Kinetics of γ″ precipitation in alloy 625

Alloy 625 is used as superheater tubes and ammonia cracker tubes in many ammonia plants and in chemical industries. The components, made of this alloy are expected to have a life of about 100,000 hours at an operating temperature in the range of 450 °C to 700 °C. The Ni₂(Cr,Mo) phase and the γ″ phase were found to be the main strengthening phases at these operating temperatures. The Ni₂(Cr,Mo) phase was stable up to 600° C white the γ″ precipitation was profuse up to 700° C. Above this temperature, γ″ precipitates nucleate heterogeneously on dislocations. The results of a detailed investigation on the kinetics of precipitation of γ″ phase in the temperature range of 600-700° C are reported in this paper. The plot of γ″ particle size as a function of cube root ageing time obeyed a linear relationship indicating that these precipitates grew by volume diffusion in this alloy. The particle size distribution was found to obey a Gaussian or log normal distribution. From the measurement of activation energy, the precipitation could be attributed to the diffusion of solutes in the matrix. The density of precipitation was found to be high on γ″ precipitates that have formed on dislocations while their size is very small. The present observations are rationalized on the basis of the available models for precipitation coarsening reported in the literature. (author)