1. The history of ductile iron pipes
In 1947, H. Morrogh from the UK discovered that when cerium was added to hypereutectic gray cast iron, the graphite would be spherical with the content above 0.02wt%. In 1948, the A.P. Ganganebin and others from the US pointed out that added magnesium to cast iron followed by inoculation with ferrosilicon; when the residual magnesium content was greater than 0.04wt%, spheroidal graphite would be obtained. Since then, large-scale industrial production has begun for nodular cast iron. The history of using ductile iron pipes can date back to the water pipeline with 21.14 Km from the Seine to Versailles in the suburbs of Paris in 1668. After 300 years, the main body is still in use today, except for the repair and replacement of some pipes and joints.
 
2. Current status and prospects of ductile iron pipes
China's ductile iron pipe industry started in the early 1990s and developed rapidly with the great support of the Chinese Urban Water Supply Association. After nearly 20 years of practical use, its safety and practicality have been generally recognized by the water supply industry. In 2008, the domestic annual output had reached 2.2 million tons, which was 11 times that of 1990. Since China is a country with a water shortage, the current urban water supply capacity is obviously insufficient, and some cities are seriously short of water. Among the 668 cities in China, more than 300 cities lack water, accounting for 46%; more than 140 cities seriously lack water, accounting for 21%. The water supply and water saving business is in the ascendant, and ductile iron pipes have broad development prospects.
 
According to the Market Research and Investment Strategy Report of Chinese Ductile Iron Pipes (2019 Edition), ductile iron pipes are mainly used for municipal construction projects such as urban water supply, urban pipeline transformation, and sewage treatment. At present, the number of large-scale enterprises in the domestic ductile iron pipe industry is about 300. There are big differences in the production capacity and quality of each factory. With the acceleration of urbanization, the investment in water conservancy construction continues to increase, and the demand for ductile iron pipes will increase rapidly in the future in China.
 

3. The characteristics of ductile iron pipes

A ductile iron pipe is a type of cast iron pipe. In terms of quality, the spheroidization grade of cast iron pipes is required to be controlled at Grade 1 to 3, so the mechanical performance of nodular cast iron pipes can be better improved, which has 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 makes it have good mechanical properties, excellent anti-corrosion performance, good ductility, good sealing effect, easy installation, and is mainly used for water supply, gas and oil transportation in municipal, industrial and mining enterprises. A certain amount of spheroidal graphite is distributed on the ferrite and pearlite matrix. According to the nominal diameter and requirements for elongation, the ratio of ferrite and pearlite in the matrix structure is different. The ratio of pearlite of small diameters is generally not more than 20%, and that of big diameters is generally controlled at about 25%.
 

4. The performance of ductile iron pipes

A ductile iron pipe is a type of cast iron, which is an alloy of iron, carbon and silicon. Graphite in spheroidal graphite cast iron is in the form of spheroids. Generally, the size of graphite is Grade 6 to 7. The quality requires that the spheroidization level of the cast pipe is controlled to be Grade 1 to 3 (spheroidization rate being greater than and equal to 80%). A ductile iron pipe is the first choice for water supply pipes due to its good performance, and it also has a good cost performance. Compared with PE pipes, ductile iron pipes are easier and faster to install and have better internal and external pressure after installation. From the perspective of airtightness and corrosion resistance, ductile iron pipes have better airtightness after installation, and a variety of anti-corrosion methods can be used to improve anti-corrosion performance. From the perspective of hydraulic performance, the ductile pipe can obtain a greater volume of runoff with the same specifications. From the perspective of comprehensive installation and maintenance costs, the ductile iron pipe has a more superior cost performance.
 

5. The anti-corrosion treatment of ductile iron pipes

Asphalt paint coatings
Asphalt paint coatings are used for pipelines for transporting gas. Preheating the pipe before painting can improve the adhesion of the asphalt paint and accelerate the drying.
 
Cement mortar lining plus special coatings
This kind of internal anti-corrosion measure is suitable for pipelines conveying sewage and can improve the corrosion resistance of the inner lining.
 
Epoxy coal pitch coatings
The epoxy coal pitch coating is suitable for both gas pipelines and sewage pipelines. It is a two-component coating that has good adhesion and a very smooth surface.
 
The epoxy ceramic lining
The epoxy ceramic lining is suitable for sewage pipelines and gas pipelines, it is not applied extensively due to its difficult manufacturing process and high cost. The epoxy ceramic lining has good adhesion and smoothness, and is an excellent anti-corrosion coating.
 
Aluminate cement coatings or sulfate cement coatings
Both of these special cement coatings are suitable for the internal anti-corrosion of ductile iron pipes used in sewage pipelines to improve the corrosion resistance of the acid and alkali in the sewage.
 
Polyurethane coatings
It is a new type of green special coating developed to meet the needs of environmental protection. It has good wear resistance and corrosion resistance.