A wide range of HAZ microstructures can be
produced because, close to the fusion boundary,
the HAZ transformation to austenite on heating
will be followed on cooling by transformation
to give either a ferrite-carbide microstructure or
martensite, depending on material composition,
peak temperature, and cooling rate. Farther from
the weld, the material will be exposed to a lower
peak temperature, so only partial reaustenization
occurs, and those areas heated below the ferriteto-
austenite transformation temperature (Ac1)
will not be significantly affected, other than by
some carbide coarsening and tempering. Despite
these variations, in the majority of applications,
there is little influence on the corrosion performance,
and preferential HAZ corrosion is relatively
rare. Where preferential HAZ attack has
been reported, it is more common in carbon and
carbon-manganese steels than in higher-alloy
grades (Ref 1).