Laboratory Classes No.4 4. Study of Softened Carbon Steel and Construction Cast Iron 4.1. Aim of the Lab Classes No.4: 4.1.1. Exploring of the alloys’ microstructure and its dependence on the composition of alloy’s components 4.1.2. Study Fe – Fe3C diagram; 4.1.3. Exploring steel and cast iron structure at equilibrium condition. 4.2. General Information Ferrum - carbon alloys (carbon steels) represent in most designs produced. Usually do not address to all compositions of ferrum - carbon diagram but only to a small part of it where the carbon content is in the range from 0% to 6.67%. The alloys with higher carbon content are hardly used. At 6.67% content of carbon in iron a chemical compound Fe3C formed, which is named iron carbide or cementite. Iron - carbon alloy eutectic is called ledeburite. Looking at the diagram. Fig.4.1. Ferrum - Carbon diagram. It can be assumed that at the range of temperature from 910 0C until 1539 0C only γ iron is exist and at the temperature lower than 723 0C exists only α – iron. Critical points of iron - carbon alloy or trend of its curves is labeled with the letter A and by subscripted figures indicating the transformation. At 7700C temperature, magnetic transformation of α-iron is accrued and alloys with carbon content not more than 0.83% becomes non - magnetic above this temperature. This line of magnetic transformation is labeled as A2. While the magnetic transformation of cementite starts at 210 0C and is labeled as A0. ACD – line of the onset of alloy solidification or liquidus line (see Fig. 4.3); AC – line of the onset temperature of austenite crystallization; CD - line of the onset temperature of cementite crystallization; AECF – line of the end of alloy solidification, or Solidus line; AE – line of the end of austenitic crystallization temperature, ECF – line of the onset temperature of eutectic crystallization; GOS - a cooling temperature when grains of ferrite begin formed from austenite A3; PSK - at 0.8 % of carbon in alloy, γ - iron starts to transfer into α – iron. Austenite, containing 0.83 % carbon, transforms into ferrite and cementite mechanical mixture and this mixture is called perlite A1; SE – line of ultimate solubility of carbon in γ - iron, depending on a temperature; PQ – line of ultimate solubility of carbon in α - iron, depending on a temperature; MO - line of magnetic transformations A2. In some areas of Fe-C diagram, there are the following structures: I. Homogeneous liquid mixture: (Fe ↔ C); II. The liquid mixture and austenite crystals: Feγ (C) + (Fe ↔ C); III. Liquid mixture and the original cementite crystals: Fe3CI + (Fe ↔ C); IV. Austenite: Feγ(C); V. Austenite and secondary cementite: L Feγ (C) + Fe3CII; VI. Austenite, secondary cementite and eutectic: Feγ (C) + Fe3CII + [Feγ (C) + Fe3C]; VII. Original cementite and eutectic: Fe3CI + [Feγ (C) + Fe3C]; VIII. Austenite and β – ferrite: Feγ (C) + Feβ (C); IX. Austenite and α – ferrite: Feγ (C) + Feα (C); X. Ferrite: Feα (C); XI. Ferrite and triple-cementite: Feα (C) + Fe3CIII; XII. Ferrite, triple-cementite and perlite: Feα (C) + Fe3 CIII + [Feα (C) + Fe3C]; XIII. Perlite and secondary cementite: [Feα (C) + Fe3C] + Fe3CII; XIV. Perlite, secondary cementite and ledeburite: [Feα (C) + Fe3C]+Fe3CII + ([Feα (C) + Fe3C] + Fe3C); XV. Initial cementite and ledeburite: Fe3CI + ([Feα (C) + Fe3C] + Fe3C). Iron - carbon alloys are divided by a carbon content and a structure into two groups steel and cast iron. Cooling curve gives to indicate the phases’ transformation that occur when the temperature of alloy is changed. As an example, consider the alloy transformation with 0.025 % of carbon. This alloy above line AC is a homogeneous liquid mixture. If cooling process is applied to its liquid mixture above the line AC until temperature lower then line AC then at the temperature (t0) austenitic crystallization starts. Rapidly cooling process of the alloy is lowered its crystallization rate due to latent heat of the alloy and the cooling curve changes its curvature right up to the horizontal line. Figure 4.2. Visualized Fe-C diagram. Figure 4.3. Fe–C diagram. Let us consider the influence of the composition of alloys on the cooling curves of these alloys (see Fig.4.4). Firstly, study the curve presented in Fig.4.4 a). Figure 4.4. Ferrum - carbon alloy cooling curve. At the temperature t0 alloy starts to solidify and at the temperature t1 it is fully solidified. Then alloy is rapidly cooling, as austenite, to the temperature t2 in the line of GOS. At this temperature, the austenite begins to form carbon solid solution or β ferrite, which is accompanied by a small heat release. At the temperature of t3 equal to 768 0C non-magnetic β - ferrite transfers into magnetic α - ferrite and this process occurs at a constant temperature. On a cooling curve this process is reflected as a horizontal straight line t3 – t3 '. At the temperature t4 γ – ferrite transforms fully into α - ferrite. At a temperature t5, located on the PQ curve, a triple-cementite starts to form. The final structure of the alloy will be ferrite and triple-cementite in grain boundaries (Figure 4.5.). a) b) c) d) Figure 4.5. Annealed steel microstructure. Quantity of hypoeutectic pearlite steels varies proportionally to the quantity of carbon content in alloys. Such steels form a hypothetical triangle of carbon concentrations vs properties and it is easy to determine the carbon content in steel in this case. To do this necessary to take into consideration a part of perlite structure, microscopically visible area. Suppose that perlite is 50% of the total visible area of a specimen under a microscope (Figure 4.5.b.). Fix a point of pearlite’s content (in percentage) of the specimen on the axis of pearlite’s content of triangle, using figure 4.6 and then this point is projected to the left and then up thereby determining the amount of carbon in steel. There is a second way to determine the amount of carbon in steel, knowing the pearlite content, by using the formula: Kx 0.8 Aper 100 ,% Where, A – an area, which is occupied by pearlite microstructure in observation area, in percentage. Figure 4.6. Diagram Fe - Fe3C. Steel part. 4.3. Methodology of lab classes and Performance Order 4.3.1. Draw the diagram of Fe - Fe3C with scale and separately the steel part of it; 4.3.2. Explain in written form each lines of diagram and diagram’s areas; 4.3.3. Each student will be issued micro-sections which should be explored by metallographic analysis and compared with the pictures of microstructure of research laboratories’ magazine and choose the correct number of the relevant image; 4.3.4. Should identify microstructure, carbon content and the potential grade of steel; 4.3.5. Draw the diagrams of cooling process of studied alloys. Show the critical points during their cooling from liquid stage until room temperature and describe the processes taking place in steel at these temperatures. LABORATORY CLASSES No. 4 "Properties of Softened Carbon Steel and Structural Cast Iron" Protocol 1. The aim of the lab: 1.1. Study the Fe – Fe3C diagram; 1.2. Exploring of the alloys’ microstructure and its dependence on the composition of alloy’s components; 1.2. Exploring steel and cast iron structure at equilibrium condition. 2. Task content: 2.1. Learn, what processes are take place on the Fe - C diagram’s curves? 2.2. Show the phase composition for each areas of the Fe - C diagram; 2.3. Draw the steel’s part diagram of carbon steel diagram using the full (Fe – Fe3C) diagram and identify state of steel micro-structure for each diagram’s areas (perform this as independent homework after laboratory classes); 2.4. Take micro-sections kit of alloys to research. Describe the structure of these micro-sections. Show the critical points of the studied alloys and describe processes that take place during their cooling from liquid state until room temperature; 2.5. Determine the composition of the steel’s microstructures and be able to distinguish difference between structure of cast iron, steel with high carbon content, steel with low carbon content and iron. Students are offered micro-sections kit of steel alloys to research. Specimens of Fe – Fe3C system are etched with 5% HNO3 solution in alcohol. All micro-sections are studied using microscope at 400 magnification. 3. Student knowledge required: Know properties of pure iron, iron allotropy, cementite and its properties. Know carbon solubility in - Fe structures, - Fe, austenite and ferrite; classification of Fe – Fe3C alloys. Know processes that occur in slowly cooled hypoeutectic steel. Know perlite properties and its formation conditions; processes that occur in slowly cooled hypereutectic steel. Know cementite properties and its formation conditions; processes that occur in slowly cooled hypereutectic pig iron; processes that occur in slowly cooled hypoeutectic cast iron. Know the differences between white and gray iron. Diagram of state of Fe – Fe3C alloy composition (present state of the studied micro-sections) Research protocol of the Laboratory results: Carbon, % Explain the graph curves and areas ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ ___________________________________________________________________ Temperature, 0C State diagram of steel alloy (Fe - Fe3C) (Indicate position of the investigated alloys on the Fe – Fe3C diagram, steel part) Carbon, % No.1 Number of micro-section ____________________ Steel grade ______________________ Micro structure ______________________ _________________________________________ Carbon content % _______0.025____________ (from analysis of alloy) No. 2 Number of micro-section ____________________ Steel grade ______________________ Micro structure ______________________ Amount % perlite ______________________ Carbon content % _________0.4_________ (from analysis of alloy) No. 3 Number of micro-section _____________________ Steel grade _______________________ Micro structure _______________________ Amount % perlite _______________________ Carbon content % __________0.8_________ (from analysis of alloy) No. 4 Number of micro-section ___________________ Steel grade _______________________ Micro structure _______________________ Carbon content % ________1.2____________ (from analysis of alloy) No. 5 Number of micro-section _____________________ Steel grade _______________________ Micro structure _______________________ Carbon content % __________4.0__________ (from analysis of alloy)
