Manufacturing Processes Chapter 3 Structural & Manufacturing

M2794.001800 M A T E R I A L A N D M A N U F A C T U R I N G P R O C E S S E S

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

Prof. Ahn Sung-Hoon ( )

School of Mechanical and Aerospace Engineering Seoul National University 2 Historical casting parts

Korean bronze dagger( ( )) Bronze bell( ) & molds( )

. Casting is a manufacturing process by which a molten material such as metal or plastic is introduced into a mold made of sand or metal, allowed to solidify within the mold, and then ejected or broken out to make a fabricated part.

. Advantages . Making parts of complex shape in a single piece. . Producing large number of identical castings within specified tolerances. . Good bearing qualities and jointless product.

. Disadvantages . Limitations of mechanical properties because of the polycrystalline grain structure. . Poor dimensional accuracy due to shrinkage of metal during solidification. . If the number of parts cast is relatively small, the cost per casting increases rapidly.

. Fundamental aspects in casting operations . Solidification of the metal from its molten state. . Flow of the molten metal into the mold cavity. . Heat transfer during solidification and cooling of the metal in the mold. . Mold material and its influence on the casting process.

. Solute( ) . Solvent( )

. When the particular crystal structure of the solvent is maintained during alloying, the alloy is called solid solution. . Substitutional solid solution( ) . Interstitial solid solution( )

. 5.2.2 Intermetallic compound( ) . Complex structures in which solute atoms are present among solvent atoms in certain specific proportions.

. 5.2.3 Two-phase system( ) . Phase: a homogeneous portion of a system that has uniform physical and chemical characteristics

Graphically illustrates the relationships among temperature, composition, and the phase present in a particular alloy system.

Lever Rule S C  C  O L S  L CS  CL L C  C  S O S  L CS  CL

. Pure iron( ) : 0.008% C . Steels( ) : 2.11% C . Cast irons( ) : ~6.67% C . a-ferrite( ): BCC, soft and ductile . d-ferrite: BCC, stable only at very high temperatures . Austenite( ) : FCC, ductile

. Cementite( ): Fe3C, C 6.67%, iron carbide( ), brittle

a-ferrite (x 90) Austenite (x325)

. a- ferrite: white

. Fe3C: dark

. Lamellar structure (pearlite)

. (x 500)

. a- ferrite - white . eutectoid - cementite - blue . proeutectoid - cementite - violet

. (x 500)

Casting 1040 steel 10kg, calculate a phase and g phase at (a) 900 C, (b) 728 C and (c) 726 C

(a) Austenite:100% g

 Cg  Co   0.77  0.40  (b) a(%)   100  100  50%, that is 5kg,      Cg  Ca   0.77  0.022   C  C   0.40  0.022  g (%)   o a 100  100  50%, that is 5kg,      Cg  Ca   0.77  0.022 

 6.67  0.40  (c) a   100  94%, that is 9.4kg  6.67  0.022 

. Fe, C 2.11~4.5%, Si ~3.5%

. According to solidification morphology :

Gray cast iron( ) . Flake graphite( ) . Gray fracture surface( ) . Damping( ) Ductile(nodular) iron( ) . Ductile White cast iron( ) . Large amount of Fe3C . Brittle . White fracture surface( ) Malleable cast iron( ) . Obtained by annealing white cast iron Compact graphite iron( )

. Fe-Cr-Ni

. Pure metal vs. alloys

v  c 2gh p v2 h    constant g 2g p v 2 p v 2 h  1  1  h  2  2  f 1 g 2g 2 g 2g

Q  A1v1  A2v2 A h 1  2 A2 h1 vD Re  

. Chvonrinov’s rule Solidification time = C(volume/surface area)2

. Shrinkage occurs at . Molten metal . Phase change . Solid metal

. Cast iron expands . Graphite has high volume/mass . Net expansion during precipitation

. Similarly Bi-Sn alloys expand

. Expendable mold, permanent pattern . Sand casting . Shell-mold casting . Plaster mold casting . Ceramic mold casting . Vacuum casting

. Expendable mold, expendable pattern . Evaporative-pattern casting (lost foam) . Investment casting (lost wax)

. Permanent mold . In permanent-mold casting, a mold are . Slush casting made from materials such as steel, bronze, refractory metal alloys, or . Pressure casting graphite. Because metal molds are better . Die casting heat conductors than expendable molds, . Centrifugal casting the solidifying casting is subjected to a . Squeeze casting higher rate of cooling, which turn affects . Semisolid metal forming the microstructure and grain size within . Casting for single crystal the casting. . Rapid solidification . Cooling methods : water, air-cooled fin . Used for aluminum, magnesium, and copper alloys due to their lower melting points . Pros : good surface finishing, close dimensional tolerances, and uniform and good mechanical properties . Cons : not economical for small production runs, not good for intricate shapes

. Hot-chamber process . Cold-chamber process

Crystal-pulling method floating-zone (Czochralski process) method

. Pearlite . Spheroidite . Bainite . Martensite . Quenching( ) . Body Centered Tetragonal(BCT) . Retained austenite . Tempered martensite

Austenite

Slow cooling Quenching

Moderate cooling Pearlite Martensite (a+Fe3C) (+proeutectic a) Bainite reheat (a+Fe3C) Tempered martensite

. The white strikes are excess proeutectoid cemetite . Cream color is retained austenite . Gray area is bainite . Blue/brown regions are martensite

. (x 320)

Precipitation hardening Age hardening( ) ( ), Al-Cu alloy

. Solution treatment . Precipitation hardening . Aging . Maraging(martensite + aging)

. Surface hardening . Carburizing ( ) . Carbonitriding ( ) . Cyaniding ( ) . Nitriding ( ) . Boronizing ( ) . Flame hardening ( ) . Induction hardening ( )

. Normalizing( )

Another bridge