Giáo trình Thí nghiệm Vật liệu học: Phần 2

Phần 2 cuốn giáo trình Thí nghiệm Vật liệu học cung cấp cho người đọc các kiến thức: Microstructure analysis of graphite - cast iron, hardness testing and quenching steel process, tempering quenched – steel. Mời các bạn cùng tham khảo nội dung chi tiết.
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Giáo trình Thí nghiệm Vật liệu học: Phần 2 Lesson 3 MICROSTRUCTURE ANALYSIS OF GRAPHITE - CAST IRON3.1. PURPOSE - Able to observe the microstructure of graphite-cast iron by usinga metallurgical microscope. - Able to classify graphite-cast irons through the microstructure.3.2. THEORETICAL BACKGROUND3.2.1. Definition Cast iron is generally defined as an alloy of Iron with 2.14% to6.67% carbon, more commonly 3 - 5% C. Cast iron has low meltingtemperature, brittle, and so easiest to cast. Graphite is an allotropic form of carbon that occurs as a mineral insome rocks and can be made from coke. Graphite has simple layeredhexagonal type a = 2.5Å; c/a = 2.74. The covalent bond strength in eachlayer is very strong, however, the force between layers is very weak,because the distance between layers is too far so graphite is very soft andeasy to separate (Figure 2.6). Graphite cast iron is carbon-iron alloys in which a part or all of itscarbon exists under the free state, due to the concentration of carbon atoms.3.2.2. Graphite forming conditions Through the equilibrium diagram for iron and carbon, only steeland white cast iron exist without any presence of graphite. The formationof graphite depends on many factors such as temperature, cooling rate,chemical composition. Graphite is an allotropic form of carbon that occurs as a mineral insome rocks and can be made from coke. Graphite is more stable thancementite. Cementite is a metastable phase; Graphite formation is promotedby being added Si > 1% and slowly cooling with the form of flakesGraphite. Cementite decomposes to ferrite and graphite: Fe3C → Fe (α)+ C (graphite)74 The addition of Si in the alloy Fe and carbon will inhibit the formationof Fe3C. Carbon will tend to react with Si if the amount is sufficient.Therefore, the addition of Si is only 1-4% which aims to decompose Fe3C topromote graphite formation. The slower cooling rate leads to a highergraphitization rate rather than s rapid cooling rate. If the graphitizationprocess takes place, it will form graphite in cast iron.3.2.4. Classification in graphite cast iron Basing on the shape of graphite, it is classified into three groups: - Grey cast iron: laminate graphite (Figure 3.1) - Malleable iron: graphite has the shape of dark rosettes. (Figure 3.2) - Nodular iron: spheroidal graphite iron is cast iron in which thegraphite is present as tiny balls or spheroids. (Figure 3.3) Ferrite matrix Graphite Figure 3.1. Laminate graphite (gray cast iron- ferrite matrix) (4% Nital) Graphite Figure 3.2. Dark rosettes (malleable cast iron - ferrite matrix) (4% Nital) 75 Graphite Figure 3.3. Spherical form (Nodular cast iron - ferrite matrix) (4% Nital)3.2.5. Gray cast iron It is a kind of graphite cast iron in which graphite has a plate shape.Gray iron has low hardness and is brittle due to the flake-like graphite,weak and brittle under tension, stronger under compression, excellentvibrational damping capacity, good wear resistance Depending on the level of graphitization, there are three types ofgray iron: - Ferritic gray cast iron: The microstructure is the graphite plate onthe ferrite matrix (Figure 3.1). - Ferrite + pearlite gray cast iron: The microstructure is the graphiteplate on the ferrite and pearlite matrix (Figure 3.4). - Pearlitic gray cast iron: The microstructure is the graphite plateon the pearlite matrix (Figure 3.5). Figure 3.4. Ferrite + pearlite gray cast iron (4% Nital)76 Figure 3.5. Pearlitic gray cast iron (4% Nital)3.2.6. Malleable cast iron A cast iron which has graphite cluster shape is formed by heatingwhite cast iron at high temperature ± 800°C for a prolonged time, ± 30hours. Cementite will be decomposed to graphite precipitates, clusters orrosettes. Malleable iron has higher durability than grey cast iron andmore ductile than white cast iron. The heating process of white cast iron to become malleable castiron: Figure 3.6. Process of heating white cast iron → Malleable cast iron Malleable iron has higher durability than grey cast iron. Basing onthe level of graphitization by time, there are three groups: 77 - Ferritic malleable cast iron (Figure 3.2). - Ferrite + ...