Martensite is a hard form of steel created through martensitic transformation, which makes it popular for tools and swords. It is brittle and made up of austenite, which transforms into ferrite and cementite when cooled naturally. Rapid cooling in quenching traps carbon atoms and changes the crystal structure, increasing hardness. Tempering partially transforms martensite into ferrite and cementite, making it tougher and more malleable. Martensite is used in tool steels, machine parts, forging dies, and spring steel. Martensitic stainless steel is stronger and easier to machine, and precipitation hardening martensitic stainless steel has high corrosion resistance and strength. Martensitic transformation is a type of dislocative transformation.
The term “martensite” usually refers to a form of steel with a distinctive atomic structure created through a process called martensitic transformation. Martensite is very hard, which means it doesn’t dent or scratch easily; this makes it a popular choice for tools, such as hammers and chisels, as well as swords. It is brittle, however, so it will break rather than bend if subjected to too much pressure. Martensite is made up of austenite, a solid solution of iron with a small amount of carbon.
Phase changes
Austenite has a particular crystalline structure known as face-centered cubic (FCC). This means that each cubic unit has a lattice point in the center of each side and at each corner; with the lattice points connected, the crystal would look like a square box with an X on each side. This type of steel begins to form at temperatures of approximately 1,350°F (732°C). Austenite can contain more carbon than other forms of iron. When allowed to cool naturally, austenite transforms into ferrite (alpha iron or pure iron) and cementite (iron carbide).
Martensitic transformation occurs when austenite is rapidly cooled in a process known as quenching. The rapid drop in temperature traps the carbon atoms within the crystalline structures of the iron atoms. This causes the crystals to change from FCC to body-centered tetragonal (BCT); the crystals are stretched so they are square on each end but longer on the sides (like a shoebox), and the lattice points that were in the center of each face are now joined together at a point in the center of the crystal. This new structure is what greatly increases the hardness of the steel.
temper
The resulting martensitic steel is extremely hard, meaning it won’t scratch, but it is very brittle, so it breaks under stress. To address this weakness, martensite is heated in a process called tempering, which causes the martensite to partially transform into ferrite and cementite. This tempered steel is not quite as hard, but becomes tougher (less likely to break) and more malleable, and therefore more suitable for industrial use.
it is used
The hardness of tempered martensite makes it a good material for tool steels, as resistance to abrasion and deformation are important in such applications. It is a common component in machine parts and forging dies. Hardened silicon-containing steels are often used for spring steel, which can be used to make springs, musical instrument strings, and components on model trains and other toys. Spring steel can be twisted or bent without permanent deformation, making it a good choice for components that require the steel to move repeatedly without degrading.
Stainless steel, which contains chromium as well as iron and carbon, can also be made with a martenistic crystalline structure. This form is less resistant to corrosion than other forms of stainless steel, but it is also stronger and easier to machine in most cases. One method of making it, called precipitation hardening (or age hardening), adds impurities such as chromium and nickel during an extended heat treatment process; Precipitation hardening martensitic stainless steel has even greater strength along with high corrosion resistance. Such steel is often used in military and aerospace applications.
Dislocative transformation
The martensitic transformation is the best-known example of a dislocative transformation, a type of phase change in which the atoms of a material move in unison for short distances rather than spreading individually over greater distances. A phase change occurs when a substance changes from one state, such as a solid, to another, such as a liquid. Because they are so well known as a type of displacement transformation, the terms “martensite” or “martensitic” are sometimes used in a broader sense to describe any material produced by displacement transformation.
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