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Cold-formed or Hot-Finished?

29 June 2018 No Comment

Here, Stewart Jones, Senior Customer Technical Services Field Engineer, Conveyance Tubes at Tata Steel discusses the benefits of hot-finished tubes in air conditioning applications.

When choosing a suitable air conditioning system for an installation, the operational stresses, system pressures and level of use required are taken into account before a system specification is defined. Similarly, these same factors along with legislation compliance, installation and service life benefits need to be considered when choosing the system’s steel tube.

Air conditioning pipework can be exposed to both internal and external corrosive conditions.  External localised corrosion may occur as a result of condensation on the exterior of the pipework. Internal corrosion may be seen as a result of Ph variations, oxygen content or poor inhibitor use.  If not addressed this may result in the need to replace sections of pipe.  Furthermore, in hard water areas, scale can build up impacting on system performance, or resulting in costly maintenance work. Therefore, it is of vital importance to select the correct steel tube for the application to avoid the need for any costly repairs as well as to reduce the risk of product failure.

However, in today’s market there is unfortunately an influx of commodity, non UK manufactured carbon steel tubes on offer. Although many of these tubes are produced according to similar product standards and have comparable mechanical properties they may not behave in the same way in service. Whilst such tubes may look the same, appearances can be deceptive as these tubes may have a very different performance and hence be less suitable to use.  It is therefore critical that specifiers and engineers understand the different tube manufacturing processes to ensure that the correct tube specifications are used in order to ensure tracability, compliance, operational performance and service life.

Carbon steel tubes are generally produced in one of two ways, either cold-formed or hot-finished. Cold-formed products are created by shaping steel strip into a cylinder and then welding the edges together to create the final tube product. Whilst undergoing this process the material hardens due to the cold working, making it more brittle and the welding stage also introduces internal stresses into the material structure.

The heat from the welding also creates a distorted region commonly known as the Heat Affected Zone (HAZ). The altered grain structure of the HAZ results in a harder, more highly stressed region with an increased risk of corrosion, cracking and service failure. In addition, any further product working or manipulation carried out on site can increase the level of internal stress, heightening the risk of product failure.

Hot-finished tubes follow a similar manufacturing process to cold-formed products, but with a vital additional process step. These tubes, following welding, are heated to a high temperature sufficient to remove both the internal stresses and the distorted grain structure in the HAZ. This removes any brittleness and results in highly ductile tubes with an uniform fine grain structure throughout the entire product.

Hot-finished steel tubes can provide the following 10 benefits for air conditioning applications over cold-formed alternatives:

  1. A more ordered consistent microstructure
  2. No internal stresses that can promote cracking
  3. More consistent and reliable mechanical properties
  4. Improved structural integrity and ductility
  5. Improved and more consistent toughness, particularly at low temperature
  6. A higher pressure integrity
  7. A greater factor of safety
  8. No loss of strength due to additional welding or heating
  9. An improved performance against corrosion
  10. An improved ability to be threaded, grooved and bent to tight radii without splitting, creasing or collapsing

Typically, air conditioning applications refer to tubes manufactured to BS EN10255, the successor to BS1387 (which was withdrawn in 2004). However, specifying this standard alone will not guarantee that a hot-finished steel tube will be supplied. It is only by also requesting a BS EN10217 Part 2 compliant product, with its GH (Get Hot) grade, that a hot-finished tube will definitely be supplied.

An example of such a product is Tata Steel’s INSTALL PLUS 235 hot-finished, multi-certified carbon steel tube. Specifically developed and manufactured within the UK, ensuring alignment with both the Construction Products Regulations (CPR) and the Pressure Equipment Directive (PED) as per BS EN10255 and BS EN10217 Part 2.

Furthermore, the superior durability and strength of hot-finished tube products compared to cold-formed alternatives means that Tata Steel INSTALL PLUS 235 range can withstand internal temperatures as low as -20oC, ideal for most cooling systems operating between 6-10oC.

In addition, to reduce the risk of corrosion by external condensation, the INSTALL PLUS 235 hot-finished tube range is available with the enhanced Tata Steel red paint finish as standard. This coating has undergone significant testing, including accelerated salt spray and humidity susceptibility, to ensure optimum performance for the tube within its lifespan.

To highlight the importance of the steel tube production process within the tubes and pipework sector, Tata Steel has formed a partnership with the School of Civil and Building Engineering at Loughborough University to establish the Building and Industrial Services Pipework Academy (BISPA). Focused clearly on issues of interest to industries such as air conditioning, BISPA offers courses and training to support on-going professional development.

To conclude, in order to ensure the longevity and durability of the steel pipework for air conditioning applications it is therefore necessary to take a deeper look beyond the initial product’s specification, to the actual product’s manufacture process and the key manufacturing standards.  By removing all the internal stresses and weaknesses, hot-finished tubes and their GH (Get Hot) grades, have a multitude of benefits over their cold-formed counterparts, thereby offering a longer, more reliable service life. Not all tubes are the same, and in order to reduce the risk of different types of steel tubes being installed, resulting in system contamination, compliance, warranty and service life issues, it is vital that pipework specifications are up to date and correct, and that product being delivered to site are checked against the initial hot-finished, multi-certified specifications to ensure it is correct.

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