BES - Fundamentals
of Welding and Joining.
Modeling Diffusion-Controlled Phase Transformations
in Steel Weldments
During the welding process, base material near the
weld is subjected to a heating and cooling thermal cycle that may induce
phase transformations. In addition, in the case of multi-pass welds, early
passes are subjected to thermal cycles when later passes are deposited.
It is desirable to quantify the extent of any transformations that occur
during these thermal excursions since they will affect the final microstructure
and may also affect the microstructural development during solidification
of the molten zone. An implicit finite-difference model was developed to
evaluate the effect of thermal cycling on the ferrite-austenite transformation
in austenitic stainless steel welds. The model considers multi-component
diffusion in a ternary system. Calculations using the model have shown
that consideration of multi-component diffusion is important. Results from
these calculations often are significantly different from predictions using
approximate binary diffusion analyses. The model shows that the phase transformations
that take place during heating are not completely reversed during cooling.
The calculations also show that the extent of any transformation is very
sensitive to the maximum temperature in the thermal cycle. The model can
determine whether there is a net increase or decrease in ferrite content
in weldments as a function of thermal exposure. In many cases, a determination
of the extent of the transformation is impossible by experimental means.
This model provides a means for evaluating this behavior. Representative
results showing ferrite growth and dissolution can be found at "http://engm01.ms.ornl.gov/BES/FDSimulation.html".
Significance:
A thorough understanding of the development of
the microstructure at high temperatures is necessary in order to understand
and control the final microstructure and properties. The current model
provides the basis for developing such an understanding and can be used
to predict transformation behavior in stainless steel welds. The same model
can be applied to other multi-component systems and is preferred over simple
binary diffusion models that do not properly account for transformation
behavior in these systems.
The research was conducted by J. M. Vitek and S. A. David at Oak Ridge
National Laboratory. The research was sponsored by the Division of Materials
Sciences, Office of Basic Energy Sciences of the U. S. Department of Energy.
