cast irons Flashcards
depending on …..cast iron may
undergo metastable or stable transformations, or even both in
succession
cooling rate (casting process and wall
thickness) and the added alloying elements
(sequence of stable solidification (at highest temperatures)
and a subsequent metastable progression in the region close
to the eutectoid is also common)
effects of carbon on cast iron colour
✓ C precipitated as cementite gives fracture surfaces a light-coloured metallic appearance, therefore white cast iron
✓ graphite makes them look grey, thus the name of grey cast iron
✓ Further characteristic microstructural features are the shape of the graphite precipitates (flaky to nodular) and the steel-like matrix generally consisting of pearlite and ferrite
✓ The carbon activity increases with the silicon content
how can we obtain gray microstructures
by alloying with 1 to 3 %Si
how can we enhance (migliorare) nucleation of graphite
adding oxide-forming and sulphide-forming elements
such as Al, Ca, Ce, Ti, Zr, etc.
which shape does graphite naturally assumes growing
shape of flakes
how can we induce the growth of nodular graphite
adding magnesium (also Ca,Ce)
balance between flake graphite and nodular graphite is…
vermicular graphite
how can we form white cast irons
✓ fast solidification
(to limit C diffusion and to promote precipitation of ledeburite containing Fe3C rather than graphite)
The supercooling is also exploited to produce a wear-resistant material.
✓ quenching against iron chills in the mould (chill casting) or in thin cross-sections (durante il processo di colata, il materiale fuso viene raffreddato rapidamente utilizzando inserti di raffreddamento in ferro nella forma di colata (chill casting) o in sezioni trasversali sottili del pezzo. Questo aiuta a controllare la solidificazione del materiale)
✓ increasing (Mn, Cr, Mo)/Si ratio to promote carbide formation, as opposed to graphite
✓ white cast iron also contains less Si (0.5 to 1.5 %) which is a graphite promoter element
how diamond is build
Diamond is built by the 3D arrangement of C atoms sharing electrons with other four carbon atoms to create strong
covalent bonds
why graphite has a high degree of anisotropy
because In graphite, the C atoms are set hexagonally in a planar ring system and the layers are stacked parallel to each other. The atoms within the rings are bonded covalently, while the layers are linked by weak van der Waals forces
modifications induced by graphite in cast irons
✓ Density of graphite is much lower and this makes cast irons lighter than steels
✓ Graphite is soft and brittle with respect to Fe, this leads to decreased mechanical properties and much lower ductility
✓ The lower coefficient of friction and brittleness of graphite lead to improved machinability of cast irons
✓ For similar reasons, also the wear behaviour (sliding on a second body) is improved
✓ The large amount of interfaces in the composite structure also provides high damping properties (friction generated due to relative movements between phases)
✓ Higher thermal conductivity and lower CTE of graphite help making the material more resistant to thermal fluctuations (e.g. thermal fatigue)
✓ Not shown in the table, addition of C lowers the melting point of cast irons, making them more suitable for casting
✓ Moreover, the formation of graphite from dissolved C in the liquid results in expansion, which partially compensates for the shrinkage of the Fe matrix upon solidification
how european standards classify different types of grey cast irons
with the shape of the graphite precipitates
✓ cast iron with flake graphite, GJL series
✓ cast iron with spheroidal graphite (ductile cast iron), GJS series
✓ cast iron with vermicular graphite, GJV series
✓ black-heart malleable cast iron, GJMB series
✓ white-heart malleable cast iron, GJMW series
which is the natural precipitate shape in hypoeutectic Fe-C-Si alloys
Flake (lamellar) graphite
effect of graphite flakes on tensile strenght and elongation at fracture
Graphite flakes act as internal notches and thus have a negative effect
GJL exhibits dependence on the wall thickness.
- GJL exhibits a pronounced dependence of the mechanical properties on the wall thickness. The cooling rate in thin walls is high so that the graphite precipitates remain small and the matrix is pearlitic
- In large-thickness walls, C can diffuse to higher extent to the graphite flakes, leaving a softer ferritic matrix
Cast iron with flake graphite is often used because of its….
(tell also the consequences of this good property)
Cast iron with flake graphite also exhibits…
- …high thermal conductivity (λ)
- λ decreases with increasing pearlite fraction in the metal matrix
- high thermal conductivity is also responsible for a good thermal fatigue resistance.
- … a remarkable wear behavior (particularly under sliding wear conditions) The graphite flakes provide an internal reservoir of lubricant that guarantees certain emergency service
what is GJS
Cast iron with spheroidal graphite (GJS), also known as ductile cast iron, nodular cast iron or spheroidal cast iron
advantages of GJS
notch effect of nodules is significantly lower than that of flakes, so that the mechanical properties of are superior
in GJS how can we obtain the spherical shape of C
treating the melt with Mg, Ca or Ce.
what is GJV
Cast iron with vermicular graphite (GJV) properties of GJV represent a compromise between those of GJL and GJS
what are malleable cast irons
materials that have originally solidified from a hypoeutectic Fe-C-Si melt as white cast iron, that are then subjected to annealing
✓ This annealing treatment brings the material back towards (or close to) the equilibrium state and the cementite
decomposes
✓ If annealing is carried out in an inert atmosphere, the cementite transforms into graphite aggregates and blackheart malleable cast iron is produced
✓ If the white cast iron is annealed in an oxygen-containing atmosphere, the carbon is oxidised and mostly forced
out of the surface zone (as CO2
, decarburisation) to produce whiteheart malleable cast iron
how do we produce whiteheart malleable cast irons
Whiteheart malleable cast irons (3.1 to 3.4%C) are produced by annealing white cast irons for several hours between
1000 at 1050°C in a weakly oxidising atmosphere
✓ The cementite in the surface that is accessible to the oxygen thus decomposes into CO2 whereas the Fe3C in the
remaining cross-section is transformed into temper graphite
✓ For very long annealing times (several days), castings with small wall thicknesses (< 15mm) exhibit a completely
ferritic microstructure, while for larger wall thicknesses, the core consists of temper graphite in a predominantly
pearlitic matrix
how can we tailor (personalizzare) mechanical properties of cast irons
mechanical properties of cast irons may be tailored after casting by subsequent quenching and tempering, or
transformation in the bainite range
problem of QeT in cast irons (apart spheroidal graphite)
other graphite shapes act as sharp internal notches and would cause cracks during quenching
what are austempered ductile irons (ADI)
Transformation in the bainite range produces bainitic spheroidal graphite cast irons as an independent group of materials also referred to as austempered ductile irons (ADI).
Casting properties of cast irons
✓ grey cast iron has a small solidification interval and a low melting temperature (due to proximity to the eutectic) This is why cast iron components can be produced much more cheaply than steels
✓ The good mould filling capacity of cast iron melts
✓ The volume reduction during solidification is small owing to the small solidification interval and decreases even further for large specific volume of the precipitating graphite (the solidification shrinkage can even be completely compensated)
machining behaviour of cast irons
✓ Functional surfaces and mounting dimensions of castings have to be adjusted by machining
✓ Grey cast iron is very suitable for machining because the graphite precipitates produce chips that break into small pieces and act as a lubricant on the working surfaces of the cutting tools
✓ Machinability decreases in the order GJL→GJV→GJS
applications of cast irons
- engine blocks for big engines as the ones of trucks or boats
- valves and pumps
- GJV is a reference materials for brake disks owing to damping properties (less noise), thermal conductivity and thermal fatigue resistance
- GJS it is used in automotive components such as steering gears, rear axles, steering knuckles, brake pads for commercial vehicles also used for crankshafts and camshafts
applications of ADI cast irons
✓ ADI is particularly suitable for applications in the automobile industry where it is used for crankshafts and
wheel hubs
Thermal fatigue
Strains can be induced in materials not only by an external applied stress field, but also due to thermal gradients.
When thermal stresses are fluctuating over time, cycling strains can induce fatigue damage
Thermal fatigue additional variables wrt mechanical fatigue
✓ The high temperature plays a role on material (oxidation, creep, softening, grain growth and other microstructural
modifications)
✓ In several components, mechanical (MF) and thermal fatigue (TF) are acting together (thermo-mechanical fatigue, TMF) on the structure. MF and TF can be in phase or not
why cast iron can be produced much more cheaply than steels
grey cast iron has a small solidification interval and a low melting temperature (due to proximity to the eutectic) This is why cast iron components can be produced much more cheaply than steels
advantages of ADI cast irons
ADI cast irons have advantages over cast steels:
* good fluidity during casting
* short ageing time during heat treatment
* lower specific weight allows lighter designs at the same strength
* high fatigue strength
* better damping properties
