Final answer:
Steel transforms into a harder and stronger form called martensite during austenitizing and rapid quenching, which involves heating steel above its critical temperature and then quickly cooling it.
Step-by-step explanation:
During the process of austenitizing and rapid quenching, commonly known as hardening, steel is transformed into a much harder and stronger form known as martensite. Austenitizing involves heating the steel above its critical temperature to change its crystal structure to austenite, after which it is rapidly cooled or quenched. This quenching traps carbon atoms within the iron lattice and causes a transformation to martensite, a highly strained and supersaturated form of ferrite that has a body-centered tetragonal (BCT) structure, which significantly increases the hardness and tensile strength of the steel.