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MATERIALS SELECTION & DESIGN Failure Analysis of Type 316L SS Drain Nozzle Elbow in H2
S Service FIGURE 3
fuses into the steel with susceptible mi- crostructure and causes SSC. The major difference between hydrogen-induced cracking (HIC) and SSC is the morphol- ogy of the cracks generated. The HIC cracks are a series of stepwise cracks that link up as they initiate and propagate.6 The SSC cracks propagate as
Sensitized microstructure along the ID of the tube. FIGURE 4
branched cracks since the crack propaga- tion is dependant on the hydrogen per- meation ease within the microstructure. The microstructure revealed a mixed mode of cracking, characteristic of SSC. SSC was initiated by intergranular cor- rosion and thereafter the synergistic ef- fect of residual stress and environment propagated SSC in the elbow.
Conclusions The failure of the elbow was caused
by SSC. The presence of carbon inside the elbow led to sensitization of the mi- crostructure. The sensitized microstruc- ture underwent intergranular corrosion, thus providing a susceptible microstruc- ture for initiation of SSC in the elbow. The large grain size as well as the defor- mation bands were responsible for infe- rior mechanical properties and lesser resistance to SSC. Non-compliance of the XW[\ NIJZQKI\QWV IVVMITQVO [XMKQÅKI\QWV[ by the manufacturer was observed.
Branched cracks showing mixed mode of cracking (indicated by arrow).
ID of the elbow was a consequence of improper cleaning procedure adopted at the pilgering stage. The lubricant used during the pilgering operation was left behind in the elbow; this became dissoci- ated during subsequent heat treatment, causing the formation of free carbon and sulfur. At this temperature, sulfur aids in the diffusion of carbon along the grain boundaries.4
The sensitized microstruc-
ture along the transverse section of the elbow was at a depth of ~100 µm.
64 MATERIALS PERFORMANCE March 2012 Water saturated with H2 S at a tem-
perature of 130 °C was present in the environment, which is conducive for SSC occurrence in the presence of a sensitized microstructure in the elbow. The SSC is a hydrogen embrittlement phenomenon where the failures occur below the yield strength of the material. The hydrogen generated on the surface of steel from the reaction of Fe with H2
S
NWZU[ QZWV []TÅLM .M; IVL PaLZWOMV 5 The hydrogen, in an atomic form, dif-
Recommendations z Tube suppliers should ensure that
the product is free of carbon depos- its on the ID.
z Qualified fabrication procedures should be adopted.
z Appropriate solution annealing should be carried out to relieve the cold work effects.
Acknowledgments The authors would like to acknowledge
the help extended by Dr. Pranesh Sen- gupta in making the EPMA. The support and guidance given by Dr. P.K. De in carrying out the work is highly appreciated.
NACE International, Vol. 51, No. 3