No disrespect meant to a fellow electrician, but I'm going to get nitpicky here.
There is no magic number above which steel will harden or below which it will not. The maximum attainable hardness of a given piece of steel versus the carbon content forms a curve on the graph. The more carbon available, the more martensite which can be formed. (Up to a point, anyway. Maximum attainable hardness levels out around 60 or 65 points. Retained austenite can actually diminish maximum attainable hardness at carbon levels beyond this.) Run an internet image search with the terms "carbon" and "hardness" and a few renditions of this graph will show up. Or look in chapter 6 of Principles of Heat Treatment of Steel by Krauss for a very good graph.
A spike or other bit of steel with 20 points of carbon can indeed be hardened. But no, it won't get as hard as another bit of steel with 30 points.
Hot work punches. Bottle openers. Sculptures of funny little people. Tent stakes. Cobras.