Structural applications of steel have long made use of elongated beams with carefully designed profiles, whose geometry gives them specific mechanical properties. Today, the range of cutting and joining techniques available means increasingly intricate steel sections can be produced to meet an ever-expanding set of applications. In this article, we summarise some of the main cutting and joining techniques used in the production of customised steel sections.
Near the end of the 19th century, engineer Benjamin Baker made a bold decision: To construct the world’s longest single-cantilever bridge out of steel. Previously untrialled in any applications of this scale, Baker knew that the strength and malleability of steel made it the perfect material for load-bearing applications in engineering. The Forth bridge is still regarded as an engineering marvel, at the time made an undeniable statement of steel’s potential. Since then, the role that steel plays in contemporary construction and engineering would be difficult to overstate – Structural steel is a vital part of any engineer’s toolkit, and without it the skyline of the modern world would be drastically different (not to mention much lower).
Steel is now one of the most commonly used engineering materials, owed in no small part to its versatility.1 The highly tuneable nature of steel both in alloying and manufacturing lends it to a wide range of applications.
Steel production starts at the chemical level, where a host of alloying elements in trace quantities can impart desirable properties. Carbon often defines the purpose of a steel, controlling strength and ductility. Elements such as copper, chromium and nickel add corrosion resistance, making alloys suited to applications such as highway bridges.2 Increasing the mass fraction of chromium to 10.5% yields stainless steel, whose famed resistance corrosion has earned it application in surgical and laboratory equipment worldwide.3
The material properties of steel are also determined by geometry. The ubiquitous I-beam is a prime example: often most economical for conventional structural applications, the I-beam can efficiently handle bending or shear loads in one direction. It can also be manufactured easily, along with several other cross-sections, by rolling from a single piece of metal. However, while rolled sections suffice for many applications, a far greater range of properties can be achieved by using cutting and welding techniques to fabricate custom sections.
Different Cutting Processes for Steel Sections
Steel can be cut in a number of ways:
Flame cutting is a simple yet effective method that has been used for over 100 years to cut carbon and high-strength low-alloy (HSLA) steels.4 A combination of oxygen with propane or acetylene is burned to produce a hot flame capable of cutting steel ranging from 6mm sheets to several-inch-thick pieces. Flame cutting produces a clean and accurate cut face, but can, for some applications, require additional machining due to the creation of a heat-affected zone (HAZ) around the cut edges. This poses a problem for very thin materials, which can be cut more precisely using other methods.
Plasma cutting was developed in the 1950s as an alternative to flame cutting that can cut high-alloy steels such as stainless steel. It uses an arc of plasma (ionized gas) to apply heat to the steel. The thermal and kinetic energy of the focused plasma jet melts the steel and expels material from the kerf. As well as being highly efficient and accurate, boasting a typical tolerance of 0.5mm, plasma cutting can be used to cut any metal. 5 6
The most accurate steel cutting process is laser cutting, which applies heat in a very precisely controlled area. Depending on the steel being cut, the heat can ignite an oxygen burning process (as in flame cutting) which fully evaporates the steel, or will be sufficient to melt the steel, in which case high-pressure nitrogen is used to blow the molten material away. While not suitable for cutting reflective metals, the highly focused laser beam enables huge reduction of the HAZ and can achieve very high tolerances – typically 0.1-0.8mm depending on the sheet thickness. 7
The most versatile option for cutting steel is water cutting. A high-pressure water jet carrying abrasive particles can cut through steel with no heat damage to the workpiece, and with a surprisingly low tolerance of around 0.5mm.8 Water cutting is an incredibly adaptable and precise cutting process for virtually any material, although the costs are relatively high.
Welding Steel Sections Together
Once cut, steel sections are typically joined by welding. Since the invention of electric arc welding in the late 19th century, a multitude of welding processes have been devised for various applications, most of which use an electric arc to supply sufficient energy to melt the base metal and facilitate fusion.
Recent improvements in “energy beam” welding techniques such as electron beam welding and laser welding offer vast improvements in speed and accuracy compared to electric-arc-based techniques: The strength and low distortion of laser weld seams lends itself to high-performance technological applications such as the International Thermonuclear Experimental Reactor currently under construction in France.9 However, the high cost of equipment required has limited use of these techniques.10
Laser Cutting and Welding Services from Masteel
Masteel UK is one company putting the advantages of laser welding to use. In conjunction with flame, laser, plasma and water cutting techniques, Masteel utilises laser welding to produce stainless steel profile sections with very small weld seams. This high precision facilitates fast and reliable joining of intricately cut profiles. The process is automated and efficiently scalable, making small-volume projects and one-off prototypes economical as well as large-scale manufacturing runs.
References and Further Reading
- Principles of Structural Design – W.F. Chen, E.M. Lui, Imprint CRC Press, 2005
- Design of Steel Structures – MIT Department of Civil and Environmental Engineering Spring Semester, 1999
- What is flame cutting? – http://www.esabna.com/us/en/education/blog/what-is-flame-cutting.cfm
- Waterjet Cutting Process Basics – http://www.esab.co.uk/gb/en/education/blog/waterjet-cutting-process-basics.cfm
- Standard metal cutting processes: laser cutting vs. plasma cutting – http://www.teskolaser.com/laser_cutting2.html
- Laser Cutting Tolerances – http://www.daysteel.co.uk/laser-cutting/laser-tolerances/
- Waterjet Machining Tolerances – https://waterjets.org/archive/getting-the-most/tips/waterjet-machining-tolerances/
- Review of candidate welding processes of RAFM steels for ITER test blanket modules and DEMO – Aubert et al, Journal of Nuclear Materials, 2011
- Laser Beam Welding – http://www.esab.co.uk/gb/en/automation/lbw/index.cfm