In any installation, welded joints are of major concern as they are frequently the sites of localized damage. This may be because they are associated with geometrical discontinuities and because they contain more inclusions than plate. one of the most important factors which determine the upper limit of service temperature of components made of austenitic stainless steels is the inferior micro structural stability of the weldment at elevated temperatures as compared to that of the base metal. The gas tungsten arc welding process with the welding direction perpendicular to the rolling direction of the base metal was used to make weld deposits using 308L stainless steel welding rods of 2.6㎜ diameter. Welding was carried out using a current of 80-140A and 10-16V with a torch travel speed of 8-15 cm/min. Typically, filling of this joint required 36 weld passes. All-weld specimens were taken from the weld pads prepared with a 20 included angle V-groove joint geometry. In order to prepare weldment specimens which were exactly located in the gage length of specimens, welded plates were cut perpendicular to the direction of welding travel. Strain-controlled low cycle fatigue tests have been conducted for 316L base metal and 308L weldment at 550℃ and 600℃ in air. A strain amplitude of ±0.4%-0.6% and a symmetrical triangular waveform at a constant strain rate of $4×10^{-3}s^{-1}$ were employed for all tests. The cyclic stress responses and Coffin-Manson plots were analyzed. Crack initiation and propagation modes were evaluated, and the deformation and damage mechanisms which influence the cyclic stress response and fatigue life identified. The micro-structure of base metal and weldment was γ-austenite/δ-ferrite with different ferrite number(FN). Primary ferrite solidification was expected since the ratio of chromium equivalent and nickel equivalent is greater than 1.5. The solidification structure of the weldment consisted of the cellular and equiaxed dendrites. Immediate cyclic hardening in 316L base metal during both continuous low cycle fatigue and creep fatigue tests was observed. For 308L weldment and HAZ, continuously softening through the fatigue life in continuous low cycle fatigue test was occurred. Whereas, in creep-fatigue test, saturation stage before the final failure was observed. The initial cyclic hardening for 316L base metal is attributed to the increasing of dislocation density and the softening of 308L weldment is resulted from the rearrangement of dislocations. Fully developed sub-grains and cell structures might be the reason for the occurrence of saturation stage during creep-fatigue test. DSA has been considered to cause a faster reduction of the cyclic life over its temperature range of operation, as a consequence of the smaller number of cycles to crack initiation and propagation. The lifetimes of weldment and HAZ were always inferior to those of base metal specimens. In all HAZ specimens, fatigue failure occurred in the weldment region. The fracture surfaces of base metal and weldment show typical of the trans-granular failure with ductile dimple in continuous low cycle fatigue test. However, inter-granular fracture is evident for the weldment in creep-fatigue test. This difference in fatigue behaviour may be explained in terms of crack initiation and propagation. Failure occurred due to crack propagation along the cellular or dendrite boundaries in the weldment. The long cellular or dendrite boundaries tend to make the crack propagate more easily with less deformation. At the high temperatures, the δ-ferrite transforms to carbides and brittle inter-metallic phases which adversely affect the toughness of the material. Also formation of an inter-metallic phase known as σ phase is a severe problem when using standard austenitic stainless steels at elevated temperatures. The presence of σ phase not only affects mechanical properties of the material detrimentally, but also reduces its corrosion resistance by removing chromium and molybdenum from the austenitic matrix. It is thought that the transformation of δ to σ and associated micro-cracking at the γ/σ interfaces led to the linking with the major crack, thus offsetting the advantage associated with crack deflection, and hence shortening the crack propagation stage, leading to an overall reduction in fatigue lifetime. The rate controlling step in transformation can be considered to be boundary diffusion of Cr through the δ/γ boundaries, during the initial stages when the δ-ferrite is present as an almost continuous network, leading to the formation of $M_{23}C_6$ carbides, and the lattice diffusion of Mo during the later stages when the δ-ferrite network has broken down, leading to the formation of σ phase. Also the precipitations of interface σ phase in duplex stainless steel are presumed to be closely related with the non-coherency of δ/γ and $δ/M_{23}C_6$ interfaces. However, the nucleation of σ phase at the $γ/M_{23}C_6$ interface is inhibited due to the coherent $γ/M_{23}C_6$ interface with cubic-cubic orientation relationship.
