Cheesegrater bolts and ‘hydrogen embrittlement’ – what you need to know

With this week’s news that one more bolt had fallen from Richard Rogers’ Cheesegrater tower in the City of London, we also now have a technical explanation from the developer as to why the bolts failed – a process called “hydrogen embrittlement”.

This latest incident follows the similar failure of two bolts in November last year, just four months after the building was completed. This resulted in debris falling from the fifth storey to the pavement below and triggered an investigation by main contractor Laing O’Rourke and Arup, the structural engineer.

According to a statement released on 14 January by British Land, the joint developer of the 222m-high building, “a number” of bolts had been replaced in addition to the three that failed, although it did not say how many. 

Anyone who isn’t a metallurgist, materials scientist or structural engineer naturally wants to know what all this means … 

The nature of the problem

Construction Manager spoke to Paul Lambert, technical director of materials and corrosion engineering at Mott MacDonald. He told us: “Hydrogen embrittlement causes fear among engineers, because it attacks the fundamental reason for using steel in the first place.”

It is also apparently the subject of intense interest and controversy among the materials science community, owing to the competing theories of what is occurring at the atomic level.

The basic mechanism is that single atoms of hydrogen enter the steel, migrate through the crystal lattice, and are attracted to the areas of highest stress.

Here they cause tiny fractures to propagate, and in a worst-case scenario, a cascading effect can take place, as the stress increases and attracts more free hydrogen atoms, leading to a sudden catastrophic failure – such as the shearing of a bolt.

One of the peculiarities of this process is that it particularly affects very hard steel developed for high stress uses, Lambert explains. “The problem is well known and there are magic numbers – such as 320 Vickers HV – above which the risk of hydrogen embrittlement becomes greater. The first thing to ask is whether the hardness of your bolt is above that value, and if it is, you’d expect an engineer to take certain precautions.”

It is well known that hydrogen can be introduced into steel by the use of acid, so it is advised that any plating or coating procedures using acid be avoided.

It is also recommended that very hard steel components be “baked”, at about the same temperature as a sponge cake. This drives incipient hydrogen out of the steel’s lattice. 

“You have to know what you’re doing, because if you take the temperature too high for too long you can weaken the bolt,” says Lambert. “But all of this is well known and it’s in the international standards, for example ISO 15330. You could argue that the problem is being aggravated by the fact that we have very fast production, delivery and use times, which means that the processing has to be very precise: if it’s done too quickly, you risk letting the hydrogen in and you may not be getting it out afterwards.”

How easy is it to detect?

The international standard ISO 15330 says that before bolts of a certain hardness are used, they should be subjected to a “preloading test for the detection of hydrogen embrittlement”.

But once they are in situ, testing apparently becomes problematic. “It’s very difficult to do non-destructive testing for bolts that might have tiny cracks because the nature of a bolt is that it has many tiny cracks built into it anyhow, so unless you have a large one forming you’re not going to detect it,” says Lambert. “Inspection is easier with very large bolts if you can get to both ends. It’s hard to use radiography for all sorts of practical reasons, so you’ll probably be using ultrasound, and it’s hard to get a good image. If you were to take them out you could use a combination of X-rays and ultrasound.”

The job is an arduous one, because only about 5% of bolts are typically susceptible to hydrogen embrittlement. Lambert says: “If it were me, I’d try to find records on the sourcing of the bolts, because you might be able to isolate a bad batch.”

The unforgiven

The obvious solution to the susceptibility of very hard steel to hydrogen embrittlement is to simply avoid using it in situations where failure might have serious consequences. So Lambert, only perhaps half joking, says the fault lies with the aesthetic sensibilities of architects – if buildings were designed by engineers, he says, “they’d have whopping great bolts made from low strength steel”.

That being an unlikely development, it seems that design and construction teams will just have to learn to live with hydrogen embrittlement. “The reason that hydrogen embrittlement causes fear among engineers is because it risks undermining one of the fundamental reasons for using ferrous materials in the first place. If you have a steel ladder and an aluminium ladder, steel might rust and bend and you could see that. The aluminium ladder would be pristine right up until the point when it breaks with no warning whatever. It’s the ductility and forgivingness of steel that we like, so we get worried when this forgiving material with all its faults suddenly lets us down.”