Which of the following improves steel's toughness?

Tempering is a heat treatment process applied to steel that significantly enhances its toughness. After the steel is hardened through quenching, it typically becomes very hard but also brittle. The tempering process involves reheating the hardened steel to a temperature below its critical point and then allowing it to cool, which helps to relieve internal stresses and reduce brittleness. During this process, the microstructure of the steel transforms, leading to improved ductility and toughness while still retaining much of the hardness gained from the quenching process. This balance of properties is critical for applications where steel must endure impact without fracturing, making tempering a vital step in producing tougher steel materials.

In contrast, hot working does improve some mechanical properties but primarily affects formability and grain structure rather than significantly enhancing toughness. Adding sulfides often leads to poor toughness and ductility due to the formation of inclusions, while alloying with lead is primarily used to improve machinability and does not contribute positively to toughness. Therefore, tempering stands out as the effective method for improving the toughness of steel, making it the correct choice in this context.