Metallurgical Study on Low-temperature Aging Characteristics and SCC Behavior of Type 316L Welds

Two types of 316L SS weld metal with different solidification modes were prepared using two kinds of filler metals having tailored Ni equivalent and Cr equivalent. The changes in nanoscale microstructure of d-ferrite, in hardness, and in stress corrosion cracking (SCC) behaviors of the weld samples with isothermal aging at 335 ^ºC have been investigated. The mottled aspect has been observed in δ-ferrite of both the ferrite-austenite (FA) and austenite-ferrite (AF) solidification mode aged samples by transmission electron microscope (TEM) observation. This aspect might be attributed to spinodal decomposition into chromium-rich a’ and iron-rich a. The hardness of ferrite phase has increased with aging time, while the hardness of austenite phase has stayed the same. This characteristic suggests that spinodal decomposition has occurred in δ-ferrite. The age-hardening rate of δ-ferrite was faster for AF mode sample than FA mode sample in the initial stage of the aging. It has been suggested that solidification mode can affect kinetics of spinodal decomposition. The number of cracks and the average crack depth of both solidification mode specimens have been slightly increased by the aging at 335 ^oC for 13200 hours.