Hardness
Hardness is a material's resistance to penetration by another body. In steels, it's usually measured in HRC (Rockwell) or HB (Brinell). Hardness comes from carbon plus heat treatment (hardening and tempering).
Physicochemical mechanism of action
Hardness is nothing but the resistance to a body trying to press in. When measuring, that's exactly what is done: Rockwell presses a diamond cone and gives HRC, Brinell takes a ball for HB, Vickers a diamond pyramid for HV. In steel, hardness comes from the fact that during the carbon is stuck in the lattice and distorts it. That is . Then come the hard , which sit in the softer matrix like gravel in concrete. Without carbon, nothing gets hard, you can quench until the water boils.
Metallurgical relevance & risks in welding
Hardness and work against each other. What gets harder becomes more brittle, always. In welding, you don't want hardness, but you get it delivered: as a hardness peak in the . In practice, a value around 350 HV is considered the limit beyond which it gets unpleasant. Conversely, too soft is just as bad. A softened zone gives way under load, and on a tool, the cutting edge simply sinks in.
Workshop practice & action instructions
Measure, don't guess. For a quick test, the file is enough: if it bites, the steel is soft; if it slides, it's hard. For numbers you can write down, take the hardness tester and measure next to the weld, not in the middle. If it got too hard, or helps. And always ask which scale is meant. HRC, HB, and HV cannot be converted in your head, and whoever does it anyway is talking about numbers that don't exist.