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Christophe Daffos
Abstract
Damage in nitrided tool steels : analyse and modelisation
Christophe Daffos - 21 january 2004
The fatigue life of tool steels is controlled by internal and superficial damage. On the surface, the initiation of crack can be described by fatigue and environment interaction. Differences between cyclic and static oxidation rule have been observed. In the core, microstructural evolutions due to temperature and mechanical sollicitations exist and can be quantified. In fact, under cyclic stress and higher temperature, the microstructural composition (precipitate distribution) and the physical properties change during the test (dislocation density).
The use of nitrided coating altered the surface oxidation kinetic. Moreover, nitruration treatment can increase the fatigue life while the imposed deformation is menus than a critical deformation value (about 1%). Quasi-fragile behavior of nitrided coating is observed for strain higher than 1%.
In order to predict the fatigue life, two predictive models have been elaborated in laboratory depending on thermo-mechanical conditions. The first based on Manson Coffin formulation introduces plastic strain as main factor. The second describes precisely cracking phenomenon with initiation and propagation phases. The effect of oxidation is introduced.
These two models permit, with thermal fatigue configurations, to predict the duration of life. The validation of our model uses results from other laboratories.
Moreover, the comparison between these two predictive models, permits to better understand the damaging mechanical mechanism. In conclusion, Manson Coffin approach defines the number of cycles to propagate the main crack.
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