Low-temperature aging characteristics and SCC behaviors 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. Differences between the two weld metals in morphology of microstructure, in composition of d-ferrite, in hardening behaviors with isothermal aging at 335 oC, and in stress corrosion cracking (SCC) behaviors have been investigated. The hardness of ferrite phase increased with aging time, while the hardness of austenite phase stayed the same. This characteristic suggests that spinodal decomposition has occurred in d-ferrite. The age-hardening rate of δ-ferrite was faster for primary austenite solidification mode (AF mode) sample than primary ferrite solidification mode (FA mode) sample in the initial stage of the aging. It has been suggested that solidification mode can affect kinetics of spinodal decomposition. The results of SCC test would suggest that threshold stress level for SCC initiation is lower for the AF mode weld compared to the FA mode weld. (author)