Issues in stress corrosion testing of welded super martensitic stainless steels for oil and gas pipelines

Abstract

The test method most commonly adopted by the oil and gas industry for stress corrosion testing of welds is the four-point bend test. Superficially a relative simple test, there are a number of variables that can potentially influence the results. These include root profile, specimen width and thickness, nonuniform thickness associated with “stepped” welds, test configuration, use of tensile or flexural data to set strain levels, temperature dependence of material properties when testing at elevated temperature, hardness, and residual stress distribution. This chapter presents a detailed assessment of the methodology of 4-point bend testing, combining experimental measurement with finite element analysis. X-ray diffraction measurements on the super martensitic stainless steels specimens used yielded an austenite volume fraction of 12%. Microhardness scanning was carried out on specimens taken through the cross-section of girth welds. In order to assess the impact on the strain distribution, measurements were made at ambient temperature with 4 mm thick specimens that had controlled steps machined at the midpoint of the weld root reducing the thickness on one side of the specimen.