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Combined methods for microstructure characterization

Comparison of EBSD measurements (local misorientation, upper row) and microhardness mappings (lower row) of pipes subjected to different weld annealing time-temperature cycles
Comparison of EBSD measurements (local misorientation, upper row) and microhardness mappings (lower row) of pipes subjected to different weld annealing time-temperature cycles

Mannesmann Line Pipe is expanding its methodological spectrum by systematically combining various approaches to microstructure characterization with a view to optimizing the properties of HFI-welded steel pipes.

New and innovative analytical methods and tools can broaden and deepen the understanding of new materials and production processes. This includes, in particular, the full utilization of the information contained in the data, e.g. by refined evaluation processes. In cooperation with Salzgitter Mannesmann Forschung GmbH, it is our objective to fully exploit synergetic effects on macro-, micro- and nanoscopic scales in the deployment and evaluation of methods for microstructure characterization and to correlate them with the mechanical properties of the materials. HFI-welds are examined for the influence of process and heat treatment parameters on their properties.

In this context, electron microscopic methods are used to a great extent. By means of electron backscatter diffraction (EBSD), for example, mean local misorientation, micro-hardness (or penetration hardness) and Charpy toughness can be inter-correlated and transferred to various weld annealing treatments. The micrographs show the local misorientation (upper row) and the penetration hardness (lower row) for two different annealing time-temperature cycles. In the two diagrams below, the corresponding density distributions are represented, from which the direct correlation between the two measurands emerges: the higher microhardness is reflected in the higher local misorientation.

Based on these results and other extensive work, various methods have been derived which can be used for characterizing the microstructure of production specimens, depending on the respective criteria as well as mechanical and technological properties. In view of the more and more stringent requirements on the service behavior of HFI pipes optimized to a specific application profile, it is absolutely essential that all standard characterization methods – from light microscopy to electron microscopic examinations – are available and used to best effect. Only then is it possible to recognize potentially unfavorable microstructural components on all size scales and derive suitable corrective measures. This is how we ensure ongoing improvement of the quality of our products.

The density distributions reflect the correlation of the two measured variables.
Density distribution

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