Hardening
Thermal process for increasing mechanical strength and wear resistance by heating into the austenite region followed by rapid quenching to form martensite.
When hardening, you heat the steel completely to austenitizing temperature until it glows through and is no longer magnetic. Then you quench it rapidly in water or oil to form martensite. This makes the material hard but also brittle and prone to cracking.
If you harden too much, the part can break off; if you leave it too soft, the edge won't hold. When welding, there's a high risk that the heat-affected zones nearby will harden uncontrollably and crack.
In practice, this means: heat cleanly and evenly, then quench quickly and completely. If it's too brittle, you need to temper after hardening to relieve stress and restore some toughness.