Scanning Microscopy


Three commercial weldable fine grained structural steels and four experimental melts with lowered contents of trace elements were subjected to a welding simulation treatment followed by stress relaxation (SR) tests. After testing times of up to 8 hours the samples were removed from the testing rig, broken in the fracture stage of an Auger instrument, and the fracture surface was examined in situ for segregation of elements. SEM investigations of the fracture surfaces and light microscope served to characterize fracture mode and microstructure.

The microfractural appearance of the grain boundaries exposed by the impact loading in the Auger instrument and of the ones separated by SR-testing was significantly different. While the fracture surfaces originating from SR-testing were flat, the samples subsequently broken in the Auger instrument showed a dimpled structure.

It could be shown that cracks always started at MnS - precipitates, and that the intergranular crack propagation was enhanced by the segregation of phosphorus. The segregation of elementary sulphur was initiated by the stress field of the cracks already formed and, contrary to the phosphorus enrichment, could be prevented by lowering the S-content in the melt. The other trace elements seemed to play no part in the stress relief cracking of the steels investigated.

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