Austenitising

Austenitising is a heat treatment step used for iron-carbon alloys, particularly steels, in which the material is heated into the austenite phase region. In this domain, the iron matrix becomes gamma-iron (austenite) and carbon is held in solution. The process dissolves carbides and homogenizes carbon distribution, preparing the steel for subsequent transformations such as quenching or diffusion-based hardening.

Temperatures for austenitising are chosen to place the steel fully into the austenitic region, typically well above the upper critical temperature (Ac3) for carbon steels and depending on alloying content. The required hold time depends on section size, carbon content, and alloying elements, ranging from a few minutes to several hours. Prolonged heating can promote excessive grain growth and, in oxidizing environments, decarburisation of the surface.

The primary purpose is to create a uniform austenitic structure so that subsequent cooling produces a predictable microstructure and mechanical properties. If the austenitised steel is quenched rapidly, the austenite transforms to martensite, producing a hard, brittle matrix that can be tempered to improve toughness. If cooled more slowly, pearlite or bainite may form, yielding different combinations of hardness and ductility. Austenitising is also used in diffusion-based processes such as carburising and cases hardening, where carbon is introduced at the surface by first dissolving carbon into austenite before diffusion into the surface layers.

Cautions during austenitising include minimizing grain growth by controlling time and temperature and protecting the surface from decarburisation through protective atmospheres or coatings.