I've been milling pre hard 4140 steel. Carbide machines it readily but I also found even with 4 flute end mills, it's dulling the cutting edge (mostly the ends and the corners) rather readily. I have been touching them off with a dremel diamond wheel, it's cheap and it sharpens the cutter to a serviceable edge but every so often I would need to touch off again. Eventually it messes with the geometry but I can still maintain a good cutting edge...
Are there better tools for milling pre hard 4140 steel? It machines with HSS but dulls it even faster, but it still dulls carbide. It doesn't dull the flutes so much but it dulls the ends much faster and that often messes up the machining characteristics when the ends get dull.