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Preparation and analysis of production control samples by a two-step method

: Vrenegor, J.; Sturm, V.; Noll, R.; Hemmerlin, M.; Thurmann, U.; Flock, J.

Angeli, J. ; European Committee for the Study and Application of Analytical Work in the Steel Industry -CETAS-:
Progress in analytical chemistry in the steel and metal industries : Seventh International Workshop on Progress in Analytical Chemistry in the Steel and Metal Industries, 16 to 18 May 2006 in Luxembourg
Essen: Verlag Glückauf, 2006
ISBN: 3-7739-6016-6
International Workshop on Progress in Analytical Chemistry in the Steel and Metal Industries <7, 2006, Luxembourg>
Fraunhofer ILT ()

Production control samples are used in steel works to control the steel composition. During the solidification and cooling down of the samples taken from a steel melt a scale layer is formed at the surface having a thickness of several hundreds of microns. The composition of the scale layer differs from the composition of the bulk material. Especially carbon shows an enhanced concentration in the inhomogeneous scale layer. Hence the analysis of the bulk material requires the removal of the scale layer. In the conventional method for process control this layer is removed by grinding or milling. The subsequent analysis is carried out by e.g. spark discharge optical emission spectrometry (SD-OES) or by X-ray fluorescence analysis (XRF).
In this paper a method is presented where the process control samples, like low-alloy steel, are prepared locally using a sequence of laser pulses in a first step and analysed with laser-induced breakdown spectroscopy (LIBS) in the second step. This method has the potential to replace the conventional mechanical preparation of the samples and to combine sample preparation and analysis in a single set-up. It could simplify the process control and shorten the process control time with the effect of a gain in efficiency. Temporal pulse sequences for an enhanced laser ablation and the analytical performance of the method will be demonstrated. Production control samples are analysed which are not prepared at one side and ground at the other side. The performance of the method is assessed by comparing the analysis of the ground side as reference with the analysis of the scale side using the same laser and measuring parameters. For example the residual value of the laser analysis of carbon describing the mean quadratic difference of the results of the analysis gained on the scale side and the ground side amounts to 0.03 wt-% for 8 samples with carbon concentrations of up to 0.7 wt-%. This value is by a factor of 50 smaller in relation to the analysis carried out without the laser preparation step. The time for the preparation and analysis at one sample position can be reduced to about 35 s using a laser with a repetition frequency of 50 Hz. With the LIBS set-up including a Paschen-Runge spectrometer, detection limits below 10 µg/g are determined for the elements C, P, S. Al, Cr, Cu, Mn and Mo.