The effect of low temperature aging of type 316L austenitic stainless
weld metal on the transformation of ferrite phase was studied. Some specimens
were aged at temperature 400ºC at various holding times ranging from 100 to
5000 hrs. The ferrite content of the specimens was estimated before and after
aging. It can be seen that the ferrite content decreased very slightly with
increasing the aging time under low temperature aging when it was measured by
Magne Gauge. Microstructure changes before and after aging were noticed.
When observing the microstructure change of ferrite content being evaluated by
optical metallography, it can be noted that the ferrite phase content reduced to
30% after aging for 500 hrs, while the reduced ferrites phase content reached at
60% after aging for 5000 hrs. Also, the microstructure evolution showed that
the ferrite/austenite interface changed from smooth line into corrugated
interface like saw-teeth after 5000 hrs aging.
In the present study, the observed contradiction between the appearance
of ferrite phase microstructure and the measured ferrite content using a Magne
Gauge could be resolved by assuming that the dissolved ferrite content
decomposed into α and ά, which kept the magnetic effect as a ferrite phase. It
can be concluded that the ferrite number (FN) measurements were not sensitive
to the decomposition of ferrite into α and ά, since they were a measure of the
amount of presented ferromagnetic phase, and both α and ά were ferromagnetic
constituents. These were significantly observed for early stage of aging till 1000
hrs, while at 5000 hrs aging, a slight change of FN could be attributed to G
phase precipitate, which was a FCC phase.