Ductile iron is distinguished from ordinary grey iron by its remarkable mechanical properties (elasticity, impact strength, elongation...). These are due to the spheroidal nature of the graphite particles.
Definition of irons
A ferrous product classification can be established in terms of the carbon content of the basic metal:
- iron : 0 to 0.1% C,
- steel : 0.1 to 1.7% C,
- cast iron or ductile iron: 1.7 to 5% C
Below 1.7 % carbon, solidification produces an austenitic single phase material, with the carbon present in the structure in solid solution. Above 1.7% carbon, the carbon cannot all be dissolved in the iron structure and consequently solidifies in the form of a secondary phase, either as graphite (pure C), or iron carbide (Fe3C). Iron is a multi-phase material, of complex structure: the most common constituents are ferrite (Feα) and pearlite (Feα + Fe3C). Other elements present in iron in very low proportions have an effect on the structure, mechanical properties and casting of the metal. Silicon (usually 1 to 3 %) plays a particular part and makes the iron in reality, a ternary alloy: iron, carbon, silicon.
Different types of cast iron
The term “Cast iron” covers a wide variety of Fe-C-Si alloys. It is usual to class them in groups according to the graphite condition, making an additional differentiation based on the structure of the metal matrix (ferritic, pearlitic).
Effect of graphite shape
In so-called “grey irons” the graphite is present in the form of flakes, hence their metallurgical name: flake graphite irons (sometimes called lamellar graphite irons). By concentrating abnormal stresses at certain points, each of these flakes may initiate cracking.
Metallurgists have therefore sought to diminish, or eliminate, this effect by changing the size or distribution of the flakes. In a first stage, the adoption of the centrifugal process to cast flake graphite iron pipes (so-called “grey iron pipes”), led to an appreciable improvement, by producing very fine graphite flakes. A decisive advance was then made in 1948 when research in both the U.S.A. and Great Britain led to the discovery of spheroidal graphite iron, more commonly known as ductile iron.
The graphite no longer exists in flake form but precipitates in a spherical form. The possibility of crack propagation lines is therefore eliminated. Graphite precipitation in spheroidal form is obtained by the controlled addition of a small amount of magnesium to the previously desulfurized base iron.
Ductile iron properties
Ductile iron owes its remarkable mechanical properties to the spheroidal shape of its graphite:
- tensile strength,
- impact resistance,
- high elastic limit,
- good elongation.
These properties are further enhanced by control of the chemical analysis and heat treatment of the metal matrix.
Ductile iron maintains the traditional qualities of cast irons, resulting from the high carbon content:
- compression strength,
- abrasion resistance,
- fatigue strength.
Saint-Gobain PAM ductile iron pipes and fittings
All PAM pipes and fittings are manufactured from ductile iron complying with Standards EN 545 and ISO 2531.
|Type of product||Minimum tensile|
|DN 40 to 2000||DN 40 to 1000||DN 1100 to 2000|
|Pipes centrifugally cast||420||10||7|
|Pipes not centrifugally cast,|
fittings and accessories
By agreement between the manufacturer and the customer, the conventional elastic limit at 0.2 % (Rp0,2) can be measured. It should not be less than :
– 270 MPa when A ≥ 12 % for DN 40 to 1000 or ≥ 10 % for DN > 1 000 ;
– 300 MPa in all other cases.
Comparison of properties of different types of cast iron for spun pipes
The Brinell hardness should not exceed 230 HB for pipes and 250 HB for fittings and accessories. For welded components, a higher Brinell hardness is admissible in the area affected by heat adjacent to the weld.