Assessment of the hardening and embrittlement of pressure vessel steels and welds as a function o... more Assessment of the hardening and embrittlement of pressure vessel steels and welds as a function of neutron dose uses trend curves derived from surveillance specimens. For some materials in the UK typical of those used in older plant, these curves incorporate an empirical factor to describe the temperature-dependence of matrix hardening. In the present work, molecular dynamics (MD) simulations have
Consideration of the structural integrity is one of the inputs when evaluating potential solution... more Consideration of the structural integrity is one of the inputs when evaluating potential solutions to plant problems. Structural integrity assessments of components forming the pressure boundaries of nuclear plant evaluate safety margins against cleavage fracture. These assessments consider the reserve factors between the applied stress and fracture toughness of the material as well as temperature margins between the operating temperature and the temperature at which the steel is ductile as defined by upper shelf behaviour. To carry out these structural integrity assessments, estimates of cleavage fracture toughness are required. The approach presented in this paper allows for differences between cleavage fracture toughness properties of different materials associated with material to material variability. Mean cleavage fracture toughness properties are described as a function of temperature, section thickness and ductile crack growth. The random scatter is described by standard statistical probability distributions with variance a constant percentage of the mean cleavage fracture toughness. Micromechanisms of cleavage fracture and numerical modelling show that fracture toughness is determined by the microstructure of the steel which also defines the work hardening characteristics, the yield stress and its temperature dependence. On this basis, temperature and thickness dependence, and scatter of cleavage fracture toughness are specific to a given type of material. This paper presents methods for estimation of cleavage fracture toughness from small data sets using simple statistical tools and cleavage fracture toughness curves fitted to large databases for materials with the specification similar to the small data set. Applications of this methodology are presented. The methodology is validated by comparing the predicted values with data that were not used in the analyses. One of the applications illustrated in1 British Nuclear Group (BNFL),Engineering and Technology Services, Berkeley Centre, Berkeley Glos. GL13 9PB, UK. 2 Also Interface Analysis Centre, University of Bristol,Bristol. BS2 8BS, UK. volves predictions of cleavage fracture toughness for the neutron irradiated condition. This is validated by comparing predictions with data measured on specimens removed from ex-service reactors.
Systematic investigation of the properties of solids requires an understanding of their microstru... more Systematic investigation of the properties of solids requires an understanding of their microstructure, extending from the nanoscale (atomic arrangement) to the millimetre scale (grain size, crack lengths). In this sense, the prefix "micro" is really the geometric mean of "nano" and "milli" – it refers to structure that has, until recently, been ignored by physicists more concerned with electronic structure and by chemists more concerned with molecular structure. Its study has been the domain of metallurgists and engineers and, by revealing the mechanisms underlying physical processes, it complements the proof testing that guarantees the safety of engineering structures. Nowadays it plays a considerable role in forensic science, conservation of museum pieces, bioengineering and in practically every activity where the state of a material crucially affects human safety or well-being.
The present work extends the understanding of the role of thermo-mechanical history on the relaxa... more The present work extends the understanding of the role of thermo-mechanical history on the relaxation of residual stresses. Different thermo-mechanical pre-treatments have been developed and applied to an ex-service Type 316H austenitic stainless steel to produce both microstructures and residual macro-stresses typical of those in the heat affected zone (HAZ) of thick section attachment weldments, where reheat cracking has been observed during high temperature service (~550°C). The microstructures have been characterised after each step of pre-treatment. The through section residual macro-stresses have been measured by neutron diffraction. For the range of thermo-mechanical pre-treatments adopted, the relaxation of the initial residual macro-stresses have been measured following thermal exposure at a temperature of 550°C for times of 1h and 1800h. The results are discussed, based on the relaxation characteristics of the residual macro-stresses, in terms of the role of initial microstructure on the creep deformation behaviour and the susceptibility to the service reheat cracking.
Assessment of the hardening and embrittlement of pressure vessel steels and welds as a function o... more Assessment of the hardening and embrittlement of pressure vessel steels and welds as a function of neutron dose uses trend curves derived from surveillance specimens. For some materials in the UK typical of those used in older plant, these curves incorporate an empirical factor to describe the temperature-dependence of matrix hardening. In the present work, molecular dynamics (MD) simulations have
Consideration of the structural integrity is one of the inputs when evaluating potential solution... more Consideration of the structural integrity is one of the inputs when evaluating potential solutions to plant problems. Structural integrity assessments of components forming the pressure boundaries of nuclear plant evaluate safety margins against cleavage fracture. These assessments consider the reserve factors between the applied stress and fracture toughness of the material as well as temperature margins between the operating temperature and the temperature at which the steel is ductile as defined by upper shelf behaviour. To carry out these structural integrity assessments, estimates of cleavage fracture toughness are required. The approach presented in this paper allows for differences between cleavage fracture toughness properties of different materials associated with material to material variability. Mean cleavage fracture toughness properties are described as a function of temperature, section thickness and ductile crack growth. The random scatter is described by standard statistical probability distributions with variance a constant percentage of the mean cleavage fracture toughness. Micromechanisms of cleavage fracture and numerical modelling show that fracture toughness is determined by the microstructure of the steel which also defines the work hardening characteristics, the yield stress and its temperature dependence. On this basis, temperature and thickness dependence, and scatter of cleavage fracture toughness are specific to a given type of material. This paper presents methods for estimation of cleavage fracture toughness from small data sets using simple statistical tools and cleavage fracture toughness curves fitted to large databases for materials with the specification similar to the small data set. Applications of this methodology are presented. The methodology is validated by comparing the predicted values with data that were not used in the analyses. One of the applications illustrated in1 British Nuclear Group (BNFL),Engineering and Technology Services, Berkeley Centre, Berkeley Glos. GL13 9PB, UK. 2 Also Interface Analysis Centre, University of Bristol,Bristol. BS2 8BS, UK. volves predictions of cleavage fracture toughness for the neutron irradiated condition. This is validated by comparing predictions with data measured on specimens removed from ex-service reactors.
Systematic investigation of the properties of solids requires an understanding of their microstru... more Systematic investigation of the properties of solids requires an understanding of their microstructure, extending from the nanoscale (atomic arrangement) to the millimetre scale (grain size, crack lengths). In this sense, the prefix "micro" is really the geometric mean of "nano" and "milli" – it refers to structure that has, until recently, been ignored by physicists more concerned with electronic structure and by chemists more concerned with molecular structure. Its study has been the domain of metallurgists and engineers and, by revealing the mechanisms underlying physical processes, it complements the proof testing that guarantees the safety of engineering structures. Nowadays it plays a considerable role in forensic science, conservation of museum pieces, bioengineering and in practically every activity where the state of a material crucially affects human safety or well-being.
The present work extends the understanding of the role of thermo-mechanical history on the relaxa... more The present work extends the understanding of the role of thermo-mechanical history on the relaxation of residual stresses. Different thermo-mechanical pre-treatments have been developed and applied to an ex-service Type 316H austenitic stainless steel to produce both microstructures and residual macro-stresses typical of those in the heat affected zone (HAZ) of thick section attachment weldments, where reheat cracking has been observed during high temperature service (~550°C). The microstructures have been characterised after each step of pre-treatment. The through section residual macro-stresses have been measured by neutron diffraction. For the range of thermo-mechanical pre-treatments adopted, the relaxation of the initial residual macro-stresses have been measured following thermal exposure at a temperature of 550°C for times of 1h and 1800h. The results are discussed, based on the relaxation characteristics of the residual macro-stresses, in terms of the role of initial microstructure on the creep deformation behaviour and the susceptibility to the service reheat cracking.
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