Effect of Low Temperature Aging of Type 316L Austenitic Stainless Weld Metal on Transformation of Ferrite Phase

Abstract

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.