Unknown steel small parts?? Spark test might be useful, otherwise;
Heat to your cherry red, quench in oil, File test for hardness.
Full hard, temper to hardness required.
Sorta hard, maybe A-2, Reharden at higher temperature and temper or use as is.
No hardness, mild steel, caseharden the part, probably the best.
Casehardening medium to high carbon steel will almost guarantee failure due to brittleness.
Chuck