Z-phase formation and its effect on long-term creep strength in 9–12%Cr creep resistant steels

Abstract

Precipitation behavior of Z-phase after long-term creep was investigated in ASME Gr.91, Gr.92, Gr.122 and 12Cr steel with delta-ferrite. Many of Z-phase particles were located around prior austenite grain boundaries (PAGBs) and interface between delta-ferrite and martensite after long-term creep. Part of MX cabonitrides around PAGB and the interface disappeared due to the Z-phase formation, indicating that the deformation resistance around PAGB and the interface is low. Time-Temperature-Precipitation (TTP) diagrams of Z-phase were obtained for all steels examined. The time for nucleation of Z-phase was shorter in higher Cr steel. In order to retard the Z-phase formation, the Cr content of the steel should be reduced. Creep strength abruptly decreases in higher Cr steel since the increase in Cr content promotes Z-phase formation together with disappearance of MX carbonitrides. Many of Z-phase particles formed around PAGB in fine-grained heat-affected zone (FGHAZ) of welded joints after long-term creep. The size of Z-phase was similar to those of M23C6 and Laves phase after creep. For FGHAZ, the number density of Z-phase particles in Gr.122 was four times higher than that in Gr.92. The creep strength of welded joint of Gr.122 was lower than that of Gr.92, indicating that a large number of Z-phase in Gr.122 contributes to decrease in creep strength.