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Alexandre Martinier
Abstract
Life time of refractory steels for SPF tools: tests and simulations
Alexandre Martinier - 6 December 2005
Superplastic forming is an innovative material forming process, making profitable the alloy superplastic properties. The companies, mainly in aeronautical industry, knowing this technological advantage are already end-users. However an extreme forming environment is imposed on the casted heat-resistant Ni-Cr-Fe steel tooling, generating very high thermomechanical stresses and causing their premature damage. The main objective of this work is to study the high temperature behaviour and the life-time of heat-resistant steel superplastic forming moulds. In this frame, work aims at introducing computer simulation tools in order to assist the design and the dimensioning of the moulds. A bibliographic study on the operational conditions and a state of the art on high temperature properties of heat-resistant steels were performed. A cyclic fatigue test campaign at high temperature (25°C-950°C) was performed to identify, on the one hand, a cyclic elasto-visco-plastic constitutive model, and on the other hand to propose a methodology for life-time prediction. To this extend, a strain formulated creep damage model was modified in order to take into account the effects of fatigue-relaxation at high temperature, by introducing a frequency effect. In parallel, the microstructural investigations at different observation scales have shown a heterogeneous mechanical behaviour of material at the grain scale. All these results were used in a validation stage that compares high temperature fatigue tests on notched samples with the finite element simulations.
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