The prediction of residual stresses in ferritic welds using finite element techniques requires materials properties to describe the thermal, tensile, cyclic and phase transformation behaviour that the material undergoes during welding and also during creep as the effect of post weld heat treatment is also of interest. Ferritic steels will transform at a temperature above about 850°C to austenite. As the steel is cooled, a further phase transformation in the structure occurs. The precise structure formed depends on the detailed chemical make-up of the steel and on the rate at which it is cooled. On slow cooling from above 850°C a pearlite-ferrite microstructure is formed. On more rapid cooling, other microstructures, particularly bainite at intermediate cooling rates and martensite at the highest cooling rates are formed. Predicting the phase on cooling requires a Continuous Cooling Transformation diagram that is suitable for welding thermal cycles and reflects the time spent above the austenitisation threshold which influences the austenitic grain size formed and subsequently the phase of material on cooling. Material properties for a SA508 Grade 3 steel and a low carbon SD3 filler metal have been generated and fitted to constitutive models that are available in the finite element codes ABAQUS and SYSWELD. The choice of hardening model and its associated parameters have been evaluated on the basis of the observed cyclic behaviour in materials testing. Validation of these models has then been carried out by finite element simulations of welded mock-ups which have been measured using neutron diffraction. These include an autogenous weld beam and groove weld specimens containing up to eight weld passes. The rationale for using these simple specimens has been to:
• Validate the capability of the model to predict the correct phase transformation behaviour and resulting stresses.
• Account for the different behaviour of the parent and filler material.
• To develop the capability for representing the material cyclic behaviour.
On the basis of these simulations recommendations have been made on the material models (and their parameters) that may be used for the finite element simulation of the welding process.
