Rolled and Welded Tubulars in Stainless Steels

Stainless steel is a vital material in applications which require strength,  exceptional corrosion resistance and ductility. As a result, tubular sections of stainless steel are perfect for the transportation of corrosive or pressurized fluids.

Stainless steel pipes are widely used in the petrochemical industry for this purpose, as well as in brewing, water treatment, manufacture of pharmaceuticals and a range of other industries.1

This article takes a detailed look at the production of a tubular stainless-steel section by rolling and welding, and how these can be further modified to serve a variety of purposes.

Global Steel Demand Continues to Rise

The Beijing National Stadium is thought to be the largest steel structure in the world currently.2 Not only is the primary structure of the striking building made of steel, but the outside of the building is clad with an enormous web of steel, to conceal the supports for its roof.

Twenty four trussed steel columns surround the central bowl of the stadium, each weighing 1,000 tonnes, and the entire stadium is made up of an eye-watering 110,000 tonnes of steel.3,4

To put it into perspective, global steel consumption in 2015 was 1,400 million tonnes: enough to build 12,700 exact replicas of the Beijing National Stadium.5 Furthermore, the Stainless Steel Association predicts that this is set to rise, and global steel demand will reach 1,648 million tonnes in 2018.6

Due to the global reliance on steel, there is increasing pressure on researchers and steel manufacturers to streamline the production of various steel components so that they can be produced efficiently and to increasingly demanding specifications.

Even seemingly unremarkable steel components are the result of decades of research and development, and can often only be produced using unique technologies. The production of a simple tubular section of stainless steel is an ideal example.

Manufacturing Tubular Stainless Steel Parts

Of course, the first step in the manufacture of a stainless-steel pipe, is  the production of the steel itself. Most of the world’s steel is produced using a basic oxygen furnace (BOF), in this process pure oxygen is blown through molten iron ore – the highly exothermic reaction between iron and oxygen is used to remove impurities and reduce the carbon content to around 1%.7

Then, the chemistry of the resulting steel is changed to imbue the alloy with specific properties. Stainless steel is characterized by low carbon content and relatively high chromium content, with the latter making up at least 10.5% of the alloy by mass.

Reducing the carbon content reduces strength and brittleness, while adding chromium improves resistance to corrosion.8

When the stainless steel has been produced, it is then shaped into a pipe. While it is possible to produce seamless steel pipe by extruding and other methods, it’s harder to control the thickness of a seamless pipe than a welded pipe.

Welded pipes have an extra advantage as they are generally easier to produce and are therefore more cost-effective.9 To make a welded pipe, manufacturers use steel plate of the desired thickness, which is shaped into a tube by rolling.

Once rolled, the tube can be welded. Welding has existed for over one hundred years, but still a lot of research is going on around it, and some welding techniques exist for virtually any type of steel in any application.

The most widely used welding process for stainless steels is Gas Tungsten Arc Welding (GTAW), due to its versatility, availability, and relatively low cost. At the other end of the scale is laser welding, which is an expensive but very precise method of joining that utilizes a carefully focused laser beam to melt the base metal, without the need for a filler.

Laser welding needs expensive equipment but can provide very accurate and fast welding. It’s widely believed that the weld seam of a welded steel pipe is a weak point, however, if accurately welded, the seam can be as strong as the rest of the pipe wall.10

Modifying Tubular Steel Sections

Once a tubular section has been made, its shape can be further modified by the formation of an elbow. This can be done in several ways, typically by either cold forming or hot forging. In hot forging, the tubular section is heated to 1130-1150°C and bent around a semi-circular mandrel before being quenched.

Cold forming involves shaping the pipe in a die using a hydraulic ram; it is then annealed at high temperatures. Cold forming is considered to have several advantages over hot forging, in particular it is significantly less expensive, and hot forging tends to result in an excessively thick wall section in the bend. 11

Induction bending is another less commonly used method for forming elbows in tubular sections of steel. This involves locally heating the pipe using an induction coil, then gradually applying a bending force as the induction coil is moved along the pipe.

The pipe is water cooled immediately after passing through the induction coil, so that at any given time only a small part of the pipe (around 35 mm) experiences high temperatures. This makes induction bending a much more versatile process, capable of forming almost unlimited bend radii and arc angles.

Steel for Masteel

Masteel, UK steel producers routinely supply a variety of welded and seamless tubular steel sections up to diameters of 2500 mm.12 Manufactured to specified dimensions on request, pipes can be formed via induction bending to fit any application and shipped worldwide.

References and Further Reading

  1. Stainless Steel Pipe Applications http://pearlitesteel.com/stainless-steel-pipe-applications/
  2. Global Steel Consumption and the Largest Steel Structures in the World - https://www.viatechnik.com/blog/global-steel-consumption-an-steel-structures-in-the-world/
  3. “The China Syndrome” – Arthur Lubow, The New York Times, 2006
  4. Olympic Structures of China - http://www.thestructuralengineer.info/library/OlympicStructures08.pdf
  5. World Steel Association - https://www.worldsteel.org/en/dam/jcr:0474d208-9108-4927-ace8-4ac5445c5df8/World+Steel+in+Figures+2017.pdf
  6. World Steel Short Range Outlook - https://www.worldsteel.org
  7. Still the Iron Age: Iron and Steel in the Modern World – Vaclav Smil, 2016, Elsevier
  8. International Stainless Steel Forum - http://www.worldstainless.org/Files/issf/non-image-files/PDF/TheStainlessSteelFamily.pdf
  9. Welded Pipe vs. Seamless Pipe - http://www.stlpipesupply.com/blog/welded-pipe-vs-seamless-pipe/
  10. American Iron and Steel Institute - https://www.steeltank.com/Portals/0/pubs/Welded%20Steel%20Pipe%2010.10.07.pdf
  11. A comparison of cold forming, hot forging, and induction bending as methods of producing duplex stainless-steel elbows for high-pressure pipework, G J Collie & I Black, 2007, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK
  12. Masteel Tubular Steel - https://masteel.co.uk/pipes-tubulars/

This information has been sourced, reviewed and adapted from materials provided by Masteel UK Ltd.

For more information on this source, please visit Masteel UK Ltd.

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