The chapter also discusses the concepts of transformation kinetics, minimum free energy, and nucleation and growth, and provides information on alloying, interphase precipitation, and various types of transformations.
Sign In or Create an Account. User Tools. Sign In. Skip Nav Destination Close mobile search navigation. ASM Technical Books. So, the final microstructure will contain cementite at the grain boundaries pro-eutectoid cementite and pearlite eutectoid. Although this is an unwanted effect the reverse effect is commonly used. A component is placed in a hot carbon-rich environment which encourages diffusion of carbon into the surface of the steel, increasing the surface hardness. This is called case carburising.
For example, strength increases with carbon content up to the eutectoid composition but then starts to drop as a grain-boundary network of brittle cementite is formed. The specimen has been metallographically prepared and etched in boiling alkaline sodium picrate very dangerous and explosive! In their simplest form, steels are alloys of Iron Fe and Carbon C. Unlike decomposition to ferrite and pearlite, the transformation to martensite does not involve atom diffusion, but rather occurs by a sudden diffusionless shear process.
The term is not limited to steels, but can be applied to any constituent formed by a shear process which does not involve atom diffusion or composition change. The martensite transformation normally occurs in a temperature range that can be defined precisely for a given steel. The transformation begins at a martensite start temperature M s , and continues during further cooling until the martensite finish temperature M f is reached.
Many formulae have been proposed to predict the martensite start temperature. Most are based on the composition of the steel, and a selection are listed in the following table:.
More recently, M s models have been developed through the use of neural networks, trained on experimental data and using further data to validate and test the model, a reasonable approximation of M s can be identified. Such models are available on the web [2] and can be used with compositional information. Neural networks based on the relationship between the chemical composition, transformation temperature and kinetics during continuous cooling enable calculation of a CCT diagram for the steel.
These also take into account the influence of alloying elements on the phase transformation curves, as well as the resulting hardness. It is also possible to predict quantitatively the microstructure of the steel e. Models combining the kinetics of martensitic transformation with mechanics, in view of microstructural development are also applicable. Therefore, the martensite phase consists of a metastable iron phase oversaturated in carbon.
Since the more carbon a steel has, the harder and more brittle it is, a martensitic steel is very hard and brittle.
Notice again that this structure has a very large area of phase boundary between ferrite and cementite, so there is a surface energy penalty in forming this plate-like structure. It is important to note that pearlite is not a phase, but a mixture of two phases: ferrite and cementite. It is formed by eutectoid decomposition of austenite upon cooling by diffusion of C atoms , when ferrite and cementite grow contiguously, C precipitating as Fe 3 C between laths of ferrite at the advancing interface, leaving parallel laths of Fe and Fe 3 C which is pearlite.
Pearlite is a two phase material with iron and carbon as its constituents. The white areas are ferrite, an interstitial solid solution of carbon in bcc iron, and the dark areas are cementite, Fe 3 C a binary compound of carbon and iron containing 6. At the temperature of about C, austenite in many steels decomposed to lower bainite, a type of BCC iron ferrite with finely dispersed carbide cementite. Austenite is stable above C and is an interstitial solid solution of carbon in the face-centered cubic iron lattice, Ferrite is a stable interstitial solid solution of carbon in body-centered cubic iron below C, and cementite is the ceramic-like compound Fe 3 C.
Primary cementite is the cementite formed already during the solidification of the molten metal figure 1. It is the hardest structure that appears on the diagram, exact melting point unknown. Its crystal structure is orthorhombic.
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