Also, from Section 5.5, we know that:
g When the temperature reached in the heat affected zone is more than about 900°C, the
steel changes to 100% austenite phase, that is, it is fully transformed. The width of this
region is rather small, from a fraction of a millimeter to a few millimeters depending on the
weld heat and workpiece thickness. Once the welding arc has passed by such a location,
the austenite phase formed changes to phases such as ferrite, pearlite, or martensite
depending on the steel composition (hardening capacity) and the cooling rate. The
properties (strength, hardness, toughness) of this part of the heat affected zone, also called
the supercritical heat affected zone, depend on the microstructure.
g The region next to the supercritical heat affected zone that gets heated to a temperature
between about 700°C and 900°C is called the intercritical heat affected zone or the partially
transformed heat affected zone. The latter term indicates that the temperature did not
exceed about 900°C in this region and therefore, the amount of austenite formed was less
than 100%, the other phase present being ferrite. Therefore, this region cannot form 100%
martensite on cooling.
g The next region after the partially transformed or the intercritical heat affected zone is the
untransformed (no austenite formed at all) or the subcritical heat affected zone. The
maximum temperature reached in this region is about 700°C. The microstructural changes
in this region can be hard to detect with an ordinary microscope. For quenched and
tempered steels, the region of the subcritical HAZ that reaches peak temperature above
the tempering temperature (say 620°C) can suffer some reduction in strength. Also, in the
presence of microalloying elements (Nb, V), there is potential for some reduction in notch
toughness in the subcritical heat affected zone.