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Influence of cooling rates and pickling on intergranular corrosion in sanicro 31HT tubes
2001 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

This Master of Science thesis has been performed at the R&D centre of AB Sandvik Steel in Sandviken, Sweden. During heat-treatment of tubes, the grain boundaries sometimes obtain precipitates due to insufficient quench. The cooling rate during quench affects precipitation which in turn might result in sensitisation, because the material gets sensitive to intergranular corrosion. A too slow cooling rate gives enough time for precipitates to develop in grain boundaries. The precipitates may affect the properties of the material negatively, resulting in material damage in the tubes during subsequent treatment. Subsequent treatment in this case is pickling, which takes away the surface oxide. If the grain boundary contains a large amount of precipitates the pickling liquid will corrode not only the oxide but also the grain boundary and the area adjacent to it. A high cooling rate prevents precipitation, but instead the tubes tend to be distorted because the tubes are more sensitive to non- uniform quench at higher cooling rates. Severely distorted tubes are straightened in a gag-straightener, which is a complicated process, and therefore avoided if possible. The aim of this thesis was to investigate the precipitation in grain boundaries during quench. Investigation of the cooling rate needed to obtain an microstructure free from precipitation, and the cooling rate that gave an amount of precipitates that could be accepted without the material being sensitive to intergranular corrosion. It was also important to examine in what part of the cooling cycle the precipitation occurred, to be able to improve the cooling cycle in production. A study was done to improve the cooling rate and to make the quench uniform. This work will determine the design specifications for a new quench facility. The material that has been investigated was Sanicro 31HT. Sanicro 31HT is an austenitic, high alloyed stainless steel. The thesis contains a literature survey, to gain an insight into the technical expertise and research in the field. The work also includes experiments both in production and in the laboratory, and the results from experiments have been evaluated by light optical microscopy and scanning electron microscopy.

Place, publisher, year, edition, pages
Keyword [en]
Technology, Cooling rate, Pickling, Intergranular Corrosion, Sanicro, 31HT, High alloyed stainless steel
Keyword [sv]
URN: urn:nbn:se:ltu:diva-47937ISRN: LTU-EX--01/114--SELocal ID: 56d83a2b-b6ab-4e24-a082-39cdd50d8648OAI: diva2:1021273
Subject / course
Student thesis, at least 30 credits
Educational program
Mechanical Engineering, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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