Stress Relief Cracking and Strain Ageing of 347H and In-740H

Many alloys that are used at high temperature may exhibit strain age cracking during long term ageing in service. This cracking typically occurs for precipitation strengthened alloys where a locally soft precipitate free zone (PFZ) forms around grain boundaries. Stress relaxation of service stresses and welding residual stresses will be localized to the PFZs, leading to intergranular failure with low ductility. Acceleration of precipitation and coarsening behavior through strain induced precipitation means that cold worked or welded components are more susceptible to this form of cracking.

There is a significant industrial need to determine the influence of plastic strain on precipitation kinetics during service, the resultant hardening response, and the change in cracking susceptibility. Samples of nickel based superalloy IN 740H and stainless steel 347H were welded, strained, and aged. The sample’s hardness as well as precipitate size and phase fraction were measured in combination with modeling studies using ThermoCalc in order to determine the change in precipitation kinetics. Thus, the effect of plastic strain, composition, microstructure, temperature, and time on the kinetics of nucleation, growth, and coarsening for NbC (in 347H) and γ’ (in In740H) were determined.

The long term ageing studies showed that the samples with the greater strain had a different coarsening rate. This is due to strain induced precipitation hardening as greater dislocation density 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. Both 740H and 347H showed this effect through precipitation of γ’ and NbC respectively. The hardness data was analyzed with a precipitate strengthening model to account for recovery effects at high temperature.