Abstract Scope |
Introduction
Many alloys that are used at high temperature exhibit stress relief cracking (SRC) during post weld heat treatment (PWHT) or cracking during long term ageing in service. This cracking typically occurs for precipitation strengthened alloys where a precipitate free zone (PFZ) forms around grain boundaries at high temperature. Stress relaxation of service stresses and welding residual stresses will be localized to these grain boundaries, which have been shown to fail intergranularly and with low ductility. Acceleration of precipitation kinetics through changes in chemistry or strain may cause plasticly strained components to be more susceptible to these forms of cracking. More work is needed to determine the direct effect of strain on cracking susceptibility. There is a significant industrial need to develop a greater understanding of the mechanism of SRC and cracking due to ageing during service. This project will analyze the how different variables, such as welding, level of pre-strain, and thermal history will change the material properties through changes in the precipitation kinetics, and therefore change the cracking susceptibility.
Experimental Procedures
The influence of plastic strain on precipitation and the resultant hardening response of austenitic stainless steel 347H and nickel base superalloy Inconel 740H will be analyzed to understand the mechanism for cracking during long term ageing in service. Samples were welded, cold rolled to induce a controlled pre-strain, and aged at two different temperatures. Full hardness ageing curves were measured for each material in the fusion zone (FZ), heat affected zone (HAZ), and the base metal (BM). Using SEM and TEM microscopy to characterize precipitates, the changes to the kinetics of coarsening from strain, welding, temperature, and base metal condition were determinedTensile samples were strained and aged to replicate the microstructure previously analyzed, and the cracking susceptibility of those microstructures were measured.
Results and Discussion
The long-term ageing studies measured how strain, temperature, welding, and base metal condition influence precipitation kinetics. The SRC tests then measured how these changes in precipitates will change the cracking susceptibility. Strain, from welding and forming, increases the dislocation density, which increases the number of nucleation sites and increases diffusion rate via the pipe diffusion mechanism. As a result, the kinetics of nucleation, growth, and coarsening are increased. This increase in kinetics is observed in strain aged samples of 740H, which shows faster coarsening of γ’ precipitates. However, the coarsening rate decreases back to the unstrained level after a few thousand hours at temperature, due to a decrease in dislocation density at high temperature. This was identified through a precipitate strengthening model based on theoretical calculation, which was verified with experimental results. This model separated the contribution of precipitation hardening from strain hardening and identified regions where strain hardening no longer had a significant impact.
Conclusion
The SRC and strain age cracking phenomenon are both associated with precipitation hardening, which will increase strength within the grain, but reduce ductility. This is due to a weakened grain boundary microstructure due to PFZ formation, which localizes strain. The kinetics of coarsening are much faster for strained than unstrained samples at short times at high temperature. This will change the cracking susceptibly of the samples in the peak aged condition. However, overaged samples have a lower cracking susceptibility. After recovery, there is no significant difference in strength, hardness, or precipitate kinetics between unstrained and strained samples. This suggests there can be a heat treatment path to eliminate higher dislocation density caused by plastic strain, which will reduce cracking susceptibility. |