The microstructure formed by a diffusionless, shear-type transformation when austenite is quenched. In steel it is body-centered tetragonal, supersaturated with carbon, and very hard.
Japanese name: マルテンサイト
Martensite appears in 11 practice questions across 5 topics, including Heat treatment and metallic materials, Materials processing and joining, Surface treatment and tribology.
When steel is cooled from the austenite region faster than the critical cooling rate, carbon atoms have no time to diffuse and form cementite. Instead, a shear-type transformation occurs in which iron atoms shift cooperatively by small amounts, and FCC austenite changes into body-centered tetragonal (BCT) martensite supersaturated with carbon. Atoms move less than an interatomic distance, and the transformation proceeds at nearly the speed of sound. The composition does not change across the transformation, and a distinctive feature is that the amount transformed is set by the temperature reached, not by the holding time.
The martensitic transformation starts at the Ms temperature during cooling, progresses as the temperature falls, and is nearly complete at the Mf temperature. Ms and Mf fall as the carbon and alloy contents increase, and in high-carbon steels Mf is below room temperature, so retained austenite that has not transformed remains. Retained austenite transforms over time and causes dimensional changes, so precision parts are given a subzero treatment, cooling below 0 °C. The volume expansion and temperature differences during quenching also cause quench cracking and distortion.
The hardness of martensite is determined mainly by its carbon content; the large lattice strain from supersaturated carbon and the high dislocation density strongly obstruct dislocation motion. On the other hand, martensite is very brittle, so it is normally tempered before use. During tempering, the supersaturated carbon precipitates as fine carbides and internal stresses relax, so hardness falls but toughness recovers substantially. Low-temperature tempering is used for tools and high-temperature tempering for shafts and gears (combined with the preceding quench, this is called quenching and tempering), giving a combination of hardness and toughness suited to the application.
The martensitic transformation is not unique to steel. In Ni-Ti alloys, the martensitic transformation and its reverse give the shape memory effect and superelasticity, used in medical wires and actuators. The tetragonal-to-monoclinic transformation of zirconia is also martensitic and is exploited in transformation toughening. All share the same mechanism of cooperative atomic shifts without diffusion, and the crystallography of the transformation determines macroscopic properties. Transformation temperatures vary greatly with alloy composition, so composition control is important.
Answer: B. It is a diffusionless shear transformation producing body-centered tetragonal (BCT) martensite supersaturated with carbon
When austenite is quenched faster than the critical cooling rate, carbon has no time to diffuse, and a shear-type transformation occurs in which atoms shift cooperatively by small amounts, producing body-centered tetragonal (BCT) martensite supersaturated with carbon. Because the carbon distorts the lattice along the c axis, martensite is very hard and brittle. The transformation starts at the Ms temperature and progresses as the temperature falls, with almost no dependence on holding time. Pearlite and bainite are the structures that separate into ferrite and cementite with carbon diffusion, and it is pearlite that forms more readily the slower the cooling.
Answer: A. A TTT diagram shows transformation when the steel is quenched to a fixed temperature and held there, while a CCT diagram shows transformation during continuous cooling
A TTT diagram shows the start and finish times of transformation when austenitized steel is quenched to a given temperature fast enough to avoid transformation and then held at that temperature; it is used to design isothermal treatments such as austempering. When the temperature falls continuously, as in actual quenching, the CCT diagram applies, and its transformation start line is shifted to longer times and lower temperatures than in the TTT diagram. The choice assigning continuous cooling to the TTT diagram and isothermal holding to the CCT diagram swaps the two definitions. Both are separate diagrams with temperature on the vertical axis and time (logarithmic) on the horizontal axis, and cooling faster than the nose gives martensite.
Because carbon dissolved in supersaturation greatly distorts the lattice, and the transformation also introduces many dislocations and twins, making dislocations very hard to move. Its hardness is determined mainly by the carbon content.
If the cooling rate is slower than the critical cooling rate, pearlite or bainite forms along the way. The centers of thick sections cool slowly, so alloy steels with high hardenability or quenchants with high cooling power are used.
In steels whose Mf is below room temperature, it is austenite that remains untransformed after quenching. It transforms over time and changes dimensions, so it is reduced by subzero treatment or tempering.
When austenite is quenched faster than the critical cooling rate, carbon has no time to diffuse, and a shear-type transformation occurs in which atoms shift cooperatively by small amounts, producing body-centered tetragonal (BCT) martensite supersaturated with carbon.
Which statement about the martensitic transformation in ste…Both are separate diagrams with temperature on the vertical axis and time (logarithmic) on the horizontal axis, and cooling faster than the nose gives martensite.
Which correctly describes the difference between a TTT (iso…Hardenability is the depth to which martensite can be obtained by quenching (how readily the steel hardens).
Adding alloying elements such as Mn, Cr and Mo to steel imp…As-quenched martensite is hard but very brittle and carries large internal stresses.
What is the main purpose of tempering a quenched steel?In the annealed condition it is nearly nonmagnetic, but heavy cold working converts part of it to strain-induced martensite, and it becomes attracted to a magnet.
Which correctly describes the properties of the 18%Cr-8%Ni …Heat-affected zone Pearlite Hardenability Stainless steel Tempering Carbon equivalent Passivity Sensitization Superelasticity Dislocation