Final answer:
Tempering is the heat treatment process that produces the maximum toughness in mild steels. It involves controlled reheating and cooling to relieve internal stresses and improve toughness.
Step-by-step explanation:
The heat treatment process that produces the maximum toughness in mild steels is tempering. Tempering is a process in which the hardened steel is heated again to a temperature below its critical point and then cooled at a controlled rate. This process helps to relieve the internal stresses in the steel and increases its toughness. The controlled reheating and cooling process during tempering allows for the formation of a fine structure in the steel, which results in increased toughness.