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Sébastien Mauriès
Abstract
Characterization and modelling of the sequence of precipitation of carbides during the heat treatment of alloy martensitic steels
Sébastien Mauriès - 18 December 2008
Martensitic steels containing 5% chromium, mainly used for forging and high-pressure die casting tools show limited lifetime due to severe thermo-mechanical working conditions. The resistance to stress at high temperatures is directly related to the stability of nanometer sized alloyed carbides which are formed beyond 450°C during tempering. The study aims to characterize alloyed carbides in the early stages of tempering and determine how the sequence of precipitation is changed. In that way, carbides forming elements (V, Mo) as well as influencing the precipitation (Ni, Co) were added to change the precipitation of a low-silicon AISI H11 steel previously studied, especially at temperatures near the peak hardness.
The characterization of carbides formed during the heat treatment was carried out using complementary techniques such as transmission electronic microscopy, X-rays diffraction, small angles neutron scattering and atom probe. These techniques have allowed access to relevant parameters of microstructure, which are distribution size, chemistry, and volume fraction of alloyed carbides. The dissolution of iron carbide M3C type is responsible for the precipitation of chromium carbides M7C3 type during tempering. The addition of nickel, molybdenum and vanadium promotes the precipitation of alloyed carbides at the peak hardness.
In order to systematize the studies on understanding the mechanisms of precipitation and the influence of alloying elements on these mechanisms, it is necessary to use modelling tools, thus know the sequence of precipitation. Kinetics simulations were carried out using MatCalc software developed at the University of Graz (Austria). The influence of input parameters of the model was studied in order to show the parameters that can be determined experimentally with sufficient accuracy (grain and subgrain size) and adjustable parameters that are difficult to measure experimentally (dislocations density and interfacial energy). The sequence of precipitation during the heat treatment of steels containing 5% chromium was simulated with differences between the volume fraction experimental and simulated a factor not exceeding 2.
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