. Bhadeshia regarding the phase transformations in high-silicon 300M steel. It focuses on the transition from austenite to martensite, particularly when preceded by a partial transformation to bainitic ferrite.

1. Overview of 300M steel and experimental objectives

300M steel is a high-silicon, medium-carbon commercial steel ($\text{Fe}\text{-}0.44\,\text{C}\text{-}1.74\,\text{Si}\text{-}0.67\,\text{Mn}\text{-}1.85\,\text{Ni}\text{-}0.83\,\text{Cr}\text{-}0.39\,\text{Mo}\text{-}0.09\,\text{V}$). High silicon concentrations are utilised because they demonstrate potential for excellent combinations of strength and toughness.

The primary objective of the research was to model the athermal kinetics of martensitic reactions in samples already containing bainitic ferrite. The study also aimed to investigate:

2. Relationship between bainite and martensite

The research challenges early assumptions that the presence of bainite necessarily deteriorates ductility and strength. Instead, the study highlights several key interactions:

Microstructural refinement

When lower bainite forms, it subdivides regions of parent austenite. This effectively refines the austenite grain size and the subsequent martensite packet size. This refinement leads to a strengthening of the martensite via an effective grain size effect. Furthermore, the strength of the bainite is enhanced by the constraint provided to its deformation by the surrounding stronger martensite matrix.

Chemical changes and $M_s$

The formation of bainitic ferrite results in the enrichment of carbon in the residual austenite. This carbon enrichment is a critical factor because it lowers the martensite start temperature ($M_s$) of the residual austenite. The study found that while the volume fraction of martensite increases with undercooling below $M_s$, the presence of bainitic ferrite does not significantly alter this relationship once carbon enrichment is accounted for.

Heterogeneity and reaction range

Chemical segregation, common in commercial steels, extends the temperature range over which the martensite reaction occurs. Heterogeneous samples exhibit a higher $M_s$ for residual austenite compared to homogenised samples because less bainite forms in the dilute regions of segregated samples, leading to lower average carbon enrichment in the austenite.

3. Kinetic modelling and autocatalysis

The Koistinen and Marburger equation

The kinetics of athermal martensitic transformation are often described empirically using the classic Koistinen and Marburger relation:

$$1 - f = \exp\{-C_1(M_s - T_q)\}$$

Where $f$ is the volume fraction of martensite and $T_q$ is the quenching temperature to which the sample is cooled. However, the study found that this model often fails at the very early stages of reaction because it neglects the driving effect of autocatalysis.

Autocatalysis in martensite formation

Autocatalysis refers to the process where the rapid formation of initial martensite plates induces new operational embryos, which are then available for further transformation loops. These sites are attributed to structural imperfections, such as arrays of dislocations created by the initial plates.

The new proposed model

The authors derived a new relationship that accounts for autocatalysis and treats the average volume of a martensite plate ($\bar{V}$) as a constant. This model is found to be in reasonable agreement with experimental data and can accurately predict martensite kinetics at all stages, including for samples partially transformed to bainite.

4. Short-answer quiz

Instructions: Review each question prompt and evaluate its metallurgical kinetics before expanding the panel to check the answer key.

1. What are the primary chemical components of 300M steel as specified in the study?
300M steel is a high-silicon, medium-carbon steel containing iron, $0.44\,\text{wt%}$ carbon, $1.74\,\text{wt%}$ silicon, and smaller specified concentrations of manganese, nickel, chromium, molybdenum, and vanadium. It is chosen for its potential for high strength and toughness driven by the silicon content.
2. How does the formation of bainitic ferrite affect the carbon concentration of the remaining austenite?
The formation of bainitic ferrite causes the residual austenite to become heavily enriched with carbon. This occurs because bainitic ferrite features an exceptionally low carbon solubility limit, forcing excess carbon solutes to partition across the interface into the remaining untransformed austenite matrix.
3. Why is the concept of a "martensite finish temperature" ($M_f$) considered fundamentally meaningless in this study?
The $M_f$ temperature is not a fundamental constant because the martensite reaction progresses in decreasing increments as temperature reduces and never truly reaches a sharp completion point. Instead, the transformation of the last traces of austenite becomes increasingly difficult due to mechanical stabilization and geometric constraints.
4. What is the "grain size effect" mentioned in relation to mixed bainite-martensite microstructures?
Isothermal bainite subdivides the prior austenite regions, which refines the effective grain size and the resulting martensite packet size. This refinement increases strength in the martensite, while the bainite plates are strengthened by the deformation constraints of the surrounding stronger martensite.
5. How does chemical segregation influence the $M_s$ temperature of residual austenite compared to homogenised samples?
Chemical segregation leads to a higher initial $M_s$ in residual austenite compared to homogenised samples because less bainite forms in the alloy-dilute regions. This results in lower carbon enrichment in those specific zones, allowing the martensitic transformation to begin at higher temperatures.
6. What experimental technique was used to record length, time, and temperature data at millisecond intervals?
The researchers performed high-resolution dilatometric experiments utilizing cylindrical rods $3\,\text{mm}$ in diameter. These runs were carried out on a custom high-speed dilatometer interfaced with a microcomputer to capture dilation signals at millisecond intervals.
7. What role does autocatalysis play in the early stages of martensite formation?
Autocatalysis accounts for the rapid transformation acceleration seen in early stages where existing martensite plates form and induce new operation embryos through localized strain fields. This creates additional operational nucleation sites that pre-existing defects cannot account for alone.
8. How does the stability of residual austenite change as the degree of bainitic transformation increases?
Residual austenite becomes increasingly stable against subsequent martensitic transformation as the volume fraction of bainite increases. This enhanced stability is driven by the higher carbon content partitioned into the austenite and the finely divided state of the phase blocks trapped between bainite sub-units.
9. According to the abstract, does the presence of bainitic ferrite significantly change the law governing the volume fraction of martensite produced by undercooling?
No, the study proved that the way the volume fraction of martensite increases with undercooling is not fundamentally altered by pre-existing bainitic ferrite. The transformation obeys the same operational kinetic law regardless of the presence of bainite, provided carbon enrichment in the residual austenite is accounted for.
10. What is the difference between "residual austenite" and "retained austenite" as defined in the study's terminology?
"Residual austenite" refers to the untransformed austenite phase that exists at the elevated isothermal reaction temperature during bainite growth. "Retained austenite" refers specifically to the final volume fraction of austenite that remains untransformed after the specimen has been cooled to ambient temperature.

5. Essay format questions

Instructions: Formulate comprehensive technical explanations based on solid-state transformation kinetics, using the guidelines in the hints for structural reference.

1. The impact of microstructural heterogeneity

Discuss how chemical segregation in commercial 300M steel alters the kinetics and thermodynamics of phase transformations compared to homogenised laboratory samples.

Key points for formulation: Address how solute banding (segregation of Mn, Ni, Cr) creates parallel fields of high and low hardenability. Detail how dilute zones transform first to bainite, causing localized carbon partitioning profiles that differ from the macroscopic average, thereby extending the temperature range of subsequent martensite curves.
2. Modeling martensite kinetics and autocatalytic terms

Compare the empirical Koistinen and Marburger equation with the new model proposed by Khan and Bhadeshia. Specifically, explain why the inclusion of autocatalysis is necessary for a more accurate prediction of the transformation.

Key points for formulation: Show that the standard Koistinen–Marburger model assumes a fixed, passive embryo density. Contrast this with the Khan–Bhadeshia approach, which models the dynamic expansion of nucleation sites triggered by the elastic strain field around newly formed plates, explaining the initial acceleration blips in high-speed dilatometry data.

6. Glossary of key terms

Term Definition
300M Steel A high-silicon, ultra-high-strength commercial steel alloy based on the AISI 4340 composition, optimized to suppress cementite precipitation during tempering.
Athermal Transformation A phase transformation that progresses solely as a function of temperature changes (undercooling steps) rather than being dependent on elapsed time at a constant temperature.
Autocatalysis The kinetic phenomenon where the rapid, displacive growth of an initial martensite plate creates new dislocation arrays, providing extra nucleation embryos for subsequent plates.
Bainitic Ferrite ($V_{\alpha_b}$) The acicular ferrite phase growing during intermediate isothermal holding; its growth drives excess carbon solutes directly into the remaining parent austenite phase.
Dilatometry A high-precision materials characterisation technique tracking relative length variations ($\Delta L/L$) to non-destructively monitor solid-state phase changes as a function of temperature or time.
Geometrical Partitioning The fragmentation effect where early martensite plates mechanically break up austenite domains into smaller isolated compartments, limiting the maximum volume ($\bar{V}$) of later plates.
Martensite Start ($M_s$) The critical temperature threshold at which the diffusionless, rapid transformation of austenite into metastable martensite initiates upon cooling.
Residual Austenite The volume fraction of parent austenite existing at the intermediate isothermal holding temperature during active bainite reaction.
Retained Austenite ($V_{\gamma_r}$) The final metastable volume fraction of face-centred cubic parent phase that fails to transform and persists down to ambient temperatures.
Undercooling ($M_s - T_q$) The thermal delta defining the temperature drop below the martensite start threshold down to the tracking quench level.