Does Bainite form with or without diffusion? : The experimental and theoretical evidence

With the increased interest in bainitic steels, fundamental understanding of the bainite transformationis of major importance. Unfortunately, the research on bainite has been hampered by an oldcontroversy on its formation mechanism. Over the years two quite different theories have developedclaiming to describe the bainite transformation i.e. the diffusionless and the diffusion controlledtheory. In this thesis, attention is directed towards fundamental understanding of the bainitetransformation and both experimental and theoretical approaches are used in order to reveal its truenatureIn the first part of this thesis the symmetry in the Fe-C phase diagram is studied. It is based on ametallographic mapping of microstructures using light optical microscopy and scanning electronmicroscopy in a high carbon steel. The mapping revealed symmetries both with respect to temperatureand carbon content and an acicular eutectoid with cementite as the leading phase was found andidentified as inverse bainite. By accepting that all the eutectoid microstructures forms by diffusion ofcarbon, one may explain the existence of symmetries in the Fe-C phase diagram. Additional supportof its existence is obtained from an observation of symmetries in an alloyed steel. From the performedwork it was concluded that the existence of symmetries among the eutectoid microstructures fromaustenite supports the idea that bainite forms by a diffusion controlled transformation.In the second part the growth of bainite is considered. An experimental study using laser scanningconfocal microscopy was performed and growth rates of the transformation products from austenite ina high carbon, high chromium steel was analysed. The growth rate measurements reveals the kineticrelation between Widmanstätten cementite and the acicular eutectoid previously identified as inversebainite which confirms its existence and the conclusions drawn in the first part. In addition, in-situobservations of bainite formation below Ms provide additional support for the diffusion controlledtheory for bainite formation.The final part of the work is a study of the critical conditions for the formation of acicular ferrite.Based on experimental information found in the literature a thermodynamic analysis is performed inview of the two theories. The results demonstrate that the governing process for Fe-C alloys cannot bediffusionless but both kinds of processes can formally be used for predicting Bs temperatures for Fe-Calloys.QC 20130503</p

Empirical methods to predict bainite start conditions

There are many empirical equations describing the effect of alloying elements on the bainite start temperature, Bs, without using Fe-C alloys as a reference. The numerical values of the resulting coefficients do not represent the physical effects of the elements. A new empirical equation is now derived by starting from binary Fe-C alloys from high temperatures, where acicular ferrite is regarded as Widmanstätten ferrite, and extrapolated to lower temperatures. It was accepted that this is not different from bainitic ferrite. Effects of alloying elements were added separately using information from ternary alloys. Information from sets of alloys with the same alloy content but different carbon contents proved particularly useful. Lines connecting such points were regarded as  lines for the respective alloy content and the effect of alloy elements was evaluated from their distances from the Bs line for Fe-C alloys. To compare the new equation with Steven and Haynes’, Bs information was collected and organized in two tables according to the experimental method with 258 entries of critical temperatures at fixed composition and 344 entries of critical temperatures at constant alloy content. The predictive abilities were similar but the new coefficients may be closer to the physical alloying effects.QC 20180518</p

Empirical methods to predict bainite start conditions

There are many empirical equations describing the effect of alloying elements on the bainite start temperature, Bs, without using Fe-C alloys as a reference. The numerical values of the resulting coefficients do not represent the physical effects of the elements. A new empirical equation is now derived by starting from binary Fe-C alloys from high temperatures, where acicular ferrite is regarded as Widmanstätten ferrite, and extrapolated to lower temperatures. It was accepted that this is not different from bainitic ferrite. Effects of alloying elements were added separately using information from ternary alloys. Information from sets of alloys with the same alloy content but different carbon contents proved particularly useful. Lines connecting such points were regarded as  lines for the respective alloy content and the effect of alloy elements was evaluated from their distances from the Bs line for Fe-C alloys. To compare the new equation with Steven and Haynes’, Bs information was collected and organized in two tables according to the experimental method with 258 entries of critical temperatures at fixed composition and 344 entries of critical temperatures at constant alloy content. The predictive abilities were similar but the new coefficients may be closer to the physical alloying effects.QC 20180518</p

Empirical methods to predict bainite start conditions

There are many empirical equations describing the effect of alloying elements on the bainite start temperature, Bs, without using Fe-C alloys as a reference. The numerical values of the resulting coefficients do not represent the physical effects of the elements. A new empirical equation is now derived by starting from binary Fe-C alloys from high temperatures, where acicular ferrite is regarded as Widmanstätten ferrite, and extrapolated to lower temperatures. It was accepted that this is not different from bainitic ferrite. Effects of alloying elements were added separately using information from ternary alloys. Information from sets of alloys with the same alloy content but different carbon contents proved particularly useful. Lines connecting such points were regarded as  lines for the respective alloy content and the effect of alloy elements was evaluated from their distances from the Bs line for Fe-C alloys. To compare the new equation with Steven and Haynes’, Bs information was collected and organized in two tables according to the experimental method with 258 entries of critical temperatures at fixed composition and 344 entries of critical temperatures at constant alloy content. The predictive abilities were similar but the new coefficients may be closer to the physical alloying effects.QC 20180518</p