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Title: Microstructural evolution in austenitic stainless steels for extended-life power station applications
Author: Zhu, Feng
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2011
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In this work, the microstructure evolution of Type 316H and Type 321 austenitic stainless steel has been studied by experimental methods and modelling techniques. In the experimental work, the major precipitate evolution in ex-service and laboratory aged Type 316H and Type 321 steels have been identified using multi-characterisation techniques. The precipitation sequences for both materials have been studied within a wide variety of ageing conditions. Due to the very low carbon level in the matrix, it takes far less ageing time (< 100 hours) to form sigma (s) phase in Type 321 comparing to that in Type 316H. The Type 321 sample with low Si concentration did not form G phase. Unusually, a large amount of Martensite (up to 50 vol%) has been observed in the virgin Type 321 submitted to the accelerated ageing programme. The microstructure evolution of delta (d) ferrite at different ageing temperatures for austenitic stainless steel has also been studied. A Monte Carlo model has been developed to predict detailed microstructure evolution during long term ageing. The results of the quantitative precipitate evolution measurements of Type 316H and Type 321 have been used for the calibration of this microstructural evolution model. After proper validation, the microstructural evolution model was in good agreement with the experimental measurements. The effect of precipitation kinetics on the reduction of fracture toughness has been studied by examining the various fracture surfaces with different ageing conditions. A new approach for the long term fracture property prediction in austenitic stainless steels, based on the Monte Carlo microstructural modelling, has been proposed. This fracture prediction uses microstructure modelling predictions of inter-granular precipitation to forecast the life time fracture toughness degradation, which is also in good agreement with previous work. Ultimately, a method for predicting the microstructure, the mechanical properties of these Type 316H and Type 321 austenitic stainless steel and possibly other austenitic stainless steel, over the power plant life time, could be achieved from this work.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Austenitic stainless steels ; Microstructural evolution ; Monte Carlo modelling ; AGRs ; Nuclear power life extension ; High temperature degradation ; Fracture mechanism ; Type 316H ; Type 321 ; d-ferrite