Microstructure evolution and creep resistance of a Z-phase strengthened 12% Cr steel

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dc.contributor.authorTimhagen, Johanna
dc.contributor.departmentChalmers tekniska högskola / Institutionen för industri- och materialvetenskapsv
dc.contributor.departmentChalmers University of Technology / Department of Industrial and Materials Scienceen
dc.date.accessioned2019-07-05T11:52:49Z
dc.date.available2019-07-05T11:52:49Z
dc.date.issued2019
dc.description.abstract9–12% Cr steels are a family of steels, which are the backbones for today’s fossil fuel steam power plants, which provide more than 60% of electricity worldwide. By developing more creep and corrosion resistant 9–12% Cr steels, the operation temperature of 620°C could be increased to 650°C, which would reduce the environmental impact. For improved resistance to corrosion, a higher Cr content is needed. However, an increased Cr content results in formation of stable but big detrimental Z-phase precipitates. While Z-phase traditionally results in premature creep failures, a new alloy design strategy, called Z-phase strengthening, aims to make use of Z-phase particles as strengthening precipitates rather than detrimental particles. Getting a better understanding of the correlation between microstructure evolution and creep resistance in these steels, could potentially lead to a solution to the conflict between creep and corrosion resistance at 650°C. In this study, two versions of the same 12% Cr steel have been evaluated, one version is tempered at 700°C and the other at 740°C. Investigation have been done with light optical microscopy (LOM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). Results from these methods were correlated with results from mechanical testing and creep testing, and together they provided a better understanding of the evolution of nano-sized particles at high temperatures. It was found that tempering with a lower temperature results in better creep properties. Mechanical properties are also improved by a lower tempering temperature, but eventually the values of hardness, tensile testing, and impact toughness for the two different versions coverage with ageing. M23C6 particles grow very slowly. Laves-phase is significantly affected by tempering temperature. A higher tempering temperature results in Z-phase particles that start to transform from MX particles after a shorter ageing time. The average particle size is smaller with a lower tempering temperature. A lower tempering temperature results in superior microstructure regarding nano-sized particles.
dc.identifier.urihttps://hdl.handle.net/20.500.12380/256837
dc.language.isoeng
dc.setspec.uppsokTechnology
dc.subjectMaterialvetenskap
dc.subjectProduktion
dc.subjectMaskinteknik
dc.subjectMaterialteknik
dc.subjectMaterials Science
dc.subjectProduction
dc.subjectMechanical Engineering
dc.subjectMaterials Engineering
dc.titleMicrostructure evolution and creep resistance of a Z-phase strengthened 12% Cr steel
dc.type.degreeExamensarbete för masterexamensv
dc.type.degreeMaster Thesisen
dc.type.uppsokH
local.programmeAdvanced engineering materials, MSc
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