Wednesday, 29 November 2017

What’s in a weld?

Investigation of welded materials for various applications in industry is of paramount importance. There are various test methods available to test the quality of welds such as destructive methods and methods whereby the material remains undamaged such as the use of images created by Rontgen rays.

The picture below shows a sample of a steel weld sample embedded in a polymer, where after it has been ground, polished and etched. In this way the crystal structure and the hardness of the weld itself and the connecting zone between the weld and the parent material can be investigated under the microscope and with a Vickers test apparatus.

A steel weld sample prepared for microscopic and hardness examination

With the ‘Motic Images Plus 3.0’ software the length of the diagonals of the ground plane of the Vickers imprints have been measured. From their square length ratio it shows that the hardness of the weld is about 12% higher than that of the parent material. Optically there is also a clear difference in the crystal structure of the parent material and the material of the weld. Both crystal structures run smoothly into each other, which is shown on one of the microscopic images. Welds are often "normalized" by means of a heat treatment in order to get back the original crystal structure and reduce welding stresses.

With thanks to: Gerard Janssen and Rijk Koster, mechanical engineers.

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