TPi September 2016

Not all tubes are created equal By Stewart Jones, Tata Steel

can have a serious impact. Therefore reducing the risk of failure and consequently improving the health and safety of employees is of significant importance. While the manufacturing process for hot-finished tubes is very similar to the cold-formed process, the addition of a vital step relieves any internal stress. To provide a full-bodied hot- finish process route, the steel tubes are heated to very high temperatures using a furnace or induction process. As the temperature of the steel rises, its internal stresses are relaxed and the HAZs are eradicated. Removing the internal weakness within the steel tube means that hot-finished tubes present several benefits over their cold-formed counterpart throughout the product’s lifespan. For example, with a uniform microstructure and no internal stress, the product has consistent and reliable mechanical properties throughout the entire product. Furthermore, the heating process also improves the final product’s structural integrity and ductility, its toughness and its ability to withstand high pressure. This means that hot-finished steel tubes will not suffer any loss of strength from additional welding and heating, and have an increased performance against corrosion. These characteristics are essential for tube products used in environments where product failure is not an option and employee health and safety is paramount.

S tewart Jones, senior customer technical services field engineer, conveyance tubes at Tata Steel,

outlines the distinctive differences between hot and cold formed tube products, and discusses the training required to resolve confusion surrounding British and European standards.

Stewart Jones

Specification of any product is largely based upon the available budget and ensuring compliance with key legislation. Therefore, a product’s manufacturing process can often be overlooked, with more influential drivers leading a specification. In the tube and pipework industry, this is often the case, with a tube’s manufacturing process not taken into consideration. Produced by two methods, either hot-finished or cold-formed, the two processes create products with similar specification information. However, there are several underlying factors and properties that should be considered. Formed from a flat coil, cold-formed steel tubes are shaped when cold, then welded to produce the tube shape. During this process the material undergoes work hardening, interrupting the grain structure of the material and introducing internal stresses.

As well as being structurally sounder and stronger, hot- formed tubes have added practical advantage over seamless

This occurs in the region surrounding the welded seam and is often referred to as the heat affected zone (HAZ). Within this region the steel has a distorted gain structure, producing a harder, less malleable material to work with. During fabrication and installation this distortion can be increased by additional working and product manipulation. This heightens the level of internal stress, consequently increasing the risk of the product cracking and failing. Used in heating, air conditioning and ventilation applications as well as for general conveyance purposes, a cracking and failing tube product

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september 2016 Tube ProducTs InTernaTIonal