A Doctor of Philosophy dissertation submitted to the University of Cambridge in 1979 by Harshad K.D.H. Bhadeshia, titled The Theory and Significance of Retained Austenite in Steels, contains a series of counter-intuitive insights into creating steels with unusual combinations of strength and toughness.
The star of this story is a microscopic "flaw"—a small amount of leftover material called 'retained austenite'. Conventional wisdom might suggest that any leftover ingredient from a chemical reaction is a sign of an incomplete or failed process. However, this research elegantly proves the opposite. This article will explore the most surprising takeaways from this foundational research and how a deliberate imperfection makes steel better.
The central finding of the research revolves around what is termed the "incomplete reaction phenomenon." When steel is heat-treated to transform its microstructure from a high-temperature phase (austenite) to a strong, tough phase (bainite), the process is deliberately halted. It does not go to completion, leaving behind small, stable quantities of the original austenite.
This is a counter-intuitive concept. In most manufacturing, an incomplete process is a defect. Here, it is the entire point. This process isn't stopped by an external timer; it's a self-limiting reaction. As carbon enriches the leftover austenite, it reaches a thermodynamic equilibrium point beyond which further non-equilibrium transformation is impossible, locking the desired microstructure in place. The research demonstrated that under controlled conditions, this leftover, or "retained," austenite is not a flaw but a critical ingredient. It is directly responsible for conferring exceptional properties to the final material. As the dissertation's abstract states, this microstructure is not just good, it's superior to those created by traditional methods.
It was shown that the retention, stability and morphology of austenite could be directly derived from the basic transformation mechanism. Under certain circumstances, the bainitic retained austenite conferred exceptional strength/toughness properties to silicon steels; these were shown to be superior to the properties associated with tempered martensite microstructures.
But for this reaction to halt so perfectly, leaving behind a beneficial material instead of a mess, requires a specific chemical traffic cop. That role is played by an unassuming element: silicon.
The success of this "incomplete" transformation hinges on a key ingredient: silicon. While often used in steel for other reasons, the research highlights its role in bainitic steels.
Silicon's critical function is to act as a blocker. During the heat treatment process, it actively inhibits the formation of brittle iron carbides, specifically a type called cementite. By preventing these carbides from forming, silicon forces the excess carbon atoms to go somewhere else. They migrate into the small amounts of remaining austenite, enriching it. This carbon enrichment is what makes the austenite stable enough to be "retained" as a distinct phase at room temperature.
The effect is remarkably powerful. The research notes that in the upper bainite studied, there was "no detectable carbide precipitation even after holding at 350°C for 74 hours." Silicon essentially pauses a natural chemical reaction, allowing for the creation of a sophisticated and beneficial composite structure at the micro-level.
The research makes another critical distinction: not all retained austenite is created equal. Its effectiveness depends entirely on its shape and location (its 'morphology') within the microstructure. The dissertation identifies two distinct types: a beneficial "film type" and a detrimental "blocky type."
The film type is confined as thin, stable layers between the plates of bainitic ferrite. The blocky type, in contrast, originates from the geometrical partitioning of the original, larger austenite grains. The difference is critical. The stable films act like reinforcing fibers woven throughout the material, while the unstable blocks are more like undesirable, brittle pockets that can crack under pressure. These blocks are larger, less stable, and according to the research, "must be avoided." Under stress, this blocky austenite is prone to transforming into brittle martensite, which is undesirable and harmful to the steel's toughness. This discovery elevates the science from simply retaining some austenite to precisely engineering its shape and distribution.
However, it is wrong to believe that all blocky austenite is bad -- it has to be coarse.
One might assume that the transformation of steel from one crystalline form to another is a gradual, continuous process of growth. There is, however, an alternative reality.
Bainite forms through a "displacive" mechanism. Instead of a slow, steady growth front, the structure forms through the "successive nucleation of displacive sub-units." The research uses more evocative language to describe this, stating that the bainitic ferrite forms by 'martensitic jumps' composed of smaller units.
A microscopic construction process that happens in discrete, rapid steps. Tiny sub-units form almost instantaneously, one after another, to build the larger bainitic plate clusters. The structure observed is the result of a step-by-step assembly process that is anything but smooth. This staccato growth process, happening sub-unit by sub-unit, is precisely what creates the space for the thin, stable 'films' of austenite to form, locking in the steel's toughness.
Bhadeshia's 1979 dissertation shows how properties can be tailored by understanding the atomic mechanisms of solid-state phase transformations. By understanding the fundamental mechanisms of transformation and leveraging the specific functions of common elements like silicon, it is possible to design materials that defy conventional expectations.