How martensite is formed?
Martensite is formed in carbon steels by the rapid cooling (quenching) of the austenite form of iron at such a high rate that carbon atoms do not have time to diffuse out of the crystal structure in large enough quantities to form cementite (Fe3C). A very rapid quench is essential to create martensite.
How is martensite formation promoted?
Two reasons for the promotion are stress concentrations at obstacles [18] (e.g. grain boundaries, twin boundaries, etc.), which assist the transformation [19] and the creation of new nucleation sites by plastic straining [20].
What are the phases of tempered martensite?
Tempering involves a three-step process in which unstable martensite decomposes into ferrite and unstable carbides, and finally into stable cementite, forming various stages of a microstructure called tempered martensite.
What is Spheroidite martensite?
A spheroidite refers to a microscopic constituent in some steels, composed of spherically-shaped cementite particles in an alpha ferrite matrix. Spheroidite is the most ductile and softest type of steel on the granular molecular level. Its purpose is to soften higher carbon steels and produce more formability.
What are Sorbite and Troostite?
Structures of the lower pearlite stage with very fine flakes are referred to as sorbite and troostite. Their structure can no longer be seen under an optical microscope. Generated pearlite with a ball-like or concentrated cementite phase is the exception.
What microstructure is martensite?
Martensite was originally named for a very hard, very brittle phase of steel that has needle-shaped microstructural features, with a microstructure being the arrangement of the phases on the microscopic scale. In steel, martensite forms due to the very fast cooling of a high-temperature phase called austenite.
What is the difference between martensite and pearlite?
Like martensite, pearlite is created by quenching steel, usually with water or oil. However, the key difference between it and martensite lies in the rate at which it is cooled. Pearlite is cooled more slowly than its martensite counterpart, making it softer and easier to bend.
What is the tempering process?
tempering, in metallurgy, process of improving the characteristics of a metal, especially steel, by heating it to a high temperature, though below the melting point, then cooling it, usually in air. The process has the effect of toughening by lessening brittleness and reducing internal stresses.
What steps are included in martensitic hardening and tempering?
Tempering of plain carbon steels On reheating as-quenched martensite, the tempering takes place in four distinct but overlapping stages: up to 250°C, precipitation of α-iron carbide; partial loss of tetragonality in martensite. between 200 and 300°C, decomposition of retained austenite.
What are the two different 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 sorbite is formed?
Detailed Solution If austenite is slowly cooled (furnace cooling), it will decompose to form the mechanical mixture of ferrite and cementite called pearlite. An increased rate of cooling (air cooling) of steel, leads to the formation of finely dispersed pearlite, known as sorbite (at about 600°C)
What is martensite formation?
Martensite formation is favoured by low austempering temperature and short holding time. Lower bainite is the predominant morphology in irons transformed below 330 °C. The formation of this structure is substantially independent of austempering time and iron composition. Ferrite plates grow rapidly into the austenite.
What is the end result of tempering of martensite?
The end result of tempering is a fine dispersion of carbides in an α-iron matrix, which often bears little structural similarity to the original as-quenched martensite. Martensite is a very strong phase but it is normally very brittle so it is necessary to modify the mechanical properties by heat treatment in the range 150-700°C.
What is the stability of martensite at room temperature?
This instability-increases between room temperature and 2500°C, when iron carbide precipitates in the martensite. During stage 2, austenite retained during quenching is decomposed, usually in the temperature range 230-300°C.
Where does martensite grow in the nucleus?
The martensite initially grows perpendicular to, and principally on, one side of the {1 1 1} γ slip plane associated with the nucleus, very likely corresponding to the side of the dislocations with missing half-planes since α -martensite is less dense than austenite. Figure 12.11.