Jackhammer bits are usually S-7 steel. Only about 55 points of carbon. They'll make a decent enough axe head- very tough & not prone to chipping but not very hard. S-7 is a difficult steel to work under a hammer. It needs to be HOT. DO NOT hammer at less than a bright red as it will set up cracks in the finished blade. Truck axles are also very tough but again, as a 4130 or 4140, not much carbon.
Personnaly, for a tough working knife where ultimate sharpness is secondary, I like car & truck coil strings, 5160. Enough alloy to make a tough blade & enough (barely) carbon the harden well. S-7 & 41** steels make great anvil tools, punches & hammers.
Just some random thoughts from another steel scrouging itinerant steel pounder.