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A 4-inch diameter schedule 40 pipe welded to a bolted flange connection failed in service at the weld. Design limits are specified as 185 PSIG, 375°F. There is no operational data to suggest these parameters were exceeded. Visual inspection showed the crack was at the flange-side toe of the weld, connected there both on the ID and OD, but appeared to be ID-initiated given the visible portion of the ID crack was longer than the OD portion (2-1/4" long on ID vs 1-1/2" long on ID). The materials, as confirmed by handheld XRF, were 316SS for the pipe, 304SS for the flange, and 308SS for the filler. The filler was determined to have 7.5-10% ferrite via a severn gauge. Weld was open-root, multiple pass, and appeared to be TIG based on cap profile (I do not have the weld procedure info).

The crack was broken open in the laboratory. The crack path was confirmed to traverse from toe-to-toe of the ID to OD weld. There was a distinct lip or ledge near the OD where the crack direction abruptly changed direction. The fracture surface exhibited several areas of distinct heat-tint. The shape of all of these tinted regions was more consistent with OD-initiated cracking (somewhat similar in shape/orientation progression marks emanating from the OD toward the ID). The shape of this heat-tint was especially pronounced on the OD ledge.

SEM evaluation showed what I perceived to be transgranular cleavage fracture. There was no overload observed (at least compared to the lab-generated fracture surface, which was a readily identified dimpled surface). Despite most of the fracture surface being transgranular, there were interspersed facets across the fracture that look to me like isolated intergranular fracture faces. SEM-EDS of these faces showed traces of Na and Cl which were present in discrete particles on the surface. There was no difference in the SEM between the fracture surfaces of the hint-tinted areas compared to the non-heat tinted areas (same general concentration of facets, same fracture morphologies, etc). 

Metallographic evaluation showed that, across 4 cross-sections taken, there was no preference for crack propagation with respect to microstructure. Additionally, in all 4 cross-sections, cracks were observed on both sides of the ID weld toe. The cracks always initiated at the toe of the weld on the ID, but did not travel along the fusion line or HAZ. The crack was confirmed to be transgranular in the base material, and did not exhibit any preferential microstructural path when traversing the weld metal. Crack paths were slightly branched, but not extensively. One notable feature from the microstructural evaluation is that there appeared to be some weld repair performed from the ID, evidenced by the shape of the fusion zone. 

I'm having trouble piecing this all together into a coherent picture. The heat tint on the fracture surfaces suggests that the crack at least partially formed during fabrication, but the fracture features between the heat-tinted areas and all other areas are nearly identical. The cracks initiating at the toe of the weld makes sense from a stress-concentration standpoint, probably more-so for SCC than fatigue given they were on the pipe ID and were present on both sides of the weld toe. SCC is further supported by the presence of chlorine...but why don't the cracks show more preference for a certain microstructure? And if some portion of the crack occurred during fabrication, why is the crack path so continuous/smooth (at least until it connects to the OD-initiated "ledge" mentioned earlier)? 

This image dump is not very well organized and I realize it's also captionless, but I'm hoping it's easily correlated enough to what I've typed above and at least a starting point. What other features do I need to look for to properly characterize this failure?

Thank you,