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Microstructure evolution in Nb alloyed Esshete 1250 creep resistant austenitic stainless steel. / Chia Yuin Wong
Swansea University Author: Chia Yuin Wong
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The microstructure evolution of a commercial grade creep-resistant austenitic steel, namely Esshete 1250, was investigated under different creep temperature and stress conditions, with an overall aim of exploring the micorstructural relationship to creep rupture during high temperature application....
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The microstructure evolution of a commercial grade creep-resistant austenitic steel, namely Esshete 1250, was investigated under different creep temperature and stress conditions, with an overall aim of exploring the micorstructural relationship to creep rupture during high temperature application. Creep tests data was supplied by British Energy on temperatures varied from 550°C to 700°C for periods of up to 17 years. The literature review includes the study of various creep resistant alloys and a detailed investigation on the precipitation reactions that take place in creep resistant steels. Moreover, the strengthening mechanisms in order to obtain suitable creep resistance properties for engineering materials for high temperature applications is also reviewed. Long term creep deformation for Esshete 1250 creep resistant steel is reviewed in Chapter 2. The tensile properties of Esshete 1250 parent material and weld material are included in this Chapter as well. Qualitative and quantitative metallography techniques are reviewed in order to provide the required background information for the interpretation of obtained microstructure. The experimental study involved hardness testing and scanning electron microscopy examination. The size, distribution of MX precipitates was analysed with electron microscopy techniques together with Optilab analysis, while metallographic grain evolution measurements in creep exposed samples was also carried out. As part of this study, the grain size evolution and precipitate size evolution of Esshete 1250 creep resistant steel are obtained. Attention then is given to the volume fraction, size and distribution of MX (Nb-rich) particles. It is concluded that MX precipitation is the key factor that influences the creep resistance of Esshete 1250 under service conditions, while grain size is additional to the effect of MX precipitation in solution and is of secondary importance. The obtained results can be implemented into other creep alloy design, helping to meet the challenge of developing high temperature alloy systems for greater sustainability.
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