Influence of sigma-phase precipitation on the impact behaviour of duplex stainless steel pipe fittings

Abstract

In this study, the effect of sigma-phase precipitation in duplex stainless steel (grade 2205) pipe fittings collected from top-side process systems offshore was investigated under realistic loading conditions. This was done by conducting quasi-static and dynamic impact tests on 3” fittings with a nominal outside diameter of 88.9 mm, nominal wall thickness of 3.05 mm and D/t-ratio of nearly 30. The fittings had a sigma-phase level between 0 and 15 vol.%, and were loaded by a massive steel indenter with a hemispherical nose of 25 mm radius. In the quasi-static tests, the loading rate was 2 mm/min, while in the dynamic tests the impact velocity was varied between 5 and 10 m/s. A few dynamic impact tests on 2” fittings with an increased wall thickness and low sigma-phase levels were carried out for comparison, while some 3” fittings were pre-charged with hydrogen before dynamic testing to examine the effect of sigma phase on cathodic protected subsea components. When the sigma-phase level was low (i.e. σ ⩽ 5 vol.%), no evidence of fracture was found in any of the pipe fittings, whereas at higher sigma-phase levels (σ > 5 vol.%), fracture occurred in all tests. At the highest sigma-phase level (σ ≈ 15 vol.%), extensive fracture and crack growth took place independent of the loading rate.