Speaker
Description
Thermochemical surface treatments, such as carburizing are widely used methods for case hardening in order to improve wear resistance, impact strength, and fatigue properties in gear applications. Alternatives to carburizing are nitriding or carbonitriding by diffusing nitrogen into the surface of metals. Previous works have shown that the nitrogen enrichment in austenite has drastic consequences on austenite decomposition, with accelerated kinetics and fine resulting microstructures. This is ascribed to a large variety of nitrides precipitates which form inside the austenitic matrix during the enrichment: micro-sized nitrides at prior austenite grain boundaries and nano-sized intragranular nitrides.
While previous works mostly investigated kinetics aspects (dilatometry, HE-XRD) and microstructural characterizations (by TEM), the present study exploits the possibilities offered by orientation-based imaging technique in a scanning electron microscope (EBSD), including in-situ observations at high temperature and the application of crystallographic parent reconstruction method with Merengue software. The challenges are to characterize (by EBSD) multiphase and nanoscale inherited microstructures, often having phases with the same crystal structure (such as CrN, retained austenite, MnS). The purpose is to establish the role of the nitrides on (i) the austenite grain size evolution during re-austenitization and (ii) the nucleation sites of ferrite/bainite upon austenite decomposition during cooling. Note that it seems difficult to determine nucleation sites for martensite (even without the presence of nitrides).
The nitrogen-enriched samples (after nitriding and carbonitriding) are found to show drastic austenite grain refinement from 10 µm in the as-enriched samples to 2 µm in the re-austenitised samples. The fine austenite grains combined with the presence of the nitrides have large consequences on the inherited microtextures in the bainite (formed at 400°C) and martensite in terms of spatial organization of the crystallographic variants and presence of proeutectoid ferrite.
Keywords: Nitriding, Carbo-nitriding, Enrichment, Micro-texture, Grain refinement
| Speaker Country | France |
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