Download Low temperature surface hardening of stainless steel

Low temperature gaseous surface hardening of stainless steel; science and
technology
Thomas Christiansen, Marcel A.J. Somers
Technical University of Denmark,
Department of Manufacturing Engineering and Management,
Kemitorvet b.204, DK 2800 Kgs. Lyngby,
Denmark
Abstract
The incorporation of nitrogen or carbon in steel is widely applied to provide major improvements
in materials performance with respect to fatigue, wear, tribology and atmospheric corrosion.
These improvements rely on a modification of the surface adjacent region of the material, by the
precipitation of alloying element nitrides/carbides or a continuous layer of iron-based
carbonitrides. Generally, stainless steel is not nitrided or carburized, because the development of
chromium nitrides/carbides in the case reduces the free chromium content available for the
development of the protective surface film. However, it was demonstrated that the incorporation
of nitrogen and/or carbon in austenitic stainless steel brings about the development of expanded
austenite (also referred to as S-phase), which spectacularly improves the tribological and wear
performances of stainless steel, without impairing the corrosion performance. Practically the
main process for nitriding and carburizing of stainless steels is plasma based, but a recent
(patented) development in the authors’ laboratory enables low temperature gaseous treatment
with its advantage of flexibility towards sample geometry and process conditions. A systematic
scientific investigation of expanded austenite has hitherto been impossible, because plasma-based
processes do not allow the synthesis of homogeneous expanded austenite of uniform
composition. Gaseous nitriding and carburizing do enable the synthesis of pure expanded
austenite, thus allowing a systematic investigation that provides the basis for formulating the
relation between process parameters, microstructure and properties in terms of thermodynamics,
kinetics and mechanics.
In the present contribution the following aspects of the science and technology behind low
temperature gaseous nitriding/carburizing of stainless steel and the development of expanded
austenite are discussed:
 thermodynamics of nitrogen stabilized expanded austenite;
 thermal stability and decomposition kinetics of expanded austenite;
 modeling growth kinetics of expanded austenite layers;
 hardness and residual stress in expanded austenite layers;
 simultaneous surface engineering and bulk hardening of PH stainless steel.