Contistretch


At one point during my time at Allied Steel & Wire I was asked to spend some time at the reverse twist process. This is a process which takes lengths of hot rolled rod and twist them several times one way and then the other way, thereby raising the strength of the hot rolled product to levels required by the BS spec (min. 460 MPa) or the various continental specs (min. 500 MPa) through cold deformation.

Then, one day I was told to stop working on the reverse twist process, because it was going to be replaced by the Contistretch process. Also, when I came back from my summer holidays, there had been a rearrangement of responsibilities in Tony Franks’ department, and my area became the new Contistretch department.

I’ve tried to find some background on the process by searching the internet, but found precious little for my efforts. Even Celsa, who must have inherited the process from its previous owner, hardly give it a mention on their website except for when they compare it unfavourably with another product of theirs called Celsamax.

So, failing any available diagram, I’ll try to describe the process in words instead. The machinery consisted of two big wheels, slightly different in diameter, over which the rod coil was pulled in a figure of eight configuration, the difference in diameter imparting an extension to the rod of about 8%. It was a very noisy process, and the wearing of ear muffs was understandably compulsory.

While successful in increasing the strength of the rod to the levels required by the various specifications, the process also flattened the ribs to levels that caused concern for certain continental specifications – the ribs being there to improve adherence between the rebar and the surrounding concrete. At some point there was also concern about safety on occasions when the rod snapped under tension, and the process was modified by imparting the stretch in a way that did not require use of the two big wheels.

However, the rib flattening remained an issue throughout, especially when we started to investigate the use of a 4-rib rather than the standard 2-rib profile. The former already started with a lower rib height and after deformation the remaining height was marginal at best.

My input in all this ? Although officially I was the development metallurgist, in practice I took on the role of QA manager. In this capacity I was in charge of the testhouse, the practices of which I compiled into a comprehensive QA manual. I was also responsible for dealing with all the auditors and ensuring that we passed the annual audits. The problems with even minor non-compliances is that at times dealing with them forces you into a corner, and the rib profile of the 4-rib rebar was one of those instances.

I never saw the end of where the 4-rib story went, because I got my redundancy before it came to a head. But I’m in no doubt that it would come to a head. As I may have mentioned elsewhere, the main reason why I was deemed surplus to requirements was that I hardly had done any work as a development metallurgist. There was only one attempt at seeing how a vanadium micro-alloyed would perform under Contistretch conditions (the resulting increase in strength was hardly worth it, something that I could have predicted beforehand), and for the remainder I was more in support of department objectives to improve formability, increasing the size range to 16mm rebar and improving the rib height after deformation.

I don’t know whether the Contistretch process is still there. If it isn’t then it would require a higher alloyed steel composition to achieve the strengths specified in the standards, a cost that would have to be compared with the transportation of rod coil from Castle to Tremorfa Works, and the upkeep of an additional process step.

The process appeared to fill a gap between hot rolled products and wire drawn products, but I have no idea whether this gap was economically viable.

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