Effect of the Laves phase and carbide on the creep strength of Fe–C–15Cr–W alloys

Abstract To clarify the effects of the Fe2W Laves phase and Cr23C6 carbide on the creep strength of a new ferritic heat-resistant steel with the ferrite parent phase strengthened by intermetallic compounds and carbonitrides, the creep strength at 973 K and microstructure of the ternary Fe–15Cr-(6, 9)W alloys and the quaternary Fe-0.05C–15Cr-(6, 9)W alloys (mass%) were investigated. Increasing the amount of Laves phase precipitates by increasing the tungsten content was effective in improving the creep strength due to the film-like Laves phase precipitates at the grain boundaries (GBs) of ferrite phase. Laves phase precipitates within the grains of ferrite phase became more uniform and finely distributed with the addition of carbon, but that did not improve the creep strength. Voids were formed around the Cr23C6 carbide precipitates at the GBs, and precipitate-free zones were observed around the GBs. We have concluded that these microstructures of the GBs caused creep rupture. In order to improve the creep strength of new precipitation-strengthened ferritic steel, reducing of the carbon content to suppress GB precipitation of Cr23C6 carbide, and increasing of the tungsten content to enhance the precipitation of Laves phase at the GBs are necessary.