Effect of Grain Size on the Precipitation Behaviour in Super-Ferritic Stainless Steels During a Long-Term Ageing

A 27.6Cr-3.6Mo-2Ni alloy was solution treated and then aged for a long time to study the effect of grain size on precipitation behaviour by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The experimental results demonstrated that the average grain size increased from 46.3 +/- 6.2 to 101.8 +/- 13.5 mu m and the grain boundary length per unit area decreased from 3.3 x 10(4) to 1.7 x 10(4) m/m(2) with an increasing annealing temperature from 1100 to 1200 degrees C. After ageing at 800 degrees C, the sigma-phase, chi-phase and Laves phase were observed. As the ageing time increased, the sigma-phase notably increased, while the chi-phase and Laves phase gradually decreased before finally vanishing after ageing for 400 h. The sigma-phase precipitation kinetics curves consisted of two parts, and the grain size had a significant effect on the first stage of the precipitation curves due to the abundance of nucleation sites in the specimens with finer grains. The Laves phase was transformed from Nb(C,N) particles by Nb diffusion. As the ageing time increased, the ferrite phase decreased due to the transformation of the ferrite phase to the sigma-phase, and then C was expelled into the untransformed ferrite grains. Moreover, new Nb(C,N) particles were formed by Nb diffusion from the Laves phase, resulting in the absence of the Laves phase.

Automated summary

The study demonstrated that grain size has a significant impact on the precipitation behavior of the alloy, with finer grains having more nucleation sites. As ageing time increased, the sigma phase increased while the chi-phase and Laves phase decreased before disappearing completely.