Iron-Carbon Equilibrium Project PDF

Most of the heat treatment that are carried out on carbon steels relate to the temperatures on the Iron/Carbon Equilibrium Diagram. How an Iron Carbon Phase Diagram Works For an iron-carbon phase diagram, the temperature is plotted on the Y axis and carbon content, as a percentage of the weight,...

Most of the heat treatment that are carried out on carbon steels relate to the temperatures on the Iron/Carbon Equilibrium Diagram. How an Iron Carbon Phase Diagram Works For an iron-carbon phase diagram, the temperature is plotted on the Y axis and carbon content, as a percentage of the weight, is plotted on the X axis. The graph is split up into segments, and each segment represents a di=erent phase of the microstructure. The microstructure is therefore a function of the temperature and chemical composition of the iron. This means that, although two iron-carbon alloys can have the same chemical composition, if they are exposed to a di=erent temperature, they will have di=erent microstructures. Only some special alloys can exist in multiple phases. Heating the metal to specific temperatures using heat treatment procedures results in di=erent phases. Some special alloys can exist in more than one phase at the same temperature Phase diagrams are graphical representations of the phases present in an alloy at diBerent conditions of temperature, pressure, or chemical composition. The iron-carbon phase diagram is widely used to understand the di=erent phases of steel and cast iron. Both steel and cast iron are a mix of iron and carbon. Also, both alloys contain a small amount of trace elements. The carbon in iron is an interstitial impurity. The alloy may form a face-centred cubic (FCC) lattice or a body-centred cubic (BCC)lattice. It will form a solid solution with α, γ, and δ phases of iron. The carbon in iron is an interstitial impurity. The alloy may form a face-centred cubic (FCC) lattice or a body-centred cubic (BCC)lattice. It will form a solid solution with α, γ, and δ phases of iron. Boundaries Multiple lines can be seen in the diagram titled A1, A2, A3, A4, and ACM. The A in their name stands for the word ‘arrest’. As the temperature of the metal increases or decreases, a phase change occurs at these boundaries when the temperature reaches the value on the boundary. Normally, when heating an alloy, its temperature increases. But along these lines (A1, A2, A3, A4, and ACM) the heating results in a realignment of the structure into a di=erent phase and thus, the temperature stops increasing until the phase has changed completely. This is known as thermal arrest as the temperature stays constant. Alloy steel elements such as nickel, manganese, chromium, and molybdenum a=ect the position of these boundaries on the phase diagram. The boundaries may shift in either direction depending on the element used. For example, in the iron-carbon phase diagram, the addition of nickel lowers the A3 boundary while the addition of chromium raises it. Eutectic Point Eutectic point is a point where multiple phases meet. For the iron-carbon alloy diagram, the eutectic point is where the lines A1, A3 and ACM meet. The formation of these points is coincidental. At these points, eutectic reactions take place where a liquid phase freezes into a mixture of two solid phases. This happens when cooling a liquid alloy of eutectic composition all the way to its eutectic temperature. The alloys formed at this point are known as eutectic alloys. On the left and right side of this point, alloys are known as hypoeutectic and hypereutectic alloys respectively (‘hypo’ in Greek means less than, ‘hyper’ means greater than). Phase Fields The boundaries, intersecting each other, mark certain regions on the Fe3C diagram. Within each region, a di=erent phase or two phases may exist together. At the boundary, the phase change occurs. These regions are the phase fields. They indicate the phases present for a certain composition and temperature of the alloy. Let’s learn a little about the di=erent phases of the iron-carbon alloy. DiBerent Phases α-ferrite Existing at low temperatures and low carbon content, α-ferrite is a solid solution of carbon in BCC Fe. This phase is stable at room temperature. In the graph, it can be seen as a sliver on the left edge with Y-axis on the left side and A2 on the right. This phase is magnetic below 768°C. It has a maximum carbon content of 0.022 % and it will transform to γ-austenite at 912°C as shown in the graph. γ-austenite This phase is a solid solution of carbon in FCC Fe with a maximum solubility of 2.14% C. On further heating, it converts into BCC δ-ferrite at 1395°C. γ-austenite is unstable at temperatures below eutectic temperature (727°C) unless cooled rapidly. This phase is non-magnetic. δ-ferrite This phase has a similar structure as that of α-ferrite but exists only at high temperatures. The phase can be spotted at the top left corner in the graph. It has a melting point of 1538°C. Fe3C or cementite Cementite is a metastable phase of this alloy with a fixed composition of Fe3C. It decomposes extremely slowly at room temperature into iron and carbon (graphite). This decomposition time is long and it will take much longer than the service life of the application at room temperature. Some other factors (high temperatures and the addition of certain alloying elements for instance) can a=ect this decomposition as they promote graphite formation. Cementite is hard and brittle which makes it suitable for strengthening steels. Its mechanical properties are a function of its microstructure, which depends upon how it is mixed with ferrite. Fe-C liquid solution Marked on the diagram as ‘L’, it can be seen in the upper region in the diagram. As the name suggests, it is a liquid solution of carbon in iron. As we know that δ-ferrite melts at 1538°C, it is evident that the melting temperature of iron decreases with increasing carbon content. Phases in the Iron Carbon Phase Diagram 1. δ-ferrite ẟ-ferrite is a low-carbon (almost pure iron) phase that has a body-centered cubic crystal structure. With an increase in carbon content, cementite will begin to form creating ferrite + cementite. δ-ferrite is stable up to 912 ºC. Above 912 ºC, δ-ferrite transforms into a face-centered cubic austenite. δ-ferrite is very magnetic and ductile but has a low strength. 2. γ-austenite y-austenite is a solid face-centered cubic phase, which is stable up until 1,395 ºC, at which point it transforms into a body-centered cubic ferrite. Most iron alloys that receive heat treatment start in the γ-austenite phase. y-austenite is non-magnetic, soft, and ductile. 3. α-ferrite α-ferrite is a high-temperature iron-carbon phase that is created by cooling a low- carbon concentration in the liquid state. The liquid state is then cooled to an austenite phase. The presence of α-ferrite in iron-carbon resists lattice dislocation and therefore slows grain growth, reduces vulnerability to fatigue, and increases strength. 4. Fe3C (Cementite) Fe3C is the chemical name for cementite, also known as iron carbide, which starts to be present at any carbon content above zero, but no iron is fully cementite until it includes 7% carbon content. Cementite can be produced through the cooling of austenite or the tempering of martensite. Cementite is hard, but also brittle, but cementite’s best property is its corrosion resistance which can be achieved when dipping cementite in a solution of 1–3% sodium chloride. 5. Ledeburite Ledeburite is a mix of austenite and cementite and has a carbon content of 4.3% carbon. The carbon content of ledeburite is too high to be found in steel but is found in cast iron. Ledeburite has a melting point of 1,147 ºC. 6. Pearlite Pearlite is formed when an austenitic phase of an iron-carbon alloy is cooled slowly. Pearlite is formed of alternating layers of ferrite and cementite, which gives pearlite significant toughness and strength. 7. Martensite Martensite is created by rapidly cooling a face-centered cubic austenite phase. This process is referred to as a martensitic transformation. The martensitic transformation creates great hardness and strength. Quenching is used to invoke a martensitic transformation. Heat Treatment in the Context of the Iron-Carbon Phase Diagram The purpose of heat-treating iron-carbon alloys is to change the microstructure of the alloy and therefore its properties. By changing the rate of cooling, the properties of an iron alloy can be altered. When cooling pearlite at a rate of 200 ºC per minute, a hardness of 300 DPH can be achieved. Cooling at 400 ºC can achieve a hardness of 400 DPH. The reason there is an increase in hardness is due to the formation of fine pearlite and ferrite microstructure in the pearlite. This is because carbon has less time to move through the lattice structure of the pearlite as it cools. Therefore, when cooling, using liquid quenching at 1,000 ºC, the carbon has no time to move. This forms a martensitic microstructure that has a hardness of 1,000 DPH. Martensite has the hardest and most brittle microstructure of all iron alloys, but after tempering by holding martensite at 400 ºC it becomes less hard and brittle. There are many important temperatures in the iron-carbon phase diagram. One of the most important is 723 ºC, which is known as the critical or A1 temperature. It is the point at which any austenite in iron, cast iron, or an iron alloy transforms into the eutectoid pearlite. At this point, the microstructure has double the amount of ferrite as it does pearlite. Reactions in the Iron Carbon Phase Diagram The two factors that a=ect the microstructure of iron-carbon alloys are temperature and carbon content. For a given carbon content, the heat treatment will a=ect the structure of the iron-carbon mix by changing the bonds between the elements. There are three types of bonding which are discussed below: 1. Eutectoid A eutectoid reaction occurs at the eutectoid point at which one single solid phase transforms into two solid phases simultaneously upon cooling. An example of the eutectoid point on the iron-carbon phase diagram is at 0.8 %wt and 723 ºC. At this point, during cooling, austenite is transformed into ferrite and cementite. 2. Eutectic A eutectic reaction is very similar to an eutectoid transformation. However, an eutectic reaction starts with a liquid and ends up as two solids, whereas a eutectoid starts as a single solid phase. The eutectic reaction will occur at the melting point of the mixture as it cools, and so the specific temperature will depend on the composition of the mixture. 3. Peritectic A peritectic reaction occurs when a molten phase and a solid phase react to form a secondary solid phase. Peritectic reactions are not as common as eutectoid or eutectic reactions and there is only one form of peritectic reaction in the iron-carbon system. How the Iron-Carbon Phase Diagram Helps Engineers and Metallurgists The iron-carbon phase diagram can be used to predict the phase states within the material if the heat treatment it has received and the chemical composition is known. Since the physical properties of an iron-carbon alloy depend on the phases of the iron- carbon alloy, the iron-carbon phase diagram can aid engineers and metallurgists in predicting the properties and behavior of such an alloy. For example, if an iron-carbon alloy of 0.8% carbon is heated above 723 ºC and then liquid quenched, then it is known that martensite is formed and the iron-carbon alloy will be hard but brittle. Types of Steel Used in the Iron Carbon Phase Diagram The types of steel used in the iron-carbon phase diagram are discussed below: 1. High-Carbon Steel High-carbon steel, also known as carbon tool steel, has a carbon content of 0.60– 1.50%. The high carbon content means it is harder and stronger than lower-carbon steels but it is also less ductile. High-carbon steel is also more corrosion-resistant than lower-carbon steel, if there is no addition of other alloying elements, such as chromium which can a=ect the corrosion resistance of steel. High-carbon steel is used in cutting tools, dies, springs, and high-strength wire. 2. Medium-Carbon-Steel Medium-carbon steel has a carbon content of between 0.30–0.60%. Medium-carbon steel is the range that has the best balance between the properties of low- and high- carbon steel. Although medium-carbon steel is harder and stronger than its low-carbon counterparts, it often requires quenching to achieve the desired hardness. Medium- carbon steels are commonly used in: pressure vessels, gears, shafts, axles, and machinery parts. 3. Low-Carbon Steel (or Mild Steel) Low-carbon steel has a carbon content between 0.05–0.30%. Low-carbon steels are known for their ductility and malleability. Due to the small amount of alloying carbon, low-carbon steel is the cheapest and most widely used as a general-purpose steel. Low-carbon steel is used for: fasteners, building frames, bridges, body panels, and pipework. Advantages and Disadvantages of Using the Iron-Carbon Phase Diagram Iron carbon diagrams are in widespread use due to their advantages which include: 1. It is relatively easy to interpret. 2. It can display information on a large number of phases. 3. It is accurate. 4. The information is well supported by research. There are also disadvantages to the iron-carbon phase diagram which are: 1. The diagram does not include all information, such as the non-equilibrium martensite phase. 2. There is no time indication for rates of heating and or cooling. 3. There is no indication of the exact properties of the di=erent phases. P is the peritectic point