Abstract
Precipitation process of nitride and its effect on creep properties of a carbon steel for high temperature use were studied. A hydrogen extraction method was used for the determination of mobile nitrogen concentration at creep testing temperatures. The results obtained are as follows;
1) It is confirmed that the method is effective to extract mobile nitrogen which has close relationship with creep properties.
2) From isothermal annealing curves of mobile nitrogen as a function of time, the order of chemical reaction, γ in -df/dt=fγk (f: fraction of nitrogen unprecipitated, t: time, k: reaction rate constant) is found to be about two and apparent activation energy for precipitation of nitride coincides with that of diffusion of silicon in α-iron. This suggests that the precipitation is controlled by diffusion of silicon.
3) The creep curves consist of two steady-state stages. Transition times from the first to the second correspond with marked reductions in mobile nitrogen. This correspondence and transmission electron microscope observation indicate that the end of fresh precipitation of nitride on dislocation may be primarily responsible for the increase of creep rate and the transition.
