Ductile iron is the successor to cast iron. Cast iron pipes were once frequently used in municipal utilities due to their cost-effectiveness and overall strength. However, this material was also heavy, prone to corrosion, and relatively brittle. To address these problems, ductile iron was introduced in the 1940s.
It was found that the addition of magnesium alloy to molten iron significantly changed the microstructure of the material and improved its mechanical properties. According to the ASTM standard for ductile iron casting, the chemical composition of ductile iron consists of 3.5-3.9% carbon and 2.25-3% silicon along with small percentages of manganese, magnesium, sulfur, phosphorus, etc.
Ductile iron maintains all the advantages of cast iron while improving on strength and ductility. This material also has a high strength to weight ratio, allowing lighter piping structures to be fabricated.
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In 1947, British H. Morrogh discovered that when cerium was added to hypereutectic gray cast iron, the graphite was spherical when the content was over 0.02wt%. In 1948, A. P.Ganganebin and others in the United States pointed out that adding magnesium to cast iron and then inoculating with ferrosilicon, when the residual magnesium content is greater than 0.04wt%, spherical graphite is obtained. Since then, nodular cast iron has begun large-scale industrial production. The history of the use of ductile iron pipes can be traced back to the 21.14km water pipeline from the outskirts of Paris from the Seine to Versailles in 1668. 300 years have passed. Except for the repair and replacement of some pipes and joints, the main body is still in use today.
China's ductile cast iron pipe industry started in the early 1990s and developed rapidly under the strong support of China Town Water Supply Association. After nearly 20 years of practical use, its safety and practicality have been generally recognized by the water supply industry. The annual output has reached 2.2 million tons, 11 times that of 1990. As China is a water-scarce country, China's urban water supply capacity is obviously insufficient at present, and some cities suffer from severe water shortages. Among the 668 cities in the country, there are more than 300 water-deficient cities, accounting for 46%; more than 140 cities are severely water-deficient , Accounting for 21%, water supply and water saving business is in the ascendant, ductile iron pipes have broad development prospects.
Ductile iron pipe is a kind of cast iron, an alloy of iron, carbon and silicon. Graphite in ductile cast iron is in the form of spheroids. Generally, the size of graphite is 6-7. The quality requires that the spheroidization level of the cast pipe be controlled to be 1-3 (spheroidization rate ≥80%), so the material itself is mechanical The performance has been better improved, with the essence of iron and the performance of steel. The annealed ductile iron pipe has a metallographic structure of ferrite plus a small amount of pearlite, which has better mechanical properties. Ductile iron pipes are mainly called centrifugal ductile iron pipes. It has the essence of iron, the properties of steel, excellent anti-corrosion performance, good ductility, good sealing effect, easy installation, and is mainly used for water supply, gas transmission, and transportation of municipal, industrial and mining enterprises. Oil etc. It is the first choice for water supply pipes and has a high cost performance. Compared with PE pipes, ductile ink pipes are easier and faster to install than PE pipes in terms of installation time, and have better internal and external pressure after installation; from the perspective of airtightness and corrosion resistance, ductile ink tubes have better airtightness after installation, and A variety of anti-corrosion methods can be used to improve the anti-corrosion performance; from the perspective of hydraulic performance, because the specifications of ductile pipes generally refer to the inner diameter, the specifications of PE pipes generally refer to the outer diameter. From the perspective of comprehensive installation and maintenance cost, the ductile tube has a more superior cost performance. The inner wall is sprayed with zinc, cement mortar and anticorrosive materials.
• Machinability
Ductile iron pipes can be bent or cut without producing chip-breaking effects. The silicon and graphite content in ductile iron offers flex, lubrication, and abrasion resistance during manufacturing. Compared to steel pipes, the ductile iron pipes present several advantages during machining, including higher productivity, extended tool life, faster spindle speed, and reduced cost of operation.
• Safety and Reliability
Other pipeline materials, such as PVC, can experience catastrophic failures due to ground movement and high pressures. For potable water pipelines, this failure presents the risk of drinking water contamination. Ruptured wastewater pipes can also contaminate the surrounding environment.
Ductile iron pipes are known for their outstanding tensile strength, bending strength, and impact resistance. Pipes constructed from this material are usually manufactured with a minimum ultimate tensile strength of 60000 psi strength, a yield strength of 30000 psi, and a 10% minimum elongation. These strengths, combined with standard wall thicknesses, allow ductile iron pipelines to resist ground movement due to heaving, settlement and seismic activity.
Ductile iron pipes also have exceptional bursting strength, allowing them to withstand sudden high-pressure pressure surges, such as water hammer.
• Corrosion Resistance
Various studies and field tests confirm that the longevity and corrosion resistance of ductile iron pipes exceeds that of cast iron. According to a report by the American Water Works Association (AWWA), the service life of a ductile iron pipe is expected to be over 100 years. This outlasts any other pipeline material used in water and wastewater infrastructure.
By contrast, steel pipes and concrete cylinders rely on external bonded coatings, e.g., cement-mortar for corrosion resistance. When bonded coatings are used, any damage to the coating can expose the pipe surface, making it susceptible to corrosion. Ductile iron pipes, on the other hand, are inherently resistant to corrosion in most environments.
In extremely harsh environments, ductile iron pipes may be encased in polyethylene for enhanced corrosion control.
• Easy Installation
Due to its flexibility and strength, the installation of ductile iron pipes is easier and less expensive than other pipe materials. For example, the strength and flexibility of ductile iron pipes allow them to be installed with less stringent bedding and backfill specifications.
Other pipe materials, like PVC, concrete, and clay, require rigorous engineered fill and bedding specifications to minimize settlement and long-term creep. Although steel pipes also possess ductility, they may also need additional braces and external soil support during installation.
Additionally, since ductile iron pipes are machinable, they can be easily cut, drilled, and directly tapped in the field.
• Cost-Effectiveness
Standard ductile iron pipes also have internal diameters that are larger than other standard pipe materials. Additionally, the texture of the internal surface of the pipe remains consistent even after many years of service.
These factors contribute to increased and consistent flow capacity, as well as lower head loss. Consequently, the energy required for pumps to overcome these losses is also relatively low. Hence, ductile iron pipes can also reduce overall utility costs and also prolong the life of pumping equipment.
• Easy Maintenance
Because of the superior durability of ductile iron pipes, the maintenance required is negligible. In the event of a leak, inexpensive pipe sleeves can be used as a form of repair, eliminating the need for complete pipe replacement.
• Sustainability
Last but not least, ductile iron pipes are environmentally friendly and recyclable. They can be produced from iron salvaged from recycled steel scraps, demolished buildings, or other sources.
Since the pipes themselves are also entirely recyclable, they produce zero net waste. This recyclability ultimately reduces environmental contamination and carbon dioxide emissions.
This is in addition to the environmental benefits of ensuring proper waste management while facilitating the distribution and disposal of water and wastewater, respectively.
As a result of its environmental, economic, and social benefits, ductile iron pipes have earned SMaRT certification from ‘The Institute for Market Transformation to Sustainability (MTS).' This is the only pressure pipe available to achieve such certification to date.
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