Weldable Martensitic Stainless Steel (WMSS) line pipe can be susceptible to internal Intergranular Stress Corrosion Cracking (IGSCC) in the Heat Affected Zone (HAZ) of welds when exposed to hydrocarbon transport environments in the presence of water and CO2. It has been reported that a Post Weld Heat Treatment (PWHT) is beneficial in reducing susceptibility to IGSCC but immunity has not been proven.

Earlier work on test welds showed that a PWHT changes the composition at former austenite grain boundaries in the HAZ of rich-grade WMSS (with typically around 6% Ni and 2% Mo).1,2  The present program demonstrates the same phenomenon on welds fully representative for off-shore pipeline installation.

Transmission Electron Microscopy (TEM) element mapping was carried out on different IGSCC susceptible rich-grade WMSS weldments. A molybdenum enrichment, in some cases combined with a chromium enrichment, was found at the former austenitic grain boundaries after the application of a PWHT, in line with the phosphorus segregation mechanism proposed earlier, 1,2  and correlated with four-point bend test results.

Together with the corrosion test work by various parties and the field experience with installed WMSS pipelines, these TEM mappings provide confidence in full IGSCC resistance, after application of a PWHT, of rich-grade WMSS for critical pipeline applications.

Subject
Foil, Materials, Piping, Intergranular stress corrosion cracking, Molybdenum, Chromium, Transmission electron microscopy, Pipelines, Phosphorus, Titanium, Elements, Grain boundary, Austenite
Keywords:
Weldable martensitic stainless steel, Intergranular stress corrosion cracking, TEM
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2013
Association for Materials Protection and Performance (AMPP)
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