Knowledge of microstructure stabilities is essential for reliable estimations of the lifetime limitations of materials and welded joints at higher temperatures. This information may be deduced from material phase diagrams and from heterogeneous weld joint simulations. The phase diagrams can be calculated using CALPHAD method [1]. This method complemented with appropriate diffusion approach [2], [3] can help us to simulate phase and element profile evolutions inside diffusion-affected zone of weld joint too. The heat-resistant steels (alloying base: Cr, Ni, Mo, V, C, N) and their weld joints were objects of our study. Thermodynamic database STEEL [4] and kinetic database DIF mentioned in [5] were used.
The results of the equilibrium calculations are presented as the phase diagram cross-sections of the selected heat-resistance steels. The same equilibrium calculations offer element activities too. The carbon activity temperature dependences were calculated for individual steels. The observed difference between the carbon activities of two different steels (at given temperature) enabled us to predict diffusion flow direction, which can be opposite then deduced from the carbon content in some steel combinations (carbon up-hill diffusion).
There are presented the selected simulation results of the heterogeneous weld joints produced by joining the heat-resistant steels at elevated temperatures. The simulations respect the coexistence of different carbide phases in the materials and they are conducted on the assumption that under the examined conditions the local phase equilibrium holds and that diffusion is the control process of phase transformation. Phase and element profiles in weld joints are given.
The phase diagram calculations and diffusion couple simulations involving time-distance dependences for phase and element redistributions in diffusion-affected zones enabled to obtain a basic information, which allow to estimate thermodynamic and diffusion stabilities of the studied heat-resistant steels and weld joints.
Grateful acknowledgement goes to the Grant Agency of the Czech Republic for the financial support under project No. 106/02/0876.
References:
[1] Saunders N. and Miodovnik A. P.: CALPHAD (calculation of phase diagram) – A
Comprehensive Guide, Elsevier Science, Amsterdam, 1998.
[2] Kirkaldy J. S., Young D. J.: Diffusion in the Condensed State, The Inst. of Metals,
London, 1987.
[3] A. Borgenstam, A. Engstrőm, L. Hőglund, J. Ågren: DICTRA, a Tool for Simulation of Diffusional
Transformations in Alloys, J. of Phase Equilibria, 21/3 (2000), 269-280.
[4] A. Kroupa, J. Havránková, M. Coufalová, M. Svoboda, J. Vřešťál: Journal of Phase Equilibria, 22/3 (2001),
312-323.
[5] J. Sopoušek, V. Jan, R. Foret: Simulation of Dissimilar Weld Joints of Steel P91, Science and Technology of
Welding and Joining, 2004, 9, March.