What is the microstructure of martensite?
Microstructure is the arrangement of the phases on the microscopic scale. A microscope can be used to observe a material’s microstructure. The microstructure of martensite contains many needle-shaped features, which cause martensite to be very brittle.
What is isothermal martensite?
Isothermal (i.e. time dependent) martensite formation in steel was first observed in the 40ies of the XXth century and is still treated as an anomaly in the description of martensite formation which is considered as a-thermal (i.e. independent of time).
What is the grain structure of martensite?
Martensite is a supersaturated solid solution of carbon in ferrite with a body-centered tetragonal (BCT) structure. Upon rapid cooling, carbon is trapped in the crystal structure.
What are the two morphologies of martensite?
In this study, two different types of martensite were observed: lenticular (Chelyabinsk LL5, Odessa IAB) and packet/lath (IVB and ungrouped ataxites, Seymchan PMG). These structures are formed at different temperatures and nickel content.
How do you form a martensite microstructure?
Martensitic steels have a predominantly lath martensite microstructure, as shown in Figure 2.9, and are formed by continuous annealing in the austenite region followed by rapid quenching of the steel. Higher hardenability is achieved by increased carbon content, typically of the order of 0.25 weight percent carbon.
What determines the hardness of martensite?
This is based on describing the strength contributions of the dislocation density in lath and plate martensite, precipitates and retained austenite. The strength of the matrix is the main contributor to the overall hardness, followed by precipitation, which effects increase with reducing the austenite volume fraction.
What is the use of time temperature transformation curve?
Time-temperature-precipitation diagrams and time-temperature-embrittlement diagrams have also been used to represent kinetic changes in steels. Isothermal transformation (IT) diagram or the C-curve is associated with mechanical properties, microconstituents/microstructures, and heat treatments in carbon steels.
What are the characteristics of martensite transformation?
The transformation is diffusionless and fast, proceeding at rates near the speed of sound. This implies that large numbers of atoms transfer rapidly and in an orderly fashion from the high-temperature structure (austenite) to the low-temperature structure (martensite).
What phases are present in martensite?
The pseudocubic martensite (Fe-C alloy) is typically described as tetragonal phase.
What kind of structure is a martensite made of?
Martensite Formation Martensite is a hard, brittle form of steel with a tetragonal crystalline structure, created by a process called martensitic transformation.
What is the mode of transformation of martensite?
The mode of transformation is called athermal transformation (meaning without thermal activation). Martensite transformation starts to transform at the martensite transformation temperature, Ms, and ceases on reaching a given temperature. Only additional cooling can drive the transformation further.
Why is martensite not shown in equilibrium phase diagram?
Martensite is not shown in the equilibrium phase diagram of the iron-carbon system because it is not an equilibrium phase. Equilibrium phases form by slow cooling rates allowing sufficient time for diffusion, whereas martensite is usually formed by fast cooling rates.
What is the difference between martensite and austenite?
One of the differences between the two phases is that martensite has a body-centered tetragonal (BCT) crystal structure, whereas austenite has a face-centered cubic (FCC) structure. Martensite has a lower density than austenite, so that the martensitic transformation results in a relative change of volume.