The preferred metallurgical term is transformation temperature. It is the temperature at which a phase change can occur. Sometimes it is used to denote the limiting temperature of a transformation range. Such transformations are expected with all metals and their alloys, and when reversible, transformations on heating occur at higher temperatures than during cooling.

In the case of steels, some of the critical temperatures are identified as:

Acl-The temperature at which austenite begins to form during heating

Ac3-The temperature at which the transformation of ferrite to austenite is completed during heating

Arl-The temperature at which austenite completes transformation to ferrite on cooling

Ar3-The temperature at which austenite begins to transform to ferrite on cooling

Ms-The temperature at which austenite begins to transform to martensite on cooling. Note that this transformation is not reversible. On heating, martensite first transforms to ferrite.

The critical temperature is customarily associated with the following phenomena: hardening when quenched; loss of magnetism; absorption of heat; formation of coarse grain on cooling.

On heating a metal, the eutectoid change at approximately 720°C (1328°F) is known as Acl (lower critical temperature), while the final temperature of transformation to austenite (upper critical temperature) is known as Ac3. On cooling, the temperatures are somewhat lower due to hysteresis, and are known as Ar3 (corresponding with Ac3) and Arl (corresponding with Acl) respectively

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