cast irons

Question 1

Solidification sequence of cast irons

Answer 1

-As exist different types of cementite (eutectic, primary), can be applied to graphite.
-Depending on cooling rate, alloying elements cast irons may undergo a metastable or stable transformation.
-The sequence of stable solidification and metastable progression is common.

Question 2

effects of carbon
kind of cast irons

Answer 2

• C precipitate as cementite –> white cast iron
• Graphite makes the surface grey–> grey cast iron
• Grey cast iron is made by alloying 1% - 3% Si
• carbon eq= %C + 1/3%Si

Question 3

graphite formation

Answer 3

• nucleation: enhanced by oxide and sulphite forming elements Al, Ca,Ce,Ti,Zr.
• natural growing of graphite in flakes form.
• adding of magnesium induce forming of nodular graphite
• a balance between flakes and nodular si vermicular

Question 4

white irons are formed by:

Answer 4

-fast solidifications–> quencing limits the C diffiusion and promote precipitation of ledebourite rather then graphite (also using iron chills).
-Increasing (Mn Cr Mo)/Si ratio to promote carbide formation
-Less concentration of Si.

Question 5

Diamond bonds

Answer 5

3D Strong covalent bonds,
each atom is bonded with 4 other atoms.

Question 6

graphite lattice

Answer 6

2D covalent hexagonal layer,while different layer are bonded by van der walls forces wich are very weak

Question 7

mechanical modification induced by graphite

Answer 7

• lower density
• lower ductility
• lower coeff of friction
• better wear behaviour
• large amount of interfaces high damping properties.
• higher thermal conductivity and lower coeff of thermal expansion
• addition of C lowers melting point
• graphite compensate Fe matrix shrinkage.

Question 8

Grey cast irons

Answer 8

• used for near net shape
• C from 2.5% to 4%
• Ni Cu act as pearlitizing effect

Question 9

EU standards based on shapes of graphite carbides (grey cast irons)

Answer 9

• GJL for flakes
• GJS spheroidal
• GJV vermicular
• GJMB black heart
• GJMW withe heart

Question 10

flake GJL

Answer 10

• Graphite flakes act as internodular notches and it gets worse the ductile and tensile strength.
• in large thickness C has a higher diffusivity–> graphite flakes
• in small thickness pearlitic matrix and small grains
• allow high thermal conductivity means good thermal fatigue resistance
• highthermal conductivity
• good thermal fatigue

Question 11

Nodular or spherical GJS

Answer 11

• better mechanical properties than flakes irons
• the spherical shape is obtain bytreating the melt with Mg
• The basic composition of GJS si the same of GJL with 1.7-2.8% of Si.

Question 12

vermicular graphite GJV

Answer 12

• its mechanical properties are in the middle between GJS and GJL

Question 13

malleable cast irons (whiteheart and blackheart)

Answer 13

• anneling treatment of white cast irons, cementite decomposes.
• inert atmosphere –> blackheart
• oxygen atmosphere (T 1000-1500 °C)–> whiteheart

Question 14

Q&T

Answer 14

is suitable only for spheroidal graphite cause his less internal tension

Question 15

application of cast irons

Answer 15

-Tombini fontanelle

Question 16

applications of flakes irons

Answer 16

-engines blocks
-caloriferi
- tombini
- pipes

Question 17

Applications of nodular

Answer 17

• Mechanical part subjected to loaded
• Crankshaft, brake pads

Question 18

Applications of vermicular

Answer 18

_ Brake disks cause damping properties

Question 19

applications of ADI cast irons

Answer 19

• ADI means austempering (heating and quencing)
• fliudity in molten phase allow complex shape
• low weight
• weel hubs and crankshaft

Question 20

Thermal fatigue

Answer 20

• change in thickness, thermal cycling induce thermal stresses
• the thermal fatigue is similar to mechanical but there are some modifications on microstructure