Thermal decomposition of N-expanded austenite in 304L and 904L steels

Abstract

In this study, expanded austenite was prepared by an industrial low-temperature plasma nitriding process in 304L and 904L austenitic stainless steels. The current investigation focuses on the assessment of the thermal stability and related phase evolution of the expanded austenite layer during isothermal annealing in protective argon atmosphere at temperatures ranging from 450 to 600°C for 24 and 168 h. Characterisation of the original expanded austenite and the decomposed surface layers was performed. Denitriding, inward N-diffusion and Cr-compounds precipitation occurred at different extent, depending on annealing conditions and alloy composition. Expanded austenite in 304L exhibited a near complete eutectoid decomposition after a short annealing time, while 904L showed significantly better thermal stability. A fine dispersion of small CrN precipitates resulting from expanded austenite decomposition at relatively low annealing temperature or short duration could further positively affect the surface hardness of both materials. Precipitate growth reduces hardness at higher annealing temperatures/times.

Keywords:

• Expanded austenite
• Plasma nitriding
• S-phase
• Thermal stability
• Discontinuous decomposition
• Eutectoid decomposition

Acknowledgements

Financial support from the National Graduate School of Materials Science, Area of Advance Materials Science, Chalmersska forskningsfonden, hosted by Chalmers University of Technology, the National Swedish Energy Administration and Stiftelsen Axel Hultgrens Fond is gratefully acknowledged. The authors also thank Miguel Angel Esneider Alcalá of CIMAV Unidad Monterrey, Mexico for technical support and Bodycote Värmebehandling AB (Sweden) for providing the nitriding treatment.

ORCID

G. Maistro http://orcid.org/0000-0003-0883-2154

S. A. Pérez-García http://orcid.org/0000-0002-6947-4728

M. Norell http://orcid.org/0000-0001-9945-9177

Y. Cao http://orcid.org/0000-0002-1965-5854

Additional information

Funding

This work was supported by Chalmers Tekniska Högskola, Energimyndigheten and Jernkontoret.