Stainless steel is an indispensable material in applications which demand strength, ductility, and excellent resistance to corrosion. As a result, tubular sections of stainless steel are ideal for transporting corrosive or pressurized fluids.
Stainless steel pipes are used extensively in the petrochemical industry for this purpose, as well as in water treatment, brewing, manufacture of pharmaceuticals and a host of other industries.1 In this article, we take 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 different purposes.
Global Steel Demand Continues to Increase
The Beijing National Stadium is considered to be the single largest steel structure in the world today.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 in an effort to conceal the supports for its roof. The central bowl of the stadium is surrounded by 24 enormous trussed steel columns, each weighing 1,000 tonnes, and the entire stadium comprises an eye-watering 110,000 tonnes of steel.3,4
To put this 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 What’s more, the Stainless Steel Association forecasts that this number is set to increase, and global steel demand will reach 1,648 million tonnes in 2018.6 In a world so reliant on steel, the burden rests 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 products of decades of research and development, and can often only be produced by the use of unique technologies. The production of a simple tubular section of stainless steel is a perfect example.
Manufacturing Tubular Stainless Steel Parts
The first step in the manufacture of a stainless-steel pipe, is of course, the production of the steel itself. The majority of the world’s steel is produced using a basic oxygen furnace (BOF), wherein pure oxygen is blown through molten iron ore – the highly exothermic reaction between iron and oxygen is used to remove impurities and lower the carbon content to around 1%.7 Next, the chemistry of the resulting steel is altered to imbue the alloy with specific properties. Stainless steel is characterised 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 decreases strength and brittleness, while adding chromium improves resistance to corrosion.8
Once the stainless steel has been produced, it’s 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 additional advantage in that they are generally easier to produce and therefore more cost effective.9 To produce 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 a century, but is still an active area of research, and a number of welding techniques exist for virtually any type of steel in any application. Gas Tungsten Arc Welding (GTAW) is the most widely used welding process for stainless steels due to its versatility and availability, and relatively low cost. At the other end of the scale is laser welding, an expensive but very precise method of joining that uses a carefully focused laser beam to melt the base metal, without the need for a filler. Laser welding requires expensive equipment, but can provide very accurate and fast welding. It’s commonly believed that the weld seam of a welded steel pipe is a weak point, however if properly 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 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, before annealing it at high temperatures. Cold forming is considered to have a number of advantages over hot forging, namely it is significantly less expensive, and hot forging tends to result in an excessively thick wall section in the bend. 11
Another less commonly used method for forming elbows in tubular sections of steel is induction bending. 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 35mm) experiences high temperatures. This makes induction bending a much more versatile process, capable of forming almost unlimited bend radii and arc angles.
Steel from Masteel
UK steel producers Masteel routinely supply a variety of welded and seamless tubular steel sections up to diameters of 2500mm.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
- Stainless Steel Pipe Applications http://pearlitesteel.com/stainless-steel-pipe-applications/
- 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/
- “The China Syndrome” – Arthur Lubow, The New York Times, 2006
- Olympic Structures of China – http://www.thestructuralengineer.info/library/OlympicStructures08.pdf
- World Steel Association – https://www.worldsteel.org/en/dam/jcr:0474d208-9108-4927-ace8-4ac5445c5df8/World+Steel+in+Figures+2017.pdf
- World Steel Short Range Outlook – https://www.worldsteel.org/media-centre/press-releases/2017/worldsteel-Short-Range-Outlook-2017-2018.html
- Still the Iron Age: Iron and Steel in the Modern World – Vaclav Smil, 2016, Elsevier
- International Stainless Steel Forum – http://www.worldstainless.org/Files/issf/non-image-files/PDF/TheStainlessSteelFamily.pdf
- Welded Pipe vs. Seamless Pipe – http://www.stlpipesupply.com/blog/welded-pipe-vs-seamless-pipe/
- American Iron and Steel Institute – https://www.steeltank.com/Portals/0/pubs/Welded%20Steel%20Pipe%2010.10.07.pdf
- 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
- Masteel Tubular Steel – https://masteel.co.uk/pipes-tubulars/