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How to Achieve Accurate Temperature Measurement of Aluminium with Thicker Alloy-Dependent Oxide Layers in Reheating and Secondary Processing Applications

Thursday, October 15, 2020 | Dr. Fiona Turner
Categories : Industry

Aluminium is a complex subject for infrared temperature measurement, as its emissivity and infrared radiance change non-linearly with temperature, surface structure and surface composition. 

While the SPOT AL EQS has become an industry leading choice for temperature measurement at Extrusion press exit, post-Quenching, or Strip rolling mill exit positions, the E, Q and S algorithms are only appropriate for freshly formed surfaces. On a freshly extruded or rolled surface of any aluminium alloy, the oxide layer that forms within the first few seconds is a thin amorphous aluminium oxide layer.  We are now introducing F and F Mg algorithms for reheating and secondary processing applications, with thicker alloy-dependent oxide layers.

The SPOT E Q and S algorithms were developed and tested on many different alloys, from all series 1000 to 8000.  During site trials, it became apparent that for each application, one algorithm could indeed be used to cover the whole emissivity / temperature / radiance band for all alloys, on freshly formed surfaces. However, these algorithms could not provide accurate measurements for the same alloys on exit from a reheating furnace, after extended periods of oxidation at high temperature.

We investigated the reason behind this finding in collaboration with Bridgnorth Aluminium Ltd, while performing site trials at their hot rolling mill.  Spectroscopic measurements revealed that despite relatively small differences in nominal composition from alloy to alloy, the type of surface oxide film formed in a preheat furnace over many hours varied dramatically with alloy type, with even trace amounts of magnesium selectively diffusing towards the surface to dominate the oxide layer. These results were published in the Proceedings of the Aluminium Two Thousand World Congress 2017 [S.F. Turner, R.W. Gagg, D.C. King, P. Drögmöller, L.K. Muman, R.D. Hunt: Practical Infrared Thermometry for Aluminium Hot Rolling]. 

After extensive further work in collaboration with other industrial and academic partners, new algorithms were developed to enable infrared measurements of reheated aluminium with thicker oxide layers.  As a heat-treated oxide layer can be very different for alloys with a high magnesium content, two distinct algorithms were developed: the new AL F and AL F Mg algorithms. 

The AL F and AL F Mg algorithms are designed for Forming and Forging applications – preheated and reheated billets and slabs prior to rolling or extruding, Forged / Formed products as well as induction heated or heat treated parts for coating, bending, mounting and shrinking applications. 

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The AL F algorithm should be used if the alloy is pure aluminium, or aluminium alloyed with less surface active metals such as iron, copper and zinc.  For these alloys the surface is predominantly aluminium oxide, AlO or Al2O3, and the surface exhibits only a small variation of emissivity with wavelength.

The AL F Mg algorithm is required for any alloy where magnesium oxides dominate the surface, whether they be MgO or MgAl2O3.  These tend to be the harder, shinier alloys.

In most cases, the choice between the AL F or AL F Mg algorithm will be clear however the decision should factor in the time spent at high temperature as well as the percentage of magnesium in the alloy – even a trace amount of magnesium can come to dominate the surface emissivity if a billet is kept at 500˚C / 932˚F for several hours, allowing magnesium to diffuse from the bulk to the surface.  The AL F Mg algorithm may therefore be required for an alloy with <1% magnesium after extended preheating, but the AL F algorithm would be more suitable for the same sample measured quickly after cold aging.

In the case of uncertainty between the AL F or AL F Mg algorithms, comparison with a surface thermocouple or roll-nip measurement is recommended to determine the characteristics of the surface.  Comparison measurements can also be used as a simple emissivity calibration to adjust the algorithms to the exact emissivity behaviour of a particular oxide, as explained in the ‘Configuration of SPOT AL EQS According to Alloy Type’ post.

To learn more about our new SPOT AL EQS modes – AL F & AL F Mg click here

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