10.1 General Conclusions
Tensile and fracture properties of two microstructures (as-deposited (AD) and reheated (RH)) and the possible detrimental effects through prestraining and static strain ageing treatments were assessed for two (simulated) reactor pressure vessel steel weldments. Materials characteristics were assessed through hardness profiles, microstructure and inclusion analysis and GDS. Weld metal toughness was assessed in terms of Charpy impact and tensile tests, and crack-tip opening displacement tests. The cleavage fracture resistance of the two weld metals was measured through slow blunt notch bending tests carried out at -196oC. These tests were carried out for four microstructural conditions ADAR, RHAR, AD5%SA and RH5%SA.
1. Tanh curves were fitted to the impact transition data for both the AD and RH microstructures in both conditions, AR and 5%SA to compare the data in terms of ductile-to-brittle transition temperature (DBTT) at 40J energy level. Strain ageing had the effect both to shift in the transition curve towards higher temperatures for the AR condition and reduce the lower shelf energy level, which is more prominent for the 5%SA condition, the extent of these effects depends on the weld metal microstructure, either AD or RH. Extremes in behaviour were noted between the RHAR and AD5%SA weld metal conditions with a shift of 65oC (at the 40J value). For these extremes of condition a reduction in upper shelf energy level of approximately 53J.
Thus these two conditions can be considered as the ‘best’ (RHAR) and the ‘poorest’
(AD5%SA) conditions.
2. The upward shift in the ductile-to-brittle transition temperature of both the RH and the AD weld metal microstructures, implies that the 5%SA condition indirectly promoted brittle fracture of these types of weld metals. Strain ageing reduces the upper-shelf energy level and shift the DBTT curves for the AR condition towards higher temperatures.
3. Impact energy values obtained from the Charpy tests for both the AD and RH microstructures in both the AR and 5%SA conditions were found to be subjected to significant variation in the transition region, characteristic of this type of test. Lower shelf impact energy values are much less scattered and are insensitive to both microstructure and treatments. However, at all other temperatures there is a clear trend for specimens with the notch root location in the RH microstructure having greater impact toughness than specimens with a notch located in AD microstructure.
Scatter was more prominent in the RH microstructure and this is deduced to be the notch being more difficult to locate in the critical region of the microstructure, due to the fine scale of the weld beads
4. Performing cold deformation and static strain ageing treatments proved to have similar trends in terms of impact energy transition curves as irradiation, hence reducing the upper-shelf energy levels and shifting the transition curve towards higher temperatures. The magnitude of these effects can be related to weld metal microstructure, with the AD microstructure being slightly more sensitive than the RH microstructure.
5. The microstructure is also found to have a marked influence on the slope of the load-displacement curves for the CTOD results, in both the AR and the 5%SA conditions.
The RH microstructure showed higher CTOD values compared to the AD
microstructure, in both weld metal conditions, for similar stable crack growth extensions. The extreme effect of static strain ageing on the fracture toughness of the AD microstructure has been confirmed with these CTOD tests. This result strongly supports the idea that the straining and ageing treatment considerably affects the deformation characteristics of the weld metal material.
6. Static strain ageing on the notch toughness of the Charpy specimens affected the AD weld microstructure more compared with the RH microstructure; therefore attention was paid towards the cleavage fracture resistance of the weld metal for different conditions. In terms of local cleavage fracture stress results showed that the AD microstructure had slightly lower cleavage fracture resistance than the RH microstructure, however both of these weld metal microstructures after straining and ageing had similar values of local cleavage fracture stress but with increased values of yield stress. Thus, either cleavage fracture resistance is deduced to be reduced compared with the as-received microstructures. This is consistent with the easier transition from ductile crack extension to cleavage fracture at a given temperature in both impact and CTOD tests, and is the single most important factor in reducing toughness in strain aged material (when such local mechanisms of fracture are in competition).
7. Cleavage fracture initiates from individual non-metallic inclusions at positions close to that of the maximum local tensile stress present ahead of a stress concentration. These are observed to be located in relatively large allotriomorphic ferrite grains for the AD microstructure and equiaxed ferrite grains for the RH microstructure. It was also noted that the distance to the critical initiation site (X0) is dependent on microstructure as the AD exhibited lower values compared with the RH microstructure in both the AR and 5%SA conditions.
10.2 Suggestions for Future Research
Throughout this thesis numerous topics have been discussed and it is clear that many of the ideas and thoughts have not been fully exhausted. At this current time a complete “simulation procedure” of neutron embrittlement and its effects has yet to be developed. Cold deformation (prestraining) and static strain ageing at 300oC have been used throughout the work in this thesis to suggest the effects of neutron irradiation on this weld metal. Reduction of cleavage fracture stress though grain boundary segregation and the corresponding intergranular fracture mode that may be caused by neutron irradiation, cannot be assessed.
Other areas of interest would be:
I. To determine cleavage fracture, this is defined by a “microstructural unit” meaning the position, facet/ grain and inclusion size, are analysed. It has also been shown using fractography that cleavage fracture was initiated in large allotriomorphic ferrite grains, this was associated with long dislocation pile up lengths. However this was not measured as part of this current work, therefore the measurement of the density of dislocations and length would be of great interest, experiments could be carried out on both the AR and 5%SA conditions using transmission electron microscopy (TEM) to confirm this theory.
II. CTOD tests were preformed on Charpy - size specimens which had been precracked, the tests were carried out with limited effectiveness for obtaining valid fracture toughness parameters. Larger specimens would need to be employed for future experiments although care over the crack tip would need to be employed to ensure that the desired microstructure/ region of weld metal was being sampled.
III. More work should be carried out on the modelling of the transition data for the Charpy results. The data collected during this work was analysed using a Tanh fit model, which is strongly influenced by the upper and lower shelf data. Todinov [236, 237]
and Novovic [13] have proposed using the Avanmi fit, which model the transition region separately from the upper and lower shelves, this may provide a better fit for the data presented in this thesis.