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Steel microstructure Steel comprises Iron and Carbon. The way in which these elements combine depends on many parameters and determines the properties of steel. Here are main known "combinations". Their obtaining depends on thermal cycles (Température - Time couple) applied to steel. Utility What is the interest to study the microstructure of steels? It is simply because the mechanical properties of steel depend on its composition and very strongly on its microstructure! Microstructure and heat treatments are closely dependent! Impossible to study one without the other. It is an important point to understand and thus progress! Unfortunately the observation of the microstructure of steels requires special equipment of which the least exotic is the optical microscope... Ferrite Ferrite is a-Iron with a negligible part of carbon. Carbon is then in solid solution of insertion in the ferrous crystalline matrix. The solubility of Carbon in Ferrite is very low. It is at a maximum of 0.022% at 727°C and decreases when the temperature drops to reach 7x10-7% at 200°C. In the matter, Ferrite is present in the form of grains. Its basic structure is Body Centered Cubic (BCC). Ferrite is ferromagnetic. Ferrite is a soft and flexible structure. Its hardness is about 10 to 15 HRC. Cementite The Cementite is not an Iron structure but carbide: Fe3C. The Cementite contains 6.67% of Carbon (in mass). The Cementite does not form a Grain on a macroscopic scale. It can be in different forms: > Fine needles, > Plates, > Microscopic grains, > Without particular form, as in the grain boundaries of hypereutectoid steels. See for example the Pearlite below. Pearlite The Pearlite is a two-phases component including Ferrite and Cementite. The Pearlite contains 0.77% of Carbon. In the Pearlite, the Cementite can exist in the form of plates or of globules. In fact the thermal cycles make possible to pass from a lamellate structure to a globular structure or the opposite. On the drawing below: On the left Lamellate Pearlite. On the right Globular Pearlite. In cutlery, one will tend to seek the lamellate pearlite which is harder. It is the ratio between the quantity of Ferrite and Cementite which determines the average Carbon rate. In the matter, the Pearlite is present in the form of Grains. The Pearlite is ferromagnetic. The Pearlite is a half-hard structure. Its hardness is about 40 to 43 HRC. Austenite Austenite is a solid carbon solution in g-Iron. Its crystalline structure is Face Centered Cubic (FCC). The solubility of Carbon in Austenite strongly depends on the temperature: 0.77% at 727°C and until 2.11% at 1148°C. The opposite diagram gives the corresponding curve. This solubility is important because the interstices between the Iron atoms are larger in a FCC structure than BCC, making possible more Carbon atoms to place themself there. Austenite is paramagnetic. Bainite The Bainite is a very particular structure. It appears as needles or plates of Cementite ordered in a ferrite matrix. It is obtained by cooling Austenite with an intermediate speed between cooling in air and hardening. We can distinguish Higher Bainite, obtained than a higher temperature, which has a coarse structure from Lower Bainite where the Cementite needles are tighter. The Higher Bainite is a half-hard structure. Its hardness is about 40 to 45 HRC. The Lower Bainite is a rather hard structure. Its hardness is about 58 to 60 HRC. Martensite Martensite is a structure obtained by a fast cooling of Austenite. The carbon atoms do not have time to migrate to recompose Pearlite or Cementite. They are trapped inside the crystalline matrix. When the temperature passes below a certain threshold named Ms (for Martensite Start), Martensite is created. Progressively with the fall of temperature, the percentage of created Martensite increases to reach 100% at a temperature called Mf (for Martensite Finish). Temperatures Ms and Mf strongly depend on the composition of steel! If Mf is lower than the ambient temperature, all Austenite will not be transformed into Martensite. This part is called residual Austenite. It is to reduce this residual Austenite that certain steel alloys undergo a cryogenic hardening (to pass below Mf). Time does not influence the creation of Martensite. Only the temperature reached determines the percentage of Martensite created! In steels with less than 0.6% of carbon, Martensite creates is presented in the form of needles. For steels with more than carbon 1%, Martensite is presented in the form of plates. The intermediate nuances contain a mixture of needles and plates. Martensite is a very hard and very fragile structure (breakable). Its hardness is about 65 to 66 HRC.