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Mechanical Design and Operation of Alkanolamine Plants 21 7

process leads to cracking. A summary of each cracking mechanism follows. Examples of
each cracking mechanism are provided in API 945, Appendix A (API, 1990).
SSC (Sulfide Stress Cracking)

Carbon steel is embrittled by atomic hydrogen dissolved in the metal lattice. In the heat-
affected zones adjacent to welds there are often very narrow hard zones combined with
regions of high residual tensile stress that may become embrittled to such an extent by dis-
solved atomic hydrogen that they crack. Figure 3-14 shows sulfide stress cracking originat-
ing at a heat-affected zone of a weld (API, 1990). SSC is directly related to the amount of
atomic hydrogen dissolved in the metal lattice and usually occurs at temperatures below
90°C (194°F) (Gutzeit, 1990). SSC is also dependent on the composition, microstructure,
strength, and residual and applied stress levels of the steel (Buchheim, 1990). SSC has been
found in attachment and seam welds in the amine regenerator overhead system, in the bot-
tom of the amine absorber, in the top of the amine regenerator column, and on the rich side
of the leadrich amine exchanger (Gutzeit, 1990). These locations suggest that SSC is due
mainly to wet acid gas corrosion. See Figure 3-1. This form of cracking can generally be
prevented by limiting the carbon steel weld metal hardness to less than 200 Brinell (BHN)
and by restricting the steel tensile strength to less than 621 MPa (90 ksi) (NACE, 1994B;
1987). Post weld heat treatment (PWHT) is beneficial in mitigating SSC because it reduces
hardness and relieves stresses (Menick, 1989; Buchheim, 1990).

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Figure 3-14. Sulfide stress cracking (SSC) in an existing hardened heat-affected zone
on a weld. (AH, 7994