Ragnhild Aune

Hydrogen Assisted Cracking in Welding of 13% Cr Supermartensitic Stainless Steels

Supermartensitic stainless steels are known to be prone to hydrogen induced cold cracking. Therefore, the objective of this work was to assess the susceptibility to hydrogen cracking in Gas Metal Arc Welding (GMAW) with use of matching base and filler materials (supermartensitic stainless steel), using the Instrumented Restraint Cracking (IRC) test. Root welding in the IRC test did not result in hydrogen induced cracking, neither for low nor high weld metal hydrogen content. Because of the martensitic transformation, the global residual stresses are very low after welding (below 100 MPa). Since the yield strength (Rp0.2% ) of the material is about 600–720 MPa, it implies that the IRC test method is not very suitable for supermartensitic stainless steels. However, by performing IRC test multi-layer welding, micro-cracks were found in the last pass. An increase in the weld metal hydrogen content resulted in reduced fracture stress and ductility, as observed in tensile testing of IRC test specimens directly after welding. Investigation of the fracture surfaces of the specimens with high hydrogen contents showed fish eyes, which are strong indications of hydrogen embrittlement. By performing heat treatment (225°C for 24 hours) of specimens with high hydrogen contents and subsequent tensile testing, the fracture stress and ductility were restored to the initial base metal level. Slow Strain Rate Testing (SSRT) with and without Cathodic Protection (CP) was performed on test specimens sampled transverse to the welding direction. CP has detrimental effect on the fracture stress and ductility due to the high weld metal hydrogen pick up.© 2003 ASME

Hydrogen Pick Up and Diffusion in TIG Welding of Supermartensitic 13% Cr Stainless Steel With Superduplex Wire

Supermartensitic 13% Cr stainless steels have been in use in offshore satellite flowlines for several years. Since they contain microstructure that is susceptible to hydrogen cracking, the pick up of hydrogen in welding with subsequent transport to critical areas may be very important, also with respect to hydrogen embrittlement when hydrogen is coming from other sources than welding (e.g., cathodic protection). In the present investigation the pick up of hydrogen has been assessed using mechanized TIG welding with superduplex 25% Cr wire. The WM and HAZ hydrogen levels were analyzed. With addition of hydrogen in the shielding gas in multipass welding, the mean WM hydrogen contents were found to be approximately 10 and 6 ppm in the cap layer and root pass, respectively. The corresponding mean HAZ concentration was 3.1 ppm (scatter between 1.3 and 4.8) immediately after welding. Post weld hydrogen diffusion heat treatment showed that hydrogen diffusion was retarded at room temperature, even for 1 month storage. Limited diffusion took place at 90°C, particularly for the cap region. The results indicate that superduplex weld metal with high hydrogen content (6–10ppm) will act as a hydrogen reservoir supplying H to the 13% Cr HAZ as long as 2–3 years after welding. Fitting the data by using the uniaxial diffusion model gave diffusion coefficients in the range of ∼3–5×10−13 m2 /s at room temperature for the superduplex WM. At 90°C a diffusivity of 5.5×10−12 m2 /s for the cap area and 2.5×10−11 m2 /s for the root area were found. For a holding temperature of 150°C, diffusion from the WM was much more significant. The hydrogen WM cap content was reduced from an initial level of 10 ppm down to 2 ppm within 3 months giving a diffusion coefficient of 1.0×10−11 m2 /s. The supermartensitic HAZ samples contained up to 5 ppm hydrogen a short time after welding. This is an important observation, since it may provide sufficient amount of hydrogen in the HAZ to cause cold cracking in the as welded condition. The uniaxial model indicated diffusivities of D = 8.0×10−11 m2 /s at 20°C and D = 2.0×10−10 m2 /s at 90°C in the HAZ.Copyright

High Heat Input Welding of 12Cr-6Ni-2.5Mo Supermartensitic Stainless Steel

In conventional welding of 13% Cr supermartensitic stainless steels, the normal microstructure that forms on cooling is martensite. Although high heat input tends to give a certain coarsening of the final microstructure, the eventual accompanying loss in toughness is not known. The present study was initiated to examine the effect of heat input on weld metal and heat affected zone mechanical properties of a 12Cr-6Ni-2.5Mo grade. The results obtained showed that the notch toughness is low (25 J) and independent of heat input for the weld metal, while it is reduced with increasing heat input for fusion line and the heat affected zone locations. Subsequent post weld heat treatment gave a substantial increase in toughness for all notch locations. Based on these results, indications are that a specified maximum heat input is not applicable in welding of supermartensitic stainless steels, allowing more production efficient techniques to be used, both in longitudinal seam and girth welding.Copyright