Decomposition of expanded austenite in AISI 316L stainless steel nitrided at 723K

Abstract

Expanded austenite (γ_N), which can be produced during plasma nitriding of austenitic stainless steels, provides high levels of strength, toughness and corrosion resistance by comparison with traditional nitride layers. However, expanded austenite properties can be lost due to decomposition caused its thermodynamic metastability. In the present work, austenitic stainless AISI 316L steel was plasma nitrided at 723 K for 5 h at 500 Pa and microstructurally characterised by X-ray diffraction (XRD), and optical and transmission electron microscopy (TEM) which confirmed the presence of fcc expanded austenite with a lattice parameter up to 9·5% larger than untreated austenite. TEM analyses of thin foils showed that fine nitrides were formed in the γ_N layer and some areas were observed with a singular lamellar morphology very similar to the pearlite colonies found in carbon steels. Selected area electron diffraction (SAED) analysis suggests that these areas are composed of bcc ferrite and cubic chromium nitrides produced after a localised decomposition of the expanded austenite layer. Amorphous expanded austenite was observed in some areas of the investigated samples. The occurrence of γ_N decomposition was associated with microsegregation of ferrite stabilisers (Cr, Mo) and depletion of an austenite stabiliser (Ni) in localised regions of the expanded austenite layer.

Keywords:

• Austenitic stainless steel
• Plasma nitriding
• Expanded austenite
• Decomposition
• Nitride formation
• Amorphous regions
• Transmission electron microscopy
• EDS microanalysis

The authors would like to thank Brazilian research agencies CNPq (LCC and JG) and CAPES (FAPF) for grants received which supported this work.