Corrosion fatigue of a superduplex stainless steel weldment

Abstract

Superduplex stainless steels have superior mechanical and corrosion properties compared to austenitic stainless steels such as the grade 300 series. This is a result of a microstructure consisting of roughly equal percentages of austenite (y) and ferrite (a) and negligible inclusion content. As a result, super duplex stainless steels are increasingly being used in the offshore oil and gas industries. It is also envisaged that they will find application in the emergent renewable energy sector in areas such as offshore wind, wave and tidal electricity / hydrogen generation. Corrosion fatigue (CF) conditions are expected in such applications. Of critical concern are weld joints where inherent sub critical surface/embedded flaws diminish crack initiation resistance enhancing the probability of subsequent crack propagation. The current research investigates the CF crack propagation performance of weld metals produced by two welding techniques. Since sub sea components are always cathodically protected, this condition was simulated in the CF tests. In addition, high positive potentials were simulated, as this condition is possible in the absence of cathodic protection. One weldment was completed using the expensive and relatively slow gas tungsten arc (GTA) welding method. The other weldment was achieved using the GTA method for the root pass and subsequently filled using the cheap and relatively quick shielded metal arc (SMA) welding method. The resultant crack propagation life was derived from the crack propagation tests by means of a numerical model. Fatigue life of the weld metals (assuming negligible residual stress influence) is similar to standard design curves for class D, carbon and carbon-manganese structural steel butt welds. Thresholds for the onset of crack growth in Zeron 100 base and weld metals are similar and were shown by means of the numerical model to correspond with the endurance limit specified in the standard design curve. Cathodic over protection is much more deleterious than high positive potentials above a critical stress level for Zeron 100 base and weld metals leading to an increase in crack propagation rates on average by a factor of 4.3 over rates in air. The GTA root/SMA fill weld metal performs equally well as the GTA root/GTA fill weld. Therefore, a potential economic saving is evident. Finally, a new model for hydrogen assisted subcritical brittle crack propagation in ferrite is proposed