The effect of Heating Temperature in Steel Bridge Repairs

650oC is the maximum heating temperature specified in most international fabrication standards for the steel grades typically used in bridge construction. It is also the temperature recommended by Federal Highways in their technical guide for heat straightening damaged steel bridges.

 

It’s an operating temperature that gives a good balance between effectiveness and caution. Nothing detrimental happens at 650oC, it’s simply a temperature that provides a practical operational margin against exceeding the more significant “lower critical temperature” of 7210C where the composition of steel does start to change.

 

It has been recognized for a long time that the effectiveness of the heat straightening process increases with temperature.  It’s a process that relies on rapid localized heating to induce expansion and plastic deformation in a manner that will permanently change the overall shape of a component when it cools down. So obviously a greater operating temperature leads to more local expansion and potentially more adjustment to the overall shape of the component per heating cycle.

 

However, the gains from increased temperature don’t end there. Up to around 65oC the coefficient of expansion of steel is for all practical purposes a constant, but above that it increases significantly with temperature. As a result even more expansion and plastic deformation is to be gained.

 

There hasn’t been a great deal of research done on this topic, but based on data in the Federal Highways Technical Guide I have prepared the chart below. It is referenced to an operating temperature of 650oC and gives a clear and simple indication of how the effectiveness of heat straightening varies with temperature. 

If the operating temperature is raised to around 720oC  (and I certainly do not recommend that you do this) the gain in effectiveness from 650oC is in the order of 80%. This is why there is a great temptation amongst less conscientious operatives to overheat, and why discipline in the repair team is so important.

 

Here in the UK the authorities have imposed an upper limit of 600oC for the heat straightening of damaged steel bridges in order to provide an additional level of caution. That’s a reduction of a little less than 8% against the FHWA recommendation, and on the face of it does not seem unreasonable. However, from the chart above it can be seen that the reduction in effectiveness is in the order of 35%, and that aligns very much with my own experience in the field. It therefore leads to a requirement for 50% more heating patterns or cycles than would be required at 650oC.

 

Development in hand-held infra-red pyrometers has come a long way in recent years. Temperatures can now be monitored continuously from a safe distance clear of the operative. With effective management I think the use of such equipment can greatly reduce the risk of over-heating.

 

I therefore think its time for us here in the UK to revert to an operating temperature of 6500C for heat straightening operations to damaged bridges. It would probably also reduce the proportion of cut-outs currently undertaken in repair projects.

 

This would of course not apply to Q&T steels, but there is not much of that in the UK bridge stock.

 

 

 

 

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