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Zhanping Zhang
Abstract
Anisothermal cyclic behaviour modelling with taking into account the tempering effect of a martensitic tool steel 55NiCrMoV7
Zhanping Zhang - 2 may 2002.
This study is focused on the modelling of the anisothermal cyclic behaviour of a tempered martenstic tool steel 55NiCrMoV7 in taking into account the ageing ef fect under thermomecanical loading in service.
The ageing during the tempering was studied by hardness measurement for temperature in the range 100°C to 700°C and the tempering time between 90 seconds and 665 hours.
Microstructure evolution was investigated at different scales, such as the ex- austenitic grains, the martensitic laths and the carbides using XRD, SEM, TEM and quantitative image analysis. Results indicate that the mean diameter of the intra-lath carbides is linearly well correlated to the hardness.
A kinetic ageing law was proposed, by means of a concept « ageing ratio » based on the quenched, annealed and tempered hardness, in the form of Johnson-Mehl- Avrami equation. This law was verified by some double-tempering tests. Moreover, the proposed law allows to get the well known Hollomon-Jaffe relationship between the tempering parameter and tempering conditions which is traditionally used in the industry for the determination of the tempering conditions to obtain an expected hardness.
Besides the usual isothermal and anisothermal low cycle fatigue tests, two types of special isothermal cyclic deformation tests were realised to identify all the parameters for the anisothermal cyclic behaviour model. The identification methodology for the constitutive equations was discussed in detail. The influence of temperature, strain amplitude and initial hardness of the steel, as well ageing have been particularly studied.
An anisothermal cyclic elastoviscoplasticity model was proposed, under the classical framework of thermodynamics of irreversible processes with internal variables. The anisothermal cyclic model considering the ageing effect has been verified by experimental under two conditions where the microstructure is not stable.
Keywords:
Anisothermal cyclic elastoviscoplasticity model, Cyclic softening,
Thermomechanical fatigue, Tempering, Softening ratio, Kinetic ageing law,
Identification, Simulation
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