Surface Engineering by Expanded Austenite
S-Phase Formation
Expanded Austenite Phase in Stainless Steel
The microstructure responsible for both the increased strength and corrosion resistance is called the expanded austenite zone, commonly referred to as the S-phase. This increased solubility results in a supersaturation of the carbon atoms which act as interstitials within the crystal lattice. This creates an expansion of the lattice, hence the name ‘expanded austenite.’ The expansion which can be up to 3% in an S-phase causes high residual stresses within the layer and is a prominent feature of expanded austenite. However this can lead to the occurrence of fatigue cracking initiating between the expanded austenite layer and the substrate when subject to cyclic stresses.
This interstitial solid solution is a form of solid solution strengthening. The stresses caused by the introduction of the solute atoms impede the movement of dislocation within the lattice, resulting in an increase ib the yield strength of the material. The expanded layer can exhibit a hardness level between 700HV – 1000HV which is greater or equal to the hardest tempered iron-carbon or nickel-carbon alloys.
An S-Phase can be formed by a low temperature carburising or nitriding process, but for the purpose of these investigations only carburised treated samples were used. The LPC process gives a more sparse distribution of interstitial atoms compared to a nitriding process, but the depth of the carburised expanded austenite layer is thicker. Typical carbon content values in the expanded austenite layer of a stainless steel are between 5% - 12%.
The solubility is further enhanced by the presence of strong carbide forming elements; the most common being chromium, but titanium, aluminium, manganese and molybdenum can also be used. The presence of these elements form ‘trapping sites,’ further increasing the diffusion and supersaturation of the carbon, thus promoting the formation of an S-phase. Due to the relatively strong affinity between chromium and carbon and the low mobility of the solvent chromium atoms, the carbides do not precipitate and the carbon is maintained in a solid solution. Whereas a high temperature carburising process forms precipitates within a few hundred hours. The phase can also be described as metastable.