Chapter 10 Phase Diagrams: The Road Map to Phases and Structures 215 Very Rapid Cooling—Martensite Metal cooled below 500°F (260°C) so fast that it does not begin to transform to pearlite or bainite is said to “miss the nose” of the IT di agram. Such rapidly cooled metal undergoes a radically different transformation. There is literally no time for carbon atoms to diffuse anywhere, so the austenite instead undergoes a diffusionless transformation. The iron-carbon alloy forms small, sharp-edged plates with needlelike particles, called martensite, Figure 10-23. The particles consist of iron in a body-centered tetragonal unit cell with carbon in some of the unit cells. The carbon atoms impose high internal stresses at the unit cell level, so dislocation motion is effectively absent. For this reason, martensite is the hardest product of any transformation from austenite, with the highest tensile strength. Martensite and BCT Unit Cells When carbon steel is cooled rapidly from the austenite phase, the “extra” carbon in the alloy forces the iron that would normally become body-centered cubic (bcc) ferrite to instead become body-centered tetragonal (bct) martensite, Figure 10-24. The carbon in the alloy distorts the cubic cell into a tetragonal cell, which is longer in one dimension. DID YOU KNOW? DID YOU KNOW? No diffusion occurs during the martensite transformation, so it starts when steel cools to the martensite start temperature, Ms, before any pearlite or bainite begins to form. The martensite transformation then proceeds through the sample at the speed of sound. As the sample is cooled further to the martensite finish temperature, Mf, the sample will form as much martensite as allowed by the 10 μm ASM International Figure 10-23. This sample of an alloy with 1.39% carbon was quenched rapidly to form martensite. The dark portions are very fine martensite needles, with sharp points at the ends. Goodheart-Willcox Publisher Figure 10-24. Body-centered cubic ferrite can hold no more than 0.022% carbon. When a higher-carbon alloy is quenched rapidly to form martensite, the body-centered cubic unit cell becomes longer in one direction, forming a body-centered tetragonal cell. This change allows more space for the carbon atoms to fit in, as illustrated by the two tetragonal unit cells here. Higher carbon content makes the tetragonal distortion larger. Copyright Goodheart-Willcox Co., Inc.