By Wendy Barnaby

When blacksmiths hammer steel on a forge, they are making use of the properties of steel which resulted in the collapse of the Twin Towers.

They are able to mould the steel because it becomes soft.

Steel loses its strength at temperatures higher than 300^oC, but does not melt until 1500⁰C.

The Twin Towers collapsed because their steel lost its strength.

"The mystery was what happened, given that the steel didn’t melt," Dr Sergei Dudarev from the UK Atomic Energy Authority Culham Laboratory told the BA Festival of Science.

"Steel–making is based on a curious phenomenon – a transition from one crystal phase to another. Without knowing it, blacksmiths use this phase transition all the time," he said.

"As you approach this phase transition, the elastic properties of steel change."

It is the elastic properties which determine the strength of the steel.

When a load presses down on a steel bar, it tends to deform. Repulsion between defects formed during deformation resist the load and hold the bar together.

As the temperature approaches the phase transition, this resistance vanishes. This results in a runaway process where the material offers no resistance to the load. It becomes soft, but has nothing to do with melting.

"It is an extremely unusual state," said Dr Dudarev.

"The problem in the collapse of the Twin Towers wasn’t about the fire in the building. The real cause was that the impact of the planes knocked the thermal insulating panels off the steel structures," he said.

This meant that the steel heated up, approached the phase transition and became soft.

"Had those panels stayed, nothing would have happened," said Dr Dudarev.

If the Twin Towers had been made of the steel used in kitchen knives, they would have stayed standing. This sort of steel can withstand temperatures of up to 1000⁰C, but it wobbles and is therefore not used for construction.

The reason for the loss of strength in the Twin Towers steel has only been understood since earlier this year.

Knowing what the problem was means that researchers will alter the chemical composition of steel to make a better type which retains its strength at higher temperatures.

Dr Dudarev forecasts that this should only take "a few years".