Low-temperature gas-phase carburization of 316L austenitic stainless steel was developedin recent years by the Swagelok company. This process generates great mechanical
and electrochemical surface properties. Hardness, wear resistance, fatigue behavior,
and corrosion resistance are dramatically improved, while the formation of carbides is
effectively suppressed. This new technique is of technical, economical, but especially
of scientific interest because the surface properties of common stainless steel can be
enhanced to a level of more sophisticated and more expensive superalloys.
The consequential continuation of previous research is the application of the carburization
process to other steel grades. Differences in chemical composition, microstructure,
and passivity between the various alloys may cause technical problems and it is
expected that the initial process needs to be optimized for every specific material.
This study presents results of low-temperature carburization of AL-6XN(superaustenitic
stainless steel) and PH13-8Mo (precipitation-hardenedmartensitic stainless steel). Both
alloys have been treated successfully in terms of creating a hardened surface by introducing
high amounts of interstitially dissolved carbon.
The surface hardness of AL-6XN was increased to 12GPa and is correlated with a
colossal carbon supersaturation at the surface of up to 20 at.%. The hardened case develops
a carburization time-dependent thickness between 10µm after one carburization
cycle and up to 35µm after four treatments and remains highly ductile. Substantial
broadening of X-ray diffraction peaks in low-temperature carburized superaustenitic
stainless steels are attributed to the generation of very large compressive biaxial residual
stresses. Those large stresses presumably cause relaxations of the surface, so-called
undulations. Heavily expanded regions of carburized AL-6XN turn ferromagnetic. Noncarburized
AL-6XN is known for its outstanding corrosion resistance, which is not impaired
upon carburization. The passive film as analyzed by XPS is fully intact.
Carbon concentration levels in PH13-8Mo reach 10 at.% and correlate with a surface
hardness of up to 14GPa. Indication for the transformation frommartensite to austenite
during the process are observed. In this context, the shape of the carbon concentration–
depth profile can be explained. Also the absence of carbides, as analyzed by TEM, can be
rationalized. Upon cooling to room temperature, most of the austenite backtransforms
into martensite and the surface regains its ferromagnetic properties. Compressive biaxial
residual stresses in carburized PH13-8Mo aremeasured around (2-2.5)GPa.
The applied low-temperature carburization process gives rise to a substantial loss in
corrosion resistance of PH13-8Mo. Possible reasons including the observed formation
of internal and external oxides as well as the change in alloy composition are discussed.
Due to the penetration depth of X-rays into the probed specimen surface, a carbon
concentration gradient may cause detectable asymmetry of diffraction peaks for certain
alloys and under certain conditions. For the first time, this effect is rationalized,
explained, and demonstrated on the basis of measured data.