M y first experience of the heat-straightening process in steel bridgework was in 1971. I was working with a very experienced plater setting up the joint between a pair of large box girders. As the junior site engineer my job was to align the boxes correctly with theodolite and level. The plater and his helper would then fair and tack the joint ready for welding.

All was going well except that the plate alignment on one corner was not good. I suggested that we should cut back the corner weld on one box by a couple feet, realign the plates and re-weld. I had clearly overstepped the mark. “No need for that” he replied. “We’ll fix it with a couple of line heats, one here, the other there”. He produced a stick of plater’s chalk and marked exactly where the heat lines were to be applied.

“Won’t that send it the wrong way’, I asked. “Leave it to me son”, he replied sharply.

I did, and 20 minutes later the plates aligned perfectly. He didn’t apply external force in any form, and it was all done very quickly using only a heating torch.

Having been proved absolutely correct his attitude towards me softened. I asked him how it worked. I didn’t find his explanation of the science very convincing, but there was absolutely no doubt that he was a master of its application, and knew exactly what would happen in any set of circumstances when heat was applied.

I think it was that day that more than any other got me hooked on steel bridging. I also learnt the very important lesson of respecting the skills and knowledge of the workforce. I worked with that plating and welding team for around 3 years, and they taught me a great deal about the behaviour of steelwork.

As far as I can determine the process of heat-straightening dates back to the early 1900’s and the invention of the oxy-acetylene torch, which enabled intense heating patterns to be applied to the surface of a steel plate. But the process didn’t take off until the 1930’s when welded construction was beginning to gather pace around the world, and fabricators were faced with the problem of correcting weld distortion.

An Australian delegate at the first International Welding Symposium held at the ICE in London in 1935 made the earliest reference that I have found to the process. It’s a number of years since I read those proceedings and I regret to say that I failed to record his name, and the organisation he represented.

Those proceedings, which I must revisit, gave the impression of a hugely successful and ground breaking conference. According to the preface it had been planned around an assumption of 80 responses to the call for papers. However, in the event 150 papers were submitted on an impressive range of topics all related to welded construction. Over 1,000 delegates attended, representing many countries around the world.

Quite a number of papers were written on heat-straightening in the 1950’s and 60’s, mostly in the USA. In the UK it remained little used outside the workshop, and was widely regarded as part of the “black art” of fabrication. In the USA it was being used much more widely and I intend to go into that next time round.

It’s important to understand the difference between hot bending and heat-straightening.

For hot bending the whole of the cross section along the entire length of the planned bend line is elevated to a temperature that significantly lowers the yield stress. That temperature is maintained while an external force is applied to bend the item by the required degree in one operation. The maximum temperature is usually 650°C. This process is little used by bridge fabricators, and is totally unsuitable for repairing damaged steelwork.

Heat-straightening is a process in which a heating torch is tracked along a predetermined path at a steady rate ensuring that plate temperature directly under the torch at any time stays safely below the lower phase transition temperature for the material. For structural steels that’s 700°C, and this is why most specifications limit heating temperatures to 650°C, except for quenched and tempered steel where the maximum temperature is lower.

Normally practitioners control temperature by closely observing the development of colours on the plate surface as the torch progresses.  Much less heat is used compared to hot bending, and when carried out correctly the risk of adversely affecting the mechanical properties of the material is much less.

The reduction in yield stress of structural steels between normal ambient temperatures and 450°C is around 20%, and the relationship between yield stress and temperature in that range is more or less linear. Over temperatures of 450°C yield stress falls rapidly, and at 650°C has dropped by around 65% relative to normal ambient temperature.

In heat-straightening the heating torch is always moving in a forward direction, only the small area directly under the torch at any time is at the maximum allowable temperature. After the torch has passed the temperature at that point drops to the 300°C to 400°C range in a few seconds, and cools at a much slower and steady rate thereafter. Consequently the yield strength of the whole section during the application of heat is impaired much less than in the hot bending process. For this reason in the USA it is quite common for impact damaged bridges to be heat-straightened with only live load restriction and no additional supports.

The photographs below were taken during some experimental work about 10 years ago in preparation for a bridge repair job. I think they give a good sense of the effectiveness of the heat-straightening process. (The process is generally called heat-straightening but it can be used to create a bend in a  plate as was the case here).

The photograph on the left shows the grid lines for three heating cycles A, B & C on a flat 20mm thick test plate. This was one of a number of heating patterns tested to find an effective way of inducing a large radius bend with the minimum number of heat cycles. The photographs centre and right show the plate after the three cycles of heat were applied and it had cooled to ambient temperature. Note that no external force whatsoever was used, and that heat was only applied to what would become the concave side of the bend line, the issue that greatly confused me in 1971.

This is a very useful process, and next time round we will look at how it works and some of the issues in using it to repair impact damage to bridges, or indeed any other steel structure.

I have come to realise that there are two very important requirements in applying this technology that are in my view understated in the literature. They are patience and discipline.

If your heat-straightening crew are short on either, you are likely to have problems.

Next - Heat-straightening technology (2 of 3)

Questions or comments to     geoff@codorus.co.uk


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