A certain temperature, different for different gases, but always the same for each gas, regarded as the temperature above which no amount of pressure can produce condensation to a liquid; the temperature above which a gas cannot be liquified.
Above this temperature the steel crystal structure changes from Body Centered Cubic to Face Centered Cubic. Quenching carbon steel after it has reached this point freezes the steel in its harder Face Centered Cubic state.
The temperature at which a phase change occurs in steel. The exact value of this temperature depends on the particular phase change occurring eg austenite to ferrite or cementite or the reverse, on the chemical composition of the steel, on whether the steel is being heated or cooled and on the rate of heating or cooling.
(Tc) The temperature at the critical point. A gas above the critical temperature will never condense into a liquid, no matter how much pressure is applied. Most substances have a critical temperature that is about 1.5 to 1.7 times the standard boiling point, in kelvin.
The point at which carbide particles in steel begin to dissolve into their surrounding matrix, creating the phase called austenite. The temperature range in which this austenizing, takes place is called the critical range.
The critical temperature, Tc, of a material is the temperature above which distinct liquid and gas phases do not exist. As the critical temperature is approached, the properties of the gas and liquid phases become the same resulting in only one phase: the supercritical fluid. Above the critical temperature a liquid cannot be formed by an increase in pressure, but with enough pressure a solid may be formed for materials other than water.