The Tempcore Process


For the first few years at Allied Steel & Wire, I was the development metallurgist for the Tremorfa Bar Mill, which used an interesting type of process to produce high strength reinforcement bar with the addition of alloying elements such as molybdenum or vanadium. We called it the Quench-and-Self-Tempered (QST) process, although officially its name was the Tempcore process, a process originally developed by the Centre de Recherche Métallurgique (CRM) in Liège.

The way the process works is that after a billet has been hot rolled to the correct diameter rebar it then enters a quenching chamber when still white hot, and high pressure water jets quench the bar from all directions until the outside reaches temperatures below 200°C. When the bar leaves the quenching chamber there is still enough heat left in the centre of the bar for the outside to be reheated to temperature in the order of 400°C, after which ii is allowed to cool down to room temperature on the cooling bed.

What this does to structure of the steel is that the outside turns to martensite, a hard and brittle phase of the iron-carbon diagram, during the quench, and the subsequent reheat or temper phase softens the martensite and makes it more ductile. The resulting microstructure displays various types of bainite near the surface, which gradually grades into a ferrite-pearlite structure in the centre.

My first job was to collect the data for qualification trials on the newly developed 40mm QST bar, something that went without too much of a problem as far as the mechanical properties were concerned. It did, however, run into some issues with end splitting in what had started off as the north end of the billet, where most of non-metallic inclusions congregated. Since the defect confined itself to the very ends of the bar, this turned out not to be of much importance, since an extra crop got rid of that portion of the bar.

I started investigating different types of rebar, of various dimensions and chemistry, which ultimately led to trials on a high strength type of bar which aimed to emulate Macalloy tie bars without the need for expensive alloys (I intend to have this as the topic of another blog). Unfortunately that work only came to fruition after I had left for the Contistretch process, so was not involved in subsequent approval trials.

And then came the moment when the directors of the steel plant and of the rod mill ambushed the third director, and carved up his domain amongst themselves, the bar & section mill going to the steel plant, and Tremorfa Bar Mill to the rod mill. The last I heard of it was that the bar mill was to be closed and its product range produced on the rod mill, which was to be converted into a rod-and-bar mill. Not sure what the reasoning behind it was, and how the modification of the rod mill to handle bars was effected, but presumably something must have been done to achieve the required strength without re-introducing expensive alloys in the chemistry.

All I know is that when you go on the Celsa website, there is no mention of the Tremorfa Bar Mill, and hence the Tempcore process is no longer in use in the UK. Presumably the economics must have dictated this, and presumably if the properties are acceptable, economics dictate which way the production goes.

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