Effect of residual stress on the fracture of ferritic steels

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To accurately study the Residual Stress (RS) effect on fracture, the first step is to be able to create the expected Stress in the component while properly controlling of the intensity of the stress, which is the main difficulty. In order to develop a methodology to tune residual stress CEA-LISN has devised a heat treatment process which can adjust the residual stress induced in the material through careful adjustment of the heat treatment. The objective of this project is to help design and subsequently validate and optimise the heat treatment process. A combination of finite element simulation and experimental techniques such as deep hole drilling and neutron/x-ray diffraction will be used. As the first step, the heat treatment process designed by CEA will be simulated at Bristol. Both Abaqus and Code_Aster will be considered in the start of the project before a final decision is made within 6 months on which software to be used. The material of interest is a Framatome RPV pressure vessel steel with complex microstructure. A thorough literature review on the thermal treatment of ferritic-martensitic RPV steels will be carried out to identify the key influencing parameters (e.g. is there a phase transformation during the heat treatment). While awaiting material and the final design of the heat treatment process from CEA, the student at Bristol will be using a model material and a known heat treatment to assist with learning finite element simulation and residual stress measurement. Once the finite element simulation of the project is developed and validated, optimisation processes can be applied to it to fine tune the target residual stress through change in the heat treatment.