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Quantitative phase analysis of duplex stainless steels as a function of heat treatment
2009 (English)Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

During heat treatments the microstructure of metals changes and heat treatment times, heating rates and cooling rates all affect the final microstructure of metals. In the duplex stainless steels chromium nitrides have been seen after quenching from higher temperatures. The precipitates have negative effects on the material properties and decrease the fracture toughness at lower temperatures and reduce pitting corrosion resistance. To study the precipitates a method for quantification is needed and a method for this will be developed. The methods used are x-ray diffraction, magnetic balance, scanning differential calorimetry and electron microscopy with image analysis. Due to too small volume fraction the β-Cr2N phase could not be detected with x-rays or in the differential scanning calorimetry. However, with electron microscopes they could be quantified and this method together with image analysis gave adequate results. Samples of SAF3207HD were heat treated with different heat treatment solution temperatures and cooling rates. Higher solution heat treatment temperatures resulted in higher fractions of chromium nitrides, but an increased cooling rate has an even larger effect on the volume fraction. Phase fractions up to 2.33 vol% were registered. Depending on the solution heat treatment temperature a minimum cooling rate exists which produces β-Cr2N precipitates and these rates are strongly dependent in the solution heat treatment temperature. The nitrides form in three shapes, as needles, clouds or along grain boundaries. Faster cooling cause clouds to form while at slower cooling the chromium nitrides are preferably formed in grain boundaries. Finally the results are plotted in two diagrams, where the heat treatment temperature, the cooling rate and the fraction of precipitates are the three parameters. If two of these three parameters are fixed, the third can be predicted from these diagrams. These diagrams can be a very useful tool when working with chromium nitrides.

Place, publisher, year, edition, pages
Keyword [en]
Physics Chemistry Maths, Phase analysis, quantification, electron microscopy, stainless steels, heat treatments, chromium nitrides
Keyword [sv]
Fysik, Kemi, Matematik
URN: urn:nbn:se:ltu:diva-45922ISRN: LTU-EX--09/129--SELocal ID: 3906207d-3428-4bc1-9e94-32389ff34bb6OAI: diva2:1019223
Subject / course
Student thesis, at least 30 credits
Educational program
Engineering Physics, master's level
Validerat; 20101217 (root)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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