A cast iron case for hot formed steel tubing
Most users of steel tubing have traditionally taken the view that a tube is a tube is a tube. In other words, one steel tube is much the same as any other. The reality, however, is very different, as David Martin explains
Different sizes and chemical formations of steel tube make certain types of tubing more suitable for safety critical applications than others. Nowhere is this distinction more important than with hot form tubing, which is commonly used in commercial heating or air conditioning systems up to and including 4in diameter.
The hot form tubing manufacturing process permanently changes the micro structure of the pipe, which means it can withstand more manipulation and extremes of temperature. Hot formed tube is heated to specifically selected furnace temperatures which recrystallise the fine grain microstructure of the tubing. This means that the structure is more 'relaxed' and it is this 'that makes hot form tubing more suitable for extreme temperatures and demanding pressures.
Hot formed tube is more suitable in safety critical applications or where the risk of failure would result in highly expensive repair or refurbishment costs. The unstressed grain structure of hot formed tubing enables this hot product to operate efficiently, even under immense pressure. And it can operate in temperatures ranging from -7 to +260 deg C.
The stretch reduction process offers increased manoeuvrability, rigidity and strength, even when under varying temperatures and thermal loads. This is especially important when used in vertical runs. The added strength and rigidity also reduces the risk of sagging between supports.
A further issue is the mixing of different types of tubing in a single application. It is not uncommon to find installations where a contractor has used a cold formed product at the end of a run of hot formed tube for no other reason than 'we had some left over from another job'. This is a mistake because mixing different types of steel tubing in an air conditioning or central heating system leaves the system at risk of failure as the different types of steel tube rarely work well together.
However, there are other issues surrounding the use of different types of tube which simple differentiation by application will not cure. In recent years, there has been a steady increase in imported cold form tube from destinations as far afield as India, Dubai and Turkey.
These imports still only account for around 15 per cent of the market, but it is often impossible to understand the exact chemical composition of the tube and forged certification is not uncommon. Often, imported tube fails to conform to the minimum technical requirements and is not sufficiently malleable or strong enough for commercial applications, such as a central heating system. Unfortunately, the use of sub-standard tube often only becomes clear once an office or industrial unit fit-out has been completed and the tube fails, leaving the potential for costly remedial work.
What's more, British Standards, while reducing the risk of non-conformity, offer only limited security for tube users. The variation in product quality from one end of EN 10253 and EN 10217 to the other is vast.
My advice to tube users is therefore two-fold. First, in safety-critical applications, hot formed tube sourced from market-leading manufacturers can mitigate the risk of failure.
Secondly, it is important only to use tube that can be fully traced throughout the supply chain. Cheap imported steel tube may appear to offer a cost-effective alternative, but its lack of traceability could leave contractors at the mercy of costly legal proceedings if the tube fails.
In the current economic environment, the temptation for contractors is to try and buy more cost effectively in order to try and maintain a margin and steel tubing would appear to offer just such an opportunity. However, this policy is not without serious implications if the wrong tube is used in an application or, worse, low grade imported tube is used.
Cold formed tube, manufactured by a reputable company, bought through a reputable distributor and used in a suitable application is perfectly acceptable, but its use must be application specific. I would urge all steel tube users to take the advice of a good distributor before specifying steel tube as not all tube is the same.
// The author is an expert in steel tubing at BSS Industrial //
8 November 2012