Introduction to Gray Cast Iron Properties & Production
By N.Ramamurthi
1. Standards for gray iron specify
a) Mechanical properties :
Hardness
Strength
b) Chemical composition (in special application)
2. Mechanical properties demand on
a) chemical composition (this includes alloying when used)
b) cooling rate (section thickness, mold material)
c) process of melting and inoculation
d) raw materials used
3. Normally used gray irons (unalloyed) have the following rang of
Composition
C – 2.8 – 40%
Si – 1.0 – 3.2
Mn – 0.25 – 0.9%
S – 0.03 – 0.15%
P – 0.05 – 0.2% (up to 1.0% in special cases)
4. Approx. estimate of UTS in 30mm (1.2) test bar [equivalent to 15mm
(0.6)casting thickness]
UTS (MPa) = 787 - 150 x C%- 47 x Si% + 55 x Mn% + 219 x S%
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IIF-WR Electrotherm Casting Clinic
5. Approx. estimate of Hardness in 30mm(1.2”) test bar *equivalent
to 15mm (0.6”) casting thickness+
BHN= 444 – 71 x C% - 14 x Si% + 21 x Mn% + 170 x S%
6. Actual values of UTS and BHN will depend on other factors also.
a) Cooling rate of mold (property of sand)
b) Melting and inoculation process
c) Raw materials used
Actual values will fall between 90% and 110% of estimated values.
7. Two empirical values have been defined
a) C.E. = C%+ Si%/3 + P%/3
C.E. affects physical and mechanical properties of solidified gray iron
C.E. affects microstructure also.
b) C.E.L = C% + Si%/4 + P%/2
This CEL affects solidification behavior of liquid gray iron. CEL affects
molting temperature, shrinkage tendency. For in temperature
depends on CEL
8. Effects of Carbon :
a) Decreases UTS and hardness
b) Improves machinability
c) Reduces melting temperature (Liquidus Temperature) and increases
fluidity
d) Reduces shrinkage tendency
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IIF-WR Electrotherm Casting Clinic
9. Effects of Silicon :
a) Silicon goes into solution in ferritic matrix, increasing hardness of
ferrite
b) Supplements effects of carbon. Hence, the terms – C.E & C.E.L
c) Promotes gray solidification and reduces chilling
d) Acts as a carrier for inoculating elements
Ca, Ba, Al, Zr, Sr, etc
10. Effects of Manganese
a) Nullifies the bad effects of Sulphur
Mn = 1.7 x S% is required for this.
b) Mn above this increases hardness up to a certain value and then
reduces.
c) Suggested value for Mn - 0.6 – 0.7% for most iron castings
11. Effects of Sulphur
a) Increases UTS and hardness
b) Improves efficiency of inoculation suggested S range – 0.06-0.08%
c) At higher levels (above 0.1%) reduces strength at high temperature.
12. Effects of Phosphorus
a) Increases of fluidity
b) Presence of phosphide eutectic causes shrinkage porosity, high
hardness and poor machinability
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IIF-WR Electrotherm Casting Clinic
c) Since phosphorus forms a low melting compound (fe3 p), metal
penetration and finning increases
d) Suggested values - <0.10%
13. Another element of importance in Nitrogen C% this is mostly from
Coke or Carbon additions
a) Nitrogen increases UTS up to 90 ppm.
b) Above 90ppm, pinhole porosity increases
c) Above 150ppm, fissure defects may occur. this can be seen under
microscope since graphite flakes appear thick and shorts.
14. Melting Practice :
a) Do not change raw material often – particularly for cupola melting
b) Do not increases metal temperature beyond the required values of
nucleation is reduced and chilling tendency increases for same
composition. This is particularly true for induction furnace melting.
c) If alloying elements are to be added, make sure metal temperature is
sufficiently high. Advisable to add to the charge rather then to the
liquid metal
d) Use good inoculant of proper size. Method of addition must ensure
proper mixing with liquid metal. If necessary stir with steel rod
Up to 50kg metal – use 1-3mm size
50 – 250 kg – 1-4mm
250 – 500 kg – 2-6mm
>500kg – 5-10mm
INOCULATE AS LATE AS POSSIBLE
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IIF-WR Electrotherm Casting Clinic
15. Pouring Practice :
a) Each casting requires a specific pouring temperature ideally. Actual
pouring temperature must be close to this. If by chance, temperature
at pouring point is higher, add clear returns/spillage to reduce
temperature
b) Keep ladles dry, hot and clean to reduce temperature drop in ladle.
c) Reduce time of metal transfer from furnace to mould as much as
possible.
d) In winter/ rainy season, molds may become colder on standing. This
will require metal to be poured at a slightly higher temperature.
e) Keep the lip of ladle as close to mould sprue as possible. Pouring
height must be as low as possible.
f) Left over metal in the ladle will cool the metal during next tap.
Preferably, pig this metal by transfer to the furnace back .
g) Use an insulated (glass wool) cover on ladle to reduce temperature
loss.
16 Quality control in melt shop :
(i) Chill / Wedge
a) Use a standard wedge to estimate chilling and grain structure of
metal. This is the best judge of metallurgical quality.
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IIF-WR Electrotherm Casting Clinic
b) Metal must be stirred well before sampling and pouring
of wedge. Do not take metal sample from the top, but about
2-3” below surface.
c) Chill/wedge samples must be cooled in air/sand until about
700 °C (dull red). Then it should be cooled in green sand up to
200 °C /300 °C and then only dipped in water
(ii)Thermal Analysis :
a) Pour sample into the thermal cup as per recommendation of
equipment supplier
b) Accuracy of C% in about +/- 0.05% . accuracy for Si% is about +/-
0.15%
c) If major additions are made to furnace for correction, repeat
sample must be taken to ensure that correction has taken place.
(iii) Furnace temperature & pouring temperature
Both must be measured and recorded. This factor affects
metallurgical quality in a big way.
(iv) Pouring time :
It is advisable to measure and record pouring time for sample
casting.
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