Vigdis Olden

The Effect of PWHT on the Material Properties and Micro Structure in Inconel 625 and Inconel 725 Buttered Joints

This report describes investigations performed on as welded and post weld heat treated samples of AISI 8630 steel, buttered with Inconel 625 and Inconel 725. The investigations have focused on the properties and microstructure in the partial mixed zone between the buttering and the steel before and after post weld heat treatment. The samples were heat treated for 4 1/2 hours at 640°C, 665° and 690°C and investigated with respect to mechanical properties and microstructure near the fusion line. A range of testing and analyses were performed including notch impact toughness testing, identification of fracture initiation and propagation in impact specimens, hydrogen measurements, examination of the micro structure in steel and Inconel using light microscope, hardness testing and electron micro-probe analysis of the alloying elements across the fusion line. Additional investigations in TEM on samples from an actual joint, post weld heat treated at 665°C were also performed. The results show that post weld heat treatment at 665°C and 690°C reduced the impact toughness in coarse grained heat affected zone, caused by decarburisation, ferrite formation and grain growth. The partially mixed zone (5–10μm) of the Inconel buttering, gained partly extremely high hardness caused by carbon enrichment, reaustenitization and formation of virgin martensite. As welded samples gave more favorable properties and microstructure than the post weld heat treated ones.Copyright

A review of cohesive zone modelling as an approach for numerically assessing hydrogen embrittlement of steel structures.

Simulation of hydrogen embrittlement (HE) requires a coupled approach; on one side, the models describing hydrogen transport must account for local mechanical fields, while, on the other side, the effect of hydrogen on the accelerated material damage must be implemented into the model describing crack initiation and growth. This study presents a review of coupled diffusion and cohesive zone modelling as a method for numerically assessing HE of a steel structure. While the model is able to reproduce single experimental results by appropriate fitting of the cohesive parameters, there appears to be limitations in transferring these results to other hydrogen systems. Agreement may be improved by appropriately identifying the required input parameters for the particular system under study. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

PREDICTION OF HYDROGEN EMBRITTLEMENT IN 25% CR DUPLEX STAINLESS STEEL BASED ON COHESIVE ZONE SIMULATION

Laboratory experiments and cohesive zone simulation of Hydrogen Induced Stress Cracking in SENT tests specimens of 25% Cr duplex stainless steel have been performed. A polynomial formulation of the traction separation law and hydrogen dependent critical stress was applied. Best agreement with the experiments was found for an initial critical stress of 2200 MPa and a critical separation of 0.005 mm. Proposed threshold stress intensity factor and lower bound net section stress is 20 MPa√m and 480 MPa. High crack growth rates and typical hydrogen influenced fracture topography suggest large influence of the stress and strain in the fracture process zone on the hydrogen diffusion rate.Copyright

A Cohesive Zone Modeling Approach to Hydrogen Induced Stress Cracking in 25%Cr Duplex Stainless Steel

Hydrogen influenced cohesive zone elements are implemented in finite element (FE) models of rectangular U and V notched tensile specimens. The material description outside the cohesive zone is representative of a fine grained 25% Cr duplex stainless steel, UNS32760-S. A three step procedure consisting of conventional elastic plastic stress analysis, stress driven diffusion analysis and finally cohesive zone fracture initiation analysis makes the basis for the presented work. The applied boundary conditions are representative of mechanical stresses and environmental loads on an oil and gas pipeline in subsea conditions. A linear traction separation law gives reasonably good fit with experimental results for gross stress levels of 0.85–0.9 times the material yield stress. Hydrogen concentration of 40 ppm at the surface and 1 ppm in bulk always gives crack initiation at the surface despite the peak normal stress localized in front of the notch tip.Copyright

FE Simulation of Cold Cracking Susceptibility in X70 Structural Steel Welded Joints

Fracture mechanics SENT testing and FE simulation to establish hydrogen influenced cohesive parameters for X70 structural steel welded joints have been performed. Base metal and weld simulated coarse grained heat affected zone have been included in the study. The base metal did not fail at net section stresses lower than 1.29 times the yield strength and reveals low sensitivity to hydrogen embrittlement. The weld simulated coarse grained heat affected zone was prone to fracture at stresses above 64% of the yield strength, which indicates hydrogen embrittlement susceptibility. The cohesive parameters best fitting the experiments are δc = 0.3 mm and σc = 1700 MPa (3.5·σy ) for the base metal and δc = 0.3 mm and σc = 2100 MPa (2.6·σy ) for the coarse grained heat affected zone.Copyright

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