CARBON CONTENT
Automotive coil and leaf springs are often 5160 (or close to it). 4160 and 5160 both have 60 points (0.60%) of carbon, which would qualify them as medium carbon steels. The “1” in the 2nd digit usually means chromium and/or magnesium has been added for “toughness” (less likely to deform or shatter, but might not stay as sharp for as long). 5160 is great for swords/sabers. It’s also good for camp knives which tend to get subjected to a fair amount of abuse, and where toughness is more important than holding an edge for a long time.
WHAT SHOULD I USE?
For a fine knife such as an EDC fixed blade/pocket knife or kitchen knife, I’d recommend 80+ points of carbon for a durable edge, so the 1095 would be a better choice.
Fully annealing the steel before grinding will save you a lot of time (and grinder belts). The slower it cools, the softer it’ll get: I have a 35 gallon can of wood ash which I use for the annealing slow cool. Vermiculite is another option.
PLASMA DECARBURIZATION
Note that if you use the plasma cutter to cut out the knife blank, you’ll have to deal with decarburization (loss of carbon) in the Heat Affected Zone (HAD). The burning which takes place in the plasma HAD strips carbon out of the steel, leaving less carbon in the discolored areas. This can also happen if you let the metal get too hot while grinding.
This is less of an issue on the spine of the knife, but you should make the edge side of the knife blank a little chunky (oversized) and plan to carefully grind back the HAD until you hit good steel.
Keep water handy while grinding and dunk often to prevent overheating. Keeping fingers on the back of the blank as you grind will encourage you to dunk before it gets too hot.
CARE OF CARBON STEEL KNIVES
Note also that carbon steel blade are prone to develop a patina and/or rust if put up wet. Leaving a carbon steel knife in the sink where it can pick up some spray is enough to rust it in minutes. It is also not recommended to leave a carbon steel knife in a leather sheath when not in use - it can trap moisture, rust the blade, and foul the sheath interior.
I use some carbon steel knives I made (one from a farriers rasp (1095ish) and one from a coil spring (5160ish) as my daily use kitchen knives, and love them. Due to its lower carbon content, I have to sharpen the coil spring knife about 2X as often as the former to keep it sharp. I also keep a quarter sheet of 500 grit wet/dry sandpaper on the sink shelf to clean off the flash rust when they don’t get dried properly.
If the knife will be stored with camping gear, rubbing on a little oil and storing where there is airflow is a good idea.
