Lecture18 - ME 382 Lecture 18 CAST IRONS Alloys of iron...

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ME 382 Lecture 18 22/ii/06 1 C AST IRONS Alloys of iron with > 1.7 wt % C The eutectic reaction provides a low melting point needed for casting E.g. , Medium carbon steel: T m = 1500 ° C 4% cast iron: T m = 1160 ° C White cast iron : α + Fe 3 C Large volume fraction of cementite, which is brittle Hard and brittle Cannot be machined, must be ground to final shape Cannot be used in tension Excellent resistance to wear & abrasion Grey cast iron: Contains 2 - 5 wt % Si (ternary alloy) Alters thermodynamics - Fe -C more stable than Fe-Fe 3 C Contains graphite flakes Graphite flakes result in planes of weakness Fairly brittle However, grey cast iron also (i) Self lubricates (good wear properties) (ii) Is machinable (chips form at graphite flakes) (iii) Has high damping coefficients (absorbs vibrations) Spheroidal graphite irons : Graphite converted to spherical form (from flakes) relatively ductile cast iron Promoted by addition of Mg, Ce or Ca
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ME 382 Lecture 18 22/ii/06 2 A LLOY S TEELS Steels that contain other elements besides carbon. These elements are added (i) To improve hardenability of steel (ii) To give additional solid-solution strengthening (iii) To give additional precipitation hardening (iv) To form carbides (v) To give corrosion resistance (vi) To stabilize austenite (f.c.c. austenitic steel at room temperature) or ferrite (vii) To help processing by, for example, reacting with embrittling elements Hardenability Very strong steels need to be quenched to 100 % martensite & then tempered Critical cooling rate (c.c.r.) depends on composition and microstructure e.g., Fine-grained materials have more nucleation sites for formation of
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Lecture18 - ME 382 Lecture 18 CAST IRONS Alloys of iron...

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