Ordinary steel is a mixture of iron & carbon. Steel does not generally respond to hardening until around 30 points of carbon are reached.
30 points = .3%. Thus for every 100 lbs of iron, there would be about 4.8 oz of carbon. With a 20 point steel, there would be about 3.2 oz of carbon per 100 lbs of Iron.
Annealing, Normalizing, Hardening, etc are all forms of Heat-Treating. These barrels of 20 points or less of carbon "May" have gone through some form of heat-treating, but they were not "Hardened".
