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MATERIALS SELECTION & DESIGN FIGURE 1
the ID of the elbow (Figure 1). The crack observed on the outside diameter (OD) of the elbow was very near the weld, whereas the crack on the ID side had passed through the weld (Figure 2). The visual examination revealed undulations across the elbow. The thickness variation measured using vernier calipers was ~0.33 mm at the crack region with maximum thickness at the edges.
Optical Microscopy
The optical micrograph showed the base metal microstructure containing grains with deformation bands. The crack propagated in a mixed-mode fashion (i.e., intergranular cracks fol- lowed by transgranular cracks) (Figure 3). Figure 4 shows a sensitized micro- structure on the ID side of the elbow near the carbon spots.
Microhardness Measurement Microhardness measurement was
made using a load of 120 gf for a dwell time of 15 s. The measurements were done in the base metal, weld metal, and near the crack. The Vickers hardness values ob- tained were in the range of 156 to 185 HV.
Electron Probe Microanalysis
EPMA was carried out on the black
spots in the elbow. Figure 5 shows the backscattered electron (BSE) image as well as the x-ray image of the black spot.
elbow was cracking. The inference of this failure was that it was environmentally induced SSC. For SSC to occur, three
NACE International, Vol. 51, No. 3
Crack extending into the weld at the ID surface of the elbow. (Original magnification 75X.)
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ence of large grains and a large number of twins and slip bands, which indicated the material has not been solution an- nealed properly after cold forming
(i.e., either a lower annealing temperature or lesser annealing time were used). The large grain size could have come from solution annealing at either a very high temperature or over a very long time period prior to the cold-forming opera- tion. The ID of the elbow revealed black spots, which were found to be free carbon when analyzed by EPMA. The micro- structure around these areas showed sensitization. The carbon pick-up on the
March 2012 MATERIALS PERFORMANCE 63 Scattered black spots seen on the ID of the elbow. (Original magnification 75X.) FIGURE 2