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Cracking of ferritic stainless steel tubes during production process
Majtás, Dušan ; Kreislová, K. ; Viani, Alberto ; Pérez-Estébanez, Marta ; Geiplová, H.
From the economic reasons many products originally made from austenite stainless steel are nowadays made from ferritic stainless steel. Ferritic steels have relatively low yield strength and the work hardening is limited. They cannot be hardened by heat treatment and only moderately hardened by cold working. Commercially made stainless steel tubes from ferritic steel, used for industrial plumbing was examined on presence of cracks. The cracking was present on the inner side of the convoluted tube shape. The tube manufacturing process consisted of continual bending of the sheet to tube shape, weld the tube, then of cold shaping by pulling through rib-forming frames, which is done in several steps. Then thermal treatment applies to the nearly finished product to remove stress remaining in the structure. Prime suspect was deformation beyond the ductility of used material. However the stress-strain tensile testing does not approved this hypothesis. Several samples of failed material were taken together with reference, and were examined by optical microscopy, and X-Ray Diffraction structure analysis. The structure of the cracked tubes does not show the signs of deformation over the limit, except the location near to the crack itself. Interestingly enough the failed material showed more homogenous structure than the original one. Needle like structures were found when the material is “overetched”, on these structures concentration of stress under bending occur. This structure was identified as δ-ferrite, however its presence in α-ferrite matrix is unclear.
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Removing of failures on high-pressure gas conduits
KUBÍČEK, Petr
In my work I have addressed the issue of troubleshooting failures on high-pressure pipelines, different kinds of failures, according to the type of damage, the physical nature of gas transportation, pumping stations; I have included also a proposal to prevent the occurrence of failures. In my paper I furthermore present two comprehensive failures which occurred in the Prachatice district and which I described already in my bachelor degree thesis. As regards one of the failures, I only assisted, very remotely. However, as regards the other failure, it was my responsibility to resolve the problem, in the most expedient way, from tits detection to modification of the project documentation and subsequent replacement of the damaged high-pressure pipeline.
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