27 January 2020

Steel tubes - it’s all in the corner

Why are corners so important and what can you do to improve your next project by paying more attention to the corners?

MAIN Corner celsius steel structural hollow section


When we talk about steel tubes, or more specifically structural hollow sections, we spend a lot of time talking about corners. So why are corners so important and what can you do to improve your next project by paying more attention to the corners?

Most structural hollow sections, whether used in building structures or a multitude of engineering applications are either square or rectangular. This isn’t because these are more structurally efficient shapes than circular tubes, but because they are much easier to deal with. Flat sides make for much easier connections with other elements and often just fit better into designs. But square and rectangular hollow sections, obviously, include four corners and it is at these corners where the differences between the quality of steel tubes can be most evident.

The differences in corners can be broadly summarised around:

a) Differences in the shape or profile of the corner.

b) The impact of the corner on the metallurgy and physical properties of the final product.

As we explore the various types of steel tube, we will explain how these two factors are impacted, and what this means for how you can design with, and use structural hollow sections.

The simplest, cold-formed tubes tend to be made by first bending a flat strip of steel into a round tube and applying a high-frequency induction weld to join the edges together and form a hollow. This hollow is then run through a set of rollers which squeeze it into a square or rectangular shape. Looking at the corner, there are two very important implications from this:

1. Because the shape is formed from a cold steel hollow, there is only so much pressure that the squeezing operation can do and so the corner profile tends to be quite ‘slack’ or, in other words, the corner will have a relatively large radius.

2. Even if the corner is relatively slack, the cold forming operation puts a lot of stress into the steel, giving differential hardening around the corner and forming weak spots where the tube could fail.

A true hot finished, or normalized hollow section avoids both of these issues. Normalized steel tubes also start their life as round hollows, formed from strip steel, but in this case, these hollows are heated to normalizing temperatures (in excess of 850°C) and the final shaping operation is done on the hot tube. Because it is hot steel that is being shaped, it is much easier to shape into a precise square or rectangle and there is no effect on the metallurgy around the corner, irrespective of how ‘tight’ the corner profile is.

There are of course some steel tubes that use a hybrid process – forming the square or rectangle while cold and then heating (but not reaching the full normalizing temperature) afterwards. Tubes made in this way will have ‘slack’ corners because they are still manufactured as cold formed and, while some of the stress is relieved, the steel is not true hot finished normalized. So it still retains areas of differential hardness, particularly around the corner and weld region, which could result in unpredictable behaviour.

So we can see that there are really three clear types of manufacture for structural hollow sections. Confusingly, both the 2nd and 3rd of these are specified under the standard EN 10210, but only the fully normalized one has all the benefits, designated by the ‘NH’ suffix, as in S355NH.

  Corner profile Corner metallurgy Standard
Cold formed Slack Stressed = unpredictable EN 10219
Partially stress relieved Slack Partially stress relieved = unpredictable EN 10210 S355J2H
Normalized Tight Normalized = same properties throughout EN 10210 S355NH

So now we know more about the effects of different manufacturing routes of hollow sections, why is this important? Let’s start with the corner profile.

You might think that having a ‘tight’ corner profile is just about aesthetics, but you would be wrong. It is correct that a tight corner profile does generally give better aesthetics, but there are functional benefits too.

To start with, there is more steel in a tight corner than in a slack one. This gives a greater sectional area. It may seem a small effect, but taking an example of a square section 120x120x8mm in size, a fully normalized hollow section has a 3% greater sectional area than a cold formed one or only partially stress-relieved one, giving a 7% increase in moment of inertia and elastic modulus for the section.

A tight corner profile also maximises the flat surface area on each side of the section. Again, this may not seem like a big issue, but taking the same example of a 120x120x8mm section, the flat area on each side of a fully normalized section will be in the region of 22% larger than on a cold formed or partially stress relieved section. This gives much greater flexibility for welded joints design on the normalized section.

Looking at the impact on metallurgy, though, makes this even worse for sections that are not fully normalised. EN 1993-1-8 clause 4.14 (the design standard for structural steelwork connections) takes into account the manufacture and makes very specific recommendations for both cold formed and partially stress relieved hollow sections to allow for the unpredictability of the metallurgy in their corner regions. This effectively means that, unless very strict requirements are met, by more stringent control of the chemical composition a cold formed hollow section should not be welded within a distance 5T (where T is the thickness) of the corner. The standard EC 3-1-8 recognises that fully normalized hollow sections have uniform metallurgical properties throughout the full section, so there is no need to use this clause.

However, as stated above, you do need to be careful because partially stress relieved hot finished hollow sections, which can only be designated as S355J2H, do retain some metallurgical inhomogeneity and so should be treated, from this point of view, in the same way as cold formed.

Due to the manufacturing process, the stresses locked into the steel in cold formed hollow sections can also lead to unpredictability and potentially the risk of failure in-use which can be avoided by using normalized sections. Our recent blog showed how both cold formed and also partially stress relieved hollow sections have increases in micro-hardness in the corner region which are not seen in their normalized equivalents. These regions of increased hardness can be the source of micro-cracking and eventual failure, particularly in applications where fatigue is important.

So there is a reason that whenever we talk about hollow sections, we end up talking about corners. Corners are important. Tight corners look nice, but more importantly leave wide open faces to normalized hollow sections. But it is what is going on within the steel that really matters. Fully normalized hollow sections have uniform steel throughout, so they give excellent performance in-use. Meanwhile, cold-formed tubes, including those which have been partially stress-relieved, have an inhomogeneous steel grain structure, leading to weak spots, particularly around the corners.

To avoid any issues is simple though – always specify fully normalized hollow sections, EN 10210 S355NH or the  brand from Tata Steel.

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