I just ruined a 5/16 NC 18 tap trying to thread a hole in a shop made t-nut. It never started cutting, and the lead in threads got flattened. The tap was a no name one with the black finish and only the size stamped on the shank. The t-nut was forged from a piece of a railroad spike and it was as forged, not annealed. I tried annealing it, but was probably too late and the damage was done. The forging sparked as low to medium carbon, definitely not mild steel. The tap sparked as high carbon, about 95 points of carbon. It was not HSS, but those generic taps almost never are, even if they are marked tungsten. A photo is attached. Maybe the end can be cut off and it could be used as a bottoming tap.