Austenite is a non-magnetic and highly ductile phase of iron-carbon alloys. In metallurgy, it is sometimes referred to as gamma iron. Austenite exists at high temperatures and it is the face-centered cubic (FCC) form of iron. At temperatures below 912°C (1674°F), an iron-carbon alloy will form a mixture of ferrite and cementite, known as pearlite. As the temperature of the iron-carbon alloy is increased further above 912°C (1674°F), the pearlite will completely transform into austenite.
The transformation of ferrite (body-centered cubic (BCC) form of iron) to austenite occurs at a certain temperature and composition, which is known as the eutectoid temperature. For a given composition of iron-carbon alloy, the eutectoid temperature typically ranges from 700 to 900°C (1292 to 1652°F). The transformation to austenite is rapid and exothermic. As the austenite is cooled, it is converted to the ferrite-cementite mixture of pearlite. At a temperature of 727°C (1341°F), the austenite is transformed to pearlite and a body-centered cubic (BCC) lattice forms.
When austenite is cooled slowly, the carbon in solution remains in the solid solution and crystallizes in a random formation as relatively large interlocking crystals, known as austenite grain. These grains exist in several distinct shapes, depending on the rate of cooling. The slow cooling rate and carbon content of austenite are important factors that affect the formation of austenite grain size and shape. Typically, the slower the cooling rate of the austenite, the larger and the less uniform the grain size.
Austenite grain size influences the hardness and strength of steel, as larger austenite grain sizes are harder and stronger, while smaller grain sizes are softer and weaker. Austenite grain size also affects the weldability of steel, as larger austenite grain sizes are more prone to cracking or other welding defects. During welding, the high localized temperature of a weld can cause the large austenite grain size to transform into heavy ferrite and form coarse pearlite, leading to cracking and poor weld strength.
Generally, oil-quenched tool steels contain a large proportion of austenite grain, as the high oil quench rates are capable of producing large austenite grain size quickly and uniformly. Air-quenching, on the other hand, is generally inadequate for producing large austenite grains and is rarely used for steels containing more than 0.7% carbon.
In order to obtain maximum hardness, strength, and ductility in a steel, a fine and uniform grain size with a minimum of non-uniformity is typically desired. This can be achieved by controlling the rate of cooling throughout the transformation process, typically through a process known as atmosphere annealing. In this process, the steel is heated to an austenitizing temperature, followed by a slow cooling process in a protective atmosphere, such as an inert gas. This ensures that the steel cools slowly and uniformly, and the austenite grains form in a fine and uniform structure that is beneficial for achieving the desired mechanical properties of the steel.
In conclusion, austenite is the FCC form of iron-carbon alloys and it exists at high temperatures. At temperatures below the eutectoid temperature, austenite is transformed to a ferrite-cementite mixture of pearlite. The grain size of austenite is an important factor that affects the hardness and strength of steel, as larger grain sizes are stronger and harder, whereas smaller grain sizes are weaker and softer. The rate of cooling is also a critical factor that affects the formation of the austenite grain size and shape, as slow and uniform cooling is required in order to achieve a fine and uniform grain size and maximum hardness, strength, and ductility.
