  A comprehensive study of pulsed nitriding in AISI H13 tool steel at low
temperature (400{\deg}C) is reported for several durations. X-ray diffraction
results reveal that a nitrogen enriched compound (Epsilon-Fe2-3N, iron nitride)
builds up on the surface within the first process hour despite the low process
temperature. Beneath the surface, X-ray Wavelength Dispersive Spectroscopy
(WDS) in a Scanning Electron Microscope (SEM) indicates relatively higher
nitrogen concentrations (up to 12 at.%) within the diffusion layer while
microscopic nitrides are not formed and existing carbides are not dissolved.
Moreover, in the diffusion layer, nitrogen is found to be dispersed in the
matrix and forming nanosized precipitates. The small coherent precipitates are
observed by High-Resolution Transmission Electron Microscopy (HR-TEM) while the
presence of nitrogen is confirmed by electron energy loss spectroscopy (EELS).
Hardness tests show that the material hardness increases linearly with the
nitrogen concentration, reaching up to 14.5 GPa in the surface while the Young
Modulus remains essentially unaffected. Indeed, the original steel
microstructure is well preserved even in the nitrogen diffusion layer. Nitrogen
profiles show a case depth of about ~43 microns after nine hours of nitriding
process. These results indicate that pulsed plasma nitriding is highly
efficient even at such low temperatures and that at this process temperature it
is possible to form thick and hard nitrided layers with satisfactory mechanical
properties. This process can be particularly interesting to enhance the surface
hardness of tool steels without exposing the workpiece to high temperatures and
altering its bulk microstructure.
