WIRE PRODUCTION

Especially in case of dynamic load, such errors are potential points to break and lead to a reduction of fatigue strength. In case of welding wire, scale residuals will cause an increased production of slag which will strongly impair the quality of welding seams. Therefore descaling with mechanical or chemical methods (pickling) is a decisive process step in wire drawing and is essential for the quality of the product wire. Methods like bending, grinding, brushing or blasting are applied for mechanical descaling. High-alloy wires, aluminium or special alloys are usually cleaned by chemical pickling in acid solutions or liquid salt bathes, combined with mechanical bending and blasting. For choosing a suitable technology and for optimising the process parameters it is necessary to know about the concrete characteristics of the scale layers.
In the scope of a development project of Kieselstein GmbH together with Leibnitz University in Hannover and KEG Kanalreinigungstechnik GmbH in Burgstädt one analysed wire rod supplies for the production of welding electrodes (qualities 10MnSi7 resp. 11MnSi7) in the period January 2006 to July 2007. That material was provided by company ISAF Drahtwerke Brielow. Different analytic methods have been used to characterise the scale layer:
– Evaluation by means of a stereo microscope
– X-ray diffraction – Phase composition
– Scanning electron microscope (REM) and microprobe (EDX) element analysis
– Light microscopy – crystalline structure, layer thickness
– Current density potential measurements
– Video-supported micro-bending test

Radiographical phase analysis

Different phases develop in dependance on the chemical composition of the basic material and in dependance on the concrete conditions during development like temperature, air humidity or pH-value. This again results in a wide spectrum of electro-chemical and mechanical characteristics of the scale layer to be removed. As the phases have different lattice structures they can be demonstrated by means of X-ray diffraction (diffraction analysis) and it is possible to determine their percentage.
One example is shown in figure 1, the main phases contained are Fe2O3 (54%) and e3O4 (42%). Besides that one could find a small amount (aprox. 3%) of FeO.
Phases Hämatit – Fe2O3 , Magnetit – Fe3O4 and Wüstit – FeO have been demonstrated in all scale samples with different percentages.

Light microscopic analysis

For analysing the layer composition and for determining layer thickness one has made microsections and has measured them in a light microscope (figure 3).
Figure 4 summarises the results of layer thickness measuring. The diagram shows that the individual charges coming from different supplies over a longer period feature partially substantial differences in layer thickness. As a deduction from these analysis results it is necessary to optimise process parameters during descaling.
Microsection patterns show that scale layers have no homogeneous structure but consist of several layers with different thickness. This divided layer composition has been found, more or less evident, in all samples tested.
Figure 5 shows a section of the scale layer (SE figure) and oxygen distribution (element analysis EDX). The area marked “lower layer” contains less oxygen than the marginal areas.
Measuring area percentage [Atom %]
iron oxygen relation Fe/O
surface 19 51,5 0,36
Upper layer 30,5 55 0,56
Lower layer 35,5 48 0,74
Basic material 63 10,5

Summary

With the radiographical method one determined in this sample 55 % Fe2O3 , 20 % Fe3O4 and 24 % FeO.
After bending descaling, the relative percentage of Fe3O4 increases. Therefore it can be assumed that phases Fe2O3 and FeO are removed much easier than Fe3O4 during mechanical descaling. This fact should be considered when ordering wire rod.
On the other hand metallographic microsections (figure 6, left-hand side etched, right-hand side not etched) clearly show that, partially, the scale layer deeply “grows” into the basic material. A removal of material of some µm (in this example approx. 100µm) is necessary to remove these scale scars completely. This can only be done by a metal-cutting process, e.g. by peeling. Such removal can be effectively achieved neither by a blasting process nor by other mechanical methods (compare figure 7) or pickling.
Within the scope of this project, comprehensive investigations were made over a longer period on the creation and the real structure of scale layers on wire material coming from running production of different rolling mills.
There it became apparent that scale does not only adhere to the surface but apparently, depending on the technological parameters during rolling, „grows“ into the basic material, and this leads to the creation of so called scale scars. Those scale parts cannot be completely removed when using the presently common descaling methods.
The mentioned peeling resp. draw-peeling is an alternative thereto. With this method, the marginal area of wire is removed by a chip-creating geometry. Here one achieves surfaces qualities of less than Ra 1µm. Kieselstein Group disposes of two test plant for producing samples for interested customers. That equipment can treat both steel wire rod in the diameter range Ø 5,5 to Ø 8,5mm and also non-ferrous metals and wires up to a diameter of 1mm. Ater a successful sample production, Kieselstein prepares the respective production and plant concept.

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