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Nadia Mebarki (V.2)
Abstract
Relationship between microstructure and mechanical properties of tempered martensitic tool steel
Nadia Mebarki - 19 february 2003.
The tempered martensitic steels with 5% chromium, mainly used for the forging and high-pressure die casting tools show a limited lifetime due to their severe thermomechanical working conditions.
Indeed, the thermomechanical history of these steels leads to a complex microstructure at several scales and especially at the nanometric scale where dislocations resulting from quenching and carbides resulting from tempering take part in the hardening of these alloys.
The main goal of the present study deals with a martensitic steel characterization at the nanometric scale. Transmission Electron Microscopy and the X Ray Diffraction were chosen in order to have access to the relevant parameters of the initial microstructure with various heat treatments.
However, previous studies showed that these steels present a strong cyclic softening behavior at any temperatures. Because softening can reach 300MPa in the extreme cases (high temperatures + severe request), it is important to take into account these phenomena in the choice of the steel and its heat treatment. This present study shows that the coalescence of carbides, the reduction of the dislocation density as well as the increase of the heterogeneity of the dislocation structure are responsible for this softening.
A relation between the variation of yield stress and the variation of the relevant microstructural parameters is proposed.
This work consists in the first stage in our aim to guide the steel manufacturers to make the chemical composition (in particular in carbide-forming elements) and the heat treatment of their hot work tool steels evolve, after several years of steadness.
Keywords:
Martensitic steel - Heat treatment - Low cycle fatigue - Microstructure -
Carbide - Dislocation - Softening assessment
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