Lesson 6 - Cast Irons Flashcards
How is cast iron produced?
By the processing of pig iron, which is the ‘raw iron’ produced from the reduction of iron ore.
When making cast irons, particularly;lar efforts must be made to remove what and what?
Sulphur and Phosporus.
How does cast irons melting point compare to that of carbon steel?
It is lower at about 1150 to 1200 C
What is ledeburite? And when does it exist?
A eutectic mixture of austenitic and cementite.
It exists when 2.06-6.67%C content present.
Cast irons contain significant amounts of silicon, how does this affect it?
Has a big effect on microstructure of cast iron as it solidifies, and thus, influence on mechanical properties.
Also helpful in the casting process as it increases the fluidity of the mo;ten iron and reduces shrinkage during solidification of the cast iron.
Properties of grey cast iron.
- Pearlite/Ferrite matrix
- Rather brittle (not as bad a white cast iron)
- Low tensile strength
- Contain graphite flakes
- Graphite is good lubricant, so grey cast iron is
easily machinable - Graphite has low mechanical strength so the
flakes are like ‘voids’ reducing effective cross
sectional area of the cast iron. Meanwhile
sharp corners can cause stress concentration
- Graphite is good lubricant, so grey cast iron is
What is white cast iron?
Type of carbon-iron alloy that contains about ~1% silicon and more than 2% carbon in the form of cementite.
When it solidifies, the cementite cannot be decomposed into ferrite and graphite and therefore, white cast iron contains cementite.
What are the two main processes for malleable cast iron?
- Whiteheart process
- Blackheart process
Why is white cast irons of almost no use as an engineering material?
It is very brittle due to presence of cementite
What promotes the formation of white cast iron?
Rapid cooling (for example in thin sections)
What happens in the whiteheart process?
- Castings are cooked between 900 and 950C in any oxidising atmosphere to remove carbon from the surface for 5 to 6 days, and are then cooled very slowly.
- In addition to being very malleable, whiteheaert ct iron can also be welded fairly easily
What happens in the blackheart process?
-The neutral atmosphere is used, and the cooking temperature between 850-950*C
- The microstructure produced in blackheart process is similar to that of whiteheart process, but more graphite is present.
What happens in spheroidal graphite (SG) cast irons (Ductile cast iron)?
- Calcium carbide is used to remove the sulphur traces (so it has low sulphur content and good ductility)
- A little cerium/magenisum is added immediately prior to casting to increase the nucleation sites which cans result in a higher number of nodules
-The graphite precipitates as spheroids, which avoids stress concentration
-Hence, SG cast irons is more ductile than ordinary grey cast iron.
What are the general attributes of cast iron?
- Most contain 2-4% carbon, 1-3% silicon
- Less shock resistance and tensile strength than
those of steel. - Compressive strength is similar to those of low and
medium carbon steels.
What are the plus points of cast iron? And then the plus points of all apart from white cast irons?
Plus point of all:
- Relatively low melting point
- Good fluidity and hence easy to cast
- Low cost
Non white cast iron:
- Good machinability
- Good resistance to deformation and wear
- Good corrosion resistance
What can white cast iron be used for?
- Too brittle for structural components but can be used in rough applications due to even better hardness, abrasion resistance and cost.
- For example the wear surfaces of slurry pumps
- Further heat treatments may be used to improve the mechanical properties of white cast iron to make it malleable.
What is wrought iron? and what has it been used for?
- An iron alloy with a very low carbon content in comparison to cast iron and carbon steel.
- Has good strength, ductility and toughness, and it is easy to use in fabrication (malleable, machinable, weldable)
- Versatile material that has been used for application such as crane hooks, chains and anchors or even ornamental iron work such as gates and fences.
