Abstract
For performance assessment of oil and gas field pipe materials their susceptibility to hydrogen assisted stress cracking is to be evaluated. In the present paper Slow Strain Rate Tests (SSRT) are applied to a Supermartensitic Stainless Steel (SMSS) in the unwelded and welded condition. The welds were carried out by EB welding with slightly overmatching filler wire. For consistent comparison of the cracking susceptibility at both conditions the partially masked tensile test pieces were subjected to galvanostatic conditions at various nominal cathodic current densities in natural formation water at aerated and dearated conditions, thus simulating respective operating and possible shutdown conditions. As a result increased cathodic charging densities enhance hydrogen levels and reduce fracture strains. At a constant strain rate of 5.14E-06 the reduction of fracture strains from 0.22 to 0.1 occurs within a narrow current density range of between 0 and - 0.002 mA/mm2. The “Time-Strain-Fracture (TSF)”- diagrams with variable strain rates show the reduction of fracture strains with increasing failure times and decreasing strain rates due to increasing diffusible hydrogen pick up. The TSF-failure limits are shifted to shorter failure times by deaerated formation water as compared to aerated conditions. The SSR test results transverse to the welds indicate fracture locations changing from the heat affected zone to the weld metal with decreasing strain rates and respectively increasing diffusible hydrogen levels. As a general conclusion, the TSF (Time - Strain – Fracture) diagrams provide quantitative information on critical strain rates for cracking as well as the sensitivity of the material to increased hydrogen pick up during the SSR test.
