Duplex Stainless Steel Welds and their Susceptibility to Intergranular Corrosion

Abstract

Duplex stainless steels (DSS) as alternatives to conventional austenitic stainless steels for the construction of pipelines is becoming more wide-spread, particularly for sour service applications where corrosion resistance / stress corrosion cracking resistance is required in aggressive chloride /sulphide environments. While these steels show many superior characteristics, limitations are associated with the welding of these steels, particularly controlling the weld structure and properties and understanding how the weld metallurgy may influence the susceptibility to intergranular corrosion (IGC). The focus of this paper is to report on the findings from a detailed study of the various weld sections within a DSS pipeline, as a function of heat input and type of weld, in terms of the metallurgical structure, composition and mechanical properties and to assess the susceptibility to IGC. Welding was performed using the manual Gas Tungsten Arc Welding (GTAW) technique at both high and low heat input conditions. Two different join configurations (double bevel single V bevel and double bevel single U joint configuration) were adopted. Structural analysis consisted of (i) optical microscopy of welded specimens; (ii) ferrite content determination; (iii) Vickers hardness measurements; (iv) Charpy impact studies and (v) transverse tensile testing. Two test methods namely a modified ASTM A262 and a modified Double Loop Electrochemical Potentiokinetic Reactivation (DL-ERP) test was used to determine susceptibility to IGC. The electrolyte solution used was 0.5M H 2 SO 4 + 0.001 M TA (thioacetamide). The test was conducted at 60 °C. The potential was scanned from -500 mV (SCE) to +200 mV (SCE) and back to 500 mV (SCE) at a rate of 1.67 mV/s. The ratio of the reactivation charge to the passivation charge was calculated. From the results obtained it can be shown that the fill region was most susceptible to IGC for all weld conditions.