Effect of Aging at 700°C on Ferrite Transformation in a 316L/308L Weldment

This article deals with the study carried out on the 316L steel plates welded using a 308L filler material of less stable microstructure than the base plate. The phase transformations in these welds aged at 700°C were investigated by means of optical metallography (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). It was found that the σ-phase forms even at isothermal holding time less than 140 h. During aging, the growth of σ-phase occurs continuously within δ-ferrite grains. After 1,000 h aging, all of delta-ferrite was decomposed and transformed into σ-phase and secondary austenite. EDX revealed that the σ-phase was enriched in Cr and Mo. The molybdenum content of the weld originated from the regions of the base metal adjacent to the fusion line. It exerted a prevailing effect on the σ-phase precipitation.