Stress corrosion cracking (SCC) sensitivity of the girth welds of supermartensitic stainless steels (SMSS) tubes seems closely linked to the thermal history encountered by the heat affected zones and chemical composition. The aim of the work presented in this paper was to investigate the influences of welding parameters (welding process, cooling rate ...), chemical composition of the base plate and environment on the SCC sensitivity. Results of four point bend tests performed on plate with weld beads in simulated formation water with and without H2S are presented and discussed. It appears that the SCC mechanism is enhanced by a decrease in the pH value, an increase in the temperature and/or the presence of H2S. On the contrary a decrease of the cooling rate (high welding heat input) used for the last deposited bead or an increase of the chromium and molybdenum concentrations improves the SCC resistance. This study, supported by the European Community, was developed to improve the understanding of this new corrosion mechanism and to propose solution for the safer use of SMSS in oil and gas applications.
During the last several years there has been a growing interest for the use of weldable supermartensitic stainless steels (SMSS) for flowline application in the oil and gas industry due to the high mechanical properties and good sulfide stress cracking (SSC) resistance in slightly sour condition at ambient temperature. However stress corrosion failures were reported during qualification testing[1], [2] as well as during research work prior to qualification testing[3]. The failure of a flow line due to intergranular stress corrosion cracking (IGSCC) in the heat affected zone (HAZ) of the girth welds was also reported[4].
Both SMSS tubes made with superduplex (SDSS) filler metal and with matching filler metal had shown cracking (including secondary cracking in case of SDSS) at the heat affected zone when tested in four point bending (4PB). Crack propagation was mainly intergranular[3].
Stress corrosion cracking (SCC) sensitivity of the girth welds of SMSS tubes seems closely linked to the thermal history encountered by the heat affected zone[5], [6], [7], [8] and to chemical composition[9], [10]. It is also dependent on the operating conditions such as pH, CO2-pressure, H2S-pressure and chloride concentration[11]. SCC was also reported in environment without H2S[2], [12]. Finally, several authors reported that post welding heat treatment (PWHT) strongly reduce the susceptibility to SCC[2], [3], [5], [6], [10], [11].
The aim of the following experimental work was to study the influence of the welding parameters (process type, cooling rate etc.), the chemical composition of the base plate and environment on the SCC sensitivity.
Table 1 gives the chemical analysis of martensitic and supermartensitic stainless steels considered in this study. The specimens were removed from 12 (heats B, S and V), 10 (heats C, D and E) or 8 mm (heat A) thick hot rolled plates produced from industrial or laboratory heats. For laboratory heats (references C, D and E), about 45 kg of each new composition were produced and hot rolled to 10mm thick coupons.