Microstructure of steel

The term "microstructure" refers to the distribution grains in steel. The description of the microstructure includes concepts such as grain size and shape., as well as identification of microscopic constituents, which are present in steel.

What is the microstructure of steel?

Usually, microstructure is understood as a microstructure at room temperature.. At elevated temperatures, various phases and their mixtures also constitute the microstructure of the steel. – high temperature. This microstructure cannot be seen in light microscope, but you can only simulate, eg, on the computer.

The microstructure is determined mainly with an optical microscope by examining the polished and etched steel surface. Each microscopic component of steel has a characteristic appearance under the microscope, which allows you to evaluate and analyze the microstructure. These microscopic components include:
– phases - ferrite, austenite, cementite and untempered martensite;
– phase mixtures - perlite, bainite and tempered martensite.

Mechanical properties of steel are largely determined by its microstructure.

Microstructure of steel: ferrite, perlite, cementite

Microstructure at room temperature of unhardened steels – which slowly cooled from the austenitizing temperature - uniquely depends on the carbon content in them.

Eutectoid steels (0,77 % carbon) are usually completely pearlite.

Microstructure hypoeutectoid steels – carbon less 0,77 % – usually are arrays of pearlite grains with ferrite along the grain boundaries.

Hypereutectoid steels – carbon content over 0,77 % – contain pearlite grains with cementite along grain boundaries.

In hypoeutectoid steels with carbon content less than 0,2 % the dominant component of the microstructure becomes ferrite grains with pearlite, which is distributed in various ways between the ferrite.

Rolled steel often exhibits a striped microstructure, which is an alternating strip of ferrite and pearlite grains, elongated in the direction of mechanical deformation during rolling, forging or other types of metal forming.

Spheroidization of cementite

Cementite in unhardened steels is usually found in thin plates, which are a constituent part of perlite. However, it is possible to change this perlite cementite into small, isolated spheres, which are in a matrix of ferrite grains. For this, a special heat treatment is performed - spheroidization. This structure is called spheroidized steel.. The spheroidized state of steel is usually the delivery state of high carbon steels. In this state, the steel is easier to cut., than then, when cementite is in perlite.

When austenite is cooled quickly, two additional components of the microstructure appear - bainite and martensite.

Martensite - metastable phase

Martensite is a nonequilibrium - metastable - phase. Therefore, it is absent in the iron-carbon phase diagram. You can say, that martensite would like to have a body-centered cubic structure, like ferrite, but the carbon in austenite distorts its crystal structure to cubic tetragonal. Wherein, the more carbon in austenite, the greater this distortion and the stronger - harder - martensite.

Martensite has two varieties under the light microscope: rack and plate. Reiki is formed when the carbon content is from 0 to 0,6 %, and plates - from 1 % and more. When the content of carbon in steel is from 0,6 to 1 % mixed lath-lamellar martensitic structure prevails.

Martensite leave

Martensite, which is formed in the quenching tank, called unreleased or fresh martensite. If the carbon content in steel is greater than about 0,3 %, then martensite will be too brittle and its use is severely limited. Therefore, most "martensites" are tempered by heating to relatively low temperatures.

Tempering martensite leads to the formation of very small carbides, which reduce strength, but increase plasticity. After tempering, martensite turns dark under an optical microscope.

Temperatures of the beginning and end of the martensitic transformation

For, to form martensite, austenite must be cooled - hardened - to a temperature below Mn - temperatures of the onset of martensitic transformation. With a decrease in the quenching temperature below the Mn point, the amount of martensite formed increases, while at a temperature of Mk all austenite is 100 % – will not turn into martensite.

At a quenching temperature between points Mn themto steel structure is a mixture of martensite and retained austenite.

Both temperatures of martensitic transformation - the beginning of Mn and end Mto - decrease with increasing carbon content in steel. Therefore, samples made of high-carbon steels, hardened at room temperature, may contain significant amounts of retained austenite. Retained austenite in common carbon steels, hardened at room temperature, appears already at a carbon content of approximately 0,4 %.

Bainit

At quenching rates slightly less, what is required for the formation of martensite, a structure is formed, which is called bainite. Bainite is similar to perlite in that, that it is composed of ferrite and carbides. The carbide component of bainite has the form of ordered filaments or chains of particles, in contrast to pearlite, where carbides are present in the form of ordered plates.

Bainite occurs in two forms - upper bainite and lower bainite - depending on temperature, at which it was formed. In the lower bainite, carbides are finer and dispersed. The strength of bainite approaches the strength of martensite, and the viscosity is often higher, than tempered martensite at the same hardness.

Martensite Formation Rate

A unique property of martensite is its rate of formation. She is much higher, than the rate of formation of any other austenite decomposition product - ferrite, cementite, perlite or bainite. Martensite grows at about half the speed of sound in steel. therefore, when the austenite temperature falls below the Mn point, martensite is formed instantly, in a matter of milliseconds.

When the steel is cooled from the austenitic region, then the phase diagram tells us which products of austenite decomposition will be formed first. This is ferrite in hypoeutectoid steels, perlite in eutectoid steels and cementite in hypereutectoid steels. These phases are formed first along the boundaries of austenite grains. To form 100% martensite, the steel must be cooled so quickly, so that these austenite decomposition products do not have time to "fall out" along the grain boundaries. maybe, that one of these phases will have time to form at the boundaries of austenite grains before, How does austenite reach temperatures below the M pointn. In this case, this phase will remain there., at grain boundaries, completely surrounded by martensite, which forms very quickly.

Isothermal transformation of austenite

If steels are cooled at a constant temperature in salt baths so quickly, that austenite does not have time to decay, then austenite will transform isothermally , at constant temperature. This isothermal transformation of steels is very well studied.. For all steels, clear dependences of the types of austenite decomposition products on the temperature of isothermal transformation and the carbon content in steel have been established..