A few observations on Laves phase precipitation in relation to its effects on creep rupture strength of ferritic steels based on Fe–9Cr (wt%) alloys at 650 °C

Creep rupture strengths, Laves phase precipitation and growth kinetics were measured at 650 °C for three Fe–9Cr (wt%) alloys in which the concentrations of Co, W and Mo were varied to investigate their effects on creep rupture strength and Laves phase precipitation behaviour, but C and N were not added to avoid the precipitation of carbide and carbonitride phases so that the effects of Laves phase precipitation on creep rupture strengths can be exclusively identified. In the two alloys containing about 3 wt% W, Laves phase was found to precipitate only on grain boundaries and hence contributed little to precipitation strengthening. A reduction in stress exponent for rupture time occurred long before the Laves phase precipitation was completed in these two alloys, which was attributed to the coarsening of Laves phase particles at grain boundaries. In an alloy with 4.53 wt% W+Mo added, Laves phase precipitated both at grain boundaries and within grains with a precipitation free zone present on both sides of grain boundary. The creep rupture strength of this alloy was higher than the other two alloys, which was considered to be mainly due to the solution strengthening effect of combined addition of W and Mo. Its stress exponent for rupture time did not change in the whole stress range tested, which was attributed to the fast growth kinetics of Laves phase particles precipitated on grain boundaries. Co had no solid solution strengthening effect, but it can increase the Laves phase precipitation kinetics in the alloys, which can also be increased by increasing the concentration of W+Mo.