Steel - Phase Transformations

910 Steel: Phase Transformations Types of Phase Transformations Lo diffusive: occurs through diffusion Lo displacive: occurs through rapid atom displacement Diffusive Le thermally activated needs T>0.3-0.4Tm Lo atoms diffuse from site to site Lo atom move from 1 to 10 atomic spacing Lo composition of phases can change due to diffusion Lo speed is strongly dependent on temperature Lo extent of transformation depends on temp. & time Displacive Lo atoms move short distances < interatomic spacing Lo atoms move by breaking and remaking atomic bonds Lo atoms move in preuse sequence (military transformation) La composition cannot change since there is no time for diffisuion Lo speed of transformation very rapid Lo extent of transformation depends on temp. only Examples of Phase Transformations Martensite (Displacive) Lo martensitic transformation displacive transformation of austenite Lo formed when avstentised Fe-C alloys are rapidly cooled or quenched to a relatively low temp (amount of transformed phase depends on final temp not time) Lo occurs when quenching rate is rapid enough to prevent carbon diffusion Lo there is only a slight displacement of each atom relative to its neighbours Lo Austenite (FCC) Martensite (BCT) Lo properties: plate-like or needle-like microstructure dislocation motion very difficult in highly strained structure hard & brittle (hardness tests) (Content Res) application: wear resistance components Bainite (Displacive & Diffusive) Lo a product of austenite transformation Le initial transformation is displacive (doesn't require diffusion) Le first ferrite phase rejects excess carbon, forcing the carbon to diffuse, & this forms very fine cementite layers. Le microstructure consists of ferrite & cementite phases formed as needles or plates, depending on the temp of the transformation Hardness: Pearlite < Bainite < Martensite 9.11 Steels TTT Diagram Lo Time-Temperature-Transformation Equilibrium vs Non-Equilibrium Lo equilibrium: assume very slow cooling rates / thermodynamically stable phases non-equilibrium transformations which have both temperature & time dependence formation of martensite & bainite in non-equilibrium plot (TTT plot), we can conditions these phases can form see under what Isothermal Phase Transformation