Introduction
Materials and Methods
Welding Experiments and Sample Extraction
Material | C | Mn | Si | Cr | Ni | Mo | P+S |
---|---|---|---|---|---|---|---|
Base material (BM) | 0.10 | 1.53 | 0.51 | 0.04 | 0.03 | 0.01 | 0.01 |
Deposited Weld metal (WM) | 0.09 | 1.58 | 0.31 | 0.04 | 0.02 | 0.01 | 0.01 |
Material | SiO2+TiO2 | CaO + MgO | Al2O3 | CaF2 | . | . | . |
---|---|---|---|---|---|---|---|
Welding flux | 15 | 40 | 20 | 25 | . | . | . |
Determination of Diffusion Coefficients Via Electrochemical Permeation and Carrier Gas Hot Extraction
Electrochemical Permeation/Base Material and Weld Metal Diffusion Coefficients at Ambient Temperature
Carrier Gas Hot Extraction/Weld Metal Diffusion Coefficients in the Range From 120 to 400 °C
Numerical Simulation of MWT/Hydrogen Diffusion at Ambient Temperature
Results and Discussion
Diffusion Coefficients
Electrochemical Permeation, Diffusion at Ambient Temperature
Material | L in mm | imax in 10−7 A/mm2 | ϕ in 10−12 mol/mm*s | tlag in s | ai in 10−10 A/(mm2*s) | Dlag in 10−5 mm2/s | DIP in 10−5 mm2/s |
---|---|---|---|---|---|---|---|
BM | 0.50 | 2.37 ± 1.18 | 1.23 ± 0.61 | 410 ± 79 | 9.70 ± 4.39 | 9.47 ± 2.15 | 17.60 ± 2.82 |
1.00 | 4.22 ± 1.57 | 4.37 ± 1.63 | 657± 98 | 8.51 ± 0.75 | 25.80 ± 4.05 | 34.60 ± 2.67 | |
WM | 0.50 | 5.41 ± 0.96 | 5.61 ± 0.97 | 882 ± 154 | 5.56 ± 1.52 | 4.83 ± 0.94 | 4.31 ± 0.77 |
1.00 | 3.05 ± 0.74 | 3.16 ± 0.78 | 1.030 ± 192 | 3.04 ± 1.95 | 16.60 ± 3.11 | 17.40 ± 3.34 |
Carrier Gas Hot Extraction: Weld Metal Diffusion Coefficients from 120 °C to 400 °C
Temperature in °C | t0.5-time in s | DCGHE in 10−3 mm2/s |
---|---|---|
120 | 33.4 ± 1.8 | 2.69 ± 0.14 |
200 | 25.4 ± 2.0 | 3.55 ± 0.28 |
315 | 20.9 ± 1.4 | 4.31 ± 0.30 |
400 | 21.1 ± 1.7 | 4.27 ± 0.34 |
Analytical Description of Diffusion at Ambient Temperature and from 120 to 400 °C
Case 1: Estimation of MWT Based on Diffusion at Ambient Temperature
Specific DC and LC combination | t50% [h] | |
---|---|---|
BM | WM | |
Dmax, LC1 = 10 mm | 23 | 47 |
Dmin, LC1 = 10 mm | 85 | 188 |
Dmax, LC2 = 30 mm | 211 | 419 |
Dmax, LC2 = 30 mm | 769 | 1.691 |
Case 2: Estimation of Dwell Time for HRHT at Elevated Temperatures
Temperature [°C] | t50% [h] (LC1=10 mm) | t50% [h] (LC2=30 mm) |
---|---|---|
120 | 3.0 | 27.1 |
200 | 2.3 | 20.5 |
315 | 1.9 | 16.9 |
400 | 1.9 | 17.1 |
Numerical Analysis of Hydrogen Diffusion in the Welded Joint at Ambient Temperature
Concentration after time t | Time t for diffusion to reach corresponding hydrogen concentration | |||||||
---|---|---|---|---|---|---|---|---|
C = 1.00 (100%) | C = 0.50 (50%) | C = 0.25 (25%) | C = 0.10 (10%) | |||||
Corresponding t using: | [103 s] | [d] | [103s] | [d] | [103 s] | [d] | [103 s] | [d] |
Dmin | 600 | 0.007 | 1,356 | 15.7 | 2,733 | 31.6 | 4,878 | 56.5 |
Dmax | 600 | 0.007 | 424 | 4.9 | 979 | 11.3 | 1,905 | 22.0 |
Conclusions
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The S420G2+M was investigated the first time in terms of hydrogen diffusion coefficients for BM and multi-layer WM. The coefficients are within the range of 10−5 mm2/s to 10−4 mm2/s at ambient temperature (BM and WM) and 10−3 mm2/s at elevated temperatures (WM).
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In that connection, permeation experiments are a suitable tool to evaluate a MWT via the use of diffusion coefficients. However, the diffusion coefficients changes with sample thickness by up to an order of magnitude. This means that MWT derived from vary accordingly. That means a diffusion coefficient-based MWT does not adequately account for a time-delayed HAC and should be critically discussed.
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The CGHE methodology represents a valuable tool for the determination of high-temperature diffusion coefficients and, e.g., for estimating the dwell times for HRHT. However, for the considered thickness up to 30 mm, very conservative dwell times (longer than one day) result. That means especially for large plate thicknesses, the diffusion path plays a crucial role and can result in long calculated diffusion time.
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A robust analytical calculation of the hydrogen concentration profiles is possible, and only the diffusion coefficient and the initial hydrogen concentration must be known. However, it is disadvantageous as it only represents an approximate solution (assumption: hydrogen concentrated at one point). Further simulations allowed the determination of the local, microstructure and time-dependent hydrogen concentration. Both models clearly showed that reliable diffusion coefficients are necessary for the calculation of the diffusion time.