When you say c.r. low carbon steel, are you thinking 1010, etc... are corrosion reisistant. That class of steel is definitely not corrosion resistant. If you are looking for relatively small sizes and/or non-complex shapes, you can get some through McMaster-Carr..I might as well .....case hardening is the addition of carbon ,nitrogen, or boron to a thin layer of the steel . It is done with a solid ,molten salt ,or gas. Around the shop it's handy to have powder such as Kasenit with which you can apply a thin case with a propane torch.