for a steel alloy, the minimum temperature above which under equilibrium conditions only austenite is present; the temperature at which austenite begins to transform to ferrite during cooling (see: decomposition of austenite; compare: lower critical temperature), teraste korral minimaalne temperatuur, millest ülevalpool tasakaaluolekus eksiteerib ainult austeniit; temperatuur, millel algab austeniidi lagunemine ferriidiks jahutamisel (vt austeniidi lagunemine, faasimuutustemperatuur Ac3; vrd alumine kriitiline temperatuur)