Modelling internal nitridation in an alumina-forming austenitic stainless steel

• An austenitic alumina forming stainless steel was exposed in a nitriding atmosphere comprising 5 vol% H2/N2 for 100 and 1000 h. After 100 h, the sample displayed aluminium nitrides down to 450 μm and chromium nitrides down to 200 μm. Thermodynamic equilibrium calculations and kinetic modelling of the system to simulate 100 h of exposure time were performed using the software ThermoCalc 4.1 including DICTRA. The order of appearance of nitrides in the sample starting from the surface can be understood from the thermodynamic equilibria as well as from results of kinetic modelling. However, the nitridation depth predicted by the kinetic modelling, was larger than experimentally observed, and the cubic chromium nitride predicted to be present on the surface was not detected in the exposed sample. One difference between model and experiment was a thin oxide layer present on the surface of the sample. As this oxide layer can decrease the inwards flux of nitrogen, attempts were made to describe the effects of this layer in the model. The model was also used for qualitative comparison of varying alloy compositions, temperatures and initial nitrogen contents of the matrix.

Ämnesord

TEKNIK OCH TEKNOLOGIER  -- Materialteknik -- Metallurgi och metalliska material (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Materials Engineering -- Metallurgy and Metallic Materials (hsv//eng)

Nyckelord

alumina forming austenitic stainless steel

high temperature corrosion

internal nitridation

kinetic modelling

thermodynamic modelling

Alumina

Austenitic stainless steel

Chromium

Kinetic theory

Kinetics

Nitridation

Nitrides

Nitrogen

Thermodynamics

Alloy compositions

High temperature corrosions

Software Thermo-Calc

Thermodynamic equilibria

Thermodynamic equilibrium calculation

Stainless steel

Kemi

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