Acicular martensite, tempered martensite, lower bainite and retained austenite

AUSTENITE

Austenite is an alloy made up of iron and carbon atoms, containing those in solid solution. It is an important form of steel, used to create a steel that is both strong and malleable. It is also known as gamma-phase iron, or γ-Fe. In metallurgical terms, it is defined as a solid solution of carbon and iron with a face-centered-cubic-lattice structure, the most universal and common type of metallic alloy.

Austenite is formed when a steel alloy is heated to high temperatures, about 725 degrees Celsius (1300 degrees Fahrenheit) or greater. At high temperatures, austenite is form, as opposed to ferrite, which is formed at lower temperatures and has a body-centered-cubic-lattice structure. At lower temperatures, both types of alloys can form, but compromise is reached with an intermediate form of steel, called pearlite, which consists of alternating layers of austenite and ferrite.

Austenite forms the basis of high-grade stainless steels, which are designed to resist corrosion, high temperatures, and wear. It is also used to enhance the strength of certain types of steel, such as tool and die steels (for example, A-2), which are prone to becoming brittle and hard at low temperatures. In some cases, austenite is also used in particular types of corrosion-resistant steels, such as CRES.

Austenite has a certain number of advantages over ferrite, the most notable beingits relatively high ductility and plasticity. This means that it is better able to endure stress without becoming brittle or hard. It can also take on higher levels of strain or distortion before breaking or crumbling. Its strength is also higher than that of ferrite, as well as its thermal conductivity.

In contrast to ferrite, austenite does not become brittle with lower temperatures. This means that it is better able to withstand sudden drops in temperature, making it ideal for a wide range of uses. It is also able to withstand high levels of heat, and can be used to create a steel alloy that is both strong and malleable.

Austenite is one of four key phases of steel, the other three being ferrite, martensite, and bainite. These phases have varying levels of hardness, strength, and ductility, and are used when making different types of steel alloys. Martensite and bainite are formed during the cooling of austenite and are used to create ultra-hard, but brittle, alloys, while ferrite is an ideal material for forming softer and more malleable steels.

In conclusion, austenite is a highly versatile alloy, used to create extremely strong and malleable metals. It is one of the four main phases of steel, and can take on a range of different characteristics depending on how it is processed and treated. As such, it is an essential material used in the manufacture of many different types of steel alloys.

吗？

Pinning martensite is an austenite-to-martensite transition of iron-based alloys caused predominantly by cooling. It is also referred to as white martensite. The sharp pointed chemical and structural features that characterize pinning martensite are relative low energy chemical microstructure features referred to as ‘needles’ by some metallurgists and ‘tinkles’ by others. Pinning martensite is composed of needles of martensite, which are dispersed though an austenitic matrix. The needles contain high carbon concentrations, and are characterized by very high hardeness, as well as excellent retention of strength after tempering or aging. The needles are formed by the localized growth of martensite, initiated by a super saturated carbon concentration in the alloy. As the alloy cools, the carbon diffuses inward from the surface of the component and forms precipitation sites, which act as the nucleation point for the growth of the needles.

The needles are very thin and acicular in shape, and when observed under the microscope, the pinning martensite typically appears to lay in a paralleled pattern. Pinning martensite also exhibits a ‘self-ordering’ effect, where the needles orient themselves in a regular, orderly, lattice-like manner. This is because each of the needle’s higher surface energy is balanced out by the lower energy of their neighbor’s in the martensite lattice.

There are several different varieties of pinning martensite, including reversion martensite, lower bainite, and residual austenite. Reversion martensite is the most common form of pinning martensite and it is composed of needles that are more randomly dispersed than in the other varieties. Lower bainite needle shapes tend to be more pointed than in reversion martensite, and their needle pattern is further ordered. Residual austenite is typically made up of the smallest needles and can contain higher amounts of carbon than other types of pinning martensite. Finally, residual austenite is the least ordered form of pinning martensite, and can contain some austenitic as well as martensitic matrixes.