Several main stages are involved in the manufacture of ductile iron:
Molten iron preparation: blast furnace, intermediate frequency furnace, spheroidizing
Pipes heat treatment
Molten iron preparation
Molten iron can be obtained by reduction reaction of iron ore in a blast furnace or melting pig iron and scrap steel in an intermediate frequency furnace. All of raw materials have to be checked and sampling inspection strictly, in order to produce high purity molten iron for next step process treatment. After desulfurization, the temperature of molten iron will be adjusted in an intermediate frequency furnace for providing the optimum casting temperature.
Meanwhile, according to the chemical composition of the molten iron, scrap steel or ferrosilicon alloy was added to molten iron for adjusting the chemical composition, then addition of magnesium wire, so that the molten iron meets requirements for casting.
Ductile iron is different from ordinary grey iron by its remarkable mechanical properties (high elasticity, high impact strength, high elongation etc.). These are due to the nature of spheroidal graphite.
The graphite of grey iron is in the form of flake; when abnormal stress concentrating at certain points, this flake graphite may cause cracking.
When the graphite become spherical shape instead of flaky shape. The possibility of crack propagation is eliminated.
The primary pipes casting methods are the “de LAVAUD” and the “WET SPRAY” process. With the “de LAVAUD” process, the pipes need annealing heat treatment for graphitizing and ferritizing, and meet the requirement of structure and mechanical properties.
With the “WET SPRAY” process, pipes only need a ferritizing heat treatment. Through suitable heat treatment process, the ductile iron pipes could be produced, which is spheroidal graphite in metal matrix without cementite and pearlite.
Ductile iron properties
Ductile iron has remarkable mechanical properties due to the spheroidal shape of graphite:
- High tensile strength,
- Good impact resistance,
- high elastic limit,
- good elongation.
Through chemical content of metal matrix and heat treatment control, These properties are further enhanced.
- High compressive strength,
- Good castability,
- Good abrasion resistance,
- Good machinability,High fatigue resistance.
Saint-Gobain PAM ductile iron pipes and fittings
All of ductile iron pipes and fittings from PAM are complying with GB/T 13295 and ISO 2531 standards.