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Low-alloyed CrMoV steels, such as T24, are widely used for welded components in fossil power stations due to their excellent creep-strength. Spectacular failure cases in the recent years exhibited severe cracking in T24 welds. The results showed that hydrogen-assisted cracking (HAC) occurring up to 200 degree Celsius cannot be excluded. Hence, a basic understanding is necessary on how hydrogen affects the material properties of welded microstructures. In this regard, each weld microstructure (HAZ and weld metal) has influence on the HAC susceptibility and respective hydrogen diffusion. Thus, the present contribution summarizes different results obtained from experiments with grades T24 (CrMoV alloy) and T22 (CrMo) and thermally simulated HAZ. Tensile tests were conducted with hydrogen charged specimens and compared to hydrogen-assisted stress corrosion cracking results obtained from slow strain rate tests (SSRT) up to 200 degree Celsius. Electrochemical permeation and degassing experiments were performed to identify a particular weld microstructure influence on hydrogen diffusion and trapping (especially in the HAZ). The results showed that T24 base material has improved resistance to hydrogen-assisted degradation/cracking. In contrast, the as-welded HAZ had remarkably increased susceptibility (tesnile tests at hydrogen concentration of 1 to 2 ppm). SSRT experiments confirmed this at elevated temperatures for both the T24 and the T22. Hence, the evaluation of a particular degradation of the mechanical properties should be performed independently for each weld microstructure. In addition, the HAZ showed decreased diffusion coefficients (at room temperature) of approximately one magnitude compared to the base materials. Trapped hydrogen was determined in the T24 at temperatures up to 120 degree Celsius compared to 75 degree Celsius in the T22. This has to be considered in case of changing operational temperatures, e.g. in the case of start-up and shutdown processes of boiler components.
Hydrogen-assisted cracking (HAC) represents a significant failure risk for (high strength) creep resistant low-alloyed steel components in fossil-fired power plant applications at temperatures of up to and above 200 °C. This particularly applies to respective start-up and shut-down processes associated with alternating service-conditions in terms of load flexible power plants. For quantitative determination of localized crack critical hydrogen concentrations, the temperature dependent hydrogen diffusion coefficients have to be determined as exactly as possible.
However, available literature provides a wide range of hydrogen diffusion coefficients for low alloyed steels with similar microstructures. Additionally, the available diffusion data seem not to be very reliable and their scatter increases with decreasing temperature. One reason is that the experimental boundary conditions can have a major impact on the determination of respective effective diffusion coefficients. Hence, the scope of this study is to evaluate the influence of the experimental boundary conditions on the derived diffusion coefficients. In addition, different methods for calculating diffusion coefficients are discussed. To elucidate such influences and to draw a line to practical application, the diffusion and trapping behavior in the creep resistant steel 7CrMoVTiB10 10 has been studied.
For such purpose, hydrogen charged specimens were isothermally degassed at different temperatures using carrier gas hot extraction (CGHE). Based on experimental data, a numerical model has been developed by which the hydrogen transport behavior and the respective hydrogen distribution during CGHE can be assessed.
It is demonstrated that the specimen heating rate has a large influence on the calculated diffusion coefficients under assumption of isothermal degassing which elsewhere has been underestimated in the assessment of diffusion data in creep-resistant steels. The numerical results suggest that calculation methods for diffusion coefficients are limited if compared to experimental results. It also turned out that the sample preparation time before CGHE can enormously influence determined diffusion coefficients. Consequently, non-homogeneous hydrogen concentration profiles have to be anticipated in the simulations to arrive at characteristic effusion curves consistent to respective CGHE experiments. In turn, validated diffusion coefficients are now available for the low-alloyed Cr-Mo-V steel which might be helpful to calculate appropriate hydrogen removal heat treatment procedures, for instance.
Modeling of Hydrogen-Assisted Cracking (HAC) in Duplex Stainless Steels (DSS):
- Hydrogen causes significant degradation in DSS and consequently HAC
- Mechanisms of HAC have not been fully understood in two-phase microstructures
- Mesoscale numerical modeling for HAC in DSS was created, coupled to the macroscale, enabling simulation of HAC in both phases independent from predefined crack paths
Hydrogen-assisted cracking (HAC) represents a significant failure risk for (high strength) creep resistant low-alloyed steel components in fossil-fired power plant applications at temperatures of up to and above 200 °C. This particularly applies to respective start-up and shut-down processes associated with alternating service-conditions in terms of load flexible power plants. For quantitative determination of localized crack critical hydrogen concentrations, the temperature dependent hydrogen diffusion coefficients have to be determined as exactly as possible.
However, available literature provides a wide range of hydrogen diffusion coefficients for low alloyed steels with similar microstructures. Additionally, the available diffusion data seem not to be very reliable and their scatter increases with decreasing temperature. One reason is that the experimental boundary conditions can have a major impact on the determination of respective effective diffusion coefficients. Hence, the scope of this study is to evaluate the influence of the experimental boundary conditions on the derived diffusion coefficients. In addition, different methods for calculating diffusion coefficients are discussed. To elucidate such influences and to draw a line to practical application, the diffusion and trapping behavior in the creep resistant steel 7CrMoVTiB10 10 has been studied.
For such purpose, hydrogen charged specimens were isothermally degassed at different temperatures using carrier gas hot extraction (CGHE). Based on experimental data, a numerical model has been developed by which the hydrogen transport behavior and the respective hydrogen distribution during CGHE can be assessed.
It is demonstrated that the specimen heating rate has a large influence on the calculated diffusion coefficients under assumption of isothermal degassing which elsewhere has been underestimated in the assessment of diffusion data in creep-resistant steels. The numerical results suggest that calculation methods for diffusion coefficients are limited if compared to experimental results. It also turned out that the sample preparation time before CGHE can enormously influence determined diffusion coefficients. Consequently, non-homogeneous hydrogen concentration profiles have to be anticipated in the simulations to arrive at characteristic effusion curves consistent to respective CGHE experiments. In turn, validated diffusion coefficients are now available for the low-alloyed Cr-Mo-V steel, which can be helpful to calculate appropriate hydrogen removal heat treatment procedures.
Offshore wind turbines (OWT) are a key factor of the sustainable energy generation of tomorrow. The continuously increasing installation depths and weight of the OWTs require suitable foundation concepts like monopiles or tripods. Typically, mild steels like the S420ML are used with plate thicknesses up to several hundreds of mm causing high restraints in the weld joints. Due to the large plate thickness, submerged arc welding (SAW) with multiple wires is the state-of-the-art welding procedure. As a result of the very high stiffness of the construction, a certain susceptibility for time-delayed hydrogen-assisted cracking (HAC) may occur. The evaluation of crack susceptibility is very complex due to the component size and stiffness of real offshore structures. For this purpose, a near-component test geometry was developed to transfer the real stiffness conditions to laboratory (i.e., workshop) scale. The investigated mock-up, weighing 350 kg, comprised heavy plates (thickness 50 mm, seam length 1,000 m) joined by a 22-pass submerged arc weld. Additional stiffeners simulated the effect of high restraint or shrinkage restraint of the weld. Extreme scenarios of hydrogen absorption during welding were simulated via the use of welding fluxes in dry (HD < 5 ml/100g Fe) and moisture condition (HD > 15 ml/100g Fe). The residual stresses were determined by a robot X-ray diffractometer. Areas of critical tensile residual stress (at the level of the yield strength) were found in the weld metal and heat affected zone. To identify possible delayed cracking, the welds were tested by phased array ultrasonic testing (PAUT) after 48 h. Summarized, no significant occurrence of HAC was detected, indicating the high crack resistance of the welded joint, i.e., a suitable combination of base material, welding consumable and welding parameters.