Supermartensitic stainless steels are now in use in offshore satellite flowlines. Due to their martensitic microstructure, these steels may be prone to cold cracking, depending on the hydrogen supply and residual stress development. The latter subject is addressed in the present investigation, where numerous thermal cycles, using the Satoh test approach, have been run to assess residual stress evolution in super 13% Cr steel, welds in super 13% Cr steel with superduplex wires, and finally, welds between 13 and 25% Cr steels with superduplex wire. The results obtained clearly demonstrate that the austenite to martensite phase transformation on cooling has remarkable effects on the final tensile residual stress level in welding of super 13% Cr steels. Specimens that go through full austenitisation on heating will have very low residual stresses after cooling due to the lattice atom rearrangement during the γ→α' transformation. This fact is in sharp contrast to the situation observed for 25% Cr superduplex steels, where the solid state transformation from δ-ferrite to α-ferrite and γ-austenite takes place at high temperature without any influence on the transient tensile thermal stresses. This causes much higher final residual stress level. Residual stress plateau between 470 and 630 MPa has been found, with the highest values for specimens with high peak temperature where full ferritisation has taken place.
Supermartensitic 13% Cr stainless steels have been qualified for and taken into use in offshore subsea satellite flowlines (Enerhaug, 1997, 1999; Olsen, 1999). This is due to much lower material costs than alternative duplex grades, and also that they possess a certain corrosion resistance. In some cases, problems have been reported related to pick up of hydrogen, either from welding (Rørvik, 1999; Aune, 2003; Akselsen, 2004a) or from the environments (Rogne, 2001, 2002a, 2002b, 2003, 2004).
