Spheroidal graphite cast irons
Spheroidal graphite cast irons is also known as nodular cast iron, ductile cast iron, highdutycast iron, etc.

The addition of magnesium or cerium to molten grey cast iron prevents the formation of flake graphite upon cooling and solidification. Instead, the uncombined carbon is distributed as fine spheroids throughout the mass of the casting. This results in a more homogeneous structure having greater strength and ductility and less susceptibility to fatigue failure.

BS EN 1563: Spheroidal graphite cast irons
BS EN 1563 specifies the requirements for spheroidal or nodular graphite cast irons. Again, the standard does not specify the chemical composition of the iron, its method of manufacture or any subsequent heat treatment. The standard is solely concerned with the properties, testing and quality control of the finished castings. How this is attained is left to the discretion of the foundry in consultation with the customer. It is a very comprehensive standard and it is only possible to review briefly some of its more important points within the scope of this chapter. The standard itself should be consulted for more detailed study.

Mechanical properties measured on test pieces machined from separately cast samples

Material Designation		Tensile strength Rm (N/mm2) min.	0.2% s proof stress Rp0.2 (N/mm2) min.	Elongation A (%) min.
Symbol	Number
EN-GJS-350-22-LTa	EN-JS1015	350	220	22
EN-GJS350-22-RTb	EN-JS1014	350	220	22
EN-GJS-350-22	EN-JS1010	350	220	22
EN-GJS-400-18-LTa	EN-JS1025	400	240	18
EN-GJS-400-18-RTb	EN-JS1024	400	250	18
EN-GJS-400-18	EN-JS 1020	400	250	18
EN-GJS-400-15	EN-JS1030	400	250	15
EN-GJS-450-10	EN-JS1040	450	310	10
EN-GJS-500-7	EN-JS1050	500	320	7
EN-GJS-600-3	EN-JS1060	600	370	3
EN-GJS-700-2	EN-JS1070	700	420	2
EN-GJS-800-2	EN-JS1080	800	480	2
EN-GJS-900-2	EN-JS1090	900	600	2

^aLT for low temperature.

^bRT for room temperature.

Note:

1. The values for these materials apply to castings cast in sand moulds of comparable thermal diffusivity. Subject to amendments to be agreed upon in the order, they can apply to castings obtained by alternative methods.
2. Whatever the method used for obtaining the castings, the grades are based on the mechanical properties measured on test pieces taken from samples separately cast in a sand mould or a mould of comparable thermal diffusivity.
3. 1 N/mm² is equivalent to 1 MPa.
4. The material designation is in accordance with EN 1560.

Minimum impact resistance values measured on V-notched test pieces machined from separately cast samples

Material Designation		Minimum Impact resistance values (in J)
		At room temperature		At		At
Symbol	Number	Mean value from 3 tests	Individual value	Mean value from 3 tests	Individual value	Mean value from 3 tests	Individual value
EN-GJS-350-22-LTa	EN-JS1015	–	–	–	–	12	9
EN-GJS-350-22-RTb	EN-JS1014	17	14	–	–	–	–
EN-GJS-400-18-LTa	EN-JS1025	–	–	12	9	–	–
EN-GJS-400-18-RTb	EN—JS1024	14	11	–	–	–	–

^aLT for low temperature.

^bRT for room temperature.

Note:

1. The values for these materials apply to castings cast in sand moulds of comparable thermal diffusivity. Subject to amendments to be agreed upon in the order, they can apply to castings obtained by alternative methods.
2. Whatever the method used for obtaining the castings, the grades are based on the mechanical properties on test pieces taken from samples separately cast in a sand mould or a mould of comparable thermal diffusivity.
3. The material designation is in accordance with EN 1560.

Alloy cast irons

The alloying elements in cast irons are similar to those in alloy steels:

Nickel is used for grain refinement, to add strength and to promote the formation of free graphite. Thus it toughens the casting.

Chromium stabilizes the combined carbon (cementite) present and thus increases the hardness and wear resistance of the casting. It also improves the corrosion resistance of the casting, particularly at elevated temperatures. As in alloy steels, nickel and chromium tend to be used together. This is because they have certain disadvantages when used separately which tend to offset their advantages. However, when used together the disadvantages are overcome whilst the advantages are retained.

Copper is used very sparingly as it is only slightly soluble in iron. However, it is useful in reducing the effects of atmospheric corrosion.

Vanadium is used in heat-resisting castings as it stabilizes the carbides and reduces their tendency to decompose at high temperatures.

Molybdenum dissolves in the ferrite and, when used in small amounts (0.5%), it improves the impact strength of the casting. It also prevents ‘decay’ at high temperatures in castings containing nickel and chromium. When molybdenum is added in larger amounts it forms double carbides, increases the hardness of castings with thick sections, and also promotes uniformity of the microstructure.

Martensitic cast irons contain between 4% and 6% nickel and approximately 1% chromium (e.g. Ni-hard cast iron). This is naturally martensitic in the cast state but, unlike alloys with rather less nickel and chromium, it does not need to be quench hardened, thus reducing the possibility of cracking and distortion. It is used for components which need to resist abrasion. It can only be machined by grinding.

Austenitic cast irons contain between 11% and 20% nickel and up to 5% chromium. These alloys are corrosion resistant, heat resistant, tough, and non-magnetic.

Since the melting temperatures of alloy cast irons can be substantially higher than those for common grey cast irons, care must be taken in the selection of moulding sands and the preparation of the surfaces of the moulds. Increased venting of the moulds is also required as the higher temperatures cause more rapid generation of steam and gases. The furnace and crucible linings must also be suitable for the higher temperatures and the inevitable increase in maintenance costs is also a significant factor when working with high alloy cast irons.

The growth of cast irons is caused by the breakdown of pearlitic cementite into ferrite andgraphite at approximately 700°C. This causes an increase in volume. This increase in volumeis further aggravated by hot gases penetrating the graphite cavities and oxidizing the ferrite grains. This volumetric growth causes warping and the setting up of internal stresses leading to cracking, particularly at the surface. Therefore, where castings are called upon to operate at elevated temperatures, alloy cast irons should be used. A low cost alloy is Silal which contains 5% silicon and a relatively low carbon content. The low carbon content results in a structure which is composed entirely of ferrite and graphite with no cementite present. Unfortunately Silal is rather brittle because of the high silicon content. A more expensive alloy is Nicrosilal. This is an austenitic nickel-chromium alloy which is much superior in all respects for use at elevated temperatures.

Three typical alloy cast irons are listed in Section below together with their properties, composition and some uses.

Composition, properties and uses of some typical cast irons

Type of iron	Composition (%)	Representative mechanical properties
		Tensile strength (N/mm2)	Hardness (Brinell)	Uses
Grey iron	3.30C 1.90Si 0.65Mn 0.10S 0.15P	Strength vary with sectional thickness but generally are in the range 150-350N/mm2		Motor vehicle brake drums.
Grey iron	3.25C 2.25Si 0.65Mn 0.10S 0.15P	Strength vary with sectional thickness but are generally in the range 150-350 N/mm2	–	Motor vehicle cylinders and pistons
Grey iron	3.25C 1.25Si 0.50Mn 0.10S 0.35P		–	Heavy machine castings
Phosphoric grey iron	3.60C 1.75Si 0.50Mn 0.10S 0.80P		–	Light and medium water pipes
Chromidium	3.20C 2.10Si 0.80Mn 0.05S 0.17P 0.32Cr	275	230	Cylinder blocks, brake drums clutch casings, etc.
Wear and shock resistant	2.90C 2.10Si 0.70Mn 0.05S 0.10P 1.75Ni 0.10Cr 0.80Mo 0.15Cu	450	300	Crankshafts for diesel and petrol engines (good strength, shock resistance and vibration damping capacity)
Ni-resist	2.90C 2.10Si 1.00Mn 0.05S 0.10P 15.00Ni 2.00Cr 6.00Cu	215	130	Pump castings handling concentrated chloride solutions: an austenitic corrosion resistant alloy.