Chapter 5 Metal-Casting Processes and Equipment Heat...

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Chapter 5 Metal-Casting Processes and Equipment; Heat Treatment Questions 5.1 Describe the characteristics of (1) an alloy, (2) pearlite, (3) austenite, (4) martensite, and (5) cementite. (a) Alloy: composed of two or more elements, at least one element is a metal. The al- loy may be a solid solution or it may form intermetallic compounds. (b) Pearlite: a two-phase aggregate consisting of alternating lamellae of ferrite and ce- mentite; the closer the pearlite spacing of lamellae, the harder the steel. (c) Austenite: also called gamma iron, it has a fcc crystal structure which allows for a greater solubility of carbon in the crys- tal lattice. This structure also possesses a high ductility, which increases the steel’s formability. (d) Martensite: forms by quenching austen- ite. It has a bct (body-centered tetrag- onal) structure, and the carbon atoms in interstitial positions impart high strength. It is hard and very brittle. (e) Cementite: also known as iron-carbide (Fe 3 C), it is a hard and brittle intermetal- lic phase. 5.2 What are the effects of mold materials on fluid flow and heat transfer? The most important factor is the thermal con- ductivity of the mold material; the higher the conductivity, the higher the heat transfer and the greater the tendency for the fluid to solid- ify, hence possibly impeding the free flow of the molten metal. Also, the higher the cooling rate of the surfaces of the casting in contact with the mold, the smaller the grain size and hence the higher the strength. The type of surfaces developed in the preparation of mold materi- als may also be different. For example, sand- mold surfaces are likely be rougher than those of metal molds whose surfaces can be prepared to varying degrees of roughness, including the directions of roughness (lay). 5.3 How does the shape of graphite in cast iron af- fect its properties? The shape of graphite in cast irons has the fol- lowing basic forms: (a) Flakes. Graphite flakes have sharp edges which act as stress raisers in tension. This shape makes cast iron low in tensile strength and ductility, but it still has high compressive strength. On the other hand, the flakes also act as vibration dampers, a characteristic important in damping of machine-tool bases and other structures. (b) Nodules. Graphite can form nodules or spheroids when magnesium or cerium is 57
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added to the melt. This form has in- creased ductility, strength, and shock re- sistance compared to flakes, but the damp- ing ability is reduced. (c) Clusters. Graphite clusters are much like nodules, except that they form from the breakdown of white cast iron upon anneal- ing. Clusters have properties that are ba- sically similar to flakes. (d) Compacted flakes. These are short and thick flakes with rounded edges. This form has properties that are between nodular and flake graphite.
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