I've asked several people and gotten mixed answers, so I've decided to post the question here. Can steel be electroplated onto steel to build up worn surface areas (I'm talking about less than a millimeter). For example, could a worn knife edge be electroplated back on? I know for large worn areas, hardfacing is used. What, if any, technique can be used to rebuild small, worn steel surfaces?
For specifically a knife edge - no. A steel blade is iron but with specific amounts of carbon ,manganese, chrome , and possible other alloy elements. They either do not , or do not predictably transfer in electroplate. Then it would need to be heat treated. Steel build-up is done with welding processes. I once built up lawnmower blades edge with steel hardfacing. The weld left a smooth rounded surface which was nearly impossible to grind to a cutting edge. So welding is not useful either.
building up worn parts like the bearing surfaces of a crankshaft is commonly done by electroplating- after plating, the part is then finish-ground to the exact size required. But note that there is a risk of ruining the fatigue resistance of the built-up part if the electroplating process happened to generate hydrogen on the plated part. This will diffuse into any microcracks in the part's surface and significantly embrittle it.
BTW hard-faced lawnmower blades are almost impossible to grind on purpose, because they are supposed to be abrasion-resistant. You have to use diamond grinding wheels to sharpen hardened parts like that.
BTW^2 a clever trick on a mower blade is to hard-surface only one side of the blade. the hard-surfaced side is more resistant to abrasion than the other side, which takes most of the wear, and leaves behind a sharp edge consisting of the hard-surfacing. This can also be done by sputtering a metal like tungsten onto one side of the blade- yielding a so-called "self-sharpening* blade!