Full text
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| Author(s): |
Carlos Mario Garzón Ospina
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Escola Politécnica (EP/BC) |
| Defense date: | 2001-08-30 |
| Examining board members: |
Andre Paulo Tschiptschin;
Helio Goldenstein;
André Luiz Vasconcellos da Costa e Silva
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| Advisor: | Andre Paulo Tschiptschin |
| Abstract | |
AISI 410 (11,5 wt-%Cr-0,13 wt-%C) and 410S (13.0 wt-%Cr-0.07 wt-%C) stainless steels were nitrided at temperatures between 1273 and 1473K and then direct-quenched in water. The N2 pressures varied between 0.02 and 0.38 MPa and the nitriding timeswere between 0.25 and 48 h. After these treatments it were obtained both thin foils (up to 0.75 mm in thickness) with homogeneous nitrogen content and thick plates with nitrogen gradient from surface to nucleus. The nitrogen content of the thin foils varied between 0.2 and 2.14 wt-%, and the nitrogen content at the surface of the thick plates was 0.13 to 2.4 wt-%. Chemical and microstructural characterization of the nitrided specimens was carried out to evaluate the nitriding processes. The microstructure was studied by using Optical and Scanning Electron Microscopy, X-Ray Diffraction and Micro-Hardness. Chemical characterization was made through Fusion under Inert Gas, Optical Spectrometry and WDX Spectrometry. The microstructure of the direct-quenched specimens was composed by martensite, retained austenite and type MX, [(Cr,Fe)(C,N)] and M2X [(Cr,Fe)2(C,N)] precipitated nitrides. The thermodynamics of nitriding and the relation between microstructure and nitriding parameters (pressure and temperature) was studied with the aid of Thermocalc. The use of this tool allowed obtaining phase diagrams and diverse calculations involving state variables as a function of nitriding conditions. The results showed thatby increasing nitriding time and pressure, as well as by decreasing temperature, both the nitrogen content at the surface of the steels and the tendency to form precipitates increased. It was also observed that the M2N nitride was more stable for lower temperatures, N2 pressures and times than the MN nitride. ) The kinetics of nitrogen uptake during high-temperature nitriding (nitriding temperature > Ac1) was modelled by solving the mass-transport equations for the thermodynamic system through the apparent diffusion coefficient concept. The diffusion coefficient of nitrogen in austenite and the mass-transfer coefficient between gas and austenite were previously calculated by the sorption method. (AU) | |