I was just thinking that....
The Rockwell scale is a hardness scale based on indentation hardness of a material. The Rockwell test determines the hardness by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.[1] There are different scales, denoted by a single letter, that use different loads or indenters. The result is a dimensionless number noted as HRA, where A is the scale letter
The Metallurgy
Take some iron ore, primarily iron oxide, and some carbon, in the form of charcoal or coke and heat them up together. Get the combination hot enough and the oxygen in the iron ore combines with the carbon and you get metallic iron and carbon dioxide. If you get it barely hot enough you get a spongy mass of almost pure iron that can be shaped by pounding. If you get it really, really hot you get molten iron and an excessive amount of the carbon is dissolved in the iron, resulting in cast iron. The challenge is to obtain the desired amount of carbon in the iron so it can be heat treated to form a tool or weapon with superior edge-holding capability and sharpness. Too little carbon and it won't harden, too much and you get cheap, brittle and easily melted cast iron.
What is the difference between soft and hard? A relatively soft metal such as copper, aluminum, iron or even lead consists of an aggregation of small randomly oriented crystals. Each little crystal is a regular array of atoms stacked together and holding on to each other. The atoms can shift position, letting go of their nearest neighbors and then reattaching to a new set of neighboring atoms. That process allows a piece of metal to bend without breaking.
When you add carbon atoms to iron they tend to be located inside the crystal lattice in places where they fit easily and have relatively little influence on the strength of the iron-carbon alloy known as carbon steel. If only we could stick those carbon atoms in locations which would prevent the iron atoms from shifting from one location to another. That's where the magic occurs. When you heat carbon steel above a certain temperature a very minor change in its crystal structure happens that has profound consequences. By making a very small shift in position the carbon atoms are relocated to places where they normally wouldn't fit. If only they would stay there then they would prevent the iron atoms from shifting and we would have a much harder material. What happens next is one of the miracles on which modern civilization is based. Simply cool the carbon steel fast enough (quenching) and the carbon atoms remain locked in place and you have a material that is orders of magnitude harder. The knives, axes, razors, automobile gears that we all take for granted depend on this miracle of heat treatment.
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