What process is commonly used to harden tool steels?

The process commonly used to harden tool steels is quenching. Quenching involves heating the steel to a high temperature until it reaches its austenitizing range, where the microstructure transforms. When the steel is then rapidly cooled, typically by immersing it in water or oil, the transition to a harder and more brittle microstructure occurs, which is known as martensite. This hardening process is crucial for tool steels, as it enhances their wear resistance and strength, making them suitable for various applications in tool-making.

While annealing, tempering, and forging are all important processes in the treatment of metals and tool steels, they serve different purposes. Annealing is primarily used to soften the metal and relieve internal stresses, rather than to harden it. Tempering, on the other hand, follows the hardening process and is done to reduce brittleness and improve ductility, as hard tool steels can be too brittle for practical uses. Forging is a shaping process that involves striking the metal while it's in a hot state, but it doesn't inherently change the hardness of the tool steel in the way quenching does. Thus, quenching is the key process specifically aimed at hardening tool steels.