Which process can increase the deformation resistance of steel?

The correct choice is hardening. This process significantly increases the deformation resistance of steel by altering its microstructure. In hardening, steel is heated to a high temperature and then rapidly cooled, typically through quenching in water or oil. This rapid cooling results in the formation of hard phases such as martensite, which enhance the material's strength and hardness.

The increased hardness from the hardening process means that the material is less susceptible to deformation under applied loads. This characteristic is crucial for components that must withstand heavy stresses and maintain their shape during use.

Tempering, while it is a process that follows hardening, serves to reduce brittleness and improve toughness rather than increase deformation resistance. Hot working involves shaping steel at high temperatures, which can lead to a reduction in strength. Adding alloying elements can enhance certain properties of steel, such as corrosion resistance or strength at high temperatures, but hardening is specifically focused on increasing hardness and resistance to deformation through the alteration of the microstructure.