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 and coarsening behavior through strain induced precipitation causes cold worked and welded components to be more susceptible to these forms of cracking. More work is needed to determine the direct effect of strain on cracking susceptibility. This project will analyze the how different variables, like level of pre-strain and thermal history, will change the material properties through changes in the kinetics of precipitation and coarsening, and therefore change the cracking susceptibility.
The influence of plastic strain on precipitation and the resultant hardening response of austenitic stainless steel 347H and nickel based 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). Several samples were characterized, and the precipitate size, number, and volume fraction was determined. A precipitate strengthening model was developed to interpret hardness results.
Results and Discussion
The long term ageing studies measured how strain and temperature influence precipitation kinetics via the strain induced precipitation hardening (SIPH) mechanism. SIPH increases the nucleation rate of precipitates and accelerates coarsening. Dislocations can enhance diffusion via the pipe diffusion mechanism, which increases with dislocation content. This means that as the dislocation content increases (through increased cold work), the precipitate hardening response of the material is also accelerated. The quantitative change in the precipitation kinetics due to strain will be described for γ’ in 740H and NbC in 347H. From the precipitate strengthening model, the changes in hardness due to precipitation were interpreted as changes in precipitate size and volume fraction. The individual effects of strain and precipitation on hardness was delineated.
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. Increasing the amount of cold work will result in faster precipitation. Combined with the weakened grain boundary, this is known to increase the susceptibility to cracking by strengthening the grain interior, causing stress relaxation to occur primarily at the grain boundary. Although this mechanism is qualitatively known, this work quantified the effect of cold work on precipitation kinetics. Thus, the change in precipitation due to strain can be better predicted.