Steel Quenched and Tempered Microstructure T(t)
The sorbitic microstructure resulting from hardening followed by high-temperature tempering, offering an optimal ratio of tensile strength to toughness. It is the standard microstructure for heavily loaded machine components.
Tempering means: harden and then heat high. After hardening, you have hard, brittle martensitic structure. By tempering, you make it tougher again without the strength completely going down. The result is called sorbite or tempered structure. When welding such material, the heat-affected zone is the problem. There the structure can harden again or become soft, depending on how fast it cools. If you don't pay attention, you get cracks or the spot is glass-hard. So always do proper preheating and post-heating, then it's fine.