Young's Modulus Profile in Kolsterized AISI 316L Steel

AISI 316L steel, subjected to a low temperature carburizing treatment (kolstering), has
been examined by Mechanical Spectroscopy (MS) and nanoindentation to determine the Young's
modulus of the surface hardened layer (S phase). MS results showed that the average value of
elastic modulus of S phase is 202 GPa, a little higher than that of the untreated material.
Nanoindentation tests, carried out with loads of 5, 15 and 30 mN, evidence a modulus profile vs
depth: E is ~ 400 GPa at a distance from the surface of ~ 110 nm, then decreases to reach the value
of the steel substrate (190 GPa) at 33 ?m.
These results, together with X-ray Photoelectron Spectroscopy (XPS) and Auger Electron
Spectroscopy (AES) measurements of carbon concentration profile, can be explained by
considering the presence of a very thin surface layer, different from S phase and consisting of a
mixed structure of Diamond-like carbon (DLC) and tetrahedral carbon (taC).
Furthermore, the same experiments have been carried out also after heat treatments at 450 °C to
correlate the modulus change to the decomposition of the metastable S phase leading to the
formation of (Cr,Mo)C and Cr23C6 carbides in a Cr-depleted austenitic matrix.