Publication details
Dissimilar Weld Joints of Steels for High-Temperature Exploitations in Energy Production
| Authors | |
|---|---|
| Year of publication | 2007 |
| Type | Article in Proceedings |
| Conference | EUROMAT 2007 |
| MU Faculty or unit | |
| Citation | |
| Field | Physical chemistry and theoretical chemistry |
| Keywords | DICTRA;phase;diffusion;CALPHAD;aktivita |
| Description | Decreasing the amount of carbon dioxide emitted per unit of energy is the main goal of energy production today. This requires looking for a technology with higher efficiency that works at high temperatures and pressures (USC boilers and turbines, for example: 610oC/30 MPa). Older steels are exploited at lower temperatures, 540 565oC, and are stepwise replaced by advanced 9-12% steels (P91, P92, E911, VM12,...) because the other high-temperature materials are more expensive or less trustworthy in long-time operation. The new creep-resistant steels are combined with the foregoing steels. The stability of dissimilar weld joints is therefore an important factor that influences any long-time service of boilers, pipes or other high-temperature parts. At high temperatures, element redistributions and phase transformations are frequently diffusion-controlled. The weld joint microstructure and mechanical properties are in close relation to phase transformations occurring in the diffusion-affected zone. The degradation processes frequently result in carbon-depleted and carbon-rich zones. The weld joint design method presented is based on the CALPHAD approach, which enables calculating the phase diagrams and the carbon activities of both the base alloys and the weld metal. The time- and temperature-dependent element redistributions and phase transfomations across the weld can be obtained from diffusion simulation, which is based on the assumption of local phase equilibrium in each part of the system and on the assumption that diffusion controls the mass transport and phase transformations in the diffusion-affected zone. The simulation can also help us to optimize the chemical composition of the weld metal. The dissimilar weld joint design can also be performed easily by a method that compares the temperature dependence of the carbon activity in the base materials and in the weld metal. The support of the Ministry of Education, Youth and Sports (projects No. MSM0021622410, MSM 0021630508, and OC164) is gratefully acknowledged. The calculations were performed using the ThermoCalc and the DICTRA software. |
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