Transformations into hypoeutectoid steel
If steel with carbon content less than 0,8 % heat to austenitic state and then cool slowly, then the first phase, which will stand out, there will be ferrite. This transformation occurs as a result atomic lattice changes from face-centered in austenite to body-centered in ferrite. Since ferrite is only able to dissolve a very small amount of carbon, then the concentration of carbon in the remaining austenite during cooling will increase until, while at a eutectoid temperature of about 720 ºC all remaining austenite will not turn into pearlite. Ferrite type, precipitated along the boundaries of austenite grains during cooling of a steel sheet 45, shown in the figure 1. This steel has been normalized by heating to a temperature 1095 ºС and cooling in air.
Picture 1 - Sheet thickness 3 mm made of steel 45. Normalization: heating to temperature 1095 ºС and air cooling. The structure is made of perlite (grey) with ferrite mesh along grain boundaries, as well as several plates of ferrite.
Transformations in hypereutectoid steel
Slow cooling of hypereutectoid steel with a carbon content of more than 0,8 % similar to cooling mild steel, except that, what's the first phase, which precipitates from austenite is not ferrite, and cementite. Cementite usually forms at the boundaries of austenite grains and forms a network structure, which can be seen in steel at room temperature after the transformation is complete. On the picture 2 cementite is visible along the boundaries of austenite grains in a structural steel bar with a carbon content 1,0 % after hot rolling at a temperature of 1175 to 925 ºС and cooling in air. With exactly the eutectoid composition, austenite turns into pearlite without forming as primary ferrite, and primary cementite.
Picture 2 - Microstructure of steel bar thickness 125 mm made of steel ШХ15, hot rolled at a temperature of 1175 to 925 ºС and cooled in air. Cementite is located along the boundaries of primary austenite grains. The inner part of the grains is perlite.
Ferrite-pearlite structure of steel 45
Since to form as ferrite, and cementite requires carbon diffusion, then with an increase in the cooling rate, the thickness of ferrite and cementite plates in pearlite. Pearlite in a ferritic steel matrix 45 shown in the figure 3. In general, at a higher cooling rate, thinner plates are formed., what, in its turn, increases the hardness and strength of steel.
Bainitic transformation into steel
At a higher cooling rate, than that, which is achieved by air cooling, it is possible to avoid the transformation of austenite into ferrite and pearlite in many carbon and low alloy steels. Transformation of austenite at temperatures below 540 ºS, but above about 200 ºС gives structure, which consists of ferrite and cementite, however, these phases are not ordered into plates. Instead, ferrite and pearlite have a pinnate or acicular, and no component of the microstructure can be seen through an optical microscope. This structure is known as bainite..
Upper and lower bainite
When the transformation occurs somewhere between 370 to 540 ºS, bainite structure has a feathery appearance and is called upper bainite. When the transformation occurs at temperatures ranging from about 200 to 370 ºS, bainite has a more acicular structure and is called lower bainite. Bainite hardness depends on its structure, but usually ranges from 45 to 60 HRC. Bainite in steel 45, which was brought to the austenitic state by heating at 1205 ºС during 10 minutes and then held for 10 minutes at temperature 340 ºС for partial isothermal transformation is shown in the figure 4. After partial isothermal transformation, the steel was cooled in air to room temperature.
Picture 4 - Steel 45 after heating to temperature 1205 ºС during 10 minutes, cooling at temperature 340 ºС during 10 minutes for partial isothermal transformation and further cooling in air. Lower bainite (dark) surrounded by martensite (White).
Martensitic transformation in steel
Transformations of austenite at lower temperatures, below 200 ºС leads to the formation of an iron phase with a body-centered tetragonal structure, which is called martensite. Usually martensite is present in fully hardened steels. Microstructurally, martensite has an acicular appearance as shown in the figure. 5. This steel has been heated to 850 ºС for obtaining a solid solution of carbon in austenite, quenching in water and tempering at a temperature 230 ºС during 1 o'clock. Rapid cooling of austenite suppresses ferrite formation, perlite and bainite and provides a fully martensitic structure. Martensite occupies a larger volume, than austenite. Therefore, the formation of martensite leads to changes in size, what can cause warpage of the product. In extreme cases, warpage can lead to cracking during quenching..
Retained austenite in a martensitic structure
In some cases, not all of the austenite is converted to martensite upon sharp cooling during quenching of the steel.. When it happens, the structure of the steel consists of austenite and martensite needles. Residual (unconverted) austenite is soft and therefore reduces the hardness of martensite-hardened steel. This retained austenite can later turn into martensite - by machining or even already in use.. Expansion of material, which occurs during the martensitic transformation, creates internal tensions, which can lead to warpage of the product and loss of tolerances of the machined product. This untempered martensite is brittle and can lead to cracking during product service.. For this reason, the presence of retained austenite in a martensite hardened product is generally undesirable..