When quenching a steel, how does the cooling rate influence the final microstructure by suppressing diffusion-controlled transformations and favoring non-diffusional ones?

When quenching a steel, which is a rapid cooling process, the cooling rate profoundly influences the final microstructure by directly impacting the ability of atoms to diffuse. Diffusion is the movement of atoms within the material, driven by thermal energy and concentration gradients, and it is a time-dependent process. Diffusion-controlled transformations, such as the formation of pearlite or bainite from austenite, require significant atomic rearrangement over time. Austenite is the face-centered cubic (FCC) crystalline structure of iron, stable at high temperatures, which can dissolve a considerable amount of carbon. Pearlite is a lamellar (layered) mixture of ferrite (body-centered cubic, BCC, low carbon iron) and cementite (iron carbide, Fe3C), formed when carbon atoms diffuse out of the austenite lattice and iron atoms rearrange. Bainite is another diffusion-controlled product, forming at lower temperatures than pearlite, involving partial diffusion of carbon atoms. When the cooling rate is very high, as in quenching, the steel is rapidly cooled past the temperatures where these diffusion-controlled transformations typically occur. This rapid cooling minimizes the time available at elevated temperatures, thereby suppressing the long-range diffusion of carbon atoms and the substantial rearrangement of iron atoms required to form pearlite or bainite. Essentially, the atoms do not have enough time or thermal energy to move into their equilibrium positions to form these more stable, softer phases. Instead, the rapid cooling favors non-diffusional transformations, primarily the formation of martensite. Martensite is a supersaturated solid solution of carbon in iron with a body-centered tetragonal (BCT) crystalline structure. It forms when the austenite is supercooled below its martensite start temperature (Ms) before significant diffusion can occur. Below Ms, the transformation proceeds by a shear-like, cooperative shift of iron atoms from the FCC austenite lattice to the BCT martensite lattice. This transformation is effectively instantaneous, or athermal, meaning it depends on temperature rather than time, and does not require long-range diffusion of carbon. The carbon atoms, unable to diffuse out, become trapped interstitially within the distorted BCT lattice, causing significant internal stresses and high hardness. Therefore, a sufficiently high cooling rate, exceeding a critical cooling rate that bypasses the 'nose' of the TTT (Time-Temperature-Transformation) diagram, ensures that austenite transforms predominantly into hard, brittle martensite by suppressing the slower, diffusion-dependent transformations.