Microstructure and Property Changes in Fe-C Alloys
Isothermal Transformation Diagrams We use as an example the cooling of an eutectoid alloy (0.8 % C) from the austenite (γ- phase) to pearlite, that contains ferrite (α) plus cementite (Fe3C or iron carbide). When cooling proceeds below the eutectoid temperature (727 oC) nucleation of pearlite starts. The S-shaped curves (fraction of pearlite vs. log. time, fig. 10.3) are displaced to longer times at higher temperatures showing that the transformation is dominated by nucleation (the nucleation period is longer at higher temperatures) and not by diffusion (which occurs faster at higher temperatures). The family of S-shaped curves at different temperatures can be used to construct the TTT (Time- Temperature-Transformation) diagrams For these diagrams to apply, one needs to cool the material quickly to a given temperature To before the transformation occurs, and keep it at that temperature over time. The horizontal line that indicates constant temperature To intercepts the TTT curves on the left (beginning of the transformation) and the right (end of the transformation); thus one can read from the diagrams when the transformation occurs. The formation of pearlite indicates that the transformation occurs sooner at low temperatures, which is an indication that it is controlled by the rate of nucleation. At low temperatures, nucleation occurs fast and grain growth is reduced (since it occurs by diffusion, which is hindered at low temperatures). This reduced grain growth leads to fine-grained microstructure (fine pearlite). At higher temperatures, diffusion allows for larger grain growth, thus leading to coarse pearlite. At lower temperatures nucleation starts to become slower, and a new phase is formed, bainite. Since diffusion is low at low temperatures, this phase has a very fine (microscopic) microstructure. Spheroidite is a coarse phase that forms at temperatures close to the eutectoid temperature. The relatively high temperatures caused a slow nucleation but enhances the growth of the nuclei leading to large grains. A very…
Time-temperature transformation (TTT) diagrams
(TTT) diagrams measure the rate of transformation at a constant temperature. In other words a sample is austenitised and then cooled rapidly to a lower temperature and held at that temperature whilst the rate of transformation is measured, for example by dilatometry. Obviously a largenumber of experiments is required to build up a complete TTT diagram. • An increase in carbon content shifts the TTT curve to the right (this corresponds to an increase in hardenability as it increases the ease of forming martensite - i.e. the cooling rate required to attain martensite is less severe). • An increase in carbon content decreases the martensite start temperature. • An increase in Mo content shifts the TTT curve to the right and also separates the ferrite + pearlite region from the bainite region making the attainment ofa bainitic structure more controllable.
The Influence of Other Alloying Elements
Alloying strengthens metals by hindering the motion of dislocations. Thus, the strength of Fe–C alloys increase with C content and also with the addition of other elements.
Development of Microstructures in Iron—Carbon Alloys
The eutectoid composition of austenite is 0.8 wt %. When it cools slowly it forms perlite, a lamellar or layered structure of two phases: α-ferrite and cementite (Fe3C). Hypoeutectoid alloys contain proeutectoid ferrite plus the eutectoid pearlite. Hypereutectoid alloys contain proeutectoid cementite plus pearlite. Since reactions below the eutectoid temperature are in the solid phase, the equilibrium is not achieved by usual cooling from austenite
Phase compositions of the iron-carbon alloys at room temperature
- Hypoeutectoid steels (carbon content from 0 to 0.83%) consist of primary (proeutectoid) ferrite (according to the curve A3) and Pearlite. - Eutectoid steel (carbon content 0.83%) entirely consists of Pearlite. - Hypereutectoid steels (carbon content from 0.83 to 2.06%) consist of primary (proeutectoid) cementite (according to the curve ACM) and Pearlite. - Cast irons (carbon content from 2.06% to 4.3%) consist of cementite ejected from austenite according to the curve ACM , Pearlite and transformed ledeburite (ledeburite in which austenite transformed to pearlite). When the liquid of eutectic composition is cooled, at or below eutectic temperature this liquid transforms simultaneously into two solid phases (two terminal solid solutions, represented by αand β). This transformation is known as eutectic reactionand is written symbolically as: Liquid (L) ↔solid solution-1 (α) + solid solution-2 (β) In the solid state analog of a eutectic reaction, called a eutectoid reaction, one solid phase having eutectoid composition transforms into two different solid phases. Another set of invariant reactions that occur often in binary systems are - peritectic reaction where a solid phase reacts with a liquid phase to…
Critical temperatures
- Upper critical temperature (point) A3 is the temperature, below which ferrite starts to form as a result of ejection from austenite in the hypo-eutectoid alloys. - Upper critical temperature (point) ACM is the temperature, below which cementite starts to form as a result of ejection from austenite in the hyper-eutectoid alloys. - Lower critical temperature (point) A1 is the temperature of the austenite-to-Pearlite eutectoid transformation. Below this temperature austenite does not exist. - Magnetic transformation temperature A2 is the temperature below which α-ferrite is ferromagnetic.
The Iron–Carbon Diagram
The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram This is one of the most important alloys for structural applications. The diagram Fe—C is simplified at low carbon concentrations by assuming it is the Fe—Fe3C diagram. Concentrations are usually given in weight percent. The possible phases are: • α-ferrite (BCC) Fe-C solution • γ-austenite (FCC) Fe-C solution • δ-ferrite (BCC) Fe-C solution • liquid Fe-C solution • Fe3C (iron carbide) or cementite. An intermetallic compound. The maximum solubility of C in α- ferrite is 0.022 wt%. δ−ferrite is only stable at high temperatures. It is not important in practice. Austenite has a maximum C concentration of 2.14 wt %. It is not stable below the eutectic temperature (727 C) unless cooled rapidly (Chapter 10). Cementite is in reality metastable, decomposing into α-Fe and C when heated for several years between 650 and 770 C. - δ-ferrite: – It is solid solution of carbon in δ-iron. Maximum concentration of carbon in δ- ferrite is 0.09% at 2719 ºF (1493ºC) which is the temperature of the peritectic transformation. The crystal structure of δ-ferrite is BCC (cubic body centered). - Austenite: – Austenite is interstitial solid solution of carbon in γ-iron. Austenite has FCC (cubic face centered) crystal structure, permitting high solubility of carbon i.e. up to 2.06% at 2097 ºF (1147 ºC). Austenite does not exist below 1333 ºF (723ºC) and maximum carbon concentration at this temperature is 0.83%. - α-ferrite: – It is solid solution of carbon in α-iron. α-ferrite has BCC crystal structure and low solubility of carbon…
Solid Solutions
A solid solution may be formed when impurity atoms are added to a solid, in which case the original crystal structure is retained and no new phases are formed. • Substitutional solid solutions: impurity atoms substitute for host atoms, and appreciable solubility is possible only when atomic diameters and electronegativities for both atom types are similar, when both elements have the same crystal structure, and when the impurity atoms have a valence that is the same as or less than the host material. • Interstitial solid solutions: These form for relatively small impurity atoms that occupy interstitial sites among the host atoms
Kinetics of nucleation and growth
From a micro structural standpoint, the first process to accompany a phase transformation is nucleation- the formation of very small particles or nuclei, of the new phase which are capable of growing. The second stage is growth, in which the nuclei increase in size; during this process, some volume of the parent phase disappears. The transformation reaches completion if growth of these new phase particles is allowed to proceed until the equilibrium fraction is attained. As would be expected, the time dependence of the transformations rate (which is often termed the kinetics of a transformation) is an important consideration in the heat treatment of materials. With many investigations, the fraction of reaction that has occurred is measured as a function of time, while the temperature is maintained constant. Transformation progress is usually ascertained by either microscopic examination or measurement of some physical property. Data are plotted as the fraction of transformed material versus the logarithm of time; an S-shaped curve, represents the typical kinetic behavior for most solid state reactions.
Precipitation reactions
A precipitation reaction is a reaction in which soluble ions in separate solutions are mixed together to form an insoluble compound that settles out of solution as a solid. That insoluble compound is called a precipitate
