Martensite and martensite transformations: what it is, martensitic steel
Martensite, the condition for the emergence of which is martensitic transformation, is a characteristic formation for alloys, containing from 11 to 17% chromium and not less 0,15% carbon. The composition of such alloys, in addition, include nickel, tungsten, molybdenum and vanadium (their number is very small).
Martensitic steel grade 10x13 is used in products, exposed to weakly aggressive environments
Properties and structure of martensite
Martensite is a needle-shaped grain in the microstructure of the metal, representing a supersaturated solid solution of carbon in the alpha gland. This structure is typical for steels, hardened, as well as for some pure metals, possessing polymorphism. Martensite owes its name to Adolf Martens, a German scientist, who devoted most of his life to the study of metals and their properties. It should be noted, that martensitic steels due to the peculiarities of their structure have the highest hardness among such materials.
microstructure of martensite
With such a phenomenon, as martensitic transformations, occurring when heating and cooling steel, related unique metal memory effect, discovered and described by scientists GV. Kurdyumov and LG. Handros c 1949 year. The essence of this effect is, that the deformation of the metal, created in it at that time, when there is a direct martensitic transformation, disappears completely during the reverse transformation. Thanks to this effect, scientists have managed to create alloys, having a memory of their form. Products from such alloys, which have been subjected to deformation in the martensitic state, take their original form, if they are heated to temperature, causing martensitic transformation in steel.
Crystal lattice of martensite, formed in the structure of hardened metal, is not cubic, and the tetragonal. Each of its elements has the shape of a rectangular parallelepiped. The central part of such a cell (as well as its tops) occupy iron atoms, in the inner space between which are carbon atoms.
Martensitic steels, as mentioned above, differ in high hardness and durability, and this is due to the fact, that the structure of martensite, being nonequilibrium, characterized by the presence of strong internal stresses. In martensitic steels, carbon atoms are redistributed when heated. This phenomenon is diffuse in nature. As a result of this distribution, two phases are formed in the steel structure, each of which differs in carbon content and the shape of its crystal lattice.
Crystal lattice of martensite
In such phases, which characterize all martensitic grade steels when heated, is:
- ferrite, which contains a very small amount of carbon - up to 0,02% (the elementary cells of the ferrite crystal lattice have the shape of a cube, the vertices and the center of which are formed by iron atoms, everything else in such cells is carbon);
- cementite, in which the carbon content is much higher - up to 6,67% (rhombic crystal lattice of cementite is formed by elementary cells, which have the shape of a rectangular parallelepiped).
The initial structure for the formation of martensite is austenite. Crystal lattices of these formations, simultaneously present in the microstructure of steel, are related by orientational relations. That is the connection, that the plane of the lattices of austenite and martensite, which have certain crystallographic indices, parallel to each other.
Martensite, which forms the microstructure of steels, can be present in it in two forms.
Different types of martensite, which are formed during hardening of austenite
Plate parts (twins) martensite
This structure is formed at lower temperatures 2000. It is characteristic of carbon and alloy steels. Properties of martensite of this type, present in the structure of the metal in the form of plates, determines the presence on such plates of the so-called midrib - the middle line, which is characterized by increased persecution. This martensite is called double, that the fraction of each of its plates is formed by many twins. Such twins, located on the planes of martensite plates, have a thickness 5-30 nm.
Optical micrograph of martensite of lamellar structure
Rail (dislocation) martensite
This formation is characteristic of the structure of steels, relating to high-alloy, few- and medium carbon. Temperature threshold, in which in such steels is the formation of martensitic structure, is above the mark 3000. Martensite of this type in full accordance with its name has the form of elongated in one direction rails, the thickness of each of which is in the range 0,2-2 micron (while their length is greater than the width of about 5 times). Metal structure, formed from martensite of this type, presented ??in the form of a combination of groups (packages) such parallel to each other crystals-rails. In this structure you can see the layer between the martensite rails, consisting of residual austenite. The thickness of such layers in alloys of different types can be from 10 to 20 nm.
Optical micrograph of martensite rack structure
Optical micrograph of martensite rack structure (in particular, Optical micrograph of martensite rack structure) Optical micrograph of martensite rack structure. Optical micrograph of martensite rack structure, Optical micrograph of martensite rack structure, Optical micrograph of martensite rack structure, Optical micrograph of martensite rack structure. Optical micrograph of martensite rack structure, Optical micrograph of martensite rack structure, Optical micrograph of martensite rack structure.
Optical micrograph of martensite rack structure, in which there is virtually no residual austenite, and the formed martensite has only a rail form. Temperature, in which martensitic transformations are observed in such steels, is close 4000 WITH.
Features of martensitic transformation in steels
The condition for such a phenomenon, as martensitic transformations, not fixed temperature, and a certain temperature range. The upper limit of this interval corresponds to the temperature, which is less than the temperature of the beginning of austenitic decay by several hundred degrees. The end of this process occurs at temperature, which is much lower than the room. Such conditions for the formation of martensite are related to this, that in the structure of the alloy there is also residual austenite.
The amount of martensite in the steel structure can be increased, if subjected to an alloy of plastic deformation. This must be done at room temperature, required for martensitic transformation. Austenite can turn into martensite in that case, if the alloy is subjected to plastic deformation at room temperature.
Scheme of changes in martensite during heating
Considered education in the structure of steel can take shape, which is called leave martensite. Conditions for its formation are heating the metal to temperature, which is lower, than the temperature of conversion of ferrite to austenite. A characteristic feature of the process, at which martensite of holiday is formed, there is that, that martensite, which has a needle or plate shape, turns into carbide inclusions of spherical configuration.
The essence of the transformation of the initial structure of the alloy into martensitic is, that the molecules in the crystals of such an alloy begin to move in an orderly manner, changing their position relative to each other and, in accordance, forming crystal lattices of a new configuration. So, there is no destruction, but only the deformation of the cells of the crystal lattice, which leads to the formation of a new alloy structure.
Formation of martensite crystals in austenite grains
For martensitic transformation of the alloy structure, in which there is no destruction, and modification of the crystal lattices of cells, which form its structure, requires very little energy. This contributes to that, that such changes are happening at a high rate. The results of such transformations, as well as the conditions of their course allow you to effectively change the characteristics of alloys, in which they occur, using for this purpose methods of thermal or mechanical influence.
Properties of steels with martensitic structure
Steels with martensitic structure, in addition to high carbon content, are also characterized by the presence of chromium. Such steels are often alloyed elements, which are able to provide high heat resistance of the metal (tungsten, molybdenum, niobium and others.).
Chemical composition of chromium martensitic steels
Become, the internal structure of which is formed by martensite, differ in the following features:
- high corrosion resistance to high humidity, alkaline and acidic media;
- high heat resistant (if alloys of this category are subjected to hardening at temperature 10500, and then perform leave on troostite or sorbitol);
- such a useful feature, as self-hardening;
- high hardness at rather low plasticity (which is typical, on the hardness of martensite, which is originally owned by such alloys, practically do not affect alloying elements, introduced into their composition);
- increased resistance to hydrogen (this is the difference between individual brands of such steels, in particular X5M, X5VF and X9M);
- resistance to cutting due to high hardness.
Mechanical properties of martensitic steels
Because steels with a martensitic structure after their hardening become very brittle and prone to destruction, the technology of their welding is much more complicated. You can perform this procedure only after that, as a product of such steel is heated to 200-4500, it is important, so that the ambient temperature is above zero. In addition to manual arc welding, performed using electrodes, covered with special compounds, The following technologies are used to join products from such alloys:
- Electroslag welding;
- welding in argon;
- welding under a protective layer of flux.
Recommended modes of welding martensitic steels
If we talk about the scope, then martensitic steels are used for the production of such products, as:
- housings and rotors for gas complete sets, as well as steam turbines;
- details of welding machines, vessels for various purposes, working under pressure, not exceeding 16 MPa;
- diaphragms for equipping steam turbines;
- parts and components for the production of pumping equipment;
- shoulder blades, which are equipped with steam turbines;
- springs for various purposes;
- details of pipelines, collectors, boilers, which undergo significant heating during operation;
- measuring instruments for various purposes, cutting and surgical;
- plates, which are equipped with compressors.
