Manufacturing process of Ductile Pipes by the process of Casting

 

Manufacturing process of Ductile Pipes by the process of Casting

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Introduction

The term Ductile refers to the ability of a metal to undergo deformation before failure or the ability to be stretched in to wires. Ductile pipes are used heavily in the world today and are key components in the water supply infrastructure.

The underground pipes that transport cities drinking water and the sewer pipes that take away the household waste are casted from ductile iron because it is more flexible than the ordinary gray ironunder pressure so, it will bend before it breaks. Different types of ductile iron water pipes photo(2012) is shown below as an example.

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Since ductile iron pipes are mostly fitted underground, they have to be manufactured in such a way to withstand pressure and overcome

corrosion etc. Therefore, these pipes are manufactured using quality standards and have gone various types of tests before finally launching the product in to the market.

In this report, you will get a brief idea about the manufacturing process of Ductile Iron Pipes that utilizes the process “casting” for its manufacture. You will also get an idea about the materials used for the pipes, the machines involved, the tools required, quality considerations andpossible defects that arise during product’s manufacture.

 

What is ductile iron?

Ductility refers to the ability of a metal to deform before breaking or can be simply stated as the ability of a component to be stretched in to wires. Ductile iron basically consists of various materials that can be produced to have a wide range of properties. Adding a small amount of magnesium or cerium to gray iron before casting produces a distinctly different microstructure and set of mechanical properties.

Instead of forming flakes, graphite still forms as sphere like particles or nodules and the resulting alloy is called nodular or ductile iron. The image below is shown to get a close view of the graphite form inductile iron.

imagePhotomicrograph Showing Graphite Form in Ductile Iron

The manufacture of iron pipes in the UK has undergone 3 major changes in the last century and these are summarized below (UK water industry, 2006).

Grey cast iron (vertically cast) -> 1920s

Grey cast iron (spun) 1920s – 1960s

Ductile cast iron (spun) –> 1960s

 

Why choose ductile iron pipes?

Ductile iron is a development of an earlier cast iron pipe that has superseded due to its many advantages as follows,

  • · Ductile iron has excellent mechanical properties under the influence of forces, such as   high resistance to tensile stresses and high impacts.
  • · Above and below ground applications.
  • · Conservation of energy and low pumping costs.
  • · Extensive size ranges.
  • · Assured long term reliability and single rated system.
  • · Installations and testing are easy.
  • · Tolerates ground movement.
  • · Different pressure capabilities and high safety factor.
  • · Gives constant performance in hydraulics flow.
  • · Corrosion is less.
  • · Surge resistance.

Following information shows the typical properties of ductile iron pipe. (Electro steel castings ltd, 2012).

Properties of Ductile Iron Pipe

Tensile strength (min): 420 MPa

Elongation (min) at break 10 %

Coefficient of thermal expansion 11x10-6 per oC

Modulus of elasticity 1.7x1010 kg/m2

Hardness (max): 230 BHN

Density: 7050 kg/m3

Bending/Beam strength Over 200 MPa (appx).

Factor of safety against bursting is 8 to 10.

 

Reasons why we have chosen ductile iron instead of gray iron

 

Advantages

Disadvantages

· Greater ductility than grey iron

· Greater impact resistance than grey iron

· Greater strength than grey iron

· Lighter and easier to lay than grey iron

· Simplicity of joints

· Joints can accommodate some angular deflection

· Low pumping cost due to larger nominal inside diameter

· Similar rate of corrosion to grey iron and steel

· Prone to external and internal corrosion

· Internal and external protection systems required

· Limited number of protection systems available in U.S.

· Polyethylene wrappings can be damaged

(Pure technologies, 2014)

Materials used for manufacturing process

  • Recycled iron.
  • Magnesium.
  • Coke.
  • Cement (mortar).
  • Water based paint.
  • Zinc and aluminum alloy and blue epoxy (for coatings).

Machines used for manufacturing process

1. Blast furnace.

Blast furnaces are typically in the form of towers in appearance. Furnaces of this type are used in steel making processes, and it is ideal for mixing charcoals and iron ore together. The extreme heat in the furnace makes it easy for both of the substances to melt in to an integrated liquid metal.

Blast furnaces are used for the process of iron smelting. Since heat is created by the furnace, it is possible for the iron to produce iron oxide. The blast of air contained within the furnace helps to intensify the amount of heat produced.

Blast furnaces are typically equipped with drawers at the base that makes collection of slag. Secure doors are mounted on to the front of the furnace, making it easier for the raw materials to be inserted in to the furnace by a conveyor belt or a trough. A blast furnace is shown with its major components in the image (2010).

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Blast furnace image (2010)

2. Centrifugal casting machine.

Metal casting can be simply stated as the process, which solid materials are heated to become molten by a furnace such as a blast furnace and are poured in to a mold cavity that contains a desired shape. When it cools down after a period of time, the molten solidifies and can be removed from a casting machine.

Centrifugal casting process uses inertia forces caused by rotation or spinning to distribute the molten metal in to the mold cavity. Centrifugal castings are typically three types,

· Centrifuging casting.

· Semi centrifugal casting.

· True centrifugal casting.

We use true centrifugal casting in manufacturing ductile iron pipes. Ahorizontal true centrifugal casting machine is shown in the image (2009).

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Image (2009)

There are advantages of casting. And few of them are as follows,

  • It is possible to cast any material.
  • Complex geometries can be produced easily, internally and externally.
  • Very large parts can be produced.
  • Material wastage is less.
  • Casted parts have same properties in all directions.

3. Annealing furnace.

Annealing is simply a heat treatment process. Annealing furnaces are used to change the internal structure of metals such as to remove coarseness of grain of pipes making it harder and flexible. An annealing furnace is shown below in the image (2011-2012).

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Designing Process

Few years ago, the designing process of iron pipes were all drawn by engineers by hand and all the calculations have been done manually. This wastime consuming. With the development of new technologies and software such as AutoCAD, the designing process have been made so easy as engineers and designers could easily design pipes for the needs of the consumers saving a lot of time. Due to this advance technological breakthroughs, custom made pipes and fittings are possible and could be designed and manufactured within a short period of time. Various pipe companies use different software.

Designing tools are as follows (Saint-Gobain Pam UK, 2014),

 

Pipespec software

PipeSpecdesign software is a support tool to assist engineers in the design and specification of pipeline schemes. The software features five analytical tools that can be utilized throughout the planning and design stages of the project:

  • · Hydraulics - Full Pipe.
  • · Hydraulics - Part-full Pipe.
  • · Embedment.
  • · Anchorage.
  • · Installation Cost.
  •  

PAMCAD Design software

To assist engineers in the creation and modification of a pipework design, a complete database of water pipeline products is available. The software enables accurate drawings to be produced quickly and easily by calling up pipeline components and arranging them on screen. PAMCAD is compatible with the latest version of AutoCAD. PAMCAD features a floating menu for easy, efficient pipework design.

Benefits:

  • · Enables pipe runs to be drawn quickly and easily.
  • · Make-up pipe facility.
  • · Use of standard products to minimize cost and lead time.
  • · Automatically allows joint gaps.
  • · Bill of materials function gives a clear list of products used.

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Diameters and lengths for ductile iron pipes

 

Ductile iron pipes are usually manufactured at a length of 6 meters.Diameters for ductile iron pipes range from 60mm to 2m or 3m. But these values differ according to countries and factory to factory. The image (2013-2015) shows pipes of various diameters but at same length, that is 6m.

 

Manufacturing Process

Ductile iron is made from 90% of the recycled iron where production begins in backyards of pipe factories. Old cars are put in to a shredder that chews them up to bits and pieces, and the shredded metal is analyzed by its chemical composition and sorted accordingly. It discards the plastics and sells aluminum and copper but keeps the iron and steel for manufacturing purposes. The factory also gets scrap steel from demolished buildings and other sources.

A crane operator uses an industrial magnet to gather precise amount of steel and iron. See image (2011)

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Image showing a magnetic crane carrying shredded metal (2011)

The shredded metals then go in to a blast furnace with coke, a form of coal at 1,500 shown by the image (2004-2010). The iron and steel liquefy while impurities are carried away.

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Image (2004-2010)

Workers then add magnesium to the low-sulfur base iron under closely controlled conditions.When magnesium is added to the molten iron, the graphite forms in spheres rather than in flakes (Saint-Gobain pam UK,2012).Change in metal is characterized by the free graphite in ductile iron being deposited in spheroidal or nodular form instead of flake form as in gray iron.

With the free graphite in nodular form, the continuity of the metal matrix is at a maximum.The formation of a far stronger, tougher ductile material greatly exceeds gray iron in strength, in ductility, and in impact characteristics. This turns the metal from ordinary gray iron in to stronger and flexible ductile iron.

The molten iron then travels down a trough in to a spinning mold of a centrifugal casting machine where centripetal force spreads the iron against the mold walls as shown in image below

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A cooling system cools the walls and the iron solidifies within seconds. Then the extractor pulls out an iron pipe mold to the standard industry length, which is 6m. The image below shows the solidification process

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Before each casting, workers insert a round form of a core in one end of the pipe mold. The molten iron fills the void between the core and the mold forming a flared edge called a “bell”. The core then seals off that end of the mold preventing molten iron from flying out during the casting.

When it’s time to connect the pipe, installers will fit the bell of one pipe over the other. Then a rubber gas seals the links together. The casting machine can make pipes of various diameters by changing the mold inside the centrifugal casting machine.

After the pipe is extracted, the inspectors weigh it and measure the wall thickness just to make sure that everything meets specifications accordingly.

Then the core is removed from the bell end and since it’s made from sand and plastic resin, the core will disintegrate simply.

Regardless of the size, pipes can be made in different diameters and the casting process is always the same for these pipes. It takes less time to manufacture smaller pipes as they harden faster because of the smaller surface area.

A freshly casted pipe is around 850 and it cools down quickly after leaving the mold. Such rapid cooling makes the iron brittle so the pipes go directly in to a gas filled annealing furnace that reheats to about 950 . This alters the internal structure making it strong and flexible. Acooling pathgraph (2009) is shown to get an idea of the temperature change with time as the full annealing process of ductile iron is similar to this full anneal curve.

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Cooling path graph (2009)

The pipe then runs through a cooling chamber that showers it with cold water. Ductile iron is somewhat resistant to internal corrosion, but for the sewer water it is less aggressive to corrosion. A variety of internal linings are available to reduce or eliminate corrosion, such as, Cement mortar and Polyurethane (PUR). Where cement mortar is more common in manufacturing ductile iron pipes.

To prevent the iron from corroding inside of the pipe, cement is sprayed called cement mortarbuilding up a lining of 1/8 of an inch thick. Then the pipe is spun for a few seconds to smooth out the cement. This seals the surface enabling the cement to cure over the next 24 hours and it also provides some extra rust protection.

The entire pipe is painted inside and the outside. For external coating, polyethylene, zinc or bituminous coating is applied.

Finally a robot paints a stripe around the straight end of each pipe. This is a depth guideline so the installation crews know when they insert the straight end of one pipe as far as it can go in to the bell end of another.

Quality considerations and standards

Quality of products is essential for a business if it is to succeed. From detailed metallurgical analysis of the molten metal to tight control of coating and lining applications, procedures have been developed to ensure consistent high quality of each individual pipe and fitting (Saint-Gobain pam UK,2012).

Iron pipes have been prepared to coincide with the publication of two new European standards covering ductile iron pipes, fittings, accessories and their joints for water pipelines (BS EN 545: 1994) and sewerage pipelines (BS EN 598: 1994), which supersede the former British Standard for such products BS 4772:1988.

Every pipe is pressure tested according to British standard BS EN 545/ BS EN 598 (BSI 2006).

Comparison of European Standards with BS 4772 (Water Industry,2006)

Quality control

ü Dimensional checks. (BS EN 545, BS EN 598)

ü Tensile property determinations. (BS EN 545, BS EN 598, BS 4772*)

ü Hydrostatic pressure tests. (BS EN 545, BS EN 598, BS 4772*)

ü Zinc coating mass determinations. (BS EN 545, BS EN 598, BS 4772*)

ü Paint coating thickness checks. (BS EN 545, BS EN 598)

ü Cement mortar lining thickness checks. (BS EN 545 BS EN 598 BS 4772*)

ü Cement mortar lining compressive strength checks. (BS EN 545, BS EN 598)

During production, each ladle of ductile iron is checked for exact content of magnesium and other elements with a computer controlled optical emission spectrometer.

The “quality-is-key” principle applies to every stage of the manufacturing process and includes(Saint-Gobain pam UK, 2012):

● Validation of suppliers and/or their materials.

● Continuous assessment of quality systems.

● On-going monitoring of product quality.

● Technical support prior to and after sales.

● On-time delivery of products and supporting information.

 

Possible defects

  • v When carrying away impurities in the blast furnace, some impurities can be left in the molten that proceeds to the centrifugal casting machine.
  • v When the mold rotates about the axis the molten iron can fly outside of the machine although the core is attached to the bell end of the pipe due to cracks and holes.
  • v There can be systematic errors when measuring the length and diameters of the pipes and when the pipes are detected by inspection, they are sent back for remanufacturing, which increases the costs.
  • v When anti corrosive fluids are sprayed on the pipe there might be spots that have been missed or scratches on the surface can cause oxidization.
  • v Defects in blast furnaces and centrifugal cast iron machines such as holes, cracks etc.