Kolsterizing

It is carburizing conducted at lower than usual temperatures, which avoids the formation of carbides.
See
How to increase 304 SS hardness
thread330-49696
for info and link re the Kolsterising process from Bodycote.

If you are interested in more details, it is a process offered by Bodycote Heat Treatment, see There is also more information on their site about the process.

As Kenvlach says, Kolsterizing is a carburization process carried out at a temperature below that necessary for chromium to diffuse to form chromium carbides, which would reduce the corrosion resistance of the stainless steel. It has a hardness of about 70 Rc and a case thickness of 22 to 33 microns. There is no distortion or coating of the part. The process is only applicable to fully austenitic alloys, or alloys that can be heat treated to a fully austenitic surface structure. It is a very effective process, having corrosion resistance at least equal to untreated stainless in most applications.

There is also a Japanese nitriding process that gives good results, although not quite as good as Kolsterizing. See paper by Thomas Bell and Chen X. Li, "Stainless Steel - Low Temperature Nitriding and Carburizing", in Advanced Materials and Processes, June, 2002. This reviews many of the current and developing processes.