Solidification sequence of cast irons
-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.
effects of carbon
kind of cast irons
- 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
graphite formation
- 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
white irons are formed by:
-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.
Diamond bonds
3D Strong covalent bonds,
each atom is bonded with 4 other atoms.
graphite lattice
2D covalent hexagonal layer,while different layer are bonded by van der walls forces wich are very weak
mechanical modification induced by graphite
- 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.
Grey cast irons
- used for near net shape
- C from 2.5% to 4%
- Ni Cu act as pearlitizing effect
EU standards based on shapes of graphite carbides (grey cast irons)
- GJL for flakes
- GJS spheroidal
- GJV vermicular
- GJMB black heart
- GJMW withe heart
flake GJL
- 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
Nodular or spherical GJS
- 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.
vermicular graphite GJV
- its mechanical properties are in the middle between GJS and GJL
malleable cast irons (whiteheart and blackheart)
- anneling treatment of white cast irons, cementite decomposes.
- inert atmosphere –> blackheart
- oxygen atmosphere (T 1000-1500 °C)–> whiteheart
Q&T
is suitable only for spheroidal graphite cause his less internal tension
application of cast irons
-Tombini fontanelle
applications of flakes irons
-engines blocks
-caloriferi
- tombini
- pipes
Applications of nodular
- Mechanical part subjected to loaded
- Crankshaft, brake pads
Applications of vermicular
_ Brake disks cause damping properties
applications of ADI cast irons
- ADI means austempering (heating and quencing)
- fliudity in molten phase allow complex shape
- low weight
- weel hubs and crankshaft
Thermal fatigue
- change in thickness, thermal cycling induce thermal stresses
- the thermal fatigue is similar to mechanical but there are some modifications on microstructure
