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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.
High-power laser welding of austenitic stainless steel with electromagnetic control of weld pool
(2014)
Laser deep-penetration welding became a widely applied tool in industrial applications due to available laser power of 20 kW and more for the single-pass welding of steel plates of up to 20 mm thikness. Above a critical limit, liquid metal tends to drop out of the bead due to hydrostatic pressure. Laser welding, in contrast to electron beam welding technique, allows for an electromagnetic manipulation of fluid flow in the weld pool. AC electromagnetic system for compensation of the hydrostatic pressure by induced Lorentz forces in the melt was experimentally and numerically investigated for single-pass full-penetration welding of up to 20 mm thikness austenitic stainless steel plates of grade AISI 304. It was shown that the application of 200-234 mT magnetic fields at oscillation frequency of around 2.6 kHZ lead to a full compensation of hydrostatic forces in the melt for plate 10-20 mm thick, respectively. Coupled fluid flow, thermal and electromagnetic finite element simulations were done with different applied magnetic flux densities and oscillation frequencies calculating for the optimal magnetic field strength to avoid melt sagging in the weld pool. The simulation results point to a lower magnetic field density needed for that purpose. The reason for that can lie in the magnetic properties of the material not being totally non-ferromagnetic. 17 Ref., 1 Table, 5 Figures.